Synthesis of Heterocycles via Palladium-Catalyzed ... - ACS Publications

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Synthesis of Heterocycles via Palladium-Catalyzed Carbonylations Xiao-Feng Wu,*,†,‡ Helfried Neumann,‡ and Matthias Beller*,‡ †

Department of Chemistry, Zhejiang Sci-Tech University, Xiasha Campus, Hangzhou, Zhejiang Province, P. R. China 310018 Leibniz-Institut für Katalyse e.V. an der Universität Rostock, Albert-Einstein-Strasse 29a, 18059 Rostock, Germany

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5. Palladium-Catalyzed Carbonylative Synthesis of Other Heterocycles 6. Summary Author Information Corresponding Author Notes Biographies Acknowledgments References

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1. INTRODUCTION Palladium-catalyzed coupling reactions have become a powerful tool in organic synthesis.1 Today, there are numerous applications of palladium catalysts in the preparation of pharmaceuticals, agrochemicals, and also advanced materials both on laboratory and industrial scale. The importance of palladium catalysis was underlined by the 2010 Nobel Price to R. Heck, A. Suzuki, and E. Negishi for their pioneering work in this field.2 Among all the palladium-catalyzed coupling reactions, carbonylation reactions also have experienced impressive improvements since the first work of R. Heck and co-workers in 1974.3 The advantages of carbonylations are (i) it is the most potent methodology in the synthesis of carbonylcontaining chemicals, which increases the carbon number at the same time, and (ii) carbon monoxide (CO) can be used as an inexpensive and readily available C1 source, which is also in agreement with the green chemistry principles.4 The progress in carbonylation chemistry has been achieved not only in academic laboratories but also in industry. Hence, it is not surprising that there are many carbonylation reactions being employed on an industrial scale.5 Heterocyclic compounds are an integral part of many biologically active molecules, and many currently marketed drugs hold heterocycles as their core structure (Scheme 1). Numerous efforts in recent years focused on the development of improved methods for the synthesis of heterocycles.6 Considering the synthetic value of carbonylation reactions and the preparation of heterocycles, the merging of these two topics offers interesting possibilities for organic synthesis. Indeed, advancements in this area have been proven by numerous publications. Although a number of reviews on catalytic carbonylations7 as well as on the synthesis of heterocycles already exist, no general summary on palladiumcatalyzed carbonylative syntheses of heterocycles has been published so far.8 Considering the importance of both topics, and the lack of a more general compilation, here we report a

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Received: March 8, 2012

CONTENTS 1. Introduction 2. Palladium-Catalyzed Carbonylative Synthesis of Four-Membered Heterocycles 2.1. Palladium-Catalyzed Carbonylative Synthesis of Four-Membered Lactones 2.2. Palladium-Catalyzed Carbonylative Synthesis of Four-Membered Lactams 3. Palladium-Catalyzed Carbonylative Synthesis of Five-Membered Heterocycles 3.1. Palladium-Catalyzed Carbonylative Synthesis of Five-Membered Oxygen-Containing Heterocycles 3.1.1. Palladium-Catalyzed Carbonylative Reactions of Alkynols 3.1.2. Palladium-Catalyzed Carbonylative Reactions of Alkenols 3.1.3. Palladium-Catalyzed Carbonylative Reactions of 2-Halo-alke(y)nes 3.1.4. Other Palladium-Catalyzed Carbonylations to Lactones 3.2. Palladium-Catalyzed Carbonylative Synthesis of Five-Membered Nitrogen-Containing Heterocycles 3.3. Palladium-Catalyzed Carbonylative Synthesis of Other Five-Membered Heterocycles 4. Palladium-Catalyzed Carbonylative Synthesis of Six-Membered Heterocycles 4.1. Palladium-Catalyzed Carbonylative Synthesis of Six-Membered Oxygen-Containing Heterocycles 4.2. Palladium-Catalyzed Carbonylative Synthesis of Six-Membered Nitrogen-Containing Heterocycles 4.3. Palladium-Catalyzed Carbonylative Synthesis of Other Six-Membered Heterocycles

© XXXX American Chemical Society

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Scheme 1. Selected Examples of Heterocyclic Drugs

materials.12 Lactones, together with esters, dienes, and allylic alcohols, were produced from alkenyloxiranes under the assistance of palladium catalysts. The selectivity of this reaction depended on the nature of the alkenyloxiranes (Scheme 3).

summary of the main achievements in this area. To make it easier for the reader to find the respective synthetic applications, we organized this review with respect to the size and type of heterocycle. Hence, it was unavoidable that publications that focused primarily on methodology or catalyst development will be mentioned in different subsections.

Scheme 3. Palladium-Catalyzed Carbonylative Synthesis of Lactones from Alkenyloxiranes

2. PALLADIUM-CATALYZED CARBONYLATIVE SYNTHESIS OF FOUR-MEMBERED HETEROCYCLES 2.1. Palladium-Catalyzed Carbonylative Synthesis of Four-Membered Lactones

Lactones, which occur widely in nature, are known to possess potent biological activities.9 The application of palladiumcatalyzed carbonylation reactions in the synthesis of fourmembered lactones was first reported by Cowell and Stille in 1980.10 They used PdCl2(PPh3)2 as catalyst, and the lactones were synthesized in high yields under mild conditions (1−4 bar of CO; 25−60 °C) from the corresponding halo-substituted alcohols (Scheme 2a). Not only four-membered rings but also

Besides the mentioned processes, palladium-catalyzed carbonylation of alkynols represents another powerful methodology for the preparation of lactones. As early as in 1994, Gabriele and co-workers presented a PdI2/KI-catalyzed oxidative carbonylation of α-substituted hydroxyalkynes to four-membered lactones in good yields under mild conditions. Later on, they extended this methodology to but-3-yn-1-ols for the synthesis of five-membered lactones (Scheme 4a).13

Scheme 2. Palladium-Catalyzed Carbonylative Synthesis of Lactones

Scheme 4. Palladium-Catalyzed Carbonylative Synthesis of Lactones from Alkynols

five- and six-membered lactones can be achieved. Later on, Qing and Jiang modified this methodology for the preparation of trifluoromethyl-substituted four- and five-membered lactones (Scheme 2b).11 In 1993, Shimizu et al. showed that lactones can also be formed using readily available allylic compounds as starting B


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lactams in good yields (Scheme 6a).23 Interestingly, Brickner and co-workers modified the solvent [using N,N-dimethylfor-

Alternatively, Dupont and co-workers developed a Pd(OAc)2/ 2-PyPPh2 system for the production of various lactones from alkynols (Scheme 4b).14 A mild and efficient methodology for the PdCl2-catalyzed cyclocarbonylation of 2-alkynols with CuCl2 affording (Z)-α-chloroalkylidene-β-lactones was developed by Ma and co-workers.15 Good regio- and stereoselectivity were observed in the latter reaction. The optically active (Z)-α-chloroalkylidene-β-lactones could be easily prepared from readily available optically active propargylic alcohols. The Pd(II)-catalyzed cyclocarbonylation of 2-alkynols with CuBr2 was also studied. Although the yields of (Z)-αbromoalkylidene-β-lactones were low, because of the relatively higher activity of the C−Br bond, the coupling reactions of (Z)α-bromoalkylidene-β-lactones proceeded smoothly to afford the corresponding products in high yields (Scheme 4c).

Scheme 6. Palladium-Catalyzed Carbonylative Synthesis of Lactams

2.2. Palladium-Catalyzed Carbonylative Synthesis of Four-Membered Lactams

Lactams have a long and important history in the field of medicinal chemistry.16 Because of their importance, many methodologies have been developed for their construction. Nevertheless, today still the Staudinger reaction is a fundamental method for their production.17 Palladiumcatalyzed carbonylation of aziridines represents another convincing procedure for the synthesis of lactams. The first report in this area was published in 1981 by Alper and Perera.18 This novel methodology allowed the production of lactams under mild conditions, albeit in low yield. Furthermore, it was used for the synthesis of hetero and carbon analogues of penicillin (Scheme 5a). The power of this procedure was also

mamide (DMF) instead of hexamethylphosphoramide (HMPA)] and made the methodology suitable for unprotected primary 2-bromoallylamines.24 In addition, Crisp and Meyer succeeded to produce lactams from the corresponding amino vinyl triflates by carrying out the reaction in CH3CN. Four-, five-, and six-membered lactams were produced in good to excellent yields.25 Besides the mentioned starting materials, 4amino-2-alkynyl carbonates were also applied for the production of lactams by Tsuji, Mandai, and co-workers.26 In the presence of palladium catalysts, lactams were produced in moderate yields under mild conditions (Scheme 6b). The methodologies developed by Brickner and Tsuji and their co-workers successfully avoid the use of aziridines in the preparation of lactams,23−25 but the specificity of starting materials limited the scope. Notably, Torii and co-workers succeeded to apply more easily available allyl phosphates and imines for the synthesis of lactams.27 3-Vinyl-β-lactams were produced in high yields from the corresponding imines and allylic compounds in a highly stereoselective manner (Scheme 7a). Troisi and co-workers developed methodologies for the [2 + 2]-carbonylative cycloaddition of chiral imines with various allyl halides. In the presence of a catalytic amount of Pd(OAc)2 and PPh3, using NEt3 as base, under 27.5 bar of CO, chiral alkenyl-β-lactams were isolated in good yields.28 PdI2 and Pd/ C−KI catalyst systems were also developed for β-lactams preparation by Gabriele and co-workers.29 A new palladiumcatalyzed synthesis of 3-amido-substituted β-lactams was reported by Arndtsen and co-workers in 2006.30 This process involves the one-pot coupling of four components, two imines, CO, and an acid chloride, providing a flexible route to construct β-lactams. Notably, two different imines can be used for the generation of lactams, making the independent control of all the separate substituents possible (Scheme 7b). More recently, Wang and co-workers investigated the carbonylation of diazo compounds. Starting from diazo compounds, via ketenes as key intermediates, β-lactams were formed after coupling with imines (Scheme 7c).31 Interestingly, neither base nor ligand was needed in this methodology. Besides β-lactams, 1,3-dioxin4-ones could also be synthesized by applying α-diazocarbonyl compounds and imines as substrates. The experiments were supported by a density functional theory (DFT) study. In addition to typical imines, 1,3-thiazines were also used as coupling partners with allyl compounds by Zhou and Alper.32 In their report, bicyclic β-lactams were synthesized by a

Scheme 5. Palladium-Catalyzed Carbonylative Synthesis of Lactams from Aziridines

proved by the carbonylation of methyleneaziridines to the corresponding lactams in moderate yields (Scheme 5b).19 Ohfune and co-workers developed a highly stereoselective process for the synthesis of 3-vinyl-2-azetidinone via a ringopening, carbonylation, and ring-closure sequence (Scheme 5c).20 This methodology was also applied in the total synthesis of carbapenem (+)-PS-5.21 A mechanistic rationale for this unusual reaction was proposed by Aggarwal and co-workers.22 Through their analysis, they have been able not only to switch from β-lactams to δ-lactams but also to switch the stereochemical outcome of the reaction by simply modifying the reaction parameters. Palladium-catalyzed carbonylative intramolecular cyclization of 2-bromoamines provides an interesting alternative for the synthesis of lactams. Ban and co-workers started from 2-bromo3-aminopropene derivatives, which in the presence of Pd(OAc)2 and PPh3 led to the corresponding α-methylene βC


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Scheme 7. Palladium-Catalyzed Carbonylative Synthesis of Lactams

carbonylative coupling and cyclization reaction of 2-aryl-1,3thiazines with allyl phosphates, catalyzed by bis(benzonitrile)palladium dichloride, using N,N-diisopropylethylamine (DiPEA) as base in tetrahydrofuran (THF). Several rhodium complexes were also effective for this process. These transformations are stereospecific, with the aryl and vinyl groups on the β-lactam ring being cis to each other. This methodology provides a novel route for the preparation of the cepham analogues, cis-7-vinyl-5-thia-1-azabicyclo[4.2.0]octan-8-ones (Scheme 7d).

Scheme 8. Palladium-Catalyzed Carbonylation Reactions of Propargyl Alcohols

3. PALLADIUM-CATALYZED CARBONYLATIVE SYNTHESIS OF FIVE-MEMBERED HETEROCYCLES 3.1. Palladium-Catalyzed Carbonylative Synthesis of Five-Membered Oxygen-Containing Heterocycles

3.1.1. Palladium-Catalyzed Carbonylative Reactions of Alkynols. The presence of a hydroxyl group and a triple bond in the same molecule makes alkynols suitable substrates for the synthesis of heterocycles. As early as 1969, Nogi and Tsuji reported the palladium-catalyzed carbonylation of propargyl alcohols in methanol to yield the corresponding five-membered γ-lactones and also various esters. When 2,5dimethyl-3-hexyn-2,5-diol was applied in benzene, diisopropylidenesuccinic acid was formed as the main product in 49% yield (Scheme 8).33 A general methodology for the carbonylative synthesis of αmethylene-γ-lactones was developed by Norton and co-workers in 1981.34 Ethynyl alcohols were prepared from epoxidation and ethynylation of olefins, which was followed by palladiumcatalyzed carbonylation reaction, leading to α-methylene-γlactones in good yields (Scheme 9). In this methodology various functional groups are tolerated, such as isolated double bonds or bromine substituents. Another catalytic system, making use of a thiourea ligand, was developed by the same group. As with the first methodology, which proceeds in an asymmetric manner, the corresponding products were formed

in moderate yields.35 Drent and co-workers applied 2pyridylphosphine as ligand for the carbonylation of propargyl alcohols. In the presence of 2.5 mol % Pd(OAc)2, ligand, and methanesulfonic acid, under 60 bar of CO and at 60−90 °C, αmethylene-γ-lactones were formed in good yields.36 Later on, Dupont and co-workers carried out the reaction in ionic liquids (1-n-butyl-3-methyl imidazolium).37 Quantitative yields of lactones were obtained, and with the possibility to reuse the ionic catalyst system, this work constitutes an important improvement. Alternatively, a cationic palladium catalyst system was also developed.38 In this report, the cyclocarbonylation of 3-butyn-1-ols was studied. Six-membered ring lactones were produced preferentially in acetonitrile using cationic palladium complexes coordinated by certain chelating diphosphines (dppb) as catalyst. A triphenylphosphine-coordinated cationic D


