4110 Chemical Reviews,2004,Vol 104,No.9 ten Brink et al. (PPna)4Pt(O) natlcaheoldhenpedteeathatatackpcnortdi xide on the ketone are cordination of 2 PPh en eAga-platmum he rea ange metric BV (see above and section 4.1.5). c 3.6.Electrophilic Activation of Hydrogen Peroxide bodtigtothecapoolftenctionaliafEarcd Figure 15.BV oxidation with [(dppb)Pt(u-OH sted)acid.It should be noted,however.that the which is highly acidic (similar to the molybdenun the Revi metals can co naking talysis with late tr metal such as ole the same toke suc perox h din be for Ne on ketone However.such platinum 72 When platinum salts are used in combination with delectro the ppb)Pt(H ent makes hydro en ide mor Ket again.the plat center gives roph note attac tion.In this case.hydrogen peroxide is also believed t ar aly /was subsequent山y ted by to t The digack on tod th the previous platinum system( hee)is that ed BV ed by assical mech more nuc philic than HOOH. course of the reaction in(CF)CDOD withCNMR g又r 0 HF+BF2OH Figure 16.BF3-catalyzed oxidation of acetone with hydrogen peroxide
sted) acid. It should be noted, however, that the active rhenium complex contains one aqua ligand, which is highly acidic (similar to the molybdenum complex). This acidity may still account for part of the activity of MTO under catalytic conditions. Examples of catalysis with late transition metal complexes are the Pt systems with bridging hydroxy ligands developed in the group of Strukul.40 The work is based on the premise that platinum-η2-peroxo complexes, which can be formed from (ligand)Pt(0) in a reaction with dioxygen, give a nucleophilic attack on ketones. However, such platinum-η2-peroxo species only react in stoichiometric reactions (Figure 14). When platinum salts are used in combination with hydrogen peroxide, a platinumhydroperoxo complex may be active. In the [(dppb)Pt(µ-OH)]2 2+-catalyzed oxidation of ketones (Figure 15), again, the platinum center gives an electrophilic activation of the ketone via coordination. In this case, hydrogen peroxide is also believed to coordinate to the platinum center and attack on the ketones proceeds intramolecularly. The difference with the previous platinum system (Figure 7) is that the platinum center may activate the hydrogen peroxide if (dppb)PtOOH]+ is indeed more nucleophilic than HOOH. It should be noted, however, that attack of coordinated hydrogen peroxide and attack of free hydrogen peroxide on the ketone are indistinguishable in kinetic investigations. Alternatively, coordination of platinum to hydrogen peroxide makes the latter more acidic, which might also promote BV oxidation. This would constitute an electrophilic activation of hydrogen peroxide. Again, platinum facilitates the rearrangement step by coordinating with the hydroxide leaving group () electrophilic activation).41 The platinum systems will be discussed further in asymmetric BV (see above and section 4.1.5). 3.6. Electrophilic Activation of Hydrogen Peroxide Interestingly, in a recent study, Brinck et al.42 showed that in the BF3-catalyzed reaction of acetone and hydrogen peroxide the Lewis acid facilitated the reaction via coordination to hydrogen peroxide, making the latter more acidic and increasing hydrogen bonding to the carbonyl functionality (Figure 16). The coordination of BF3 to acetone was calculated to lead to stabilization of the adduct, rendering it nearly unreactive! The same Lewis acid also facilitated the rearrangement step after migration to the outer peroxygen, creating a BF2OH leaving group rather than a hydroxide leaving group. As was pointed out above, many early transition metals can coordinate to hydrogen peroxide, making it more electrophilic and more eager to attack electronrich substrates such as olefins. By the same token, such electrophilic activation would decrease the tendency to attack already electron-poor ketones in a BV reaction. Neumann recently reported on the electrophilic activation of hydrogen peroxide by 1,1,1,3,3,3-hexafluoro-2-propanol in the oxidation of olefins and ketones.43 This solvent may form hydrogen bonds with hydrogen peroxide, but it is not able to receive hydrogen bonds back, due to the decreased electron density on CF3CHOHCF3 itself. This particular solvent, therefore, makes hydrogen peroxide more electrophilic, which should indeed promote attack of the peroxygens on olefins but not on ketones. This apparent anomaly was subsequently explained by Berkessel and co-workers31 who showed that Brønsted acid-catalyzed BV oxidations with hydrogen peroxide proceed by a nonclassical mechanism in CF3- CHOHCF3 (Figure 17). The intermediacy of spiro bisperoxide (1) was established by following the course of the reaction in (CF3)CDOD with 13C NMR. Figure 14. Nucleophilic reaction of platinum-η2-peroxo complex on ketone. Figure 15. BV oxidation with [(dppb)Pt(µ-OH)]2 2+. Figure 16. BF3-catalyzed oxidation of acetone with hydrogen peroxide. 4110 Chemical Reviews, 2004, Vol. 104, No. 9 ten Brink et al
