Phthalocyanine Metal Complexes in Catalysis - Chemical Reviews

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Phthalocyanine Metal Complexes in Catalysis Alexander B. Sorokin* Institut de Recherches sur la Catalyse et l’Environnement de Lyon IRCELYON, UMR 5256, CNRS−Université Lyon 1, 2 avenue Albert Einstein, 69626 Villeurbanne cedex, France 9.1. Diiron Complexes on Phthalocyanine Platform: Reactivity and Mechanistic Considerations 9.1.1. μ-Oxo Bridged Diiron Phthalocyanines 9.1.2. N-Bridged Diiron Phthalocyanines: Emerging Catalysts for Oxidation and Other Reactions 10. Conclusion Author Information Corresponding Author Notes Biography Acknowledgments Abbreviations References

CONTENTS 1. Introduction 2. Preparation of Homogeneous Phthalocyanine Complexes 3. Preparation of Heterogeneous Phthalocyanine Catalysts 4. Catalytic Applications of Phthalocyanine Metal Complexes in Oxidation 4.1. Oxidation of Aliphatic C−H Bonds in Alkanes 4.1.1. Homogeneous Catalytic Reactions 4.1.2. Heterogeneous Catalytic Reactions 4.2. Oxidation of Methane 4.3. Oxidation of Olefins 4.3.1. Homogeneous Catalytic Reactions 4.3.2. Heterogeneous Catalytic Reactions 4.4. Oxidation of Aromatic C−H Bonds 4.4.1. Homogeneous Catalytic Reactions 4.4.2. Heterogeneous Catalytic Reactions 4.5. Oxidation of Phenols 4.5.1. Homogeneous Catalytic Reactions 4.5.2. Heterogeneous Catalytic Reactions 4.6. Oxidation of Alcohols 4.6.1. Homogeneous Catalytic Reactions 4.6.2. Heterogeneous Catalytic Reactions 4.7. Oxidation of Polysaccharides and Ligninocelluloses 4.8. Oxidation of Sulfur Compounds 4.8.1. Homogeneous Catalytic Reactions 4.8.2. Heterogeneous Catalytic Reactions 4.9. Miscellaneous Oxidations 5. Preparation of Nitrogen-Containing Compounds 5.1. Homogeneous Catalytic Reactions 5.2. Heterogeneous Catalytic Reactions 6. C−C Bond Formation 6.1. Homogeneous Catalytic Reactions 7. Reduction 7.1. Homogeneous Catalytic Reactions 8. Miscellaneous Reactions 9. Mechanisms and Active Species Involved in Reactions Catalyzed by Metal Phthalocyanines © XXXX American Chemical Society

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1. INTRODUCTION Phthalocyanine metal complexes1,2 (MPc’s) are structurally related to porphyrin complexes (Figure 1), which are widely used

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Figure 1. Structures of porphyrin and phthalocyanine complexes.

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by nature in the active sites of enzymes responsible for catalytic aerobic oxidations, reduction and transport of dioxygen, and destruction of peroxides.3 Although phthalocyanines are purely synthetic ligands they can, therefore, be regarded as related to bioinspired chemistry usually associated with porphyrin complexes. Among a large variety of porphyrinoid macrocyclic complexes such as porphyrins, porphyrazines,4 corroles,5 and corrolazines,6 MPc’s are probably the most accessible from a preparation point of view. Porphyrin catalytic chemistry closely relevant to cytochrome P-450 modeling is well-documented.7−9 Synthetic metalloporReceived: January 7, 2013

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dx.doi.org/10.1021/cr4000072 | Chem. Rev. XXXX, XXX, XXX−XXX

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catalytic activity. Different aspects of preparation of supported MPc’s and their recycling are discussed. Investigation of the reaction mechanisms and active species involved is essential for the development of efficient catalysts. Mechanistic background for successful applications of MPc catalysts in a variety of reactions is considered. A special emphasis is put on the single atom bridged diiron phthalocyanine complexes as an emerging class of catalysts.34 Consequently, this review covers different aspects of the catalytic chemistry of phthalocyanine metal complexes. In the first part, general synthetic methods for the preparation of metal phthalocyanines and different strategies for their immobilization are delineated. In the second part, catalytic applications of MPc’s in various reactions are discussed. Homogeneous and heterogeneous versions for each reaction type are given separately. In the third part, mechanistic features determining the catalytic activity of MPc’s are considered. This review covers the literature up to the end of 2012, with emphasis on the recent results.

phyrin complexes have been largely used for a variety of catalytic transformations.10 Not surprisingly, the catalytic properties of metalloporphyrins have been reviewed in numerous books and reviews.11−13 In turn, MPc’s are very attractive as catalysts not only because of structural analogy with porphyrin complexes but also due to their accessibility in terms of the cost and straightforward preparation on a large scale as well as their chemical and thermal stability (Figure 1). These complexes are widely used in different fields of material science, including semiconductor, electrochromic and non-linear optical devices, information storage systems, liquid crystal applications.14−18 One of the most important applications of MPc’s has been in catalysis, including large-scale industrial processes. For instance, the Merox process, referred to as “sweetening” in the petroleum refining industry, involves catalytic oxidation of mercaptans in the presence of sulfonated cobalt phthalocyanines to remove a major part of sulfur from petrol.19 To date, only a limited number of reviews covering selected catalytic applications of MPc’s are available.20−23 Noteworthy, there is a striking contrast between the state of art of the porphyrin and the phthalocyanine catalytic chemistry from a mechanistic point of view. Extensive efforts have been directed to cytochrome P-450 modeling using metalloporphyrins to elucidate the mechanisms of dioxygen activation and the structures of reactive intermediates. Elusive active species involved in oxidation catalytic cycle were proposed, identified, and characterized by different spectroscopic methods. Mechanisms of catalytic oxidation of various organic substrates (e.g., alkanes, olefins, etc.) by cytochrome P-450 and its chemical models based on porphyrin metal complexes were proposed, evidenced, and now generally accepted. Although MPc’s have intensively been studied as catalysts, mostly in the oxidation field, their catalytic chemistry in terms of mechanisms and active species involved in these catalytic oxidations is not yet developed. Recently, much effort has been focused not only on the development of novel catalytic methods using MPc’s but also on the mechanistic studies and characterization of active species involved in the catalysis. In particular, iron tetrasulfophthalocyanine (FePcS) was shown to be able to perform oxidative degradation of recalcitrant chlorinated phenols by H2O2 in aqueous solutions under mild conditions with partial mineralization.24−28 This topic has been developed by many researchers toward degradation of different pollutants29 and has recently been reviewed.30 Hence, the oxidative degradation of pollutants in the presence of MPc’s will not be covered here. Photocatalytic and electrocatalytic processes in the presence of MPc’s are not included in this review. Recent reviews on these specific topics are available.31−33 The use of MPc-based catalysts to prepare valuable products is a very attractive topic which will be the scope of this review. Although the field is dominated by oxidation, the scope of reactions catalyzed by metal phthalocyanine complexes is rapidly expanding. A large range of various transformations including reduction, preparation of nitrogen-containing compounds, and various C−C bond formation reactions can be efficiently catalyzed by MPc’s. The catalytic properties of MPc’s can be used in large-scale processes for the preparation of bulk chemicals as well as for the synthesis of elaborated fine chemicals up to applications in total synthesis. The catalytic properties of MPc’s depend on the metal and complex structure and can be tuned by appropriate structural modifications. Principal synthetic strategies to the preparation of MPc’s are briefly described. Immobilization of MPc’s onto various supports can provide recyclable catalysts with increased

2. PREPARATION OF HOMOGENEOUS PHTHALOCYANINE COMPLEXES The accessibility of the complexes in terms of their price and availability at a large scale plays an important role, in particular, for the economic viability of the processes. Synthetic access to MPc’s is based on the cyclotetramerization of cheap precursors like phthalic acids, phthalic anhydrides, or phthalonitriles in the presence of a metal salt, which functions as a template during the macrocycle formation (Scheme 1). Scheme 1. The Most Important Precursors for the Synthesis of Phthalocyanine Complexesa

a

The possible substituents are omitted for clarity.

Other precursors, e.g., phthalimides, phthaloamides, and 1,3diiminoisoindolines, can be also used. As a rule, regular symmetrical MPc’s can be prepared in one step with good yields that are superior to those of related macrocyclic complexes. The general structure of substituted phthalocyanines and their abbreviations used in this review are given in Scheme 2. To overcome the low solubility of the unsubstituted MPc’s in common organic solvents, substituents should be introduced at the periphery of phthalocyanine core. This can be achieved either by using appropriately substituted precursors or by postsynthetic modification of the phthalocyanine moiety. The former approach is preferred because it allows one to obtain well-identified complexes. Postsynthetic modification of MPc’s typically leads to a mixture of differently substituted complexes. One classical example is sulfonated cobalt phthalocyanine employed in the Merox process, which is prepared by direct sulfonation with an average sulfonation degree of 2. The substituents can be placed in peripheral (2, 3, 9, 10, 16, 17, 23, 24) or nonperipheral (1, 4, 8, B


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other metals, in particular Fe, Ru, and Mn, might provide complexes with interesting catalytic properties. In addition, functional groups at the phthalocyanine moiety are often used for covalent anchoring of the complexes onto supports. The nature of the metal plays an most important role in the catalytic activity. Although phthalocyanine complexes of more than 70 metals have been described, the most interesting MPc’s for catalysis are those with Fe, Co, Cu, Ru, Mn, Cr, Al, and Zn. Thus, the synthesis of different substituted MPc’s allows one to obtain the complexes with tailored catalytic properties. The preparation of watersoluble MPc’s suitable for reactions in aqueous solutions has recently been reviewed.42 Low-symmetry MPc’s containing differently substituted isoindole units can be also prepared,43 but their use in catalysis is still rare. For more detailed information on the synthesis of phthalocyanine metal complexes, the reading of detailed recent reviews35,42,44 is recommended.

Scheme 2. General Structure of Substituted Phthalocyanine Complexes and Their Abbreviations

3. PREPARATION OF HETEROGENEOUS PHTHALOCYANINE CATALYSTS The principal motivation for the preparation of heterogeneous catalysts is the possibility of their easy separation from the reaction mixture and their reuse for successive reactions, provided that the catalysts retain their catalytic properties. However, too few papers focus on rigorous studies of the catalysts’ stability and recycle. Both organic polymers and inorganic materials can be used as supports. More robust inorganic solids are more suitable, in particular, for oxidation reactions and should be preferred. Several factors should be considered for the appropriate choice of the support: stability of the support under reaction conditions, possible involvement with the reaction, capacity to readily introduce functionality for covalent anchoring, degree of functionalization, and availability and cost of the support. Accessible amorphous silica is most frequently used, whereas other oxides like alumina or zirconia have a limited scope. Porous materials with high surface area, e.g., zeolites, mesoporous silicas, and layered structures (clay minerals, pillared clays, and layered double hydroxides), provide a high catalyst loading and a possibility to use confinement effects. In this case, a relation between pore dimensions and substrate size is an important factor. The method of preparation of the supported catalyst can influence the catalyst structure in terms of distribution of the active sites on the surface and their accessibility and of the state of the complex. Consequently, the appropriate choice of the method has a primary importance for obtaining active and selective catalysts. Synthetic strategies for the immobilization of MPc’s are similar to those traditionally used for supporting organometallic complexes (Figure 2).

11, 15, 18, 22, 25) positions. Tetra-, octa-, and hexadecasubstituted phthalocyanines are the mostly used complexes. It should be noted that tetrasubstituted MPcR4 complexes consist of four constitutional Cs, C2ν, C4h, and D2h isomers with statistical ratio 4:2:1:1 given that bulkiness of substituents does not disturb this ratio in favor of the less sterically crowded positional isomers. To the best of our knowledge, there is no data in the literature showing different properties of positional isomers in catalysis. Indeed, catalytic steps take place at the metal site and should not be influenced by remote substituents occupying one of the two possible arrangements at periphery of the phthalocyanine core. In addition, there should not be any appreciable difference in their electronic influence, since all the four substituents are the same and situated in the same position on the aromatic ring. The only difference is their relative position with respect to each other, which could be an issue while studying physical features like liquid crystalline properties, etc. However, the presence of four constitutional isomers makes difficult the preparation of crystals for X-ray structural determination. Peripheral substituents play the key role in the catalytic chemistry of MPc’s.35 The appropriate choice of the substituents allows one not only to obtain a desired solubility profile but also to tune the catalytic properties by varying the electron-withdrawing or electrondonating character of the substituents. The nature of substituents also influences on the stability of the phthalocyanine complexes. For instance, introduction of the fluorinated substituents into MPc molecule increases their stability to nucleophilic, electrophilic, and radical attacks.36 In addition to MPcF16 (M = Ru,37 Fe,38 Co, and Cu39), all-fluorinated phthalocyanines bearing perfluoroalkyl groups such as ZnPcF8(i-C3F7)840 and CoPcF8(iC3F7)841 have been prepared. The extension of this family to

Figure 2. Schematic representation of immobilization methods. C


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Scheme 3. Supporting of MPcS onto Positively Charged Ion-Exchange Resins by Electrostatic Interaction and Coordination

Scheme 4. Monomeric and Dimeric FePcS Catalysts Supported on Amino-Modified Silicasa

Method A: 18 h incubation of FePc(SO2Cl)4 in Py, followed by slow addition to a suspension of NH2−SiO2 in Py and stirring at 80°C for 24 h. Method B: rapid addition of FePc(SO2Cl)4 in Py to a suspension of NH2−SiO2 in Py, followed by stirring at 20°C for 24 h. a

catalytic properties because the coordination sphere of the grafted complex differs from that of homogeneous MPc’s. Resting after grafting free silanol groups can be in turn modified to confer to the surface the desired hydrophilic/hydrophobic properties. In some cases the support is noninnocent in catalysis and its properties can play an important role. For example, its hydrophobic/hydrophilic properties can influence the local concentration of the substrate near the supported catalytic site, thus changing the reaction rate. The support properties can be tuned by appropriate modification of the support surface to achieve an optimal hydrophilic/hydrophobic profile for efficient mass transfer of reagents. There are numerous successful examples where immobilized catalysts exhibit a higher reaction rate compared to their homogeneous counterparts. Immobilization of the MPcS via coordination and electrostatic interaction is shown in Scheme 3. Uncharged polyvinyl(pyridine) resin can be used for coordinative fixation.45 After methylation of the free pyridine groups, the additional electrostatic interaction reinforces the complex fixation. The resulting MPcS−polymer solids are very stable to leaching in water or common organic solvents. MPcS can be anchored onto the acrylic copolymers Met−expansin− NH2 and Met−expansin−piperazine via covalent sulfamide bond between amine groups of the resin and SO2Cl groups of the phthalocyanine ligand.46 The length of the linker influences the catalytic properties of these materials. Immobilization of MPc’s on chitosan aerogel microspheres (polysaccharide derived from chitin, the amine groups of which were obtained by removing most of the chitin acetyl groups) afforded oxidation catalysts containing, in addition to the oxidation sites of the complex, the

Several general approaches have been used to immobilize MPc’s: (i) physical adsorption onto support surface, (ii) encapsulation within porous materials, (iii) electrostatic interaction between oppositely charged complexes and surface, (iv) grafting via direct coordination of metal to support, and (v) covalent anchoring to support. Physical adsorption of MPc’s from solution onto the support surface is the most simple and straightforward approach to supported catalysts, which does not necessitate modification of the support and complex. Due to the flat aromatic structure, MPc’s show strong affinity for the hydrophobic surfaces. However, such impregnated materials may contain different and not well defined catalytic sites, especially when very high complex loadings are used. This can result in the adsorption of several layers because of stacking, and a significant part of the MPc molecules becomes unavailable for the reaction. Obviously, the physically adsorbed solid catalysts cannot be used in the solvents in which MPc’s are soluble because of their possible leaching. Despite these limitations, these readily available catalysts can successfully be used if aforementioned issues are taken into account. The coordination, covalent, and electrostatic fixation approaches often need a preliminary modification of the support and/or MPc’s. The modification of the silica surface to introduce the functionality can conveniently be performed with a variety of commercially available silylating agents, such as 3-aminopropyltriethoxysilane or 3-choropropyltrimethoxysilane. Other functional mono- and oligosilanes can be used to anchor diamine, thiols, pyridines, imidazole, cyanopropyl, and epoxide groups onto the silica surface. Coordination bonding of MPc’s to the modified surface provides heterogeneous catalysts with modified D


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phthalocyanine molecules, resulting in a particularly active catalyst for alkane oxidation.56 The precise location of phthalocyanine complexes in zeolites has been a matter of debate. The aperture of the main channels of zeolite Y is 7.4 Å and the diameter of the cage is about 12 Å. The diameter of a planar MPc molecule is estimated to be about 13 Å. Consequently, entrapment of the MPc in a zeolite Y cage requires deformation of the complex or of the cage (Figure 3).57

basic sites of the chitosan support that could be advantageous for some reactions, e.g., aerobic oxidation of isophorone.47 An additional reason to immobilize homogeneous complexes onto appropriate supports is the possibility to anchor a catalytically active form. This is especially true for phthalocyanine complexes, solutions of which often contain several monomeric, dimeric, and even oligomeric species.48 By appropriate choice of the grafting conditions, FePcS was anchored onto mesoporous or amorphous amino-modified silica either in monomeric or in dimeric form (Scheme 4).49 Perhalogenated MPcF16 and MPcCl16 complexes are also amenable for covalent anchoring onto amino-modified silicas via nucleophilic substitution. A direct one-pot procedure for covalent anchoring of sulfonated complexes activated by triphenylphosphine ditriflate onto unmodified silica surface has been developed.50 This method allows one to obtain a more homogeneous distribution of (FePcS)2O onto the silica surface. Complexes containing free acid residues can be anchored to mesoporous titania by a one-pot hydrolytic process.51 The first step includes a modification of acid residues with titanium alkoxide, followed by hydrolysis and condensation under mild conditions to form mesoporous material. The loading of the complex in these hybrid materials can be easily modulated by addition of various amounts of Ti(OiPr)4 during the hydrolysis. Very high complex loadings can be easily reached (in the case of FePcS, 214 μmol g−1 or 19.2 wt %). FePcS was directly incorporated during the synthesis of hydrophobic or hydrophilic sol−gel materials.52 However, the incorporation of FePcS occurred in the aggregated state, which resulted in lower conversion and selectivities of aromatic oxidations compared to catalysts prepared by covalent anchoring of FePcS onto silicas. A more elaborated approach is necessary to avoid aggregation phenomena and to access separated single catalytic sites. The encapsulation approach is often associated with “ship-ina-bottle” constructions. Historically, phthalocyanine complexes were first immobilized by encapsulation in zeolites. Romanovsky and co-workers prepared MPc’s inside the supercages of zeolite NaY for the first time.53 MPc was constructed inside the zeolite pores from template metal ions and o-dicyanobenzene. The formed phthalocyanine molecules are believed to be sterically entrapped inside the pores because the size of the MPc exceeds pore entrance dimensions. MPc’s can be incorporated by template synthesis and zeolite synthesis methods. The scope and limitations of these methods are discussed in the literature.54 The zeolite surface was proposed to catalyze redox and acid− base reactions involved in the synthesis of the phthalocyanine moiety.55 When the template synthesis of MPc’s is performed inside the pores, it can result in the incomplete formation of the complex. Such materials may contain precursor molecules and metal-free phthalocyanine, the relative amounts of which depend on the bulkiness of MPc’s. To remove these impurities that compromise the catalytic activity, an extensive Soxhlet washing is necessary. MPc’s can be added during crystallization of the zeolite as a template agent. Faujasite zeolites have been modified with CoPcF16 and CuPcF16 by synthesizing NaX around the complexes.39 Alternatively, the CoPcF16 and CuPcF16 were prepared by template synthesis inside NaY zeolites ionexchanged with Co2+ and Cu2+. The zeolite synthesis approach provided better defined materials, since there were no free metal ions or ligand left in the zeolite, which could complicate characterization or reactivity. Using ferrocene as iron source has also enabled a complete chelation of all the iron by

Figure 3. Schematic representation of RuPcF16 and a NaY supercage containing RuPcF16. Reprinted with permission from ref 57. Copyright 1995 American Chemical Society.