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propane (dppp), and monodentate ligands, such as PPh3 or PCy3, were equally effective for this reaction. Conjugated enynols could also be carbonylated, affording 3-alkenyl-2(5H)furanones in good yields. However, double-bond isomerization (cis−trans) occurred if the enynol containing a cis olefinic substituent was used as the substrate. The latter cyclocarbonylation reaction is believed to proceed via an allenylpalladium intermediate, which is formed by initial insertion of Pd(0) into the C−O bond of the alkynol followed by rearrangement. This methodology was applied for the carbonylation of trifluoromethyl-substituted propargylic alcohols to give the corresponding 3-trifluoromethyl-2-(5H)furanones in high yields.41 The group of Gabriele succeeded in producing substituted furans from the corresponding alkynols under oxidative conditions (Scheme 11a).42 4-Yn-1-ols bearing a terminal triple

Scheme 9. Palladium-Catalyzed Carbonylative Synthesis of Methylene Lactones

Scheme 11. Palladium-Catalyzed Carbonylative Synthesis of Tetrahydrofurans

palladium complex, on the other hand, effected the formation of five-membered α-alkylidene lactones exclusively in DMF (N,N-dimethylformamide). A mechanism involving palladium hydride species as active catalysts has been presumed for the formation of six-membered ring lactones. The palladium-catalyzed carbonylative preparation of αvinylidene-γ-lactones in good yields from 5-hydroxy-2-alkynyl methyl carbonates was described by Tsuji and co-workers.39 The reaction was particularly rapid with tertiary substituted carbonates and slower with secondary carbonates. However, the product could not be isolated selectively when a primary carbonate group was used. In 1997, Yu and Alper succeeded in applying palladium catalysts for the synthesis of 2(5H)-furanones from corresponding substituted alkynols under carbonylation conditions in 67− 98% yield (Scheme 10).40 This reaction required catalytic amounts of Pd2(dba)3·CHCl3 (4 mol %) and 1,4-bis(diphenylphosphino)butane (dppb) (8 mol %) in dichloromethane under an atmosphere of CO (40 bar) at 95 °C. In addition, 14 bar of hydrogen were needed for this reaction. Other bidentate ligands, such as 1,3-bis(diphenylphosphino)-

bond undergo oxidative cyclization−alkoxycarbonylation in methanol at 70 °C and 100 bar of a 9:1 mixture of carbon monoxide and air in the presence of catalytic amounts of [PdI4]2− in conjunction with an excess of KI to give 2E[(methoxycarbonyl)methylene]tetrahydrofurans in good yields. A competing reaction, the cycloisomerization−hydromethoxylation leading to 2-methoxy-2-methyltetrahydrofurans, could be easily prevented by increasing the KI excess. The latter products can be prepared from 4-yn-1-ols and methanol in high yields using the same catalytic system without KI excess in the absence of carbon monoxide. Akita, Kato, and co-workers developed another system that avoids the use of KI and high pressure and leads to different products (Scheme 11b). Following this, they succeeded in performing the reaction in an asymmetric manner by applying chiral bisoxazolines as ligands.43 With respect to the reaction mechanism, the studies of Norton and co-workers are noteworthy. It is proposed that the Pd(II)-catalyzed cyclocarbonylation of acetylenic alcohols to αmethylene-γ-lactones proceeds through carboalkoxypalladium intermediates, followed by intramolecular syn addition to the triple bond (Scheme 12).44 Such intermediates have been independently synthesized, isolated, and found to undergo appropriate interconversions. A PdI2/Bu3P/CH3CN catalyst system gave rates first-order in CO pressure, with the ratedetermining step evidently being the uptake of CO by Pd. The use of a SnCl2 cocatalyst promoted ligand exchange on the

Scheme 10. Palladium-Catalyzed Carbonylative Synthesis of 2(5H)-Furanones

Scheme 12. Cyclocarbonylation of Deuterated Alkynol

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palladium center, dissociating Cl− and forming a Pd cation, which is a much faster cyclocarbonylation catalyst. The rate is now independent of CO pressure and first-order in Pd and in substrate. The rate-determining step is coordination of the substrate, followed by rapid uptake of CO and completion of the cyclocarbonylation reaction. As the carboalkoxy intermediates also react intermolecularly with terminal acetylenes, yields in the catalytic cyclocarbonylation reaction improved substantially when it was run at concentrations below 0.3 M. Besides the carbonylative cyclization of alkynols, the carbonylative reaction of propargyl alcohols with additional amines and thiols were also described. Ogawa and co-workers developed the reaction of propargylic alcohols with diaryl disulfides and carbon monoxide. In the presence of tetrakis(triphenylphosphine)palladium, a novel thiolative lactonization reaction lead to β-(arylthio)- α,β-unsaturated lactones in moderate to good yields (Scheme 13a).45 Similar conditions

As reported by Gabriele and co-workers, aside from lactones, furans can also be prepared by a similar process with different substrates.49 Hence, a variety of (Z)-2-en-4-yn-1-ols have been carbonylated under oxidative conditions to give substituted furan-2-acetic esters in good yields (Scheme 15a). The Scheme 15. Palladium-Catalyzed Carbonylative Synthesis of Furans

Scheme 13. Palladium-Catalyzed Carbonylation of Alkynols with Amines and Thiols cyclization−alkoxycarbonylation process occurred in alcoholic media at 50−70 °C under 100 bar pressure of a 9:1 mixture of carbon monoxide and air in the presence of catalytic amounts of PdI2 in conjunction with KI. The proposed reaction pathway involves the in situ isomerization of the initially formed (E)-2[(alkoxycarbonyl)methylene]-2,5-dihydrofuran species, which in some cases have been isolated and shown to be the intermediates. Under similar reaction conditions, 3-yne-1,2diols were transformed into the corresponding furan-3carboxylic esters in good yields (Scheme 15b).50 The palladium-catalyzed carbonylation of alkynols resulted in synthetically interesting lactones and furans. Similarly, 2hydroxy-substituted phenylacetylenes may give the corresponding benzofurans. Indeed, in 1994 Sakamoto and co-workers reported the palladium-catalyzed carbonylation reaction of 2alkynylanilines and 2-alkynylphenols in methanol. As expected, the corresponding indoles and benzofurans are obtained in moderate yields. The reaction of 2-alkynylbenzamides gave 3alkylidenisoindoles (Scheme 16).51 A similar methodology was

can be employed with homopropargylic alcohols, giving the corresponding δ-lactones with a β-arylthio group successfully. The reaction using diaryl diselenides in place of diaryl disulfides also led to a similar one-pot selenylation/lactonization sequence to provide the corresponding β-selenobutenolides. Meanwhile, Xiao and Alper described the carbonylation of propargyl alcohols with thiols.46 By changing the reaction conditions, sulfur-containing furanones were produced in good yields (Scheme 13b). In 2004, the group of Gabriele published the carbonylative synthesis of 4-dialkylamino-5H-furan-2ones.47 Starting from alkynols and dialkylamines, in the presence of CO and air, the target products were produced in good yields by applying PdI2 and KI as the catalytic system (Scheme 13c). The mentioned developments of palladium-catalyzed carbonylations of alkynols were also applied in the total synthesis of bioactive molecules (Scheme 14).48 Examples constitute the syntheses of Vernolepin, Germacranolide, (+)-Asimicin and (+)-Bullatacin, and so on.

Scheme 16. Palladium-Catalyzed Carbonylative Synthesis of Benzofurans and Indoles

Scheme 14. Application of Palladium-Catalyzed Carbonylations of Alkynols in Total Synthesis

reported by Lütjens and Scammells for the synthesis of XH-14 and its derivatives, which contains a benzofuran as the main skeleton.52 Efforts from Yang’s group improved the efficiency of this novel benzofuran process.53 By applying a PdI2−thiourea catalytic system and using CBr4 as oxidant, methyl benzofuran3-carboxylates were produced in 78−84% yields from the corresponding 2-hydroxyarylacetylenes. Both electron-rich and electron-deficient groups were tolerated. Afterward, the same group developed palladium-mediated carbonylative annulations F


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of o-alkynylphenols on silyl linker-based macrobeads and produced 2,3-disubstituted benzofurans in good yields.54 On the basis of their continuing interest on this topic, they also reported the carbonylative annulation of o-alkynylphenols to benzofuran-3-carboxylic acids and benzofuro[3,4-d]furan-1ones. These five-membered heterocycles were synthesized in good yields, but 1 equiv of palladium catalyst was needed for successful coupling (Scheme 17).55

readily available (Z)-2-en-4-yn-1-ols was reported by Gabriele and co-workers.59 This method is based on the PdI2-catalyzed oxidative aminocarbonylation of alkynes to give the corresponding 2-ynamide intermediates, which undergo intramolecular conjugate addition to give 2-(5H-furan-2-ylidene)acetamide derivatives. Spontaneous or one-pot acid-promoted aromatization of 2-(5H-furan-2-ylidene)acetamides eventually leads to the final furanacetamide derivatives. Other carbonylation approaches toward 3-acyl-substituted furans were also reported. For example, the palladium-catalyzed carbonylative coupling of aryl iodides with 1-aryl-2-alkyn-1ones led to furans in 36−75% yields (Scheme 19a).60 The

Scheme 17. Palladium-Mediated Annulation of oAlkynylphenols

Scheme 19. Palladium-Catalyzed Carbonylative Synthesis of Furans

In 2002, a general methodology for the palladium-catalyzed carbonylative annulation of o-alkynylphenol to construct 2substituted-3-aroyl-benzofurans was reported by Yang, Fathi, and co-workers.56 Good yields of the desired products were obtained (Scheme 18a). Interestingly, related to that work, Arcadi, Cacchi, and co-workers reported the synthesis of 3alkylidene-2-coumaranones via carbonylative coupling of oethynylphenols and vinyl triflates (Scheme 18b).57 All three studies showed that the presence of terminal alkynes and vinyl coupling partners negatively influences the selectivity. Moreover, Chaplin and Flynn reported the synthesis of 3-alkylidene2-coumaranones by multicomponent reaction using vinyl iodide as the coupling partner (Scheme 18c).58 A simple and convenient synthesis of 2-furan-2-ylacetamides starting from

contribution from Li and Yu proved that furans can be prepared by carbonylative coupling of aryl iodides with γ-propynyl-1,3diketones (Scheme 19b).61 The group of Kato, Akita, and coworkers developed a new type of PdII-catalyzed carbonylative dimerization of allenyl ketones.62 The resulting difuranylketones were obtained in moderate to good yields (Scheme 19c).

Scheme 18. Palladium-Catalyzed Carbonylative Coupling of o-Alkynylphenols

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The electrophilicity of the acylpalladium species was proposed to allow for the oxypalladation of an additional molecule of allenyl ketones. Costa and Gabriele and co-workers presented a direct synthesis of 1-(alkoxycarbonyl)methylene-1,3-dihydroisobenzofurans and 4-(alkoxycarbonyl)benzo[c]-pyrans by oxidative palladium-catalyzed cyclization/alkoxycarbonylation of 2-alkynylbenzyl alcohols, and of 2-alkynylbenzaldehydes or 2alkynylphenyl ketones.63 Reactions were carried out in ROH or CH3CN/ROH (R = Me, iPr) mixtures as the solvent at 70− 105 °C in the presence of catalytic amounts of PdI2 in conjunction with KI under a 4:1 or 3:1 CO/air mixture (20 or 32 bar total pressure at 25 °C). The reaction occurs through intramolecular attack by the nucleophilic oxygen atom (either already present in the starting material or generated in situ by ROH attack on carbonyl group) on the triple bond coordinated to PdII, followed by alkoxycarbonylation. The presence of substituents at the alkyne terminal position and at the carbon atom α to the hydroxy group played a key role in the selectivity of the process toward the formation of a five- or six-membered ring (Scheme 20). Alternatively, the reaction of alkynyloxiranes

Scheme 21. Palladium-Catalyzed Synthesis of 3(2H)Furanones

reactions: carbonylative coupling of iodobenzene with 2methyl-3-butyn-2-ol forms 4-hydroxy-4-methyl-1-phenyl-2-pentyn-1-one, which undergoes hydrogenolysis to yield 4-methyl-1phenyl-2,3-pentadien-1-one. Subsequent cyclocarbonylation of the latter intermediate leads to the final product (Scheme 22). Scheme 22. Palladium-Catalyzed Carbonylative Synthesis of Furanones

Scheme 20. Palladium-Catalyzed Carbonylative Synthesis of Furans and Pyrans Concerning the formation of 3-alkylidenefuran-2-ones, the group of Alper established a palladium catalyst system for the carbonylative coupling of aryl iodides with benzyl acetylenes.68 More recently, our group developed a general and efficient method for the synthesis of these furanones. Starting from aryl bromides and aryl triflates, after double carbonylation with benzyl acetylenes, furanones were produced in good yields. Methylated BE-23372M, a kinase inhibitor, was also produced in a one-pot sequence with 65% yield (Scheme 23).69 Scheme 23. Proposed Mechanism and the Synthesis of Methylated BE-23372M

could also lead to 1,3-dihydroisobenzofurans and tetrahydrofurans.64 Moderate to good yields of the products were produced under similar reaction conditions (PdI2/KI/CO/O2). Because of the interesting biological properties of 3(2H)furanone derivatives, many methodologies have been developed for their synthesis. In 1988, Inoue and co-workers synthesized 3(2H)-furanones from the coupling of α-ethynyl tertiary alcohols and acyl chlorides in the presence of a palladium catalyst and CO2.65 Later on, they did the same reaction under the pressure of CO and CO2, but started from aryl halides instead of acyl chlorides.66 It was revealed that there was the intermediate formation of an acetylenic ketone from the acetylenic alcohol, CO, and the aryl halides and subsequent formation of a cyclic carbonate from the acetylenic ketone and CO2 and decarboxylation gave the 3(2H)-furanones (Scheme 21). Alternatively, Kiji and co-workers succeeded to form the same type of product in the absence of CO2, but with 3isopropylidene-5-phenyl-2(2H)-furanone as the main product.67 Carbonylative coupling of iodobenzene and 2-methyl-3butyn-2-ol in aqueous NaOH/benzene was carried out by using Pd(OAc)2/PPh3/Bu4PBr as catalyst. In sharp contrast to a homogeneous Et3N solution, this biphasic solvent system gave 3-isopropylidene-5-phenyl-2(2H)-furanone in moderate yield and 2,2-dimethyl-5-phenyl-3(2H)-furanone and benzoic acid as byproducts. The formation of 3-isopropylidene-5-phenyl2(2H)-furanone is explained by the following sequential

