Science of Synthesis

Matsumoto, K.; Katsuki, T.; Arends, I. W. C. E., Science of Synthesis: Stereoselective Synthesis, (2011) 1, 116.

Epoxidation with monooxygenases depends on cofactors. The enzymes usually consist of several subunits that, in a series of cascades, provide the reducing equivalents to the iron-active site. The need for a cofactor is eliminated in the case of peroxidases, for which hydrogen peroxide acts as the oxidant. The hydrogen peroxide results in a shunt pathway that directly restores the [P+•Fe(IV)=O] species from iron(III) (see ‌Scheme 61‌). Because of its ease of handling, chloroperoxidase (CPO) from Caldariomyces fumago is an attractive option; it is commercially available, and displays a reasonable substrate range.[‌163‌] The main problems are its instability under the reaction conditions and its sluggish reaction rates. Typical turnover rates are 0.1–2 s−1. The epoxidation of styrene in the presence of 0.8% of surfactant is, at 5.5 s−1, the fastest on record.[‌164‌] The advantage of this reaction is that the enantioselectivities are, in most cases, good to excellent. For example, hept-2-ene can be oxidized with 1700 turnovers relative to enzyme in 30% tert-butyl alcohol to give the corresponding epoxide with 96% ee. The total turnover number of this reaction can be increased from 1700 to 11 500 when hydrogen peroxide is generated in situ by a glucose oxidase mediated reaction of glucose and oxygen. A wide range of 2-methylalk-1-enes can be oxidized with enantiomeric excess values ranging from 50 to 95% (‌Scheme 66‌).[‌165‌–‌167‌]

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