Advances in Synthetic Applications of Hypervalent Iodine Compounds

Review pubs.acs.org/CR

Advances in Synthetic Applications of Hypervalent Iodine Compounds Akira Yoshimura* and Viktor V. Zhdankin* Department of Chemistry and Biochemistry, University of Minnesota Duluth, Duluth, Minnesota 55812, United States ABSTRACT: The preparation, structure, and chemistry of hypervalent iodine compounds are reviewed with emphasis on their synthetic application. Compounds of iodine possess reactivity similar to that of transition metals, but have the advantage of environmental sustainability and efficient utilization of natural resources. These compounds are widely used in organic synthesis as selective oxidants and environmentally friendly reagents. Synthetic uses of hypervalent iodine reagents in halogenation reactions, various oxidations, rearrangements, aminations, C−C bond-forming reactions, and transition metal-catalyzed reactions are summarized and discussed. Recent discovery of hypervalent catalytic systems and recyclable reagents, and the development of new enantioselective reactions using chiral hypervalent iodine compounds represent a particularly important achievement in the field of hypervalent iodine chemistry. One of the goals of this Review is to attract the attention of the scientific community as to the benefits of using hypervalent iodine compounds as an environmentally sustainable alternative to heavy metals.

CONTENTS 1. Introduction 2. Structure and Reactivity of Polyvalent Iodine Compounds 3. Synthetic Applications of Trivalent Iodine Compounds 3.1. Iodosylarenes 3.2. Hypervalent Iodine Halides 3.2.1. (Difluoroiodo)arenes 3.2.2. (Dichloroiodo)arenes 3.3. [Bis(acyloxy)iodo]arenes 3.4. [Hydroxy(sulfonyloxy)iodo]arenes 3.5. Aryliodine(III) Derivatives with Nitrogen Ligands 3.5.1. [Acyloxy(amido)iodo]arenes 3.5.2. [(Diamido)iodo]arenes 3.5.3. [(Azido)iodo]arenes 3.6. Heterocyclic Iodine(III) Compounds 3.6.1. Benziodoxole and Derivatives 3.6.2. Benziodazoles and Derivatives 3.6.3. Other Heterocyclic Iodine(III) Derivatives 3.7. Iodonium Salts 3.7.1. Alkyl- and Fluoroalkyliodonium Salts 3.7.2. Aryl- and Heteroaryliodonium Salts 3.7.3. Alkenyliodonium Salts 3.7.4. Alkynyliodonium Salts 3.8. Iodonium Ylides 3.9. Iodonium Imides 4. Synthetic Applications of Pentavalent Iodine Compounds 4.1. Noncyclic and Pseudocyclic Iodylarenes 4.2. Heterocyclic Iodine(V) Compounds

4.2.1. 2-Iodoxybenzoic Acid (IBX) and Derivatives 4.2.2. Dess−Martin Periodinane (DMP) and Analogues 5. Enantioselective Reactions Using Chiral Hypervalent Iodine Reagents 5.1. Chiral Iodine(III) Reagents 5.2. Chiral Iodylarenes 6. Iodine Compounds as Organocatalysts 6.1. Catalytic Cycles Based on Iodine(III) Species 6.1.1. Iodoarenes as Precatalysts 6.1.2. Iodide Salts as Precatalysts 6.2. Catalytic Cycles Based on Iodine(V) Species 6.3. Chiral Iodine Species as Organocatalysts 7. Recyclable Hypervalent Iodine Compounds 7.1. Polymer-Supported Iodine(III) Reagents 7.2. Polymer-Supported Iodine(V) Reagents 7.3. Recyclable Nonpolymeric Iodine(III) Reagents 7.3.1. Iodine(III) Reagents with Insoluble Reduced Form 7.3.2. Application of Ion-Exchange Resins for Recycling 7.3.3. Ion-Supported Iodine(III) Reagents 7.3.4. Fluorous Iodine(III) Reagents 7.4. Recyclable Nonpolymeric Iodine(V) Reagents 8. Conclusion Author Information Corresponding Authors Notes

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Received: September 16, 2015 Published: February 10, 2016 © 2016 American Chemical Society

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DOI: 10.1021/acs.chemrev.5b00547 Chem. Rev. 2016, 116, 3328−3435

Chemical Reviews Biographies Acknowledgments References

Review

The following areas of hypervalent organoiodine chemistry attracted especially strong interest and recent research activity: (i) applications of 2-iodoxybenzoic acid (IBX) and derivatives in organic synthesis, (ii) development of new iodine(III) reagents with nitrogen ligands, (iii) enantioselective catalytic applications of iodoarene compounds and iodide salts, (iv) chemistry of heterocyclic benziodoxole derivatives, (v) chemistry of iodonium salts, and (vi) structural studies of hypervalent iodine compounds. This Review summarizes the literature data that appeared after publication of our earlier reviews in 1996, 2002, and 2008.13−15 Presentation of material is arranged on the basis of the general classification of hypervalent iodine reagents. This Review covers literature published through the summer of 2015.

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1. INTRODUCTION Iodine is one of the heaviest elements in the Periodic Table, which is generally classified as a nonmetal. However, due to the large size of the iodine atom, the bonding in iodine compounds is different from that of the light main-group elements. The interatomic π-bonding common in the compounds of light pblock elements is not observed in iodine compounds. Instead, a linear, three-center-four-electron (3c-4e) bond (L−I−L) is formed by the overlap of the 5p orbital on iodine atom with the orbitals on the two ligands L. This 3c-4e bond is commonly named as a “hypervalent bond”. The presence of a weak, highly polarized hypervalent bond explains the special structural features and reactivity pattern of polyvalent iodine compounds.1−3 Compounds of iodine in higher oxidation states, which are known under common name of “hypervalent iodine compounds”, have emerged as versatile and environmentally benign reagents for organic chemistry. Structure and reactivity of hypervalent iodine compounds are generally similar to that of the transition metal derivatives. In particular, the reactions of hypervalent iodine reagents are usually rationalized as ligand exchange, oxidative addition, reductive elimination, and ligand coupling, in terminology typical of the chemistry of transition metal. However, in contrast to the heavy metals, iodine is an environmentally benign and a relatively inexpensive element (bulk price of iodine in the last 10 years was between $20 and $100 per kg). The annual production of iodine is about 30 000 tons with an estimated world’s total reserves of 15 million metric tons located mainly in Chile and Japan.3,4 One of the goals of this Review is to attract the attention of the scientific community as to the benefits of using hypervalent iodine compounds as an environmentally sustainable alternative to heavy metals. The chemistry of polyvalent iodine compounds has been previously summarized in four books,2,5−7 several book chapters,1,8−12 and numerous comprehensive reviews.13−32 Many specialized reviews on synthetic applications of specific classes of polyvalent iodine compounds have been published.33−114 These specialized reviews cover the following areas of hypervalent iodine chemistry: synthetic applications of [hydroxy(tosyloxy)iodo]arenes,33,34 chemistry of iodonium salts,25,35−50 the chemistry of iminoiodanes,51,52 chemistry of iodonium ylides,53−57 fluoro and perfluoroorgano hypervalent iodine reagents,58−60 chemistry and application of benzoiodoxoles,61−64 coordination chemistry of hypervalent iodine reagents,66,67 chemistry of polymer-supported hypervalent iodine compounds,68 chemistry of hypervalent iodine-mediated oxidative dearomatized reactions,69−78 hypervalent iodineinduced oxidative coupling reactions,79,80 ring contraction reactions,81,82 phosphorolytic reactivity of benziodoxolones,83 radical reactions of hypervalent iodine reagents,84−86 transition metal-catalyzed oxidations using hypervalent iodine reagents,87−90 stereoselective reactions of hypervalent iodine reagents,91−95 catalytic application of organoiodine compounds, 96−108 catalytic application of inorganic iodide salts,109−111 and synthetic application of pentavalent iodine reagents.112−114

2. STRUCTURE AND REACTIVITY OF POLYVALENT IODINE COMPOUNDS General aspects of structure and reactivity of polyvalent iodine compounds have been previously discussed in several books and reviews.1,2,5−12 Main classes of organohypervalent iodine compounds are shown in Figure 1. Key features of molecular structure and the general reactivity pattern of hypervalent iodine compounds are summarized below. The known organic compounds of polyvalent iodine belong to the following general classes: (1) trivalent iodine derivatives 1 and 2, named as λ 3 -iodanes according to IUPAC recommendations, and (2) pentavalent iodine derivatives 3, known as λ5-iodanes (Figure 2). The iodine(III) compounds (RIX2 1) have 10 electrons at the iodine atom and the overall trigonal bipyramidal geometry with the heteroatom ligands X in the apical positions, and the less electronegative carbon substituent R and two lone pairs of electrons occupying the equatorial positions. Iodonium salts 2 have a similar pseudo trigonal bipyramidal geometry with inclusion of a weakly bonded anionic part of the molecule. The λ3-iodanes RIX2 1 have an approximately T-shaped structure, and the I−X bond length is longer than an average I−X covalent bond length. Bond angles R−I−R in iodonium salts, iodonium ylides, and iodonium imides are close to 90°. The bonding orbital in λ3iodanes is occupied by two electrons from the nonhybridized 5p orbital of iodine and one electron of each ligand X in the linear X−I−X bond. The resulting hypervalent bond is highly polarized, longer, and weaker as compared to the usual covalent bond between two atoms. The presence of this hypervalent bond in λ3-iodanes explains the high electrophilic reactivity of hypervalent iodine compounds. In computational studies of λ3iodanes, Kiprof, Ochiai, and Suresh have found that the phenomenon of trans influence of ligands X in the hypervalent X−I−X bond plays an important role in stabilization of hypervalent iodine derivatives.115−117 The iodine(V) compounds (RIX4 3) have a square bipyramidal geometry with the organic substituent R and the lone pair of electrons in the apical positions and the four electronegative ligands X in the basal positions. Two orthogonal 3c-4e hypervalent bonds connect all ligands X to iodine, and the apical substituent R is linked by a regular covalent bond. In a study of ligand effects on organohypervalent iodine(V) compounds, Suresh reported the trans influence of ligands in λ5-iodanes derived from Dess−Martin periodinane.117 The organo λ3- and λ5-iodanes with aromatic substituent R (R = aryl or hetaryl) generally are stable compounds. Several examples of alkyliodine(III) derivatives 3329


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Figure 1. Main classes of organohypervalent iodine reagents.

3.1. Iodosylarenes

stabilized by a rigid structure (tricyclene or cyclopropyl) and strong electron-withdrawing groups have also been isolated. Hypervalent iodine reagents are extensively used as oxidants and electrophilic reagents in organic synthesis. In general, three typical reactivity patterns have been observed for hypervalent iodine reagents: (i) ligand exchange, (ii) reductive elimination, and (iii) ligand coupling (Scheme 1). The radical type reactions, homolytic reactions, and single-electron transfer (SET) reactions are also observed for hypervalent iodine reagents under appropriate conditions. These typical reactivity patterns of organoiodane reagents were summarized by Ochiai in a book chapter.118 It should be mentioned that the term “hypervalent” and the concept of hypervalency have been sharply criticized by theoretical chemists. In particular, the concept of hypervalency was criticized by Gillespie and Silvi who, based on the analysis of electron localization functions, stated that “as there is no fundamental difference between the bonds in hypervalent and nonhypervalent (Lewis octet) molecules there is no reason to continue to use the term hypervalent.”119 Despite all of the criticism, the term hypervalent has been overwhelmingly accepted by synthetic chemists, and the concept of hypervalency is currently widely used to describe the structural features and special reactivity of polycoordinated main-group elements.

Iodosylarenes 7 are effective oxidizing reagents; however, these reagents are usually insoluble in organic solvents except for methanol and DMSO. Iodosylarenes 7 are most commonly

Figure 2. Typical structural types of λ3-iodanes and λ5-iodanes.

Scheme 1. Simplified Reaction Mechanism of λ3-Iodanes 1 with Nucleophiles

3. SYNTHETIC APPLICATIONS OF TRIVALENT IODINE COMPOUNDS

obtained by hydrolysis of (diacetoxyiodo)arenes with aqueous NaOH.120 Under these conditions, various substituted iodosylarenes 7 can be prepared from the corresponding

Trivalent iodine compounds of types 1 or 2 are named λ iodanes according to IUPAC rules.2,5,6,13−15 In general, the λ3iodanes with an aryl substituent (1, R = aryl) are relatively stable compounds and are commonly used as reagents in organic reactions. This section of this Review covers the preparation, structural features, and reactions of the important classes of hypervalent iodine reagents: iodosylarenes, iodine(III) halides, acyloxy or sulfonyloxy aryl-λ3-iodanes, mono- or diamido-λ3-iodanes, heterocyclic iodine(III) compounds, iodonium salts, iodonium ylides, and iodonium imides. 3

Scheme 2. Typical Procedure for Preparation of Iodosylarenes 7 from (Diacetoxyiodo)arenes 6

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same group reported the aqua complexes of iodosylarenes 17− 19 with one molecule of water coordinated to the hypervalent iodine, which were prepared by treatment of (diacetoxyiodo)arenes with trimethylsilyl triflate or bis(trifluoromethylsulfonyl)imide in the presence of 18-crown-6 ether in dichloromethane.132,133 Single-crystal X-ray study revealed that these aqua complexes also have the T-shaped structure with two apical positions of the iodine(III) atom occupied by OH and one molecule of water. The presence of 18-crown-6 ether also has a stabilizing effect on iodine complexes (Figure 4). Oligomeric iodosylbenzenes 20 and 21 were prepared from λ3-iodanes via ligand exchange under acidic condition. Both compounds were characterized by X-ray diffraction, and the formation of these structures has been explained by selfassembly.134,135 The oligomer 20 was prepared by the reaction of PhI(OH)OTs and MgClO4, and X-ray analysis of compound 20 revealed a polymeric structure formed by dicationic pentaiodanyl units connected by secondary I···O bonds into a linear array of 12-atom hexagonal rings. The oligomer 21 can be prepared from PhI(OAc)2 and NaHSO4. X-ray crystal analysis of the (PhIO)3 fragment revealed an oligomeric structure with the T-shaped intramolecular geometry typical of trivalent iodine with O−I−O and O−I−C bond angles in the range of 166.54−177.99° and 79.18−92.43°, respectively. The I−O bond distances in the (PhIO)3 fragment of this oligomer are within 1.95−2.42 Å (Figure 5).135,136 The ortho-substituted iodosylbenzenes 8−11 were prepared by Protasiewicz and co-workers by basic hydrolysis of the respective ortho-substituted diacetoxy iodoarenes.51,121,122 Single-crystal X-ray diffraction analysis revealed that 2-tertbutylsulfonyl-substituted iodosylbenzene has a pseudocyclic structure, and the iodine center has intramolecular I···O secondary bonds with sulfonyl oxygen atom and intermolecular I···O secondary bonding with neighboring iodosyl oxygen atom.67 The introduction of intramolecular secondary bonding in ortho-substituted iodosylbenzenes 8−10 results in improvement of solubility in common solvents (0.08 M in chloroform). Iodosylbenzene is an effective oxidant, but it has poor solubility in common organic solvents because of the polymeric feature. Reactions with iodosylbenzene require initial depolymerization by adding a hydroxylic solvent or an appropriate catalyst (Lewis acid, bromide or iodide anion, transition metal complex, etc.). Examples of oxidation reactions with iodosylbenzene reported in our previous reviews13−15 include the following: oxidation of alcohols,137,138 oxidation of sulfides,139,140 Baylis−Hillman reaction with the combination of (PhIO)n/KBr/H2O,141 radical fragmentation reaction of alcohols or amines with the PhIO·I2 system,142−144 oxidative Grob rearrangement of 3-hydroxyl piperidine with iodosylbenzene in water,145 preparation of imidazoles, thiazoles, and imidazo[1,2-a]pyridines via α-tosyloxy carbonyl compounds using iodosylbenzene and p-toluenesulfonic acid, etc.146

Figure 3. ortho-Substituted iodosylarenes 8−11.

(diacetoxyiodo)arenes (Scheme 2).51,121−125 This procedure can also be used for the synthesis of 4-methoxyiodosylbenzene,124 4-nitroiodosylbenzene,124 and the pseudocyclic iodosylarenes 8−11 bearing 2-tert-butylsulfonyl,51,121,122 or 2diphenylphosphoryl groups (Figure 3).123 The alkaline hydrolysis of (dicholoroiodo)arenes 12 in the aqueous tetrahydrofuran solution can produce the paraiodosylarenes 13 and ortho-nitro iodosylbenzene 14 (Scheme 3).126 This reaction is also acceptable for the preparation of pseudocyclic iodosylarene 14 bearing a 2-nitro substituent.127 Iodosylbenzene is isolated as a yellowish amorphous powder that cannot be recrystallized from organic solvents because of the polymeric nature. In methanol solution of iodosylbenzene, the depolymerization of intermolecular network occurs to give PhI(OMe)2 15 via ligand exchange.128 Spectroscopic studies revealed that in the solid state iodosylbenzene has a μ-oxozigzag polymeric and asymmetrically bridged structure, with the PhIO units connected by intermolecular I···O secondary bonds.2,14,15,129 The primary I−O bond distances, the secondary intermolecular I···O bond distances, and the overall iodine center geometry were studied by EXAFS analysis.130 The μ-oxo-bridged zigzag polymeric structure of iodosylbenzene is also in agreement with the trans-influence concept.116 Iodosylbenzene can be depolymerized by addition of a hydroxylic solvent (water or alcohol) or a catalyst (Lewis or Brønsted acid, iodide or bromide anion, transition metal complex, etc.) to produce the monomeric iodosylbenzene species, which are known to be effective reagents in organic reactions. Several iodosylarene derivatives with the polymeric network disrupted due to intermolecular or intramolecular secondary interactions have been isolated, and their reactivity has been studied. The preparation, X-ray crystal structures, and the reactivity of activated iodosylbenzene monomeric complexes with 18crown-6 ether were reported by Ochiai and co-workers.66,107,131 In particular, the treatment of iodosylbenzene with HBF4· Me2O in the presence of 18-crown-6 ether afforded stable activated iodosylbenzene species 16, which can be dissolved in MeCN, MeOH, water, DMSO, and dichloromethane. Singlecrystal X-ray diffraction of protonated iodosylbenzene 16 revealed a T-shaped structure in which the secondary intermolecular interaction of iodine with 18-crown-6 ether replaces the intramolecular I···O hypervalent interactions in original (PhIO)n. This secondary intermolecular interaction is responsible for increasing the stability of this complex. The Scheme 3. Alkaline Hydrolysis of (Dichloroiodo)arenes

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Figure 4. Examples of activated iodosylarenes compounds 16−19.

Figure 5. Oligomeric iodosylbenzenes 20 and 21.

Scheme 4. Benzylic C−H Oxidation of 22 Using (PhIO)n/KBr Combination

Scheme 5. Cyclization of Michael Adducts 24, 26, and 29

Iodosylbenzene has also been used as a reagent for nucleophilic epoxidation of electrophilic alkenes.147,148 Several new oxidation reactions with iodosylbenzene have been recently reported. The benzylic C−H bond in compound 22 can be oxidized with (PhIO)n in water in the presence of montmorillonite-K10 (M-K10) clay and KBr to give carbonyl compounds 23 in good yields (Scheme 4).149 The mechanism of this C−H oxidation probably involves a radical initiator, which is generated from the (PhIO)n/KBr system. The oxidative cyclizations of Michael adducts 24, 26, and 29 promoted by iodosylbenzene and tetrabutylammonium iodide provide functionalized fused dihydrofurans 25, cyclopropanes

Scheme 6. Fluorinative Cyclization of 31

27, oxetanes 28, and azetidine derivatives 30 in good yields with excellent diastereoselectivities (Scheme 5).150−152 The reaction of N-(but-3-en-1-yl)-4-methylbenzenesulfonamide 31 with iodosylbenzene in the presence of BF3·Et2O affords 3-fluoro-1-tosylpyrrolidine 32 as the major product via a cyclic carbocation intermediate, which is generated by the 3332


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Scheme 7. Monofluorination of 1,3-Dicarbonyl Compounds 33

Scheme 8. Preparation of Oxazoles 36

Scheme 9. Intramolecular Azirizination Reaction of 37

Scheme 10. Isolation of Iodosylarene Adduct of a Manganese(IV)−Salen Complex

iodosylbenzoic acids. 2-Iodosylbenzoic acid (IBA) has the actual structure of benziodoxolone and is discussed in section 3.6 dealing with hypervalent iodine heterocycles. 3-Iodosylbenzoic acid is used as an efficient recyclable reagent for oxidative iodination of aromatic compounds (see section 7.3).157−161 Derivatives of transition metals have a significant catalytic effect on oxidations with iodosylarenes.2,14,15,90 Iodosylbenzene is widely used as a terminal oxidant for transition metalcatalyzed oxygenation of various substrates with metalloporphyrins and other transition metal derivatives via the intermediate oxo−metal complexes.162−168 Examples of oxidation reactions with iodosylarenes using various transition metal catalysts include hydroxylation of hydrocarbons,169−171 epoxidation of alkenes,172−175 and oxidation of alcohols176−178 or sulfides.179−181 The key active intermediates in these reactions are metal oxo complexes, which are generated from iodosylbenzene and transition metal catalysts, and these species are the actual oxygen transfer reagents to various substrates. However, details of the initial interaction of transition metal catalysts with hypervalent iodine(III) reagents are still unclear. It has been demonstrated that, in some reactions, the initial reaction of iodosylbenzene with metalloporphyrins generates unstable

intramolecular nucleophilic attack of the amino group on the iodine(III) species. In this reaction, BF3·Et2O plays a dual role, such as the activation of iodosylbenzene and a fluorine atom source (Scheme 6).153 The reaction of ethyl-3-oxo-3-phenylpropionate 33 using the combination of iodosylbenzene and aqueous HF affords 2fluoro-3-oxo-3phenylpropionate 34 in good yield (Scheme 7).154 The reaction of phenylacetylene 35 in acetonitrile with iodosylbenzene in the presence of trifluoromethanesulfonic acid affords 2-methyl-4-phenyloxazole 36 in good yield. The mechanism of this reaction probably involves an intermediate vinyl iodonium salt formed in situ from alkyne and iodosylbenzene, and the structure of this iodonium salt was confirmed by X-ray crystallography (Scheme 8).155 Moriarty and co-workers reported a metal-free intramolecular aziridination of alkenes 37 by treatment with iodosylbenzene and a catalytic amount of camphorsulfonic acid. This reaction mechanism probably involves formation of a sulfonyliminoiodane followed by intramolecular [2+2] cyclization to give sulfonylaziridine 38 in good yield (Scheme 9).156 Several acids other than iodosylbenzene ArIO have been used as oxidants, and the most important of these are 2- and 33333


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adducts, which can serve as the actual oxidants of substrates to afford the final products of oxygenation.182−185 Fujii and co-workers reported the preparation, structure, and useful reactivity of the iodosylarene adduct with manganese(IV)−salen complex 39. These iodosylarene−metal complexes in dichloromethane can perform oxidation of sulfides and epoxidation of alkenes in moderate yields and with low

Scheme 13. Oxidation by Using Activated Iodosylbenzenes 16 or 17

Scheme 11. Oxidations Using Oligomeric Iodosylbenzene Sulfate 21

Scheme 14. Oxidative Cleavage Reaction Using Activated Iodosylbenezene 16

enantioselectivity (Scheme 10).186 McKenzie and co-workers reported the crystal structure of an iron(III) iodosylbenzene complex. This complex is a protected precursor of FeO(V) species and a selective oxygenation reagent for sulfides.187 The reactivity of oligomeric iodosylbenzene sulfate 21 is similar to the acid-activated iodosylarenes. Reagent 21 reacts with alcohols, sulfides, and alkenes in aqueous acetonitrile at room temperature to give the corresponding oxidation products 40−43 in good yields (Scheme 11).135,188,189 The oligomeric iodosylbenzene sulfate 21 can also serve as a useful terminal oxidant for catalytic oxidation of anthracene 47 using metalloporphyrin complexes 44−46 as catalysts to afford the corresponding oxygenation products 48 (Scheme 12).88,190,191 Monomeric iodosylbenzene complex and aqua iodine(III) complexes 16−19 have been utilized as oxidants in several organic reactions. Activated iodosylbenzenes 16 and 17 are useful oxidants for oxidation of phenols, sulfides, styrene, and

silyl enol ethers in water to afford the corresponding oxidation products (Scheme 13).107,131−133 The activated iodosylbenzene 16 reacts with alkene 52 in water to give the product of double bond cleavage 53 in good yield (Scheme 14).133,192 This is a convenient procedure that can serve as a safe alternative to the ozonolysis reaction. 3-Phenylpropanol 54 reacts with activated iodosylbenzene species 16 affording the 6-chromanyl(phenyl)iodonium salt-18crown-6 ether complex 55 in good yield with excellent regioselectivity (Scheme 15),193 while the reaction of 16 with 3-phenylpropyl methyl ether 56 in the presence of the water and BF3−Et2O gives major product 57 resulting from paraselective aromatic fluorination with BF3 being the source of fluorine atom (Scheme 16).194 The ortho sulfonyl iodosylarenes 8−10 with intramolecular I···O secondary bonds are effective oxidants due to the good solubility in common organic solvents.51,67,121,122 The reactions

Scheme 12. Catalytic Oxidation of Anthracenes 47 Using Metal Catalysts 44−46

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Scheme 15. Synthesis of Chromanyl(phenyl)-λ3-iodane 55

Scheme 16. para-Selective Aromatic Fluorination of 3-Phenylpropyl Ether 56

unstable yellow solid insoluble in conventional solvents.203 IF5 is a powerful fluorinating reagent.12,204−206 Hara and co-worker reported the trifluoromethylation reaction of (methylsulfanyl)methyl tolylsulfone compounds,207 and the oxidative desulfurizing fluorination reactions with IF5/pyridine/HF complex,208,209 which is an air- and moisture-stable reagent. Iodine trichloride, ICl3, 65, can be prepared at low temperature from iodine and liquid chlorine.210 The solid-state structure of iodine trichloride was characterized by X-ray crystallography, which showed the dimeric structure, I2Cl6, with a planar geometry containing two bridging I−Cl···I bonds (I−Cl distances 2.68 and 2.72 Å) and four terminal I−Cl bonds (2.38−2.39 Å) (Figure 6).211 Experimental evidence supporting formation of iodine tribromide, IBr3, in solution was reported; however, this product could be isolated in the solid state.212

Scheme 17. Oxidation of Phosphine 58 and Sulfides 60

Scheme 18. Epoxidation of Alkenes 62 Using Metal Catalysts

Scheme 19. Typical Approaches to the Preparation of (Difluoroiodo)arenes

of phosphines 58 and sulfides 60 with iodosylarene 10 give corresponding products 59 and 61 in good yields (Scheme 17).49 Iodosylarenes 8 and 10 are also useful reagents for the oxidation of metal derivatives affording new metal−oxo species. Some of the generated metal−oxo species react with olefins 62 to give epoxidation products 63 (Scheme 18).51,195−198 Jones and Templeton reported the synthesis, structure, and oxygen atom insertion reactions of the complex of iodosylarene 10 with Rh catalyst.199 These iodosylarene−metal complexes can promote dimerization of metal complexes leading to the oxobridged dinuclear species.

Organic hypervalent iodine halides in general have higher thermal stability and are commonly used as reagents in organic synthesis. This section covers the preparation, structure, and Scheme 20. Synthesis of (Difluoroiodo)arenes 66 Using Xenon Difluoride

synthetic applications of organic difluoro- or dichloroiodoarenes. 3.2.1. (Difluoroiodo)arenes. Two general procedures are used for preparing (difluoroiodo)arenes 66: (1) oxidative addition of fluorine to iodoarenes 67 using strong fluorinating reagents, and (2) ligand exchange in iodine(III) compounds 68 using a fluorine anion source (Scheme 19). A mild and selective method for the preparation of (difluoroiodo)arenes according to the first approach consists of the treatment of aryl iodides with xenon fluoride and anhydrous HF (Scheme 20).213−215 This direct oxidative fluorination procedure is acceptable for the synthesis of various (difluoroio)doarenes with electron-donating or electron-withdrawing substituents; however, the reaction time is longer for the electron-deficient iodoarenes.