The red-shift of the Q-band of FePcF16 to 644 nm compared to 622 nm for the outer surface physisorbed complex and to 616 nm for solution in acetone has been ascribed to a slight distortion of the macrocycle inside the supercage, although it may also arise from other interactions between the complex and the zeolite lattice. The electronic spectrum of CoPc encapsulated in zeolite Y exhibited Q-band splitting into bands of nearly equal intensity, indicating a lowering of the symmetry.58 Upon encapsulation, IR modes of CoPc became Raman-active and vice versa, indicating the loss of center of inversion as a consequence of the nonplanar geometry of encapsulated phthalocyanine ring. The formation of local defects in the zeolite structure was suggested on the basis of electron microscopy results.59 Extensive Soxhlet extraction with several solvents can give an indication of the location of the MPc’s. The loading of the FePc(NO2)4 was lower than that for the unsubstituted FePc.60 When FePc(NO2)4−Y material was subjected to the extensive extraction procedure, all phthalocyanine molecules were removed from the zeolite, clearly indicating that the complex was not formed inside the zeolite cages. On the contrary, it was impossible to remove all unsubstituted complex FePc from FePc−Y by the same procedure that could be compatible with encapsulation. These “ship-in-a-bottle” catalysts have been studied by various physicochemical and spectroscopic techniques (N2 adsorption, FTIR, XRD, EPR, SEM, etc.), but some aspects are still obscure and not all questions have been adequately addressed.54,59 An interesting approach was proposed by Jacobs and coworkers. Iron phthalocyanine encapsulated in the supercages of zeolite Y was embedded in a polydimethylsiloxane membrane (PDMS).61 Such a hierarchic structure resembles the structural organization of the enzymatic systems where active cytochrome P-450 is situated in phospholipid membrane. Occlusion of MPc E


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to related zeolite-based “ship-in-a-bottle” materials containing MPc,71 there is no diffusion limitation in catalytic oxidations in the liquid phase in the FePcS−MIL-101 case.69 Recent progress in the preparation of organized porous solids constructed from the porphyrin and the phthalocyanine molecules has provided access to materials with controlled porosity and a very high concentration of active site.72 Twodimensional covalent organic frameworks consisting of NiPc(OH)8 connected with phenylene diboronic acid formed a layered structure of planar sheets with uniform microporous channels and large surface area.73 Variation of the linking block allows one to obtain materials with different porosity and properties.74,75 Such materials are believed to be very useful in the catalytic applications,76 but the key issue for successful application of such materials is their structural stability. A spirolinked cobalt phthalocyanine polymeric network was prepared from CoPcCl16.77 A linkage by rigid nonplanar units provided amorphous microporous materials (diameter of pores 250 °C). Porous metal organic frameworks (MOF) possess a unique ensemble of properties: high surface area, crystalline open structures, tunable pore size, and functionality. Rapid progress is being made in developing MOF-based catalysts possessing metallic catalytic centers.67 Another approach includes using MOF as a host matrix for immobilization of catalytically active complexes. MIL-101 is an especially suitable material due to its good resistance to water, common solvents, and temperature (up to 320 °C).68 MIL-101 has a rigid zeotype crystal structure, consisting of quasispherical cages of two modes (2.9 and 3.4 nm) accessible via windows of 1.2 and 1.6 nm, respectively. In spite of very close dimensions of FePcS (ca. 1.7 × 1.7 nm) and MIL-101 windows (1.5 × 1.6 nm), FePcS has been rapidly and irreversibly inserted into nanocages of the MIL-101 framework with retention of both FePcS and MIL-101 structures, as evidenced by XRD, Raman, XPS, and EDX methods.69 FePcS could not be extracted from the material by intensive washing and ion exchange. XPS analysis revealed that the surface [Fe]:[Cr] atomic ratio was close to the bulk value found by ICP-AES elemental analysis and practically did not change after argon ion sputtering. The local elemental analyses of different particle places performed by EDX also showed a homogeneous Fe distribution in the MIL-101 material. These results suggest that FePcS molecules can be located inside the MIL-101 cages. The size of MPc’s strongly influenced the location of the complex in MIL-101 matrix. The MPc−MIL-101 materials were prepared by wet infiltration.70 While FePcF16 and RuPcF16 were incorporated inside MIL-101 pores, a bulkier μ-nitrido diiron phthalocyanine, (FePctBu4)2N, was strongly absorbed at the outer surface of the MIL-101 crystallites. The different organization of these materials had an incidence on the catalytic properties. In contrast

4. CATALYTIC APPLICATIONS OF PHTHALOCYANINE METAL COMPLEXES IN OXIDATION Catalytic reactions in the presence of MPc’s will be reviewed according to the reaction types in separate sections devoted to homogeneous and heterogeneous catalysis. Neat unsubstituted MPc’s have been often used as catalysts in different reactions. Owing to their poor solubility in common organic solvents, it is not always clear whether MPc’s are soluble or not under reaction conditions. These examples as well as reactions performed in ionic liquids and fluorinated solvents will be described in sections on homogeneous catalytic reactions. Only catalysts prepared by immobilization of MPc’s on different solid supports are considered as heterogeneous, and their applications will be discussed in sections on heterogeneous catalytic reactions. 4.1. Oxidation of Aliphatic C−H Bonds in Alkanes

Selective oxidation of alkanes under mild conditions is still an open challenge in catalysis, although much effort has been devoted to develop different approaches.78 Phthalocyanine metal complexes have provided promising results. A wide range of the oxidants may be used in combination with MPc’s. For evident reasons, molecular oxygen is the most attractive oxidant. However, special attention should be paid to the security of the reactions involving hydrocarbons and organic solvents in the presence of oxygen, especially at high temperature. In addition, the free radical oxidation mechanism is typically operating in these reactions, and achieving a high selectivity is often a problem. Accessible hydrogen peroxide and organic peroxides are widely used oxidants. Along with high active oxygen donor content, H2O2 has other advantages: it is cheap, safe, and clean, F


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KHSO5 system for 48 h. The oxidation of cyclohexanol and cyclohexanone were also performed to furnish ε-caprolactone, 6hydroxyhexanoic, and adipic acid. A bicyclic pentadentate 14,28[1,3-diiminoisoindolinato]phthalocyaninato iron(III) complex (“helmet” phthalocyanine) was used in combination with H2O2 for the oxidation of cycloalkanes under an inert atmosphere.85 The catalyst exhibited a strong selectivity for alcohol vs ketone formation. Alcohol to ketone ratios were 6.7 and 21.0 for the cyclohexane and cyclooctane oxidations, respectively. Total turnover numbers of 101 and 122 were obtained for cyclohexanol and cyclooctanol, respectively, with high overall yields based on the H2O2. Unexpectedly, the catalytic oxidation of indan containing weak C−H bonds was inefficient.85 MPc’s in combination with ionic liquids can be used for the aerobic oxidation of alkyl aromatic compounds.86CoPc in [bmim]Br showed the highest activity compared to Ru, Pd, Ni, Cu, and Zn phthalocyanines. Alkylarenes were converted to carbonyl compounds under 1 atm O2 at 100 °C with 74−93% yields (Table 1). The oxidation was less efficient in [bmim]Cl and [bmim]PF6 ionic liquids, whereas common organic solvents (MeOH, MeCN, xylene) and water completely suppressed the reaction. The origin of the strong promotion of the catalytic activity by ionic liquids is not clear. The similar approach was used for aerobic oxidation of nitrotoluenes to nitrobenzoic acids.87 FePc in 2:1 ionic liquid−NaOH aqueous solution biphasic system mediated oxidation of o-, m-, p-nitrotoluenes and 2,4dinitrotoluene at 90 °C and 2 MPa O2 with 92−93% isolated yields after 12 h. The catalytic system was used in six successive runs with the same activity. Fluoro-tagged CoPc(C6F13)4 was used for aerobic oxidation of PhEt under fluorous biphasic conditions (PhEt:C8F18 = 5:2, 2000:1 substrate:catalyst ratio, 90 °C).88 The catalyst in fluorinated solvent can be separated and reused four times, affording 35% conversion and 86% selectivity to acetophenone. The μ-nitrido diiron phthalocyanine−tBuOOH system showed a strong preference for the oxidation of benzylic C−H bonds over aromatic oxidation.89 Among the products of toluene and p-xylene oxidation, benzylic acids were the principal products (Table 2). However, (FePctBu4)2N containing electron-donating groups afforded PhCHO as the main product. Only one methyl group of p-xylene was oxidized and selectivity to acid was lower. The oxidation of hydrocarbons to ketones and epoxidation of olefins by (Bu4N)HSO5 in the presence of hydrophobic CuPc particles has been described.90 MPc (M = Fe, Co, Ni, Ru) complexes were compared with porphyrin complexes and polyoxometalates in liquid phase aerobic oxidation of methyl isobutyrate to methyl α-hydroxyisobutyrate used for the preparation of methyl methacrylate.91 The oxidation at 120 °C provided a 24% selectivity to target product at 19% substrate conversion. Although several catalytic systems have been developed for the efficient oxidation of alkanes with high TON values, their stability and recycling should be improved. 4.1.2. Heterogeneous Catalytic Reactions. Mild oxidation of alkanes is still an open challenge in heterogeneous catalysis, despite significant efforts devoted to develop different approaches. Supported MPc complexes seem to provide promising results. Encapsulated MPc−zeolites often suffered from problems with preparation of well-defined solids containing no free metal ions and/or free organic ligand.54 For instance, FePc−VPI-5 material contained only 40% of phthalocyanine molecules in the metalated form.65 Nevertheless, FePc−VPI-5

forming water as byproduct. More sophisticated oxidants like PhIO, m-chloroperbenzoic acid (m-CPBA), and (Bu4N)HSO5 have also been employed. 4.1.1. Homogeneous Catalytic Reactions. The use of phthalocyanine complexes for the oxidation of alkanes has attracted much of interest. Both the nature of the metal and the structure of the phthalocyanine complex as well as the oxidant used influence the outcome of the oxidation. Lyons and Ellis showed that azidometallophthalocyanine complexes (M = Cr, Mn, Fe) mediated aerobic oxidation of isobutane at 80 °C and propane at 125−150 °C, whereas MPc’s without azide ligand were inactive.79 The efficiency of the oxidation of isobutane to t BuOOH with ∼90% selectivity followed the trend Fe(N3)PcF16 (TON = 990) > Mn(N3)PcF16 (400) > Fe(Cl)PcF16 (260) > Cr(N3)PcF16 (220). Similar to porphyrin complexes, the introduction of the strong electron-withdrawing substituents increases the catalytic activity. The most efficient Fe(N3)PcF16 catalyst was used for the oxidation of propane to i-PrOH and acetone (0.6:1 ratio) with turnover number [TON; mole of product(s)/mole of catalyst] of 1210 in PhCN at 150 °C for 3 h. The reason for the strong promotion of oxidation by N3− is not clear. However, the formation of azide radical with concomitant reduction of metal ion might initiate radical processes.80 The oxidation of cyclohexane continues to attract much attention because cyclohexanone is an important precursor for the production of adipic acid and caprolactam used for manufacturing nylon-6,6 and nylon-6, respectively. The catalytic activity of 21 phthalocyanine complexes was examined in the aerobic oxidation of alkanes in the presence of CH3CHO.81 Among five phthalocyanine ligands and Fe, Co, Mn, and Ni and inactive Mg, Zn, Sn, and Al complexes, FePcCl16 was found to be the most efficient. Cyclohexane was oxidized to a 1:1 mixture of cyclohexanol:cyclohexanone with TON of 88 under the following conditions: 40 mmol of substrate, 4 mmol of CH3CHO, 0.01 mmol of catalyst, 1 atm of O 2, room termperature. n-Hexane, adamantane, and indan were oxidized to alcohols and ketones with a total TON of 22, 188, and 377. The catalytic oxidation of neat indan with 0.01 mol % catalyst loading gave a TON of 8 500. The Hammett correlation obtained in competitive oxidation of para-substituted ethylbenzenes showed a ρ value of −1.34, which was close to that reported for the iron porphyrin species (−1.69) but different from those for alkoxyl radical (−0.4).81 Kinetic isotope effect values for competitive oxidation of cyclohexane/cyclohexane-d12 and ethylbenzene/ethylbenzene-d10 were determined to be 6.2 and 6.7, respectively, close to the values typical for metalloporphyrin oxo species.82 However, the oxidation of cyclohexene by FePcCl16/O2/CH3CHO system has resulted in 70% allylic oxidation products and 30% epoxide, suggesting the involvement of a radical pathway.81 Comparative oxidation of cyclohexane by H2O2, tBuOOH, and m-CPBA was studied using FePcCl16 and FePcS catalysts.83 Higher yields of cyclohexanol, cyclohexanone, and cyclohexanediol were obtained using FePcS. As for oxidant, tBuOOH provided the best yields and less degradation of the catalysts. Under optimal conditions a total TON of 29 has been obtained. Fast degradation of the phthalocyanine core was detected using m-CPBA oxidant. In the presence of H2O2, the FePcS was more stable, though some degradation was still observed. Several MPcS (M = Fe, Co, Ni, Cu, Ru) were examined in the oxidation of C6H12 using H2O2 and KHSO5 oxidants.84 Adipic acid was formed with 95% selectivity and 21% yield in the RuPcS− G


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Table 2. Oxidation of Toluene and p-Xylene by tBuOOH Catalyzed by μ-Nitrido Diiron Phthalocyaninesa

Table 1. Aerobic Oxidation of Alkylarenes Catalyzed by CoPc in [bmim]Br (1 atm O2, 100 °C, 6−11 h)a

catalyst (FePctBu4)2N [FePc(SO2tBu)4]2N [FePc(SO2C6H13)4]2N [FePc(SO2tBu)4]2N [FePc(SO2C6H13)4]2N

TON

RCH2OH, %

Oxidation of Toluene 137 16 115 7 197b 5 Oxidation of p-Xylene 312 17 587 15

RCHO, %

RCOOH, %

47 16 12

37 77 83

28 19

55 66

a

Conditions: 1 mL of substrate, [catalyst] = 1 mmol, [tBuOOH] = 206 mmol, 60 °C, 24 h. bT = 40 °C.

a

of cyclohexane by tBuOOH to a mixture of ∼2.5:1 cyclohexanone:cyclohexanol were 400 and 4500 in the presence of FePc−Y and FePc(NO2)4−Y, respectively.60 Catalyst containing only FePc molecules within zeolite and with no noncomplexed iron ions was particularly active.56 The site isolation in the protective zeolite matrix led to a TON of 6000 in the oxidation of cyclohexane by tBuOOH, whereas corresponding homogeneous catalysts were deactivated by bimolecular oxidative selfdestruction after a few turnovers. Zeolite- and MCM-41encapsulated CoPc, CoPcF16, and CuPc complexes were used for the oxidation of cyclohexane by tBuOOH and H2O2, affording about 1:1 alcohol:ketone selectivity.92 FePc supported onto activated carbon black by impregnation was more efficient in the oxidation of cyclohexane by tBuOOH as compared to the zeolite-encapsulated complex.93 This was attributed to the increased hydrophobicity of the carbon surface, which favorably changed the adsorption behavior of the reactants toward substrate. Using 0.5 g of catalyst, 50 mmol of C6H12, and 100 mmol of tBuOOH, ∼34% and ∼14% conversions were achieved after 24 h with FePc−carbon black and FePc−Y catalyst, respectively. No data on the product composition were given. The leaching of the FePc from the carbon support to the solution and oxidative degradation of the complex were observed.93 A series of Cu, Co, Fe, Ni, and Al phthalocyanines was tested in the oxidation of cyclohexane in the presence of alkyl hydroperoxides (tBuOOH and cyclohexyl and cumyl hydroperoxide) at 70 °C.94 Substituted chloro- and nitrophthalocyanines of Cu, Co, and Fe were more efficient, either in the neat state or when incorporated in zeolites X and Y. The most active CuPcCl16−NaY−tBuOOH system exhibited a high turnover frequency of 400 h−1 with tBuOOH efficiency up to 90%. Cyclohexanol, cyclohexanone, and adipic acid were obtained with 30, 48, and 9% selectivity, respectively. Succinic, glutaric, and valeric acids and valeraldehyde were the side reaction products, suggesting a free radical oxidation mechanism. Neat CuPcCl16 and a physical mixture of CuPcCl16 and zeolite Na-Y behaved in a manner similar to that of CuPcCl16−Na-Y. Turnover numbers as high as 3200 were obtained.94 Catalytic activity of heterogeneous catalysts often depends on the method of immobilization. RuPcF16−MCM-41 materials were prepared by (1) ship-in-the-bottle synthesis of RuPcF16 within the MCM-41 mesopores, (2) addition of RuPcF16 during the synthesis of MCM-41, and (3) anchoring RuPcF16 onto the functionalized silica surface of MCM-41.95 Method 2 afforded a very low RuPcF16 loading. RuPcF16−MCM-41-(1) catalyzed the oxidation of n-hexane by tBuOOH mainly to 2- and 3-hexanone with up to 1350 turnovers. No products of terminal oxidation were detected. The catalytic activity of RuPcF16−MCM-41-(3)

Adapted with permission from ref 86. Copyright 2008 Elsevier.

(250 mg) catalyzed the oxidation of cycloalkanes (25 mmol) by t BuOOH (addition at a rate of 0.3 mL h−1) at room temperature. Conversions of 20% were typically achieved with a 15:85 alcohol:ketone selectivity. Assuming that every FePc molecule was accessible, TONs of 125 and 313 were obtained in the oxidation of cyclododecane and cyclohexane, respectively. In opposite case, if only the channel ends are accessible, TON was in the range of a few 100 000.65 Severe diffusion problems of substrates and products in confined materials can also occur. Although FePc(NO2)4−Y and FePc−Y catalysts contained a significant part of metal-free phthalocyanines, TON for oxidation H


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can be explained by a better balanced concentration of the substrate, the peroxide, and the ligand around the catalytic site. Although no recycling study was published, this catalytic system seems particularly promising. However, further development of this approach was only briefly published.97 Some deactivation of the catalyst was observed, most probably owing to the sorption of the polar products within zeolite as the reaction progresses. Regeneration of the membrane by treating at 150 °C under vacuum was sufficient to re-establish the initial catalytic activity. Noteworthy, when FePc was directly occluded in PDMS without zeolite matrix, the material underwent a very rapid degradation.61 Balkus and co-workers have prepared FePcF16 and RuPcF16 zeolite-entrapped materials by a zeolite synthesis method around MPcF16.57 While FePcF 16 intrazeolite material was not sufficiently stable in the presence of a high concentration of peroxide, RuPcF16 catalyst with a loading of ∼1 complex per 125 NaX supercages showed improved catalytic properties and stability. The low loading of RuPcF16 was essential in maintaining diffusion pathways during the catalysis. The oxidation of cyclohexane (0.66 mL, 6 mmol) was carried out at room temperature in the presence of tBuOOH (0.3 mL, 2.4 mmol) and RuPcF16−zeolite (100 mg, 0.0002 mmol) (substrate:oxidant:catalyst = 30 000:12 000:1). The encapsulated catalyst was much more active, providing about 3000 turnovers/day (than the homogeneous RuPcF16, 250 cycles per day) with no sign of deactivation after >20 000 turnovers, corresponding ∼70% cyclohexane conversion. Incorporation of the RuPcF16 in zeolite increased the selectivity to cyclohexanone (98.4% vs 67% for the homogeneous reaction). The RuPcF16−zeolite material showed a substrate size selectivity, indicating the intrazeolite location of the active sites. The homogeneous oxidation occurred with comparable rates for cyclododecane and cyclohexane: 243 and 258 turnovers/day, respectively. The RuPcF16−zeolite catalyst was much more active in the oxidation of cyclohexane, 2933 vs 295 turnovers/day, in accordance with the steric constraints exerted by the zeolite matrix.57 Encapsulation of CuPcCl16 and FePcCl16 in zeolites strongly increased their catalytic activity in the oxidation of propane by tBuOOH to afford a mixture of ∼1:1 i-PrOH:acetone with TON of 316 and 371, respectively (as compared to 5.0 and 4.8 obtained with the neat complexes).98 A positive effect of encapsulating FePcF16 and RuPcF16 in MIL-101 metal−organic framework by wet infiltration was shown in the aerobic oxidation of tetralin to 1-tetralone.70 The complex loading was ∼0.8 FePcF16 or ∼1.2 RuPcF16 molecules per large cage, respectively. The homogeneous distribution of complexes inside of MIL-101 material was confirmed by EDS analysis. The catalytic experiments were performed with substrate:catalyst molar ratio of 147 000:1 for FePcF16@MIL101 and 92 000:1 for RuPcF16@MIL-101. While pure MIL-101 showed no catalytic activity, FePcF16@MIL-101 and RuPcF16@ MIL-101 exhibited very high TON values: 48 200 and 46 300 as compared with 3400 and 5200 for the homogeneous FePcF16 and RuPcF16, respectively (Table 4). FePcF16@MIL-101 exhibited a high selectivity (up to 80%) to the desired 1-tetralone, a diesel fuel additive and an intermediate for the synthesis of agricultural chemicals. The induction period and low values of kinetic isotope effect (KIEFe = 1.99, KIERu = 1.76) in the oxidation of adamantane-d2 probe82 are consistent with a free radical mechanism. Although no leaching of the complexes was detected, the catalytic activity of the heterogeneous catalyst decreased on reuse, presumably because of the blockage of the pores by the side overoxidation products.

in the oxidation of cyclohexane was low (TON = 150 after 150 h) compared with that of RuPcF 16 −MCM-41-(1) (several thousands turnovers per day), presumably owing to too high complex loading in RuPcF16−MCM-41-(3) material (5 wt %). This could lead to partial blockage of the pores and thus make a significant part of the complex inaccessible.95 The influence of the method of catalyst preparation, the nature of the support, and oxidant on the oxidation of cyclohexane and n-decane was investigated in details.96 CuPcCl16 and CoPcCl16 immobilized onto MCM-41 by impregnation and onto aminomodified MCM-41 and SiO2 by covalent anchoring were tested in combination with tBuOOH, O2/isobutyraldehyde (IBA) or O2/benzaldehyde (Table 3). Table 3. Oxidation of Cyclohexane Catalyzed by Differently Prepared CuPcCl16 and CoPcCl16 Catalystsa TON catalyst

obtained with t BuOOH

obtained with O2/IBA (O2/PhCHO)

CuPcCl16 CuPcCl16−NH2−SiO2 CuPcCl16−MCM-41 CuPcCl16−NH2−MCM-41 CoPcCl16 CoPcCl16−NH2−SiO2 CoPcCl16−MCM-41 CoPcCl16−NH2−MCM-41

57 85 196 330 120 160 240 424

47 (50) 70 (88) 120 (196) 196 (297) 62 (89) 107 (132) 123 (214) 188 (364)

a

Conditions: 50 mg of supported catalyst or 2 mg of neat complex, 5 mmol of cyclohexane, 15 mmol of co-oxidant, 1 atm of O2, MeCN, 50 °C, 24 h.