The double carbonylation of benzyl acetylenes offers an interesting pathway for furanone synthesis. By changing the reaction conditions, maleic anhydrides can be formed form terminal alkynes via insertion of two CO molecules (Scheme 24a). The report from Alper’s group using PdCl2 and CuCl2 as the catalytic system produced maleic anhydrides from the corresponding terminal alkynes in the presence of formic acid or water.70 Afterward, several reported methods have been H


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temperature could be decreased to 110 °C. In the presence of (−)-BPPM [(2S,4S)-tert-butyl-4-(diphenylphosphino)-2((diphenylphosphino)methyl)pyrrolidine-1-carboxylate], the reaction proceeded in an enantioselective manner (Scheme 26a).80 Originally, the method was limited to only terminal

Scheme 24. Palladium-Catalyzed Synthesis of Maleic Anhydrides

Scheme 26. Palladium-Catalyzed Enantioselective Carbonylation of Alkenols

published from other groups with different oxidants.71 Notably, the study from Jiang’s group states that the reaction could proceed smoothly in the absence of external oxidant by using PdCl2 as the catalyst. Nevertheless, the presence of CuCl2 as oxidant can improve the yield from 76% to 99%.72 According to the authors, traces of water in dioxane might play a role as oxidant. In addition to maleic anhydrides, 3-substituted furan2(5H)-ones were also prepared from the carbonylation of alkynes. Under oxidative reaction conditions, in the presence of a suitable palladium catalyst, both terminal and internal alkynes can be transformed into the corresponding 3-substituted furanones (Scheme 24b).73 3.1.2. Palladium-Catalyzed Carbonylative Reactions of Alkenols. In 1984, Semmelhack and co-workers studied the intramolecular alkoxycarbonylation/lactonization of allylic alcohols.74 The effects of solvents and remote substitutents on the stereoselectivity were carefully studied, and good yields of the corresponding lactones were achieved (Scheme 25).

alkenes; however, by variation of palladium salts and ligands, internal alkynes could also be used as substrates in this reaction. In the presence of a catalytic amount of Pd(OAc)2 and dppb, α,β-substituted-γ-butyrolactones have been isolated in 42−85% yields (Scheme 26b).81 Depending on the structure of the allylic alcohol used, the formation of the corresponding alkene or β,γ-unsaturated carboxylic acid as byproduct was observed. Interestingly, the group of Zhang developed a highly enantioselective palladium-catalyzed asymmetric cyclocarbonylation of geminally disubstituted allylic alcohols. Thereby they demonstrated the first highly enantioselective cyclocarbonylation of β,γ-substituted allylic alcohols lacking dialkyl substituents at the α-position. These results demonstrate that the so-called BICP ligand is a unique chiral ligand for this kind of palladium-catalyzed asymmetric carbonylation (Scheme 27).82

Scheme 25. Palladium-Promoted Carbonylation of Allylic Alcohols

Scheme 27. Palladium−BICP-Catalyzed Carbonylation of Allylic Alcohols Later, they studied the intramolecular alkoxycarbonylation of hydroxyalkenes. Tetrahydrofurans were successfully produced in a stereoselective manner in moderate to excellent yields.75 In 1985, Alper and Leonard reported a palladium-catalyzed carbonylation of alkenols into five- and six-membered lactones.76 The reaction was conducted in acidic THF with PdCl2 as catalyst and CuCl2 as oxidant at room temperature and under atmospheric pressure of CO. On the basis of this achievement, they extended their methodology to secondary and tertiary allylic alcohols.77 The reaction was also performed in an asymmetric manner by applying poly-L-leucine or 2,2′bis(diphenylphosphino)-1,1′-binaphthyl (BINAP) as chiral ligand.78 The use of CuCl2 as oxidant was avoided by using Pd(dba)2 as catalyst and dppb as ligand in 1,2-dimethoxyethane (DME) under 40 bar of CO and at 190 °C. The desired lactones were isolated in 45−92% yields, and 2(5H)furanones were isolated in 60−80% yield in the case of alkynols.79 Notably, in this contribution the use of additional oxidant is avoided, but elevated temperature was needed. However, by carrying out the reaction in dichloromethane (DCM) with a mixture of 27 bar of H2 and 27 bar of CO, the reaction

Tamaru and co-workers reported the dialkoxycarbonylation of 3-butenols, to provide γ-butyrolactone 2-acetic acid esters in moderate yields under 1 bar of CO (Scheme 28a).83 In this stereospecific reaction, 1−50 mol % of PdCl2 and 3 equiv of CuCl2 were needed as the catalyst system, and also propylene oxide and ethyl ortho-acetate were required as additives. Besides I


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Scheme 28. Palladium-Catalyzed Carbonylative Synthesis of Lactones

preparation of natural products such as Kumausyne, Crisamicin A, Deoxynojirimycin, and so on.93 The report from Dixneuf in 1996 demonstrated the carbonylation of alkynyl carbonates to 5- or 6-membered lactones in the presence of a palladium catalyst.94 It is assumed that this reaction proceeded via an allenylpalladium intermediate. More recently, a mild and efficient methodology for the PdCl2-catalyzed chlorocyclocarbonylation of 2,3- or 3,4allenols with CuCl2 for the synthesis of 3-chloromethyl-2(5H)furanones and 3-chloromethyl-5,6-dihydropyran-2-ones was developed by Ma and co-workers (Scheme 30a). Optically

3-butenols, 3-butyn-1-ols could also be used as starting materials and furnished α-methylene-γ-butyrolacetones in excellent yields.84 The work from Inomata’s group excluded the use of propylene oxide and ethyl ortho-acetate; instead, O2 was applied as oxidant. In the presence of 10 mol % of PdCl2 and 1.5 equiv of CuCl2, under CO/O2 (v/v 1:1; 1 bar),85 in methanol at room temperature, 6 different γ-butyrolactone 2acetic acid esters were formed in 76−94% yields.86 At the same time, the palladium-catalyzed decarboxylative carbonylation of 3-vinyl-1-oxo-2,6-dioxacyclohexanes to 2-vinyl-γ-butyrolactones was established by the same group.87 In the presence of a catalytic amount of Pd(PPh3)4, lactones were produced in high yields at room temperature under 1 bar of CO (Scheme 28b). On the basis of the carbonylation of allylic alcohols, the related cyclocarbonylation of 2-allylphenols was also investigated.88 Different catalytic systems, such as Pd-clays and Pd(OAc)2/dppb, were developed, and ionic liquids (1-butyl-3methylimidazolium (BMIM) PF6 and BMIM NTf2) and dimethyl carbonate (DMC) were applied as solvents besides the usual organic solvents. Five-, six- and even seven-membered lactones were produced in good yields, and even in an asymmetric manner. The reaction of allylic alcohols allows for the construction of one ring in one step. More interestingly, two conjugated fivemembered rings can be easily formed by the palladiumcatalyzed carbonylation of 4-penten-1,3-diols. The first palladium-promoted (stoichiometric) oxycarbonylation of 5hexen-1,4-diols was reported by Semmelhack and co-workers.70,89 Later on, the group of Yoshida made the method catalytic in palladium by using CuCl2 as the reoxidant.90 cis-3Hydroxytetrahydrofuran acetic acid lactones were synthesized in good yields under 1 bar of CO at room temperature (Scheme 29). This methodology was applied by Kitching and

Scheme 30. Palladium-Catalyzed Cyclocarbonylation of Allenols

active 3-chloromethyl-2(5H)-furanones could be prepared from readily available optically active 2,3-allenols.95 Meanwhile, Li and Shi reported the palladium-catalyzed cyclocarbonylation of α-allenic alcohols to give the corresponding lactones (Scheme 30b). The α-allenic alcohols were prepared from vinylbromohydrin derivatives under mild conditions.96 3.1.3. Palladium-Catalyzed Carbonylative Reactions of 2-Halo-alke(y)nes. The works by Larock and co-workers in the 1970s led to an efficient procedure for the synthesis of chloro-substituted lactones.97 Here, the reaction of propargylic alcohols with mercuric chloride gave β-chloro-γ-hydroxyvinylmercuric chlorides as key intermediates. In the presence of a stoichiometric amount of palladium chloride and 1 bar of CO, the corresponding products were synthesized in good yields (Scheme 31a). When applying CuCl2 as reoxidant and using Scheme 31. Palladium-Catalyzed Carbonylation of Organomercurials

Scheme 29. Palladium-Catalyzed Carbonylation of Diols

benzene as solvent, only catalytic amounts of palladium chloride were needed. Kocovský and co-workers obtained organomercurials from cyclopropyl alcohols, which were converted into lactones in the presence of a catalytic amount of palladium catalyst using BQ as oxidant (Scheme 31b). The reaction was done in a stereoselective manner under 1 bar of CO.98 These methodologies offer other choices for the synthesis of functionalized lactones, but the main drawback of these protocols is the need for stoichiometric amounts of toxic mercury compounds. In this respect, the use of organic halides as substrates, which avoids the need for mercury, is advantageous.

co-workers in the preparation of plakortone cores, a novel class of activators of cardiac SR-Ca2+-pumping ATPase.91 The group of Gracza developed an asymmetric oxycarbonylation of pent-4ene-1,3-diols.92 In the presence of palladium salts and chiral bis(oxazoline) ligands, 1,4-benzoquinone (BQ) in acetic acid, under 1 bar of CO, the desired product were produced in a chiral manner in low yields. They also extended the substrates to 4-benzyloxyhepta-1,6-diene-3,5-diols. Besides the synthesis of plakortones, these methodologies were also applied in the J


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Scheme 32. Palladium-Catalyzed Cyclocarbonylations

In 1991, Shibasaki and co-workers reported the asymmetric synthesis of α-methylene lactones starting from prochiral alkenyl halides.99 In the presence of 5 mol % of Pd(OAc)2 and chiral ligand [(S,S)-chiraphos], under 1 bar of CO, the reaction was finished in 1 h at 80 °C in dimethylsulfoxide (DMSO) with K2CO3 as base. The group of Negishi reported the palladium-catalyzed cyclic carbometalation−carbonylation in 1994, and also the carbonylative cyclization of 1-iodo-2alkenylbenzenes, 1-iodo-substituted 1,4-, 1,5-, and 1,6-dienes, and 5-iodo-1,5-dienes.100 Moderate yields of five- or sixmembered heterocycles were achieved under CO pressure. 3Iodohomoallylic alcohols were synthesized from 3,4-pentadien2-one, tetra-n-butyl ammonium iodide, and aldehydes in the presence of ZrCl4 as catalyst in good yields. These 3iodohomoallylic alcohols can be further transformed into α,βunsaturated γ-lactones by palladium-catalyzed cyclocarbonylation (Scheme 32a).101 In a similar manner, allylic alcohols could also be produced from aldehydes and acrylate via Baylis− Hillman reaction. Following the same idea, 3-alkenyl phthalides were produced in good yields from the Baylis−Hillman adducts (Scheme 32b).102 With respect to the increasing importance of trifluoromethyl-substituted compounds, the report of Qing and Jiang on the cyclocarbonylation of 3-iodo-3-trifluoromethyl allylic alcohols is noteworthy.103 Several 3-trifluoromethyl2(5H)-furanones were isolated in good yields (Scheme 32c). Interestingly, Ryu and co-workers reported the influence of light on the carbonylation of alkyl iodides.104 The reaction proceeds by a radical pathway (Scheme 32d). In the same report, carboxylic acid esters and α-keto amides were also synthesized from the corresponding alkyl iodides under the same reaction conditions. More recently, the same group described the Pd/light-induced carbonylation of alkenes to esters and lactones. In 1982, Larock and Fellows reported the thallation− carbonylation of benzyl alcohols.105 Thallium(III) trifluoroacetate was used for the ortho-thallation of arenes, which are subsequently carbonylated with 10 mol % of PdCl2, 2 equiv of LiCl, and MgO in either methanol or THF under 1 bar of CO. Moderate yields of phthalides were obtained (Scheme 33). Besides these two-step reactions, more efficient protocols were also reported. For example, Crisp and Meyer reported the palladium-catalyzed intramolecular coupling of hydroxyl vinyl

Scheme 33. Thallation−Carbonylation of Arenes

triflates.106 Here, α,β-butenolides were formed in good yields. Furthermore, the palladium-catalyzed cyclocarbonylation of 2halobenzyl alcohols was also developed.107 The application of Mo(CO)6 as CO source, using microwave to promote this palladium-catalyzed carbonylation system, has been reported too.108 The usefulness of these methodologies was proven by numerous applications in total synthesis, such as dihydromahubanolide B, (+)-homopumiliotoxin 233G, CP-263,114, and phomoidrides.109 3.1.4. Other Palladium-Catalyzed Carbonylations to Lactones. There are many other procedures known for palladium-catalyzed carbonylative lactone formation that were developed besides the cyclocarbonylation of alkynols, alkenols, and 2-halo-alkens; for example, in 2003, Chatani’s group reported the palladium-catalyzed carbonylation of 2(propargyl)allyl phosphates.110 On the basis of this method, unsaturated γ-lactones were synthesized in good yields (Scheme 34). Two reaction mechanisms were proposed (a and b), and pathway a was developed further by Negishi’s group at the end of the 20th century.111 Starting from 2-halo aryl alkenyl ketones, in the presence of CO and palladium catalyst, the corresponding five- and six-membered heterocycles were formed in good yields. Recently, Cho and Kim reported the palladium-catalyzed cyclocarbonylation of β-bromovinyl aldehydes and ketones.112 The reactions of ketones needed higher temperature compared to the aldehydes, and moderate to good yields were observed (Scheme 35). Palladium-catalyzed cyclocarbonylation was also applied in the total synthesis of uncinine and its analogues.113 A convenient synthesis of 3-spiro-fused benzofuran-2(3H)ones was reported by Arcadi’s group, and shortly after Grigg’s group extended this procedure toward 3-spiro-2-oxindoles.114 After palladium-catalyzed carbonylation of vinyl triflates with oiodophenols (or o-iodoanilines), intramolecular Heck reaction gave the terminal products in good yields. Notably, the K


