Baeyer reaction, mechanism

Density functional theory has been used to model the Baeyer-Villiger reaction mechanism for Ti(IV)-H202 and Sn(IV)-H202 systems. These calculations have shown... 

Fig. 14.37. Regioselective Baeyer-Villiger rearrangement of an electron-poor aromatic aldehyde. This reaction is part of the autoxidation of benz-aldehyde to benzoic acid. Both alternative reaction mechanisms are shown the [1,21-rearrangement (top) and the /3-elimination (bottom).

The reaction mechanism, illustrated in Scheme 7.13, does not include cyclohex-andione as the reaction intermediate moreover, the Baeyer-Villiger ring enlargement on cyclohexanone was negligible. 

The epoxidation of enones by alkaline hydrogen peroxide is well known. Prolonged reaction causes C-C bond fission, probably by a Baeyer-Villiger mechanism. Applied to the 1 o,2a-epoxy-4,6-dien-3-one system (231), this... 

The Baeyer-Wlliger oxidation of ketones affords esters (from open-chain ketones) and lactones (from cyclic ketones), respectively. It is typically carried out using nucleophilic oxidants, such as peracids or dihydrogen peroxide in the presence of bases (29). Nucleophilic oxidants are required because the first step of the reaction mechanism is the addition of the peroxidic oxidant to the ketone s carbonyl function, affording the so-called Criegee-intermediate (Scheme 10). The subsequent cleavage of the 0 0 bond in the latter, and the concerted shift of yield the ester (lactone) product. In enzymatic Baeyer-Vlliger oxidations, flavin hydroperoxides such as 26 act as the nucleophilic oxidant (30). 

Possibly, furfural was oxidized to 2-formyloxyfuran via acid-catalyzed Baeyer-ViUiger oxidation by H2O2, and then 2-hydroxyfuran and formic acid (FA) were formed [35]. The 2-hydroxyfuran has isomers of 2(3H)-furanone and 2(5H)-furanone, the former providing SA, whereas the latter yields MA and FA. From and NMR spectra recorded during the reaction to determine the intermediate species, the presence of MA, FA, 2(5H)-furanone, and SA was detected. However, further investigation to reveal the detailed reaction mechanism of furfural oxidation to SA over acid catalysts in the presence of H2O2 remains necessary. 

Quantum, by contrast, converted an ethylene—carbon monoxide polymer into a polyester-containing terpolymer by treatment with acidic hydrogen peroxide, the Baeyer-Villiger reaction (eq. 11). Depending on the degree of conversion to polyester, the polymer is totally or partially degraded by a biological mechanism. 

The mechanism of this degradation has received considerable attention, and for some species the reaction is equivalent to a nonenzymatic Baeyer-Villager reaction, producing first the 17j8-acetate. This functionality can then be hydrolyzed and oxidized to the ketone and may undergo a second Baeyer-Villager reaction to produce a lactone ... 

The Dakin reaction proceeds by a mechanism analogous to that of the Baeyer-Villiger reaction. An aromatic aldehyde or ketone that is activated by a hydroxy group in the ortho or para position, e.g. salicylic aldehyde 12 (2-hydroxybenzaldehyde), reacts with hydroperoxides or alkaline hydrogen peroxide. Upon hydrolysis of the rearrangement product 13 a dihydroxybenzene, e.g. catechol 14, is obtained ... 

In another kind of reaction, an aromatic aldehyde ArCHO or ketone ArCOR is converted to a phenol ArOH on treatment with alkaline H202, but there must be an OH or NH2 group in the ortho or para position. This is called the Dakin reac-The mechanism may be similar to that of the Baeyer-Villiger reaction (18-19) ... 

Synthesis of all four 8,8a-secobenzophenanthridine alkaloids was carried out chiefly by Baeyer-Villiger oxidation of appropriate benzophen-anthridines (Scheme 32). Thus, arnottianamide (206) was obtained from chelerythrine (210) (172,175), iwamide (207) from N-methyldecarine (211) (168,172), integriamide (208) from avicine (212) (171,172), and isoarnottiamide (209) from nitidine (213) (172,175). The proposed mechanism of this reaction (168,172,175) consists of initial attack of the peroxide ion on the C=N+ double bond followed by rearrangement and hydrolysis. 

To date, reports have involved palladium catalysts for Suzuki and Sono-gashira coupling reactions [63-66], rhodium catalysts for silylations of alcohols by trialkylsilanes [67,68], and tin-, hafnium-, and scandium-based Lewis acid catalysts for Baeyer-Villiger and Diels-Alder reactions [69]. Regardless of exact mechanism, this recovery strategy represents an important direction for future research and applications development. Finally, a particularly elegant protocol where CO2 pressure is used instead of temperature to desorb a fluorous rhodium hydrogenation catalyst from fluorous silica gel deserves emphasis [28]. 

Fig. 23. Mechanism of CHMO-catalyzed Baeyer-Villiger reaction (173,177,178,182).

In 2001, Albrecht Berkessel and Nadine Vogl reported on the Baeyer-Villiger oxidation with hydrogen peroxide in 1,1,1,3,3,3-hexafluoroisopropanol (HFIP) as solvent in the presence of Brpnsted acid catalysts such as para-toluenesulfonic acid (equation 85) . Under these conditions cyclohexanone could be selectively transformed into the corresponding lactone within 40 min at 60 °C with a yield of 92%. Mechanistic investigations of Berkessel and coworkers revealed that this reaction in HFIP proceeds by a new mechanism, via spiro-bisperoxide 234 as intermediate, which then rearranges to form the lactone. The study illustrates the importance of HFIP as solvent for the reaction, which presumably allows the cationic rearrangement of the tetroxane intermediates. 

The same Pt species that epoxidize double bonds are active in Baeyer-ViUiger oxidation of ketones. Strukul has shown that this synthetically interesting process can be carried out also enantioselectively, in the presence of appropriate diphosphine ligands For this reaction a mechanism has been proposed that involves again a quasi-peroxo metallacycle intermediate, even though in this reaction the metal catalyst plays primarily the role of a Lewis acid while the real oxidant is H2O2 itself (Scheme 9). 

Mechanism of Baeyer-Villiger Oxidation Both aldehydes and ketones are oxidized by peroxy acids. This reaction, called the Baeyer- Villiger oxidation, is especially useful with ketones. 

The conversion of benzaldehyde in the presence of air to benzoic acid was reported in 1832 by Wohler and Liebig, and in 1900 Baeyer and Villiger proposed perbenzoic acid as an intermediate in the reaction. The currently accepted free radical chain mechanism for the process was proposed by Backstrom in 1934 (equation 34). Bates and Spence already in 1931 had proposed that photolysis of CH3I forming CHs in the presence of O2 led to peroxyl radicals CHaOO-. ... 

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