In all cases, higher TON was obtained with tBuOOH oxidant, and CoPcCl16−NH2−MCM-41 was the most efficient catalyst. While the oxidation of cyclohexane resulted in a mixture of ∼1:1 cyclohexanol:cyclohexanone, the oxidation of n-decane afforded a mixture of isomeric ketones with almost 100% selectivity. The TON values in the oxidation of n-decane with tBuOOH were significantly higher than those of cyclohexane: up to 671 and 525 obtained with CuPcCl16−NH2−MCM-41 and CoPcCl16− NH2−MCM-41, respectively. The higher reactivity of n-decane was explained by its stronger adsorption onto siliceous supports.96 The promising catalytic system was developed by Jacobs and co-workers, who incorporated FePc encapsulated in the supercages of zeolite Y in the polydimethylsiloxane membrane (PDMS).61 This zeolite-encaged iron phthalocyanine embedded in turn in polymer membrane showed an efficient oxidation of cyclohexane. Occlusion of MPc−Y in PDMS membrane led to increased catalytic activity compared to the neat and zeoliteencapsulated complexes. This elaborated catalyst showed a turnover frequency of 3.3 min−1 in the oxidation of cyclohexane by tBuOOH at room temperature. A heterolytic nonradical oxidation mechanism was proposed on the basis of (i) the absence of bicyclohexyl, tert-butylperoxocyclohexane, and tertbutyl cyclohexyl ether products, which are typical for the radical oxidation; (ii) a kinetic isotope effect of 9.0 obtained in the concurrent oxidation of C6H12 and C6D12; and (iii) regioselectivity in adamantane oxidation. Using 0.32 g of FePc−zeolite Y−PDMS (0.096 g of FePc−zeolite Y; the amount of FePc was not indicated), 40 mmol of tBuOOH, and 300 mmol of cyclohexane in a counter-current membrane reactor, a TON of 1000 was obtained after 5 h. The increase in the catalytic activity I


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metalloorganic CH4 activation, the mononuclear iron porphyrin complex of cytochrome P-450 and nonheme diiron construction of soluble MMO generate powerful oxidants capable of attacking the C−H bond of methane.103,104 Mononuclear and binuclear phthalocyanine complexes have been examined for the oxidation of methane as bioinspired catalysts. Raja and Ratnasamy have studied the oxidation of CH4 by t BuOOH in the presence of neat CuPc, CoPc, CuPc(NO2)4, CuPcCl16, CoPcCl16, FePcCl16, and NiPcCl16 as well as the same complexes encapsulated into zeolites Na-X and Na-Y.105 The reactions were performed at 4.46 bar of CH4 in CH3CN containing 0.5 g of tBuOOH at 0 °C during 12 h. In the experiments in the presence of CuPcCl16, CoPcCl16, or FePcCl16 complexes encapsulated in zeolite Na-X, CH3OH, CH2O, HCOOH, and CO2 were obtained in ratios indicated in Table 5.

Table 4. Aerobic Oxidation of Tetraline to 1-Tetralone Catalyzed by Homogeneous and MIL-101-Encapsulated FePcF16 and RuPcF16 TON catalyst

after 6 h

after 24 h

selectivity to 1-tetralone, %

MIL-101 FePcF16 FePcF16@MIL-101 RuPcF16 RuPcF16@MIL-101

0 3400 24200 5200 30900

6300 48200 nd 46300

68 80 (80) 70 62 (74)

The primary products of the oxidation of alkanes by MPc−tBuOOH systems are often hydroperoxides. However, these hydroperoxides can be overlooked during standard GC analysis because of the degradation to alcohols and ketones at high-temperature GC analysis conditions. The procedure developed by Shul’pin, consisting of the GC analysis of the reaction mixture after the reduction99 and titration of the hydroperoxides, is highly recommended. Zeolite-encaged CuPc, CoPc, FePc, and MnPc catalyzed the oxidation of cis-pinane by t BuOOH mainly to 2-pinane hydroperoxide and trace amounts of 2-pinalol.100 The catalytic activity increased in the order MnPc−Y < FePc−Y < CoPc−Y < CuPc−Y, reaching up to 80% selectivity to 2-pinane hydroperoxide at 90% conversion. Aerobic oxidation of alkyl arenes was performed in the presence of silica-supported CoPcS and N-hydroxyphthalimide (NHPI), serving to produce phthalimide N-oxyl radical (PINO).101 The supported CoPcS catalyst was more efficient than the neat CoPc, FePc, CuPc, and NHPI as well as supported FePc and CuPc materials in the oxidation of benzylic methyl and methylene groups to the corresponding carboxylic acids and ketones (Scheme 5).

Table 5. Amounts of C-1 Products Found in Experiments on CH4 Oxidation by tBuOOH in CH3CN Solvent products (mol %) catalyst

CH3OH

CH2O

HCOOH

CO2

TON

CuPcCl16−Na-Y CoPcCl16−Na-Y FePcCl16−Na-Y

51.5 12.5 52.6

41.7 72.2 42.3

4.1 10.9 3.2

2.7 4.4 1.9

48.5 30.5 107.2

However, it should be noted that the same C-1 products could be obtained in the oxidation of CH3CN solvent owing to the well-known radical decomposition of tBuOOH catalyzed by phthalocyanine complexes. The following observations are in agreement with this suggestion. It was found that the amount of products depended primarily on the tBuOOH amount and was insensitive to the amount of the catalyst above 1 g. An increase in the catalyst amount influenced only the ratio of the products but not their total amount. The origin of C-1 products could be unambiguously determined by using labeled 13CH4. However, in the absence of labeling experiments with 13CH4, the formation of C-1 products from the oxidation of organic solvent and/or decomposition of tBuOOH cannot be excluded as follows from the study described below. Taking into account the ability of μ-nitrido diiron phthalocyanine to generate high-valent diiron oxo species (Pc)FeIVNFeIVO(Pc+•) (vide infra), this complex has recently been proposed for the oxidation of methane.106,107 Initial experiments were performed in CH3CN containing 0.1 mM (FePctBu4)2N and 678 mM H2O2.106 The main product was found to be HCOOH along with minor amounts of CH2O and CH3OH. Small amounts of acetic acid and acetone were also detected. It should be pointed out that the oxidation products containing one carbon atom can be derived either from the oxidation of methane or from the oxidation of acetonitrile or ligand of the complex. To distinguish between the oxidation of CH4 and the possible oxidation of CH3CN, the reaction was performed in CD3CN. Formic acid obtained in 6 mM concentration consisted of 68% of HCOOH and 32% of DCOOH, indicating a parallel oxidation of CH4 and CD3CN solvent. This is not surprising because, if the active species is strong enough to oxidize CH4, it should oxidize any organic molecule containing weaker C−H bonds. The products of the oxidation of organic solvents might be the same as products of CH4 oxidation. This issue has not usually been discussed in the related literature. The above example shows that a careful control and labeling experiments should be performed to determine the

Scheme 5. Aerobic Oxidation of Alkylarenes in the Presence of CoPcS−SiO2 and NHPI

Polymeric Co(salphen)/CuPc material obtained by the condensation polymerization was applied for the aerobic oxidation of cumene to cumene hydroperoxide at 100 °C with 76% conversion and 89% selectivity.102 The catalytic performance of this bimetallic catalyst was superior than those of Co(salphen) or CuPc and remained high during four consecutive runs. 4.2. Oxidation of Methane

Oxidation of methane is particularly difficult compared to other alkanes, owing to the very high dissociation energy of the methane C−H bond (435 kJ mol−1). Not surprisingly, current approaches to transform methane mainly involve high-temperature processes. A direct, low-temperature oxidation of CH4 continues to be a major challenge both from fundamental and practical points of view. In nature, cytochrome P-450 and methane monooxygenase (MMO) enzymes constitute particularly exciting and challenging examples of oxidation of the strongest C−H bonds under ambient conditions. In contrast to J


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blue. Thus, the presence of acid also increased the stability of the complex. Importantly, the Fe(μ-N)Fe structural motif of (FePctBu4)2N was essential for the catalytic activity. Mononuclear FePctBu4 as well as corresponding diiron μ-oxo (Fe−O−Fe) and μ-carbido (FeCFe) complexes were not active in the oxidation of methane. An important feature of this system is the ability to use H2O2 as the oxidant. The replacement of H2O2 with m-CPBA resulted in a decreased TON value of 12.5 because of the concurrent oxidation of m-CPBA and products of its transformations.109 The (FePctBu4)2N−H2O2 system was applied for the oxidation of propane to propan-1-ol, propan-2-ol, propionaldehyde, acetone, and propionic, acetic, and formic acids.110 Hutchings and co-workers have studied in details the product composition and the stability of the (FePctBu4)2N−SiO2 supported catalyst under modified reaction conditions.111 In the presence of 5000 equiv of H2O2 with respect to the catalyst, instead of 678 equiv used in ref 106, they have performed oxidation of methane in pure water. Using 1H NMR and GC methods, the formation of CH3OOH and CO2 that originated from the phthalocyanine ligand degradation was observed. Two pathways responsible for free radical degradation of phthalocyanine complex and leaching of the iron ions to solution have been proposed: (i) generation of OH radicals in the close vicinity of the absorbed complex catalyzed by silica support containing 20 ppm of iron and (ii) generation of OH radicals by (FePctBu4)2N, which would lead to inherent instability. The SiO2 support alone with 20 ppm of Fe content showed significant H 2 O 2 decomposition, which led to OH radical formation. Hydroxyl radical induced degradation of the complex provides more iron ions, thus leading to autocatalytic decomposition of the complex. The possible generation of OH radicals by the (FePctBu4)2N will be discussed in details in the mechanistic section. When oxidizing CH4, particular attention should be paid to product analysis and to the study of the origin of the products. In order to evidence CH4 oxidation a labeling study should be performed. The important challenge in methane oxidation is to develop a catalytic system with high selectivity to methanol, i.e., to avoid the formation of overoxidation products (CH2O, HCOOH, CO2).

origin of the oxidation products and to confirm the occurrence of CH4 oxidation. Although the catalytic oxidation of CH4 does occur with a high turnover number of 40.8, the reaction is masked by the oxidation of CH3CN solvent. To avoid this problem, the oxidation of CH4 was carried out in water, a clean and inert solvent, using silicasupported (FePctBu4)2N (complex loading of 20−40 μmol/g, specific surface 180−185 m2/g).106,107 The formation of labeled 13 CH3OH, 13CH2O, and H13COOH from labeled 13CH4 was evidenced by 13C NMR and GC−MS techniques. This finding unambiguously indicated that these products were formed in the oxidation of methane. Acetic acid and acetone did not contain 13 C label and were most probably formed from the oxidative degradation of the complex containing tBu substituents. Importantly, unsubstituted (FePc)2N complex exhibited significant catalytic activity in the oxidation of CH4 (TON = 103), indicating no significant contribution to C-1 products due to ligand degradation.108 Formic acid was the main product because of easier oxidation of the intermediate CH3OH and CH2O products with respect to methane presented in the aqueous solutions in comparable concentrations. When the reaction was performed in 97% H218O, methanol did not contain 18O label, whereas 52.5% of 18O and 47.5% of 16O were found in the formic acid. Using 0.925 μmol of catalyst and 678 μmol of H2O2, methane (32 bar) was oxidized in pure water even at 25 °C, though with a moderate TON of 13. The optimal reaction temperature interval was 40−60 °C to provide turnover numbers of 26−29. The oxidation was less efficient at a higher temperature, most probably because of more rapid catalyst degradation. The significant improvement of the (FePctBu4)2N−SiO2− H2O2 system in terms of the activity and stability of the catalyst was achieved in the presence of small amounts of acid (Figure 4).

4.3. Oxidation of Olefins

Numerous industrially important products can be obtained by the oxidation of olefins. Selective oxidation of double bonds provides epoxides and diols, whereas the oxidation of allylic positions leads to the formation of allylic alcohols and ketones. In many cases, the oxidation of olefins results in the formation of several products in comparable amounts. The important goal is, therefore, to obtain either epoxides or products of allylic oxidation with high selectivity. In general, metal-centered active species like high-valent metal oxo complexes show a high selectivity in epoxidation. Allylic alcohols and ketones are typical products when one-electron processes and radical intermediates are involved. 4.3.1. Homogeneous Catalytic Reactions. Aerobic oxidation of olefins was performed in the presence of μ-oxo dimeric (FePctBu4)2O and (MnPctBu4)2O.112 α-Pinene was oxidized to the mixture of α-pinene oxide, trans-verbenol, verbenone, and pin-3-en-2-ol (Scheme 6). The oxidation of 1-hexene, cyclohexene, and cycloheptene afforded epoxides (12−29%), allylic alcohols (27−45%), and ketones (30−45%) with TONs of 120 and 80, 520 and 580, 120,

Figure 4. Dependence of the efficiency of CH4 oxidation on the H2SO4 concentration. TON values are shown above the bars. Reaction conditions: 2 mL of water, 32 bar of methane, 0.925 μmol of supported catalyst (18.5 μmol/g), 678 μmol of H2O2, 20 h.

A very high performance was obtained in 0.075 M H2SO4 aqueous solution: 223 CH4 molecules were oxidized by each (FePctBu4)2N molecule.107 The yield of the reaction products on H2O2 was also high, reaching up to 92%. Interestingly, when the reaction was performed in pure water, the complex was completely bleached after 20 h of reaction. In the presence of diluted acid, the recovered supported catalyst after 20 h was still K


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Scheme 6. Aerobic Oxidation of α-Pinene in the Presence of μ-Oxo Dimers

Complete conversion and 82% selectivity with 0.1 mol % catalyst loading was obtained in the epoxidation of trans-stilbene. A distinct reactivity in the oxidation of terminal olefins by O2 was observed with the dimeric (PcRu)2.119 In the presence of PdCl2(PhCN)2 (olefin activator), the oxidation of 1-octene to 2octanone (up to TON = 12) and 1-decene to 2-decanone was obtained in THF at 20−25 °C and 50 atm O2 using a Ru:Pd:olefin molar ratio of 1:1.7:40. Aerobic oxidation of 3,5,5-trimethylcyclohex-3-en-1-one (βisophorone) to 3,5,5-trimethylcyclohex-2-en-1,4-dione was performed at 25 °C in the presence MnPc (0.33 mol %), FePc (1.3 mol %), or CoPc (3.3 mol %) to afford 59, 55, and 28% yields, respectively.120 Manganese porphyrin complex was more efficient in this reaction, giving ketoisophorone in 93% yield. Oxygenation of styrene by O2 in the presence of FePc (10 mol %) and NaBH4 (1.5 equiv) provided 1-phenylethanol with 97% yield.121 FePc was more efficient than various Co complexes. Aerobic oxidation of aromatic olefins in the presence of NaBH4 was carried out using 0.1 mol % of Mn(OAc)PctBu4, Fe(OH)PctBu4, or CoPctBu4.122 The best yields of 1-phenylethanol and 2-phenyl-2-propanol were obtained in the presence of Mn(OAc)PctBu4: 88 (12 h) and 94% (4 h), respectively. The transformation of unactivated olefins to alcohols was further developed to introduce tertiary alcohol group during the total synthesis of 20′-vinblastine analogues, which are regarded as important drug targets for the treatment of cancer (Scheme 8).123

and 100 for (FePctBu4)2O and (MnPctBu4)2O, respectively. The product compositions in the oxidation of these olefins are typical for the autoxidation radical mechanism. However, cis,transcyclodeca-1,5-diene and dicyclopentadiene gave only corresponding cis-epoxide in the former case and three epoxides in the latter, while allylic oxidation products were obtained in radical oxidation.112 The dimeric [Pc(CF3)8Fe]2O complex catalyzed the oxidation of stilbenes, styrenes, cyclohexenes, and butanes by PhIO with TON of 3−57.113 Oxidation of cyclohexene by the IBA−O2 system in the presence of MPctBu4 [M = Mn(III), Fe(III), Co(II)] complexes occurred with a higher reaction rate compared to the noncatalyzed reaction to provide 50−55% yields of the epoxide.114 The addition of TEMPO radical scavenger inhibited the reaction, suggesting the involvement of acyl and/or acylperoxy radicals. m-CPBA and tBuOOH were used in combination with FePc, CoPc, and FePcCl16 for the oxidation of cyclohexene to epoxide, allylic alcohol, and ketone with TON up to 743.115 Oxidation of olefins by peracetic acid was investigated in the presence of manganese complexes of porphyrins, tetraazaporphyrin, and 3,5-octanitrophthalocyanine.116 cis-Stilbene and 1dodecene were oxidized quantitatively and stereospecifically (for cis-stilbene) to epoxides using an AcOOH:olefin ratio of 1:1 without formation of side products, suggesting a concerted oxygen transfer mechanism. Using an olefin:catalyst ratio of 3700, a TON of 2860 was achieved in 1 h at 20 °C. CoPc(NO2)8 was less active than Mn porphyrins but, in difference to the latter, was able to stereoselectively epoxidize trans-stilbene with a TON of 1400 due to the absence of the steric hindrance of the phthalocyanine plane. The CoPc(NO2)8−AcOOH system epoxidized a less reactive terminal 1-dodecene, though with lower TON of 130.116 High yields of epoxides were obtained in the oxidation of aliphatic and aromatic olefins (15 examples) by H2O2 (2 equiv) in the presence of iron helmet phthalocyanine.117 Epoxidation of stilbenes with 2,6-dichloropyridine N-oxide using sterically hindered Ru phthalocyanines with nonperipheral hexyl, octyl, dodecyl, isopentyl, and 2-cyclohexylethyl substituents furnished epoxides with high yields and turnover numbers (Scheme 7).118

Scheme 8. Oxidation of Unactivated Olefins by the FePc− NaBH4−O2 Systema

a

Adapted with permission from ref 123. Copyright 2012 American Chemical Society.

Treatment of 1,6-dienes with FePc (10 mol %) in the presence of NaBH4 (3 equiv) and 1 atm of O2 resulted in radical cyclization to form five-membered carbo- or heterocyclic compounds bearing an hydroxyl group with up to 81% yield (seven examples).124 4.3.2. Heterogeneous Catalytic Reactions. CuPcCl16 was encapsulated in MCM-41 channels either by impregnation or by covalent anchoring onto amino-modified material.125 The anchoring method provided a more uniform distribution of the complex on support surface, which led to a higher catalytic performance compared with those of neat CuPcCl16 and materials prepared by physical adsorption. The oxidation of olefins by the O2−IBA system was more efficient in terms of selectivity and turnover numbers compared to tBuOOH oxidant (Table 6). CuPc encapsulated in the supercages of zeolite Y showed a similar selectivity to epoxide in the oxidation of styrene by t BuOOH (24%), but the conversion was higher (95%).126 CoPc−zeoliteY material was much more stable than homogeneous CoPc during epoxidation of cyclohexene with PhIO, though the reaction rate was slower because of incorporation of CoPc inside the zeolite pores.127

Scheme 7. Epoxidation of Stilbenes by 2,6-Dichloropyridine N-Oxide Catalyzed by Ru(CO)Pc(C6H13)8

L


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Table 6. Heterogeneous Oxidation of Olefins Catalyzed by Covalently Anchored CuPcCl16−NH2−MCM-41 Materiala product distribution, % olefin styrene styrene cyclohexene 1-decene a

oxidant

conversion, %

epoxide

aldehyde

others

TON

BuOOH O2 + IBA O2 + IBA O2 + IBA

47 100 80 44

25 74 77 40

19 20

3 6 3 4

1472 3144 2532 1410

t

Conditions: 5 mmol of alkene, 5 mmol of tBuOOH or 15 mmol of IBA, 10 g of MeCN, 1 atm of O2, 40 °C, 8 h.

octanethiol.132,133 Mesoporous TiO2 containing FePcS prepared by a one-pot hydrolytic process from a modified Ti alkoxide provided 57% yield of ketoisophorone (Scheme 9).51 Better catalytic properties of FePcS−TiO2 were due to a cooperative action between TiO2 and FePcS catalytic sites. On reuse, a 10− 20% decrease of the catalytic activity was observed between runs, but the selectivity to ketoisophorone remained fairly constant for the successive runs. FePc intercalated into lamellar ZrPO 4 was used in combination with hydroquinone and Pd2+ in a triple catalytic system for Wacker-type oxidation of terminal olefins to ketones (Scheme 10).134

Selective epoxidation of aliphatic olefins with n-Bu4NHSO5 was catalyzed by CuPcS supported onto Fe3O4 magnetic particles coated by amino-modified silica to give 40−96% yields of epoxides.66 Aromatic olefins underwent C−C bond cleavage to provide aldehydes or ketones. Allylic oxidation of cyclohexene rather than epoxidation was the predominant pathway in the presence of d-FePcS−SiO2 and t BuOOH.128 Using a 0.5 mol % catalyst loading, 2-cyclohexen-1one, 2-cyclohexen-1-ol, and cyclohexene oxide were obtained with 57, 5, and 2% yields, respectively. The related FePcCl16− SiO2−tBuOOH system was reported for mild oxidation of cyclohexene, limonene, and α-pinene to 2-cyclohexen-1-one, verbenone, and carvone with 47, 22, and 12% yields, respectively.129 The selectivity of the olefin oxidation can be governed by the choice of the oxidant. Replacement of tBuOOH with O 2 −IBA led to selective epoxidation. 130 With a catalyst:substrate:(CH3)2CHCHO ratio of 1:265:530, cyclooctene oxide, cyclohexene oxide, and styrene oxide were prepared with 90, 78, and 74% yields, respectively. Turnover number was as high as 240 cycles. The influence of the support was shown in the aerobic oxidation of β-isophorone to ketoisophorone, which is an important precursor for the preparation of flavors and fragrances. While homogeneous protocols for this oxidation have been published, efficient heterogeneous methods are still rare. Immobilization of FePcS and CoPcS on chitosan aerogel microspheres afforded bifunctional catalysts with basic and oxidation active sites.47 These catalysts showed moderate selectivity in oxidation of β-isophorone to ketoisophorone (Scheme 9).

Scheme 10. Catalytic Cycle of Terminal Olefin Oxidationa

a

Adapted with permission from ref 134. Copyright 2002 Taylor & Francis.