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Scheme 34. Palladium-Catalyzed Carbonylation of 2(Propargyl)allyl Phosphates

Scheme 37. Palladium-Catalyzed Cyclocarbonylation of Alkynones

good carbonyl leaving group among the groups tested. Following a similar concept, the cyclocarbonylation of 2propynyl-1,3-dicarbonyls with organo halides or triflates lead to furans in good yields (Scheme 37b).117 The oxidative cyclocarbonylation of 2-alkyl-2-propargylcyclohexane-1,3-diones mediated by palladium catalysts was developed by Kato, Akita, and co-workers.118 Bicyclic-β-alkoxyacrylates were produced in 51−74% yields with 72−82% enantiomeric excesses (ee's) (Scheme 37c). The authors also extended their palladium-catalyzed cyclocarbonylation to propargylic esters, propargylic acetates, 4-yn-1-ones, and allenyl ketones. This methodology was applied by Mukai and co-workers in the total synthesis of naturally occurring diacetylenic spiroacetal enol ethers.119 A related mechanistic study was also done by Carfagna and co-workers that was supported by both experiment and DFT study.120 The competition between a Pd(0)-promoted deallylation catalytic cycle and a Pd(II)-promoted heterocyclization catalytic cycle (they have named this “sequential homobimetallic catalysis”) has been shown to occur starting from 1-(2allyloxyphenyl)-2-yn-1-ols to afford 2-benzofuran-2-ylacetic esters and β,γ-unsaturated esters in high yields under carbonylative conditions by Gabriele’s group.121 In view of the conceptual as well as the synthetic importance of the process, the mechanistic aspects and the synthetic scope of the reaction were investigated in detail. All the experimental evidence was in agreement with the sequential homobimetallic mechanism, and the reaction proved to be of general synthetic applicability. Instead of esters, amides could also be produced in the presence of amines under the same conditions (Scheme 38a).122 Applying similar reaction conditions, they also

Scheme 35. Palladium-Catalyzed Cyclocarbonylation of βBromovinyl Aldehydes and Ketones

reactions were carried out under atmospheric pressure of CO and at ambient temperature. Interestingly, the group of Miura developed the palladiumcatalyzed carbonylation reaction of naphthols and phenols having appropriate substituents with aldehydes in the presence of CF3CO2H as cocatalyst.115 Hence, benzofuran-2(3H)-one derivatives were produced in good yields (Scheme 36). The reaction mechanism is believed to start with the nucleophilic addition of naphthol to the protonated aldehyde, which is promoted by CF3CO2H, to give dihydroxy intermediate (2hydroxybenzyl alcohols, for examples), followed by palladiumcatalyzed carbonylation. Indeed, the reaction of 2-hydroxybenzyl alcohols also took place effectively under similar conditions. Chatani and co-workers reported the carbonylation of yne esters in 2005.116 Lactones were produced in good yields under 1 bar of CO (Scheme 37a). The 2-pyridinyloxy group was a

Scheme 36. Palladium-Catalyzed Carbonylation of Naphthols with Aldehydes

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catalyzed heterocyclization−alkoxycarbonylation process. A number of indoles were prepared in good yields (Scheme 39b).

Scheme 38. Palladium-Catalyzed Carbonylative Cascade Reaction

3.2. Palladium-Catalyzed Carbonylative Synthesis of Five-Membered Nitrogen-Containing Heterocycles

Nitrogen-containing heterocycles show a variety of biological activities and represent privileged structures compared to other types of heterocycles. In this respect it is interesting that Arcadi, Cacchi, and co-workers reported the palladium-catalyzed cyclocarbonylation of 2-alkynyltrifluoroacetanilides with aryl halides and vinyl triflates.129 2-Substituted 3-acylindoles were synthesized in fair to good yields (Scheme 40a). The succeeded to extend their methodology to the synthesis of coumarins (Scheme 38b).123 Starting from readily available 2(1-hydroxyprop-2-ynyl)phenols, the reactions were carried out in the presence of catalytic amounts of PdI2 in conjunction with an excess of KI in MeOH at room temperature and under 90 bar of CO to give 3-[(methoxycarbonyl)methyl]coumarins in good isolated yields (62−87%). Grigg and co-workers reported palladium-catalyzed multicomponent reactions to interesting benzofurans and related nitrogen heterocycles. 124 Cyclocarboformylation can be achieved regioselectively and provides novel access to aldehydes by applying silane as hydride source. Alternatively, organostannanes and NaBPh4 were also used as coupling partners and gave the corresponding ketones in good yields (Scheme 39a).

Scheme 40. Palladium-Catalyzed Cyclocarbonylation of 2(1-Alkynyl)benzenamines

Scheme 39. Palladium-Catalyzed Cyclocarbonylation Using Silanes or Borates As Coupling Partner and PdI2-Catalyzed Carbonylative Synthesis of Indoles

mechanism is believed to begin with the oxidative addition of the organohalide to active Pd(0) species, followed by the coordination and insertion of CO to form the corresponding acylpalladium intermediate, which undergoes an intramolecular nucleophilic attack of the nitrogen atom after coordination of the triple bond to R−Pd−X. The final product is formed after reductive elimination. Interestingly, 2-(1-alkynyl)benzenamines were converted into 3-(halomethylene)indolin-2-ones in the presence of PdX2 and CuX2 (X = Br, Cl).130 The products are achieved in moderate to good yields (Scheme 40b). The latter reaction mechanism is proposed to start with the coordination of PdCl2 to the triple bond and nitrogen, followed by cis- and trans-halopalladation to generate a vinylpalladium species. Afterward, the coordination and insertion of CO occurred, and the terminal product is formed after reductive elimination. The active Pd(II) species can be regenerated by the oxidation of Pd(0) with CuX2 to start a new catalytic cycle. Notably, when carrying out the reaction of 2-(1-alkynyl)benzenamines in methanol under oxidative conditions, (E)-3(methoxycarbonyl)methylene-1,3-dihydroindol-2-ones were produced in 48−64% yields (Scheme 41a).131 1-(2-Aminoaryl)-2-yn-1-ols, which can be easily obtained by the Grignard reaction between 1-(2-aminoaryl)ketones and alkynylmagnesium bromides, were carbonylated into quinoline-3-carboxylic esters or indol-2-acetic esters, depending on the reaction conditions (Scheme 41b).132 Using similar reaction conditions, the same authors also described the palladium-catalyzed cyclocarbonylations of 2-ynylamines to 4-dialkylamino-1,5dihydropyrrol-2-ones (Scheme 41c)133 and (Z)-(2-en-4-ynyl)amines to the corresponding pyrrols (Scheme 41d).134 A novel one-pot construction of pyrazoles and isoxazoles was reported by Mori’s group in 2005, and later by Stonehous’s group. Starting form terminal alkynes, hydrazines or hydroxylamines, CO, and aryl iodides, the corresponding heterocycles were formed in good yields under the assistance of a palladium

They also described the multicomponent reaction of bicyclopropylidene, CO, and aryl iodides or aryl thiols, leading to different heterocycles in fair yields.125 The group of Larock developed a palladium-catalyzed carbonylative annulation of internal alkynes. Five-, six-, seven-, and eight-membered ring lactones were synthesized by this procedure.126 This kind of concept was recently modified and used for total synthesis of the marine ascidian metabolite perophoramidine and other natural products.127 The group of Gabriele developed also a novel carbonylation procedure for the synthesis of indoles.128 Their multicomponent cascade reaction is initiated by a nucleophilic attack to the imine moiety, which is followed by a palladiumM


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advance, followed by addition of hydrazine to furnish the products.136 As early as in 1983, Danishefsky and Taniyama reported the palladium-mediated cyclization of acrylanilide (Scheme 42a).137 The group of Tamaru had a long-term interest in the related palladium-catalyzed oxidative aminocarbonylation of unsaturated ureas and carbamates (Scheme 42b).138 The Pd(II)catalyzed intramolecular aminocarbonylation of olefins bearing many types of nitrogen nucleophiles has been examined in detail in their group. The cyclizations proceeded either under acidic conditions [A: typically PdCl2 (0.1 equiv) and CuCl2 (3.0 equiv) under 1 atm of CO at room temperature in methanol] or buffered conditions [B: typically PdCl2 (0.1 equiv) and CuCl2 (2.3 equiv) under 1 atm of CO at 30 °C in trimethyl orthoacetate]. Among the different substrates tested, endocarbamates displayed distinctive reactivity: they smoothly underwent intramolecular aminocarbonylation under conditions B to furnish 4-[(methoxycarbonyl)methyl]-2-oxazolidinones in good yields. Other nitrogen-containing substrates (exoureas, endoureas, exocarbamates, and exotosylamides), on the other hand, satisfactorily underwent aminocarbonylation only under conditions A to give the corresponding products in good yields. Under conditions B, they are unreactive and provided the expected products either in poor yields or as intractable mixtures of products. Similar to the mentioned Wacker-type oxidation conditions, N-tosylhomoallylamines furnished 3-methyl-2-pyrrolidones as products.139 In the presence of palladium and copper salts, the reactions were carried out at 1 bar of CO and at room temperature (Scheme 43a). Meanwhile, this reaction was also realized in an enantioselective manner by application of spiro bis(isoxazoline) as chiral ligands. dppb and BINAP were also used as ligand under pressure of CO and H2 (Scheme 43b).140 The combination of aminocarbonylation with Friedel−Crafts acylation was reported by Cernak and Lambert in 2009.141 In the presence of palladium and indium salts, α-pyrrolidinyl ketones were produced in good yields with CuCl2 as oxidant (Scheme 43c). This type of oxidative aminocarbonylation was

Scheme 41. PdI2-Catalyzed Carbonylations Leading to Nitrogen Heterocycles

catalyst. The reaction was conducted at room temperature using ambient pressure of CO in an aqueous solvent system. Concerning the reaction mechanism, the first step is the oxidative addition of the aryl iodide to the palladium center, followed by coordination and insertion of CO to form the acylpalladium species. After reductive elimination, alkynones are formed, which condensate with hydrazines or hydroxylamines to give pyrazoles or isoxazoles as the final products.135 Some one-pot, two-step procedures for generating pyrazoles were also reported. Here, the alkynones were produced by carbonylative coupling of aryl halides with terminal alkynes in Scheme 42. Palladium-Catalyzed Oxidative Aminocarbonylation

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significantly to this area. In the presence of a palladium catalyst, allenic amines underwent cyclization under mild conditions in methanol. Additional oxidation reagents were needed to reoxidize the intermediate Pd(0) species. This method was also used in the preparation of pumiliotoxin 251D by the same group. Palladium-catalyzed carbonylative C−H activation reactions of aromatic rings have obvious advantages compared with the similar carbonylation reactions of aryl halides. Already in 1967, Takahashi and Tsuji published the palladium-mediated carbonylation of azobenzenes.148 They first prepared azobenzene− palladium chloride complexes, and then the complexes reacted under an atmosphere of CO to indazolinones in excellent yields, which could be further carbonylated into quinazolinedions in the presence of cobalt carbonyl (Scheme 45a). Benzyl

Scheme 43. Palladium-Catalyzed Aminocarbonylation

Scheme 45. Palladium-Promoted Carbonylative C−H Activation

also applied in the total synthesis of (±)-ferruginine, (±)-anatoxin-a, and 1,4-iminoglycitols.142 [Pd(CH3CN)4](BF4)2 is a strongly electrophilic complex, which can activate olefins to undergo nucleophilic attack by nitriles to give nitrilium salts. These nitrilium salts undergo subsequent reactions with a variety of nucleophiles, including electron-rich aromatics, alcohols, and amines, ultimately producing a variety of heterocyclic ring systems.143 The report from Alper’s group demonstrated that the use of 2-aminostyrenes as reactants in the presence of catalytic quantities of palladium acetate and tricyclohexylphosphine affords fivemembered ring lactams in high yield and selectivity. Bicyclic and tricyclic heterocycles containing six-membered ring lactams can be synthesized from the related reaction of 2-allylanilines with CO/H2 using the catalytic system Pd(OAc)2/PPh3, whereas the use of 1,4-bis(diphenylphosphino)butane instead of PPh3 in the latter process results in the formation of the seven-membered benzazepinones in good yield. The regioselectivity control depends on the nature of the palladium catalyst, the relative pressures of the gases, and the solvent.144 Beller and co-workers described the palladium-catalyzed carbonylation of aldehydes with urea derivatives providing a remarkably simple, pharmacologically interesting method for the preparation of 5-, 3,5-, and 1,3,5-substituted hydantoins in good to very good yields.145 Besides the cyclocarbonylation of alkenyl and alkynyl amines, the palladium-catalyzed aminocarbonylation of allenic amines constitutes a versatile methodology for the construction of nitrogen-containing heterocycles. The groups of Gallagher (Scheme 44a)146 and Tamaru (Scheme 44b)147 contributed

amine derivatives were also carbonylated by the formation of the corresponding palladium complexes in advance (Scheme 45b).149 Palladium-catalyzed carbonylation of benzyl amines to benzolactams were also reported. In the presence of catalytic amounts of palladium catalyst, and using Cu(II) and air as oxidants, N-monoalkylated benzyl amines or phenethylamines were transformed into benzolactams in good yields, and N,N′dialkylureas were obtained from primary amines (Scheme 45c).150 More recently, Yu and co-workers reported the carbonylation of C(sp3)−H bonds to succinimides. Notably, only a catalytic amount of palladium catalyst was needed together with 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO) as oxidant. However, the corresponding products were isolated in moderate yields. The carbonylative C−H activation of arenes by using sulfonamide as directing group was also reported by the same group (Scheme 45d).151 In addition to C−H activation reactions of arenes, the palladium-catalyzed carbonylation of o-bromoaminoalkylbenzenes was reported by Ban and co-workers in 1978.152 In the

Scheme 44. Palladium-Catalyzed Cyclocarbonlyation of Allenic Amines

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presence of catalytic amounts of Pd(OAc)2 and PPh3 under atmospheric pressure of CO, five-, six-, and even sevenmembered benzolactams were prepared in good yields (Scheme 46a). A similar sequential reaction was also developed by Grigg