Figure 6. Structures of IF3 64 and ICl3 65.

3.2. Hypervalent Iodine Halides

Inorganic iodine halides (iodine fluorides, iodine chlorides, iodine bromide) in general have low thermal stability. Iodine trifluoride is an unstable compound decomposing at −28 °C.200 Even at low temperatures, IF3 64 quickly disproportionates to IF5 and IF or I2.201 Hoyer and Seppelt attempted the recrystallization of IF3 from anhydrous HF at low temperature, and performed single-crystal X-ray structure determination (Figure 6).202 The X-ray analysis of IF3 revealed that this compound has a polymeric structure with distorted T-shaped molecular geometry. Iodine pentafluoride, IF5, is isolated as an 3335


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CH3CN by treatment of (diacetoxyiodo)benzene with Bu4NF in absolutely anhydrous media.232 Sanford and co-workers have also reported the preparation of p-MeC6H4IF2 in situ from pMeC6H4I(OPiv)2 and AgF.233 (Difluoroiodo)arenes can be used as reagents for the synthesis of iodonium salts. Hara and co-workers reported the reaction of para-(difluoroiodo)toluene 71 with terminal alkynes 72 leading to stereo- and regioselective formation of synthetically useful (E)-2-fluoro-1-alkenyliodonium salts 73, which could be further converted in situ to (E)-2-fluoro-1-iodo1-alkenes 74 by the KI/CuI- or Pd-catalyzed coupling reaction (Scheme 22).224,234−236 Ochiai and co-workers reported the reaction of compound 71 with alkynylcarbocycles 75 affording the ring-expanded iodonium salts 76 in high yields with excellent stereoselectivity (Scheme 22).237 The mechanism of this reaction probably involves a sequence of electrophilic iodanation, 1,4-halogen shift, ring enlargement, and fluorination of the alkynes. Frohn and co-workers reported the reaction of parafluoro(difluoroiodo)benzene 77 and trimethylsilyl cyanide 78 to give the para-fluoro[cyano(fluoro)iodo]benzene 79, pFC6H4IF(CN), or para-fluoro(dicyanoiodo)benzene 80, pFC6H4I(CN)2, via ligand exchange (Scheme 23).238 Some of the (cyanoiodo)arene derivatives are relatively unstable; however, structures of p-FC6H4IF(CN) 78 and p-FC6H4I(CN)2 80 have been confirmed by X-ray analysis.238 Various (difluoroiodo)arenes have been used as powerful and selective fluorinating regents. β-Ketoesters and β-dicarbonyl derivatives are selectively fluorinated at the α-position by difluoroiodotoluene.227,229,239 In a specific example, reactions of β-ketoesters, β-ketoamides, or β-diketones 81 with para(difluoroiodo)toluene 71 give the respective products of monofluorination 82 under mild conditions in good yields (Scheme 24).239 The oxidative monofluorination products 84 can also be prepared from α-(phenylthio)acetamides 83 by treatment with (difluoroiodo)toluene 71 under mild conditions (Scheme 25).240,241 Likewise, the reaction of α-phenylthio esters 85 under similar conditions affords fluorides 86 (Scheme 25).242 The reaction mechanism probably involves a fluoro-Pummerertype reaction. This monofluorination reaction using para(difluoroiodo)toluene 71 also works for a selective and clean fluorination of α-substituted selenides 87 to give products 88 in moderate yields (Scheme 26).231 β-Dicarbonyl derivatives are selectively α-tosyloxylated by reagent 71 and para-toluenesulfonic acid monohydrate in dichloromethane solution.243 This reaction is also very effective for introducing various other oxygen-containing functionalities,

In general, (difluoroiodo)arenes are difficult to handle because of the highly hygroscopic properties. The preparation of 66 using XeF2 is very convenient because xenon gas is the only byproduct in the reaction, and the obtained products 66 are sufficiently pure for further use as reagents. This useful procedure is also suitable for the synthesis of heteroaromatic iodine fluorides; for example, treatment of 4-(C5F4N)I with XeF2 affords 4-(C5F4N)IF2 in 84% yield.216 A similar reaction of 3-iodo-4-methylfurazan with XeF2 in acetonitrile at room temperature affords the 3-(difluoroiodo)-4-methylfurazan.217 Other strong fluorinating agents, such as elemental fluorine, XeF2/BF3, CF3OCl, ClF, BrF5, C6F5BrF2, and C6F5BrF4, have been used for preparing (difluoroiodo)perfluoroarenes.218−222 Shreeve and co-workers developed a useful procedure for the synthesis of (difluoroiodo)arenes utilizing Selectfluor as a fluorinating reagent in acetonitrile solution.223 The same group has also improved the one-pot oxidative iodination/fluorination reaction for preparing (difluoroiodo)arenes from the corresponding arenes using elemental iodine with Selectfluor. Several (monofluoroiodo)arene triflates, ArIF(OTf), have been prepared in situ from the respective iodoarenes and xenon fluorotriflate, FXeOTf.224,225 Another synthetic method reported by Fuchigami and Fujita involves electrochemical fluorination of iodoarenes to (difluoroiodo)arenes.226−228 This procedure is especially effective for preparing para-substituted (difluoroiodo)arenes. Some electrochemically generated Scheme 21. Preparation of (Difluoroiodo)arenes 70 Using Aqueous HF

(difluoroiodo)arenes can be employed as in-cell mediators for subsequent fluorinations.226−229 In a classical one-step ligand exchange procedure, mercuric oxide and aqueous hydrofluoric acid in dichloromethane are used to prepare (difluoroiodo)arenes from (dichloroiodo)arenes.230 The use of HgO for removal of the chloride ion from reaction mixture is a drawback of this method. Hara and coworkers have developed a convenient mercury-free modification of this procedure by using the reaction of freshly prepared iodosylarenes 69 with aqueous HF to give products 70 in good yields (Scheme 21).126,231 Because of the moisture-sensitive nature, (difluoroiodo)arenes usually are prepared in situ and used in solution without isolation. DiMagno and co-workers have developed a method for a very clean generation of (difluoroiodo)benzene in

Scheme 22. Reactions of Terminal Alkynes with para-(Difluoroiodo)toluene 71

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Scheme 23. Synthesis of (Cyanoiodo)arenes 79 and 80 from para-Fluoro(difluoroiodo)benzene 77 and TMSCN 78

Scheme 24. Monofluorination of β-Dicarbonyl Compounds 81

Scheme 26. Monofluorination of Organic Selenides

recyclable hypervalent iodine reagents 97−100 (Figure 7).250,251 Alternative procedures for oxidative chlorination of aryl iodides involve the use of hydrochloric acid and an inorganic oxidant, such as MnO2, KMnO4, KClO3, NaIO3, Na2S2O8, NaBO3, Na2CO3·H2O2, CrO3, concentrated HNO3, and urea− H2O2.252−257 In a specific example, the chlorination of aryl iodides with Na2S2O8 in the presence of hydrochloric acid gives (dichloroiodo)arenes in high yields.253 A convenient one-pot oxidative iodination/chlorination of arenes with an appropriate oxidant in hydrochloric acid has also been developed.253 A useful procedure for chlorination of iodoarenes using the urea− H 2 O 2 complex under solvent-free condition has been reported.257 A mild and convenient procedure for the synthesis of (dichloroiodo)arenes 101 involves the use of aqueous sodium hypochlorite in concentrated hydrochloric acid (Scheme 30).258 Originally reported X-ray crystal structures of (dichloroiodo)arenes, PhICl2259 and 4-ClC6H4ICl2,260 were imprecise, and 40 years after the original publication a better quality crystal structure of PhICl2 has been reported by Chaloner and co-workers.261 The single-crystal X-ray crystallography data for PhICl2 show the T-sharped geometry around the iodine center with two primary I−Cl bond distances of 2.47 and 2.49 Å. The molecules of PhICl2 are arranged into a zigzag network due to the intermolecular secondary interaction between chlorine and iodine atoms with a I···Cl distance of 3.42 Å. The hypervalent iodine center has overall square planar

such as mesyloxy, acetoxy, phosphoryloxy, methoxy, ethoxy, and isopropoxy. Murphy and co-workers reported the α,α-difluorination reaction of phenylacetate derivatives 89 with para(difluoroiodo)toluene 71 (Scheme 27).244 This is a metal-free reaction useful for the synthesis of α-carbonyl difluorides 90. Fluorinated cyclic ethers 92 can be prepared from the iodoalkyl-substituted precursors 91 using reagent 71 via a stereoselective fluorinative ring expansion (Scheme 28).245 Some five-, six-, or seven-membered ring-contracted products 94, 96 were synthesized from the cyclohexene, cycloheptene, or cyclooctene derivatives 93, 95 using (difluoroiodo)toluene 71 (Scheme 29). (Difluoroiodo)toluene 71 reacts with arylsubstituted alkenes to afford the rearranged, geminal difluorides, formed by migration of the aryl group.82,228,246,247 3.2.2. (Dichloroiodo)arenes. (Dichloroiodo)arenes are widely used as chlorinating reagents. Among (dichloroiodo)arenes, (dichloroiodo)benzene, PhICl2, is the most commonly used reagent, which can be conveniently prepared by direct chlorination of iodobenzene with chlorine in dichloromethane or chloroform.248 The direct chlorination of iodobenzene with chlorine has been used for the kilogram-scale preparation of PhICl2.249 (Dichloroiodo)arenes are heat- and light-sensitive yellow solids, and, in general, these reagents cannot be stored for extended periods of time. The direct chlorination of iodoarenes is also useful for the synthesis of the efficient

Scheme 25. Monofluorination of α-(Phenylthio)acetamides 83 and α-Phenylthio Esters 85

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Scheme 27. α,α-Difluorination of Phenylacetate Derivatives 89

α-hydroxy-β,β-dichloropyrrolidine 108 in high yield (Scheme 33). The proposed mechanism of this oxidative chlorination involves multiple C−H bond activation. This procedure is also acceptable for the oxidative chlorination of N-protected six- or seven-membered nitrogen heterocycles.268 In the reaction of an electron-rich aromatic ketone, such as 4aminoacetophenone 109, with (dichloroiodo)benzene, a selective chlorination of the aromatic ring occurs, instead of the α-chlorination of ketone, leading to 4-amino-3-chloroacetophenone 110 in high yield (Scheme 34).249 This reaction has been scaled up to produce 25 kg of product 110 of high purity. Prakash and co-workers have reported the analogous chlorination of 2-aryl-2,3-dihydro-4(1H)-quonolone 111 giving products of regioselective chlorination 112 in moderate yield (Scheme 34).269 The reaction of 5,10,15-trisubstituted porphyrins 113 with (dichloroiodo)benzene selectively gives the meso-chlorinated porphyrins 114 in high yields (Scheme 35).270 Under similar conditions, the reaction of 5,10,15,20-tetraarylporphyrins gave the β-monochlorinated compounds as the main products. α,α-Dichlorination reaction of phenylacetate derivatives 89 (see Scheme 27) using (dichloroiodo)benzene in the presence of pyridine has been reported by Murphy and co-workers.244 This is a fast reaction giving products of gem-dichlorination in good yields. (Dichloroiodo)benzene reacts with monoterpenes in methanol via the ionic pathway, giving products of chloromethoxylation with excellent regio- and stereoselectivity.271 Nicolaou and co-workers reported enantioselective 1,2dichlorination of allylic alcohols using (dichloroiodo)arenes in the presence of catalytic amounts of a dimeric cinchona alkaloid derivative (DHQ)2PHAL 115 as chiral auxiliary.272 For example, the (DHQ)2PHAL 115-catalyzed enantioselective dichlorination reaction of trans-cinnamyl alcohols 116 with 4Ph(C6H4)ICl2 affords products 117 in good yields and enantioselectivities (Scheme 36). A 1,3-dichlorination reaction of the donor−acceptor cyclopropanes using (dichloroiodo)benzene and leading to various chlorinated products of ring opening in good yields has been reported; a single electron transfer (SET) mechanism has been suggested for this reaction.273

Scheme 28. Fluorinative Ring Expansion Reaction of Cyclic Ethers 91

geometry with lone pairs of electrons residing in the transpositions.261 (Dichloroiodo)arenes are commonly used as efficient chlorinating reagents. In general, the two most common mechanistic pathways for the chlorination of substrates with (dichloroiodo)arenes are known: (1) radical pathway under photochemical conditions or the presence of radical initiators in low polarity solvents, and (2) ionic pathways due to the electrophilic nature of the iodine atom in (dichloroiodo)arenes or the electrophilic addition of Cl2 generated by initial dissociation of the reagents. An alternative concerted molecular addition mechanism in the reactions of PhICl2 with alkenes has also been proposed.262 The general reactivity patterns of ArICl2 were discussed in detail in several earlier reviews.13,263,264 (Dichloroiodo)benzene has been applied for a substitutive chlorination at the sp3-carbon of ketones.265,266 For example, some aliphatic and aromatic ketones can be directly converted into the corresponding α-chloroketones in moderate yields. The mechanism of α-chlorination of ketones can be either radical or ionic controlled by the reaction conditions.265 Zhang and co-workers reported the synthesis of αchloroketone acetals 103 in two steps starting from various ketones 102 by reaction with (dichloroiodo)benzene in the presence of molecular sieves in ethylene glycol solution (Scheme 31).266 (Dichloroiodo)toluene has been used as a reagent for the enantioselective α-chlorination reaction of β-keto esters 104 in the presence of titanium complex 105 in toluene, giving the corresponding α-chlorinated products 106 in moderate yield with a relatively low enantioselectivity (Scheme 32).267 Interestingly, the maximum enantioselectivity of this chlorination is observed at 50 °C. The oxidative chlorination of N-protected pyrrolidine 107 by treatment with 3 equiv of p-nitro(dichloroiodo)benzene affords

Scheme 29. Fluorinative Ring Contraction Reaction of Cyclic Alkenes 93 and 95

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Figure 7. Recyclable (dichloroiodo)arenes.

Scheme 30. A Convenient Procedure for the Preparation of (Dichloroiodo)arenes

Scheme 33. Chlorination of N-Protected Pyrrolidine 107

Pb(SCN)2 led to the formation of para-thiocyanated phenol derivative 126 in 78% yield (Scheme 39).283 (Dichloroiodo)benzene is a common reagent for the oxidation or chlorination of various derivatives of transition metals. Representative examples of such reactions include the oxidation of complexes of molybdenum,284 tungsten,284 palladium,285−290 platinum,291,292 cobalt,293 vanadium,294 iridium, 295 rhodium, 296−298 thallium, 299 cerium, 300 and gold.301−305 In addition, PhICl2 has been used for the conversion of heterobimetallic complex Pt(II)−Au(I) to the Pt(III)−Au(II) complex.306

Scheme 31. Synthesis of α-Chloroketone Acetals 103

Recently, the regioselective reactions of chloroformyloxylation and α-chlorination of alkenes 118 and 120 using (dichloroiodo)benzene have been developed.274 This methodology provides a convenient approach to either regioselectively chloroformyloxylated products 119, or α-chlorinated alkenes 121, depending on the starting alkenes (Scheme 37). (Dichloroido)benzene can also be used for the oxidation of alcohols, aldehydes,275−277 or sulfides.278 The reaction of secondary alcohols in the presence of sodium azide affords the corresponding ketones in good yields.275,277 Oxidation of primary alcohols 122 with the PhICl2−NaN3 combination gives the corresponding carbamoyl azides 123 as major product via Curtius rearrangement. This reaction is acceptable for a onepot synthesis of various ureas 124 directly from primary alcohols and amines using the PhICl2−NaN3 combination (Scheme 38). Reaction of aryl sulfides with (dichloroiodo)benzene in pyridine−water solution affords the corresponding sulfoxides in good yields. This reaction occurs almost instantaneously at room temperature.278 The combination PhICl2−Pb(SCN)2 can be used for the effective thiocyanation reaction of β-dicarbonyl compounds, silyl enol ethers, ketene silyl acetals,279 alkynes,280 phenols, and naphthols.281−283 For example, the treatment of a sterically bulky phenol derivative 125 with a mixture of PhICl2−

3.3. [Bis(acyloxy)iodo]arenes

[Bis(acyloxy)iodo]arenes, ArI(O2CR)2, belong to one of the most important classes of hypervalent iodine(III) compounds. Two representatives of these compounds, (diacetoxyiodo)benzene (DIB) and [bis(trifluoroacetoxy)iodo]benzene (BTI or PIFA), are common, commercially available oxidants. Numerous applications of [bis(acyloxy)iodo]arenes as precursors to other hypervalent iodine derivatives and as oxidizing reagents have been summarized in previously published reviews.2,6,13−15 This section provides an overview of synthetic methods for the preparation of [bis(acyloxy)iodo]arenes, a summary of structural studies, and recent examples of applications of these important reagents. [Bis(acyloxy)iodo]arenes can be prepared using two general approaches: (1) oxidation of an aryl iodide in the presence of appropriate carboxylic acid, and (2) conversion of a readily available (diacetoxyiodo)arene to a new [bis(acyloxy)iodo]arene by a ligand exchange reaction with a carboxylic acid. In particular, the practically important reagent, DIB, can be prepared by the direct oxidation of iodobenzene using peracetic acid according to the procedure of Sharefkin and Saltzman.307 A common procedure for preparing various [bis(acyloxy)iodo]arenes consists of the treatment of substituted iodoben-

Scheme 32. Enantioselective Mono-chlorination Using Titanium Catalyst 105

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Scheme 34. Regioselective Chlorination of Aromatic Ketones

Scheme 37. Regioselective Chloroformylation or αChlorination of Alkenes

Scheme 35. Chlorination of Porphyrins

zenes with corresponding peracids; this method is also acceptable for preparing the polymer-supported analogues of DIB from poly(iodostyrene) or aminomethylated poly(iodostyrene),68,308−311 and the ion-supported (diacetoxyiodo)arenes.312,313 Likewise, treatment of various iodoarenes with peroxytrifluoroacetic acid affords the respective [bis(trifluoroacetoxy)iodo]arenes in high yield.314−316 Other oxidants, such as periodates,317−319 chromium(VI) oxide,320 sodium percarbonate, 321 m-chloroperoxybenzoic acid (mCPBA),322−327 potassium peroxodisulfate,328,329 H2O2− urea,330 Selectfluor,223 and sodium perborate,331 can oxidize iodoarenes in the presence of carboxylic acids to give the corresponding [bis(acyloxy)iodo]arenes. For example, oxidation of aryl iodides using sodium perborate (NaBO3) in acetic acid at 40 °C has been used for preparing numerous (diacetoxyiodo)-substituted arenes and heteroarenes, such as N-tosyl-4-(diacetoxyiodo)pyrazole, N-trifluoromethanesulfonyl4-(diacetoxyiodo)pyrazole, 2-(diacetoxyiodo)thiophene, and 3(diacetoxyiodo)thiophene.331−337 The chiral (diacetoxyiodo)arene 127 and its derivatives were efficiently prepared by the perborate oxidation of the corresponding aryl iodides (Figure

Scheme 38. Oxidative Rearrangement of Primary Alcohols Using PhICl2−NaN3

Scheme 36. Enantioselective Dichlorination of Allylic Alcohols

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Scheme 39. Thiocyanation of Phenol 125

Scheme 41. Preparation of [Bis(trifluoroacetoxy)iodo] Compounds 132 and 133

Figure 8. Chiral (diacetoxyiodo)arenes.

8).337 Under similar conditions, the chiral (diacetoxyiodo)arenes 128 and 129 were prepared directly from the corresponding iodoarenes in the work of Fujita and co-workers (Figure 8).336 An improved procedure for the synthesis of (diacetoxyiodo)arenes in the presence of catalytic amounts of trifluoromethanesulfonic acid using sodium perborate has been reported.338 Shreeve and co-workers developed a convenient practical procedure for the preparation of (diacetoxyiodo)arenes using Selectfluor as the oxidant in acetic acid.223 This efficient synthetic procedure was used for the direct oxidation of iodoarenes with the rigid spirobiindane backbone,339 and also for the oxidation of the C2-symmetric chiral iodoarene 130 to give the chiral hypervalent iodine reagent 131 in high yield (Scheme 40).101,340 A convenient newer method for the synthesis of [bis(trifluoroacetoxy)iodo]arenes 132 from iodoarenes using commercially available and inexpensive oxidant Oxone (2KHSO5·3KHSO4·3K2SO4) in trifluoroacetic acid at room temperature has been reported.341 This procedure is also acceptable for the synthesis of [bis(trifluoroacetoxy)iodo]perfluoroalkanes 133 (Scheme 41). The ligand exchange in (diacetoxyiodo)arenes with an appropriate carboxylic acids represents the second common approach to [bis(acyloxy)iodo]arenes. The ligand exchange reaction is usually performed by heating (diacetoxyiodo)arene and a nonvolatile carboxylic acid in a solvent with a high boiling point (Scheme 42).342−347 The relatively volatile acetic acid is removed under these reaction conditions resulting in the formation of [bis(acyloxy)iodo]arene 134 as the main product. This procedure is acceptable for preparation of the glutamic acid-based [bis(acyloxy)iodo]benzenes 135,343 amino acid derivatives 136,345 cinnamic acid derivatives 137,347 and 3methylfurazan-4-carboxylic acid derivative 138.348 In general, the ligand exchange reaction of (diacetoxyiodo)arenes with stronger acids can be performed under mild conditions at room temperature. In a specific example, the reaction of DIB with trifluoroacetic acid readily proceeds at room temperature to give BTI after evaporation.349 A similar procedure can be used

for preparing a useful recyclable oxidizing reagent, 3-[bis(trifluoroacetoxy)iodo]benzoic acid 140, from 3-iodosylbenzoic acid 139 in trifluoroacetic acid (Scheme 43).157,350 Numerous X-ray structural studies of bis(acyloxy)iodo]benzenes have been discussed in the previous reviews.2,6,13−15 In most cases, bis(acyloxy)iodo]benzenes have a pentagonal planar coordination at the iodine atom including two secondary intramolecular I···O bonds with oxygen atoms of the carboxylate ligands. In 1979, Alcock and co-workers reported single-crystal X-ray structures of (diacetoxyiodo)benzene and [bis(dichloroacetoxy)iodo]benzene, in which the importance of secondary bonding in the solid-state stucture of [bis(acyloxy)iodo]benzenes has been demonstrated.351 In particular, the crystal structure of PhI(OAc)2 141 revealed a distorted Tshaped geometry at the iodine center and the overall pentagonal-planar coordination with the two primary I−O bonds and two secondary intramolecular I···O bonds (Figure 9). Bond distances of the two primary I−O bonds in 141 are about 2.156 Å, which is longer than the sum of the atomic covalent radii (1.99 Å), and the I−C bond distance (2.090 Å) in 141 is also longer than the sum of covalent radii of carbon and iodine (2.07 Å). The intramolecular I···O secondary bond distances of 2.817 and 2.850 Å are significantly shorter than the sum of the van der Waals radii of iodine and oxygen.351 In contrast to PhI(OAc)2 141, the molecules of PhI(OCOCF3)2 142 form a dimeric structure due to the additional I···O intermolecular secondary bonds (Figure 9).352 In the crystal structure of [bis(trifluoroacetoxy)iodo]pentafluorobenzene, C6F5I(OCOCF3)2, one primary C−O bond, two primary I− O bonds, two weak I···O intramolecular secondary bonds, and two I···O intermolecular secondary bonds are present, forming a heptacoordinated iodine center.353 Several examples of μ-oxo-bridged diiodanyl diacetates and their derivatives whose X-ray crystal structures were reported in the literature are shown in Figure 10. These structures include

Scheme 40. Preparation of C2-Symmetric Chiral (Diacetoxyiodo)benzene 131

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Scheme 42. Preparation of [Bis(acyloxy)iodo]benzenes from (Diacetoxyiodo)arenes

The 17O NMR spectroscopic study of bis(acyloxy)iodoarenes in chloroform confirmed the T-shaped structure of trivalent iodine compounds in solution.357,358 The carboxylic groups of [bis(acyloxy)iodo]arenes demonstrate a dynamic behavior due to a [1,3]-sigmatropic shift of the iodine between two oxygen centers in the carboxyl.358 DIB and other bis(acyloxy)iodoarenes are useful reagents for the oxidation of alcohols.2,15,104 For example, the PhI(OAc)2·I2 system has been used for efficient oxidation of secondary alcohols to ketones and primary alcohols to carboxylic acids.359 Oxidation of aldehydes or primary alcohols by treatment with PhI(OAc)2·I2 in a methanol solution gives methyl esters 147 in high yields (Scheme 44).359,360 The experimental procedure for the oxidation of alcohols using 3-nitro-(diacetoxyiodo)benzene 148 and boron trifluoride ether complex as additive has been reported by Ochiai and co-workers.361 This BF3-catalyzed oxidation reaction probably involves a rapid ligand exchange on the iodine center with alcohols 149 to generate the alkoxyiodane intermediate 150, followed by a rate-limiting reductive elimination of 3-nitroiodobenzene to produce carbonyl compounds 151 (Scheme 45). The combination of DIB and TEMPO (2,2,6,6-tetramethyl1-piperidinyloxyl), which was first proposed by Piancatelli, Margarita, and co-workers,362 can serve as an efficient reagent for oxidation of alcohols.326,363−368 Using this reagent combination, various primary and secondary alcohols can be converted to the corresponding carbonyl compounds in generally high yields.362 Under these conditions, primary alcohols are selectively oxidized to aldehydes, without overoxidation to carboxylic acids, even in the presence of other oxidizable moieties. As a typical example, oxidation of nerol 152 using DIB in the presence of catalytic amounts of TEMPO in aqueous acetonitrile affords the corresponding aldehyde 153 in high yield (Scheme 46).363 The TEMPO-catalyzed selective oxidation procedure can also employ recyclable and polymersupported hypervalent iodine reagents instead of DIB (see section 7). Several similar oxidative procedures use polymersupported TEMPO,369 fluorous-supported TEMPO,370,371 ionliquid-based TEMPO,372 or silica-supported TEMPO366 in combination with hypervalent iodine reagents. Several examples of the uncatalyzed oxidation of alcohols with hypervalent iodine(III) reagents are known.309,373−375 Fullerene diol can be oxidized to the respective fullerene dione in high yield by PhI(OAc)2 in benzene solution at 35 °C.374

Scheme 43. Synthesis of 3[Bis(trifluoroacetoxy)iodo]benzoic Acid 140

Figure 9. Single-crystal X-ray structures of PhI(OAc)2 141 and PhI(OCOCF3)2 142.

