《有机化学》课程教学资源（文献资料）The Baeyer-Villiger Reaction New Developments toward Greener Procedures.pdf

The Baeyer−Villiger Reaction: New Developments toward Greener Procedures G.-J. ten Brink, I. W. C. E. Arends, and R. A. Sheldon* Laboratory for Biocatalysis and Organic Chemistry, Department of Biotechnology, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands Received February 10, 2004 Contents 1. Introduction 4105 2. H2O2 and O2 as Green Oxidants 4106 2.1. Hydrogen Peroxide 4106 2.2. Dioxygen 4106 3. Theoretical Considerations 4107 3.1. Mechanism of the Reaction 4107 3.2. Electrophilic Activation of Substrate 4108 3.3. Electrophilic Activation of Intermediate 4108 3.4. Nucleophilic Activation of Intermediate 4109 3.5. Nucleophilic Activation of Hydrogen Peroxide 4109 3.6. Electrophilic Activation of Hydrogen Peroxide 4110 4. Catalytic Reactions 4111 4.1. Homogeneous Catalysts 4111 4.1.1. Oxidation of Aldehydes 4111 4.1.2. Oxidation of Cycloalkanones 4112 4.1.3. Oxidative Ring Contraction 4113 4.1.4. Oxidation of Linear Ketones 4113 4.1.5. Enantioselective Oxidations 4113 4.2. Biocatalysis 4116 4.2.1. Lipases 4116 4.2.2. BVMOs 4116 4.2.3. Enantioselective Reactions 4117 4.2.4. Regioselective Reactions 4118 4.2.5. Chemoselectivity 4118 4.3. Heterogeneous Oxidation 4118 4.3.1. Solid Peracids 4118 4.3.2. Solid Lewis Acid Catalysts 4118 4.3.3. Solid Catalysts for in Situ Formation of Peracids 4120 5. Outlook 4121 6. Abbrevations 4121 7. References 4121 1. Introduction In 1899, Adolf Baeyer and Victor Villiger reported the oxidation of menthone to the corresponding lactone (Figure 1) using a mixture of sodium persulfate and concentrated sulfuric acid (Caro’s acid).1 The persulfuric acid was subsequently replaced by an organic peracid, and the Baeyer-Villiger (BV) reaction became one of the most well-known and widely applied reactions in organic synthesis.2,3 Its success is largely due to its versatility: (i) A variety of carbonyl compounds can be oxidized; that is, ketones are converted into esters, cyclic ketones into lactones, benzaldehydes into phenols, or carboxylic acids and R-diketones into anhydrides. (ii) A large number of functional groups are tolerated. (iii) The regiochemistry is highly predictable with the migratory aptitude being tertiary alkyl > cyclohexyl > secondary alkyl > benzyl > phenyl > primary alkyl > CH3. 4 (iv) The reaction is generally stereoselective; that is, the migrating group retains its configuration. (v) A wide range of oxidants may be used with their activity decreasing in the order: CF3CO3H > monopermaleic acid > monoperphthalic acid > 3,5-dinitroperbenzoic acid > p-nitroperbenzoic acid > m-CPBA ∼ HCO3H > C6H5CO3H > CH3CO3H . H2O2 > t-BuOOH. Although more than a century has gone by since its discovery, the BV reaction is far from being at the end of its development. The standard protocol for a BV oxidation suffers from several disadvantages. The use of an organic peracid results in the formation of one equivalent of the corresponding carboxylic acid salt as waste, which has to be recycled or disposed of (returning it to the manufacturer of the peracid is generally not an option). Moreover, organic peracids are expensive and/or hazardous (because of shock sensitivity), which limits their commercial application. For example, the transport and storage of peracetic acid have been severely curtailed, making its use prohibitive. Consequently, increasing attention has been focused on the in situ generation of organic peracids, via reaction of either the corresponding aldehyde with oxygen or the carboxylic acid with hydrogen peroxide. In an alternative approach, the use of an organic peracid is dispensed with altogether by employing hydrogen peroxide in the presence of a catalyst. A prerequisite for success is that the method should be amenable to the use of commercially available (30 or 60%) aqueous hydrogen peroxide and preferably avoid the use of environmentally unattractive solvents such as chlorinated hydrocarbons. If successful, such a method would circumvent both the environmental and the safety issues associated with the classical BV oxidation. In short, there is a definite need for a green BV oxidation procotol, which utilizes aqueous hydrogen peroxide as the stoichiometric oxidant in an environmentally attractive solvent or (preferably) under solvent-free conditions. From the viewpoint of scope in organic synthesis, the method should also exhibit high degrees of chemo-, regio-, and enantioselectivity and broad substrate specificity. Chem. Rev. 2004, 104, 4105−4123 4105 10.1021/cr030011l CCC: $48.50 © 2004 American Chemical Society Published on Web 08/14/2004