This three-component cascade process involved the carbonylation of substituted aryl iodides to generate the respective acyl palladium species, which reacted with a primary aliphatic/ aromatic amine, amide, or sulfonamide followed by an intramolecular conjugate addition to afford 3-substituted isoindolin-1-ones in good yields (Scheme 46c). Moreover, Shim and co-workers described the cyclocarbonylation of 2-(2bromophenyl)-2-oxazolines by a palladium−nickel catalyst.158 Under 3 bar of CO and in the presence of the bimetallic catalyst, the corresponding isoindolinones were produced in high yields (Scheme 47a). Later on, they synthesized similar products by palladium-catalyzed coupling of 2-iodobenzoyl chloride with imines.159 In the presence of Pd(PPh3)2Cl2/PPh3 as the catalyst system and NEt3 as base, the corresponding isoindolinones were formed in moderate yields (Scheme 47b). More complex isoindolinones were produced by the same group through a palladium-catalyzed carbonylative coupling of 2-bromobenzaldehydes with aminoalcohols or diamines.160 These multistep reactions provided the corresponding isoindolinones in good isolated yields (Scheme 47c). The reactions of diamines were carried out under lower temperature and with lower catalyst loading. Besides that, the palladiumcatalyzed coupling of 2-bromobenzaldehydes and 2-bromocyclohex-1-enecarbaldehydes with primary amines has also been developed (Scheme 47d).161 Interestingly, no base was needed in these reactions. With respect to the mechanism, the reaction started with the formation of an imine by condensation of the aldehyde and the primary amine. Oxidative addition of the carbon−bromide bond of the imine to the active palladium(0) catalyst produces the arylpalladium(II) complex. After coordination of carbon monoxide to the metal center and subsequent insertion into the C−Pd bond, an aroylpalladium(II) intermediate is formed. Then, an intramolecular acylpalladation to the imine gives the alkylpalladium(II) intermediate. Subsequent hydrogenolysis with molecular hydrogen leads to the isoindolin-1-one. It is assumed that hydrogen is produced by the reaction of CO with H2O generated in the initial condensation stage. The group of Mori and Shibasaki reported the use of a special titanium−isocyanate complex for a novel one-step

Scheme 46. Palladium-Catalyzed Carbonylative Synthesis of Isoindolin-1-ones

and co-workers.153 Here, starting from 2-halobenzylamines, ethyl glyoxalates, and aryl boronic acids, the in situ-generated carbinolamines/imines reacted with CO to give isoindolones. Following this concept, Shim et al. developed a palladiumcatalyzed coupling of o-bromobenzyl bromides with primary amines. Initially, o-bromobenzyl amines are formed, which react further via palladium-catalyzed aminocarbonylation in DMF. The final products were obtained in fair to moderate yields (Scheme 46b).154 The latter reaction was improved by Grigg and co-workers,155 as well as in further studies by other groups.156 Applying in situ-generated palladium nanoparticles, this three-component reaction proceeded even at room temperature under 1 bar of CO and gave the desired products in good yields. Recently, the same group also described a novel palladium-catalyzed carbonylative synthesis of isoindolin-1ones.157

Scheme 47. Palladium-Catalyzed Carbonylative Synthesis Towards Isoindolin-1-ones

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Scheme 48. Palladium-Catalyzed Carbonylative Synthesis of Isoindolin-1-ones

Scheme 49. Palladium-Catalyzed Carbonylative Synthesis of Isoindolinone Derivatives

synthesis of isoindolinones and quinazolinones starting from ohalophenyl alkyl ketones.162 As shown in Scheme 48, this reaction proceeds through the oxidative addition of the enol lactone, generated by palladium-catalyzed carbonylation of ohalophenyl alkyl ketones, to the titanium−isocyanate complex A. Some other procedures for the preparation of isoindolinone derivatives were also reported.163 Examples are the carbonylation of iodoanilines with phenylacetylenes (Scheme 49a), the reaction of 1-halo-2-alkynylbenzene with amines, and the onepot reaction of 1,2-dihaloarenes, alkynes, and amines (Scheme 49b). In addition to isoindolinones, several methods for the preparation of phthalimides have been developed in the last two decades. In 1991, Perry’s group reported the carbonylative coupling of o-dihaloarenes with primary amines to phthalimides.164 As shown in Scheme 50a, using PdCl2 as catalyst and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) as base, phthali-

mides were produced in good yields in dimethylacetamide (DMAc). The same group also succeeded in applying similar reaction conditions for the carbonylative synthesis of 2arylbenzoxazoles and 2-arylbenzimidazoles.165 Here, aryl halides were coupled with o-fluoroanilines and o-phenylenediamines to give 2-arylbenzoxazoles and 2-arylbenzimidazoles, respectively. More recently, Cao and Alper extended this methodology to 1,2-dibromobenzenes as substrates by using phosphonium salt-based ionic liquids as solvent under 1 bar of CO.166 This process showed a wide tolerance for functional groups, and excellent yields of products were obtained. The recyclability of the catalytic system was also investigated. The group of Larock developed the straightforward carbonylation of o-halobenzoates and primary amines to phthalimides.167 This method gave the corresponding products in good yields and tolerated different functional groups (Scheme 50b). Later on, the group of Queiroz showed that it is also possible to perform similar reactions under CO-free conditions by using Mo(CO)6 as CO source.168 The group of Arndtsen developed a number of elegant multicomponent reactions that introduce one or two CO's into the parent molecules.169 In the presence of a palladium catalyst, alkynes, imines, acid chlorides, and CO coupled together to give pyrroles as terminal products (Scheme 51a). Using αamidoesters instead of imines and acid chlorides gave the same products (Scheme 51b). Interestingly, when the reaction was carried out with imines and acid chlorides, imidazoles were formed as the final products. By simply changing the sequence of addition of the substrates, imidazolinium salts and imidazolines were produced. In general, these methods offer convenient pathways for the production of heterocycles from easily available substrates.

Scheme 50. Palladium-Catalyzed Carbonylative Synthesis of Phthalimides

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Scheme 51. Palladium-Catalyzed Carbonylation of Imines

derivatives were developed. Thus, Alper and co-workers reported a palladium-catalyzed N−C coupling/carbonylation sequence toward 2-carboxyindoles. The catalyst system showed good functional group tolerance and gave the products in high yields (Scheme 54a). 2-Aroylindoles could also be produced from the same substrates in moderate yields (Scheme 54b).172 Finally, it is worth mentioning that in 2010 Staben and Blaquiere reported a four-component carbonylation reaction for the synthesis of 1,2,4-triazoles.173 Under mild conditions and low CO pressure, a wide substrate scope was achieved (Scheme 55). Notably, the pharmaceutically interesting product Deferasirox was also synthesized by this methodology.

In 2001, Kang and Kim demonstrated the cyclocarbonylation of allenic sulfonamides with aryl iodides.170 In their report, αallenic sulfonamides underwent a carbonylation−coupling− endocyclization sequence with aryl iodides in the presence of palladium catalyst. Pyrrolines and pyrrolidines were produced in moderate yields under 20 bar of CO. The mechanism of this reaction was proposed to start with oxidative addition of ArI to Pd(0) followed by carbonylation to give acylpalladium species. This complex adds to the central carbon of the allene moiety to provide a π-allylpalladium complex. Cyclization of the latter intermediate by endomode will give the terminal product (Scheme 52).

3.3. Palladium-Catalyzed Carbonylative Synthesis of Other Five-Membered Heterocycles

Scheme 52. Palladium-Catalyzed Carbonylative Coupling of Allenic Sulfonamides with Aryl Iodides

Besides the previously described five-membered oxygen- and nitrogen-containing heterocycles, the synthesis of five-membered heterocycles with both oxygen and nitrogen as well as other heteroatoms will be summarized in this section. As early as 1992, Meyers and co-workers reported the palladium-catalyzed carbonylative synthesis of oxazolines.174 Aryl or enol triflates made from the corresponding ketones and phenols, and also aryl halides, were used as starting materials and coupled with amino alcohols to give chiral α,β-unsaturated or aryl oxazolines in good yields. Later on, Perry’s group performed systematic studies on this one-pot, two-step process for the preparation of oxazolines (Scheme 56).175 Young and DeVita developed a novel procedure for the synthesis of oxadiazoles (Scheme 57a).176 In a one-pot procedure, oxadiazoles were prepared in moderate yields from aryl iodides and amidoximes under 1 bar of CO. Both electron-withdrawing and electron-donating substituents were tolerated. Afterward, Zhou and Chen reported a similar reaction with diaryliodonium salts as starting materials (Scheme 57b).177 Oxazolidinones are another important class of heterocycles, and many of their derivatives show interesting biological activities. Thus, in 1986 Tam reported the oxidative carbonylation of 2-aminoethanols to oxazolidinones.178 Using a combination of PdCl2 and CuCl2 as the catalytic system, reactions proceeded smoothly at 3 bar of CO. Diols and aminodiols were also used as substrates in this reaction.

Under similar reaction conditions, 6-trifluoromethyl-12acylindolo[1,2-c]quinazolines were prepared in high yields through palladium-catalyzed carbonylation of bis(otrifluoroacetamidophenyl)acetylene with aryl or vinyl halides and triflates.171 The reaction, which tolerated a variety of functional groups, probably involves the formation of a 3-acyl-2-(otrifluoroacetamidophenyl)indole intermediate, followed by cyclization to the final products (Scheme 53). Recently, two novel processes for the synthesis of carbonylated indole

Scheme 53. Palladium-Catalyzed Carbonylation of Bis(o-trifluoroacetamidophenyl)acetylene

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Scheme 54. Palladium-Catalyzed Synthesis of Carbonylated Indoles

Scheme 55. Palladium-Catalyzed Carbonylative Synthesis of Triazoles

Scheme 58. Palladium-Catalyzed Carbonylative Synthesis of Oxazolidinones

The group of Costa and Gabriele made comprehensive studies on cyclocarbonylations of propargyl amines and related compounds.183 In the presence of PdI2 and KI, oxazolidinone derivatives were synthesized in alcohols in the presence of oxygen. In the case of prop-2-ynylamides, oxazolines were prepared under similar reaction conditions (Scheme 59).

Scheme 56. Palladium-Catalyzed Carbonylative Synthesis of Oxazolines

Scheme 59. Palladium-Catalyzed Carbonylation of Propargyl Amines

Scheme 57. Palladium-Catalyzed Carbonylative Synthesis of Oxadiazoles

Gabriele and co-workers also applied their PdI2/KI system for the oxidative carbonylation of β-amino alcohols, but higher pressure was needed (20−60 bar of CO and air).179 Interestingly, Li and Xia reported that palladium on charcoal is a more efficient catalyst compared to the reported homogeneous catalyst systems.180 Albeit the catalyst could be reused for five times without losing activity and selectivity, it is very likely that the “real” active catalyst in this heterogeneous system is leached palladium nanoparticles. In addition, Pd(II) complexes have been combined with anodic recycling at a graphite electrode.181 In this process, the reaction could be carried out at room temperature under atmospheric pressure of carbon monoxide. Good yields of oxazolidinones were obtained. More recently, Troisi and co-workers developed the palladium-catalyzed cyclocarbonylation of ortho-substituted phenols, thiophenols, and anilines.182 Benzo-fused five- and six-membered heterocycles are produced in excellent yields. Notably, no oxidation reagent was needed in this reaction. Apparently, the oxidative addition of Pd(0) to the N−H or O− H bond of the substrates regenerated the required Pd(II) species. Thus, this cyclocarbonylation of amino alcohols offers a straightforward process for the preparation of oxazolidinones. High yield and selectivity are easily achieved, with many possibilities of catalytic systems available (Scheme 58).

Related cyclocarbonylations of propargyl acetates and amides were also studied by Kato and co-workers.184 They applied BQ as oxidant and bisoxazoline ligands and obtained excellent yields of the cyclization products. The authors stated that the so-called box ligand enhanced the π-electrophilicity of palladium(II) and thus promoted the coordination of the triple bond of a second substrate molecule to the acyl palladium intermediate to enable the dimerization reaction to take place. The cyclocarbonylation of o-alkenyl hydroxylamines under oxidative conditions was reported by Bates and Sa-Ei.185 Treatment of o-homoallylhydroxylamines with palladium(II) and copper(II) in the presence of a base, methanol, and carbon monoxide resulted in the formation of isoxazolidines (Scheme 60). An electron-withdrawing group on the hydroxylamine nitrogen was essential for successful cyclization. When carbamate groups were used, the products were formed exclusively as their cis isomers. A facile and selective palladium-catalyzed domino synthesis of carbonylated benzothiophenes was developed by Zeng and Alper in 2011.186 2-Carbonylbenzo[b]thiophene derivatives S


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Scheme 60. Palladium-Catalyzed Carbonylation of o-Alkenyl Hydroxylamines

reaction (Scheme 62). Huynh, Li, and co-workers tested a series of palladium carbene complexes [PdBr2(iPr2-bimy)L] Scheme 62. Palladium-Catalyzed Carbonylative Synthesis of Flavones

were produced from 2-gem-dihalovinylthiophenols in 24−73% yields (Scheme 61). This protocol involved an intramolecular Scheme 61. Palladium-Catalyzed Carbonylative Synthesis of Benzothiophenes

with different types of coligands in the carbonylative annulation of 2-iodophenol with phenylacetylene to afford the respective flavone. Complexes with an N-phenylimidazole coligand showed the best activity and also afforded high yields when the substrate scope was extended to other aryl or pyridyl acetylenes. In addition, this catalyst was also efficient in the carbonylative annulation of 2-iodoaniline with acid chlorides, giving 2-substituted 4H-3,1-benzoxazin-4-ones in good yields. Furthermore, this Pd−N-heterocyclic carbene (NHC) complex also proved to be an efficient catalyst for the hydroxycarbonylation of iodobenzenes at low catalyst loading and under low CO pressure.194 Alternatively, Miao and Yang reported a novel method for the preparation of flavones in 2000.195 Various flavones were easily synthesized via palladium-catalyzed carbonylative annulation of iodophenol acetates with terminal acetylenes in high yields (Scheme 63). This reaction provides also the possibility

C−S coupling/intermolecular carbonylation cascade sequence and allowed for access to various highly functionalized benzo[b]thiophenes.