Figure 10. μ-Oxo diiodanyl compounds with reported X-ray crystal structures.

the chiral μ-oxoiodobinaphthyl diacetate 143,333 μ-oxodiiodanyl diacetate 144,136 μ-oxodiiodanyl ditrifluoroacetate 145,354 and heterocyclic μ-oxodiiodanyl diacetate 146.355 The bond length I(III)−OCOCF3 in the molecule of 145 is considerably longer than that in BTI 142, which is consistent with a significant ionic character of 145.356 Scheme 44. Oxidation of Alcohols Using the DIB·I2 System

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Scheme 45. Reaction Mechanism for the Oxidation of Alcohols Using the 148−BF3·Et2O System

Scheme 46. Oxidation of Nerol 152 to Nepal 153 Using the DIB−TEMPO System

Scheme 47. Oxidative Cleavage of Hydrobenzoin 155

Scheme 49. Diastereoselective Diacetoxylation of Alkenes Using DIB

Scheme 48. Reactions of Alkenes with BTI

Wirth and co-workers have found that the perfluorinated analogue of BTI 154 can cleave hydrobenzoin 155 quantitatively to benzaldehyde 156 at room temperature in only 8 min (Scheme 47).376 [Bis(acyloxy)iodo]arenes are efficient reagents for various oxidative transformations of alkenes.19,377−383 For example, the reactions of alkenes with [bis(trifluoroacetoxy)iodo]benzene (BTI) in the absence of any additives afford vicinal bis(trifluoroacetoxy) derivatives, which can be converted to glycols or carbonyl compounds by hydrolysis.378,383 In particular, the treatment of cyclohexene 157 with BTI under reflux conditions leads to the cis-1,2-bis(trifluoroacetate) 158 in high yield (Scheme 48).378 The reaction of a bicyclic alkene, benzonorbornadiene 159, with BTI under similar conditions gives the product of rearrangement 160 in 95% yield (Scheme 48). Analogous products of rearrangement are also obtained in the reactions of alkenes with (diacetoxyiodo)benzene in the presence of strong acids.384 Gade and co-workers reported mechanistic studies on the protiocatalytic nature of the diacetoxylation reaction of alkenes using (diacetoxyiodo)benzene.385 Systematic studies of the catalytic activity in the presence of proton-trapping and metalcomplexing additives confirmed that strong acids act as catalysts in this reaction. Addition of trifluoromethanesulfonic acid as a catalyst showed a selectivity and reaction rate similar to or better than the most efficient metal cation catalysts, such as Pd(II) and Cu(II).385

Diastereoselective diacetoxylation of various alkenes using the DIB−BF3·OEt2 system has been investigated to demonstrate that a selective synthesis of syn- or anti-diacetoxylation products can be achieved depending on the presence or absence of water.386 As a demonstration of this efficient methodology, the diastereoselective diacetoxylation of various alkenes, including styrenes, aliphatic alkenes, cycloalkenes, and α,β-unsaturated esters, has been performed to give the corresponding vicinal diacetates 162 and 163 (Scheme 49). This procedure is also acceptable for a large-scale reaction of 161 affording final products in good yields. The proposed reaction mechanism involves 1,3-dioxolan-2-yl cation 164385−389 as key intermediate, analogously to the I2−silver salt-mediated Prevost390 and Woodward391 reactions (Scheme 49). Scheme 50. Aminobromination of Electron-Deficient Olefins

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Scheme 51. Iodocarboxylation of Alkenes with 136

Liang and co-workers reported the selective diacetoxylation of various piperidine derivatives using DIB.392 This reaction gave desired cis-diacetoxylation compounds in moderate yields. A convenient procedure for a highly regioselective 1,2acetoxysulfenylation of alkenes with disulfides using DIB has been reported.393 Wang and co-workers have developed a DIBpromoted procedure for the regio- and stereoselective aminobromination of electron-deficient olefins 166 to products 167 using the N-bromosuccinimide-tosylamide combination394,395 or bromamine-T396 as nitrogen and bromine source (Scheme 50). The same group has also reported that, when using chloramine-T as nitrogen source and chlorine source, products of aminochlorination of electron-deficient olefins were obtained with high regio- and stereoselectivity and in good yields.397,398 This heavy-metal-free protocol is superior to metal salts for the aminobromination or aminochlorination of electron-deficient olefins. (Diacetoxyiodo)benzene in combination with simple bromide salts in ethanol can be used for the regioselective ethoxybromination of a wide range of enamides giving synthetically versatile α-bromo hemiaminals.399 Stereoselective bromoacetoxylation or iodoacetoxylation of alkenes using DIB in combination with bromide or iodide anion sources has been developed by Kirschning and coworkers.400,401 The active electrophilic species in these reactions, diacetylhalogen(I) anions (AcO)2X− (X = Br or I), are generated from the corresponding halide anions and polymer-supported iodine reagents.401 The reaction of the PhI(OAc)2·I2 combination with alkenes in the presence of external nucleophiles has been used for the preparation of various β-functionalized iodoalkanes.402 Under simililar conditions, the treatment of alkenes 168 with amino acid-derived phenyliodine(III) dicarboxylates 136 and tetraphenyl phosphonium iodide led to the iodocarboxylate compounds 169 in moderate yields (Scheme 51).345 The combination of [bis(acyloxy)iodo]arenes with iodine is an excellent reagent for oxidative iodination of aromatic compounds.403 For example, the treatment of electron-rich arenes with the DIB·I2 combination in the presence of sulfuric acid at room temperature afforded the corresponding iodine compounds in high yields.404 The oxidative halogenation of arenes using DIB in a solvent-free system has been reported.405 Alkanes can be iodinated using DIB·I2 combination in the presence of t-butanol under photochemical or thermal conditions.406 The monoiodination reaction of methoxysubstituted alkyl aryl ketones using the combination of BTI·I2 in acetonitrile or methanol affords the corresponding iodinated products in moderate to good yields.407 The BTI·I2 system is also effective for iodination of electron-deficient heterocyclic compounds.408 The chemically unstable compounds, 3iodoindole derivatives, were also obtained by using the BTI·I2 system. The sensitive protective groups, such as acetyl, BOC, and TBDMS, are stable under these reaction conditions.408 The reaction of 2,3-disubstituted indoles with PhI(OAc)2·TBAI

Scheme 52. General Mechanism of BTI-Mediated Amidation

affords products of regioselective acetoxylation in good yields.409 The active species of this reaction is hypoiodite. Various oxidative reactions, such as cationic cyclizations, rearrangements, and fragmentations using [bis(acyloxy)iodo]arenes, have been summarized in previous reviews.15,17,19,21,24,34,79,81,410 The preparation of heterocycles by the BTI-induced oxidative cationic cyclization is a particularly important procedure. For example, numerous BTI-induced intramolecular amidation reactions of precursors 170 to give five-, six-, and seven-membered heterocycles 172 have been reported by ́ Tellitu and Dominguez (Scheme 52).411,412 The key active species in these reactions, N-acylnitrenium intermediates 171, are initially generated from amides 170 and BTI, and the experimental evidence supports the ionic mechanism of these cyclization reactions.413 This methodology can be used to prepare various heterocyclic compounds, such as heterocycle-fused quinolinones,414 1,4-benzodiazepin-2-one derivatives and fused indeno-1,4-diazepinones,415,416 benzo-, naphtho-, and heterocycle-fused pyrrolo[2,1-c][1,4]diazepines,417 quinolinones or pyrrolidinones,418 dibenzo[a,c]phenanthridines,419 thiazolofused quinolinone derivatives,420 isoindolinones and isoquinoScheme 53. BTI-Mediated Syntheses of Heterocyclic Compounds

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lin-2-ones, 421 indolines,422 5-aroyl-pyrrolidinone derivatives,423,424 indazolones,425 substituted indolizidinones,426,427 1-arylpyrrolopyrazinones,428 benzisothiazolene,429,430 isothiazolones,431 α-hydroxyalkyl lactam derivatives,432 structurally diverse pyrrolo(benzo)diazepines,433 and furopyrimidinones.434 Representative examples include the preparation of pyrrolidinones 174 from alkynylamides 173,423,424 benzisothiazol-3ones 176 from 2-mercaptoamides 175,429 and hydroxyl lactams 178 from o-alkyl hydroxamates 177 (Scheme 53).432 Similar hypervalent iodine-induced heterocyclizations of the appropriate amide or amine precursors have been utilized in several important synthetic transformations, such as DIBpromoted preparation of aminoindolines by inter- or intramolecular oxidative deamination of terminal alkenes and amines,435 preparation of 1-arylcarbazoles by metal-free electrocyclization,436 preparation of highly substituted pyrrolin-4-ones by BTI-promoted cyclization of enaminones,437 preparation of 2-substituted-4-bromopyrrolidines by DIBpromoted oxidative bromocyclization of homoallylic sulfonamides,438 synthesis of 1,2,4-thiadiazoles by the reaction of [bis(acyloxy)iodo]benzenes with 1-substituted thioureas,439,440 DIB-promoted synthesis of pyrazolo- and isoxazolopyrimidines,441 preparation of azaspirocycles by the BTI-promoted nitrenium ion cyclizations,442−447 synthesis of lactams and spiro-fused lactams by the reaction of N-acylaminophthalimides with BTI,448 preparation of various substituted 1,2,4-triazolo[4,3-a]pyrimidines by the DIB-induced oxidation of 2,4pyrimidinylhydrazones,449−451 preparation of pyrrolidino[60]fullerene from the DIB-induced reaction of C60 with amino acid esters,452 preparation of 1,3,4-oxadiazoles from acylhydrazones by BTI-mediated oxidations,453−455 preparation of substituted 1,2,4-triazolo[4,3-a]quinoxalines from arenecarboxaldehyde-3methyl-2-quinoxalinylhydrazones,456,457 preparation of N-substituted indoles by BTI-induced cyclization of enamines,458 the DIB-mediated preparation of indoles from N-aryl enamines,459 the DIB- or BTI-mediated synthesis of oxindoles and spirooxindoles,460,461 the BTI-mediated synthesis of carbazolones and 3-acetylindoles,462 preparation of polysubstituted pyrroles from 3-alkynyl amines with DIB,463 the one-pot synthesis of substituted pyrroles using DIB,464 and the synthesis of 2-arylproline derivatives by intramolecular oxyamination of alkenes with BTI or DIB.465 Numerous cyclizations of nonamine substrates promoted by DIB or BTI have also been reported. These DIB- or BTImediated oxidative cyclizations were used for the preparation of oxygen heterocycles, such as the one-pot synthesis of 2,3dihydrofurans,466 the synthesis of benzo- and naphthofurans via DIB-induced intramolecular oxidative cyclization of o-hydroxystillbenes,467 the preparation of isoxazolines or isoxazoles from aldoximes and alkenes or alkynes using DIB468−472 or BTI,473,474 the synthesis of 4,5-disubstituted furfuryl alcohols via BTI-mediated oxidative cycloisomerization of 2-propargyl-

substituted 1,3-dicarbonyl compounds,475 the BTI-promoted preparation of oxazoles via oxidative isomerization of propargylamides,476 the synthesis of oxazolines from N-

obtained compound 182 can be further converted to the natural product alkaloid (−)-pinidine.481 [Bis(acyloxy)iodo]arenes-promoted rearrangements have been applied in various ring expansion or ring contraction reactions,482−494 Hofmann rearrangement,495−505 and Claisen rearrangement.506−509 Stereoselective synthesis of cyclic ethers 184 has been performed via deiodonative ring-enlargement of cyclic substrates 183 by treatment with (diacetoxyiodo)toluene or BTI (Scheme 56).483

Scheme 54. DIB-Mediated Oxidative Decarboxylation of 2Aryl-Substituted Carboxylic Acids

Scheme 58. DIB or BTI-Mediated Hofmann Rearrangement of Carboxamides

Scheme 55. BTI-Mediated Oxidative Stereospecific Fragmentation of Cyclopropyl Alcohol 181

allylamides using DIB,477 and the DIB-mediated preparation of pyrimidine-annulated oxazolines.478 Various fragmentations or rearrangements at electrondeficient centers induced by hypervalent iodine reagents have been reported. For example, the reaction of 2-aryl-substituted carboxylic acids 179 with DIB in the presence of a catalytic amount of sodium azide affords carbonyl compounds 180 via oxidative decarboxylation (Scheme 54).479 Scheme 56. Hypervalent Iodine-Induced Stereoselective Ring Expansion Reactions

Treatment of tertiary cyclopropyl alcohols with BTI involves the oxidative fragmentation reaction. For example, the framentation reaction of tertiary cyclopropyl alcohol 181 with BTI gives product 182 in good yield (Scheme 55).480,481 The Scheme 57. DIB-Mediated Stereoselective Ring Contraction Reactions

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corresponding products 204 in good yields (Scheme 63).511 The reaction of alkenes 205 using DIB in the presence of tertbutylhydroperoxide gives the allylic oxidation product 206 in moderate to good yields (Scheme 63).512 Both reaction mechanisms involve intermediate radical species. Direct oxidation of aromatic ring can be achieved using BTI in combination with Lewis acids. For example, reactions of various anilides 207 with BTI and BF3·Et2O at room temperature in acetic acid or methanol afford the corresponding para-substituted compounds 208 and 209 in good yields (Scheme 64).513 The direct tosyloxylation of anilides 207 has been performed in a similar fashion by treatment of anilides with BTI under mild conditions in the presence of BF3·Et2O and toluenesulfonic acid to give para-tosyloxylated products 210 with high regioselectivity (Scheme 64).514 A similar preparation of para-triflates 211 from anilides 207 has been achieved by direct oxidative triflation using BTI as the oxidant and AgOTf as the source of triflate anion (Scheme 64).515 [Bis(acyloxy)iodo]arenes are often used as reagents in various oxidative transformations of phenols and other electron-rich aromatic substrates.308,335,516−521 (Diacetoxyiodo)benzene is a useful oxidant for the synthesis of benzoquinones from various substituted o- and p-hydroquinones.335,517,518 Particularly important are the hypervalent iodine-induced oxidative dearomatizations of p- or o-substituted phenolic substrates 212 and 215 in the presence of external or internal nucleophiles affording the corresponding cyclohexadienones 214 or 216 (Scheme 65). This reaction involves phenoxyiodine(III) intermediates 213, formed via initial ligand exchange reaction, followed by reductive elimination of iodobenzene and addition of nucleophiles to give the final cyclohexadienones 214 or 216.522 The phenolic substrates can be involved in inter- or intramolecular oxidative dearomatization with various nucleophiles, such as water, alcohols, fluoride ion, carboxylic acids, amides, oximes, and carbon nucleophiles (Scheme 65). Synthetic applications of oxidative dearomatization reactions of phenols and related substrates have been summarized in numerous reviews.70,72−74,76−78,94,523 An example of this reaction in the intermolecular mode is illustrated by the reaction of para-substituted substrate 217 with carbon nucleophiles such as allylsilane 218 to give the corresponding products 219 with a quaternary carbon (Scheme 66).524 Hypervalent iodine-induced dearomatization of phenolic substrates in the intramolecular mode is a powerful synthetic tool for the construction of spirodienone fragment. Numerous

Scheme 59. Synthesis of Symmetrical Ureas via Hofmann Rearrangement of Carboxamides

Methyl 2-aryl-2,3-dihydrobenzo[b]furan-3-carboxylate 186 was prepared by oxidation of flavanone 185 with DIB in the presence of sulfuric acid in trimethyl orthoformate via a stereospecific ring contraction by an aryl shift (Scheme 57).494 Organohypervalent iodine(III) compounds, such as DIB or BTI, are particularly useful as reagents for the Hofmann-type degradation of aliphatic or aromatic carboxamides 187 affording carbamates 189 or amines 190 via intermediate isocyanates 188 (Scheme 58).15,19,81,510 Kalesse and co-workers developed the synthesis of symmetrical urea derivatives 192 by a Hofmann rearrangement of carboxamides 191 using DIB followed by the nucleophilic addition of amines (Scheme 59).497 Yanada and co-workers reported the platinum(II)-catalyzed syntheses of indoles and isoquinolines 194 mediated by DIB in a Hofmann-type rearrangement of 2-alkynylbenzylamides 193, and a similar procedure was successfully used for the preparation of C2-symmetric macrocyclic bisindoles 196 from 2-alkynylbenzamides 195 (Scheme 60).496 DIB or BTI-mediated iodonio-Claisen rearrangement reactions present a useful approach to ortho-iodoarene derivatives. Zhu and co-workers reported the synthesis of orthoallyliodoarenes 198 and 199 from allyltrimethylsilane 197 and DIB via the reductive iodonio-Claisen rearrangement (Scheme 61).506 This approach has been used in the concise synthesis of an antifungal natural product, broussin. Vallribera and Shafir reported the BTI 201-mediated synthesis of the α-arylation product 202 by iodonio-Claisen rearrangement from α-cyanoketones 200 (Scheme 62).507 The prepared α-(2-iodoaryl)ketones 202 are versatile synthetic building blocks. Oxidations of C−H bonds at the benzylic or allylic position can be performed using DIB in the presence of iodine or peroxides. For example, alkylbenzenes 203 can be oxidized using DIB and p-toluenesulfonamide or p-nitrobenzenesulfonamide in 1,2-dichloroethane at 60 °C to give the

Scheme 60. Hofmann Rearrangement of 2-Alkynylbenzamide Derivatives

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Scheme 61. Synthesis of ortho-Allyliodoarenes Using Allyltrimethylsilane and DIB

Scheme 62. BTI-Mediated Iodonio-Claisen Rearrangement of α-Cyanoketones

Scheme 63. DIB-Mediated C−H Oxidation Reactions

Scheme 65. Oxidative Dearomatization of Phenols 212 and 215

Scheme 64. BTI-Mediated Oxidation of Anilides 207

Scheme 66. Oxidative Allylation of Phenols 217

various phenols 226 to give the corresponding dienone 228 via intermediate 227 in moderate yields (Scheme 68). The migration of an allyl group occurs stereospecifically with a retension of configuration. An oxidative ipso-rearrangement mediated by a hypervalent iodine reagent that enables rapid generation of a functionalized dienone system 230 containing a quaternary carbon center has been developed (Scheme 69).553 The process occurs through transfer of an aryl group from a silyl segment present on the lateral chain of the phenol derivative 229. This transformation was used in the total synthesis of an alkaloid sceletenone.553 Kita and co-workers have reported the reaction of 1-(phydroxyaryl)cyclobutanol 231 and DIB in hexafluoroisopropanol (HFIP) and water producing spiro cyclohexadienone lactone 232 via a domino reaction (Scheme 70).555 This

oxidative spirocyclizations of phenolic substrates containing an internal oxygen,470,471,525−532 nitrogen,72,533−536 or carbon nucleophile537−551 have been reported and utilized in natural product syntheses. Representative examples include the preparation of spirolactone 221 from amino acid 220,530 spirolactam 223 from phenolic 2-oxazolines 222,535 and spirocyclic 225 from phenols 224 (Scheme 67).541 Canesi and co-workers have developed several DIB-mediated synthetically useful tandem rearrangement protocols for the oxidative dearomatization of phenol derivatives.552−554 For example, an oxidative Wagner−Meerwein rearrangement mediated by DIB allows the allyl, aryl, and alkyl migration in 3347


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Scheme 67. Oxidative Spirocyclization Using DIB

Scheme 69. Oxidative Rearrangement of Phenol 229

Scheme 70. DIB-Mediated Domino Reaction of 1-(pHydroxyaryl)cyclobutanol 231

reaction mechanism involves four following steps: (1) oxidation of phenols forming an alkoxyiodine(III) intermediates 233, (2) rearrangement of 233 producing a spiro diene dione compound 234, (3) water attack on the ketone affording a carboxylic phenol, and (4) intramolecular cyclization of 235 giving the spiro cyclodienone lactone 232, proceeding in a domino manner. Hypervalent iodine-induced oxidative coupling reactions of electron-rich aromatic substrates in polar, non-nucleophilic solvents can proceed via a single electron transfer (SET) mechanism. The intermediate formation of cation-radical intermediates 237 has been detected by ESR spectroscopy in the mechanistic study of oxidative coupling reactions of aromatic ethers 236 published by Kita and co-workers (Scheme 71).556 The initially generated intermediates 237 further react with external or internal nucleophiles to give the final products of dearomatization 238 or coupling 239. In groundbreaking research, Kita and co-workers have also developed various synthetically useful intermolecular or intramolecular oxidative coupling reactions.557−559 The intermolecular coupling reaction has been utilized for the preparation of products 239 from N3−, AcO−, ArS−, SCN− anions, β-dicarbonyl compounds, and other external nucleophiles. The intramolecular coupling reaction provides a powerful synthetic tool for the preparation of various carbocyclic and heterocyclic compounds.560−567 Recent examples of the intramolecular oxidative coupling of phenolic ethers include the following: (1) oxidative coupling of tetramethoxysubstituted phenyl acetate 240 affording the corresponding tetramethoxydibenzooxepione 241,568 (2) cyclization of Nprotected indoles 242 to pyrroloiminoquinones 243,569 and (3)

cyclization of benzylsulfide 244 to dihydrobenzothiophene 245570 (Scheme 72). Coupling products 243 and 245 have been employed in the total synthesis of the potent cytotoxic makaluvamine F.540,571−573 Kita and co-workers have reported the intermolecular crosscoupling reaction between two different phenolic ethers 246 and 247 using the combination of [bis(trifluoroacetoxy)iodo]pentafluorobenzene and BF3·OEt2. This coupling reaction produced the mixed naphthalene-benzene biaryl compounds 248 without formation of homocoupling products (Scheme 73).574 The obtained biaryl compounds 248 have been further applied in the synthesis of the benz[b]phenanthridine products. The BTI-induced oxidative coupling reaction can also proceed with the nonphenolic electron-rich aromatic substrates and various nucleophilies to afford the corresponding coupling products.325,558,559,575−578 Kita and co-workers reported the BTI-induced oxidative homocoupling reaction of arenes 249 to give biaryl compounds 250, and the oxidative cross-coupling of naphthalene 251 and pentamethylbenzene 252 leading to the corresponding unsymmetrical biaryl product 253 without formation of homocoupling products (Scheme 74).355,575,577 Various iodinated biaryl compounds can be also prepared directly from the corresponding iodinated arenes using BTI in the presence of BF3·OEt2 under similar conditions.579 In the cross-coupling reaction of thiophenes and nucleophiles, the reaction of 3-hexylthiophene 254 with BTI and

Scheme 68. DIB-Mediated Oxidative Wagner−Meerwein Rearrangement of Phenols 226

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Scheme 71. Oxidative Coupling of Aromatic Substrates 236

affording the dimeric coupling product in 74% has also been reported by Zheng and co-workers.583 The DIB or BTI-mediated direct coupling reactions of various organic substrates are known. For example, the crosscoupling reaction between cycloalkenylsilanes 262 and the silylated nucleobase 263 promoted by DIB in the presence of trimethylsilyl triflate in dichloromethane gives the coupling products 264 in moderate yields (Scheme 78).584 This procedure was applied in the synthesis of a potential antiHIV agent having bis(hydroxymethyl)cyclohexene as a pseudosugar moiety. The treatment of the tetrahydroisoquinolines 265 with various organomagnesium compounds in the presence of BTI gives the corresponding products 266 in good yields (Scheme 79). This method represents a direct sp3 C−H bond functionalization of tetrahydroisoquinolines.585 The oxidative amidation,586,587 azidation,588−592 thiocyanation,593−595 and selenization347,380−382,596−602 have been developed as useful synthetic methodologies, and the desired products can be easily prepared from the corresponding starting materials using the combination of [bis(acyloxy)iodo]arenes and corresponding nucleophiles. A direct oxidative amination of aldehydes using (diacetoxyiodo)benzene in the presence of N-hydroxysuccinimide has been developed.586 Reactions of various aldehydes 267 with amines 268 and DIB under these conditions afford the respective amides 269 in good to excellent yields (Scheme 80). The proposed reaction mechanism involves radical species, which were detected by ESR spectroscopy.586 A similar procedure can provide convenient access to the glycosyl carboxamides from appropriate aldehydes and amines using (diacetoxyiodo)benzene in the presence of ionic liquids at room temperature.587 When a similar reaction of aldehydes 270 was performed with the sodium azide instead of amines and without Nhydroxysuccinimide, aroyl azides 271 were obtained in good yields (Scheme 81).589 This reaction probably involves initial

Scheme 72. Intramolecular Oxidative Coupling of Phenolic Ethers

TMSX (X = Cl, Br, NCS, or CN) at room temperature in methylene chloride provides the corresponding substituted products 255−257 in good yields (Scheme 75).325,558,580 The head-to-tail thiophene homocoupling product 258 can be selectively prepared from 3-hexylthiophene 254 using BTI reagent and trimethylsilyl trifluoromethanesulfonate (Scheme 76).581 In the reaction of pyrroles, cross- or homocoupling products can be also obtained under similar conditions.325,578 A combination of BTI and BF3·OEt2 provides the C−C coupling of Bodipy (4,4′-difluoro-4-bora-3a,4a-diaza-s-indacene) monomers 260 leading to mixtures of dimers 261 (when X = I or p-tolyl) (Scheme 77) and higher oligomers, when X = H.582 Bodipy dyes have attracted significant interest in recent years due to their outstanding optical properties. A similar reaction of zinc-linked chlorin monomer using BTI

Scheme 73. Intermolecular Oxidative Cross-Coupling of Phenolic Ethers

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Scheme 74. Oxidative Homo- and Cross-Coupling of Alkylarenes

reported by Tingoli and co-workers.597−600 The phenylselenated products are obtained in good yields from the corresponding alkenes, diphenyl diselenide, and DIB in acetonitrile. In particular, the reaction of cyclohexene 278 under these conditions proceeds as stereoselective transaddition to give trans-1-acetoxy-2-(phenylseleno)cyclohexane 279 in good yield (Scheme 85).600 Vesely and co-workers reported a DIB-mediated enantioselective α-selenylation reaction of aldehydes 280 using organocatalyst 281 with commercially available phenyl diselenide under mild oxidative conditions (Scheme 86).602 This transformation affords α-selenyl alcohols 282 in good yields and with excellent enantioselectivities. This reaction opens a suitable and alternative way for the preparation of biologically active building blocks such as β-hydroxy alcohols, α-amino acids, and α-hydroxy esters. [Bis(acyloxy)iodo]arenes can be used as efficient initiators of radical processes. Upon heating or irradiation, [bis(acyloxy)iodo]arenes undergo decarboxylative decomposition producing alkyl radicals, which can be trapped with various heteroaromatic bases or electron-deficient alkenes. 3 4 3 , 6 0 3 − 6 0 9 The (diacetoxyiodo)arene−iodine combination represents another important radical-generating system.610 This system is useful for generating the oxygen-centered radicals from carboxylic acids or alcohols.611−616 In particular, the oxidative cyclization of 2-substituted benzoic acids 283 using the [bis(acyloxy)iodo]arene−iodine combination under photolysis at room temperature affords lactones 284 in moderate to good yields (Scheme 87).611,612 A useful synthetic methodology is based on transformations of the alkoxy radicals generated by the reaction of alcohols with DIB and iodine under photochemical or thermal conditions. This methodology has been utilized in a number of syntheses.142,617−638 In particular, the preparation of modified cyclodextrin 286 by an intramolecular radical approach from alcohol 285 using the DIB−iodine system was reported by Martin and Suarez (Scheme 88).636

Scheme 75. Oxidative Substitution Reaction of 3Hexylthiophene 254

generation of (diazidoiodo)benzene from DIB and sodium azide. A direct decarboxylative azidation of α,β-unsaturated carboxylic acids 272 can be realized using BTI and sodium azide in the presence of a phase transfer reagent, Et3NBr, to give vinyl azides 273 in good yields (Scheme 82).590 This method is also acceptable for the synthesis of acyl azides.590 The reaction of alkenes 274 with potassium thiocyanate and DIB at room temperature affords the difunctionalized products, acetoxy-thiocyanate derivatives 275, in moderate to good yields. This transformation provides access to products 275 with anti-stereoselectivity and in good regioselectivity (Scheme 83).593 1,2-Dithiocyanates 277 can be obtained by a similar reaction from alkenes 276 and trimethylsilyl isothiocyanate using DIB as the oxidant (Scheme 84).594 Reactions of cyclohexene and 1methylcyclohexene under these conditions proceed stereoselectively affording the respective trans-adducts.594,595 The arylselenation reaction of various alkenes by diaryl diselenides in the presence of (diacetoxyiodo)benzene has been Scheme 76. Oxidative Homo-Coupling of 3-Hexylthiophene 254

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Scheme 77. BTI-Mediated Synthesis of Bodipy Dimers

Scheme 78. DIB-Mediated Direct Coupling of Cycloalkenylsilanes and Silylated Nucleobase

Scheme 79. BTI-Mediated sp3 C−H Bond Functionalization

Scheme 83. Oxidative Thiocyanation of Alkenes

Scheme 84. Dithiocyanation of Alkenes

Scheme 80. DIB-Mediated Oxidative Amidation of Aldehydes

Scheme 85. Stereoselective Selenylation Reaction of Cyclohexene 278

Scheme 81. DIB-Mediated Oxidative Azidation of Aldehydes

the oxazaspiroketal-containing cephalostatin 290 in high yield (Scheme 90).649 Chalcogen compounds (S, Se, Te) and pnictogen compounds (P, As, Sb, Bi) are readily oxidized by [bis(acyloxy)iodo]arenes to give the corresponding oxidation products.654−669 For example, the oxidation of ditellurides 291 by BTI at room temperature affords arenetellurinic mixed anhydrides 292.657 Triarylbismuthanes 293 are also oxidized by DIB at room temperature to give the pentavalent triarybismuth diacetates 294 in good yields (Scheme 91).660 [Bis(acyloxy)iodo]arenes are also useful terminal oxidants for complexes of transition metals, and the [bis(acyloxy)iodo]arenes−transition metals combinations are effective catalytic systems for various oxidations.670 The combination of (diacetoxyiodo)benzene and metalloporphyrins or other transition metal complexes is an effective oxidative system for the biomimetic oxygenation reactions.671−673 A selective oxidation of primary or secondary alcohols to the corresponding carbonyl compounds can be promoted by (diacetoxyiodo)arenes with transition metal catalysts, such as ruthenium trichloride,159,674,675 Ru(pybox) (pydic),676 polymer-micelle