Consequently, in this review, we will focus on green BV reactions using hydrogen peroxide. In the first part, some general features of reactions with hydrogen peroxide and dioxygen are delineated. In the second part, the mechanism of the BV reaction is analyzed to identify ways in which a catalyst might improve the reaction. In the third part, reactions catalyzed by homogeneous catalysts, biocatalysts, and heterogeneous catalysts are discussed. 2. H2O2 and O2 as Green Oxidants 2.1. Hydrogen Peroxide The above-mentioned drawbacks of the classical BV reaction have stimulated considerable activitys especially in the past few yearssin the development of catalysts that employ hydrogen peroxide as a clean oxidant.3b,5 The use of hydrogen peroxide has many advantages: it is safe and cheap, the active oxygen content is high, it does not require a buffer, and it is clean, since the byproduct formed is water. These points make the use of hydrogen peroxide extremely interesting from an industrial (large-scale) point of view. However, there are some disadvantages concerning the use of hydrogen peroxide.6 (i) Because water is always present in solution, hydrolysis of the product esters may occur, and not all substrates are therefore compatible with water. (ii) Hydrogen peroxide is one of the weakest oxidants of a wide range of available peroxides and peracids (see above), and a catalyst is required to activate it. (iii) Some catalysts show a low selectivity on hydrogen peroxide. This may cause the formation of unselective hydroxy or hydroperoxy radicals. Furthermore, pure dioxygen may evolve from H2O2 decomposition, causing a build-up of pressure and creating a potentially unsafe combination with flammable organic solvents. (iv) High concentrations of hydrogen peroxide (>40% mol/mol) in organic solvents are unsafe. To avoid dangerous situations as mentioned in points iii and iv above, Jones6 recommends adhering to the following checklist: (i) Avoid any contamination in the reaction vessel (which may induce an uncontrolled reaction). (ii) Avoid build-up of oxygen pressure (by venting and flushing with N2). (iii) Keep the concentration of the peroxo compound below 20% mol/mol (by presetting the reaction temperature, adding the peroxo compound last,7 stirring the reaction mixture, making sure that the peroxo compound reacts completely before adding more, and providing cooling if required). (iv) Destroy excess peroxo compound before work-up. (v) Never use acetone or other low boiling ketones as the solvent for cleaning or extraction. 2.2. Dioxygen Free radical autoxidation of an aldehyde is facile and affords the corresponding peracid. In the presence of a reactive substrate, e.g., an olefin or a ketone, Gerd-Jan ten Brink was born in Rijnsaterwoude, The Netherlands, in 1971. He received his M.Sc. degree from the Free University of Amsterdam (1995) under the supervision of Professor F. Bickelhaupt. In 2001, he received his Ph.D. degree (cum laude) for his research on “Green Catalytic Oxidations” under the supervision of Professor R. A. Sheldon. After a year of postdoctoral research spent jointly in the Sheldon group and at Avantium Technologies BV, he now works for ChemShop BV in Weert, The Netherlands. Isabel W. C. E. Arends (born 1966) studied chemistry at the University of Leiden (The Netherlands), where she received her Ph.D. in physical organic chemistry in 1993, under the supervision of Professor R. Louw and Dr. P. Mulder. Postdoctoral work followed with Professor K. U. Ingold at the National Research Council in Canada on liquid phase oxidations catalyzed by biomimetic iron complexes. She joined the group of R. A. Sheldon in 1995, where she was appointed Assistant Professor in 2001. Her research interests focus on enzyme- and metal-catalyzed redox reactions and green selective oxidations employing O2 and H2O2 in particular. Roger Sheldon (1942) received a Ph.D. in organic chemistry from the University of Leicester (United Kingdom) in 1967. This was followed by postdoctoral studies with Professor Jay Kochi in the United States. From 1969 to 1980, he was with Shell Research in Amsterdam, and from 1980 to 1990, he was R&D Director of DSM Andeno. In 1991, he moved to his present position as Professor of Organic Chemistry and Catalysis at the Delft University of Technology (The Netherlands). His primary research interests are in the application of catalytic methodologiesshomogeneous, heterogeneous, and enzymaticsin organic synthesis, particularly in relation to fine chemicals production. He developed the concept of E factors for assessing the environmental impact of chemical processes. Figure 1. Oxidation of menthone with Caro’s acid. 4106 Chemical Reviews, 2004, Vol. 104, No. 9 ten Brink et al