4. PALLADIUM-CATALYZED CARBONYLATIVE SYNTHESIS OF SIX-MEMBERED HETEROCYCLES In the previous section, we summarized the palladium-catalyzed carbonylative synthesis of five-membered heterocycles. Some closely related six-membered heterocycles have been mentioned there and will not be further described in this section. 4.1. Palladium-Catalyzed Carbonylative Synthesis of Six-Membered Oxygen-Containing Heterocycles

Flavones are a major group of secondary metabolites found throughout plants and have shown a wide variety of biological activities.187 Among all the known procedures, the palladiumcatalyzed cyclocarbonylation of o-iodophenols with terminal acetylenes is one of the most straightforward processes. As early as in 1990, Kalinin and co-workers reported the first examples of this type of reaction.188 They used 1 mol % of PdCl2(DPPF) (DPPF = diphenylphosphinoferrocene) as catalyst under 20 bar of CO at 120 °C in Et2NH, which acted both as base and solvent. About 50−81% of the corresponding flavones were formed in the presence of 2 equiv of aromatic or aliphatic acetylenes. Applying the same reaction conditions, Torii, Kalinin, and co-workers used o-iodoanilines as substrates for the preparation of quinolones.189 In this report, they showed detailed studies on the catalyst system, solvent, base, temperature, and pressure. In the past decade, palladium-catalyzed carbonylative syntheses of flavones have been carried out at room temperature, under balloon pressure of CO, and even using water as solvent.190 Simple PdCl2 (5 mol %) and PPh3 (10 mol %) were used as the catalyst system with NEt3 as base and a slight excess of aliphatic acetylenes, and flavones were produced in 35−95% yields. Furthermore, Chen and coworkers described the reaction of o-iodophenols with ethynylferrocene.191 Here, CuI (4 mol %) was used as additive together with 4 mol % of Pd(PPh3)4, using K2CO3 as base in toluene at 80 °C, giving the products in 74−80% yields. A onepot Sonogashira-carbonylation annulation reaction of aryl halides, ethynyltrimethylsilane, and o-iodophenols was also developed by Awuah and Capretta.192 By applying microwaves, the products were isolated in 46−71%. The group of Alper performed this reaction in phosphonium salt-based ionic liquids.193 No ligand was needed in this efficient and selective

Scheme 63. Palladium-Catalyzed Carbonylation of Iodophenol Acetates

for a combinatorial synthesis of flavones on solid supports. Following a similar concept, Martin and co-workers applied the carbonylative Sonogashira reaction in the total synthesis of luteolin as the key step.196 In addition to the palladium-catalyzed carbonylative coupling of o-iodophenols with acetylenes, its coupling with allenes was also developed. In 1997, Okuro and Alper reported such a methodology.197 In a regioselective process, benzopyranones were produced in fair to high yields (Scheme 64). The group of Grigg improved this methodology to be performed at atmospheric CO pressure with Pd(PPh3)4 (5 mol %) as catalyst and K2CO3 as base.198 They also extended the substrate Scheme 64. Palladium-Catalyzed Carbonylative Synthesis of Benzopyranones

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terminal alkynes.203 In the presence of a catalytic amount of Pd(PPh3)4, lactones were produced in good yields (Scheme 66b). Similar products could also be synthesized by palladiumcatalyzed carbonylation of iodoalkenes. This was demonstrated in the total synthesis of manoalide, a marine sesterterpenoid with anti-inflammatory properties.204 For the synthesis of six-membered lactones, Kalck and coworkers described an interesting cyclocarbonylation of olefinic alcohols in 1993. 205 By applying PdCl 2 (PPh 3 ) 2 or PdCl2(PPh3)2/SnCl2·2H2O as the catalytic system at 40 bar of CO and 100 °C, lactones were produced from isopulegol. With the same catalyst, allylbenzenes, propenylbenzenes, and monoterpenes were converted into the corresponding esters. Later on, they conducted this reaction in an asymmetric manner and found that Pd(H)(SnCl3)L2 is the key active catalyst species.206 Dong and Alper reported that the cyclocarbonylation of oisopropenylphenols with CO (35 psi) and H2 (7 psi) using Pd(OAc)2 and (+)-DIOP [DIOP = (2,2-dimethyl-1,3-dioxolane-4,5-diylbismethylene)bisdiphenylphosphine] as the chiral catalyst afforded 3,4-dihydro-4-methylcoumarins in 60−85% yield with up to 90% enantiomeric excess (Scheme 67a).207They also used a PCP-type palladium(II) catalyst

scope to o-iodoanilines. Meanwhile, the combination of this reaction with Diels−Alder reactions and cyclocondensations with hydrazine were also realized.199 As shown previously, the reaction of o-iodophenols with terminal acetylenes and CO gives flavones selectively. However, different products can be observed in the case of internal alkynes. In 2000, Kadnikov and Larock reported that the palladium-catalyzed annulation of internal alkynes with oiodophenols in the presence of CO resulted in exclusive formation of coumarins.200 No isomeric chromones have been observed. The best reaction conditions utilized 2-iodophenol, 5 equiv of alkyne under 1 atm of CO in the presence of 5 mol % Pd(OAc)2, 2 equiv of pyridine, and 1 equiv of n-Bu4NCl in DMF at 120 °C. The use of a sterically unhindered pyridine base was essential to achieve high yields (Scheme 65). A variety Scheme 65. Palladium-Catalyzed Carbonylative Synthesis of Coumarins

Scheme 67. Palladium-Catalyzed Intramolecular Carbonylation of Hydroxyalkenes

of 3,4-disubstituted coumarins containing alkyl, aryl, silyl, alkoxy, acyl, and ester groups were prepared in moderate to good yields. Moreover, mixtures of regioisomers were obtained when unsymmetrical alkynes were employed. 2-Iodophenols with electron-withdrawing and electron-donating substituents and 3-iodo-2-pyridone are effective in this annulation process. The reaction is believed to proceed via oxidative addition of 2iodophenol to Pd(0), insertion of the alkyne into the aryl− palladium bond, CO insertion into the resulting vinylic carbon−palladium bond, and nucleophilic attack of the phenolic oxygen on the carbonyl carbon of the acylpalladium complex with simultaneous regeneration of the Pd(0) catalyst. Notably, in this annulation process the intermolecular insertion of an alkyne into the aryl−C−Pd bond was preferred compared to CO insertion. Afterward, Cao and Xiao described this reaction under microwave irradiation, but the selectivity problem was not resolved. Hence, coumarins and flavones were obtained.201 Li, Yu, and Alper developed also an efficient ionic liquidbased protocol for the preparation of highly substituted endocyclic enol lactones via carbonylations of alkynes and 1,3-diketones.202 The reactions proceeded in excellent regioselectivity and in reasonably good yields (Scheme 66a). The catalyst system could be recycled five times with only modest loss of catalytic activity. More recently, Wu and Hua reported a similar reaction of 1,3-cyclohexanediones and

immobilized on silica and silica-supported dendrimers for the production of five- or seven-membered ring lactones. From 2allylphenols, the corresponding lactones were produced in good selectivity and high yields. These immobilized catalysts were stable toward oxygen and moisture and could be recycled by simple filtration in air. Apparently, they combine the advantages of heterogeneous and homogeneous catalysts.208 Another efficient palladium-catalyzed domino reaction for the formation

Scheme 66. Palladium-Catalyzed Carbonylation of 1,3-Diketones

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nitrobenzenesulfenate under CO pressure gave the δ-lactone in moderate yield. Zhou and co-workers demonstrated a regioselective boroxarene formation from 2-hydroxybiphenyl and 2-methoxybiphenyl under mild conditions.217 These synthetic intermediates were converted further on to 3,4-benzocoumarins through CO insertion. Application of this chemistry to commercially available 3′-methyl-biphenyl-2-ylamine resulted in an efficient two-step synthesis of phenaglydon. However, in all the reactions described, 1 equiv of Pd(OAc)2 was needed for acceptable yields (Scheme 70). In 2009, Willis’s group demonstrated that in situ-generated enolates can be employed as intramolecular nucleophiles in palladium-catalyzed aryl-carbonylation reactions to give the corresponding isocoumarins.218 At 1 bar of CO, good yields were achieved with both cyclic and acyclic ketones as substrates. Later on, they also used this methodology in a concise synthesis of the natural product thunberginol A (Scheme 71). Most recently, Yu and co-workers developed a palladiumcatalyzed carbonylation of phenethyl alcohols, which gives access to the related saturated isochromanones.219 Moderate to excellent yields of the desired products were achieved via CH activation by using amino acid ligands and overstoichiometric amounts of silver acetate (Scheme 72). The synthetic value of this method was proven by a short synthesis of a histaminerelease inhibitor. Meanwhile, a comprehensive study of the synthesis of isoquinoline and isocoumarin derivatives was performed. Intramolecular C−N and C−O reductive coupling took place under relatively mild conditions.220 In 2008, Giri and Yu reported a selective palladium-catalyzed C−H activation of aromatic carboxylic acids too.221 In the presence of a catalytic amount of Pd(OAc)2, benzoic and phenylacetic acid derivatives were converted into orthosubstituted dicarboxylic acids in good yields (Scheme 73). The authors were also able to characterize the initially formed cyclometalation complex by X-ray. It should be noted that related carbonylation products can be produced from 1,8diiodonaphthalene.222 Cyclic carbonates represent an important class of carbonyl compounds with interesting potential applications in the chemical industry.223 Hence, Gabriele’s group reported an efficient method for the oxidative carbonylation of 1,2- and 1,3diols to five- and six-membered cyclic carbonates (Scheme 74).224 In the presence of their previously described PdI2-based system, 1,2-diols underwent an oxidative carbonylation process to afford 5-membered cyclic carbonates in high yields (84− 94%) and with good efficiency for this kind of reaction (up to ca. 190 mol of product per mol of PdI2). Under similar conditions, 6-membered cyclic carbonates were obtained for the first time through a direct catalytic oxidative carbonylation of 1,3-diols (66−74% yields). Obviously in this work no oxidative addition of the active palladium complex to a C−X bond takes place. Instead, activation of carbon monoxide followed by nucleophilic attack of the alcohol and subsequent reductive elimination provides the cyclic carbonates.

of chromans and benzodioxins was described by Tietze and coworkers starting from the alkenes and allyl phenyl ethers (Scheme 67b).209 The same class of compounds was also prepared by intramolecular alkoxycarbonylations of alkenes,210 which was also applied in the total synthesis of leucascandroolide A and polyketides.211 Already in 1978, the group of Norton reported the cyclocarbonylation of 2-exo-ethynyl-7-syn-norbornanol to αmethylene γ-lactone in moderate yield.212 Two decades later, Cao, Xiao, and Alper described a palladium-catalyzed double carbonylation and cyclization reaction of enynols with thiols to form thioester-containing 6-membered ring lactones with excellent selectivity and in moderate to good yields (Scheme 68a).213 They also conducted the reaction in ionic liquids, but Scheme 68. Palladium-Catalyzed Carbonylation of Enynols

only monocarbonylated six-membered lactones were formed. Good selectivity and high yield were obtained, and the recyclability of the catalyst system was also tested (Scheme 68b).214 Negishi and co-workers reported the reaction of aryl and alkenyl halides with acidic ketones.215 In the presence of CO (40−45 bar), NEt3 (1−2 equiv), and 5 mol % of PdCl2(PPh3)2 in DMF at 100 °C, the corresponding enol carboxylates were formed. In the case where alkenyl halides were used, the initially formed products can cyclize to give the corresponding lactones. Ryu and co-workers reported the selective cyclocarbonylation of saturated alcohols to δ-lactones in which lead tetraacetate (LTA) was used as a one-electron oxidant to generate highly reactive alkoxyl radicals.216 The mechanism of this remote carbonylation likely involves (i) alkoxyl radical generation via LTA oxidation of the saturated alcohol, (ii) conversion of this alkoxyl radical to a δ-hydroxyalkyl radical by 1,5-hydrogen-transfer reaction, (iii) CO trapping of the δhydroxyalkyl radical yielding an acyl radical, and (iv) oxidation and cyclization of the acyl radical to give the final δ-lactone. Carbonylations of five classes of saturated alcohols, namely, primary alcohols having primary δ-carbons, primary alcohols having secondary δ-carbons, primary alcohols having tertiary δcarbons, secondary alcohols having primary δ-carbons, and secondary alcohols having secondary δ-carbons, were carried out to afford δ-lactones in moderate to good yields (Scheme 69). A metal salt-free system was also tested for a special substrate derived from a tertiary alcohol having a secondary δcarbon. Here, the photolysis of the respective alkyl 4-

4.2. Palladium-Catalyzed Carbonylative Synthesis of Six-Membered Nitrogen-Containing Heterocycles

Scheme 69. Cyclocarbonlyation of Aliphatic Alcohols

For the preparation of various quinoline derivatives, the palladium-catalyzed carbonylative coupling of 2-haloanilines with terminal alkynes offers straightforward access. Thus, as early as in 1991, Torii and co-workers reported this type of V