Scheme 82. Synthesis of Vinyl Azide from Vinyl Carboxylic Acid

This system is also useful for generating nitrogen radicals.144,377,639−653 For example, the nitrogen radical species, generated from the N-alkylsaccharins 287 and DIB in the presence of iodine under irradiation with a tungsten lamp, are key intermediates in the intramolecular coupling reaction to give arenesulfonamides 288 (Scheme 89).639 Lee and co-workers reported the aminyl-radical cyclization of the primary amine 289 using the DIB·I2 combination to give 3351


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Scheme 86. Enantioselective α-Selenylation Reaction of Aldehydes

Scheme 89. Synthesis of N-Alkylsaccharin Derivatives

Scheme 87. Synthesis of Lactones from Benzoic Acids

3.4. [Hydroxy(sulfonyloxy)iodo]arenes

incarcerated ruthenium catalysts,677 chiral-Mn(salen) complexes,678,679 Mn(TPP)CN/Im catalytic system,680 (salen)Cr(III) complexes,681 and iron tetraamido macrocyclic complexes.682 The intramolecular or intermolecular oxidative coupling reactions using [bis(acyloxy)iodo]arenes in the presence of transition metal catalysts have also been reported. 89,90,683−700 The mechanisms of reactions of (diacetoxyiodo)benzene and other hypervalent iodine reagents in the presence of palladium catalysts have been summarized by Sanford and co-workers.89,90 In palladium-catalyzed reactions, the hypervalent iodine reagents, such as DIB and PIFA, can oxidize palladium(II) to palladium(IV) species, which are key active species in catalytic cycles. Unusual palladium(IV) complexes have been prepared by the reaction of PhI(O2CPh)2 with palladium(II) complexes containing chelating 2-phenylpyridine ligands and characterized by X-ray crystallography.701 Pd-catalyzed reactions using hypervalent iodine reagents are useful for selective C−H acyloxylation,690,691,696,697,701−710 aminooxygenation,711−713 diamination,714,715 synthesis of cyclopropanes, 685,688,693 synthesis of N-heterocyclic compounds,687,689 and cyclization of N-alkylanilines.716 The combination of the hypervalent iodine reagents and gold catalysts can be used for effective oxidation of organic substrates.717−723 For example, direct coupling of (4fluorophenyl)trimethylsilane 295 and 1-chloro-2-methoxybenzene 296 with (diacetoxyiodo)benzene and camphorsulfonic acid in the presence of the Au(I) catalyst affords the crosscoupling product 297 in high yield (Scheme 92).722

[Hydroxy(sulfonyloxy)iodo]arenes, ArI(OH)OSO2R, belong to one of the most important classes of hypervalent iodine(III) compounds. A detailed discussion of the literature on the synthesis, structure, and uses of iodine(III) organosulfonates can be found in several books and reviews.2,13−15,33 An important representative of these compounds, [hydroxy(tosyloxy)iodo]benzene, PhI(OH)OTs (Koser’s reagent or HTIB), is a commercially available reagent. For the general preparation of [hydroxy(organosulfonyloxy)iodo]arenes 298, [hydroxy(tosyloxy)iodo]heteroarenes 299− 301, recyclable [hydroxy(tosyloxy)iodo]arenes 302−306, and a polymer-supported [hydroxy(tosyloxy)iodo]benzene 307 (Figure 11), the ligand exchange reaction with an appropriate organosulfonic acid and [bis(acyloxy)iodo]arenes is usually used.157,341,724−735 Likewise, numerous polyfluoroalkyl derivatives of the types CnF2n+1I(OH)OTs 308341,736,737 and CnF2n+1CH2I(OH)OTs 309736,738 can be prepared by treatm en t of t he respective po ly fluoro alky liod o bis(trifluoroacetates) with p-toluenesulfonic acid. Togo and co-workers reported a convenient one-pot procedure for the preparation of various [hydroxy(organosulfonyloxy)iodo]arenes by treatment of iodoarenes with mCPBA in the presence of organosulfonic acids at room temperature.730 Olofsson and co-workers reported a further modified procedure for the synthesis of [hydroxy(organosulfonyloxy)iodo]arenes 311 from arenes 310, iodine, mCPBA, and respective sulfonic acids. This convenient one-pot protocol involves the oxidative iodination-oxidation-ligand exchange of arenes 310 (Scheme 93).739

Scheme 88. Preparation of Modified Cyclodextrin 286

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Scheme 90. Radical Cyclization of Primary Amine 289 Using DIB−I2 Combination

PhI+OH and the respective sulfonate anion as fully solvated species, which do not form ion pairs with each other. One of the most typical reactions of [hydroxy(organosulfonyloxy)iodo]arenes 316 is the functionalization of carbonyl compounds 315 at α-carbon to give products 317 (Scheme 95).33,34 Synthetic applications of this α-carbon functionalization include several one-pot reactions, such as α-alkoxylation or αacetoxylation,745 α-iodination,746 α-azidation,747 and heterocyclization.34 In particular, the tosyloxylation of ketones followed by heterocyclization has been used in the syntheses of numerous heterocyclic systems, such as 2-aroylbenzo[b]furans,748 3-aryl-5,6-dihydroimidazo[2,1-b][1,3]thiazoles,748 6arylimidazo[2,1-b]thiazoles,749 (1S,2R)-indene oxide,750 2mercaptothiazoles,751 triazolo-[3,4-b]-1,3,4-thiadiazines, 752 d ih y d r o in d e n o [ 1 , 2 - e ] [ 1 , 2 , 4 ] t r i a z o l o [ 3, 4-b ] [ 1 , 3 , 4 ] thiadiazines,753 furo[3,2-c]coumarins,754,755 4,5-diarylisoxazoles,756 2-substituted 4,5-diphenyloxazoles,757 quinoxaline,758 3-carbomethoxy-4-arylfuran-2-(5H)-ones,759 thiazol-2(3H)imine-linked glycoconjugates,760 and other important heterocycles. Modified procedures for α-sulfonyloxylations of ketones (Scheme 95) include the solid-state reactions under solventfree conditions,725,761 the use of ionic liquids as solvents,762−767 and the generation of HTIB 312 in situ from iodosylbenzene and p-toluenesulfonic acid monohydrate.146 [Hydroxy(organosulfonyloxy)iodo]arenes have high reactivity toward alkenes and alkynes. In particular, the reaction of alkenes with HTIB 312 affords vic-ditosyloxyalkanes in moderate yield.756,768−773 The reaction of alkenes with intramolecular participation of a nucleophilic functional group using [hydroxy(organosulfonyloxy)iodo]arenes gives the corresponding heterocyclic compounds.774−778 Muniz and coworkers have found that a modified HTIB reagent can promote the intramolecular aminations of alkenes to give the corresponding indoles.779 In particular, this modified HTIB reagent generated in situ from PhIO and 2,4,5-tris-isopropylbenzenesulfonic acid promotes the chemoselective oxidative cyclization of 2-amino styrenes 318 to indoles 319 under mild conditions (Scheme 96). The reaction of polyalkylbenzenes with HTIB in the presence of halide salts affords the corresponding substituted halogen compounds.780−783 The polycyclic aromatic compounds react with HTIB in the presence of trimethylsilyl

Scheme 91. Oxidation of Te or Bi Compounds Using [Bis(acyloxy)iodo]benzenes

Koser and co-workers prepared several [alkoxy(tosyloxy)iodo]benzene derivatives by ligand exchange in HTIB.740,741 In particular, the treatment of HTIB 312 with trimethyl orthoformate afforded the [methoxy(tosyloxy)iodo]benzene 313 (Scheme 94). Similarly, the ligand exchange reaction of [methoxy(tosyloxy)iodo]benzene 313 and menthol gave the chiral menthyloxy compound 314 in high yield (Scheme 94). The X-ray crystal structure of [hydroxy(tosyloxy)iodo]benzene, PhI(OH)OTs, was first reported by Koser and coworkers.742 Structure analysis of HTIB 312 revealed a T-shaped geometry around the iodine atom, and two different covalent I−O bonds of 2.47 Å (I−OTs) and 1.94 Å (I−OH). On the basis of the reported X-ray structures of several [hydroxy(sulfonyloxy)iodo]arenes, it can be stated that the introduction of a substituent in phenyl ring of HTIB 312 does not have any significant effect on the geometry at the hypervalent iodine center. X-ray crystal structures of HTIB derivatives, [hydroxy(mesyloxy)iodo]benzene, and the respective oxo-bridged anhydride compounds have been reported by Richter and coworkers.743 The molecular geometry of [hydroxy(mesyloxy)iodo]benzene is very similar to that of HTIB 312. [Hydroxy(mesyloxy)iodo]benzene and the respective oxo-bridged mesylate form dimers in the solid state due to intramolecular I···O secondary bonding. The nature of species present in aqueous solutions of [hydroxy(mesyloxy)iodo]benzene and HTIB was analyzed by Richter, Koser, and co-workers.744 In water solution, both [hydroxy(mesyloxy)iodo]benzene and [hydroxy(tosyloxy)iodo]benzene are completely ionized to give Scheme 92. Gold-Catalyzed Direct Coupling of 295 and 296

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Figure 11. Examples of [hydroxy(organosulfonyloxy)iodo]arenes and [hydroxy(organosulfonyloxy)iodo]polyfluoroalkanes.

Scheme 93. Modified One-Pot Procedure for the Synthesis of [Hydroxy(organosulfonyloxy)iodo]arenes

Scheme 96. Preparation of Indoles Using Modified HTIB Reagent

Scheme 97. Preparation of 3,6-Disubstituted-1,2,4,5tetrazines Scheme 94. Synthesis of [Alkoxy(tosyloxy)iodo]benzenes

Scheme 95. Synthesis of Organosulfonates 317 from Carbonyl Compounds Using Reagents 316

isothiocyanate in methylene chloride to afford the products of regioselective thiocyanation in good yields.784 The oxidation of some substituted phenols, containing electron-withdrawing groups at the ortho position, with HTIB also leads to tosyloxylation of aromatic ring.785 HTIB is a useful oxidant for aldoximes and ketoximes.786−789 In particular, the reaction of aldoximes with alkenes or alkynes using HTIB affords the corresponding isoxazolines or isoxazoles in good yields.786,787 Wei and co-workers have reported a new synthesis of 3,6-disubstituted-1,2,4,5-tetrazines 3354


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321 from hydrazones 320 by using HTIB (Scheme 97).790 The corresponding 3,6-disubstituted-1,2,4,5-tetrazines can be easily obtained through one-step N-deprotection of p-toluenesulfonyl groups and aromatization by tetrabutyl ammonium fluoride in THF.

methoxynaphthalene 327 using HTIB in the presence of bromotrimethylsilane in hexafluoroisopropanol (HFIP) (Scheme 100).810 3.5. Aryliodine(III) Derivatives with Nitrogen Ligands

Aryliodine(III) compounds with I−N bonds are less common than those with I−O bonds. In general, the acyclic hypervalent iodine compounds with nitrogen ligand have low thermal stability and are sensitive to moisture. The cyclic compounds of this type will also be discussed in section 3.6. The chemistry of these compounds has been summarized in several reviews and books.2,13−15,52,813 Several structural types of noncyclic aryliodine(III) derivatives with one or two nitrogen ligands are known (Figure 12). Veretennikov and Gavrilov reported the synthesis of the imidazolyl-substituted hypervalent iodine(III) compounds 329 using ligand exchange of BTI with imidazole.814 Amidoiodane(III) derivatives 330−335 are readily prepared from [bis(acyloxy)iodo]arenes and appropriate nitrogen compounds via ligand exchange reactions.721,814−829 The μ-oxo bridged amidoidanes 336 can also be prepared by a similar procedure in the presence of water.825,826,830 The noncyclic azidoiodanes 337 and 338831−837 are known as intermediate species in azidation reactions, and amidoiodanes 339796,838 have been isolated as intermediate products in Hofmann rearrangement. The iminoiodane 340 and derivatives are importrant nitrene precursors, which can be obtained from [bis(acyloxy)iodo]benzenes and corresponding sulfonamides; the chemistry of iminoiodanes will be discussed in section 3.9. Some nitrogen compounds are employed as counteranions in iodonium salts 341−344.831,839−843 A specific example of such a compound is illustrated by the preparation of N-substituted indolyl(phenyl)iodonium bis(sulfonyl)imides 345 from indole derivatives and bis(sulfonyl)imides using (diacetoxyiodo)benzene. Compounds 345 are useful precursors for the bromo-amination via 1,3migration of imide in the presence of 1,3-dibromo-5,5dimethylhydantoin (DBH) to give the corresponding 3bromo-2-bis(tosyl)amino-indoles 346 in good yields (Scheme 101).843 Suna and co-workers have reported a direct regioselective C−H azidation of heterocycles using heteroaryl(phenyl)iodonium azides in the presence of a copper catalyst.831 The resulting heteroaryl azides can be further transformed to heteroarylamines by treatment with aqueous ammonium sulfide. 3.5.1. [Acyloxy(amido)iodo]arenes. Muniz and co-workers have found that [acetoxy(amido)iodo]arenes 330−332 can be easily prepared from (diacetoxyiodo)benzene (DIB) and appropriate bis(organosulfonyl)imides via ligand exchange, and the structure 330 was characterized by X-ray crystallography.820,826 ArI(OAc)NTs2 is a useful reagent for oxidative amination of alkenes and alkynes. In a specific example, a direct metal-free amination of arylalkynes has been developed, which proceeds by the reaction of terminal alkynes 347 or 349 with

Scheme 98. HTIB-Induced Oxidative Rearrangement of Arylalkenes

[Hydroxy(organosulfonyloxy)iodo]arenes are useful reagents for the oxidative rearrangement reactions. For example, HTIB serves as an efficient oxidant in the Hofmann-type rearrangement of carboxamides to amines.791−796 Togo and co-workers reported the Hofmann-type rearrangement of aromatic and aliphatic imides using in situ-generated HTIB and directly affording the corresponding amino acids.797 Reactions of HTIB with alkenes often proceed with oxidative rearrangements. Justik and Koser have published a study of an oxidative rearrangement in the reaction of arylalkenes 322 with HTIB in aqueous methanol leading to α-aryl ketones 323 in good yields Scheme 99. HTIB-Induced Ring Expansion Reaction

(Scheme 98). This regioselective oxidative rearrangement reaction is commonly observed in the reactions of acyclic and cyclic arylalkenes.798 The analogous HTIB-induced oxidative rearrangement in cyclic alkenes can be used for ring contraction reactions.799−803 The ring expansion reactions can also be induced by HTIB under mild condition.804−809 An HTIB-mediated ring expansion reaction of 1-vinylcycloalkanol derivatives has been reported by Silva and co-workers. For example, the unsaturated trimethylsilyl ether 324 reacts with excess HTIB to afford the benzocycloheptanone derivative 325 as the main product (Scheme 99).807 Analogously to [bis(acyloxy)iodo]benzenes, HTIB can be used as an efficient oxidant for the intermolecular C−C bond cross-coupling reaction. Kita and co-workers reported an HTIB-induced, metal-free cross-coupling of heteroaromatic compounds providing a variety of useful mixed biaryls.559,810−812 For example, the cross-coupling product 328 was synthesized from substituted thiophene 326 and 1Scheme 100. HTIB-Induced Metal-Free Cross-Coupling Reaction

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Figure 12. Examples of aryliodine(III) derivatives with nitrogen ligands.

Scheme 101. Regioselective C(sp2)−H Dual Functionalization of Indoles

rapid access to the important class of ynamides 348 and to cyclopentenes 350. ArI(OAc)NTs2 can be conveniently generated in situ from [bis(acyloxy)iodo]arenes and bistosylimide (HNTs2), and used without isolation in reaction with alkenes. Minakata and coworkers reported the oxidative decarboxylative amination reaction of β,γ-unsaturated carboxylic acids 351 using PhI(OAc)NTs2, which was generated in situ from DIB and HNTs2,

Scheme 102. Intermolecular C−H Amination of Terminal Alkynes Using 330

Scheme 104. Oxidative C−N Bond Formation Reaction under Metal-Free Condition

PhI(OAc)NTs2 330 (Scheme 102).822 This unprecedented intermolecular conversion of C−H to C−N bond provides

affording the imidation products 352 in moderate yields (Scheme 103).827 This reaction with saccharin instead of bistosylimide can also afford the corresponding product of imidation. Cho and Chang have also reported the metal-free intermolecular oxidative C−N bond-forming reaction using in situ-generated PhIOAc(NR) reagent from DIB and the appropriate imides. For example, the in situ-generated reagent reacts with mesitylene 353 to give the sp2 C−H bond imidation product 354 in 79% yield (Scheme 104).819 A similar oxidative

Scheme 103. Oxidative Decarboxylative Amination of β,γUnsaturated Carboxylic Acids 351

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Scheme 105. Nitrogen Transfer Reactions Using Imidoiodane 355

C−N bond formation reaction by using microwave was reported by DeBoef.844 The metal-catalyzed intermolecular or intramolecular C−N bond-forming reactions using the in situ-generated PhI(OAc)NR reagent are also known.721,818 DeBoef and co-workers have proposed a new protocol for the regioselective intermolecular amination of various arenes using DIB in the presence of a Au(I) catalyst. A direct regioselective synthesis of various substituted anilines has been achieved with yields as high as 90%. Mechanistic insight suggests that the regioselectivity can be predicted on the basis of electrophilic aromatic metalation patterns. 3.5.2. [(Diamido)iodo]arenes. [(Diamido)iodo]arene reagents can be prepared by reacting [bis(acyloxy)iodo]arenes with 2 equiv of the appropriate appropriate imides or amides via simple ligand exchange reaction. In particular, Varvoglis and co-workers have first reported the synthesis and reactions with ketones of hypervalent iodine(III) bisimidiates, such as bis(saccharino)iodobenzene, bis(succinimido)iodobenzene, and bis(phthalimido)iodobenzene.797,815−817 More recently, Muniz and co-workers have prepared new bis(imido)iodobenzenes, which were characterized by X-ray crystallography. The same group has also demonstrated that the new imidoiodane 355 is a useful nitrogen transfer reagent in

Scheme 107. Copper Triflate-Induced C−H Insertion Reaction of 2-Phenylpyridine Derivatives

The copper triflate-mediated direct amination of 2-phenylpyridine derivatives using bis(phthalimido)iodobenzene has been developed by DeBoef and co-workers.829 A series of different 2-phenylpyridine derivatives 362 were aminated to give products 363 with yields up to 88% (Scheme 107). Mechanistic studies have demonstrated that this reaction proceeds via a copper-mediated single electron transfer mechanism. Several μ-oxo-bridged amidoiodane reagents have been prepared from amidoiodanes(III) in the presence of water and characterized by X-ray crystallography.825,826,830 μ-Oxobridged amidoiodanes are useful nitrogen transfer reagents in reactions with nucleophilic organic substrates. For example, the saccharin-based μ-oxo-bridged imidoiodane 364 prepared by treatment of DIB with saccharine in the presence of water readily reacts with silyl enol ethers 365 at room temperature to give the corresponding α-aminated carbonyl compounds 366 in moderate yields (Scheme 108).830 3.5.3. [(Azido)iodo]arenes. Acyclic hypervalent iodine derivatives with azido ligands, PhI(N3)OAc or PhI(N3)2, are unstable compounds, which easily decompose to iodobenzene and dinitrogen at −25 to 0 °C. Azidoidanes, generated in situ from a hypervalent iodine reagent and a source of azide anion, are useful reagents for various azidation reactions, such as mono- or vicinal diazidation of alkenes,833,846−850 allylic azidation,835,851−854 α-azidation of carbonyl compounds,832,855 benzyl or alkyl azidation,591,836,856,857 C−H azidation of aldehydes,589 and the oxidative decarboxylative azidation of α,β-unsaturated carboxylic acids.590 For example, Antonchick and co-workers have reported that the cascade of C−N and C− C bond-forming reactions of alkenes 367 afford 2-oxindoles 368 under metal-free conditions with high reaction rates at ambient temperature providing access to complex products (Scheme 109).848 Mechanistic studies have confirmed that this reaction proceeds via azide radicals generated from the initial azidoiodane.

Scheme 106. Diastereoselective Diamination of Amidine 360

reactions with alkenes, silyl enol ethers, and allenes to give the corresponding aminated compounds 356−359 (Scheme 105). The ArI(NTs2)2 reagents can be readily generated in situ from DIB and 2 equiv of bistosylimide (HNTs2) or from the ArI(OAc)NTs2−HNTs2 system and can be used without isolation in reaction with alkenes.826 Chiba and co-workers reported that PhI(NTs2)2 and PhI(NMs2)2 can also be effective reagents for the diastereoselective anti-diamination reaction. For example, the diastereoselective reaction of amidine 360 with PhI(NMs2)2 affords the corresponding amination product 361 in good yield (Scheme 106).845 3357


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Scheme 108. α-Amination of Silyl Enol Ethers

Scheme 109. Cascade of C−N and C−C Bond-Forming Reactions of Alkenes

Scheme 111. Rh(III)-Catalyzed C−H Azidation of Arenes

Despite the lack of aromatic conjugation, the five-membered heterocyclic iodine compounds have considerably higher thermal stability as compared to the noncyclic analogues due to the bridging of the equatorial and the apical positions at hypervalent iodine center by a five-membered ring,859 and also due to the better overlapping of the nonbonding electrons on hypervalent iodine atom with the π-orbitals of the benzene ring.860 High thermal stability of five-membered iodine−oxygen heterocycles (benziodoxoles) made possible the preparation of otherwise unstable hypervalent iodine derivatives with peroxide, azido, cyano, and trifluoromethyl substituents. The most important heterocyclic λ3-iodanes are represented by five-membered heterocycles, although several examples of four-membered and six-membered heterocycles with iodine(III) atom in the ring have also been reported. The fivemembered iodine(III) heterocycles are represented by various cyclic compounds 375−382 incorporating hypervalent iodine and oxygen, nitrogen, or some other elements in the heterocyclic ring (Figure 13). Particularly important are the five-membered heterocyclic iodine compounds 375 with an oxygen atom in the ring, the so-called “benziodoxoles”. Benziodoxoles have found an important synthetic application as reagents for various oxidative functionalizations of organic substrates. The higher thermal stability of five-membered heterocyclic iodine(III) reagents made possible the preparation of stable benziodoxole derivatives with I−F,861−864 I−Br,865,866 I−OOR,116,867−872 I−N3,873−885 I−CN,875,886−888 and I−CF3 bonds.63,863,889,890 These substituted benziodoxoles have found application as the “atom-transfer” reagents for organic synthesis.61 The chemistry of benziodoxoles has been summarized in several reviews.2,61,64,65

Du and Zhao reported the intramolecular C−O bond formation using the PhI(OAc)2−NaN3-system mediated oxygenation of N,N-diaryl tertiary amines 369 (Scheme 110).858 The appealing features of this method include mild reaction conditions, absence of heavy-metal catalysts, and the direct intramolecular functionalization of the aliphatic C−H bonds adjacent to nitrogen. The metal catalyst-mediated azidation reactions have also been developed. Rhodium(III)-catalyzed C−H azidation of arenes using in situ-generated azidoiodane has been achieved by Li and co-workers.837 This C−H azidation reaction is effective for various arenes 371 to give the products 372 in good yields (Scheme 111). A copper-catalyzed Markovnikov-type intermolecular azidocyanation of aryl alkenes 373 has been developed to give a series of α-azidopropanenitriles 374 in moderate yield (Scheme 112).834 This method may provide a potential strategy for the synthesis of the corresponding 3-amino-2-arylpropanoic acid. 3.6. Heterocyclic Iodine(III) Compounds

On the basis of the available literature data, it can be stated that iodine is not capable of forming conjugated cyclic systems with aromatic stabilization because of the large atom size and the semi-ionic nature of the hypervalent I−C, I−N, and I−O bonds.2 Moreover, the high level computational studies using adaptive natural density partitioning bond modeling technique (AdNDP) reveal that the double bond between iodine atom and other elements does not exist.129 The bonds that are commonly shown in the literature as IC, IN, and IO in fact have a dative 2c-2e nature, which precludes their involvement in a conjugated aromatic system.

Scheme 110. Intramolecular C−O Bond Formation Reaction Using the PhI(OAc)2−NaN3 System

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Scheme 112. Copper-Catalyzed Intermolecular Azidocyanation of Aryl Alkenes

Figure 13. Specific examples of hypervalent iodine(III) five-membered heterocycles.

Figure 14. Recent examples of heterocyclic iodine(III) compounds characterized by X-ray crystallography.