the peracid can transfer an oxygen atom to the substrate, resulting in the formation of one equivalent of epoxide or ester and acid. Oxidations involving the in situ formation of a peracid from an aldehyde and dioxygen are generally referred to as the Mukaiyama method.8 One industrial route to -caprolactone, for instance, involves the in situ formation of peracetic acid from acetaldehyde (Figure 2).9 The use of metal catalysts is optional,10 and the combination aldehyde/dioxygen is often not significantly different from peracids, although it should be noted that radical type chemistry may take place instead of the intended BV reactions. In alcoholic solvents, radical type side reactions are suppressed to a certain extent.11 Recently, Ishii12 reported on the “aerobic” BV oxidation of a cyclohexanol/cyclohexanone mixture (KA oil) to yield lactones (Figure 3). However, in this reaction, cyclohexanol is first oxidized to give cyclohexanone and hydrogen peroxide and the latter is used as the true oxidant in the BV reaction. The latter methodsusing O2 for the in situ formation of hydrogen peroxide as the actual oxidantshas been receiving much attention over the past years because it is cheaper than hydrogen peroxide itself.13 3. Theoretical Considerations 3.1. Mechanism of the Reaction For an in-depth discussion on the mechanism of the BV reaction, we refer to the excellent reviews of Krow2 and Meunier.3a A few key issues are briefly mentioned here. The generally accepted mechanism for the BV oxidation is a simple two-step reaction that involves the so-called Criegee intermediate or adduct. In the first step, a peroxide attacks the polarized CdO bond. The second step follows a concerted pathway (Figure 4). Only with acylperoxo type oxidants can the hydroxyl proton in this intermediate migrate intramolecularly. Hence, these oxidants are more effective than alkylperoxo type oxidants, which generally require a catalyst.14 It should be noted that in many reactions the two steps have activation energies that are in the same order of magnitude. Hence, catalysts may need to facilitate both steps of the reaction. With some exceptions,15 the rearrangement step is usually rate limiting.16 In the Criegee intermediate, a proper alignment is required for the rearrangement step: The migrating group RM needs to be antiperiplanar17 to the O-O bond of the leaving group (primary stereoelectronic effect) and antiperiplanar to a lone pair of the hydroxyl group (secondary stereoelectronic effect). In 1980, Noyori18 provided evidence for the existence of the secondary effect, but compelling evidence for the primary effect was not yet available.19 Criegee rearrangements in allyl hydroperoxides20 already hinted at such an effect, but in 2000, Crudden et al. showed its existence in a true BV oxidation of trans- and cis- 4-tert-butyl-2-fluorocyclohexanone (Figure 5).21 When the 2-fluoro substituent in 4-tert-butyl-2- fluorocyclohexanone is aligned in an axial position, differences in dipole effects in the various conformations are minimal and do not influence the migration of either the CH2 or the CHF group. In this case, a normal product distribution22 is observed and the electron-rich CH2 group migrates preferentially. However, when the 2-fluoro substituent is placed in an equatorial position, the conformation with CH2 antiperiplanar to the O-O bond creates an unfavorable dipole interaction of the perester with the CHF group. In this case, the electron-poor CHF group can achieve the required alignment more easily and is “forced” to migrate. Thus, at least in these cases, the primary stereoelectronic effect is more important than the migratory aptitude.23 Figure 2. Mukaiyama oxidation of cyclohexanone to caprolactone. Figure 3. “Aerobic” BV reaction of KA oil. Figure 4. Mechanism for BV reaction as proposed by Criegee. RM is the migrating group. Figure 5. BV oxidation of trans- and cis-4-tert-butyl-2- fluorocyclohexanone. The Baeyer−Villiger Reaction Chemical Reviews, 2004, Vol. 104, No. 9 4107