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Scheme 70. Palladium-Mediated Synthesis of 3,4-Benzocoumarins

reported the same process by using 10 mol % of PdCl2(DPPF) in Et2NH under 20 bar of CO at 120 °C,226 which was later applied by Haddad et al. in the synthesis of BILN 2061 derivatives (Scheme 75a).227 Alternatively, the palladiumcatalyzed reaction of 3-(2-haloarylamino)prop-2-enoates leads to quinoline derivatives.228 A similar concept was used by Mü l ler and co-workers in their elegant synthesis of meridianins.229 On the basis of a carbonylative Sonogashira reaction and a subsequent cyclocondensation process, 2,4,6trisubstituted pyrimidines were produced that were further converted into meridianins. In 1990, Torii’s group developed the palladium-catalyzed carbonylation of 3-substituted 3-(2haloarylamino)prop-2-enoates. Various 2-substituted 1,4-dihydro-4-oxo-quinoline-3-carboxylates were produced in good yields from the corresponding enoates (Scheme 75b). A study from Ye and Alper showed that 2-iodoanilines and allenes gave similar quinoline derivatives under carbonylation conditions.230 This palladium-catalyzed cyclocarbonylation reaction of o-iodoanilines with allenes and CO in 1-butyl-3-methylimidazolium hexafluorophosphate afforded 3-methylene-2,3dihydro-1H-quinolin-4-ones in moderate to excellent yields under a low pressure (5 bar) of CO (Scheme 75c). As shown by the authors previously, the ionic liquid enhanced the efficiency of the cyclocarbonylation reaction. The recyclability of the system of ionic liquid/catalyst/ligand was also demonstrated. Quinazolines as another class of important six-membered nitrogen-containing heterocycles find numerous applications in drugs. In 1987, Tilley and co-workers reported an interesting protocol for their synthesis.231 Starting from 5-substituted 2-(2bromoanilino)-pyridine, pyrido[2,1-b]quinazolines were produced under carbonylative conditions. The reaction mechanism was believed to proceed through an acyl palladium species, which undergoes nucleophilic attack by the pyridine nitrogen, leading to ring-closure with loss of palladium(0) and a proton. The methodology has been shown to be compatible with a variety of functional groups including amides, primary alcohols, aromatic amines, and heteroaromatic rings. Hence, it allows for a flexible selection of substituents on the ultimate pyrido[2,1b]-quinazoline ring (Scheme 76a). Larksarp and Alper developed a catalytic system for the cyclocarbonylation of o-iodoanilines with heterocumulenes.232 By applying a palladium acetate−bidentate phosphine catalyst system at 70−100 °C, the corresponding 4(3H)-quinazolinone derivatives were obtained in good yields (Scheme 76b). By utilizing o-iodoaniline with isocyanates, carbodiimides, and ketenimines, 2,4-(1H,3H)-quinazolinediones, 2-amino-4(3H)quinazolinones, and 2-alkyl-4(3H)-quinazolinones were obtained, respectively. The nature of the substrates and the electrophilicity of the carbon center of the carbodiimide, as well as the stability of the ketenimine, influenced the product yields of this reaction. Urea-type intermediates are believed to be generated first in situ from the reaction of o-iodoanilines with heterocumulenes. Then, palladium-catalyzed carbonylation and intramolecular cyclization give the products. The same group

Scheme 71. Palladium-Catalyzed Carbonylative Synthesis of Isocumarins

Scheme 72. Palladium-Catalyzed Carbonylative C−H Activation of Phenethyl Alcohols

Scheme 73. Selective Palladium-Catalyzed Carbonylative C− H Activation of Phenylacetic Acids

Scheme 74. Palladium-Catalyzed Oxidative Carbonylation of Diols

carbonylation process.225 In the presence of 5 mol % of palladium catalyst and under 20 bar of CO at 120 °C, 2substituted 1,4-dihydro-4-oxo-quinolines were produced in good yields (Scheme 75). Shortly after, Kalinin and co-workers W


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Scheme 75. Palladium-Catalyzed Carbonylative Synthesis of Quinolinones

Scheme 76. Palladium-Catalyzed Carbonylative Synthesis of Quinazolines

dines to the corresponding quinazolines.236 The reactions were carried out in the presence of 1.0 equiv of CuO as oxidant under atmospheric pressure of CO and provided diversified 2aryl(alkyl)-quinazolin-4(3H)-ones in reasonable to good yields from N-arylamidines, which are readily derived from anilines and nitriles (Scheme 76f). In 2000, Knight and co-workers developed the palladiumcatalyzed decarboxylative carbonylation of 5-vinyloxazolidin-2ones.237 Good yields of 3,6-dihydro-1H-pyridin-2-ones were obtained from the corresponding 5-vinyloxazolidin-2-ones, which are readily prepared from amino acid precursors, by a

also developed carbonylation reactions of N-(2-iodophenyl)N′-phenyl-carbodiimides (Scheme 76c)233 and N,N′-di-oiodophenyl carbodiimides (Scheme 76d)234 to yield the corresponding quinazolines under similar reaction conditions. They also proved that quinazolines could be synthesized from o-iodoanilines, imidoyl chlorides, and CO (Scheme 76e).235 The latter reaction was proposed to proceed via in situ formation of an amidine, followed by oxidative addition, CO insertion, and intramolecular cyclization to give the substituted quinazolin-4(3H)-ones. More recently, Zhu and co-workers developed a palladiumcatalyzed intramolecular C−H carboxamidation of N-arylamiX


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lation of these interesting nitrogen-containing heterocycles. Alternatively, isoquinolinones can also be prepared via palladium-catalyzed carbonylation of diethyl(2-iodoaryl)malonates and imidoyl chlorides.240 Fair to good yields of the corresponding products were obtained using tris(2,6dimethoxyphenyl)phosphine (TDMPP) as ligand (Scheme 78c). In addition, Broggini and co-workers developed the palladium-catalyzed cyclocarbonylation of N-allylamides and 2iodobenzoic acids.241 At high pressure (100 bar of CO), isoquinolinones were produced in good yields (Scheme 78d). The group of Larock developed a palladium-catalyzed annulation of internal alkynes with N-substituted o-iodoanilines under 1 bar of carbon monoxide, which resulted in the formation of 3,4-disubstituted 2-quinolones (Scheme 79a).242 In this reaction the nature of the substituent on the nitrogen is crucial for obtaining high yields of the 2-quinolones. The best results are obtained using alkoxycarbonyl, p-tolylsulfonyl, and trifluoroacetyl substituents. Notably, the N-protecting group is lost during the course of the reaction. A variety of internal alkynes, bearing alkyl, aryl, heteroaryl, hydroxyl, and alkoxyl substituents, were effective in this process. Electron-rich and electron-poor N-substituted o-iodoanilines, as well as heterocyclic analogues, could be employed as annulating agents. Related 2-iodophenols were also tested under the reaction conditions; however, the corresponding coumarins were formed in low yields. Willis and co-workers developed a palladium-catalyzed carbonylative coupling of 2-(2haloalkenyl)aryl halides with primary amines.243 Quinolones were produced in good yields under atmospheric pressure of CO (Scheme 79b). More recently, a palladium-catalyzed annulation of benzamides with [60]fullerene was also reported.244 This reaction proceeded through direct sp2 C−H bond activation to form a 7-membered palladacycle interm e d i a t e , w h i c h l e d t o t h e f o r m a t io n o f [ 6 0 ] fulleroisoquinolinones in moderate yields (8−64% based on recovered C60). At the same time, palladium-catalyzed cyclocarbonylations of arylethylamines were also reported (Scheme 79c and 79d),245 and the total synthesis of teleocidin

palladium-catalyzed decarboxylative carbonylation process (Scheme 77). Scheme 77. Palladium-Catalyzed Carbonylative Synthesis of Pyridinones

In 1997, Alper and co-workers reported the cyclocarbonylation of 2-vinylanilines.238 In the presence of chiral phosphines as ligands, quinolinones were produced in a stereoselective manner (Scheme 78a). Moderate to excellent yields of quinolinones with 20−54% of ee were achieved. Recently, they developed a new route to ring-fused substituted oxazoloand pyrazoloisoquinolinones via a three-component cascade process through a one-pot carboxamidation/aldol-type condensation reaction sequence.239 A range of ring-fused oxazoloisoquinolinones and pyrazoloisoquinolinones were obtained from a variety of active methylene compounds (Scheme 78b). The products of these cascade reactions contain different functional groups that can be further functionalized. Hence, this methodology enables further molecular manipu-

Scheme 78. Palladium-Catalyzed Carbonylative Synthesis of Isoquinolinones

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carbonylative synthesis of isoquinoline-4-carboxylic esters and isochromene-4-carboxylic esters. Again they used PdI2 as catalyst and performed the reactions in alcohols (Scheme 80b).249 The group of Rossi developed a palladium-catalyzed carbonylative synthesis of 2-aryl-4-aminoquinolines and 2-aryl4-amino[1,8]naphthyridines.250 This palladium-catalyzed domino reaction started from aryl iodides and amines leading to 2aryl-4-amino-quinolines and 2-aryl-4-amino[1,8]naphthyridines (Scheme 80c). The scope of the reaction was examined using two 2-ethynylarylamines, four aryl iodides, and 10 primary amines as substrates. The selection of the appropriate catalytic system was achieved by testing several palladium/phosphine systems and allowed researchers to override previously reported drawbacks associated with the use of primary amines in related reactions. An enantioselective synthesis of tetrahydropyrrolo[1,2-c]pyrimidine-1,3-diones via palladium-catalyzed intramolecular oxidative aminocarbonylation was described by Sasai and coworkers.251 The carbon−carbon double bond of suitable substituted N-alkenylureas reacted intramolecularly with one of the nitrogen atoms in the presence of a palladium catalyst under a carbon monoxide atmosphere (Scheme 81). Notably,

Scheme 79. Palladium-Catalyzed Carbonylative Synthesis of Quinolinone Derivatives

Scheme 81. Palladium-Catalyzed Carbonylation of Alkenylureas

B4 was achieved (Scheme 79e).246 Mechanistic studies on this kind of carbonylation were performed by the group of Vicente and Saura-Llamas.247 In 2002, Dai and Larock reported the palladium-catalyzed cyclocarbonylation of o-(1-alkynyl)benzaldimines.248 A number of 3-substituted 4-aroylisoquinolines have been prepared in good yields by treating N-tert-butyl-2-(1-alkynyl)benzaldimines with aryl halides in the presence of CO and a palladium catalyst (Scheme 80a). Synthetically this methodology provides a

the use of a chiral spiro bis(isoxazoline) ligand (SPRIX) was essential to obtain the desired products in optically active forms. In comparison with the coordination ability of other known ligands, the peculiar character of SPRIX originates from two structural characteristics: low σ-donor ability of the isoxazoline coordination site and rigidity of the spiro skeleton.

Scheme 80. Palladium-Catalyzed Carbonylative Synthesis of Quinolines

4.3. Palladium-Catalyzed Carbonylative Synthesis of Other Six-Membered Heterocycles

In 1996, Cacchi, Fabrizi, and Marinelli reported a novel synthesis of benzoxazinones.252 Starting from 2-iodoaniline and unsaturated halides or triflates in the presence of K2CO3 and Pd(PPh3)4 under atmospheric pressure of CO, 2-aryl- and 2vinyl-4H-3,1-benzoxazin-4-ones were produced in good yields (Scheme 82a). This methodology was later applied in the synthesis of a new potent inhibitor of human leukocyte elastase.253 Three years later, Larksarp and Alper developed the related palladium-catalyzed carbonylative coupling of 2-iodoanilines with acid chlorides to benzoxazinones.254 In their work 2substituted-4H-3,1-benzoxazin-4-ones were produced in good to excellent yields (Scheme 82b). The reaction is believed to proceed via in situ amide formation followed by oxidative addition to Pd(0), CO insertion, and intramolecular cyclization to form the 2-substituted-4H-3,1-benzoxazin-4-one derivatives. The same reaction was done under the assistance of microwaves, applying Pd/C as catalyst.255 In addition, it was proved that benzoxazinone derivatives can be prepared from 2-

simple and convenient route to isoquinolines containing aryl, alkyl, or vinyl substituents at C-3 and an aroyl group at C-4 of the isoquinoline ring. The reaction is believed to proceed via cyclization of the alkyne containing a proximate nucleophilic center promoted by an acylpalladium complex. More recently, Gabriele and co-workers applied the same substrates in the Z


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Scheme 82. Palladium-Catalyzed Carbonylative Synthesis of Benzoxazinones

iodoanilines directly (Scheme 82c).256 More interestingly, this class of compounds was produced from the corresponding aniline derivatives as well, via C−H activation, and even at room temperature (Scheme 82d and 82e).257 Recently, our group developed a convenient and general palladium-catalyzed carbonylative synthesis of 2-arylbenzoxazinones.258 Starting from 2-bromoanilines and aryl bromides, the corresponding products were isolated in good yields (65−91%; Scheme 82f). Moreover, a one-pot synthesis of 2,3-diarylquinazolinones was also demonstrated. Xiao and Alper reported an efficient procedure for the synthesis of thiochromanones.259 Here, the desired products were obtained by palladium-catalyzed carbonylative ringforming reactions of 2-iodothiophenol derivatives with allenes and carbon monoxide. These reactions afforded thiochroman-4ones in good to excellent isolated yields with fairly high regioselectivity (Scheme 83). This catalytic heteroannulation may involve regioselective addition of the sulfur moiety on the more electrophilic carbon center of the allene, arylpalladium formation, CO insertion, subsequent intramolecular cyclization, and then reductive elimination. The regioselectivity is proposed to be governed by electronic effects. A novel synthesis of 3-substituted-3,4-dihydro-2H-1,3benzothiazin-2-ones was described by Alper and co-workers

Scheme 83. Palladium-Catalyzed Carbonylative Synthesis of Thiochromanones

in 2008. The strategy relied on an unusual palladium-catalyzed carbonylation of 2-substituted-2,3-dihydro-1,2-benzisothiazoles to give the corresponding 3,4-dihydro-2H-1,3-benzothiazin-2one derivatives in general in good yields (Scheme 84a). Finally it is worth mentioning that the group of Gabriele described a one-step synthesis of 2-[(dialkylcarbamoyl)methylene]-2,3-dihydrobenzo[1,4]-dioxines and (Z)-3[(dialkylcarbamoyl)methylene]-3,4-dihydro-2H-benzo[1,4]oxazines.260 Starting from readily available 2-prop-2-ynyloxyphenols and 2-prop-2-ynyloxyanilines, the corresponding products were obtained in good yields via tandem PdI2catalyzed oxidative aminocarbonylation and intramolecular conjugate addition (Scheme 84b). AA


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aminocarbonylation was also applied in the synthesis of C-14labeled heterocycles.264 More recently, Alper and co-workers described a convenient protocol for the synthesis of substituted benzazepine derivatives (Scheme 87).265 This protocol is based on the sequential

Scheme 84. Palladium-Catalyzed Carbonylative Synthesis of Benzothiazinone, Benzodioxine, And Benzoxazine Derivatives

Scheme 87. Palladium-Catalyzed Carbonylative Synthesis of Seven-Membered Lactams

5. PALLADIUM-CATALYZED CARBONYLATIVE SYNTHESIS OF OTHER HETEROCYCLES Although most of the carbonylative cyclizations focused on the formation of five- and six-membered rings, there are also a few examples known for the preparation of larger rings; for example, in 1999, the palladium-catalyzed synthesis of a 2,3,4,5-tetrahydro-1H-2,4-benzodiazepine-1,3-dione derivative was reported by Bocelli et al.261 Using 1-butyl-1(o-iodobenzyl)-3-phenylurea as starting material at 80 °C under CO pressure, 91% of the desired product was isolated (Scheme 85).