The same catalytic system transformed 1,3-cyclohexadiene into 1,4-diacetyl-2-cyclohexene with 50% isolated yield.134 CuPc precursor was supported onto silica with 90% isotopic enrichment) was used in the presence of nonlabeled 16O2 and H216O, phenol contained 93% of Ph18OH, thus indicating that product oxygen originated exclusively from H218O2. The formation of benzene epoxide in biological oxidation is accompanied by an NIH shift, which is a migration of the substituent from the site of hydroxylation to an adjacent carbon atom.137 1,3,5-Trideuterobenzene was used as a mechanistic probe for the detection of an NIH shift by the analysis of the isotopic composition of p-benzoquinone (Scheme 11). When no NIH shift occurs, only p-benzoquinone-d2 should be formed. The product obtained in the oxidation of 1,3,5trideuterobenzene by the (FePctBu4)2N−H2O2 system contained 75% of p-benzoquinone-d2, 19% of p-benzoquinone-d3, and 6% of p-benzoquinone-d1, thus indicating the occurrence of the NIH shift. The Fe−N−Fe structural unit was essential for this activity, since mononuclear FePctBu4 and μ-oxo (FePctBu4)2O complexes were much less efficient and stable and did not show the formation of benzene epoxide.136 The mechanistic features of the benzene oxidation mediated by the (FePctBu4)2N−H2O2 system (incorporation of 18O from H218O2 to the reaction products, formation of benzene epoxide, and NIH shift) resemble those of the biological oxidation and are compatible with the involvement of high-valent diiron oxo species. The oxidation of aromatic compounds to quinones is an important synthetic transformation for the preparation of vitamins and drugs. A particularly difficult oxidation of 2methylnaphthalene (2MN) to 2-methyl-1,4-naphthoquinone (vitamin K3, 2MQ) is still performed in industry by an extremely polluting process involving CrO3 in H2SO4. Numerous oxidation products are usually formed owing to the concurrent oxidation of

Scheme 12. Multiple Pathways in the Oxidation of 2Methylnaphthalene

reason only moderate yields (40−50%) of the target 2MQ are usually obtained. In the search for cleaner methods of the 2MN oxidation, different approaches have been proposed.139 Oxidation of naphthalene, 2MN, and 2,3-dimethylnaphthalene by peracetic acid catalyzed by MnPc(NO2)8 or [FePc(NO2)8]2O afforded 1,4-naphthoquinones with good yields at 20 °C.140 Heating of the neutralized final reaction mixtures led to an increase of product yields by 1.5−5 times to achieve 35−62%. Noteworthy, the μ-oxo dimeric [FePc(NO2)8]2O was more active than the monomeric FePc(NO2)8 (by a factor of 2). This system was very efficient: an AcOOH:substrate ratio of 3.2−4 was sufficient for the complete conversion of substrate within 15−90 min with 0.75 mol % [FePc(NO2)8]2O or 4.3 mol % MnPc(NO2)8 catalysts. Note that 3 equiv of AcOOH is necessary to convert naphthalene substrate to the corresponding quinone. The selective oxidation of 2MN to 2MQ is very difficult because of the inevitable formation of several side products, including isomeric 6-methyl-1,4-naphthoquinone. The ratio between these two quinones was not indicated in this work.140 The mechanism of naphthalene oxidation mediated by porphyrin and phthalocyanine complexes and the nature of intermediates and final products were different.141 Kinetic analysis showed that naphthoquinone and 1-naphthol were formed in parallel reactions, suggesting two reaction pathways for the MnPc(NO2)8−AcOOH system. On the basis of data obtained from product distribution, kinetic isotope effects (kH/kD), and 18O labeling studies on the oxidation of polycyclic aromatic hydrocarbons (anthracene, phenantrene, benzo[a]pyrene) by FePcS−H2O2, the involveN


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ment of FeIVO species was proposed.142 The inverse kH/kD of 0.75 on the oxidation of antracene was explained by the addition of electrophilic FeIVO to anthracene accompanied by the sp2to-sp3 hybridization change to form σ adduct followed by successive 1e− oxidations and proton eliminations to provide anthraquinone. Nemykin and co-workers reported the interesting catalytic properties of μ-oxobis[iron(III)-2,9(10),16(17),23(24)-tetratert-butylphthalocyanine] in combination with hypervalent iodine reagents.143 Using iodosylbenzene sulfate and 2iodylbenzoic acid derivative, a selective oxidation of anthracene to anthraquinone was performed (Scheme 13).

pore SBA-15-mediated aerobic oxidation of PhH to phenol with 100% selectivity and 11.6% yield in the presence of ascorbic acid.148 CoPcCl16, CuPcCl16, and FePcCl16 encapsulated in zeolites X, Y, and L were employed for oxidative chlorination and bromination of benzene, toluene, phenol, aniline, and resorcinol.149 HCl and alkali chlorides or bromides were used as sources of halogens, and H2O2 and O2 were the oxidants. CuPcCl16−Na-X showed the best catalytic performance with the following selectivity to monohalogenated products: anisole (48%), toluene (55%), phenol (64%), aniline (64%), benzene (100%), and resorcinol (100%). MPcS (M = Fe, Mn, Co) anchored onto the surface of mesoporous and amorphous silicas in monomeric or μ-oxo dimer forms150 were compared for the oxidation of 2MN by t BuOOH.151 Manganese and cobalt catalysts showed low catalytic activity. The μ-oxo dimeric FePcS−SiO2 catalyst demonstrated superior catalytic properties compared to homogeneous and monomeric supported catalysts, providing a 34% yield of vitamin K3. The FePcCl16 and FePc(NH2)4 based catalysts were less efficient.151 (FePcS)2O anchored onto MCM41 exhibited lower conversion and selectivity. This can be explained by the increased residence time of the substrate in the MCM-41 porous system that should favor overoxidation reactions and should limit the substrate conversion. Along with 2MQ main oxidation product, isomeric 6-methylnaphthoquinone (6MQ), 2-naphthaldehyde, and naphthoic acid (products of methyl group oxidation) as well as coupling and overoxidation products (epoxyquinones) were formed. All catalysts preferentially oxidized the methyl-containing ring with 2MQ/6MQ regioselectivity between 70:30 and 80:20. Less demanding aromatic oxidation of anthracene and xanthene resulted in 90% and 99% yields of anthraquinone and xanthone, respectively.152

Scheme 13. Oxidation of Anthracene to Antraquinone Catalyzed by (PctBu4Fe)2Oa

a

Adapted with permission from ref 143. Copyright 2009 John Wiley and Sons.

(PctBu4Fe)2O was more efficient than cobalt and ruthenium porphyrin complexes. This complex (5 mol %) was also used in the combination with (Bu4N)HSO5 (6 equiv with respect to substrate) for the oxidation of anthracene, 2-tert-butylanthracene, 2-methylnaphthalene, 9,10-dihydroanthracene, 1,2,3,4tetrahydronaphthalene, indan, ethylbenzene, toluene, and benzene to the corresponding p-quinones with high yields in 5−30 min.144 On the basis of preliminary data, PcFeIV−O− FeIVO(Pc) or PcFeIII−O−FeIVO(Pc+•) have been proposed as active species. The introduction of a fused pyridine ring into the phthalocyanine core led to the increased reactivity of the corresponding μ-oxo diiron complex, making possible the oxidation of 2-methylnaphthalene using oligomeric iodosylbenzene sulfate as the oxidant.145 2-Methyl-1,4-naphthoquinone (vitamin K3) was obtained with 32% yield. 4.4.2. Heterogeneous Catalytic Reactions. Solvent-free oxidation of toluene was performed in the presence of zeolites containing CoPcCl16, FePcCl16, CoPc(NO2)4, and FePc(NO2)4 using O2, H2O2, and cumene hydroperoxide as oxidants.146 These apparently free radical oxidations generated a mixture of products of benzylic oxidation and hydroxylation of the aromatic ring with no distinct dependence on the catalyst structure. The highest conversions were obtained with cumene hydroperoxide (79−89%) and then with H2O2 (16−87%) and with air (7−9%). The aerobic oxidation of toluene provided benzaldehyde as the main product with 57−79% selectivity. Systems with cumene hydroperoxide and H2O2 were less selective, providing different mixtures of products of benzylic and aromatic oxidations, mainly PhCH2OH, PhCHO, PhCOOH, o-cresol, and p-cresol.146 Vanadyl tetraphenoxyphthalocyanine showed superior catalytic properties compared with VO(acac)2, V2O5/Al2O3, V2O5/ TiO2, and V2O5/ZrO2 in direct H2O2 hydroxylation of benzene, toluene, and anisole to phenol, cresols, and methoxyphenols, respectively.147 Vanadium phthalocyanine encapsulated in large-

4.5. Oxidation of Phenols

Selective oxidation of phenols to quinones is the key step in the preparation of many vitamins and valuable synthetic precursors. Quinones are powerful intermediates in organic synthesis and show biological activity since quinone fragments often occur within the molecular frameworks of natural products. Oxidative coupling of phenols affords bisphenol derivatives. 4.5.1. Homogeneous Catalytic Reactions. (FePctBu4)2O complex catalyzed aerobic oxidation of 2,6-di-tert-butylphenol to 3,3′,5,5′-tetra-tert-butyl-p-bisphenoquinone or 3,3′,5,5′-tetratert-butyl-p-biphenyl-4,4′-diol (Scheme 14).153 Scheme 14. Oxidative Dimerization of 2,6-Di-tertbutylphenol

The product composition depended on the substrate:catalyst ratio. While bisphenol was obtained with a substrate:catalyst ratio of 1500, bisquinone was the principal product at the ratio of 100. CoPcS catalyzed the oxidation of 2,6-di-tert-butylphenol by t BuOOH to coupling products.154 Under optimal conditions, 3,3′,5,5′-tetra-tert-butyl-4,4′-diphenoquinone and 4,4′-dihydroxy-3,3′,5,5′-tetra-tert-butylbiphenyl were obtained in 84 and 49% yields, respectively. The CoPcS catalyst underwent a rapid O


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provided more selective and stable catalyst: TMQ was obtained with 87% yield at 97% TMP conversion.157 Selective oxidation of only one group in compounds containing several oxidizable functions keeping other sites intact is a significant synthetic challenge. A case example is the oxidation of functionalized phenols to quinones bearing additional functional groups. Although functionalized quinones can be prepared using multistep synthetic strategies, the selective aromatic oxidation of readily available phenols is the most direct approach. d-FePcS−SiO2 catalyst in combination with tBuOOH was found to be quite selective in the aromatic oxidation of phenols containing easily oxidizable groups such as alcohol groups, double and triple bonds, and benzylic positions (Table 8) with no need for protection/deprotection procedures.23,50,158 The presence of functional groups enhances the versatility of the quinone building blocks for the synthesis of elaborated structures. Catalyst loading was generally 0.5−1 mol % and the product yields were in the range of 40−60%. The quinones bearing double and triple bonds have been obtained for the first time. The preparation of quinoline-5,8-dione by direct oxidation of 8-hydroxyquinoline (8-HQ) with 61% yield is another important example. Metal catalysts can often be inactivated by this particulate substrate because of coordination of the metal site with the quinoline nitrogen atom.159 Iron phthalocyanine supported catalysts perform this oxidation with high turnover frequency values of 215−3570 h−1. The catalytic performance of these catalysts decreased upon recycling. The observed high selectivity of aromatic oxidation versus oxidation of oxidizable substituents can be explained by the coordination of the phenolic group to iron phthalocyanine, followed by interaction with tBuOOH and by two successive one-electron oxidations (Scheme 15). This mechanism was proposed on the basis of 18O2 labeling experiments, EPR spectroscopy with spin traps, kinetic studies, and complete analysis of all the reaction products.160 The trace amounts of coupling products, low 18O incorporation to products from 18O2, and the absence of EPR signals in the presence of 3,5-dibromo-4-nitrosobenzenesulfonic acid (DBNBS) spin trap indicate that phenoxyl radicals were not formed in an appreciate quantity. The mechanism of phenol oxidation in the presence of FePcS-based catalysts is different from that of titanium single-site solids, which are also efficient catalysts for the oxidation of phenols. The formation of phenoxyl and hydroxyl radicals during the oxidation process has been detected using DBNBS and 5,5-dimethyl-1-pyrroline N-oxide (DMPO) spin traps.161 The selectivity of phenol oxidation depends on the nature of solvent. Consequently, the solvent dependence should be studied in order to obtain a high quinone yield. In practice, acetonitrile, acetone, and dichloroethane are among the best solvents for these reactions. Appropriate change of the structure of the supported catalyst and the reaction conditions allows tuning of the catalytic activity. A detailed study of these variables was performed in the tBuOOH oxidation of 8-hydroxyquinoline to quinoline-5,8-dione.159 The catalytic oxidation showed high turnover frequency values (215−3570 h−1) depending on the catalyst structure. The crucial factors affecting the product yield were the supporting procedure, the nature of solvent, the catalyst amount, the substrate concentration, and the oxidant:substrate ratio. Under optimal conditions, the selectivity to quinone was 70% at a 94% substrate conversion. Hot catalyst filtration experiments showed a truly heterogeneous oxidation, but the conversion and selectivity decreased in the second cycle to 66%

degradation in the course of the reaction. The high selectivity to coupling products and marked instability of the catalyst suggest an involvement of one-electron radical pathways. The same selectivity of 2,6-di-tert-butylphenol oxidation was observed in the FePcS−KHSO5 system.155 Noteworthy, the selectivity of phenol oxidation was sensitive to the nature of oxidant. Oxidation of 2,3,6-trimethylphenol (TMP) with KHSO5 in the presence of FePcS using an oxidant:substrate:catalyst ratio of 1200:300:1 was claimed to result in a quantitative yield of 2,3,5-trimethylbenzoquinone (TMQ, precursor of vitamin E) after 5 min.156 The high selectivity to TMQ formation was explained by quenching of radicals by MeOH used as the solvent in an 8:2 mixture with water. When a CoPcS more proned to radical chemistry was employed as the catalyst, TMQ was obtained with 49% yield. 4.5.2. Heterogeneous Catalytic Reactions. Selective oxidation of phenols catalyzed by iron phthalocyanines can afford high yields of quinones, which are important intermediates for the preparation of vitamins and drugs. In this context, application of heterogeneous catalysts, which can be readily separated from the reaction mixture and recycled, can be advantageous. It is also important to suppress concurrent oneelectron oxidation, leading to radical intermediates and finally to coupling products. The oxidation of TMP to TMQ was performed in the presence of supported phthalocyanine complexes (Table 7).49 Table 7. Oxidations of 2,3,6-Trimethylphenol to 2,3,5Trimethylbenzoquinone Catalyzed by Silica-Supported MPcSa,b,c entry

catalyst

TMP conversion, %

TMQ yield, %

1 2 3 4 5 6 7

FePcS d-FePcS−MCM-41 m-FePcS−MCM-41 d-FePcS−SiO2 m-FePcS−SiO2 MnPcS−SiO2 MnPcS−MCM-41

96 98 98 96 95 65 94

47 24 21 77 42 22 47

a

Data adapted from ref 49. bConditions: tBuOOH, CH2ClCH2Cl, 30 °C, 2 h. cd-FePcS, μ-oxo dimeric form, m-FePcS, monomeric form

The catalytic activity of MPcS covalently supported onto mesoporous MCM-41 and amorphous Degussa aerosol 200 silicas was the same, but the selectivity to TMQ was notably higher in the latter case. The possible reason is the easier overoxidation of TMQ inside mesopores. However, manganesesupported catalysts showed the opposite behavior: MnPcS− MCM-41 provided 47% yield of TMQ vs 22% in the case of MPcS−SiO2 catalyst. Supported iron dimeric catalyst was more selective than the homogeneous FePcS and the supported monomeric catalyst, affording 77% TMQ yield compared with 47% and 42%, respectively. The d-FePcS−SiO2 was reused in the repetitive TMP oxidations without any regeneration procedure. The catalytic activity remained the same in three cycles (96, 97, and 93% conversions), but the TMQ yield was gradually decreased: 77, 59, and 51%.49 The UV−vis study of the recycled catalyst showed a progressive transformation of the μ-oxo dimer to the monomeric FePcS, resulting in a decrease of the oxidation selectivity. This lack of stability of the μ-oxo diiron structural unit under reaction conditions is the principal inactivation pathway. The stabilization of the supported dimer by covalent linking of two adjacent phthalocyanine molecules using diamine spacer P


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Table 8. Heterogeneous Oxidation of Functionalized Phenols Catalyzed by d-FePcS−SiO2d

Conditions: d-FePcS−SiO2, tBuOOH, acetone, 30 °C. bIn MeCN. cIn dichloroethane, 80 °C. dAdapted with permission from ref 23. Copyright 2011 Elsevier.

a

and 41%, respectively. According to UV−vis data, the possible reason was the oxidative degradation of phthalocyanine ligand.

The improvement of the stability of the ligand by structural modification might lead to catalysts more amenable to recycling. Q


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Scheme 15. Proposed Mechanism of Oxidation of Phenols by t BuOOH Mediated by FePcS−SiO2a

Scheme 16. Structures of Natural Naphthoquinones Prepared by Aerobic Oxidation in the Presence of FePc−ZrPO4

The FePcS was incorporated into Mg−Al hydrotalcite layered materials with different Al3+ content.164 Higher Al3+ content favored adsorption of the μ-oxo dimer form, which was more efficient in catechol oxidation. 4.6. Oxidation of Alcohols

4.6.1. Homogeneous Catalytic Reactions. The oxidation of C1−C4 alcohols was performed by FePcS and RuPcS (0.02−2 mol %) in combination with H2O2 or KHSO5 in aqueous solutions at 20 °C.165 Primary alcohols were oxidized to carboxylic acids with no evidence of intermediate aldehyde formation. Secondary alcohols were rapidly oxidized to ketones, but their overoxidation to Baeyer−Villiger products and C−C bond cleavage products was detected. The RuPcS was more efficient with KHSO5 (TON up to 500), whereas the FePcS provided higher conversions with H2O2. Cyclobutanol was oxidized to cyclobutanone and a minor amount of γbutyrolactone (from the oxidation of cyclobutanone) in MPcS−KHSO5 systems, suggesting a two-electron oxidation mechanism, since undetected noncyclic products should be formed in one-electron oxidation. No kinetic isotope effect was observed in the oxidation of MeOH and EtOH, indicating that C−H cleavage was not the rate-limiting step.165 PtPcS catalyzed mild aerobic oxidation of allylic alcohols to carbonyl compounds with low efficiency.166 In the presence of 20 mol % PtPcS, 18− 55% conversions of aliphatic, benzylic, and allylic alcohols were obtained after 14 days of reaction. CoPc in combination with powdered KOH was used for the aerobic oxidation of 13 secondary alcohols.167 In the presence of 5 mol % CoPc and 1 equiv KOH, ketones were obtained in 70− 97% isolated yields after 0.08−15 h reflux in xylene. Aerobic oxidation of primary and secondary alcohols catalyzed by CoPc in [bmim]Br ionic liquid furnished aldehydes and ketones with 81−92% yields (Scheme 17).86

a

Adapted with permission from ref 160. Copyright 2009 Centre National de la Recherche Scientifique (CNRS) and Royal Society of Chemistry.

The FePcS−MIL-101 hybride materials show a high catalytic activity in oxidation of TMP and 8-HQ by tBuOOH to corresponding quinones, do not suffer from chromium or iron leaching, and behave as true heterogeneous catalysts.69 The selectivity of oxidation depended on the choice of the oxidant. FePcS−SiO2 catalyst can also be used for the synthesis of biaryl compounds, which are important scaffolds in pharmaceuticals. For example, by using O2−IBA system TMP was oxidized to 2,2′,3,3′,5,5′-hexamethyl-4,4′-biphenol in 86% yield.130 FePctBu4 grafted via coordination onto NH2−MCM-41 was more resistant toward oxidative degradation and was more active than FePctBu3(OH) covalently anchored onto SBA-15 in the oxidation of phenol by H2O2 to catechol and hydroquinone.162 The FePc and CoPc encapsulated in NaY zeolite were more active than the neat complexes in the aerobic oxidation of hydroquinone to benzoquinone with up to 24 turnovers for 4 h.163 The FePc supported onto montmorillonite K10 or on lamellar ZrPO4 was more efficient in the oxidation of hydroquinones to quinones (five examples, 89−98% yields).134 Several important natural naphthaquinones, e.g., juglone, menadione, lawsone, and phthiocol, were prepared by oxidation of hydroquinone precursors with high yields (Scheme 16).