A more detailed mechanism (Figure 6) shows the possible mechanisms by which catalysts may improve BV reactions. Here, one can distinguish (1) electrophilic activation of the substrate, (2) electrophilic activation of the intermediate, (3) nucleophilic activation of the intermediate, (4) nucleophilic activation of (hydrogen) peroxide, and (5) electrophilic activation of (hydrogen) peroxide. 3.2. Electrophilic Activation of Substrate The action of acids (H+ or metal cations) is in part to activate the carbonyl functionality toward nucleophilic attack of peroxide or peracid via increasing the polarization of the CdO double bond (Figure 6, intermediate 1). Therefore, the combination CF3- CO3H/CF3CO2H gives one of the most reactive peracids, even though CF3CO3 - is a weak nucleophile, reluctant to attack the polarized carbonyl functionality. Indeed, in a buffered solution, the activity of CF3- CO3H is strongly diminished indicating that an improved leaving group effect of CF3COO- may not be important. Other work, however, indicated that electron-withdrawing substituents on the leaving group did actually facilitate rearrangement, an effect observed in oxidation both with peracids24 and with hydrogen peroxide.3b One example of transition metal-catalyzed electrophilic activation of substrates is the platinum-CF3 system, described below, which was developed in the group of Strukul (Figure 7).25 Activation of the ketone via coordination to Lewis acids seems to be the most general way to activate substrates for BV oxidation. In this case, the ketone coordinates to an electronpoor platinum center and becomes susceptible to attack of free hydrogen peroxide (intermediate I). This activation is somewhat reminiscent of activation of R,â-unsaturated ketones in Diels-Alder reactions. Not surprisingly, cationic platinum complexes of (chiral) diphosphines proved to be active in this reaction as well.26 To our knowledge, catalysts that are typically successful in Diels-Alder reactions, such as lanthanides, are rarely used to activate ketones for BV reactions with H2O2, 27 although these water stable Lewis acids seem to meet all of the requirements for a successful BV reaction. Other Lewis acids such as gallium(III) or tin(IV) chloride are too water sensitive and have mainly been successful under anhydrous conditions with, e.g., bis- (trimethylsilyl)peroxide as the oxidant (Figure 8).28,29 Clearly, the method is far from green. Corma et al.30 developed solid tin catalysts that are water stable and use hydrogen peroxide as the oxidant (see later under solid Lewis acids). 3.3. Electrophilic Activation of Intermediate In BV reactions with peracids as oxidants, strong acids, such as CF3CO2H, may also catalyze the rearrangement step via protonation of the carbonyl functionality of the leaving group (Figure 9). As this rearrangement step is usually rate limiting, the catalyst has a large effect here. Activation of the intermediate hydroperoxy adduct is similar to activation of the acylhydroperoxy intermediate. A Lewis acid may also facilitate the migration step, via coordination or protonation of the hydroxide (alkoxide), which is otherwise a very poor leaving group. This is again illustrated with the (dppe)Pt(CF3)]+ complex where the platinum center facilitates the rearrangement step via coordination to hydroxide (Figure 7, intermediate I). In most if not all cases, Lewis acid catalysts can facilitate both steps of the reaction. It is not always trivial to make a distinction between Brønsted and Lewis acid catalysis, as the pH of a solution may decrease when Lewis acidic metals are added to the reaction mixture. Therefore, carrying out BV reactions in buffered solutions may sometimes lead to surprising results. One important difference in BV reactions with hydrogen peroxide is Figure 6. Electrophilic and nucleophilic activation of the BV reaction. Figure 7. BV oxidation with (dppe)Pt(CF3)]+. Figure 8. Lewis acid-catalyzed oxidation of 2-(3-methyl- 2-butenyl)cyclopentanone. Figure 9. Acid-catalyzed BV oxidation with peracids as the oxidant. 4108 Chemical Reviews, 2004, Vol. 104, No. 9 ten Brink et al