Scheme 113. Electrophilic Fluorination Reaction Using Fluorobenziodoxole 390

386,911 cyclic imino iodine(III) compound 387,912 and isocyano benziodoxole 388.913 Benziodoxoles 383 and 385 were prepared via ligand exchange from 2-iodosylbenzoic acid and the appropriate silylated acetylene or the corresponding carboxylic acid, while 384 and 386 were synthesized by oxidation of the respective iodoarenes. 3.6.1. Benziodoxole and Derivatives. Halobenziodoxoles have found application as reagents for halogenation of organic substrates and oxidation of alcohols.862,864,865,914 For example, an air- and moisture-stable fluorobenziodoxole 390 can be used as an electrophilic fluorinating reagent for 1,3-diketones 389 to give the mono- or difluorinate compounds in good yields (Scheme 113).862 The stable peroxybenziodoxole (375, X = O, Y = tBuOO) is a particularly useful oxidizing reagent. Ochiai and co-workers have found that this peroxybenziodoxole can be used as a reagent for oxidation of ethers and acetals.868,870,915,916 Recently, Maruoka and co-workers have reported a practical

Single-crystal X-ray structures have been reported for various benziodoxole derivatives 375,63,830,861−863,865,867,872,875,876,889,891−901 benziodazoles 376,902−905 benziodoxaboroles 379,906 benziodoxathioles 380,907,908 and cyclic phosphonate 382.909 In general, benziodoxoles have a planar structure with a highly distorted T-shaped geometry around iodine. The I−O bond length in the cycle of benziodoxolones (375, 2X = O) can vary from 2.11 Å in a benzoate derivative (375, Y = 3-ClC6H4CO2)897 to 2.48 Å in arylbenziodoxolone (375, Y = Ph),891 which is indicative of a significant increase in the ionic nature of this bond. The observed bond angle C−I−O in benziodoxoles is about 80°, which is significantly different from the 90° angle typical of noncyclic hypervalent iodine compounds. Figure 14 shows examples of recently reported X-ray structures of heterocyclic iodine(III) compounds, such as triisopropylsilyl ethynyl benziodoxolones 383,910 tosyloxybenziodoxole 384,862 tolylvinyl carboxylic benziodoxole 385,901 arylbenziodoxolones 3359


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efficient electrophilic or radical azidating reagents toward various organic substrates.876−885 Waser and co-workers have reported the electrophilic azidation of cyclic β-keto esters using azidobenziodoxole 375 (X = Me, Y = N3) affording products of azidation in quantitative yields in the absence of any catalyst.877 In the case of the less reactive noncyclic β-keto esters or silyl enol ethers, complete conversion and good yields could be obtained by using a zinc catalyst. Jiao and co-workers have developed the copper-catalyzed oxoazidation and alkoxyazidation of indoles using azidobenziodoxole 400.880 This dearomatization reaction of 3-methylindoles 399 leading to the versatile 3-azido indolenine derivatives 401 in moderate to high yields has a good synthetic potential (Scheme 116). Very recently, Hartwig’s group reported an iron catalystmediated selective tertiary C−H azidation reaction using azidobenziodoxole 400.883 This reaction is suitable for the azidation of complex molecules in the late stages of a multistep synthesis; a specific example of azidation of a complex molecule is illustrated by Scheme 117. The azido groups in products of azidation can be further converted to vatious nitrogencontaining functional groups. Acetoxybenziodoxole and methoxybenziodoxole are stable compounds, which have been used as reagents in some oxidation reactions.919−921 For example, Togo and co-workers reported the oxidation of alcohols to aldehydes or ketones using various acetoxybenziodoxole derivatives.919 The same group has also reported selective oxidation of alcohols with acetoxybenziodoxoles. An effective C−H alkoxylation of unactivated methylene and methyl groups using palladium catalyst with methoxybenziodoxole 406 was reported by the Rao group (Scheme 118).920 This group has also reported palladium-catalyzed double alkoxylation of an unreactive C−H bond using methoxybenziodoxole 406 (Scheme 118).921 This new procedure can be used for the preparation of both symmetrical and unsymmetrical acetals. Both new procedures demonstrate good functional group tolerance, excellent reactivity, and high yields. Togni and co-workers have reported the synthesis of stable electrophilic trifluoromethylating reagents, trifluoromethylbenziodoxoles (Figure 13, structures 375, X = CF3), by treatment of the corresponding methoxybenziodoxole or acetoxybenziodoxole with trimethyl(trifluoromethyl)silane.889,890,922 The same group has also reported a simplified one-pot procedure for the synthesis of trifluoromethylbenziodoxoles from 2iodobenzoic acid in 72% yield.863 Solid-state structures of trifluoromethylbenziodoxoles were characterized by X-ray crystallography, which showed the distorted T-shaped geometry around iodine, typical for the hypervalent λ 3 iodanes.63,889,899,923 The chemistry of trifluoromethylbenziodoxoles has been summarized in a recent review by Togni and co-workers.890

Scheme 114. Oxidation of Unactivated Cyclic Alkanes by Hypervalent Iodine Compound

approach to the oxidation of unactivated C−H bonds by an ortho-nitro-substituted hypervalent iodine compound 394, Scheme 115. Formation of Thiocyanates from Thiols Using Cyanobenziodoxoles

which was generated in situ from ortho-nitro(diacetoxyiodo)benzene 393 with tert-butylhydroperoxide (TBHP).871 This oxidative reaction of cyclic alkanes 392 affords the corresponding ketones 395 in good yields (Scheme 114). Cyanobenziodoxoles are thermally stable, white, microcrystalline solids; the structure of these compounds was confirmed by X-ray diffractometry. Cyanobenziodoxoles are useful cyano transfer reagents.875,887,888,898,917 In particular, Waser and co-workers have reported the synthesis of thiocyanates 398 by treatment of aliphatic and aromatic thiols 396 with cyanobenziodoxoles 397 at room temperature (Scheme 115).887 An efficient conversion of disulfides to thiocyanates was also possible. The direct electrophilic cyanations of β-keto esters and amides using cyanobenziodoxole have been reported by Chen and co-workers.888 The procedure is accomplished within 10 min at room temperature to give the cyanated products in high yields. Recently, a similar reaction of asymmetric α-cyanation of β-keto esters using cyanobenziodoxole and cinchona alkaloids as organocatalyst has also been reported by Waser and co-workers.917 The cyclic azidoiodanes, azidobenziodoxoles, are thermally stable, nonexplosive, microcrystalline solids, which can be stored indefinitely long in a refrigerator. Azidobenziodoxoles can be readily prepared by the reaction of appropriate benziodoxoles with trimethylsilyl azide or sodium azide in good yields, and the structure of azidobenziodoxole 375 (X = CF3, Y = N3) was characterized by single-crystal X-ray diffraction.873,875−877,918 Azidobenziodoxoles can be used as

Scheme 116. Cu-Catalyzed Dearomatization of 3-Methylindoles Using Azidobenziodoxole

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Scheme 117. Fe-Catalyzed C−H Bond Azidation of Tetrahydrogibberellic Acid Derivative

Scheme 118. Palladium-Catalyzed C−H Alkoxylations Using Methoxybenziodoxole 406

Scheme 119. Intermolecular Cyanotrifluoromethylation of Alkenes

Scheme 120. Intramolecular Carbotrifluoromethylation of Alkynes

as starting material for the synthesis of pharmaceutically and agrochemically important compounds. Reactions of alkenes or alkynes with trifluoromethylbenziodoxoles in the absence of nucleophiles afford the appropriate trifluoromethylated products of cyclization.882,982−1000 Liu and co-workers have reported a mild and efficient copper-catalyzed intramolecular carbotrifluoromethylation of alkynes 413 with trifluoromethylbenziodoxolone 411 (Scheme 120).999 The reaction tolerates a range of substrates to give trifluoromethylated heterocycles 414 with high selectivities. A similar reagent, (phenylsulfonyl)difluoromethylbenziodoxole, was developed by Hu and co-workers.1001 This electrophilic reagent can effectively transfer the (phenylsulfonyl)difluoromethyl group to various thiols. The same group has also reported the (phenylsulfonyl)difluoromethylation reaction of α,β-unsaturated carboxylic

These electrophilic trifluoromethylating reagents can be used for direct transfer of the trifluoromethyl group from iodine to heteroatoms, such as nitrogen,924,925 phosphorus,926−928 sulfur,899,929,930 and oxygen.931−935 Several research groups have reported the use of trifluoromethylbenziodoxoles for the synthesis of trifluoromethylated aromatic compounds,936−946 trifluoromethylated vinyl derivatives,947−962 trifluoromethylated alkynes,963,964 trifluoromethylated allylic products,965,966 and αtrifluoromethyl carbonyl compounds967−969 starting from the corresponding nucleophilic organic substrates. The reaction of vinyl or alkynyl substrates with trifluoromethylbenziodoxoles in the presence of external nucleophiles affords the corresponding bifunctional compounds.970−981 For example, Liang and coworkers developed the copper-catalyzed intermolecular cyanotrifluoromethylation of alkenes 410 using trifluoromethylbenziodoxolone 411 (Scheme 119).976 Products 412 can be used 3361


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Scheme 121. Decarboxylative Alkynylation of Aliphatic Carboxylic Acids

Scheme 122. Regioselective Alkynylation of Indole in the Presence of Metal Catalysts

acids or β,γ-unsaturated carboxylic acids in the presence of copper catalyst.948,1002 The products of (phenylsulfonyl)difluoromethylation can be converted to gem-difluoro alkenes in good yield by treatment with lithium bis(trimethylsilyl)amide. The treatment of hydroxybenziodoxoles with alkynyl(trimethyl)silanes in the presence of BF3·Et2O affords alkynyl-substituted hypervalent iodine heterocycles, 1-alkynylbenziodoxoles, in moderate yields.894 An improved procedure for the preparation of 1-alkynylbenziodoxoles in high yields has been reported.1003 The Waser group reported the synthesis of 1-[(triisopropylsiyl)ethynyl]-1,2-benziodoxol-3(1H)-one using a similar procedure. 1004−1006 Olofsson and co-workers developed the one-pot preparative approach to 1-alkynylbenziodoxoles from 2-iodobenzoic acid and alkynyl boronic esters or alkynyl pinacol boronates in good yields.1007 Several crystal structures of 1-alkynylbenziodoxoles have been established by X-ray diffractometry.894,910,1008 The structural data showed usual T-shaped geometry about the iodine atom with the I−O bond distance significantly longer than the computed covalent I−O single bond distance, which is indicative of a highly ionic nature of this bond. Alkynylbenziodoxoles have been used as efficient electrophilic alkynyl transfer reagents to sulfides,1009,1010 sulfones,1011 amines,1012 and phosphine compounds.1013 These reagents are also useful for direct α-alkynylation of carbonyl compounds.1005,1008,1014 The reaction of aliphatic carboxylic acids 415 with 1-alkynylbenziodoxole 383 in the presence of silver catalyst affords the decarboxylative alkynylation products 416 (Scheme 121).1015 The mechanism of this C(sp3)−C(sp) cross-coupling reaction probably involves the interaction of hypervalent iodine reagent with alkyl radical generated via decarboxylation of the carboxylic acid.

Regioselective C−H alkynylation reactions of aromatic or heteroaromatic compounds using 1-alkynylbenziodoxole 383 under metal-catalyzed conditions have been reported by several groups.1004,1016−1029 For example, Waser and co-workers reported the regioselective alkynylation reactions of indoles in the presence of different transition metal catalysts. The C-2 selective alkynylation of indole 417 is achieved using 383 with palladium catalyst, while the C-3 selective alkynylation occurs under gold catalyst (Scheme 122).1004,1020 Both reactions tolerate a broad range of functional group in substrates, and the silyl group in products 418 and 419 can be easily removed to give the corresponding terminal alkynes. The direct olefinic C−H alkynylation reaction can be realized using 1-alkynylbenziodoxole 383 in the presence of transion metal.1030−1033 For the efficient olefinic C−H bond alkynylation, the presence of a metal-coordinating directing group is needed, which is critically important for the accomplishment of this transformation. Aromatic aldehydes bearing appropriate coordinating groups can also be alkynylated in the presence of iridium or rhodium catalyst to give appropriate ynone products.1034 The intramolecular alkynylation-cyclization reactions of alkenes with 1-alkynylbenziodoxoles leading to cyclic products of oxyalkynylation and aminoalkynylation have also been reported.1035,1036 Furthermore, the reaction of carbonyl allenyl substrates with 1-alkynylbenziodoxoles proceeds as the domino cyclization-alkynylation to give alkynyl furans as the products.1037 The aryl-substituted hypervalent iodine heterocycles, arylbenziodoxolones, can be obtained by the oxidation of 2iodobenzoic acid with K2S2O8 followed by addition of arenes.1038 Olofsson and co-workers have developed a modified method for the preparation of phenylbenziodoxole in 66% by a one-pot reaction from 2-iodobenzoic acid with m-chloroperox3362


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of 2-iodobenzamide derivatives.904 The oxidation of N-(2iodobenzoyl) amino acid derivatives with dimethyl dioxirane affords chiral hypervalent iodine macrocycles 424 as the final isolated products.1045 Structures of these macrocycles were established by X-ray analysis. It was suggested that the initial oxidation of N-(2-iodobenzoyl) amino acids 422 with dimethyl dioxirane gives amino acid-derived benziodazoles 423, whose self-assembly affords the final products 424 (Scheme 124). Single X-ray crystal structural analysis of macrocycles 424 revealed the presence of secondary bonding between hypervalent iodine atom and oxygen atoms of the amino acid fragment.1045 A theoretical computational study has confirmed that the self-assembly of three 423 units can be explained by the formation of I···O secondary bonding and also by the rearrangement of primary and secondary bonding at the trigonal bypyramidal hypervalent iodine centers resulting in placement of the least electronegative C-ligands in the equatorial positions.1046 3.6.3. Other Heterocyclic Iodine(III) Derivatives. Several other hypervalent iodine heterocycles incorporating phosphorus,909,1047,1048 sulfur,121,907,908,1048−1052 and boron911 atoms in the ring have been synthesized and characterized by X-ray crystallography. The reactivity of these heterocyclic iodine compounds has been less inverstigated. Alkynylbenziodoxathioles were originally prepared from benziodoxathiole and the corresponding terminal alkynes by Koser and co-workers.907 Alkynylbenziodoxathioles were further reacted with thioamides under basic condition to give the corresponding thiazoles in good yields.1050 The byproduct in these reactions, 2-iodoarylsulfonate, can be recovered from the reaction mixture in almost quantitative yield. Justik and co-workers have described the synthesis of arylbenziodoxathioles and investigated their structure by X-ray analysis.1051 The same group has also reported the reactivity of arylbenziodoxathioles. For example, a chemoselective reaction of 2-[(4-methoxyphenyl)iodonio]benzenesulfonate 425 with cresol in the presence of p o t a s s i u m t e r t-bu t oxide aff o r d s p o t a s s i u m 2 - ( 4 methylphenoxy)benzenesulfonate 426 as the major product (Scheme 125). The selectivity of this aromatic nucleophilic substitution reaction has been rationalized by the electronwithdrawing electronic effect of the sulfonate group.

ybenzoic acid, trifluoromethanesulfonic acid, and benzene in CH2Cl2 at 80 °C followed by addition of aqueous ammonia.1039 Another convenient one-pot synthesis of arylbenziodoxolones from 2-iodobenzoic acid and various arenes using Oxone as an inexpensive and environmentally safe oxidant has also been reported.911 Single-crystal X-ray structures have been published for several arylbenziodoxoles.891,911 1-Phenylbenziodoxole is known as a classical benzyne precursor under heating.1038,1040−1043 Reactions of 1-arylbenziodoxoles with nucleophiles proceed exclusively as substitution in the benziodoxolone ring giving aryl iodides and the respective ortho-substituted benzoic acids as major products. The reactivity study of 1-arylbenziodoxoles in reactions with Scheme 123. Preparation of Acetoxybenziodazole 421 from 2-Iodobenzamide

nucleophiles revealed that ortho-methyl-substituted benziodoxoles, such as 1-phenyl-7-methylbenzidoxole, are more reactive than 1-phenylbenziodoxole.911 This enhanced reactivity of 1phenyl-7-methylbenzidoxole was explained by the steric effect of ortho-substituent on the nucleophic substitution in diaryliodonium salts. 3.6.2. Benziodazoles and Derivatives. In 1965, Wolf and Steinberg reported the first preparation of the benziodazole heterocyclic system by a peracetic acid oxidation of 2iodobenzamide 420.1044 A more recent X-ray crystallographic study confirmed the structure of acetoxybenziodazole 421 for the product of this reaction (Scheme 123).905 The reaction of acetoxybenziodazole 421 with alcohols in the presence of trimethylsilyltriflate affords rearranged compounds, 1-alkoxy-3-iminiumbenziodoxoles, which were characterized by X-ray crystallography.903 The propsed reaction mechanism of this structural rearrangement most likely involves the ring opening and recyclization in the protonated benziodazole. The produced N-protonated 3-iminobenziodoxoles have a greater thermodynamic stability than the respective O-protonated benziodazole-3-ones by about 15 kcal/mol according to molecular orbital calculations.903 Acetoxybenziodazole 421 reacts at room temperature with azidotrimethylsilane to afford the corresponding azidobenziodazole, which can be used as an azidating reagent analogously to azidobenziodoxoles.905 Several benziodazole analogues can also be prepared by the oxidation

3.7. Iodonium Salts

Iodonium salts can be generally defined as 8-I-2 cationic species with two carbon substituents associated with a suitable anion, R2I+X−. The outer electronic shell of iodine atom in iodonium ion has only 8 electrons, and therefore iodonium salts formally are not hypervalent compounds. However, X-ray structural data of iodonium salts show pseudotrigonal bipyramidal geometry at the iodine center, which is similar to typical hypervalent iodine

Scheme 124. Preparation of Macrocyclic Benziodazoles 424

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Scheme 125. Reaction of Arylbenziodoxathioles with Potassium Phenoxide

Scheme 126. Trifluoroethylation of Indoles Using 2,2,2-Trifluoroethyl(mesityl)iodonium Triflate

Scheme 127. NHC Catalyst-Mediated Aldehyde C−H Bond Arylation Reaction

functional group tolerance. The mechanism of this reaction has been studied by quantum chemical calculations. Bennett and co-workers reported the air- and water-stable iodonium salt, phenyl(trifluoroethyl)iodonium triflimide, as a useful reagent for thioglycoside activation.841 Various thioglycosides rapidly undergo glycosylations in the presence of this reagent at room temperature to give respective glycosides in good yields. This simple procedure allows construction of glycosidic linkages with high efficiency. 3.7.2. Aryl- and Heteroaryliodonium Salts. Diaryiodonium salts belong to one of the most important classes of hypervalent iodine compounds, and the chemistry of aryland heteroaryliodonium salts has been extensively covered in several reviews.35,36,39,48 Symmetrical or unsymmetrical diaryliodonium salts are usually prepared by the treatment of λ3iodanes with an arene or a nucleophilic arylating reagent (e.g., arylborates, arylstannanes, or arylsilanes). Several efficient procedures for the preparation of diaryliodonium salts and heteroaryliodonium salts have been published by Kita and coworkers.37,1063,1064 Kitamura’s group and Olofsson’s group have developed a convenient one-pot procedure for preparing diaryliodonium salts from iodoarenes and aromatic substrates using appropriate oxidants,1065−1070 and the same groups have also developed a modified method utilizing elemental iodine and arenes as starting compounds in the presence of oxidants.1071−1074 X-ray crystallography of numerous diaryiodonium salts has been reported and discussed in previous reviews. Diaryliodonium salts in general have low solubility in common organic solvents; however, the triflates and the tetrafluoroborates are usually more soluble.

species. The structure of iodonium salts demonstrates the presence of a strong secondary interaction between the iodine atom and the counterion, and the R−I−R bond angle is about 90°, which is typical of hypervalent iodine species. Because of these structural features, iodonium salts are usually classified as the 10-electron hypervalent compounds accounting for the closely associated anionic part of the molecule. Significant recent research activity has been focused on aryliodonium salts bearing alkenyl, alkynyl, and fluoroalkyl groups as the second ligand. Iodonium salts are practically useful compounds with numerous applications as synthetic reagents and biologically active compounds.2,13−15,35,36,39−41,46,60 In this section, the preparation of iodonium salts and their applications in organic synthesis are summarized. 3.7.1. Alkyl- and Fluoroalkyliodonium Salts. Iodonium salts with aliphatic substituents in general lack stability. Several alkyliodonium salts with electron-withdrawing substituents in the alkyl chain have been isolated and characterized by X-ray crystallography.839,1053 Generally, these compounds are prepared by the reaction of corresponding [bis(trifluoroacetoxy)iodo]- or [hydroxy(sulfonyloxy)iodo]alkyl reagents with arenes in the presence of strong acids.736,738,839,1053,1054 Fluoroalkyliodonium salts have found some synthetic application as electrophilic fluoroalkylating reagents.840,841,1055−1062 For example, the reaction of indoles 427 with 2,2,2-trifluoroethyl(mesityl)-iodonium triflate 428 in the presence of base results in the highly C-3 selective trifluoroethylation to give products 429 in good yields (Scheme 126).1062 This reaction enables fast and efficient introduction of the trifluoroethyl group under mild conditions with high 3364


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Scheme 128. O-Arylation Reaction of Ethyl Acetohydroxamate 434

Scheme 129. Metal-Free Oxidative Cross-Coupling Reaction of α-Thienyl Iodonium Bromide 437

Scheme 130. meta-Selective C−H Arylation of Substituted Anilide 440

Scheme 131. Arylative Meyer−Schuster Rearrangement of Propargylic Alcohol

Diaryliodonium salts are effective electrophilic arylating reagents toward various carbon nucleophiles and, in particular, have been used for mono- or diarylations of carbonyl compounds.1075−1081 Gaunt and co-workers have reported the N-heterocyclic carbene 432-mediated C−H bond arylation of aldehyde 430 with diaryliodonium salt 431 giving heteroaryl ketone 433. This new strategy provides a synthetic approach to various ketones bearing a diverse array of aryl and heteroaryl substituents that can subsequently be converted into molecules displaying structural motifs commonly found in medicinal agents (Scheme 127).1082 Direct arylation of the generally unreactive C−H bonds of indoles in the absence of metal catalysts has been explored, and some of these reactions afford the corresponding products of regioselective arylation.1083−1085 Reactions of diaryiodonium salts with heteroatom nucleophiles such as oxygen,1086−1096 diaryl substituted nitrogen,1097−1102 and sulfides1103−1109 also give the corresponding arylation products in good yields. For example, Olofsson and co-workers developed the O-arylation reaction of ethyl acetohydroxamate 434 with various diaryliodonium salts 435 to give the O-arylated products 436 (Scheme 128).1092 Some of the obtained products could be further reacted in situ with ketones under acidic conditions to yield substituted benzofurans through intermediate oxime formation, [3,3]-rearrangement, and cyclization in a fast and

operationally simple one-pot fashion without using excessive reagents. This methodology has been applied to the synthesis of Stemofuran A and the formal syntheses of Coumestan, Eupomatenoid 6, and (+)-machaeriol B. Reactions of unsymmetrical diaryliodonium salts with nucleophiles can proceed chemoselectively, depending on the electronic or steric factors of the substituents in the aromatic rings of the diaryliodonium salts.1099 Kita’s group reported a unique reactivity of heteroaryliodonium salts, which were generated in situ from Koser reagent and heteroaromatic compounds, in unusual metal-free crosscoupling reactions. For example, the in situ-generated α-thienyl iodonium bromide 437 and 1,3-dimethoxybenzene 438 in the presence of TMSBr in 1,1,1,3,3,3-hexafluoroisopropanol (HFIP) solution gave the corresponding cross-coupling product 439 in high yield (Scheme 129).810 The reaction mechanism of cross-coupling reaction involved hydroarylation of nucleophiles to iodonium bromide to give the corresponding compounds via reductive elimination of iodobenzene. This developed procedure was the efficient synthesis of head-to-tail bithiophenes.811,1110 Transition metal-catalyzed arylations of organic substrates with diaryliodonium salts have been summarized in several reviews and books.2,15,35,36,39 Recently, numerous regio- and stereoselective arylations in inter- or intramolecular mode using 3365


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transition metal catalysts, such as copper,1111−1127 palladium,1128−1130 silver,1131 nickel,1132 iron,1133 iridium,1134 and ruthenium,1135 have been reported. Particularly important is the copper-catalyzed, directed meta-selective C−H arylation of aromatic compounds with amido substituents reported by Gaunt.1112 In a representative example, substituted anilide 440 is selectively phenylated by diphenyliodonium salt 441 under copper catalyst to give meta-substituted products 442 in good yields (Scheme 130). Gaunt’s group has also accomplished the copper-catalyzed arylative Meyer−Schuster rearrangement of propargylic alcohols to enones with diaryliodonium salts.1119 Treatment of propargylic alcohols 443 with diphenyliodonium salt 441 under copper catalysis in the presence of 2,6-di-t-butylpyridine (DTBP) affords the corresponding α-aryl-α,β-unsaturated ketone 444 in high yields (Scheme 131). This protocol could operate under mild conditions and provided a broad scope of the desired enone products in good yields and high selectivity for the E-isomer. Kitamura and co-workers have developed efficient benzyne precursors based on diaryliodonium salts.1136−1143 Benzyne

aryl esters by the arylation of carboxylic acids,1146 and benzo[b]seleno[2,3-b]pyridines from 2-selenoxo-2H-pyridin-1yl esters.1147 The heteroaromatic analogues of 445, phenyl[4(trimethylsilyl)thien-3-yl]iodonium triflate 448,1148,1149 and phenyl [1-tert-butoxycarbonyl-4-(trimethylsilyl)-1H-pyrrole-3yl]iodonium triflate 450,1150 have been prepared from the appropriate heteroaromatic bis(trimethylsilyl) compounds and (diacetoxyiodo)benzene in the presence of triflic acid. The respective benzyne analogues can be generated from 448 or 450 by the treatment with potassium fluoride and trapped by the reaction with furan 446 at room temperature to give the corresponding adducts 449 and 451 (Scheme 133). Two additional analogues of benzyne precursor, phenyl[o(trimethylsilyl)carboranyl]iodonium acetate1151 and phenyl[3(trimethylsilyl)bicyclo[2.2.1]-hept-2,5-dien-2-yl]iodonium triflate,1152 were also reported, and their reactivity has been investigated. 3.7.3. Alkenyliodonium Salts. The preparation, structural features, and reactivity of alkenyliodonium salts have been previously summarized in the reviews of Ochiai, 10,47 Okuyama,42,43,45 and Zefirov.46 This section will discuss recent developments in the synthesis and applications of alkenyl(aryl)iodonium salts. The following general procedures for the synthesis of alkenyliodonium salts have been reported: the silicon− iodine(III) exchange reaction of alkenylsilanes,1153−1155 the boron−iodine(III) exchange reaction of alkenylboronic acids,1156,1157 the tin−iodine(III) exchange reaction of alkenylstannanes,1158−1161 the reaction of vinylzirconium derivatives with (diacetoxyiodo)benzene,1162 the addition of hypervalent iodine reagents to alkynes,224,234−237,1163−1166 and the addition reactions to alkynyl iodonium salts.1167−1169 Numerous alkenyliodonium salts have been structurally characterized by X-ray analysis.237,1169−1176 Alkenyl(phenyl)iodonium salts are powerful electrophilic alkenylating reagents due to the exceptional nucleofugality of the phenyliodonium group (1012 times better leaving group than iodine itself) and its strong electron-withdrawing effect (the Hammett constant σm for the PhI+ group is 1.35).1177 Detailed mechanistic studies of the reactions of alkenyliodonium salts with nucleophiles have been carried out by several research groups. In particular, the ligand coupling, SN1, SN2, and Michael addition−elimination mechanisms have been proposed for these reactions.1178−1185 Synthetic applications of alkenyl(phenyl)iodonium salts as alkenylating reagents have been discussed in previous books

Scheme 132. Trapping of Benzyne Generated from Precursor 445

sources, phenyl[2-(trimethylsilyl)phenyl]iodonium triflate and other ortho-trimethylsilyl-substituted aryliodonium salts, are readily prepared from 1,2-bis(trimethylsilyl)arenes and (diacetoxyiodo)arenes in the presence of triflic acid. Treatment of phenyl[2-(trimethylsilyl)phenyl]iodonium triflate with Bu4NF in methylene chloride produces benzyne, which is further trapped with dienes or azides to afford the respective products of cycloaddition in excellent yields. In a representative example, treatment of phenyl[2-(trimethylsilyl)phenyl]iodonium triflate 445 as benzyne source and furan 446 in the presence of tetrabutylammonium fluoride (TBAF) under room temperature affords the benzyne adduct compound 447 in 100% yield (Scheme 132).1136 In additional examples of synthetic applications, the benzyne source 445 has been used in the preparation of the following products: 2-aryl-substituted nitriles by direct arylation,1144 cycloaddition product of thiophene S-oxide via cycloaddition−elimination reaction,1145

Scheme 133. Reactions of Furan with Heteroaromatic Benzyne Precursors

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Scheme 134. Enantioselective Vinylation of Aldehydes Using Vinyliodonium Salt 453

Scheme 135. Electrophilic Carbofunctionalization of Alkynes Using Vinyliodonium Salt 457

reductive elimination of a triflate group to give the final products 458. A stereoselective synthesis of (Z)-β-(tosyloxy)alkenyl iodide 460 from (aryl)[(E)-β-(tosyloxy)alkenyl] iodonium tosylates 459 by heating with catalytic CuCl has been developed (Scheme 136).1187 This methodology has been successfully applied to the synthesis of different trisubstituted olefins and to a formal synthesis of anticancer compound (Z)-tamoxifen. Several useful reactions of unstable monocarbonyl iodonium ylides, which were generated from (Z)-(2-acetoxyvinyl)iodonium salts and lithium alkoxides or triethylamine via the ester exchange reaction, have been reported by Ochiai and coworkers.1188−1195 For example, the in situ reactions of aldehydes or imines afford the corresponding epoxides or aziridines.1188,1190 Miyamoto and co-workers have investigated reactions of β-butoxycarbonyliodonium ylides, Darzen reagent analogues, which were prepared by the ester exchange reaction of β-butoxy-β-acyloxyvinyl-λ 3 -iodane 461 with lithium bases.1191 The iodonium ylide generated in situ from 461 cleanly undergoes Darzen’s condensation with aromatic aldehydes 462 to selectively give the trans epoxyester 463 (Scheme 137). 3.7.4. Alkynyliodonium Salts. The chemistry of alkynyl(aryl)iodonium salts was previously summarized in several specialized reviews.39,40,44,50,1196 In this section, the recent important synthetic applications of alkynyliodonium salts are overviewed. Alkynyliodonium salts are generally prepared by the reaction of a hypervalent iodine reagent with terminal alkynes,768,1197−1205 or silylated,1206−1212 stannylated,1213−1223 and borylated alkynes.1224−1226 A one-pot procedure for preparing alkynyliodonium salts from alkynyl precursors and

and reviews.2,15,36,46,47 In a recent representative example, MacMillan and co-workers have developed a highly enantioselective reaction for the α-vinylation of aldehydes 452 with various alkenyl(phenyl)iodonium triflates 453 using copper(I) and chiral amine 454 catalysis via a synergistic coupling mechanism (Scheme 134).1186 This new enantioselective vinylation reaction has been used for the preparation of various useful olefinic products 455. The copper-catalyzed endoselective oxyvinylations of allylic amides with a variety of vinyl(aryl)iodonium triflates have been developed by Gaunt’s group.1118 This transformation is tolerant to a wide range of functional groups and provides ready access to a broad selection of oxazine products in high yield and excellent diastereoselectivity. The same group has also explored the copper-catalyzed electrophilic carbofunctionalization of Scheme 136. CuCl-Catalyzed Synthesis of (Z)-β(Tosyloxy)alkenyl Iodides

alkynes 456 to the highly functionalized tetrasubstituted alkenes 458 using vinyl(aryl)iodonium triflates 457 (Scheme 135).1121 The mechanism of this electrophilic carbofunctionalization reaction probably includes intermediate formation of a high oxidation state Cu(III) species, which is followed by the