Scheme 17. Aerobic Oxidation of Alcohols in CoPc− [bmim]Br System at 70°C

The catalyst in ionic liquid could readily be separated from the products and reused in six successive oxidations. Efficient aerobic oxidation of α-hydroxy ketones to α-diketones (11 examples) in near quantitative yields in the presence of 2 mol % CoPc(SO2NH2)4 in refluxing MeOH was reported.168 The (PctBu4Fe)2O catalyst in combination with the esters of iodoxybenzoic acid performed oxidation of primary and secondary alcohols to carbonyl compounds (Scheme 18).169 The reaction showed an especially high selectivity with benzylic alcohols: corresponding aldehydes and ketones were R


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Scheme 18. Oxidation of Alcohols Catalyzed by (PctBu4Fe)2O

sustainable approach consists of the introduction of functional groups to the biopolymer backbone in a one-pot process in water avoiding generation of wastes, separation steps, and large energy consumption. A practical, clean, and single-step oxidative modification of starch with H2O2 catalyzed by water-soluble MPc’s has been developed.175−178 The solid starch was successively impregnated with aqueous solutions of the catalyst and H2O2 followed by intensive mixing. The oxidation process was performed at dry static conditions. The screening of different MPcS (M = Fe, Co, Mn, VO) revealed that FePcS was the most efficient, providing 95−98% yields of oxidized starch with less than 0.01 mol % catalyst loading. Desired amounts of carbonyl and carboxyl groups per 100 anhydroglucose units (typically 1− 20) can be obtained via the cleavage of the C2−C3 bond and the oxidation of the primary C6 hydroxyl group.175 Turnovers of more than 2000 and high yields based on H2O2 (around 70%) indicate the efficiency of this method. Peroxo complex PcSFeIII− OO− was proposed to cleave the C2−C3 bond of the anhydroglucose unit according to the mechanism outlined in Scheme 19. This approach was successfully applied to the modification of hydroxyethylcellulose, carboxyethylcellulose, guar gum, and inuline.179,180 Hydrophilic tailor-made materials with desired balance of hydrophilic/hydrophobic properties and solubility in water due to carboxyl groups can be obtained. In turn, carbonyl functions can be used for grafting the different molecules to provide materials with special properties. Due to clean and practical character (cheap FePcS catalyst, clean H2O2 oxidant, water with no acid or buffer additives, no separation steps, no wastes), this method shows great promise for applications in cosmetic, paper, and textile industries. The FePcS−H2O2 system was used for the delignification of holocellulose to extract 4-O-methylglucuronoxylanes.181 This procedure is the cleaner alternative to classical delignification methods based on sodium chlorite and alkaline extraction. Veratryl alcohol (VA, 3,4-dimethoxybenzyl alcohol) is a metabolite of lignin biodegradation and is often used as a lignin model. FePcS and MnPcS were used for oxidation of VA to aldehyde by m-CPBA and H2O2 with ∼30% yields, owing to the low stability of the catalyst.182 The FePcS−H2O2 system showed better catalytic properties in the oxidation of VA, 4-hydroxy-3methoxytoluene, and 3,4-dimethoxytoluene under acidic conditions.183 On the other hand, the CoPcS was more efficient in aerobic oxidation of these substrates at pH 11, providing 76− 100% yields. FePc solubilized in ionic liquid was used for aerobic oxidation of chitosan.184 The catalyst was separated by decantation and reused in seven successive runs without loss of activity.

obtained with 91−95% isolated yields. Secondary aliphatic and aromatic alcohols were oxidized by CuPc−n-Bu4NHSO5 system at 70 °C for 30 min with 55−95% yields.170 The oxidation of primary alcohols was less selective, providing aldehydes with 15− 55% yields. 4.6.2. Heterogeneous Catalytic Reactions. Immobilization of FePc, CoPc, and CuPc onto polystyrene matrix furnished the catalysts for aerobic oxidation of alcohols in refluxing toluene.171 All heterogeneous catalysts exhibited comparable activity in the oxidation of phenethyl alcohol, benzoin, mandelic acid, and benzyl alcohol, affording the oxidation products with 90−96% yields for 0.2−6.5 h at 2.5 mol % catalyst loadings. CoPc, FePc, and MnPc supported onto γ-Al2O3 catalyzed oxidation of cyclohexanol to cyclohexanone by tBuOOH, providing 88, 66, and 46% yields, respectively.172 In combination with CoPc−γ-Al2O3, tBuOOH was a more efficient oxidant compared with H2O2 in the oxidation of PhCH2OH and nhexanol to corresponding aldehydes with a TON as high as 600 to afford 65 and 48% aldehyde yields, respectively. Noteworthy, significant degradation of the catalysts was observed in the presence of H2O2. MPc’s were more stable when used in combination with tBuOOH. CuPc immobilized on MCM-41, Ti-MCM-41, and Al-MCM41 was used for the oxidation of PhCH2OH to PhCHO by O2 and peroxides.173 The most efficient CuPc−Al-MCM-41catalyst showed 47% conversion in the oxidation by tBuOOH with 100% selectivity to PhCHO. Using O2 as oxidant the selectivity to PhCHO was 90% at 45% conversion. The triple catalytic system consisting of FePc−ZrPO4, hydroquinone, and RuCl2(PPh3)3 performed aerobic oxidation of PhCH2OH to PhCHO with up to 98% yield according to the catalytic cycle shown in Scheme 10.134 4.7. Oxidation of Polysaccharides and Ligninocelluloses

The current interest in using renewables as raw materials has initiated a search for clean and cost-effective approaches to their transformation in useful products.174 In this context, polysaccharides are abundant, nontoxic, biodegradable, natural polymers already possessing a high degree of functionalization. Their elaborated polymeric nature can be useful for many purposes. However, to fulfill demand for tailored application profiles, native biopolymers often need to be modified. A

4.8. Oxidation of Sulfur Compounds

4.8.1. Homogeneous Catalytic Reactions. This topic is closely connected to the oxidative desulfurization of fuels. The overall reaction is presented in Scheme 20. Cobalt and vanadium phthalocyanines are more efficient compared to their iron, manganese, or molybdenum counter-

Scheme 19. Proposed Mechanism of the Oxidative Modification of Polysaccharides by FePcS−H2O2 System

S


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presence of CuPc furnished sulfones with 65−100% yields (nine examples).170 Aliphatic sulfides provided better yields, and the presence of double bond or hydroxyl group in the sulfide was tolerated. 4.8.2. Heterogeneous Catalytic Reactions. CoPc(COOH)4 and CoPc(NH2)4 were covalently grafted onto silica with low surface area modified with 3-aminopropyltriethoxysilane and 3-chloropropyltrimethoxysilane, respectively.193 High surface coverage was favorable for self-association between Pc molecules. The catalytic activity of supported catalysts in the oxidation of 2-mercaptoethanol was comparable with those of homogeneous complexes (TOF ∼ 570−1000 min−1). Substituted CoPc were also coordinatively loaded onto imidazolylpropyl-modified SiO2.194 Higher TOFs, up to 2290 min−1, were obtained with CoPc containing electron-withdrawing substituents in the aerobic oxidation of 2-mercaptoethanol. Oxidation of sulfides with n-Bu4NHSO5 catalyzed by CuPcS supported onto Fe3O4 magnetic particles coated by amino-modified silica provided either sulfoxide or sulfone products, depending on the solvent.66

Scheme 20. The Merox Process

parts. CoPcS2 (disulfophthalocyanine) prepared by sulfonation of CoPc is a commonly used commercial catalyst in the sweetening process. Sulfonamide complex CoPc(SO2NH2)4 was reported to show a better catalytic activity.185 Two reaction pathways involving either Co(II)/Co(III) or Co(II)/Co(I) species are possible.36 Kinetic and binding studies showed that the rate-limiting step of the latter mechanism is an electron transfer from thiol substrate to cobalt to form RS•−CoIPc intermediate.186 Dendritic cobalt phthalocyanine showed enhanced stability and catalytic activity in the aerobic oxidation of 2-mercaptoethanol with a TOF up to 339 min−1.187 The construction of a dendrimer around a catalytic site can allow possible control of the catalytic properties by the incorporation of a catalytic site at appropriate depth and by the introduction of recognition moieties for the substrates. Introduction of the cationic groups at the periphery of cobalt phthalocyanine increased the rate of the aerobic oxidation of 2-mercaptoethanol at neutral pH up to 414 min−1.188 The catalytic activity was affected by the solvent polarity and by the length of the alkyl chains bearing quaternary ammonium groups. Related to the problem of desulfurization of fuels, aerobic oxidation of dibenzothiophene was performed in the presence of FePc(NO2)4 at 100 °C.189 A deep oxidation of dibenzothiophene by H2O2 or KHSO5 has been achieved in the presence of FePcS and RuPcS, respectively.190 The recyclable CoPcS−[bmim]BF4 system was used for the aerobic oxidation of thiols at room temperature.191 Aromatic and aliphatic disulfides were obtained in 83−99% yields (12 examples). High turnover numbers have been achieved (up to 27000). Highly fluorinated CoPcF16 and CoPcF8(i-C3F7)8 were used for aerobic oxidation of 2-mercaptoethanol to corresponding disulfide showing high turnover frequencies of 0.84 and 1.74 s−1 and high TONs of 7700 and 13 000, respectively. CoPcF16 and CoPcF8(i-C3F7)8 were active even in the oxidation of perfluorothiophenol, with 32% and 53% yields of disulfide, respectively.36 The dimerization C6F5S-p-C6F4SH product owing to nucleophilic reaction was the side product. Noteworthy, the oxidation does not occur with regular CoPc. Fluorinated biphasic system was also used for oxidation of parasubstituted methyl phenyl sulfides by IBA−O2 in the presence of CoPc(C8F17)4.192 In contract to the fluorinated porphyrin counterpart, which yielded sulfones, CoPc(C8F17)4 selectively afforded sulfoxides, though with lower conversions (40−78%, six examples). Surprisingly, catalyst recycling through phase separation was unsuccessful because of the partial degradation of the complex.192 Oxidation of sulfides by n-Bu4NHSO5 in the

4.9. Miscellaneous Oxidations

An interesting example of synthetically useful catalytic application of the μ-oxo diiron phthalocyanines was published by Lindsey and co-workers.195,196 The porphyrin synthesis by the pyrrole−aldehyde condensation method requires oxidation of the porphyrinogen, which is usually carried out by p-chloroanil taken in stoichiometric amounts. The use of 5 mol % of FePc (transformed to the μ-oxo dimer) allowed decreasing the pchloroanil amount to 5 mol % without compromising the porphyrin yield in the high concentration conditions. Further improvement was achieved by using preliminarily prepared (FePctBu4)2O, [FePc(n-C6H13)4]2O, and the two isomers of (FePc)2O, μ-oxo(1) and μ-oxo(2). All μ-oxo dimers used in 0.3− 1 mol % amounts in combination with 1 mol % quinone or hydroquinone provided ∼25% yield of tetraphenylporphyrin in 60 min using O2 as the terminal oxidant. Noteworthy, the monomeric FeP was inactive when used in these amounts. This aerobic oxidation process is much cleaner than the stoichiometric oxidation with DDQ or p-chloroanil and can be performed in the presence of BF3·(Et2O) 2, CF3COOH, or under neutral conditions. The workup of the reaction mixture was substantially easier and allowed scaling up the porphyrin preparation. Another example of site-selective oxidation mediated by FePc is the preparation of α,β-acetylenic ketones by the oxidation of alkynes and propargylic alcohols without transformation of the triple bond.197 Due to the presence of the adjacent triple bond and carbonyl group, α,β-acetylenic ketones are very useful building blocks for enantioselective total synthesis and for the preparation of heterocyclic compounds, nucleosides, nonproteinogenic amino acids, pheromones, and drugs. Conjugated

Table 9. Oxidation of Alkynes by tBuOOH Catalyzed by Supported Metallophthalocyaninesa,b

a

catalyst

substrate

product

conversion, %

selectivity, %

FePcS−SiO2 FePcS−SiO2 FePcS−SiO2 FePcS−SiO2 FePcS−SiO2 FePcCl16−SiO2 FePcCl16−SiO2

C3H7CCC3H7 PhCCC2H5 HCCC6H13 HCCC(OH)C5H11 CH3CCC5H11 C3H7CCC3H7 PhCCC2H5

C3H7CCC(O)C2H5 PhCCC(O)CH3 HCCC(O)C5H11 HCCC(O)C5H11 CH3CCC(O)C4H9 C3H7CCC(O)C2H5 PhCCC(O)CH3

80 75 38 84 64 79 86

89 83 38 100 47 88 93

Data from ref 197. bConditions: catalyst:alkyne:oxidant = 1:50:200, 40 °C, 24 h. T


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ing aromatic and aliphatic ketones and aldehydes with 40−82% yields. Oxidative polymerization of 3,4-ethylenedioxythiophene201 and aniline was performed using the FePcS−H2O2 system.202 (FePc)2O showed a high activity in the aerobic polymerization of pyrrol (TON > 1000).203 Interestingly, μ-oxo diiron porphyrins and monomeric species were significantly less active.

ynones are usually prepared by the acylation of metal acetylenides and by multistep syntheses, and only few methods have been reported using direct stoichiometric α-oxidation of alkynes. Heterogeneous catalysts FePcS−SiO2 and FePcCl16− SiO2 were used for the selective oxidation of alkynes and propargylic alcohols to α,β-acetylenic ketones with high selectivity except terminal alkynes (Table 9). High intramolecular kH/kD values were measured using C3H7CCC3D7 containing C−H and C−D bonds in the same stereochemical environment.152 The formation of propargylic alcohol occurred with kH/kD of 3.5 and 6.5 for FePcS−SiO2 and FePcCl16−SiO2, respectively. In turn, the formation of ketone proceeded with a higher kH/kD of 9.3 and 18.1 for FePcS−SiO2 and FePcCl16−SiO2, respectively. The dependence of intramolecular kH/kD on the catalyst structure strongly suggests the involvement of FePc-based active species abstracting the propargylic H atom in the first rate-limiting step. Three successive oxidations of 1-phenyl-1-butyne showed that the catalytic activity of FePcCl16/SiO2 remained high, the conversions and selectivities being 86, 84, 73% and 93, 100, 95%, respectively. The catalyst recycled after three oxidations exhibited the same DR UV−vis spectrum as that of the initial supported catalyst.197 Manganese tetrakis(2-methoxy-4-formylphenoxy)phthalocyanine was covalently anchored onto amino-modified silica.198 This recoverable supported catalyst was efficient in the oxidative esterification of aryl, allyl, and alkyl aldehydes with primary alcohols using H2O2 oxidant to provide esters with TON = 170−333 (Scheme 21).

5. PREPARATION OF NITROGEN-CONTAINING COMPOUNDS 5.1. Homogeneous Catalytic Reactions

The introduction of nitrogen functionalities via amination of C− H bonds is particularly important because of their prevalence in biologically active molecules and the relative difficulty of incorporating nitrogen into molecular frameworks.204 FeIIIPc(SbF6) efficiently catalyzed intramolecular allylic amination, the reactivity of the reaction following the trend in homolytic bond dissociation energies (Scheme 23). Scheme 23. FePc-Catalyzed Intramolecular C−H Amination and Reactivity Trend of This Reactiona

Scheme 21. Oxidative Esterification of Aldehydes with Alcohols by the MnPc−SiO2−H2O2 System a

Adapted with permission from ref 204. Copyright 2012 American Chemical Society.

Hence, allylic C−H amination is strongly preferred over aziridination and amination of all other C−H bond types, making possible highly selective reactions. Noteworthy, iron phthalocyanine was a more efficient catalyst than the porphyrin, nonheme, and salen complexes of iron. Iron salen complex showed no reactivity. The better catalytic properties of FeIIIPc(SbF6) were explained by the increased electron-withdrawing character of phthalocyanine ligand, which makes a more electrophilic iron center. The FePc−PhI(OR)2 system is superior to traditional Rh- and Ru-based systems and exhibits the highest reported chemoselectivities for intramolecular allylic C−H amination over aziridination (>20:1). The reaction shows a large substrate scope and can be performed with allylic, benzylic, etherial, tertiary, and secondary C−H bonds. Furthermore, this difference in reactivity due to electronic and steric factors was remarkably used to perform very selective aminations of elaborated substrates containing numerous C−H bonds of different types.204 For polyolefin substrates, allylic C−H amination occurs at the most electron-rich, least sterically hindered site up to 14:1 selectivity ratio. Reactivity trends and mechanistic studies support a stepwise process involving an initial homolytic C−H bond abstraction followed by a rapid radical rebound.204 MPc, MPcCl16, MPcF16, and MPc(m-OC6H4−CF3)4 (M = Fe, Mn, Ni, Cu, Co) catalyzed nitrene transfer reactions.205 The azidirination of aromatic and aliphatic alkenes and the amidation

Secondary and tertiary alcohols showed low activity in this reaction. The proposed mechanism involves the oxidation of the hemiacetal intermediate by postulated Mn(IV) oxo species. FePcS−SiO2 was more efficient than Fe-pillared clays in Baeyer−Villiger oxidation of cyclohexanone to caprolactone by O2 in the presence of 2 equiv of PhCHO at 25 °C (Scheme 22).199 Scheme 22. Baeyer−Villiger Oxidation of Cyclohexanone

The product yield of 61%, selectivity above 95%, and TON = 4300 were obtained using a small amount of the heterogeneous catalyst (1.4 μmol of FePcS for 10 mmol substrate). The ICP analysis showed no iron leaching. Aerobic oxidation of 15 trimethylsilyl ethers to the corresponding carbonyl compounds was studied in the presence of Co-, Zn-, Cu-, Ni-, Pd-, and RuPc.200 The best catalyst for this transformation was CoPcS in combination with 1-butyl-3methylimidazomium chloride ([bmim]Cl) ionic liquid, providU


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of C−H bonds with (N-p-toluenesulfonyl)imino)phenyliodinane (PhINTs) were the most efficient using the FePc(m-OC6H4-CF3)4 catalyst in the presence of molecular sieves (Scheme 24).

Scheme 26. Preparation of 3,4-Dihydropyrimidinones

Scheme 24. Aziridination of Olefins and Amidation of C−H Bonds Catalyzed by FePc(m-OC6H4−CF3)4 presence of CoPc followed by an intramolecular amidation.209 The isoindolinone fragment can be found in many drugs of natural and synthetic origin. This one-pot approach is a cheaper and cleaner alternative to methods based on Pd and Pt catalysts and affords high chemoselectivity with excellent yields with most of the studied aliphatic and aromatic amines (Scheme 27). Scheme 27. Synthesis of N-Substituted Isoindolinones

Along with PhINTs, chloramine T and bromamine T can also be used as less expensive nitrene donors, but lower product yields were obtained.206 The CuPc (5 mol %) was more efficient in the aziridination of 4-methylstyrene by PhINTs, affording a 90% yield as compared to 40, 35, 20, and 10% yields for FePc, MnPc, NiPc, and CoPc, respectively. Aromatic alkenes gave better yields of aziridines (six examples, 50−92%) as compared to aliphatic alkenes (four examples, 48−72%) after 0.5−3.5 h of the reaction in the presence of molecular sieves and under N2 atmosphere at room temperature. Stereoselective one-pot preparation of N-substituted β-amido ketone derivatives by condensation of aromatic aldehydes, ketones, and acrylonitrile was catalyzed by CuPc, AlPc(Cl), and ZnPc.207 With more efficient CuPc, reaction showed a large substrate scope with no significant influence of the nature of substituent on aldehyde or ketone moieties (Scheme 25).

5.2. Heterogeneous Catalytic Reactions

Oxidative cyanation of tertiary amines in the presence of polymer-supported FePcS, NaCN, and H2O2 furnished α-amino nitriles with high yields and selectivity.210 α-Amino nitriles are versatile intermediates in organic chemistry, since they can be transformed to α-amino acids, α-amino alcohols, α-amino carbonyls, and 1,2-diamines. Most of the current methods involve multistep syntheses and/or heavy metal or halogencontaining oxidants and stoichiometric reagents. Oxidative onepot cyanation using H2O2 and recyclable heterogeneous catalyst represents a cleaner approach (Scheme 28).

Scheme 25. Preparation of N-Substituted β-Amido Ketones

Scheme 28. Oxidative Cyanation of Tertiary Amines Ortho-substituted aldehydes formed amido carbonyl compounds with ∼90% anti-diastereoselectivity irrespective of the substituent patterns of aldehydes or ketones (13 examples).207 Propionitrile (eight examples), as well as acetonitrile, benzonitrile, o-tolunitrile, benzyl cyanide, and 3-bromopropionitrile, was converted to the corresponding N-substituted β-amido ketone with moderate to good yields. Thus, a large array of functionalities present in aldehydes, ketones, and nitriles are tolerated. Biginelli condensation of aromatic aldehyde, β-carbonyl compound, and urea was performed in the presence of MPc’s (M = Co, Cu, Fe, Ru, VO) to afford 3,4-dihydropyrimidinones possessing diverse pharmaceutical properties (Scheme 26).208 The best yields were obtained using CoPc catalyst, which could be recycled three times. Aliphatic and aromatic aldehydes bearing electron-donating and electron-withdrawing substituents can be used in this clean, one-pot synthesis involving cheap and accessible catalyst. N-Substituted isoindolinones were obtained by the reductive amination of 2-carboxybenzaldehyde using diphenylsilane in the

FePcS was covalently anchored onto amino-methylated polystyrene resin and MeO−PEG-5000−NH2 via sulfonamide linkage with 0.49 and 0.12 mmol g−1 loadings, respectively. The polystyrene-supported FePcS showed a lower catalytic activity than the homogeneous FePcS, but PEG-supported catalyst was comparable in activity with the homogeneous FePcS. Both supported catalysts could be recycled five times without loss of the catalytic activity. No leaching of iron into solution was evidenced by ICP-AES analyses of the filtrates and recovered catalysts. This method was used for N,N-dimethylanilines bearing electron-donating (82−90% yields) or electron-withdrawing groups (18−84%) as well as for aromatic derivatives of cyclic amines such as piperidine (78−80%), pyrrolidine (76− V


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Scheme 29. Synthesis of Butyl N-Phenyl Carbamate

FePc to afford 47−82% yields of addition products (Scheme 32).213

80%), and tetrahydroisoquinoline (80−89%). Interestingly, a selective cyanation of the methyl rather than the methylene group of the ethyl substutuent was observed for N-methyl-Nethylaniline (83−85%).210 Zeolite Y-encapsulated MPc (M = Cu, Ni, Co) were used for the preparation of alkyl and aryl carbamates from amines, CO2 and alkyl halides in high yields under mild reaction conditions (Scheme 29).211 The catalytic activity followed the order NiPc−Y (TOF = 135 h−1) > CuPc−Y (107 h−1) > neat CuPc (85 h−1) > CoPc−Y (72 h−1). All MPc−Y catalysts showed comparable conversions and product selectivities. This approach can be considered as an alternative to the traditional phosgene/isocyanate technology.