The Baeyer-Vger Reactior Chemical Revews,04,Vol 104.No.941 00÷O0 gc 0 6 紫器等 rnted acd-catalyzed BV 4mo%(☐s 4.Catalytic Reactions Figure19.Oxidation of piperona 4.1.Homogeneous Catalysts o,N○-cH025a30wH0: 90 C.3h oan-cm Figure 20.Oxidation of 4-nitrobenzaldehyde One of the most undere imated oxidatio tions mainly the correspond ing acid eerally speaking.seleninicad catalystsshow out without a catalyst unde alkaline conditions. aide,Tereachnt by.e.gse yelerm ium notably Seo la and showed a high selectivity fo rganic only yie Figure 18.BV reaction of aldehydes Table 1.H:O:Oxidation of Aldehydes to Acid/Phenol Mixtures substrate ICHCNHSO H 4N0-9 86 4-MeO. *ArSe(O)OHis3.5-(CF)2C.H,Se(O)OH. Selectivity toacid:yield of phenol not given
4. Catalytic Reactions 4.1. Homogeneous Catalysts 4.1.1. Oxidation of Aldehydes One of the most underestimated oxidation reactions is undoubtedly the oxidation of (benz)aldehydes. A selective route to form benzoic acids is oxidation of the hydratesformed from the aldehyde and waters with strong inorganic oxidants such as KMnO4, CrO3, fuming HNO3, Jones reagent, etc. However, environmental considerations have shifted the attention to BV type reactions. In this case, the reaction can yield two products: the corresponding benzoic acid and the ester of the corresponding phenol and formic acid. Formation of the latter product from benzaldehydes is a useful alternative to direct hydroxylation of aromatics (Figure 18). With electron-donating hydroxy or amino substituents on the ortho or para position, the so-called Dakin reaction can be carried out without a catalyst under alkaline conditions. Recenty, a number of articles have appeared on the oxidation of aldehydes with aqueous hydrogen peroxide. The reaction is catalyzed by, e.g., Brønsted acids,44 MTO,45 arylseleninic acids, and SeO2. 46 Although the titles of some articles may imply that the catalysts involved are particularly effective to direct the reaction to either acid or phenol, the electron density on the phenyl ring largely determines this selectivity. Electron-donating substituents favor ring migration to yield phenols, whereas electron-withdrawing substituents favor hydrogen migration to yield acids.47 However, some differences in selectivity can be found depending on solvent type and pH. For instance, the oxidation of piperonal (Figure 19) gives the phenol under acidic conditions,48 whereas under alkaline conditions mainly the corresponding acid is formed.49 Under more or less neutral conditions, the selectivity is directed to the phenol when bis(2- nitrophenyl) diselenide is used as the catalyst (precursor).50 Generally speaking, seleninic acid catalysts show a high tendency to form phenols if electron-donating substituents are present on the aromatic ring of the substrate.51 With MTO,45 selectivity to the (electronrich) phenols is lower than with bis(2-nitrophenyl) diselenide. Table 1 gives an overview of the selectivity of several catalysts active in the oxidation of aldehydes. Several catalysts based on seleniumsnotably SeO2 and Ph2Se2 in THF52sshowed a high selectivity for the carboxylic acid. Noyori et al.44 used a simple lipophilic acid catalyst, [CH3(n-C8H17)3N]HSO4, to convert aldehydes to carboxylic acids under halide and metal-free conditions without the presence of any organic solvent (Figure 20). Although details were not given, it seems reasonable that substantial amounts of phenol were formed in those cases when only low yields of acid were reported. Possibly, the phenols are oxidized further. The system developed by Noyori is probably the easiest and greenest way to oxidize aldehydes to carboxylic acids to date, and the protocol is suitable to oxidize aldehydes on a mole-scale. The Figure 17. Nonclassical Brønsted acid-catalyzed BV oxidation in (CF3)2CHOH. Figure 18. BV reaction of aldehydes. Table 1. H2O2 Oxidation of Aldehydes to Acid/Phenol Mixtures substratea SeO2 b (ref 46) ArSe(O)OHb,c (ref 51) WO4 2- d (ref 53) [CH3(n-C8H17)3N]HSO4 d (ref 44) H+/MeOHb (ref 48) 4-NO2-æ 87/0 99/0 89 93 80/0 4-Cl-æ 83/0 50/50 86 76 87/0 4-CH3-æ 88/0 45/55 57 41 51/28 4-H-æ 97/0 NDe 84 85 NDe 4-MeO-æ 46/41 0/95 6 9 0/90 octanal 91/0 95/1 87 82 NDe a æ: -C6H4CHO. b Acid to phenol ratio. c ArSe(O)OH is 3,5-(CF3)2C6H3Se(O)OH. d Selectivity to acid; yield of phenol not given. e ND ) not determined. Figure 19. Oxidation of piperonal. Figure 20. Oxidation of 4-nitrobenzaldehyde. The Baeyer−Villiger Reaction Chemical Reviews, 2004, Vol. 104, No. 9 4111