Scheme 137. Darzen-type Condensation of Alkenyliodonium Salt with Aromatic Aldehydes

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iodoarenes and mCPBA has been developed.1007,1039,1207 The crystal structure of alkynyl(aryl)iodonium salts has been characterized by X-ray crystallography,1199,1207,1217,1227−1233 and it has been demonstrated that the aryl group occupies an equatorial position, whereas the alkynyl moiety and the counterion occupy apical positions. The thermal stability of alkynyl(aryl)iodonium salts depends on the nature of the anion and the substituent on the acetylenic β-carbon. The unsubstituted alkynyliodonium salts, ethynyl(phenyl)iodonium salts, generally have lower thermal stability, and, for example, ethynyl(phenyl)iodonium tetrafluoroborates gradually decompose when left to stand at room temperature. Ochiai and coworkers have found that the stability of ethynyl(phenyl)-

The alkynylation reaction at carbon atom has also been developed. In particular, Nachtsheim’s group has reported the efficient alkynylation of azlactones 474 using alkynyiodonium salts 475 (Scheme 140). The obtained alkynylated azlactones 476 can be easily converted into various quaternary α-amino acid derivatives.1247 Alkynyl(aryl)iodonium salts have also been used as efficient alkynylation reagents for transition metals to provide the corresponding alkynyl metal species.1214,1215,1248−1256 For example, Canty and co-workers have found that alkynyl(phenyl)iodonium triflate 478 can transfer alkynyl groups to platinum(II) species 477 to give the alkynylplatinum(IV) species 479, the structure of which was established by X-ray crystallography (Scheme 141).1249 Instead of the 1,2-migration in alkylidene carbenes, an intramolecular C−H or O−H insertion reaction can occur, and this reaction has been utilized in the synthesis of different cyclic compounds including natural products.1207,1218,1219,1221,1222,1257−1264 In a representative example, the unisolable alkynyliodonium triflate 481 obtained from alkynylstannane 480 by treatment with Stang’s reagent, PhI(CN)OTf, was used for generating alkylidene carbene 482 under base conditions, which cyclized via intramolecular C−H insertion to give the cycloheptatrienylidene product 483 in good yield (Scheme 142).44,1262 Treatment of product 483 with potassium fluoride afforded the tropoloisoquinoline alkaloid pareitropone. Lee and co-workers have discovered an intramolecular cyclization of alkenes via the in situ-generated alkylidene carbenes. The malonate precursor 484 was treated with propynyliodonium triflate 485 under basic conditions to give the angularly fused triquinanes 486 in moderate yields (Scheme 143).1264

Scheme 138. Mechanism of Reactions of Alkynyliodonium Salts with Nucleophiles

iodonium salts can be increased by complexation with 18crown-6 ether, and the resulting ethynyl(phenyl)iodonium-18crown-6 ether complex does not show any decomposition after storage under ambient conditions over one month.66,107,1232,1233 The reaction of alkynyliodonium salts 464 with nucleophilies involves the initial formation of alkylideneiodonium ylides 465. Protonation of ylides 465 can lead to the β-functinalized alkenyliodonium salts 466, which can be isolated as final products. The alternative reaction pathway involves reductive elimination of PhI from ylides 465 generating alkylidene carbenes 467, which undergo 1,2-migration or intramolecular 1,5-carbene insertions to give the corresponding alkynes 468 or the cyclic products 469 and 470 (Scheme 138). Alkynyl(aryl)iodonium salts have been used as useful alkynylating reagents for hetero atoms1207,1234−1245 to give the corresponding acetylenic products via 1,2-shift of alkylidene carbenes, and mechanisms of these reactions have been studied by using carbon-labeling experiments.1236,1245,1246 In a specific recent example, Kitamura and co-workers have developed a regioselective alkynylation of benzotriazole 471 at the 2 position with silylethynyliodonium triflates 472 (Scheme 139). The possible reaction mechanism involves the intermediate formation of alkenylidene carbene intermediate from benzotriazole anion and alkynyliodonium salts followed by the rearrangement to form an alkynylbenzotriazole 473.1241

3.8. Iodonium Ylides

In 1957, Neiland and co-workers reported the first preparation of a stable iodonium ylide by treatment of dimedone (5,5dimethyl-1,3-cyclohexanedione) with (difluoroiodo)benzene.1265 Following this pioneering work, numerous stable aryliodonium ylides have been prepared and utilized as reagents for organic synthesis. The chemistry of aryliodonium ylides has been discussed in several reviews, which mainly summarized the use of ylides as precursors for the generation of singlet carbenes or carbenoid species.53−57 In this section, the synthesis and structural studies of iodonium ylides are overviewed, and recent developments in their synthetic use are discussed. Most iodonium ylides should be handled at low temperature or generated and used in situ because of the low thermal stability. The most practically important ylides, phenyliodonium bis(sulfonyl)methides, PhIC(SO2R)2,1266−1269 and bis(carbonyl)methides, PhIC(COR)2,1265,1270−1277 are synthesized by treatment of (diacetoxyiodo)benzene with dicarbonyl compounds or disulfones in the presence of base. Ochiai and co-workers have developed a new procedure for the preparation of iodonium ylides by the transylidation reaction of halonium

Scheme 139. Regioselective Alkynylation of Benzotriazole with Alkynyliodonium Salts

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Scheme 140. Regioselective Alkynylation of Azlactones 474

Scheme 141. Synthesis of Alkynylplatinum(IV) Complex

Scheme 142. Intramolecular C−H Insertion Reaction of Alkynyliodonium Salt 480

Scheme 143. Preparation of Angularly Fused Triquinanes 486 from Iodonium Salt and Malonate

Figure 15. ortho-Substituted iodonium ylides characterized by X-ray analysis.

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ylides under thermal conditions1278 or under rhodium(II)catalyzed conditions.1279 Cardinale and Ermert have developed a simplified procedure for the synthesis of aryliodonium ylides directly from the respective aryl iodides.1274 In particular, Meldrum’s acid-based aryliodonium yildes were synthesized by the two-step one-pot procedure. Recently, several new, stable ortho-substituted iodonium ylides have been prepared by reactions of β-dicarbonyl compounds with hypervalent iodine reagents, and several structures (487−490) have been characterized by X-ray analysis (Figure 15).1275,1276,1280

Acyclic carbonyl iodonium ylides have also been used as reagents for the synthesis of heterocyclic compounds.1288−1291 Maulide and Afonso have developed the intramolecular C−H insertion reaction of β-ketoamides 496 under metal-free conditions to afford the corresponding β-lactam derivatives 497 (Scheme 146).1290 This reaction probably involves the singlet carbene intermediate generated from iodonium ylides formed in situ from β-ketoamides 496 and (diacetoxyiodo)benzene. A new electrophilic trifluoromethylthiolating reagent, trifluoromethanesulfonyl iodonium ylide (compound 499), has been developed by Shibata and co-workers. This iodonium ylide can react with nucleophiles, such as enamines,1292 arylamines,1293 allylsilanes, silyl enol ethers,1294 and pyrroles,1295 to give the corresponding products of trifluoromethylthiolation. In a representative example, reactions of substituted enamines 498 with ylide 499 under copper catalysis afford trifluoromethylthiolated products 500 in good yields (Scheme 147).1292 The key step in this reaction involves formation of the trifluoromethylthio group via reduction of the trifluorosulfonylmethyl group under copper catalysis.

Scheme 144. Metal-Free Intramolecular Cyclopropanation of Rigid Iodonium Ylide 491

3.9. Iodonium Imides

Iodonium imides or iminoiodanes, ArINR, are nitrogen analogues of iodonium ylides and useful nitrene precursors. The first example of iminoiodane was reported by Abramovitch and co-workers in 1974.1296 One year after, the general procedure for the preparation of iminoiodanes by reactions of (diacetoxyiodo)arenes with p-toluenesulfonamide under basic conditions was developed by Yamada and Okawara.1297 Since then, numerous iodonium imides have been prepared and utilized as reagents for organic synthesis. Iminoiodanes are useful nitrene or nitrenoid precursors under catalytic or thermal conditions, and synthetic applications of these reagents include aziridination of alkenes, transimidation of heteroatoms, and amination of various organic substrates.52,87,813,1298−1305 This section provides an overview of the preparation and structural studies of iodonium imides and describes recent developments in their applications as synthetic reagents. Numerous iminoiodanes have been characterized by a singlecrystal X-ray analysis.122,1306−1311 Iminoiodanes in general have a polymeric structure with distorted T-shaped geometry at the hypervalent iodine centers. The solubility of iminoiodanes in common organic solvents usually is very low because of polymeric properties. Protasiewicz and co-workers have reported the synthesis and X-ray structure of highly soluble ortho-sulfonyl-substituted aryl(imino)iodane, in which the I···N secondary bonding is redirected from intermolecular to intramolecular interaction.51,67,122 A similar redirection of secondary bonding has been observed in ortho-alkoxy iminoidanes, which also have good solubility in common organic solvents.1311

Iodonium ylides are applied as useful carbene or carbenoid sources for the reactions under thermal, catalytic, or photochemical conditions. A discussion of the reaction mechanisms and an overview of synthetic uses of iodonium ylides as carbene precursors can be found in earlier reviews.56,57,1281 The generated carbenes can be trapped with various substances to give the corresponding products of inter- or intramolecular cyclization, products of transylidation, or products of C−H insertion. Numerous recent examples of inter- or intramolecular cyclopropanation of alkenes under metal-catalyzed or metalfree conditions have been published by several groups.1273,1275,1282,1283 Moriarty and co-workers have developed an intramolecular cyclopropanation of alkenes using the sterically rigid iodonium ylides 491 under metal-free condition (Scheme 144).1282 The proposed mechanism involves initial intramolecular [2+2] cycloaddition of the ylide moiety to the double bond followed by elimination of iodobenzene from the four-membered cyclic intermediate to yield cyclopropane 492. Dimedone-derived iodonium ylides have been used in numerous synthetic transformations leading to various heterocyclic compounds.1276,1284−1287 For example, the fused dihydrofuran derivatives 495 can be obtained from iodonium ylide 493 and alkenes 494 under photochemical activation in high yields (Scheme 145).1285 The reaction using iodonium ylide 493 generated in situ from DIB and 1,3-cyclohexanedione under photochemical conditions gave products 495 in similar yields.

Scheme 145. Synthesis of Dihydrofurans from Iodonium Ylide 493 and Alkenes

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Scheme 146. Metal-Free C−H Insertion Reaction of β-Ketoamides 496

Scheme 147. Copper-Catalyzed Trifluoromethylthiolation of Enamines

Scheme 148. Metal-Free Aziridination of Syrenes Using N-Tosyliminoiodane

Scheme 149. Copper-Catalyzed Iminoiodane-Mediated Aminolactonization of Alkenes

Scheme 150. Copper-Catalyzed Direct C−H Amidation of Aldehydes

developed the synthesis of 5,5-disubstituted butyrolactones 506 from the corresponding alkenes 504 and the in situ-generated nosyliminoiodane in the presence of copper catalyst and ligand 505 (Scheme 149). This reaction presumably involves initial formation of aziridine intermediates followed by aminolactonization to give the final products. The obtained products 506 can be further transformed into novel highly functinalized spiro-heterocyclic compounds.1318 The transition metal-catalyzed C−H amidation reaction of saturated or unsaturated organic substrates is one of the important reactions of iminoiodanes.87,1299−1301,1303−1305 Recently, several new types of C−H insertion reactions using iminoiodanes have been developed.1326−1334 A direct synthesis of amides by insertion into the C−H bond of aldehydes has

Iminoiodanes are commonly used as nitrene precursors under catalytic or thermal conditions in the aziridination of alkenes. Numerous examples of inter- or intramolecular aziridination reactions using iminoiodanes have recently been reported by several groups.156,1311−1325 For example, Minakata and co-workers have reported a metal-free aziridination of styrene derivatives 501 with N-tosyliminophenyliodane 502 in the presence of a combination of elemental iodine and tetrabutylammonium iodide (TBAI) (Scheme 148).1321 This aziridination reaction probably has a radical mechanism, and TBAI3 (generated from I2 and TBAI) acts as the actual catalyst in this reaction. The in situ-generated aziridine products can be easily transformed to heterocyclic compounds. Dodd’s group has 3371


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Scheme 151. Copper-Catalyzed Amination and Aziridination of Dicarbonyl Compounds

Scheme 152. Preparation of Palladium Acetate Complex 514

been reported by Chan and co-workers.1326,1327 For example, the reaction of aldehydes 507 with iminoiodane 502 in the presence of copper catalyst and pyridine affords the corresponding amides 508 in good yields (Scheme 150).1327 This reaction most likely proceeds via rate-determining insertion of the copper-nitrenoid species into the carbon− hydrogen bond of an aldehyde. Amination of the allylic C−H bond of enolic form of dicarbonyl compound 509 affords the corresponding α-acyl-βamino derivatives in good yields. This reaction can give amination products 510, or the same reaction with increased amounts of iminoiodane 502 can selectively afford 2,2-diacyl aziridine derivatives 511 (Scheme 151).1330

Saito and co-workers have developed a metal-free [2+2+1] annulation reaction of alkynes 515 with N-tosyliminoiodane 502 in nitrile solvents to give the highly substituted Ntosylimidazoles 516 with high regioselectivities (Scheme 153).1339 The N-tosyl group in products 516 can be deprotected by treatment with trifluoroacetic anhydride and pyridine to afford the N-unsubstituted imidazole. This reaction has been used in the synthesis of catharsitoxin E, a natural product isolated from the Chinese remedy qiung laug.

4. SYNTHETIC APPLICATIONS OF PENTAVALENT IODINE COMPOUNDS Compounds of pentavalent iodine, or λ5-iodanes, represent a practically important class of hypervalent iodine compounds. Particularly useful are the cyclic iodine(V) derivatives, such as Dess−Martin periodinane (DMP), 2-iodoxybenzoic acid (IBX), and its analogues, which are widely used as selective oxidizing reagents in organic synthesis. IBX and DMP are important, but not perfect reagents. IBX is insoluble in most solvents, and it can explode violently upon heating or impact, while DMP is sensitive to moisture. In addition, IBX and DMP are normally used as the stoichiometric reagents, which can have potentially damaging environmental effects and contradict with the principles of Green Chemistry. Several polymer-supported, recyclable analogues of IBX have been introduced in the 21st century and utilized in organic synthesis; these reagents are discussed in sections 7.2 and 7.4. The chemistry of organic iodine(V) compounds has been summarized in several reviews.18,112,113,1340,1341 This section overviews the preparation and structural features of pentavalent iodine compounds and provides a summary of their use as reagents in organic synthesis.

Scheme 153. Preparation of N-Tosylimidazole 516

The sulfonamide insertion reaction into the metal−carbon bond has been reported, and the resulting metal complexes were characterized by X-ray analysis.1335−1338 In a specific example, Ritter and co-workers prepared the new palladium acetate complex by sulfonamide insertion into the palladium− carbon bond of benzoquinoline-derived palladacycle compound 512, followed by chloride−acetate ligand exchange (Scheme 152).1337 The reaction of compound 514 with different arylboronic acids resulted in transmetalation to give the aryl palladium complex, which reacted with the electrophilic fluorinating reagent, Selectfluor, to afford the regiospecifically fluorinated products. 3372


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4.1. Noncyclic and Pseudocyclic Iodylarenes

Scheme 154. Allylic Oxidation of Alkenes to Enones Using PhIO2 with Selenium Catalyst

A direct oxidation of iodoarenes with strong oxidants produces the noncyclic iodylarene compounds, ArIO2. The mechanism of this oxidation involves initial formation of iodosylarenes ArIO, which then slowly disproportionate to ArI and ArIO2 under moderate heating, or even at room temperature. For the preparation of iodylarens from iodoarenes, several common oxidants such as sodium hypochlorite, sodium periodate, dimethyldioxirane, and Oxone are used.121,123,319,1342 Another practical approach to ArIO2 employs peracetic acid in the presence of catalytic amounts of RuCl3; this method is particularly useful for the preparation of noncyclic iodylarenes with electron-withdrawing groups in the phenyl ring.674,1343 In

or in the presence of a catalyst. Because of the low reactivity and explosive properties, the noncyclic iodylarenes have received only limited practical application.2,113 For example, Scheme 155. Carbon−Carbon Double Bond Oxidative Cleavage with Iodylbenzene

Crich and co-workers have reported that aryl ketones can be oxidized by iodylbenzene in the presence of perfluorooctylseleninic acid as a catalyst to the corresponding ketoacids in good yields.1354 Benzylic methylene compounds are also oxidized under these conditions to the corresponding ketones. The same group has also reported that in combination with iodylbenzene 528 as reoxidant, perfluorooctylseleninic acid catalyzes the allylic oxidation of alkenes 527 to enones 529 in trifluoromethylbenzene at reflux conditions in moderate to good yield (Scheme 154).1355 After a reductive workup with sodium metabisulfite, the catalyst is recovered by fluorous extraction in the form of bis(perfluorooctyl) diselenide, which, itself, serves as a convenient catalyst precursor. Iodylbenzene 528 can cleave the unsaturated carbon−carbon double bonds to give the corresponding carbonyl compounds.1356 For example, the reaction of cyclic compounds 530 with iodylbenzene 528 leads to the oxidative carbon− carbon double bond cleavage affording carbonyl compounds 531 in good yields (Scheme 155). This cleavage reaction probably proceeds via a radical pathway. Chiba and co-workers have utilized the copper-catalyzed oxidation of N-allyl enamine carboxylates with PhIO2 in the synthesis of azaheterocycles.1357 This catalytic system allows stepwise cyclopropanation via carbocupration of alkenes. Oxidative cyclopropane ring opening of 5-substituted 3azabicyclo[3.1.0]hex-2-enes has also been used for the synthesis of highly substituted pyridines. The pseudocyclic iodylarenes are useful reagents due to their good solubility in organic solvents. In particular, the pseudocyclic iodine(V) compounds can oxidize sulfides and alcohols to the corresponding sulfoxides and carbonyl compounds.1345,1346,1348,1352,1353,1358,1359 These oxidations proceed without overoxidation to carboxylic acids and sulfones. Isopropyl-2-iodoxybenzoate, also known as IBX-ester, can serve as a thermally stable and efficient source of oxygen in the metalloporphyrin-catalyzed oxidations of hydrocarbons and alcohols.177,190 Isopropyl-2-iodoxybenzoate 533 also can be used as oxygen transfer reagent to iron complexes.1360−1362 For example, Rybak-Akimova’s group has reported the oxidation of iron(II) complex 532 by reagent 533 to the iron(IV)−oxo complex 534, which can oxidize cyclooctene and phosphine to

Figure 16. Pseudocyclic iodylarenes.

this reaction, iodobenzene is initially oxidized by peracetic acid to (diacetoxyiodo)benzene, and then the RuCl3-catalyzed disproportionation of PhI(OAc)2 gives PhIO2 and PhI, which are further oxidized. Because of their polymeric structure, the noncyclic iodylarenes have a low solubility in organic solvents except DMSO. According to X-ray crystallographic studies, iodylbenzene has a three-dimentional polymeric structure with a distorted octahedral geometry at the iodine(V) center with three primary I−O bonds (1.92−2.01 Å) and three secondary I···O bonds (2.57−2.73 Å).1344 Iodylarenes with appropriate substituents in the ortho position of the aromatic ring have a pseudocyclic structure due to the intramolecular secondary I···O interaction with the oxygen atom of ortho-substituent. Such pseudocyclic iodine(V) compounds have a significantly better solubility in organic solvents because their polymeric nature is partially disrupted by the redirection of secondary bonding.67,121 Protasiewicz and coworkers have first prepared a pseudocyclic iodylarene, 1-(tbutylsulfonyl)-2-iodylbenzene 517 (Figure 16), and structurally characterized this compound by X-ray crystallography.51,121 The crystal structure of 517 revealed a pseudooctahedral geometry at the iodine atom with I−O bond lengths in the iodyl group of 1.796 and 1.822 Å and the intramolecular distance of 2.693 Å between one of the sulfone oxygen atoms and the iodine center. Following this pioneering study, various types of pseudocyclic iodylarenes bearing ortho-substituent, such as ethers 518,1345 esters 519,1346,1347 amides 520,1348 phosphine oxide 521,123 sulfonate ester 522,1349 sulfonamides 523,1350,1351 nitro 524,127 and amino derivatives 525, 526,1352,1353 have been prepared from the corresponding iodoarenes using suitable oxidants (Figure 16). The noncyclic iodylarenes have a relatively low reactivity as oxidizing reagents because of insolubility in organic solvents, and their reactions are usually conducted at high temperature 3373


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Scheme 156. Oxidation of Iron(II) Complex to Iron(IV)−Oxo Complex Using IBX-Ester 533

Figure 17. Examples of heterocyclic iodine(V) compounds.

Scheme 157. Selective Oxidation of Tetraol 547 in the Total Synthesis of (−)-Erinacin E 549

convenient method using Oxone as the oxidant instead of potassium bromate was developed by Santagostino and coworkers.1376 This improved method can be also used for the large-scale preparation of IBX. IBX and its derivatives are widely applied as reagents for oxidation of alcohols to carbonyl compounds, especially useful

cyclooctane epoxide and phosphine oxide, respectively (Scheme 156).1360 4.2. Heterocyclic Iodine(V) Compounds

Among iodine(V) heterocyclic compounds, the five-membered iodine(V) heterocycles are especially important. The cyclic λ5iodanes, 2-iodoxybenzoic acid (IBX, 535) and Dess−Martin periodinane (DMP, 536), are widely used as selective reagents for the oxidation of alcohols to carbonyl compounds and some other important oxidative transformations. Additional examples of heterocyclic iodine(V) compounds are represented by structures 537−546 in Figure 17.908,1363−1373 Synthetic applications of IBX, DMP, and other heterocyclic iodine(V) reagents have been summarized in several reviews.18,112,1340,1341 4.2.1. 2-Iodoxybenzoic Acid (IBX) and Derivatives. 2Iodoxybenzoic acid (IBX) was originally reported by Hartman and Mayer in 1893.1363 About 100 years later, the initially suggested cyclic structure of IBX was confirmed by singlecrystal X-ray crystallographic study.1374 In the solid state, IBX has a three-dimensional polymeric structure due to strong intermolecular secondary I···O contacts and hydrogen bonding. The IBX samples, prepared by the original procedure using KBrO3 in sulfuric acid, can explode under impact or heating, probably because of the bromate impurities.1375 A safe and

Scheme 158. Dearomatization Reactions of Phenols Using SIBX

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Nicolaou and co-workers have utilized IBX in numerous synthetic works, such as one-step conversion of saturated alcohols and carbonyl compounds to α,β-unsaturated carbonyl systems,1385−1389 selective benzylic oxidations,1390−1392 oxidative cyclization of anilides and similar substrates,1393−1395 and synthesis of amino sugars.1395 In particular, the α,β-unsaturated compounds can be prepared in one step directly from the corresponding alcohols, ketones, and aldehydes by IBX oxidation.1386 The reaction of cycloalkanols 554 with 2 equiv of IBX 535 under moderate heating in the appropriate solvent system affords the corresponding α,β-unsaturated ketones 555 in good yields (Scheme 159).1385 Under similar conditions, the dehydrogenation of cyclic ketone 556 afforded enone 557, which was employed in the total synthesis of (−)-anominine.1396 Arimoto and co-workers reported the unexpected dehomologation reaction of primary alcohols to one carbon shorter carboxylic acid using IBX 535 and molecular iodine. This novel reaction has been used for the oxidation of various alcohols 558 to one carbon shorter carboxylic acid 559 in good yields (Scheme 160).1397 The authors of that paper isolated and investigated by 1H and 13C NMR the active species formed from IBX and molecular iodine under reaction conditions; however, the exact structure of this highly reactive intermediate remains unknown. Kirsch and co-workers have investigated oxidative functionalization of carbonyl compounds with IBX or IBX derivative (IBX-SO3K) and found that, depending on a functional group in the α-position of carbonyl compound, the reaction may lead either to the oxidative dehydrogenation or to α-functionalization.1398−1404 In particular, the direct azidation reaction of 1,3dicarbonyl compounds 560 by treatment with IBX-SO3K and NaN3 in the presence of sodium iodide as catalyst gives the corresponding azidation compounds 561 in good yields (Scheme 161).1401 This reaction can also be used for the azidation of 1,3-dicarbonyl compounds with sensitive substituents. IBX can be used as effective oxidizing reagent for the threecomponents condensation reactions, such as Ugi-type reaction1405−1407 and Strecker-type reaction.1408,1409 In a specific example, the reaction of aldehydes 562, amines 563, and TMSCN 564 using IBX 535 and tetrabutylammonium bromide affords α-iminonitriles 565 in good yields (Scheme 162). This methodology has also been employed in the synthesis of indolizidines via intramolecular cycloaddition of α-iminonitriles under microwave conditions. IBX 535 has been be used as reagent for the synthesis of heterocyclic products.1410−1415 For example, the IBX-mediated synthesis of functionalized pyridines 568 from β-enamino esters 566 and allylic alcohols 567 has been reported by Pal and Iqbal (Scheme 163). This new methodology can afford 2-substituted

in the synthesis of natural products and complex molecules with different functional groups.2,112,1340 In a representative example, Nakada and co-workers reported the IBX-promoted oxidation of the allylic secondary alcohol 547 to the enone 548, which was further converted to the natural product (−)-erinacin E 549 by stereoselective reduction in good overall yield (Scheme 157).1377 IBX is commonly used as reagent for regioselective dearomatization of phenols leading to cyclohexa-2,4- or -2,5dienone.70,78,523,1341,1378 For example, Quideau and co-workers have found that the reaction of phenolic derivatives 550 with SIBX (stabilized IBX; IBX 49%, benzoic acid 22%, isophthalic acid 29%) proceeds as the ortho-oxygenative phenol dearomatization/Diels−Alder cyclodimerization sequence of reactions to give the corresponding dimeric products 551 in good yields (Scheme 158). The oxidation of naphthols 552 under similar conditions affords ortho-quinols 553 in high yields.1379 Additional representative examples include the use of IBX or SIBX as oxidants in key steps of the following synthetic works: total synthesis of the resveratrol-derived natural polyphenols Scheme 159. Synthesis of α,β-Unsaturated Ketones Using IBX

(−)-hopeanol and (−)-hopeahainol A,1380 synthesis of carnosic acid and carnosol,1381 the synthon synthesis of epicocconone,1382 total synthesis of the wasabidienines B1 and B0,1383 and total synthesis of the bissesquiterpene (+)-aquaticol.1384 The synthetic value of IBX as a reagent has been extended to several other synthetically useful oxidative transformations. Scheme 160. Dehomologation Reaction of Primary Alcohols Using IBX and I2