Scheme 32. Oxidative Addition of Alkoxycarbonyl Radicals to α-Methylstyrene

Noteworthy, simple iron salts, FeCl3 and Fe(NO3)3, gave low product yields. The scope of this method was tested in the reaction of methyl carbazate with 13 olefins. Isolated product yields ranging from 21 to 81% were obtained. 2-Arylpropenes with conjugated double bonds were especially suitable substrates. A plausible mechanism includes the intermediate formation of alkoxycarbonyl radical via diazene, its addition to alkene followed by trapping of the resulting radical by O2. This peroxo radical reacts with FePc to afford β-hydroxy ester product via alkoxy radical abstracting hydrogen atom from carbazate or diazene. The selective formation of C−C bonds via activation of C−H bonds is a challenging task. Iron complexes in combination with peroxides usually oxidize olefins to epoxides and/or to allylic oxidation products. However, when this reaction was performed in an inert atmosphere in the presence of (PctBu4Fe)2N and t BuOOH, along with the expected products from the allylic oxidation, the ketone from the addition of the aldehyde to the olefin double bond was produced (Scheme 33).214 The preparation of methyl ketones is performed with 100% atom efficiency: the products contain all atoms of both substrates. When FeSO4·7H2O or mononuclear FePctBu4 were used instead of the (FePctBu4)2N, a product composition typical for usual radical oxidation with no formation of the methylketone was obtained. Thus, the presence of (FePctBu4)2N was essential for the hydroacylation reactivity. Under optimal reaction conditions, hydroacylation products can be obtained with up to 92% selectivity. The (FePctBu4)2N−tBuOOH system was competent in the functionalization of cyclic and linear olefins, including olefins bearing oxidizable functional groups such as allylic and benzylic positions, and phenol and alcohol functions, without the need of protecting groups. The octene-1 and 2cyclohexen-1-one afforded only anti-Markovnikov products. The anti-Markovnikov selectivity for allylbenzene and 2-propene-1-ol was 95 and 83%, respectively. In addition to the large substrate scope, the important features of this system are also a very low catalyst loading (typically 0.01 mol %), very high turnover numbers for the formation of methyl ketones achieved (3600− 5700), and a high selectivity to hydroacylation products (six examples, 52−92% depending on the olefin structure). A reaction mechanism explaining the high performance of the (FePctBu4)2N in the hydroacylation reaction was proposed.214 A very interesting approach was developed by Reetz and Jiao.215 They have prepared a series of conjugates of CuPcS with serum albumins, proteins acting as transport carriers for a large

6. C−C BOND FORMATION 6.1. Homogeneous Catalytic Reactions

The formation of C−C is typically associated with the noble metal catalysts (Pd, Rh, Ru, etc.). Heck and Suzuki reactions are catalyzed by “ligandless” Pd species released in the reaction solution from homogeneous and heterogeneous catalysts. Irreversible precipitation of Pd black provokes a drop of the catalytic activity. To overcome this, a novel concept was proposed: a controlled and reversible release of Pd from macrocyclic complexes should provide a “homeopathic Pd concentration in solution which should lead to high C−C coupling activity”.212 Compared to the PdTPP and a Robsontype Pd bimetallic complex, PdPc showed a superior catalytic activity in Suzuki coupling of aryl bromides with phenylboronic acid (Scheme 30). Scheme 30. Formation of Biaryls by Suzuki Coupling

In Heck coupling of styrene with aryl halides PdPc was better than the PdTPP but inferior to a Pd bimetallic complex (Scheme 31).212 Scheme 31. Heck Coupling of Styrene with Aryl Halides

The replacement of these expensive and toxic catalysts with more available and nontoxic complexes is highly desirable. MPc’s have successfully been used for the C−C bond formation. Oxidative addition of alkoxycarbonyl radicals derived from different carbazates (except benzyl and tert-butyl derivatives) to α-methylstyrene was performed in the presence of 10 mol % W


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Scheme 33. Hydroacylation of Cyclohexene with CH3CHO Mediated by (PctBu4Fe)2N and tBuOOH under Ar

range of compounds including hemin. Anchoring achiral catalytically active complexes to proteins may give access to enantioselective catalysts due to chiral organization of the protein surroundings around guest active site. However, examples of high enantioselectivity (>90% ee) achieved by such conjugates are rare. The CuPcS−serum albumin conjugates were examined as catalysts for the Diels−Alder reaction of azachalcones with cyclopentadiene in aqueous solutions. The best catalyst was obtained using bovin serum albumin, which provided Diels−Alder products with remarkably high enantioselectivity of 85−98% ee (Scheme 34).

Table 10. Cyclopropanation of Styrene with Ethyl Diazoacetatea

Scheme 34. Asymmetric Diels−Alder Reaction Catalyzed by CuPcS−Bovin Serum Albumin (BSA) Conjugate

catalyst

yield, %

trans:cis ratio

FeIII(Cl)Pc MnIIPc NiIIPc CuIIPc RuIII(Cl)Pc FeIIPcCl16 MnIIPcCl16 NiIIPcCl16 CuIIPcCl16 FeIII(Cl)PcF16 CuIIPcF16 RuIIPcF16 FeIIPctBu4 FeIII(Cl)Pc(m-OC6H4−CF3)4 CuIIPc(m-OC6H4−CF3)4

31 4 7 40 59 59 12 8 39 68 58 80 58 89 58

2.4 1.6 1.5 1.2 1.9 2.1 1.5 1.8 1.5 5.0 2.8 3.2 2.8 2.0 4.5

a Reaction conditions: catalyst:styrene:EDA = 1:500:750, CH2Cl2, room temperature, 4 h. Yields are calculated on the basis of the initial amount of styrene.

Interestingly, a trans:cis selectivity of styrene cyclopropanation catalyzed by FeIIPcCl16 showed a strong dependence on the solvent nature. While in regular organic solvents products with trans-configuration predominated, up to 6:1 for MeOH, in ionic liquid the main product was in cis-configuration (trans:cis = 0.67). In contrast to porphyrin complexes, no decrease in the product yield in the case of styrenes bearing electronwithdrawing substituents was observed. High yields of cyclopropanation products with strong predominance of transisomers were obtained for styrene (91% yield, 3.2 trans:cis ratio), 4-methylstyrene (90%, 5.2), 4-methoxystyrene (88%, 4.8), 4-chlorostyrene (88%, 5.3), 4-bromostyrene (82%, 5.8), 4fluorostyrene (86%, 4.5), 4-trifluoromethylstyrene (81%, 4.0), and 4-nitrostyrene (83%, 5.0). Intramolecular cyclopropanation of allylic diazoacetates was also demonstrated (Scheme 36).217

The endo/exo ratio was around 95:5 and high product yields were obtained with 2 mol % catalyst. It was shown that the efficiency of the hybrid catalyst depended on the binding place of CuPcS inside the serum albumin. Using the copper salts such as Cu(NO3)2, CuCl2, Cu(OTf)2, and Cu(BF4)2 led to racemic products most probably owing to indiscriminate binding of copper salts by protein.215 Thus, the combination of cheap commercially available CuPcS with robust, readily accessible, and easy-to-handle protein provides a highly efficient catalyst for the asymmetric C−C-forming reactions. Catalytic cyclopropanation of styrene with ethyl diazoacetate (EDA) was performed in the presence of several Co macrocyclic complexes.216 In the presence of CoPc, the cyclopropanation product was obtained in 24% yield (Scheme 35).

Scheme 36. Intramolecular Cyclopropanation of Allylic Diazoacetates

Scheme 35. Cyclopropanation of Styrene with Ethyl Diazoacetate

Cyclopropanation of aromatic and aliphatic olefins with (CH3)3SiCHN2 in the presence of MPc’s (M = Cu, Fe, Mn, Ni, Co) afforded silylcyclopropanes in moderate to good yields (Scheme 37).218 CuPc exhibited the better catalytic properties. CrIII(PctBu4)OTf was shown to be a highly efficient, recyclable Lewis acid catalyst for the regio- and stereoselective rearrange-

The influence of the nature of the metal and supporting phthalocyanine ligand on the efficiency of cyclopropanation of olefins with EDA was studied in detail. 217 Substituted phthalocyanines were more efficient in cyclopropanation of styrene (Table 10). X


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procedure involving Pd-mediated reduction.223 The FePc system does not require an ortho-substituted or activated N-aromatic scafford, and several indoles possessing interesting bioactivity and relatively limited synthetic approaches have been prepared. The intermediacy of nitrosoarene was suggested on the basis of trapping experiments.223

Scheme 37. Cyclopropanation of Olefins with (CH3)3SiCHN2 Catalyzed by CuPc

7. REDUCTION ment of epoxides to aldehydes.219 The arrangement of epoxides to carbonyl compounds is an important method for the carbonskeletal modification. While a number of metal catalysts were reported for the transformation of epoxides to ketones via hydrogen migration, only a few examples have been published on alkyl migration to form aldehydes (Scheme 38).219

7.1. Homogeneous Catalytic Reactions

The application of MPc’s for reductive transformations has received less attention as compared with the oxidation reactions.224 CoPcS was shown to be an efficient catalyst for the reduction of nitrite and nitrate by Na2S2O4.225 This reaction is important since denitrification plays an important role in the biogeochemical nitrogen cycle. In particular, bioreduction of NO2− can be performed by heme enzymes. The catalytic cycle includes the reduction of CoIIPcS to CoIPcS, which was stable in alkaline solutions for hours, followed by the reduction of the coordinated nitrite or nitrate. Interestingly, reduction of NO2− and NO3− led to the formation of different products, NH3 and N2 along with N 2 O, respectively, although NO 2 − was an intermediate in the reduction of NO3−. This difference in products was explained by the different coordination of these ligands to the CoIPcS. The nitrite was proposed to coordinate via nitrogen while the nitrate via oxygen. The corresponding iron complex was also a suitable catalyst for the NO2− reduction.226 Noteworthy, the selectivity of the reduction depended on the choice of the reductant (Scheme 44). While dithionite reduced NO2− to N2O, the use of sulfoxylate led to the formation of NH3. This different reduction selectivity was explained by the involvement of differently reduced intermediate complexes (Scheme 44). Two features of the MPc-catalyzed reduction of nitro compounds are particularly useful: (i) the possibility of converting nitro compounds to N-heterocycles in a one-pot process with reduction and condensation steps (Scheme 45) and (ii) the possibility of reduction of nitro functions in the presence of many sensitive functionalities without the need to protect them.227 Highly chemo- and regioselective reduction of aromatic nitro compounds to amines by hydrazine hydrate was catalyzed by CoPc and CuPc.228 The reaction showed a large substrate scope with remarkable tolerance to the presence of a number of functional groups, e.g., halogens, aldehyde, ketone, carboxyl, amide, ester, lactone, nitrile (Scheme 46). Only one NO2 group of dinitrobenzenes was selectively reduced to furnish o-, m-, and p-nitroanilines with 85, 92, and 95% yields, respectively. Potassium and ammonium formates can also be used as reductants, but amine yields decreased to 34− 45%. Notably, simple Co and Cu salts provided only 10−12% yields of amines. The role of phthalocyanine ligand was proposed to favor the reduction to the CoIPc state and to promote the coordination of the nitro group to MPc due to interaction of the Pc nitrogen atoms with the NO2 oxygen atoms and, probably, due to aromatic π−π interaction. The ZnPc in polyethylene glycol (PEG-400) also reduced a large number of nitroaromatics (40 examples, 46−99% yields) by N2H4·H2O.229 The reaction showed a strong solvent influence. While high product yields were obtained in EtOH, [Bmim]BF 4 , [Bmim]HSO 4 , CH2OHCH2OH, and PEG-400, the reduction was inefficient in water, THF, toluene, and ethyl acetate. The system exhibits a high selectivity and tolerance to a wide range of functional

Scheme 38. Two Pathways of Rearrangement of Epoxides to Carbonyl Compounds

Due to the electron-withdrawing character of phthalocyanine ligand improving the Lewis acidity of Cr(III), the CrIII(PctBu4)OTf performed an efficient rearrangement of a wide range of epoxides to corresponding aldehydes with almost perfect regioand stereoselectivity (Table 11). Noteworthy, chromium(III) porphyrins including those bearing strong electron-withdrawing functionalities, such as Br, CN, and C6F5, afforded insufficient conversions and product yields. In addition, a very small CrIII(PctBu4)OTf loading (100 000:1 substrate:catalyst ratio) could be applied, providing TON > 60 000. This robust and efficient catalyst was recycled and reused in five consecutive reactions with >90% yields and an excellent stereoselectivity (98% ee).219 This method is particularly useful for the practical preparation of chiral aldehydes with all-carbon-substituted quaternary stereocenters from chiral epoxides readily accessible via well-established enantioselective epoxidation methods. Aromatic and aliphatic aldehydes were converted to cyanohydrins trimethylsilyl ethers by the AlPc−Ph3PO system (Scheme 39).220 Acetophenone was also amenable to this reaction with 90% product yield. Both AlPc and Ph3PO were necessary as Lewis acid for the activation of the aldehydes and Lewis base for the activation of Me3SiCN, respectively. Aromatic aldehydes can also be transformed to olefins by the treatment with ethyldiazoacetate in the presence of Ph3P and FePc catalyst with high trans selectivity (Scheme 40).221 Vanadyl Pc catalyzed the cyclization of arylethynes.222 Interestingly, while Rh and Ru porphyrins afforded cyclotrimers, VOPc selectively provided cyclodimerization products, especially when Ar bears electron-donating substituents (Scheme 41). VOPc was proposed to catalyze this reaction via a vinylidene intermediate. Annulation of aryl hydroxylamines with alkynes catalyzed by FePc furnished 3-arylindoles with moderate to excellent yields (Scheme 42).223 N-Aryl hydroxylamines bearing electron-donating and electron-withdrawing groups as well as terminal and internal aryl alkynes are suitable substrates (Scheme 43). Yields of indoles from this one-step method are only 3−15% lower than those produced by the conventional two-step Y


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Table 11. Rearrangement of Epoxides to Aldehydes Catalyzed by CrIII(PctBu4)OTfa,b

a

Adapted with permission from ref 219. Copyright 2009 Royal Society of Chemistry. bReaction conditions: 1 mol % CrIII(PctBu4)OTf, 1,2dichloroethane, 83°C.

Scheme 39. Trimethylsilylcyanation of Aldehydes Catalyzed by the AlPc−Ph3PO System

Scheme 41. Cyclooligomerization of Arylethynes Catalyzed by VOPc

Scheme 40. Olefination of Aldehydes with N2CHCOOEt Catalyzed by FePc groups. Only one nitro group in m- and p-dinitrobenzene was reduced. An exclusive formation of benzotriazole was obtained in the case of o-dinitrobenzene (Scheme 47). ZnPc-, CuPc-, and CoPc-based systems can be reused four times before the catalytic activity decreased in the fifth cycle.229 Interestingly, FePc, which is usually applied for oxidation reactions, can also be used for the reduction of nitroarenes to Z


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Scheme 42. Preparation of 3-Arylindoles via Annulations of Aryl Hydroxylamines with Alkynesa

Scheme 46. Reduction of Aromatic Nitro Compounds to Aromatic Aminesa

a

Adapted with permission from ref 228. Copyright 2010 John Wiley and Sons.

Scheme 47. Direct Preparation of Benzotriazole from Dinitro Aromatic Compoundsa a


Scheme 43. Synthesis of 3-Indoles from NPhenylhydroxylamine and Different Arylalkynesa a

Adapted with permission from ref 229. Copyright 2012 Royal Society of Chemistry.

Scheme 48. Reduction of Carbonyl Compounds by ZnPc− NaBH4

a

NaBH4 alone can be used for the reduction of carbonyl compounds. However, excess of NaBH4 and long reaction times are usually needed. In PEG-400 solvent, 4-nitrobenzaldehyde was reduced to the corresponding alcohol with 50% yield after 12 h.231 The presence of NiCl2 allowed achieving 65% yield after 12 h, whereas 0.4 mol % NiPc strongly enhanced the reaction rate, and 99% yield of 4-nitrobenzylic alcohol was obtained after 20 min. The available and nontoxic PEG-400 was the most suitable solvent (>99% product yield, 20 min) as compared to i-PrOH (86%, 12 h), EtOH (75%, 12 h), H2O (48%, 12 h), MeOH (44%, 12 h), ethylene glycol (23%, 12 h), and THF (7%, 12 h). This method tolerates the presence of different functions, in particular, halogens, nitrile, carboxyl, and nitro groups. High turnover numbers can be achieved (Table 12). Selective reduction of only one aldehyde group was achieved in o-, m- and p-benzenedialdehydes. Heterocyclic aldehydes containing pyrrole and furan rings were also selectively reduced to alcohols (87−93% yields).231 Even unsaturated aldehydes were converted to alcohols without affecting the conjugated or isolated double bond. Finally, the method was extended to the large-scope reduction of alkyl and aryl ketones to secondary alcohols with 78−95% yields (13 examples).231 The high efficiency of this system in the reduction was explained by possible activation of the carbonyl group by NiPc having Lewis acid character and activation of NaBH4 by a crown ether-type coordination with PEG-400. This useful synthetic approach has further been developed for reductive amination of carbonyl compounds.232 The reaction of 4-bromobenzaldehyde with 4-methoxyaniline at 70 °C was used for optimization of conditions. The CoPc provided 95% yield of


Scheme 44. Dependence of the Selectivity of Nitrite Reduction on the Reductant

Scheme 45. Example of One-Pot Preparation of NHeterocycles Including Reduction and Condensation Steps

anilines.230 FePc (0.5 mol %), FeSO4·7H2O, or FePc:FeSO4·7H2O 1:1 mixture was used in combination with hydrazine hydrate in CH3OH:H2O 1:1 mixture under reflux conditions. Depending on the substrate substituents, the rational choice of the catalytic system allowed one to obtain a large range of anilines with different functionalities in almost quantitative yields (30 examples).230 The modified ZnPc−NaBH4 system was efficient in carbonyl reduction. In this complementary approach, carbonyl groups of nitro substituted aromatics were reduced to alcohols without affecting the nitro group (Scheme 48).229 AA


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occurring compounds possess significant biological activity, and their reduced metabolites have been extensively studied for their estrogenic, antifungal, and anticancer activity. The reduction of flavons to cis-flavan-4-ols occurs via initial reduction of the conjugated double bond followed by the reduction of the carbonyl group (Scheme 50).

Table 12. Reduction of Substituted Benzaldehydes by NaBH4 in the Presence of NiPca

a b

R

substrate:catalyst

yield, %

time, min

TON

H p-Cl p-Br m−OH p-CH3 m-OCH3 m-CN o-COOH o-NO2 m-NO2 p-NO2 o-CHO m-CHO p-CHO p-OH, m-OCH3 o-NO2, 3,4-CH2O2

1000:1 500:1 500:1 500:1 500:1 250:1 250:1 250:1 500:1 500:1 250:1 250:1 250:1 250:1 500:1 250:1

92 83 89 90 91 94 84 87b 87 90 92 91c 64 70 94 89

15 20 20 20 30 25 20 20 55 55 20 35 35 20 25 25

5105 1443 711 1092 2246 1019 943 855 1707 1764 895 1001 1037 1004 1813 672

Scheme 50. Reduction of Flavons to cis-Flavan-4-ols by the CoPc−NaBH4 System

Excellent product yields were obtained with 10 mol % catalyst loading. Lower catalyst loading led to a decrease of the product yield to 87 and 70% with 2 and 1 mol %, respectively. NiPc and FePc were less efficient in the reduction of flavon (61 and 50% yields, respectively). The reaction efficiency depended on the nature of cobalt phthalocyanine substituents following the trend CoPc(NO2)4 (95% yield) > CoPc (89%) > CoPc(CH3)4 (82%) > CoPc(OCH3)4 (75%) > CoPc(OC8H17)4 (40%). The presence of the OH group in the substrate prevents the reduction. The proposed mechanism involves CoIPc and hydrido HCoIIIPc species. The same system reduced isoflavons with lower selectivity, although the cis-isomer was again a major product (Scheme 51).234

Adapted with permission from ref 231. Copyright 2012 Springer. Yield of phthalide. cYield of 1,2-benzenedimethanol.

corresponding secondary amine, whereas NiPc, FePc, and CuPc were found to be less active, providing 67, 18, and 18% yields, respectively. Simple Fe, Co, Ni, and Cu salts were inefficient. The efficiency of the reductants follows the trend Ph2SiH2 (95% yield) > NaBH4 (46%) > PhSiH3 (34%) > (CH3)2ClSiH (27%) > HCOOK (17%) > HCOONH4 (15%). A large range of aromatic and aliphatic aldehydes was reductively coupled with aromatic and aliphatic amines to afford high yields of secondary amines with tolerance for many functional groups (Scheme 49).

Scheme 51. Reduction of Isoflavons to cis- and transIsoflavan-4-ols by the CoPc−NaBH4 System

Scheme 49. Reductive Amination of Aldehydes with Amines by the CoPc−Ph2SiH2 System

The selectivity of the reduction was further improved in the presence of substituted Co phthalocyanines (Table 13). NiPc and FePc showed lower selectivity: isoflavanone (product of incomplete reduction) and cis- and trans-isoflavan4-ols were obtained in similar amounts. Metal salts showed a sluggish catalytic activity. These results highlight the essential role of a phthalocyanine complex in the reduction process.