palladium-catalyzed allylic amination and a subsequent intramolecular carbonylation reaction. The substrates were obtained by Baylis−Hillman reaction. The success of the latter palladium-catalyzed one-pot reaction opened the possibility of a new synthetic route for the formation of a number of biologically interesting products containing the benzazepine ring system. Recently, an efficient method for the synthesis of 1,4-benzoand pyrido-oxazepinones was also disclosed.266 This reaction proceeds via a domino process through one-pot ring-opening/ carboxamidation reaction sequences of N-tosylaziridines with 2halophenols/pyridinol under phase-transfer conditions (benzyltriethylammonium chloride, TEBA). The method worked with a range of N-tosylaziridines and 2-halophenols/pyridinol to provide facile access to a variety of 1,4-benzo- and pyridooxazepinones (Scheme 88a). The authors also performed the reaction with 2-iodothiophenols, which led to 1,4-benzothiazepin-5-ones in good yields (Scheme 88b).267 The group of Kalck reported a chemo- and regioselective procedure for the production of a nine-membered lactone.268 Starting from dihydromyrcenol in the presence of PdCl2(PPh3)2/SnCl2·2H2O and molecular sieves, cyclocarbonylation occurred and gave the lactone as final product (Scheme 89). Cho and Larock developed a palladium-catalyzed intramolecular cyclocarbonylation of hydroxyl-substituted 3-iodofurans, leading to the corresponding lactone-containing furans.269 The 3-iodofurans are readily prepared by iodocyclization of 2(1-alkynyl)-2-alken-1-ones in the presence of various diols. Meanwhile, a neat one-pot synthesis of a cryptand was developed by using a palladium-catalyzed carbonylation reaction (Scheme 90).270 Finally, the group of Takahashi and Doi also applied elegantly carbonylations for the preparation of macrosphelide and related macrolactams.271

Scheme 85. Palladium-Catalyzed Carbonylative Synthesis of Benzodiazepinedione

Lu and Alper developed a more general and efficient method for the synthesis of oxygen-, nitrogen-, or sulfur-containing medium ring-fused heterocycles with recyclable palladiumcomplexed dendrimers on silica as catalysts.262 Their process tolerates a wide array of functional groups, including halide, ether, nitrile, ketone, and ester. The dendritic catalysts showed high activity, affording the heterocycles in excellent yields (Scheme 86). Importantly, these catalysts were easily recovered Scheme 86. Palladium-Catalyzed Carbonylative Synthesis of Benzodiazepinediones

6. SUMMARY We have tried to summarize in this review the major developments of palladium-catalyzed carbonylative syntheses of heterocycles in the last two decades. In general, in these reactions inexpensive carbon monoxide is incorporated into the parent substrate by insertion of CO into an activated C−X bond (X = Cl, Br, I) in the presence of palladium catalysts. The resulting acyl palladium complexes will react further both inter-

by simple filtration in air and could be reused up to the eight cycles with only a slight loss of activity. Recently, the same authors used PdI2 and 1,3,5,7-tetramethyl-6-phenyl-2,4,8-trioxa6-phospha-adamantane (Cytop 292) as an in situ-formed palladium complex for the intramolecular carbonylation of substituted 2-(2-iodophenoxy)anilines.263 A series of substituted dibenzo[b,f ][1,4]oxazepin-11(10H)-ones were prepared in good yields under mild reaction conditions. This type of AB


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Scheme 88. Palladium-Catalyzed Carbonylation of N-Tosyl Aziridines

palladium by other metals), what are the goals for the future in this area? From a synthetic point of view, advancements of carbonylative CH-activation processes are highly desirable. Here, more environmentally benign procedures can be foreseen if the regioselectivity can be controlled. Moreover, an important issue would be to replace the toxic and gaseous carbon monoxide by more benign and practical analogues. In this respect the use of carbon dioxide in the presence of a benign reductant would be a very interesting option. We encourage synthetic chemists to address these challenging goals.

Scheme 89. Palladium-Catalyzed Carbonylative Synthesis of a Nine-Membered Lactone

and intramolecularly with a variety of O-, N-, C-, and Snucleophiles. Advantageously, such processes allow for a substantial increase in molecular complexity of the substrate. The basis for most of today’s known methodologies was laid by R. F. Heck and co-workers already in the mid-1970s in his work on amino- and alkoxycarbonylations of simple aryl bromides, and therefore the term Heck carbonylation is sometimes used for them. Although most synthetic organic chemists are somewhat reluctant to use carbon monoxide in coupling reactions because of the necessity to use high-pressure equipment, the ongoing success of palladium-catalyzed carbonylations is documented by an increasing number of synthetic applications and publications in the past years. Clearly, the main focus of the work in most academic laboratories in the past was to increase the tool box of synthetic methodologies. Hence, today a range of carbonylative cyclizations is available. Notably, some of the procedures allow for an efficient assembly toward potentially bioactive heterocycles, which are otherwise significantly more difficult to access. Despite an extensive amount of ligands and catalysts nowadays (commercially) available as well as substantial know-how in catalyst optimization strategies, for most carbonylation reactions the catalytic efficiency (total catalyst turnover numbers and turnover frequencies) is still relatively low. Here, further improvements are desired, and it is most likely that the recent advances in the development of ligands and their improved synthetic abilities on the lab scale will result in improved processes also for kg-syntheses in the process development of the pharmaceutical industry. Apart from further catalyst improvements (improved efficiencies; replacing noble

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]; matthias.beller@catalysis. de. Notes

The authors declare no competing financial interest. Biographies

Xiao-Feng Wu was born in 1985 in China. He studied chemistry in Zhejiang Sci-Tech University (China), where he got his Bachelor’s degree in science (2007). In the same year, he went to Rennes 1 University (France) and worked with Prof C. Darcel on iron-catalyzed

Scheme 90. Palladium-Catalyzed Carbonylative Synthesis of H3L

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of Science. He is married to Dr. Anja Fischer-Beller, and they have two sons.

reactions. After he earned his Master’s degree in 2009, he joined Matthias Beller’s group in Leibniz-Institute for Catalysis (Germany), where he completed his Ph.D. thesis in January 2012. Then he started his independent research at ZSTU and LIKAT. His research interests include carbonylation reactions, heterocycles synthesis, and the catalytic application of cheap metals. He also was a fellow of the Max-Buchner-Forschungsstiftung.

ACKNOWLEDGMENTS The authors thank the state of Mecklenburg−Vorpommern and the Bundesministerium für Bildung und Forschung (BMBF) for financial support. The authors also thank Drs. Martin Nielsen and Marko Hapke (both LIKAT) for valuable advice on this review. REFERENCES (1) For selected reviews on palladium-catalyzed coupling reactions, see: (a) Alonso, F.; Beletskaya, I. P.; Yus, M. Tetrahedron 2008, 64, 3047. (b) Rollet, P.; Kleist, W.; Dufaud, V.; Djakovitch, L. J. Mol. Catal. 2005, 241, 39. (c) Zapf, A.; Beller, M. Chem. Commun. 2005, 431. (d) Frisch, A.; Beller, M. Angew. Chem., Int. Ed. 2005, 44, 674. (e) Negishi, E.; Anastasia, L. Chem. Rev. 2003, 103, 1979. (f) Surry, D. S.; Buchwald, S. L. Angew. Chem., Int. Ed. 2008, 47, 6338. (g) Doucet, H.; Hierso, J.-C. Angew. Chem., Int. Ed. 2007, 46, 834. (h) Nicolaou, K. C.; Bulger, P. G.; Sarlah, D. Angew. Chem., Int. Ed. 2005, 44, 4442. (i) Roglans, A.; Pla-Quintana, A.; Moreno-Manas, M. Chem. Rev. 2006, 106, 4622. (j) Tucker, C. E.; de Vries, J. G. Top. Catal. 2002, 19, 111. (k) Nakao, Y.; Hiyama, T. Chem. Soc. Rev. 2011, 40, 4893. (l) Enthaler, S.; Company, A. Chem. Soc. Rev. 2011, 40, 4912. (m) Kambe, N.; Iwasaki, T.; Terao, J. Chem. Soc. Rev. 2011, 40, 4937. (n) Knappke, C. E. I.; Jacobi von Wangelin, A. Chem. Soc. Rev. 2011, 40, 4948. (o) Molnar, A. Chem. Rev. 2011, 111, 2251. (p) Selander, N.; Szabo, K. J. Chem. Rev. 2011, 111, 2048. (q) Jana, R.; Pathak, T. P.; Sigman, M. S. Chem. Rev. 2011, 111, 1417. (2) (a) Wu, X.-F.; Anbarasan, P.; Neumann, H.; Beller, M. Angew. Chem., Int. Ed. 2010, 49, 9047. (b) Negishi, E. Angew. Chem., Int. Ed. 2011, 50, 6738. (c) Suzuki, A. Angew. Chem., Int. Ed. 2011, 50, 6722. (3) (a) Schoenberg, A.; Bartoletti, I.; Heck, R. F. J. Org. Chem. 1974, 39, 3318. (b) Schoenberg, A.; Heck, R. F. J. Org. Chem. 1974, 39, 3327. (c) Schoenberg, A.; Heck, R. F. J. Am. Chem. Soc. 1974, 96, 7761. (4) (a) Sheldon, R. A. Green Chem. 2007, 9, 1273. (b) Song, J. J.; Reeves, J. T.; Fandrick, D. R.; Tan, Z.; Yee, N. K.; Senanayake, C. H. Green Chem. Lett. Rev. 2008, 1, 141. (c) Tang, S.; Bourne, R.; Smith, R.; Poliakoff, M. Green Chem. 2008, 10, 268. (d) Anastas, P.; Eghbali, N. Chem. Soc. Rev. 2010, 39, 301. (5) For selected reviews on industrial applications, see: (a) Zapf, A.; Beller, M. Top. Catal. 2002, 19, 101. (b) Torborg, C.; Beller, M. Adv. Synth. Catal. 2009, 351, 3027. (c) Busacca, C. A.; Fandrick, D. R.; Song, J. J.; Senanayaka, C. H. Adv. Synth. Catal. 2011, 353, 1815. (6) For selected reviews on heterocycles synthesis, see: (a) Chopade, P. R.; Louie, J. Adv. Synth. Catal. 2006, 348, 2307. (b) Hemming, K. Annu. Rep. Prog. Chem., Sect. B: Org. Chem. 2010, 106, 136. (c) Cacchi, S.; Fabrizi, G.; Goggiamani, A. Org. Biomol. Chem. 2011, 9, 641. (d) Arndtsen, B. A. Chem.Eur. J. 2009, 15, 302. (e) Nakamura, I.; Yamamoto, Y. Chem. Rev. 2004, 104, 2127. (f) Patil, N. T.; Yamamoto, Y. Chem. Rev. 2008, 108, 3395. (g) Herrerias, C. I.; Yao, X.; Li, Z.; Li, C. −J. Chem. Rev. 2007, 107, 2546. (h) Cacchi, S.; Fabrizi, G. Chem. Rev. 2011, 111, PR215. (i) Zeni, G.; Larock, R. C. Chem. Rev. 2006, 106, 4644. (j) Brandi, A.; Cicchi, S.; Cordero, F. M. Chem. Rev. 2008, 108, 3988. (k) Schmidt, A.; Beutler, A.; Snovydovych, B. Eur. J. Org. Chem. 2008, 4073. (l) Stokes, B. J.; Driver, T. G. Eur. J. Org. Chem. 2011, 4071. (m) Vizer, S. A.; Yerzhanov, K. B.; Quntar, A. A. A. A.; Dembitsky, V. M. Tetrahedron 2004, 60, 5499. (n) Shaaban, M. R.; ElSayed, R.; Elwahy, A. H. M. Tetrahedron 2011, 67, 6095. (7) For reviews on palladium-catalyzed carbonylations, see: (a) Brennführer, A.; Neumann, H.; Beller, M. Angew. Chem., Int. Ed. 2009, 48, 4114. (b) Brennführer, A.; Neumann, H.; Beller, M. ChemCatChem 2009, 1, 28. (c) Beller, M. Carbonylation of Benzyland Aryl-X Compounds. In Applied Homogeneous Catalysis with Organometallic Compounds, 2nd ed; Cornils, B., Herrmann, W. A., Eds.; Wiley-VCH: Weinheim, Germany, 2002; pp 145−156. (d) Skoda-Földes, R.; Kollár, L. Curr. Org. Chem. 2002, 6, 1097.

Helfried Neumann studied chemistry at the University of Würzburg, Germany. He then moved to the group of Priv.-Doz. Dr. Herges/Prof. Schleyer at the University of ErlangenNürnberg, where he obtained his Ph.D. in 1995 working on the synthesis of tetradehydrodianthracene. In 1996, he became an associate researcher at the Institute for Organic Catalysis, Rostock (IfOK), and the TU Darmstadt. Since 1998 he has been a project leader in the group of Matthias Beller. His research interests include multicomponent reactions, carbonylations, and transition metal-catalyzed synthesis of fine chemicals.

Matthias Beller, born in 1962, studied chemistry in Göttingen, Germany, where he completed his Ph.D. thesis in 1989 in the group of Prof. Tietze. Then, he spent 1 year in the group of Prof. Sharpless at MIT, U.S.A. From 1991 to 1995, Beller was an employee of Hoechst AG in Frankfurt, Germany. In 1996, he moved to the Technical University of Munich as Professor for Inorganic Chemistry. In 1998, he relocated to Rostock to head the Institute for Organic Catalysis (IfOK). Since 2006 Matthias Beller is director of the Leibniz-Institute for Catalysis. His scientific work has been published in around 530 publications, and >90 patent applications have been filed in the last decade. Matthias Beller has received several awards including the OttoRoelen Medal, the Leibniz-Price, and the German Federal Cross of Merit. Most recently, he received the first “European Price for Sustainable Chemistry” and the “Paul-Rylander Award” of the Organic Reaction Catalysis Society, U.S.A. Matthias Beller is a member of the Association for Technical Sciences of the Union of German Academies of Sciences and Humanities, as well as the German National Academia AD


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