Scheme 161. Azidation Reaction of 1,3-Dicarbonyl Compounds Using IBX-SO3K and NaN3

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Scheme 162. Strecker-type Reaction Using IBX and TBAB Combination

Scheme 163. IBX-Mediated Preparation of Substituted Pyridines

Scheme 164. Oxidation−Dehydrogenation of Alcohols Using IBX-OTs 545

nicotinic acids, tetrasubstituted unsymmetrical pyridines, and precursors of azafluorenones.1413 Useful halogenation reactions of alkenes, alkynes, and aromatic compounds by treatment with IBX-halogen reagent combinations have been reported.1416−1420 In particular, the IBX·I2 combination can generate hypoiodous acid (HOI), which reacts with alkenes or aromatic compounds to give the corresponding products of iodination in good yields.1417 New reagents, IBX derivatives, IBX-OTs and IBX-OMs, were prepared by the treatment of IBX with the corresponding sulfonic acids via ligand exchange. IBX-OTs 545 is a strong oxidant and electrophile due to the electron-withdrawing tosylate substituent. For example, the reaction of alcohols 569 with 3 equiv of 545 at room temperature results in an oxidation−dehydrogenation sequence leading to α,β-unsaturated carbonyl compounds 570 (Scheme 164). 4.2.2. Dess−Martin Periodinane (DMP) and Analogues. The triacetoxybenziodoxolone 536, commonly known as Dess−Martin periodinane (DMP), was first reported in 1983 by Dess and Martin.1364 Within a few years after initial publication, DMP has gained a status of a reagent of choice for selective oxidation of alcohols to carbonyl compounds, especially in complex molecules containing other sensitive functional groups.1421 DMP can be conveniently prepared by

Scheme 166. One-Pot Synthesis of Trichloromethyl Carbinols from Primary Alcohols

the treatment of IBX with acetic anhydride and a catalytic amount of p-toluenesulfonic acid at 80 °C.1422 DMP is a commercially available reagent. The structure of DMP has been confirmed by single-crystal X-ray diffractometry.1423 Because of the mild reaction conditions (absence of acidic additives, room temperature) and high selectivity of oxidations, DMP has become one of the most commonly used oxidizing reagents. DMP can be used for oxidation of alcohols with sensitive substituents, such as double and triple bonds, amines, silyl ethers, sulfides, selenides, phosphine oxides, epimerization sensitive groups, etc. Addition of water to the reaction mixture can accelerate oxidations with DMP.1424 Synthetic applications of DMP have been summarized in several overviews.2,18,113,114 In a representative example, the oxidation of α-hydroxyboronates 571 using DMP 536 affords the corresponding acylboronates 572 in good yield (Scheme 165). No oxidative cleavage of the carbon−boron bond is observed in this reaction.1425 Snowden and co-workers have reported a simple synthesis of trichloromethyl carbinols 575 from primary alcohols 573 by treatment with DMP 536 (Scheme 166). This method allows conversion of chiral primary alcohols to the corresponding trichloromethyl carbinols with complete stereochemical fidelity, despite the intermediate formation of base-sensitive aldehyde intermediates. Trichloromethyl carbinols 575 can be further

Scheme 165. Oxidation of α-Hydroxyboronates Using DMP 536

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Scheme 167. DMP-Mediated Cyclization Reaction

Scheme 168. Synthesis of γ-Lactams from Pyrroles Using DMP

tranformed to the corresponding carboxylic acids or amides.1426,1427 DMP has been used in the synthesis of polycyclic compounds and heterocycles.1428−1433 Nicolaou and co-workers have reported the DMP-mediated cyclization of anilides affording the natural product-like heterocyclic compounds. For example, the reaction of anilide 576 with DMP leads to the formation of polycycle 577 in moderate yield (Scheme 167).1433 Dobrota and co-workers have also reported the DMP-mediated one-pot cyclization of anilides to the corresponding quinoxalines.1430 The oxidative dearomatization reaction of pyrrole to γ-lactam by treatment with DMP 536 has been accomplished.1434 This methodology allows controlled oxidation of pyrroles 578 to give the functionalized γ-lactams 579 in good yields (Scheme 168). DMP can be also used for the facile oxidative aromatization of heterocyclic compounds.1435,1436 Palladium-catalyzed oxidative cycloisomerization of 3-cyclopropylideneprop-2-en-1-ones 580 with DMP 536 has been used for the facile synthesis of a highly strained functionalized 2-alkylidenecyclobutanone 581 (Scheme 169). The obtained cyclobutanone products 581 can be easily converted to furanones with a spiro-cyclopropane unit by treatment with acid.1437

Figure 18. Chiral organohypervalent iodine reagents.

Scheme 170. Dioxytosylation of Styrene with Chiral Reagent 589

the aromatic ring. This section will summarize recent practical applications of chiral iodanes. 5.1. Chiral Iodine(III) Reagents

Chiral organohypervalent iodine(III) reagents can be prepared by using the same general methods as for the common iodine(III) derivatives. Numerous examples of chiral trivalent iodine compounds have been reported; several representative reagents 582−587 are shown in Figure 18.101,336,340,344,465,729,741,918,1316,1438−1447 Enantioselective functionalization of alkenes and ketones using chiral [hydroxy(organosulfonyloxy)iodo]arenes 583 has been reported by Wirth and co-workers. For example, the dioxytosylation of styrene 588 with reagent 589 affords product 590 in 65% ee (Scheme 170). The higher ee values in the reaction correlate with the relative population of an axial conformation of the methyl group on the asymmetric carbon atom of the pseudocyclic molecule 589.337 Fujita and co-workers have prepared the optically active lactate-based (diacetoxyiodo)arenes, which are useful reagents for the enantioselective tetrahydrofuranylation,336,389 dioxyacetylation,387 and oxylactonization reactions.1443,1448−1451 In a specific example, the asymmetric synthesis of 3-alkyl-4-

5. ENANTIOSELECTIVE REACTIONS USING CHIRAL HYPERVALENT IODINE REAGENTS Enantioselective reactions using organohypervalent iodine reagents in the presence of chiral additives are gaining increasing importance in organic synthesis. Likewise, numerous chiral organohypervalent iodine reagents have been developed for the enantioselective reactions.94,98,103 The two most common types of chiral hypervalent iodine reagents are known: (i) hypervalent iodine reagents with chiral ligands, and (ii) hypervalent iodine reagents with chiral substituent in

Scheme 169. Oxidative Cycloisomerization of 3-Cyclopropylideneprop-2-en-1-ones

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Scheme 171. Enantioselective Oxylactonization of ortho-Alkenylbenzoates 591

Scheme 172. Asymmetric Spirolactonization of a Naphthol Substrate

Scheme 173. Diamination of Styrenes Using Reagent 593 and Ms2NH

Scheme 174. Stereoselective Rearrangement of α,β-Unsaturated Ketones

oxyisochroman-1-ones 594 from ortho-alk-1-enylbenzoates 591 has been achieved by treatment with chiral (diacetoxyiodo)arenes 592 or 593. The oxidative oxylactonization of orthoalkenylbenzoate 591 proceeds with high regio-, diastereo-, and enantioselectivity to give the corresponding lactones 594 in up to 97% ee (Scheme 171). This stereocontrolled transformation has been employed in the asymmetric synthesis of a biologically active natural compound.1443 Similar reactions of alkenamides with reagents 592 or 593 afford the corresponding lactone imines with high stereoselectivity.1451

Enantioselective oxidative dearomatization of phenols to construct a chiral ortho-spirolactone structure using chiral organohypervalent iodine reagent 587 has been originally developed by Kita’s group.1442 In particular, the reaction of ortho-carboxy-substituted naphthol 595 with reagent 587 affords the corresponding chiral spirolactone 596 with high enantioselectivity (Scheme 172). Two years later, Ishihara and co-workers reported the more effective asymmetric spirolactonization of naphthols using C2-symmetric hypervalent iodine3378


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Scheme 175. Intramolecular Aminofluorination of Alkenes Using Chiral Reagent 603

Scheme 176. Enantioselective Oxidative Dimerization of 2,6-Dimethylphenol

Scheme 177. Asymmetric Hydroxylative Phenol Dearomatization Reaction Using 609

5.2. Chiral Iodylarenes

(III) reagent 597 bearing two chiral lactate-derived groups.101,340 Direct diamination of alkenes can be performed in enantioselective fashion using chiral hypervalent iodine reagents.820,825 Styrenes 598 have been converted into the corresponding (S)-diamine derivatives 599 with high enantioselectivity under metal-free conditions using chiral (diacetoxyiodo)arene 593 as the oxidant and Ms2NH as the nitrogen source (Scheme 173). Ditosylamine HNTs2 has also been successfully used as a nitrogen source in this reaction.820 Wirth and co-workers have developed an efficient and highly stereoselective method for the α-arylation of a wide range of carbonyl compounds by an oxidative rearrangement procedure. The stereoselective oxidative rearrangement of α,β-unsaturated ketones 600 by treatment with lactic acid-based chiral iodine(III) reagent 597 affords the corresponding products of α-arylation 601 with high enantioselectivity (Scheme 174).1452 The regio- and enantioselective intramolecular metal-free aminofluorination of pentenamines 602 with the chiral (difluoroiodo)arene 603 provides 2-fluoropiperidines 604 in good enantioselectivity (Scheme 175). A similar gold-catalyzed reaction of hexenamines with reagent 603 affords the corresponding azepanes. The reaction of styrenes with reagent 603 proceeds as an intermolecular regioselective aminofluorination to give 2-fluoro-2-phenylethanamines in moderate to good yields.1453

Chiral iodylarenes can be prepared using the same general methods as for common iodylarenes, and they can be used as selective reagents for the oxidation of alcohols,1454,1455 sulfides,1369,1454,1456 and for oxidative dearomatization of phenols.1446,1457 In a specific example, the pseudocyclic iodylarenes with chiral oxazoline groups can transform orthoalkylphenols into ortho-quinol Diels−Alder dimers with significant levels of asymmetric induction. In particular, the reaction of 2,6-dimethylphenol 605 with chiral iodylarene 606 affords o-quinol dimer 607 with moderate enantioselectivity (Scheme 176).1457 Pouysegu and Quideau reported the preparation and reactivity of the C2-symmetric iodine(V) reagents based on the binaphthyl and biphenyl backbones. For example, the reaction of C2-symmetrical biphenylic iodine(V) reagent 609 with phenols 608 results in asymmetric hydroxylative phenol dearomatization (HPD) leading to the corresponding orthoquinol-based [4+2] cyclodimer 610 with up to 94% ee (Scheme 177).1446

6. IODINE COMPOUNDS AS ORGANOCATALYSTS It has been widely recognized that hypervalent iodine species and transition metal complexes have similar reactivity patterns. In particular, the observed for hypervalent compounds reactions of ligand exchange, oxidative addition, reductive elimination, and ligand coupling are common in the chemistry of transition metals.1 However, the hypervalent iodine3379


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and complex spirocyclic products. The first examples of catalytic iodine(III)-mediated reactions utilizing iodobenzene as a precatalyst include the formation of α-acetoxycarbonyl compounds 612 from ketones 611 and the synthesis of spirocyclohexanones 614 from phenols 613 (Scheme 178).1458,1459 Following the discovery of α-acetoxylation of ketones by Ochiai and co-workers, several research groups have reported

catalyzed reactions remained unknown until the beginning of 21st century. In 2005, the groundbreaking studies on the hypervalent iodine-catalyzed reactions were independently published by the groups of Ochiai,1458 Kita,1459 and Vinod.1460 Following this initial research, numerous types of catalytic hypervalent iodine reactions have been developed during the last 10 years. While chemical reactions catalyzed by iodine species have been discovered only in 2005, the electrochemical generation of iodine(III) species in situ from catalytic amounts of iodoarenes and the use of these species as the in-cell mediators in electrochemical fluorination reactions have been reported by Fuchigami and co-workers much earlier, in 1994.226 The

Scheme 180. Cyclization of N-Allylamides 617 to Oxazoline Products

Scheme 178. First Examples of Catalytic Iodine(III)Mediated Reactions numerous examples of α-acetoxylation,1467 α-tosyloxylation,1468−1471 α-phosphoryloxylation,1472 and α-fluorination1473,1474 reactions using catalytic amounts of iodoarenes in the presence of appropriate nucleophiles and suitable terminal oxidants. Ochiai’s procedure has also been used for the intramolecular α-substitution reaction. In particular, Ishihara and co-workers have developed the hypervalent iodine-catalyzed intramolecular oxylactonization of ketocarboxylic acids to ketolactones based on Ochiai’s methodology.1475 The iodine(III) catalytic system has been used for various functionalizations of alkenes or alkynes.1476−1487 For example, Tan and co-workers reported the catalytic (diacetoxyiodo)benzene-mediated oxidative iodolactonization of alkenes and alkynes in the presence of Bu 4 NI. In this reaction, (diacetoxyiodo)benzene is generated from catalytic amounts of iodobenzene, and sodium perborate monohydrate is used as the oxidant. The reaction of alkenyl or alkynyl carboxylic acid 615 under optimized conditions gives the corresponding lactones 616 in high yields (Scheme 179).1476 The analogous phosphoryloxylactonization of pentenoic acids using similar catalytic conditions was reported by Zhou and co-workers.1477 Moran and co-workers have developed the hypervalent iodine-catalyzed cyclization of N-allylamides 617 to the corresponding oxazoline compounds 618 (Scheme 180). This new procedure can be used for the preparation of the five- to seven-membered ring products of cyclization.1478 Braddock and co-workers have reported the bromocyclization of alkenecarboxylic acids to bromolactones using orthosubstituted iodobenzenes as organocatalysts and N-bromosuccinimide (NBS) as the terminal oxidant and bromine source.1479 In this reaction, the electrophilic bromoiodane intermediates are generated in situ from iodoarene and NBS. Yan’s group has also developed the bromolactonization reaction using catalytic amounts of iodobenzene and lithium bromide as the bromide source with Oxone as the terminal oxidant.1480

hypervalent iodine-catalyzed reactions have been discussed in several recent review articles.18,20,96−98,102−111,1461−1466 6.1. Catalytic Cycles Based on Iodine(III) Species

In a catalytic system, the reoxidation of iodine in low valent state to higher oxidation state is the key step. The choice of the stoichiometric oxidant in these reactions is critically important for the side reaction to occur with oxidant and substrate. The oxidant has to be carefully selected to achieve effective reoxidation of the reduced iodine form under reaction conditions. Common oxidants that satisfy these requirements include m-chloroperoxybenzoic acid, hydrogen peroxide, sodium perborate, and Oxone. Iodobenzene and numerous other compounds of iodine, including the chiral iodoarenes, can be used as precatalysts in these reactions. 6.1.1. Iodoarenes as Precatalysts. Numerous examples of hypervalent iodine(III)-mediated catalytic reactions have been reported in the last 10 years. These catalytic reactions have been utilized in the synthesis of functionalized organic compounds of different types, including various heterocycles

Scheme 179. Iodolactonization of Alkenyl- or Alkynylcarboxylic Acid

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Scheme 181. Bromoiodanes-Catalyzed Regioselective Bromination of Alkenes

obtained α,α-disubstituted-α-hydroxy carboxylamides 626 can be transformed to introduce structural diversity and complexity into the carbocyclic framework. The intermediate generation of bromobenziodoxolone species from o-iodobenzamide 620 has been confirmed by NMR spectroscopy and ESI−MS experiments.1483 Miyamoto and Ochiai have developed the efficient method for iodoarene-catalyzed oxidative cleavage of carbon−carbon multiple bonds as an environmentally friendly, safe alternative to ozonolysis. Under these organocatalytic conditions, various unsaturated compounds 627, such as cyclic alkenes, acyclic alkenes, and aryl acetylenes, are selectively cleaved to carboxylic acids 629 (Scheme 182). This catalytic reaction involves in situ generation of the tetracoordinated square-planar hydroxyiodonium intermediates 628 as the reactive species.1486 Following the discovery of the intramolecular C−O bondforming reactions by Kita and co-workers, various intramolecular coupling reactions have been developed, such as the C−O,1488−1492 the C−C,1493,1494 and the C−N355,1495−1502 bond formation reaction. These new bond formation reactions are particularly useful for the preparation of heterocyclic compounds. For example, the intramolecular coupling reactions between the sp2 or sp3 C−H bond and the carboxylic acid function in substrates 630 or 632 using catalytic amounts of aryliodine and peracetic acid as terminal oxidant have been developed by Martin and co-workers. The selective insertion to sp2 or sp3 C−H bond depends on the iodoarene catalyst: 4iodotoluene as a precatalyst can be useful for the intramolecular sp2 C−H bond insertion reaction, and 1-bromo-4-iodobenzene is acceptable for the intramolecular sp3 C−H bond insertion reaction (Scheme 183).1492 Lupton and co-workers have reported the iodobenzenecatalyzed 1,2-functionalization of alkenes via a cascade C−O and C−C bond formation. The reaction provides [4.2.1]nonanes 636 and oxabicyclo-[3.2.1]-octanes 637 from

Gulder and co-workers have reported three regioselective bromination reactions of alkenes using NBS with oiodobenzamide derivatives as preorganocatalysts (Scheme 181).1481−1483 For example, the reaction of methacrylamides 619 with NBS and o-iodobenzamide derivative 620 as precatalyst results in bromocyclization to give the 3,3disubstituted oxoindoles 621. As a demonstration of synthetic Scheme 182. Iodomesitylene-Catalyzed Oxidative Cleavage of C−C Multiple Bonds

utility of this cyclization, the key intermediate of acetylcholinesterase inhibitor physostigmine has been prepared using this procedure.1481 The dibromination of alkenes or alkynes 622 can be catalyzed by N-butyl o-iodobenzamide 623 at room temperature using NBS or the KBr−Oxone combination. A similar reaction in the presence of KCl instead of KBr results in the dichlorination of alkenes in moderate to good yields.1482 α,α-Disubstituted-α-hydroxy carboxylamides 627 are easily accessible from imides 625 in a metal-free, bromobenziodoxolone-catalyzed rearrangement, which opens a selective and efficient route to these highly useful building blocks. The 3381


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Scheme 183. C(sp2)−H and C(sp3)−H Functionalization Leading to C−O Bond Formation

Scheme 184. Intramolecular Cascade C−O and C−C Bond Formation Reaction

Scheme 185. Oxidative Cross-Coupling Reaction of Sulfonanilides and Aromatic Hydrocarbons

Scheme 186. Iodine(III) Species-Mediated Synthesis of Isoxazolines and Isoxazoles

commercially available 3-alkoxy cycohexen-2-ones 634. This methodology is flexible, with the second cyclization event achieved using two alternate electron-rich aromatic groups, providing a range of carbocycles (Scheme 184). This reaction involves initial formation of alkyliodonium salts 635 from

substrate 634 and the generated iodine(III) species, followed by nucleophilic attack from the aromatic ring providing the final products.1503 The oxidative cross-coupling reactions between aromatic compounds and amides can be achieved by using in situ3382


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Scheme 188. Catalytic Iodine(III)-Mediated ipsoHydroxylation of Boronic Compounds

generated hypervalent iodine species.1504−1506 For example, Kita and co-workers have developed the organohypervalent iodine-catalyzed oxidative cross-coupling reaction of aromatic sulfonanilides with aromatic hydrocarbons. Using 2,2′-diiodobiphenyl as precatalyst is critically important for this catalytic reaction. The coupling reaction of 4-bromo-naphthylsulfonanilide 638 and aromatic hydrocarbons 639 with mCPBA as oxidant and 2,2′-diiodobiphenyl as precatalyst affords biaryl products 640 in good to high yields (Scheme 185).1504 The stoichiometric hypervalent iodine(III) reagents are commonly used as efficient reagents for oxidation of alcohols and phenols to the respective carbonyl compounds or quinones. The catalytic version of oxidation of alcohols and phenols has also been reported. The catalytic organoiodine(III) species, which are generated from iodoarenes and common oxidants, can promote oxidation of alcohols in the presence of TEMPO.1507−1510 Yakura and co-workers have prepared the new bifunctional hybrid-type catalysts bearing TEMPO and iodobenzene moieties, which can be used for the environmentally benign oxidation of primary alcohols to carboxylic acids.1508 Yakura’s group has also reported the catalytic version of phenolic oxidation using catalytic amounts of iodoarenes with Oxone as terminal oxidant. The reaction of phenols under aqueous condition affords the corresponding p-quinones or pquinols in generally high yields.1511−1515 Experimental procedures for cyclization of aldoxime 641 with alkenes 643 or alkynes 645 to isoxazolines 644 or isoxazoles 646 using catalytic amounts of aryliodine and Oxone as the

was first reported by Miyamoto and Ochiai in 2012.1521 Carboxamides 647 react with the tetracoordinated squareplanar bis(aqua) (hydroxy)phenyl-λ3-iodane complex as an active oxidant generated from a catalytic amount of iodobenzene by the reaction with mCPBA in the presence of aqueous HBF4 to give ammonium salts 648 in good yields (Scheme 187). The mechanism of this reaction is similar to the classical Hofmann rearrangement promoted by stoichiometric hypervalent iodine reagents. The Hofmann rearrangement promoted by a catalytic amount of iodobenzene and Oxone as terminal oxidant has also been reported.1522 This reaction involves hypervalent iodine species formed in situ from PhI and Oxone in the presence of HFIP in aqueous methanol solutions. Under these conditions, Hofmann rearrangement of various carboxamides 647 afforded the corresponding carbamates 649 in good yields (Scheme 187). The efficient hypervalent iodine(III)-mediated procedure for ipso-hydroxylation of diversely functionalized arylboronic compounds 650 to phenols 651 using NaIO4 as a co-oxidant has been developed (Scheme 188).1523 This reaction is also applicable to the conversion of alkylboronic acids or esters to the respective alcohols. 6.1.2. Iodide Salts as Precatalysts. Iodide salts and also elemental iodine can be used as precatalysts for hypervalent iodine(III)-mediated reactions.110 These precatalysts can be transformed in situ to the iodine species in higher oxidation state (+1 or +3) by suitable oxidants; in particular, the highly unstable iodite (IO2−) species have been detected by the ESInegative ion mass spectrometry in the reaction mixture containing Bu4NI and H2O2.1524 Synthetic application of a similar catalytic system can be illustrated by the oxidative αtosyloxylation of ketones using mCPBA as the stoichiometric oxidant in the presence of catalytic I2 or KI as a precatalyst. The reaction of ketones 652 with toluenesulfonic acid under these conditions affords α-tosyloxyketones 653 in moderate yield (Scheme 189). This reaction can also give similar results under basic conditions using a catalytic amount of alkyl iodide instead of I2 or KI.1525 Zhang and Su reported the NaI-catalyzed direct αalkoxylation of ketones with alcohols via oxidation of the αiodoketone intermediate. The reaction of ketones 652 with the NaI−-mCPBA combination in methanol gives α-methoxyketones 654 in generally good yields (Scheme 190). The key intermediate in this reaction, α-iodoketone, is generated by oxidative iodination of ketone with NaI and mCPBA, and then the generated α-iodoketone is converted to product 654 by oxidatively assisted nucleophilic substitution with methanol.1526 An efficient one-pot sulfonyloxylactonization of alkenoic acids mediated by hypervalent iodine species generated from catalytic ammonium iodide and mCPBA has been developed. This procedure provided the corresponding sulfonyloxylactones in moderate to good yields via intermediate formation of hypervalent iodine intermediates.1527

Scheme 187. Iodine(III)-Catalyzed Hofmann Rearrangement of Carboxamides

oxidant have been develoved.1516,1517 These reactions proceed via initial formation of nitrile oxides 642, which react with alkenes or alkynes to give corresponding isoxazolines and isoxazoles (Scheme 186). Yan and co-worker have developed the convenient one-pot synthesis of isoxazolines from aldehydes using similar catalytic conditions.1518 Anilines can be oxidized to azobenzenes under hypervalent iodine catalysis using peracetic acid as the terminal oxidant. This metal-free oxidation system demonstrates wide substituent tolerance; alkyls, halogens, and several versatile functional groups, such as amino, ethynyl, and carboxyl substituents, are compatible, and the corresponding products are formed with good to excellent yields. This procedure is also acceptable for the synthesis of asymmetrical azo compounds.1519 The use of stoichiometric amounts of hypervalent iodine species formed in situ from PhI and Oxone for Hofmann rearrangement of carboxamides was reported in 2010.88,1520 The hypervalent iodine(III)-catalyzed Hofmann rearrangement 3383


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Scheme 189. α-Tosyloxylation of Ketones Using mCPBA with Catalytic Amounts of I2 or KI

Scheme 190. α-Alkoxylation of Ketones Using NaI−mCPBA Combination

Scheme 191. TBAI-Mediated Synthesis of Imidazo[1,2-a]pyridines

Scheme 192. IBX-Mediated Catalytic Oxidation of Alcohols Using Oxone as Oxidant

Scheme 193. Modified IBX-Based Catalytic Oxidation of Alcohols Using Oxone

Scheme 194. Oxidative Cleavage of Olefins by in Situ-Generated Catalytic Modified IBX

Yu and Han reported the efficient oxidative C−N coupling reaction between aminopyridines 655 and β-keto esters or 1,3diones 656 using a catalytic amount of tetrabutylammonium iodide (TBAI) and tert-butyl hydroperoxide (TBHP). Under these reaction conditions, imidazo[1,2-a]pyridines 657 were formed in moderate yields (Scheme 191). The β-diketo-αiodine(III) species, generated from diketones 656 and the TBAI−TBHP combination, were proposed as the initial key intermediates in this reaction. A subsequent reaction of β-

diketo-α-iodine(III) species with aminopyridines 655 affords the final product 657.1528 6.2. Catalytic Cycles Based on Iodine(V) Species

IBX and its analogues are well-known as effective stoichiometric oxidants for oxidation of alcohols to the corresponding carbonyl compounds and for other oxidative transformations. The first catalytic procedure involving hypervalent iodine(V) active species was reported by Vinod and co-workers in 2005. This reaction employed catalytic amounts of 2-iodobenzoic acid and Oxone as terminal oxidant in aqueous acetonitrile at 70 °C. 3384


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The IBS-catalyzed selective oxidations of phenols to oquinones with Oxone in nonaqueous solvents have also been developed by Ishihara’s group. Various phenols are oxidized to the corresponding o-quinones in generally good yields.1536 The catalytic 5-Me-IBS-mediated procedure has been used for the oxidative conversion of tertiary allylic alcohols 668 to enones 669 (Scheme 196). Cyclic and acyclic substrates afford the corresponding enones in moderate to high yields.1537 The efficient catalytic system for oxidation of hydrocarbons and alcohols using catalytic amounts of PhI and RuCl3 and Oxone as a stoichiometric oxidant has been developed. This RuCl3-mediated tandem catalytic reaction involves reoxidation of ArIO to ArIO2. In particular, the oxidation of various alcohols using this catalytic system gave the corresponding carbonyl compounds or carboxylic acids in excellent yields. Likewise, propylbenzene 670 is effectively oxidized under these conditions to propiophenone 671 in high yield (Scheme 197). In this reaction system, PhIO is initially formed from PhI and Oxone, and then the key activated species of this catalytic cycle, PhIO2, is generated by the RuCl3-catalyzed disproportionation of PhIO.88,1538

Under these catalytic conditions, primary alcohols 658 were effectively oxidized to carboxylic acids 659 (Scheme 192).1460 A similar catalytic oxidation of alcohols with tetrabutylammonium Oxone as a soluble stoichiometric oxidant was reported by Giannis and co-workers in 2006. Using this system, a variety of benzylic alcohols 660 were transformed to aldehydes 661 in good yields, whereas secondary alcohols were converted to ketones (Scheme 192).1529 Likewise, the Page group has developed the catalytic oxidation of alcohols to carbonyl compounds using tetraphenylphosphonium monoperoxysulfate (TPPP) as a soluble terminal oxidant.1530 Goddard and co-workers have theoretically investigated the oxidation of alcohols with IBX by DFT calculations and found that the rate-determining step in this reaction is twisting of the alkoxyiodine(V) intermediate, the so-called “hypervalent-twisting”.1531 The same group has also suggested that the orthosubstitution of IBX is enhancing reactivity because of the repulsion between alkoxy group and ortho-substituent.1531 On the basis of this prediction of increased reactivity of the orthoScheme 195. Catalytic Cascade Oxidative Dehydrogenation Reaction of Cyclic Alcohols