One of the typical problems of reductive amination is the intolerance of CC bonds. In this approach the reductive amination of aldehydes containing isolated or conjugated double bonds occurred with almost quantitative yields.232 The reaction of 2-carboxybenzaldehyde and aniline afforded N-phenylisoindolone, a product of biological importance, with 92% yield. Aliphatic amines (five examples) and ketones (14 examples) were also amenable to efficient reductive amination in the presence of CoPc−Ph2SiH2 system with good to excellent yields. On the basis of labeling experiments and competitive reactions, it was proposed that hydrosilylation of the imine should be accelerated due to its activation by CoPc, having Lewis acid character. The reduction of substituted flavons233 and isoflavons234 with NaBH4 catalyzed by CoPc’s has been described. These naturally

Table 13. Influence of the Catalyst Structure on the Selectivity of Isoflavone Reduction yield, % catalyst CoPc CoPc(tBu)4 CoPc(CH3)4 CoPc(NO2)4 CoCl2 NiPc FePc AB

isoflavanone

cis-isoflavan-4-ol

trans-isoflavan-4-ol

42 28 21

68 68 69 76 14 28 39

30 32 28 23 6 18 30


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Regioselective reduction of isosorbide dinitrate to isorobide 5mononitrate, a vasodilator with advantageous pharmacokinetics, was carried out with the CoPc−NaBH4 system (Scheme 52).235

Scheme 54. Preparation of Oximes from Olefins

Scheme 52. Regioselective Reduction of Isosorbide 2,5Dinitrate to Isosorbide 5-Mononitrate

hydrazones and azines in the presence of AlClPctBu4 (Scheme 55).240 Scheme 55. Hydrophosphorylation of Hydrazones and Azines Catalyzed by AlClPctBu4 This method shows advantages over the enzymatic method and the reduction involving Pt, providing 52% isolated yield of the target pure isorobide 5-mononitrate. Reduction agents like NaBH4 are capable of reducing cobalt phthalocyanines to the [CoIPc]− state, which can be studied by a UV−vis method. Reduction of CO2 by the [CoIPc]− and [CoIPc•−]2‑ species was attempted.236 An organic/fluorous biphasic system was used for Pd(II) tetrakis[heptadecafluorononyl]phthalocyanine-mediated reduction of olefins by H2 (15 bar) at 80 °C.237 Styrene was reduced to ethylbenzene with 100% conversion and TON = 634 after 6 h with the possibility of catalyst reuse in nine consecutive runs. This recyclable system was active for the reduction of 1-octene (92% conversion, TON = 596) to provide n-octane (41%), 2octene (32%), and 3-octene (19%) but showed a low activity in hydrogenation of trans-2-octene and cyclohexene. The advantage of reduction reactions described in this section is the use of unsubstituted MPc’s, which are cheap and readily available on a large scale. On the other hand, the catalytic properties of MPc’s can be tuned by the introduction of appropriate substituents that might lead to catalytic systems with novel reactivity and improved catalytic activity.

8. MISCELLANEOUS REACTIONS CoPc catalyzed aerobic deprotection of aldoximes and ketoximes in [bmim]Br to regenerate aldehydes and ketones with 70−92% yields (Scheme 53).238

This approach was extended to provide an access to pyridinecontaining hydrazine phosphanates combining two pharmacophores (phosphanate group and pyridine moiety) in one molecule.241 The compounds bearing picolinic, nicotinic, and isonicotinic acids were prepared in 75−85% yields. The combination of the peptide unit and the phosphonate group in one molecule was realized in α-hydrazino phosphanates derived from hydrazides of natural amino acids.242 These biologically active mimetics of amino acids were obtained in the presence of AlClPctBu4 with 46−75% yields. Recyclable aluminum phthalocyanines supported onto silica were used for one-pot synthesis of aminophosphanate from 1-indanone, benzylamine, and diethyl phosphate with 85% yield.243 Titanium imido complex Pc#TiIVNMes [Pc# = tetrakis(1,1,4,4−6,7-tetralino)porphyrazine, Mes = mesityl] performed nitrene transfer reaction to p-chlorobenzaldehyde and nitrosobenzene to form p-chlorobenzylidene mesitylamine and mesityl phenyl diazene, respectively.244 The related Pc#TiIVCl2 complex catalyzed polymerization of ethylene. Titanyl phthalocyanine PcTiO was shown to mediate the catalytic formation of carbodiimides from isocyanates (Scheme 56).245

Scheme 53. Preparation of Carbonyl Compounds from Aldoximes and Ketoximes

Scheme 56. Formation of Carbodiimides by Metathesis from Isocyanates Catalyzed by PcTiO

It can also be considered as a method for the preparation of aldehydes and ketones, because oximes can be prepared from noncarbonyl compounds. For example, oximes were obtained from aryl-substituted olefins by the treatment with tert-butyl nitrite and NaBH4 with moderate to high yields (Scheme 54).239 FePc (1 mol %) provided the best catalytic performance among several Co, Mn, and Ni complexes and Fe salts. Thus, this sequence presents an original approach for the preparation of carbonyl compounds, both steps of which being catalyzed by phthalocyanine complexes. Biologically active α-hydrazino phosphanates can be prepared by catalytic hydrophosphorylation of aliphatic and aromatic

The proposed mechanism involves [2 + 2]-cycloaddition of RNCO with the TiO group to form the cyclic CO2 adduct (Scheme 57). This unstable metallocycle loses a CO2 to form imido complex PcTiNAr. This complex is stable in the case of bulky aryl groups. When Ar = Ph or tolyl, a further [2 + 2]-cycloaddition of RNCO to PcTiNAr takes place and the N,N′-ureato complex is formed. This complex undergoes a slow isomerization to N,Obound species followed by the formation of carbodiimide product and regeneration of PcTiO catalyst.245 This mechanism has been strongly supported by the isolation and X-ray crystal structures of PcTiNR and PcTi[κ2-(NR)C(O)(N′R)] intermediates. AC


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CCl3CHO can be added at the double bonds of olefins in the presence of CoPctBu4 or CuPctBu4 (Scheme 59).249

Scheme 57. Proposed Mechanism for the Reaction of PcTi O with Arylisocyanates and Catalytic Formation of Carbodiimidea

Scheme 59. Kharasch Addition of CCl3R to Olefins

Ni, Pt, Pd, Mn, Cr, and Zn phthalocyanine complexes showed a very low activity in this Kharasch radical addition. Using CoPctBu4 or CuPctBu4, anti-Markovnikov products were typically obtained with 60−80% yields. Intramolecular addition can also be performed.249 FePc catalyzed oxidative aerobic activation of Ph3P to afford esters from a wide range of alcohols (10 examples) and carboxylic acids (21 examples) (Scheme 60).250

a

Adapted with permission from ref 245. Copyright 2011 Royal Society of Chemistry.

Substituted 1,4-dihydropyridines (30 examples), widely used for the treatment of hypertension and atherosclerosis, were rapidly aromatized by FeIII(Cl)Pc−tBuOOH to substituted pyridines with 89−99% yields.246 Lewis acid properties of MPctBu4 (M = Co, Ni, Pd, CrCl) were used for the catalytic chlorination of benzene, toluene, and oxylene with molecular chlorine.247 Toluene was transformed in oand p-chlorotoluene with 0.05 mol % catalyst loading at 25 °C with up to 97% total yield (Scheme 58).

Scheme 60. Preparation of Esters via FePc-Catalyzed Activation of Ph3P

The involvement of acyloxyphosphonium ion intermediate in the reaction mechanism was supported by the 18O labeling study. Noteworthy, neither iron porphyrin nor phthalocyanine complexes of Co, Cu, In, Mn, Ni, TiO, and Zn catalyze this reaction. The ability of FePc to use O2 for the activation of Ph3P was proposed to be the reason for its catalytic activity.250 The hydrolysis of imines resulting in the formation of amines and carbonyl compounds was performed in the presence of thiourea dioxide and CoPc (Scheme 61).251

Scheme 58. Aromatic Chlorination of Toluene by Cl2 in the Presence of MPctBu4 (M = Co, Ni, Pd, CrCl)

Scheme 61. Hydrolysis of Imines Promoted by the CoPc− Thiourea Dioxide System

Chlorination of benzene in the presence of CoPctBu4 resulted in 10 and 15% yields of chlorobenzene and hexachlorobenzene, respectively. A complex mixture of 3- and 4-monochloro-oxylenes (76% total yield in 10:1 ratio), dichloroxylenes (16% yield), and trichloroxylenes was obtained with 0.05 mol % CoPctBu4 catalyst after 3.5 h. Chlorination of pyridine provided 3,5-dichloropyridine in 15% isolated yield after 5 h. On the basis of UV−vis data, the cation radical form of CoPctBu4 was proposed to be the active species. Phthalocyanine moieties of the complexes underwent chlorination during catalytic reactions.247 Iron phthalocyanines bearing positively charged substituents mediated oxidative chlorination of aromatic compounds by H2O2−HCl.248 Using 0.65 mol % catalyst loading, 2-methyl-1chloronaphthalene (94% chromatographic yield), 2-methoxy-1chloronaphthalene (99%), 2,3-dimethyl-1,4-dichloronaphthalene (95%), 2,6-dimethyl-1,5-dichloronaphthalene (95%), and 1-methyl-4,5-dimethoxy-2-chlorobenzene (99%) were prepared. Polychlorinated compounds such as CCl4, CCl3CO2Et, and

This reaction is extensively used in the synthesis of bioactive elaborated molecules and requires strong acid or basic conditions, which cannot be used for sensitive substrates. The CoPc−thiourea dioxide system can be applied without using basic or acidic conditions, thus increasing the scope of the hydrolysis method. Cycloaddition of CO2 to propylene oxide was performed in the presence of Al(OH)PctBu4.252 In the presence of basic cocatalysts such as Bu3N, 1-methylimidazole, and Ph3P, the unsubstituted Al(Cl)Pc was more efficient than the corresponding Mg, Fe, Ni, and Co complexes (Scheme 62) and could be recycled three times without loss of activity.253 Under optimal condition (180 mmol epoxide, 380 mmol CO2, 0.18 mmol catalyst, 0.81 mmol Lewis base, 140 °C, 12−102 min) AD


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species, but they have never been described. Only the phthalocyanine Nb(IV)−oxo complex, which is not related to oxo-transfer chemistry, was obtained by the reduction of PcNbVCl in 1,2-dimethoxyethane.259 This lack of knowledge is mainly due to the inherent reactivity of these active species under ambient conditions. Low solubility and aggregation behavior of MPc’s in solution are additional complicating factors. Nevertheless, the formation, stability, and properties of elusive active intermediates can be investigated under low temperature using different spectroscopic methods. The first high-valent iron oxo complex on the phthalocyanine platform has been obtained from FePctBu4 and m-CPBA at −60 °C.260 This elusive species was characterized by UV−vis, cryospray mass spectrometry, EPR, X-ray absorption, and highresolution X-ray emission methods. Upon addition of m-CPBA at −60 °C, a pink species with broad absorption bands at 529, 629, and 667 nm and EPR signal at g = 2.01 was generated, indicating the presence of the Fe(IV) site and the unpaired electron centered on the phthalocyanine core. Cryospray MS showed the presence of (PctBu4)Fe−m-CPBA peroxo complex (m/z = 963.2) and oxo species (PctBu4)FeO (m/z = 808.3) and (PctBu4)FeO(Cl) (m/z = 843.2). The oxo species were amenable to rapid isotopic exchange of the oxo ligand with labeled H218O, similarly to the porphyrin counterparts.261 The mechanism of the formation of the Fe(IV) oxo species and its formulation is proposed in Scheme 63. This finding suggests that mechanistic features of oxidation catalyzed by phthalocyanine complexes can be similar to those mediated by their macrocyclic analogs. Indeed, the kinetic studies of the oxidation of nine olefins by PhIO demonstrated the similarity between the reaction pathways, reaction rates, and products that iron porphyrin and iron phthalocyanine catalyze.262 Depending on the structure of the MPc’s and oxidant involved, several mechanisms of oxidation can be operational. For instance, the outcome of the oxidation of phenols depends on the oxidant used. FePcS in combination with H2O2 performed oxidative degradation of phenols, including recalcitrant polychlorinated compounds, to produce compounds derived from the cleavage of the aromatic cycle and CO2.24,25 When tBuOOH was used as the oxidant, the same FePcS complex mediated the selective oxidation of phenols to quinones.23 As for the aerobic oxidations catalyzed by MPc’s, as a rule, these reactions involve radical species and can be classified as free radical oxidation processes. Moreover, one catalytic system can show different mechanistic features. The oxidation of anthracene, TMP, and xanthene by the FePcS−SiO2−tBuOOH system occurred with a very low 18O incorporation from 18O2 to products and an inverse kH/kD, indicating two-electron oxidation as the principal pathway.152 This mechanism is consistent with the high yield of quinones, while coupling products should be obtained if a one-electron pathway is operating. The oxidation of alkynes occurred with a

Scheme 62. Preparation of Cyclic Carbonates from Epoxides and CO2

cyclic carbonates from ethylene, propylene, and chloropropylene oxides were obtained in 93, 99, and 94% yields, respectively. Disulfonated derivative of AlPc was covalently anchored onto MCM-41 and used for the formation of cyclic carbonates in 10 successive reactions without detectable loss of catalytic activity.254 The efficiency of the reaction was markedly enhanced in the presence of n-Bu4NBr, which facilitated a nucleophilic opening of the epoxide ring, whereas AlPc electrophilically activated the epoxide. Using a catalyst:n-Bu4NBr:epoxide ratio of 1:1:2500, cyclic carbonates of ethylene, propylene, chloropropylene, and styrene epoxides were obtained with TON of 830, 560, 904, and 768, respectively, at 110 °C for 2 h.254 The synthesis of cyclic carbonates from CO2 and epoxides can be performed at 25 °C and 1 bar of CO2.255 MPc’s (M = Cu, Co, Ni, Al) encapsulated in zeolite Y showed high catalytic activity for the cycloaddition of CO2 to epichlorohydrin and propylene oxide.256 The higher activity was obtained in the presence of base, e.g., N,N-dimethylaminopyridine. CuPc−Y showed a much high turnover frequency of 12 326 h−1 compared to the neat complex (502 h−1) and the silicasupported CuP (478 h−1). Cyclic carbonates were obtained with more than 90% yields under the following reaction conditions: 18 mmol of epoxide substrate, 7.2 μmol of catalyst, 7.2 μmol of Lewis base, 100 psi of CO2, 120 °C, 4 h.256

9. MECHANISMS AND ACTIVE SPECIES INVOLVED IN REACTIONS CATALYZED BY METAL PHTHALOCYANINES Phthalocyanine metal complexes belong to a large family of macrocyclic porphyrinoid complexes that include porphyrins, porphyrazines, corroles, corrolazines, etc. Do these phthalocyanine complexes behave in catalysis like their porphyrinoid analogs? Are MPc’s able to form the same type of active species as their porphyrin counterparts? In the oxidation field, high-valent iron oxo species with porphyrin, corrole, and nonheme ligands are well-established entities capable of performing a variety of reactions.9,257,258 Until recently, the corresponding mononuclear phthalocyanine complexes have often been postulated as active

Scheme 63. Proposed Mechanism for the Formation of High-Valent Iron Oxo Phthalocyanine Cation Radical Speciesa,b

at

Bu substituents are omitted for clarity. bReproduced with permission from ref 260. Copyright 2012 Royal Society of Chemistry. AE


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significant 18O incorporation from 18O2 to reaction products, suggesting an involvement of radical intermediates corresponding to one-electron oxidation. The reason for this dual reactivity is not yet clear. This finding might be reminiscent of the involvement of multiple reaction pathways in the cytochrome P450 mediated oxidations as explained by the two-oxidant and two-state reactivity model.263 Numerous examples show the complexity of MPc-mediated oxidation reactions, indicating that this feature is typical not only for porphyrin or nonheme complexes but probably a more general phenomenon. Further research should be performed in order to understand whether different species could be involved or if one active species could be able to adapt a mechanism depending on the substrate to oxidize. The catalytic activity of MPc’s and their porphyrin analogs was compared for several reactions. Barkanova et al. showed that mechanisms of naphthalene oxidation by AcOOH in the presence of porphyrin and phthalocyanine complexes were different.141 The FePc was more efficient in the allylic C−H amination compared to the iron porphyrin, the nonheme, and the salen complexes.204 The PdPc exhibited a higher catalytic activity in Suzuki coupling of aryl bromides with phenylboronic acid and in Heck coupling of styrene with aryl halides than the palladium porphyrin.212 These results can be explained by the different properties of macrocyclic supporting ligands. High oxidation states of metal ion of the complexes are more readily accessible in the porphyrin series than in the phthalocyanine counterparts.264 It means that the phthalocyanine ligand tends to stabilize the lower oxidation states of the central metal ion compared to the porphyrin environment. In turn, this implies that MPc’s in high oxidation states should be stronger oxidants as compared to their porphyrin analogs in the same oxidation states. Careful mechanistic studies are very useful to improve catalytic properties of materials by appropriate design of catalyst molecules. For example, on the basis of a detailed ESI-MS study of FePcS and its oxygen adducts, highly durable FePc catalysts for the oxygen reduction reaction in polymer electrolyte membrane fuel cells have been obtained by introduction of sterically protected electron-donating diphenyl thiophenol groups into Pc molecules.265 Much higher current density has been achieved using these catalysts compared with the commercial FePc-based materials. In addition, the ability of MPc’s to interact with peroxides can be used in chemical sensors.266 Along with catalytic activity and selectivity, the stability of MPc under reaction conditions, deactivation pathways, and the possibility of regeneration and recycling are of significant importance. Careful in-depth investigations of the catalyst stability and reuse issues have not been frequently published. A critical analysis of this problem can be found in a review by Jones.267 It should be pointed out that achieving the catalyst stability in oxidation is much more challenging compared to other reactions.268 Although phthalocyanine complexes are generally considered as robust, they can undergo degradation under strong oxidative conditions. Nevertheless, products of degradation of metallophthalocyanines can also show some catalytic activity.269,270 The stability of phthalocyanine complexes under catalysis conditions is therefore of primary importance. Scarce are the examples in which the mechanisms of the degradation of the phthalocyanine core have been studied in detail. d’Alessandro et al. investigated the degradation of NiPcS and CoPcS in the presence of KHSO5.271 Attenuation of the Q-band in the visible

spectrum was accompanied with the formation of sulfophthalimide. These conclusions were supported by the study of stability of RuPcS and FePcS in the presence of KHSO5 and H2O2.272 Meso-nitrogen oxide derivatives and metal−biliverdin-like compounds were identified by ESI-MS and NMR methods at short reaction time. Fragmentation of the phthalocyanine moiety to sulfophthalimide and metal complex with a tridentant ligand was observed at longer reaction times. The thermo- and photostability of MPcS (M = Ru, Cu, Ni, Fe, Co) have been investigated.273 In the case of supported MPc’s, the loss of their catalytic properties, preventing their reuse, can occur owing to the leaching of the complex to the reaction solution. Covalent anchoring of the complex usually makes the catalyst more stable. Nevertheless, hot filtration tests and determination of metal ion concentration in the reaction solution after the reaction should be performed to confirm the stability of the catalyst. The catalytic activity can also decrease because of the adsorption of the reaction products on the surface leading to the blockage of the active sites. In these cases the catalyst can be regenerated by washing or by heating. The importance of careful studies of all aspects of the catalyst stability and recycling is often underestimated. Detailed mechanistic studies should allow the determination of mechanisms of the reactions and provide a basis for the optimization of these catalysts in terms of activity and selectivity. 9.1. Diiron Complexes on Phthalocyanine Platform: Reactivity and Mechanistic Considerations

This binuclear macrocyclic concept is rooted in biological oxidation of strong C−H bonds. Numerous studies have been directed to chemically mimic active sites of enzymes with the objective of reproducing their catalytic activity.274−276 Significant progress has been achieved with structural and spectroscopic modeling, but functional models capable of oxidizing strong C− H bonds are still rare. As oxidation catalysts, iron phthalocyanine complexes can be considered as bioinspired models of cytochrome P-450 enzymes capable of performing the most difficult oxidation reactions.3 Active sites of many enzymes involved in biological oxidation contain binuclear O-bridged diiron structural units in nonheme environments. Toluene monooxygenases, phenol hydroxylase, alkene monooxygenase, butane monooxygenase, and others mediate demanding oxidations of alkanes, olefins, and aromatic compounds.277,278 The diiron nonheme site of soluble methane monooxygenase (MMO) catalyzes a particularly challenging oxidation of methane.279 Can a construction based on the diiron site in the macrocyclic environment of phthalocyanine ligands show catalytic activity? It is worth noting that the dimeric μ-oxo iron phthalocyanines have commonly been regarded as catalytically inactive complexes. The formation of the μ-oxo dimer species from the monomeric complexes during catalytic reactions has been considered as an inactivation pathway. However, there are some considerations that dimeric species can be even more suitable for the catalytic oxidation. Indeed, in the course of generation of active oxidizing species, iron site(s) increase(s) their oxidation state by two redox equivalents. This high oxidation state is stabilized by charge delocalization at the iron site and porphyrin ligand in the (P+•)FeIVO species in cytochrome P-450 and at two iron sites in the soluble MMO. In the case of diiron macrocyclic construction, the positive charge can be delocalized at two iron sites as well as at two macrocyclic ligands. This provides better possibilities for the formation of AF


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structures and physicochemical properties of μ-oxo, μ-nitrido, and μ-carbido dimeric complexes with an emphasis on the phthalocyanine systems.293 It is not excluded that the catalytic properties of the μ-oxo and μ-nitrido dimers can be influenced by their geometrical organization, but no experimental data supporting this proposal are yet available. The μ-oxo dimeric FePcS can covalently be fixed onto a silica support.150,151 These catalysts showed a high catalytic performance in a variety of oxidation reactions.23 The interesting catalytic activity of homogeneous μ-oxo diiron phthalocyanines has also been published.143−145 However, under oxidation conditions the μoxo diiron complexes undergo transformation to the monomeric species, which exhibit lower catalytic performance.151 This lack of the stability of Fe−O−Fe structural unit limits the reuse of these catalysts in the successive reactions. 9.1.2. N-Bridged Diiron Phthalocyanines: Emerging Catalysts for Oxidation and Other Reactions. The high potential of MPc’s in catalysis has recently been demonstrated by the discovery of the catalytic activity of the binuclear diiron complexes on the phthalocyanine platform. Surprisingly, up to 2008, similar complexes have not been considered as prospective catalysts. The μ-nitrido dimer phthalocyanines belong to another class of single-atom bridged binuclear complexes (Scheme 65).

active oxidizing species in the presence of peroxides (Scheme 64). Scheme 64. Possible Pathways for the Formation of HighValent Oxo Species on Monomeric and Dimeric Phthalocyanine Platformsa

a


Depending on macrocyclic ligand and peroxide structures, axial ligand, and solvent polarity, both homolytic and heterolytic cleavages of the peroxide O−O bond are possible for the mononuclear iron complexes.11,280,281 Homolysis of PcFeIIIOOR to form PFeIVO and OR• is often the principal pathway involving free radical intermediates. In the case of dimeric peroxocomplex PcFeIII−O−FeIII(Pc)OOR, a heterolytic cleavage of the O−O bond should be much more favorable than in the previous case due to delocalization of the charge at the two iron sites and two macrocyclic ligands. In this scenario, no formation of OR• radicals occurs, and radical processes leading to unselective reactions and degradation of the complex are avoided. The intrinsic reactivity of the species PFeIVO and PFeIV−O−FeIV(P)O generated from the mononuclear and the binuclear macrocyclic complexes, respectively, can be very different. Consequently, there are some reasons for looking for the catalytic activity of the diiron macrocyclic complexes that can exhibit even better catalytic properties than those of the mononuclear counterparts. Indeed, the examples of the superior catalytic activity of the dimeric metal phthalocyanine complexes are being increasingly reported (see refs 106, 107, 112, 113, 119, 128, 136, 140, 143−145, 153, 160, 169, 199, 203). Two principal classes of single-atom bridged complexes, μ-oxo and μ-nitrido dimers, are considered below. 9.1.1. μ-Oxo Bridged Diiron Phthalocyanines. μ-Oxo diiron phthalocyanine complexes can be prepared from the mononuclear FePc in the presence of O2.282 Interestingly, there exist two different isomers of (FePc)2O, which can be distinguished due to the difference in their X-ray powder patterns; UV−vis, NMR, IR, and Mössbauer spectral properties; and magnetic behavior.283−285 Three structures of the μ-oxo dimers have been proposed, which differ either in geometrical organization (parallel or bent phthalocyanine planes)286 or in the iron oxidation state, Fe(II) or Fe(III).287,288 This problem has been a subject of debates, and on the basis of abundant experimental data, two structures have been assigned as dimers with linear and bent Fe(III)OFe(III) units.289 Hanack and coworkers obtained the similar results in the case of alkoxysubstituted (FePc)2O.290 The existence of two μ-oxo diiron porphycene complexes was evidenced by X-ray crystallographic analysis.291 One isomer has the Fe−O−Fe angle of 145.3°, while the other dimer possesses an almost linear Fe−O−Fe motif with an angle of 178.5°. Different conformations of the μ-nitrido dimer [FePc(OC5H11)8]2N were also published.292 The recent review by Ercolani and co-workers summarizes the results on the

Scheme 65. Structure of (FePctBu4)2Na

a

Only one of the 10 possible position isomers is depicted.