that with Brønsted acid catalysts, dimeric, trimeric, or polymeric peroxides seem to be formed more easilyscompounds that are potentially explosive (Figure 10). Indeed, a BV reaction can sometimes proceed via such a dimeric peroxide intermediate as recently shown by Berkessel and co-workers (see also later Figure 17).31 3.4. Nucleophilic Activation of Intermediate On the basis of the mechanism depicted in Figure 9, it is difficult to imagine base catalysis to activate the intermediate. Base catalysis was observed when bicarbonate was added to a solution of m-CPBA and a bicyclic ketone in dichloromethane.32 The reaction rate nearly doubled, which was ascribed to an accelerated rearrangement step of an anionic Criegee adduct as compared to the neutral adduct (Figure 11). Renz and Meunier noted in their review3 that in the reaction mentioned above, bicarbonate also removed the coproduct, m-CBA, from the reaction mixture via deprotonation and precipitation. In this way, the m-CBA could not compete with m-CPBA for the substrate, resulting in an increase in rate. Although BV reactions are sometimes carried out under neutral to basic conditions to avoid acidcatalyzed side reactions, base catalysis is not commonly observed in BV reactions with hydrogen peroxide.33 3.5. Nucleophilic Activation of Hydrogen Peroxide Very few transition metals can catalyze BV reactions with hydrogen peroxide. The early transition metals (Ti, V, Mo, and W) may form peroxo complexes with hydrogen peroxide, but these are generally electrophilic in nature. Therefore, these complexes are active in, for example, epoxidation via electrophilic attack on preferably electron-rich olefins. A nucleophilic attack on the partially positively charged carbon of the CdO functionality is unlikely to occur with these complexes. Such a reaction seems to be the domain of the late transition metal peroxo complexes such as (ligand)Pt(O)2 or (ligand)Pd(O)2, which are partly nucleophilic in nature (see later). Indeed, the first example of transition metal catalysis, which involved a (dipicolinato)MoVI peroxo complex (Figure 12) in the oxidation of cyclic ketones with 90% hydrogen peroxide, later turned out to be a simple acid-catalyzed reaction, rather than the first example of nucleophilic reactivity of a group VI peroxo metal complex.34 With this in mind, the MTO-catalyzed BV oxidation of cyclobutanone with aqueous hydrogen peroxide becomes all the more suspicious.35 MTO is an extremely active catalyst for the epoxidation of olefins with aqueous hydrogen peroxide.36 The active bisperoxo intermediate gives an electrophilic attack on the double bond of the alkene. Therefore, a proposed nucleophilic attack of the same bisperoxo complex on the CdO double bond of, e.g., cyclobutanone (Figure 13), would seem unlikely. However, with the evidence available until now, it appears that MTO can exhibit electrophilic properties in epoxidation and nucleophilic properties in BV oxidation.37 The reason that MTO may change its nucleophilic/ electrophilic behavior depending on the substrate is not entirely clear. If the ketone coordinates to rhenium, then the metal plays a role as a Lewis acid and induces electrophilic activation of the substrate. The coordination of basic ligands (ketone) also increases the electron density on the metal center, which in turn increases the nucleophilic character of the peroxo groups.38 Reaction of the bisperoxo complex with thianthrene-5-oxide did reveal a partly nucleophilic character of the catalyst,39 and in the oxidation of 1,3-diketones, MTO acts as both an electrophilic and a nucleophilic catalyst.37 A similar intermediate has been proposed for the rhenium-catalyzed reaction as for the molybdenumcatalyzed reaction, but in this case, 17O NMR revealed polarization of the peroxo moiety, which might explain the nucleophilic character. Furthermore, contrary to the MoVI system, a stoichiometric reaction between the rhenium bisperoxo complex and cycloalkanones in the absence of hydrogen peroxide did lead to product formation, indicating that the rhenium bisperoxo complex is more than an expensive (BrønFigure 10. Formation of organic peroxides under acidic conditions. Figure 11. Rearrangement of anionic Criegee adduct. Figure 12. Molybdenum-catalyzed BV reaction of cyclopentanone. Figure 13. MTO-catalyzed oxidation of cyclobutanone. The Baeyer−Villiger Reaction Chemical Reviews, 2004, Vol. 104, No. 9 4109