6.3. Chiral Iodine Species as Organocatalysts

On the basis of the stoichiometric hypervalent iodine(III)mediated reactions, numerous enantioselective catalytic reactions using chiral iodoarenes as precatalysts have been developed. Currently, this is an important, hot topic, and rapidly expanding research area. The first catalytic, enantioselective α-tosyloxylation reaction of ketone 672 using chiral iodoarene 673 was reported in 2007 by Wirth and coworkers.1539 The authors obtained products of α-tosyloxylation 674 in moderate yields and modest enantiomeric excess (Scheme 198). Similar α-tosyloxylation reactions have also been tested by other researchers using various chiral iodoarenes as organocatalysts, but in most cases the enantioselectivity of these reactions was at a moderate level.1540−1547 The use of chiral organic iodides as precatalysts allows one to perform the catalytic oxidative spirocyclization of phenolic substrates as an enantioselective reaction.99,101,340,1442,1445,1463,1548 Kita and co-workers have originally reported that the chiral iodosylarene 587 with a rigid spirobiindane backbone can be used for enantioselective dearomatization of naphtholic substrates 595 giving optically active products 596 with 65% ee (Scheme 172, see section 5.1).1442 The same group has investigated a modified chiral iodoarene 675 having ortho-substituted spirobiindane backbone, which was a more effective precatalyst for the enantioselective spirocyclization reaction (Scheme 199).99 The conformationally flexible C2-symmetric iodoarene catalyst 676 designed by Ishihara’s group promoted the catalytic enantioselective spirocyclization of substrate 595 affording product 596 with enantiomeric excess of up to 92% (Scheme 199). The authors suggested that the intramolecular hydrogenbonding interactions in the in situ-generated organohypervalent

substituted IBX, numerous modified IBX precursors have been prepared and investigated as catalysts in the oxidation of alcohols.1366,1532,1533 In a specific example, the modified IBXmediated catalytic oxidation of alcohols with Oxone using 3,5dimethyl-2-iodobenzoic acid and 3,4,5,6-tetramethyl-2-iodobenzoic acid as precatalysts has been developed by Moorthy and Nair. 3,4,5,6-Tetramethyl-2-iodobenzoic acid with Oxone in CH3NO2 at room temperature worked especially well as a precatalyst for the oxidation of alcohols 662 to the corresponding aldehydes 663 (Scheme 193).1366 The oxidative cleavage of olefins has been accomplished by the in situ-generated catalytic 3,4,5,6-tetramethyl-2-iodoxybenzoic acid using Oxone. The reaction mechanism involves initial dihydroxylation of the olefin 664 with Oxone, and then oxidative cleavage of the dihydroxy intermediate by the in situgenerated 3,4,5,6-tetramethyl-2-iodoxybenzoic acid provides acid 665 in moderate to good yields (Scheme 194).1534 Ishihara and co-worker have found that 2-iodylbenzenesulfonic acid (IBS) is an especially effective catalyst in the oxidation of alcohols. The catalytic IBS-mediated oxidation of aromatic or aliphatic alcohols affords the corresponding carbonyl compounds or carboxylic acids in good yields. Remarkably, the catalytic oxidation of cyclic hexanols 666 using 2-iodobenzenesulfonic acid sodium salt with excess of Oxone can provide the corresponding enones 667 in good yields (Scheme 195).102,104,1535

Scheme 196. 5-Me-IBS-Catalyzed Oxidation of Tertiary Allylic Alcohols

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Scheme 197. Benzylic Oxidation of Propylbenzene Using PhI−Oxone−RuCl3 Combination

Scheme 198. Catalytic Enantioselective α-Tosyloxylation Reaction Using Chiral Iodoarene

Scheme 199. Catalytic Enantioselective Kita’s Spirolactonization Using Chiral Iodoarenes

Scheme 200. Enantioselective Spirocyclization of 1-Hydroxy-N-aryl-2-naphthamides

iodine(III) intermediate play a crucial role in improving enantioselectivity.101,340 Recently, Ibrahim and co-workers have prepared the C2-symmetric iodoarene catalyst 677 based on the rigid all-carbon antidimethanoanthracene framework, which promotes the spirocyclization of substrate 595 with moderate enantiomeric excess (Scheme 199).1548 Gong and co-workers reported a similar, highly enantioselective, dearomatizative spirocyclization of 1-hydroxy-N-aryl-2naphthamide derivatives using chiral iodoarene(III) catalysis. The reaction of 1-hydroxy-N-aryl-2-naphthamide derivatives 678 with Ishihara’s catalyst 676 in the presence of mCPBA resulted in oxidative dearomatizative spirocyclization to give spirooxindole derivatives 679 in good yields and with high enantioselectivity (Scheme 200). The authors claimed that the

generated chiral iodine(III) reagent is of great potential for performing new stereoselective transformations by oxidative activation of aromatic systems.1549 The asymmetric oxidative intramolecular cross-coupling of C−H bonds using catalytic chiral iodine(III) reagent has been accomplished. The anilide derivatives 680 by treatment with designed chiral iodoarene 681 as catalyst in the presence of peracetic acid proceed as stereoselective intramolecular C−H cross-coupling to give the structurally diverse spirooxindoles 682 in moderate yields and with high levels of enantioselectivity (Scheme 201).1550 Wirth and co-workers have reported the chiral iodoarene 684-catalyzed stereoselective intramolecular functionalization of alkenes via a cascade two C−N bond formation reaction. 3386


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Scheme 201. Enantioselective Synthesis of Spirooxindoles from Anilides

Scheme 202. Stereoselective Intramolecular Diamination of Alkenes

Scheme 203. Enantioselective Preparation of 2,5-Cyclohexadienones

Scheme 204. Enantioselective Hydroxylative Dearomatization of 2-Methylnaphthol

iodoarene has been developed by Quideau and co-workers. For example, the ortho-selective dearomatization reaction of 2methylnaphthol 689 using iodoarene 690 and excess mCPBA gave the dearomatized epoxide product 691 with 29% enantiomeric excess (Scheme 204).1552

This efficient metal-free method achieves the highly stereoselective intramolecular diamination of alkenes 683 to provide the bicyclic products 685 (Scheme 202). Some of the obtained products can be further transformed to free diamines under reductive conditions. Harned and co-workers have shown that the chiral iodoarene 687 can be employed as a precatalyst for the intermolecular enantioselective oxidative dearomatization reaction of phenols 686 to give the corresponding 2,5-cyclohexadienones 688 with moderate levels of enantioselectivity (Scheme 203). Iodoarene catalyst 687 can also be used for enantioselective intramolecular spirocyclization of phenols to provide spirocyclic products with moderate levels of enantioselectivity.1551 The enantioselective intermolecular hydroxylative dearomatization of phenols catalyzed by the axially chiral binaphthyl

7. RECYCLABLE HYPERVALENT IODINE COMPOUNDS Despite their unique reactivity and selectivity of reactions, the common, stoichiometric hypervalent iodine reagents have a disadvantage of generating the nonrecyclable reduced iodine products (e.g., PhI) as waste after every reaction. Polymersupported reagents, as well as recyclable nonpolymeric hypervalent iodine reagents, overcome these drawbacks, and in general show a similar reactivity with their monomeric analogues. Moreover, polymer-supported reagents can find 3387


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Scheme 206. Preparation of Polymer-Supported Iodosylbenzene Sulfate 695

SO3, 0.68 mmol/g) (Scheme 206). Reactions of alcohols or sulfides with reagent 695 under mild conditions afford the corresponding oxidized compounds in good yields, and polymeric byproduct can be easily recovered by filtration. Recycling of this reagent is possible with minimal loss of activity after several cycles.1557 The reaction of poly[(diacetoxyiodo)styrene] 706 with sodium hydroxide under solvent-free conditions leads to the polystyrene-supported iodosylbenzene 707 (Scheme 207). The elemental analysis of polymer 707 indicates that the −IO groups are partially hydrated as shown in structure 708. Polymer 707 is a good reagent for oxidation of alcohols in the presence of BF3·Et2O or using the RuCl3−Oxone combination.311 Kumar and co-workers have reported the synthesis of heterocyclic compounds 711, such as benzimidazoles, benzoxazoles, and benzothioazoles, from amines 709 and benzaldehyde 710 using poly[(diacetoxyiodo)styrene] 706 (Scheme 208). The reduced form of this reagent, poly(iodostyrene), can be converted back to 706 by treatment with acetic anhydride and hydrogen peroxide. Recycling of the reagent 706 can be repeated many times without loss of activity.1561 Wei and Zhu have prepared the magnetic nanoparticlesupported (diacetoxyiodo)benzene 713. The nanoparticle reagent is an efficient oxidant for alcohols 712 to the corresponding carbonyl compounds 714 (Scheme 209), and the reduced form of reagent can be removed from the reaction mixture with the assistance of an external magnet. The reoxidized magnetic nanoparticle-supported reagent demonstrated good loading levels after several reoxidations.1562

Figure 19. Polymer-supported trivalent iodine reagents.

application in the high throughput synthesis and solid-phase synthesis techniques. In recent years, various polymer-bound hypervalent iodine reagents have been developed. Several reviews of polymer-supported and nonpolymeric recyclable hypervalent iodine reagents have been published.20,68,106,1553 7.1. Polymer-Supported Iodine(III) Reagents

Polymer-supported trivalent iodine reagents are generally prepared by treatment of the respective polymer-supported iodide with an appropriate oxidant. Polystyrene is most commonly used as the polymeric backbone; a few examples of silica-supported hypervalent iodine reagents are also known. The first polymer-bound hypevalent iodine reagent, poly[(diacetoxyiodo)styrene], was reported by Okawara and coworkers in 1961.1554 More recently, several other polymersupported trivalent iodine reagents have been introduced. Examples of important polymer-bound hypervalent iodine(III) reagents are illustrated by structures 692−697 in Figure 19.68,311,516,735,1063,1554−1560 The reactivity of polymer-supported iodine(III) reagents in general is similar to that of the common, nonpolymeric iodine(III) reagents, and synthetic application of these reagents has previously been covered in several reviews.20,68,106 In a specific recent example, the polymer-supported (dichloroiodo)styrene 693 reacts with organic substrates 698, 700, 702, and 704 affording the same respective products 699, 701, 703, and 705 as in the reactions of (dichloroiodo)benzene with these substrates (Scheme 205). The products of these reactions can be easily separated from poly(iodostyrene) by simple filtration. Reagent 693 can be regenerated in about 90% overall yield from the recovered poly(iodostyrene) by treatment with bleach and aqueous HCl.1555 Treatment of poly[(diacetoxyiodo)styrene] 706 with sodium bisulfate monohydrate under solvent-free conditions gives polystyrene-supported iodosylbenzene sulfate 695 (loading of

7.2. Polymer-Supported Iodine(V) Reagents

The first polymer-bound hypervalent iodine(V) reagents, aminopropylsilica gel-supported IBX and Merrifield resinsupported IBX, have been reported independently by the Giannis group and the Rademann group in 2001.1563,1564 More recently, several other polymer-supported IBX derivatives have been prepared.1565−1570 The polymer-supported pseudocyclic iodylarene derivatives, IBX-esters, IBX-amides, and IBX-ethers, have also been reported.1566,1571−1576 Several representative

Scheme 205. Typical Reactions of Polymer-Supported (Dichloroiodo)styrene

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Scheme 207. Preparation of Polymer-Supported Iodosylbenzene 707

Scheme 208. Preparation of Benzimidazole, Benzoxazole, and Benzothioazole Using Poly[(diacetoxyiodo)styrene] 706

Scheme 209. Oxidation of Alcohols with Magnetic Nanoparticle-Supported Reagent 713

Figure 20. Polymer-supported hypervalent iodine(V) reagents.

Scheme 210. Chemoselective Oxidation of Alcohol with Reagent 720

Scheme 211. Chemoselective and Regioselective Oxidation of Phenols with Reagent 715

from the reaction mixture by simple filtration and reoxidized by Oxone. In a specific example, the polymer-supported IBX-ether 720 can be used for chemoselective oxidation of 4(methylthio)benzyl alcohol 721 to give aldehyde 722 in good yield (Scheme 210).1574

polymer-bound pentavalent iodine reagents 715−720 are shown in Figure 20. Similarly to common IBX, the polymer-supported IBX analogues demonstrate high reactivity toward oxidation of alcohols. The reduced form of these reagents can be separated 3389


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Figure 21. Recoverable nonpolymeric hypervalent iodine(III) reagents.

Scheme 212. α-Tosyloxylation of Ketones Using Recyclable Iodoarene 738 and mCPBA

Bernini and co-workers reported the chemoselective and regioselective hydroxylation of tyrosol derivatives 723 with polymer-supported IBX 715 to give the corresponding dihydroxybenzenes 724 in good yields (Scheme 211). Polymer-supported reagent has been recovered from this reaction by simple filtration, regenerated, and reused for several cycles of oxidation reactions without loss of efficiency.1577

Scheme 213. Oxidation of Alcohols and Sulfides Using Recyclable Reagent 728 in the Presence of Recoverable SiO2−RuCl3 Catalyst 741

7.3. Recyclable Nonpolymeric Iodine(III) Reagents

The polystyrene-supported hypervalent iodine reagents are useful oxidants, but they have several serious drawbacks as compared to the nonpolymeric analogues. The resin-supported reagents usually have lower reactivity as compared to the common nonsupported reagents, and the repeated use of resinsupported reagents leads to a significant loss of activity due to oxidative degradation of the polymeric backbone. The modified, recyclable, nonpolymeric hypervalent iodine reagents have reactivity similar to that of the original hypervalent iodine(III) reagents and are free of the drawbacks of polymersupported reagents. Recovery of the reduced form of a recyclable reagent can be performed by using biphasic separation methods summarized in the following sections. 7.3.1. Iodine(III) Reagents with Insoluble Reduced Form. Numerous recoverable trivalent iodine reagents with insoluble reduced form have been reported. Specific examples of such reagents are illustrated by compounds 725−736 shown in Figure 21.322,323,326,1578−1580 In a recent publication, Karade and co-workers have reported the synthesis of 2,4,6-tris[(4-dichloroiodo)phenoxy)]-1,3,5-

triazine 729 in two steps starting from 2,4,6-trischloro-1,3,5triazine and 4-iodophenol. Reagent 729 can be used for the chlorination of alkenes or arenes, and for the oxidative synthesis of 1,2,4-thiadiazoles under mild conditions in good yields. The product of reduction, iodoarene 738, can be recovered from reaction mixture by simple filtration after addition of methanol, and the recovered iodoarene can be oxidized back to hypervalent reagent 729 without loss of activity.1579 The same group reported that the recyclable iodoarene 738 can be used as a catalyst for the α-tosyloxylation of ketones 737 to give products 739 in good yields (Scheme 212).1581 3390


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Scheme 214. Oxidation of Optically Active Alcohol Using Reagent 730 and AZADO

Scheme 216. Recyclable System Based on Ion-Exchange Resin Amberlite IRA

Recyclable reagent 728 is an efficient reagent for chlorination of alkenes and arenes. 3-Iodobenzoic acid as the reduced form of reagent 728 can be recovered from the reaction as a solid after basic workup followed by addition of hydrochloric acid.251,1578 The SiO2−RuCl3 catalyst 741 in combination with reagent 728 is a green recyclable catalytic system useful for the oxidative conversion of alcohols 740 and sulfides 743 to the respective products 742 and 744 in good yields under aqueous conditions (Scheme 213). The SiO2−RuCl3 catalyst 741 can be recovered from the reaction mixture by filtration and reused without any significant loss of activity. The reduced form, 3iodobenzoic acid, also can be recovered and reoxidized to reagent 728 in about 90% overall yield.1582 The recyclable reagents derived from 1,3,5,7-tetraphenyladamantane 730−732 and tetraphenylmethane 733−735 have been developed by Kita and co-workers. These reagents demonstrate reactivity similar to that of the nonrecyclable hypervalent iodine(III) reagents. The reduced forms of these reagents are recyclable by a simple filtration of the solution in methanol.322,323,1583,1584 In particular, the oxidation of alcohols to carbonyl compounds by 1,3,5,7-tetraphenyladamantanebased hypervalent iodine(III) reagent 730 in the presence of nitroxy radical AZADO has been reported.1583,1584 This methodology can be applied to the nonracemic oxidation of an optically active alcohol 745 to the enolizable ketone 746 (Scheme 214). Zhang and co-workers have used the cyclic recyclable reagent 736 as an efficient coupling reagent for the direct condensation between carboxylic acids 747 and alcohols 748 to the corresponding ester 749 in good yields (Scheme 215). This methodology can also be used for the coupling between carboxylic acids and amines to give amides and peptides. The reduced form of this reagent, 2-iodoisophthalic acid, can be separated from the reaction mixture and oxidized with NaOCl− HCl to give 736 with 94% recovery.1580 7.3.2. Application of Ion-Exchange Resins for Recycling. The use of ion-exchange resins allows especially efficient separation of the reduced form of several hypervalent iodine reagents, such as 3-iodosylbenzoic acid 750 and its derivatives. 3-Iodobenzoic acid 751 can be recovered from reaction mixture by trapping with ion-exchange resin Amberlite IRA 900 752 (hydroxide form) via the formation of iodoarene complex 753. Treatment of the immobilized complex 753 with aqueous HCl results in the release of 3-iodobenzoic acid 751 with over 90% recovery (Scheme 216).

Several oxidative transformations, such as the iodination of alkenes or arenes,162,1585 the α-tosyloxylation of ketones,161 and the oxidation of alcohols,158 have been performed with this simple recyclable system using 3-iodosylbenzoic acid 750 as the recoverable oxidant. The anionic exchange resin Amberlite IRA 900 HCO3− can be used instead of Amberlite IRA 900 for the recovery of 3iodobenzoic acid from the reaction mixture.160 In a specific example, two recyclable hypervalent iodine reagents, 3[hydroxy(t osyloxy)iodo]benzoic acid and 3-[bis(trifluoroacetoxy)iodo]benzoic acid, which are prepared by treatment of 3-iodosylbenzoic acid 750 with p-toluenesulfonic acid or trifluoroacetic acid, have been used as oxidants for the iodination of arenes, oxidation of sulfides, or α-tosyloxylation of ketones. The reduced form of these reagents, 3-iodobenzoic acid, has been recovered from anionic exchange resin Amberlite IRA 900 HCO3− by treatment with hydrochloric acid and reoxidized to 3-iodosylbenzoic acid 750.157 7.3.3. Ion-Supported Iodine(III) Reagents. Numerous ion-supported (or ionic liquid-supported) trivalent iodine reagents have been developed. Most of these reagents are thermally stable solids or viscous liquids, and the reduced form of these reagents can be separated from reaction mixtures by simple filtration or by extraction with ionic liquid. Several examples of ion-supported hypervalent iodine(III) reagents 754−762 are shown in Figure 22.312,313,1586−1588 Treatment of various primary and secondary alcohols with the ion-supported [bis(acyloxy)iodo]arene 755 in ionic liquid [emim]+[BF4]− (1-ethyl-3-methylimidazolium tetrafluoroborate) in the presence of bromide anion,1589 or in water in the presence of ion-supported TEMPO,372 affords the corresponding aldehydes or ketones. Likewise, the oxidation of sulfides using reagent 755 under similar conditions gives the corresponding sulfoxides in excellent yields.1590 Togo and co-workers have prepared the tetraalkylammonium-derived ionic liquid-supported [bis(acyloxy)iodo]arenes 764 and 765, which are efficient oxidants for the Hofmann rearrangement of amides 763 to carbamates 766 in methanol solution (Scheme 217). These reagents can also be used for oxidation of alcohols and for preparation of 5-aryl-2methyloxazoles from ketones affording the corresponding products in high yields. The ion-supported iodoarenes as

Scheme 215. Coupling Reaction between Carboxylic Acids and Alcohols Using Reagent 736

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Figure 22. Typical examples of ion-supported hypervalent iodine(III) reagents.

Scheme 217. Hofmann Rearrangement of Amides Using Ion-Supported Iodine(III) Reagent

carboxylic acids 769 using ion-supported diaryliodonium salt 761 has been performed, and the reaction products 768 and 770 were separated by simple extraction with a hexane−ethyl acetate mixture (Scheme 218). The reduced ionic-liquidsupported iodoarene as byproduct can be easily isolated and reused.1588 The reactions of generated in situ ion-supported hypervalent iodine(III) reagents using stoichiometric or catalytic amounts of ionic-supported iodoarenes and mCPBA as terminal oxidant have been developed.1592−1594 In particular, Togo and coworkers reported the ion-supported iodoarenes 772 or 773catalyzed conversion of sulfonamides 771 using mCPBA as the oxidant in TFE solution to give the cyclization products 774 (Scheme 219). The catalytic quantities of ion-supported iodoarenes can be easily removed from reaction mixtures.1592 7.3.4. Fluorous Iodine(III) Reagents. Hypervalent iodine(III) reagents with a long perfluoro alkyl chain have been proposed as efficient recyclable oxidants.251,335,1595,1596 The reduced form of fluorous iodine(III) reagents can be readily

Scheme 218. Electrophilic Phenylation of Phenols and Carboxylic Acids with Reagent 761

byproducts can be easily separated from the reaction mixture and can be reoxidized to [bis(acyloxy)iodo]arenes.1591 The ion-supported diaryiodonium salts were prepared from ion-supported arenes and hypervalent iodine(III) reagents in good yields. The electrophilic phenylation of phenols 767 and

Scheme 219. Ion-Supported Iodoarene-Catalyzed Cyclization of 771

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Figure 23. Representative examples of fluorous hypervalent iodine(III) reagents.

Oxone as oxidant also worked for the conversion of various alcohols to the respective carbonyl compounds in good to high

Scheme 220. Dichlorination of Cyclooctene Using Reagent 775

Scheme 223. Oxidation of Sulfides and Alcohols Using Recyclable Reagent 789

Scheme 221. Oxidation of Phenols to Quinones Using Fluorous Iodine(III) Reagents

Scheme 222. Oxidation of an Alcohol Using Fluorous IBX 786

yields. The fluorous IBX can be readily recovered from the reaction mixture as insoluble fluorous IBA by simple filtration, and can be reused without significant loss of the catalytic activity.1598 Scheme 224. Preparation of 4-Iodylbenzenesulfonate from 4-Iodobenzenesulfonic Acid

recovered from the reaction mixture using fluorous techniques and recycled. Several examples of fluorous hypervalent iodine(III) compounds 775−780 are shown in Figure 23. Iskra and Gladysz reported the application of fluorous dichloroiodine compounds 775−777 in the chlorination of cyclooctene to 1,2-dichlorocyclooctane (Scheme 220). The reducted form of reagent 775 is separated from the reaction mixture in 95% yield by liquid−liquid biphase workup and can be reoxidized to 775 by treatment with chlorine gas.251 The fluorous [bis(trifluoroacetoxy)iodo]perfluoroalkanes and (diacetoxyiodo)arenes are useful oxidants of phenols to the corresponding quinones. The reduced forms of these reagents are recovered in high yields by using fluorous techniques and can be reoxidized and reused.335,1595−1597 In particular, the fluorous reagents 779 or 780 demonstrate good performance in the oxidation of phenols 783 to give the corresponding quinones 784 in good yields (Scheme 221). The liquid−liquid biphase workups are used for the recovery of these fluorous reagents.335 Miura and co-workers have developed the recyclable fluorous IBX 786 from the respective fluorous iodoarene using Oxone, and demonstrated its reactivity as a reagent for oxidation of alcohol 785 to ketone 787 (Scheme 222). Reagent 786 generated in situ from fluorous iodoarene as precatalyst using

7.4. Recyclable Nonpolymeric Iodine(V) Reagents

The common hypervalent iodine(V) reagents, IBX and DMP, are widely used as effective oxidants. However, these reagents are normally used as the nonrecyclable, stoichiometric oxidants, which is a disadvantage in regard to the principles of Green Chemistry. For example, the recovery of IBX after oxidation of alcohols can be achieved only in a low yield.1599 2-Iodylpyridine derivatives, the recyclable hypervalent iodine(V) reagents, have been reported in 2011. The 3-alkoxysubstituted 2-iodylpyridines were prepared from the corresponding 2-iodopyridines by oxidation with 3,3-dimethyldioxirane, and several products were structurally characterized by single-crystal X-ray crystallography. In particular, 2-iodyl-3propoxypyridine 789 shows an improved solubility in organic solvents (e.g., 1.1 mg/mL in acetonitrile) because of the 3393


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pseudocyclic structure with intramolecular secondary bonding. Reagent 789 can be used for oxidation of sulfides 788 to sulfoxides 790 and alcohols 791 to the carbonyl compounds 792 (Scheme 223). The reduced form of this reagent, 2-iodo-3propoxypyridine, can be effectively recovered from the reaction mixture by simple acid−base separation techniques.1600 Another example of a recyclable pentavalent iodine reagent, potassium 4-iodylbenzenesulfonate 794, has been prepared by treatment of 4-iodobenzensulfonic acid 793 with Oxone in water (Scheme 224). The solid-state structure of compound 794 was established by single-crystal X-ray crystallography. The thermally stable 4-iodylbenzenesulfonate 794 is a soluble in water reagent useful for oxidative iodination of arenes. Reagent 794 can be easily recovered from reaction mixtures by treatment of the aqueous solution with Oxone followed by filtration.1601

University of Utah in 1990, where he worked for three years as Instructor of organic chemistry and Senior Research Associate with Professor Peter J. Stang. In 1993, he joined the faculty of the University of Minnesota Duluth, where he is currently a Professor of Chemistry. He published about 250 research papers, gave over a hundred research presentations in many countries, edited several books, coauthored the Handbook of Heterocyclic Chemistry (3rd ed., 2010), and authored a book on Hypervalent Iodine Chemistry (Wiley, 2013). His main research interests are in the areas of synthetic and mechanistic organic chemistry of hypervalent main-group elements and organofluorine chemistry. In 2011, he was a recipient of the National Award of the American Chemical Society for Creative Research & Applications of Iodine Chemistry. He is a member of the editorial boards of five journals and a Scientific Editor of Arkivoc.

ACKNOWLEDGMENTS The work described in this Review was supported by research grants from the National Science Foundation (CHE-035354, 0702734, 1009038, 1262479).

8. CONCLUSION The literature summarized in this Review reflects a significant continuing interest in hypervalent iodine chemistry. Just in 8 years after publication of our 2008 review,15 numerous new reagents have been developed and increasingly employed in synthetic chemistry. Hypervalent iodine reagents are commercially available and environmentally benign compounds with very useful oxidizing properties. A recent discovery of hypervalent catalytic systems and recyclable reagents, and the development of new enantioselective reactions using chiral hypervalent iodine compounds, represent a particularly important achievement in the field of hypervalent iodine chemistry. Compounds of iodine possess reactivity similar to that of transition metals, but have the advantage of environmental sustainability and efficient utilization of natural resources. Iodine is an environmentally friendly and a relatively inexpensive element, which is currently underutilized in academic research and in industrial applications. We hope that this Review will attract the attention of academic and industrial researchers to the benefits of using compounds of iodine as an environmentally sustainable alternative to transition metals.

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AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies Akira Yoshimura, a native of Osaka, Japan, received his M.S. in 2007 and Ph.D. in 2010, both from Tokushima University, under the supervision of Professor Masahito Ochiai. During 2010−2014, he carried out his postdoctoral work with Professor Viktor Zhdankin at University of Minnesota Duluth, and in 2015 he was appointed to a Research Associate position at the same university. His research interests are in the fields of synthetic and mechanistic organic chemistry of hypervalent main-group elements and heterocyclic chemistry. He published 30 research papers, mainly related to the chemistry of hypervalent iodine and bromine. Viktor V. Zhdankin was born in Ekaterinburg, Russian Federation. His M.S. (1978), Ph.D. (1981), and Doctor of Chemical Sciences (1986) degrees were earned at Moscow State University. He moved to the 3394


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