The unsubstituted (FePc)2N was prepared for the first time in 1984,294 followed by the studies on its physicochemical properties.295 Further progress in the preparation and characterization of N-bridged binuclear complexes was nicely reviewed by Ercolani and co-workers.293 The use of nitrogen atom bridging the two iron sites in the phthalocyanine environment resulted in the remarkable stability of the Fe−NFe unit, as evidenced by the spectroscopic data.106,108,296 The (FePc)2N and related complexes can be formally considered as a mixed valence Fe(III)NFe(IV) systems with one unpaired electron. However, the Mössbauer spectrum contains one single doublet with δ = 0.06 mm s−1 and ΔEQ = 1.76−1.78 mm s−1, indicating two equivalent iron sites with intermediate +3.5 oxidation state.293 Thus, there is an important structural difference between the Fe(III)OFe(III) and the Fe(+3.5)NFe(+3.5) systems. The idea to use N-bridged diiron complexes in combination with active oxygen donors like H2O2 or m-CPBA is somewhat counterintuitive, because of their already oxidized starting FeIIIFeIV or Fe+3.5Fe+3.5 state. In enzymes or mononuclear model complexes the resting state is usually FeIII or even FeII. AG


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form transient elusive species. Direct investigation of these species is challenging because of their high reactivity and inherent instability. An alternative approach consists of the investigation of stable high-valent diiron phthalocyanine complexes in the same oxidation states but without oxo ligand(s).108 These model isoelectronic analogues of unstable oxo diiron complexes can be used in order to get insight into the oxidation and spin states of the iron atoms. High-resolution Xray emission spectroscopy (XES) and resonant inelastic X-ray scattering (RIXS) as well as Fe K-edge XANES were applied to study iron species in the resting [PcFe3.5+NFe3.5+Pc]0 as well as in the oxidized complexes [PcFe I V NFe I V Pc] + PF 6 and [PcFeIVNFeIV(Pc•+)]2+Br2. The XES spectra of the Kβ line are sensitive to the local iron spin density and show an unexpected difference in the spin state between the initial [PcFe3.5+NFe3.5+Pc] 0 (low spin, LS) and the one-electron oxidized [PcFeIVNFeIVPc]+PF6 (high spin, HS) and two-electron oxidized cation radical species [PcFeIVNFeIV(Pc•+)]2+Br2 (LS) (Scheme 67).

However, the possibility of the electron delocalization on the two iron centers, the two phthalocyanine ligands, and the bridging nitrogen makes the activation of H2O2 possible and the highvalent diiron oxo state accessible. There is an important difference between the mononuclear and the binuclear constructions which should be reflected in their reactivity (Scheme 66). Scheme 66. Possible Pathways for the Formation of Active Species on the Mononuclear and the Binuclear Iron Platforms

Scheme 67. Spin States of Iron in the Row of N-Bridged Diiron Species upon Increase of Oxidation State

Heterolytic cleavage of the O−O bond in mononuclear iron complexes leads to the formation of (Pc+•)FeIVO species, which was prepared and characterized using m-CPBA as oxidant. Such a species has two redox equivalents above the Fe(III) state. In the case of the μ-nitrido diiron complex, heterolytic cleavage of the O−O bond of the peroxo complex should furnish [PcFeIV(μ-N)FeIV(O)(Pc•+)]0 complex, which is 2 redox equivalents above the Fe(III)Fe(IV) state. This species should be a very strong oxidant. Low-temperature UV−visible spectroscopy is a convenient method for monitoring reactions of the complexes with oxygen donors and provides useful information. Utilization of several powerful spectroscopic techniques allows one to obtain unique information on the structure of these unstable active species. Although this high reactivity does make their detection and characterization very difficult, available spectroscopic, labeling, and reactivity data support the formation of high-valent diiron oxo species.106,297 Using porphyrin supporting ligand, we were able to prepare and characterize an elusive [TPPFeIV(μ-N)FeIV(O)(TPP•+)]0 intermediate (TPP = tetraphenylporphyrin), which was a much stronger oxidant than its mononuclear counterpart.109 Mechanistic features of benzene oxidation by the (FePctBu4)2N−H2O2 system, in particular, incorporation of 18O to the products exclusively from H218O2, intermediate formation of benzene epoxide, and NIH shift,134 strongly support the involvement of high-valent diiron oxo species. These features are characteristic of cytochrome P-450 and toluene 4-monooxygenase enzymes operating via highvalent iron oxo species.135,136 In contrast, these observations are not compatible with participation of free hydroxyl radicals. The remarkable catalytic properties of μ-nitrido diiron phthalocyanines were demonstrated in the oxidation of methane in water at ambient temperatures106,107 and in the oxidation of benzene136 and alkylaromatic compounds,89 as well as in the oxidative dehalogenation.296 In the course of formation of the diiron active species, the iron atoms of the complex change the oxidation and spin states to

Supposing that one-electron and two-electron oxidized oxo species follow the same trends as observed in the model compounds, one can suggest that the two-electron oxidation should be the main pathway for the formation of the active species via heterolytic cleavage of the O−O bond of the hydroperoxo complex. This pathway allows avoiding LS−HS− LS transformations. The main catalytic oxidation pathway should include two-electron transformation from the LS [PcFeIV(μN)FeIV(O)(Pc•+)]0 to the LS initial complex. On the basis of DFT calculations, the μ-nitrido group was proposed to stabilize low-spin iron states, which may be useful in activating the O−O bond of a diiron hydroperoxo complex.298 When H2O2 oxidant was replaced with tBuOOH, a very different catalytic activity of (FePctBu4)2N was been observed. The (FePctBu4)2N−tBuOOH system was very efficient in the C−C-forming reactions, which is in sharp contrast to the oxidizing (FePctBu4)2N−H2O2 system. In particular, hydroacylation of olefins with CH3CHO showed a high selectivity in the formation of methyl ketones and high TON.214 The antiMarkovnikov addition to the olefin double bond typical for the radical species suggests the involvement of an acyl radical in this reaction. The composition of the side products is also in agreement with the involvement of radical species (Scheme 33). However, iron salts and mononuclear iron phthalocyanine able to initiate a radical chemistry were not active in the hydroacylation reaction. This observation suggests a special role played by N-bridged diiron phthalocyanine. To rationalize the reason for the remarkable catalytic activity of the (FePctBu4)2N in hydroacylation, it was proposed that this binuclear complex reacts with tBuOOH to form a peroxo complex [FeIV(μN)FeIII− OOtBu]−. The formation of this complex was evidenced by ESIMS in the previous work. 296 Since the reactivities of AH


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reactivity of the μ-nitrido diiron complexes described in the aforementioned examples, one can conclude that the Fe(+3.5)− N−Fe(+3.5) structural unit strongly changes the properties of these phthalocyanine complexes, making them quite different from regular mononuclear Pc complexes.303 The properties of these μ-nitrido species are governed by this particular structural feature rather than supporting Pc ligand, although it also plays an important role as an electronic reservoir.

(FePctBu4)2N−H2O2 and (FePctBu4)2N−tBuOOH systems visà-vis cyclohexene in the presence of acetaldehyde are different, the [FeIV(μN)FeIII−OOtBu]− complex should not probably undergo the heterolytic cleavage of O−O bond as the hydroperoxo complex [FeIV(μN)FeIII−OOH]− does.106,297 Alternatively, O−O bond in the [FeIV(μN)FeIII−OOtBu]− can be cleaved homolytically to form tBuO· radical and [FeIVNFeIV O]− species able to react with aldehyde to produce acyl radicals (Scheme 68).

10. CONCLUSION A current challenge in the field of bioinspired catalytic chemistry is the development of efficient catalysts that should be readily accessible on a large scale. In this context, MPc’s seem to be promising candidates, combining availability and high reactivity in many reactions. A vast number of new catalytic applications of MPc’s have been published in the last 3−4 years. This increasing interest shows that these old complexes are very attractive, both for academic research and potential industrial catalytic applications. Although the field is dominated by iron phthalocyanines for oxidation and by cobalt complexes for reduction, phthalocyanine complexes of many metals have also been used as catalysts. The reactivity profile of MPc’s is quite broad and useful. Initially employed as oxidation catalysts, MPc’s are coming to find a wide range of applications in reduction, addition, and C−C- and C− N-forming reactions. Recent developments include multicomponent syntheses involving C−C and C−N formation steps. These catalytic reactions can be elaborated to a high level of efficiency by tuning the structure in terms of metals and electronic properties of the phthalocyanine ligands. An appropriate construction of MPc catalytic systems can even provide access to chiral products. For instance, conjugates of MPc’s with protein matrix of serum albumins catalyze enantioselective Diels−Alder reactions with 81−98% ee.215 Phthalocyanine complexes have been and will be playing an increasing role in the development of sustainable oxidation methods. A detailed mechanistic understanding of the reactivity exhibited by MPc’s is a prerequisite for the rational design of catalysts. The investigation of mechanisms and metal phthalocyanine based active species will not only bridge a gap between porphyrin and phthalocyanine catalytic chemistry but will also be beneficial for the development of new industrially viable catalytic methods for clean oxidation. The main problem in this area of chemistry is the development of new catalytic systems that combine a high catalytic activity with high stability under the oxidative conditions. A high potential of phthalocyanine complexes in catalysis has further been demonstrated by the remarkable catalytic properties of the μ-nitrido diiron phthalocyanines. This finding represents a novel approach to catalytic activity. The interest in (MPc)2Nbased systems stems not only from their obvious fundamental importance but also from a desire to use their great synthetic potential to develop novel, clean, and selective catalytic methods. Looking forward, the ever-expanding scope of application of MPc’s suggests that their potential usefulness could be substantial, keeping in mind the availability of these complexes. Application of MPc’s in fine chemistry is expected to make the preparation of elaborated products more efficient, resulting in the less expensive and cleaner processes. Current demand for environmentally friendly processes requires the development of green methods that employ readily available catalysts in combination with clean reagents, e.g., O2 and H2O2 for oxidation reactions. The metal complexes of phthalocyanines are

Scheme 68. Proposed Mechanism for Hydroacylation of Olefins Catalyzed by (FePctBu4)2N in the Presence of a Catalytic Amount of tBuOOHa,b

a Charges of the complexes are omitted for clarity. bReproduced with permission from ref 214. Copyright 2011 John Wiley and Sons.

Acyl radical adds to an olefin in anti-Markovnikov fashion to form the addition radical A. The hydrogen abstraction from the aldehyde by the radical A to afford the hydroacylation product is inefficient. It was proposed that this key step can be carried out by FeIV(μN)FeIII−OH issued from the reaction of FeIV(μN)FeIVO with aldehyde. Thus, the hydroacylation product is formed and the FeIVNFeIVO species is regenerated for the next reaction with aldehyde. The diiron oxo species formed from the binuclear complex (FePctBu4)2N participate in the two key reactions: (i) generation of acyl radical from the aldehyde and (ii) the formation of hydroacylation product from the addition radical A (Scheme 68). This reaction sequence forms a closed catalytic cycle with no need for other reagents. Iron salts and mononuclear iron phthalocyanine able to initiate radical chemistry showed no hydroacylation activity, most probably because of their inability to mediate the formation of the hydroacylation product from the addition radical A. Due to this unique reactivity of the (FePctBu4)2N, the tBuOOH oxidant can be used in a catalytic amount (15 mol % to olefin) just to initiate the process, which provides up to 86% conversions of olefins. In other words, tBuOOH is used for the initial formation of the FeIVNFeIVO species, which acts in combination with the FeIVNFeIII−OH to perform two steps of the catalytic cycle. The proposed mechanism is also consistent with the small catalyst loading (0.01 mol %) sufficient for the reaction. This finding significantly increases the scope of application of these emerging catalysts. From a mechanistic point of view, the different behavior of the N-bridged diiron macrocyclic complexes in the presence of H2O2 and tBuOOH indicates the rich redox chemistry of this Fe(μN)Fe platform, which can be used for the development of various catalytic reactions. Furthermore, the synthetic development in the preparation of various μ-nitrido binuclear structures should provide new examples of the catalytic applications. Recently, the complexes having electron-withdrawing299,300 and electron-donating substituents292 as well as heterometallic301 and heteroleptic302 structures have been prepared. On the basis of the unusual AI


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MMO MOF MPc MPcCl16 MPcF16 MPc(NO2)4 MPcR4 (MPcR)2O MPcS NHPI PDMS PEG PINO TEMPO TMP TMQ TPP TON VA

particularly suitable catalysts for clean catalytic oxidations as well as for other reactions and keep a great promise for the future.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Tel: 33472445337. Fax: 33-472445399. Notes

The authors declare no competing financial interest. Biography

methane monooxygenase metal−organic framework metal phthalocyanine complex metal hexadecachlorophthalocyanine complex metal hexadecafluorophthalocyanine complex metal tetranitrophthalocyanine complex metal tetrasubstituted phthalocyanine complex μ-oxo diiron phthalocyanine metal tetrasulfophthalocyanine complex N-hydroxyphthalimide polydimethylsiloxane membrane polyethylene glycol phthalimide N-oxyl radical 2,2,6,6-tetramethylpiperidyl-1-oxy 2,3,6-trimethylphenol 2,3,5-trimethylbenzoquinone tetraphenylporphyrin anion turnover number veratryl alcohol

REFERENCES (1) Phthalocyanines: Properties and Applications; Leznoff, C. C., Lever, A. B. P., Eds.; VCH: Weinheim, Germany, 1989, 1993, 1996; Vols. 1−4. (2) The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: San Diego, CA, 2003; Vols. 15−20. (3) Ortiz de Montellano, P. R. Cytochrome P-450: Structure, Mechanism and Biochemistry, 3rd ed.; KluwerAcademic/Plenum: New York, 2004. (4) Stuzhin, P. A.; Ercolani, C. The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: San Diego, CA, 2003, Vol.15, p 263. (5) Aviv-Harel, I.; Gross, Z. Chem.Eur. J. 2009, 15, 8382. (6) Goldberg, D. P. Acc. Chem. Res. 2007, 40, 626. (7) Meunier, B.; de Visser, S.; Shaik, S. Chem. Rev. 2004, 104, 3947. (8) Denisov, I. G.; Makris, T. M.; Sligar, S. G.; Schlichting, I. Chem. Rev. 2005, 105, 2253. (9) Nam, W. Acc. Chem. Res. 2007, 40, 522 and references therein. (10) Meunier, B. Chem. Rev. 1992, 92, 1411. (11) Costas, M. Coord. Chem. Rev. 2011, 255, 2912. (12) Che, C.-M.; Kar-Yan, Lo, V.; Zhou, C. Y.; Huang, J.-S. Chem. Soc. Rev. 2011, 40, 1950. (13) Lu, H.; Zhang, X. P. Chem. Soc. Rev. 2011, 40, 1899. (14) de la Torre, G.; Claessens, C. G.; Torres, T. Chem. Commun. 2007, 2000. (15) D’Souza, F.; Ito, O. Chem. Commun. 2009, 4913. (16) Martinez-Diaz, M. V.; de la Torre, G.; Torres, T. Chem. Commun. 2010, 46, 7090. (17) Bottari, G.; de la Torre, G.; Guldi, D. M.; Torres, T. Chem. Rev. 2010, 110, 6768. (18) Bottari, G.; Suanzes, J. A.; Trukhina, O.; Torres, T. J. Phys. Chem. Lett. 2011, 2, 905. (19) Basu, B.; Satapathy, S.; Bhatnagar, A. K. Catal. Rev. 1993, 35, 571. (20) Chen, M. J.; Rathke, J. W. Phthalocyanines in hydrocarbon activation. In Phthalocyanines: Properties and Applications; Leznoff, C. C., Lever, A. B. P., Eds.; VCH Publishers: New York, 1996; Vol. 4, p 183. (21) Meunier, B.; Sorokin, A. B. Acc. Chem. Res. 1997, 30, 470. (22) Kaliya, O. L.; Lukyanets, E. A.; Vorontsov, G. N. J. Porphyrins Phthalocyanines 1999, 3, 592. (23) Sorokin, A. B.; Kudrik, E. V. Catal. Today 2011, 159, 37. (24) Sorokin, A.; Séris, J.-L.; Meunier, B. Science 1995, 268, 1163. (25) Sorokin, A.; Meunier, B. Chem.Eur. J. 1996, 2, 1308. (26) Sorokin, A.; De Suzzoni-Dezard, S.; Poullain, D.; Noël, J.-P.; Meunier, B. J. Am. Chem. Soc. 1996, 118, 7410. (27) Sorokin, A.; Fraisse, L.; Rabion, A.; Meunier, B. J. Mol. Catal. A 1997, 117, 103. (28) Hadasch, A.; Sorokin, A.; Rabion, A.; Fraisse, L.; Meunier, B. Bull. Soc. Chim. Fr. 1997, 134, 1025.

Alexander B. Sorokin graduated from the Moscow Lomonosov State University, Moscow, Russia, and obtained his Ph.D. on the bioinspired oxidation of alkanes mediated by iron porphyrins in 1992 under the guidance of Prof. A. E. Shilov (Institute of Chemical Physics of the Russian Academy of Sciences, Chernogolovka, Russia). After postdoctoral work (1992−1997) with Prof. Bernard Meunier at Laboratoire de Chimie de Coordination du CNRS in Toulouse, France, he joined l’Institut de Recherches sur la Catalyse du CNRS, Villeurbanne, France, in 1997. His current research focuses on bioinspired oxidation including oxidation of methane, catalytic chemistry of phthalocyanine complexes, mechanisms of oxidation, and preparation and spectroscopic characterization of high-valent iron oxo species.

ACKNOWLEDGMENTS The author is thankful to all colleagues and co-workers whose names are given in the references for their valuable contribution to the development of phthalocyanine catalytic chemistry. I am especially grateful to Dr. P. Afanasiev and Dr. E. V. Kudrik for the fruitful collaboration and stimulating discussions. I thank Dr. S. Mishra for editing the English of the manuscript. Research in the author’s laboratory has been supported by the Agence Nationale de la Recherches (ANR, France) (ANR-08-BLANC-0183-01). ABBREVIATIONS 2MN 2-methylnaphthalene 2MQ 2-methyl-1,4-naphthoquinone bmim 1-butyl-3-methylimidazolium cation m-CPBA m-chloroperbenzoic acid DBNBS 3,5-dibromo-4-nitrosobenzenesulfonic acid DDQ 2,3-dichloro-5,6-dicyano-1,4-benzoquinone DMPO 5,5-dimethyl-1-pyrroline N-oxide EDA ethyl diazoacetate FePctBu4 iron tetra-tert-butylphthalocyanine (FePctBu4)2N μ-nitrido diiron tetra-tert-butylphthalocyanine IBA isobutyraldehyde MIL-101 mesoporous chromium terephthalate AJ


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Phthalocyanine Metal Complexes in Catalysis - Chemical Reviews

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