Acetylenes, oxidation oxirene

Acetylene is oxidized to ketene by hot air (24JCS(125)1534). Oxirene is probably not involved since molecular oxygen is not a donor of singlet oxygen atoms (see the remarks on this point in (64JA4866)). 

Parallel chemical oxidation of deuterium labeled biphenylacetylene with metachloro-perbenzoic acid yielded the same product as was obtained in the enzymatic reaction. These observations exclude the possible oxidation of the acetylenic terminal C—H bond, because such a reaction would not be compatible with complete conservation of the deuterium atom. Thus, the formation of a ketene derivative was postulated (Scheme 5). However, the nature of the intermediate from which the ketene derivative is formed remains to be elucidated. An oxirene structure was suggested in parallel to alkene epoxidation however, oxirenes are very unstable species due to electronic and steric factors It is currently believed that a complex between the acetylenic n electrons and cytochrome P450 iron-oxene moiety is formed, leading to the formation of the ketene. ... 

Acetylenes, which have shorter and stronger 7t-bonds than olefins, can also be oxidized by cytochrome P450 enzymes. The oxidation of terminal acetylenes gives ketenes in which the terminal hydrogen has quantitatively migrated to the internal carbon of the triple bond (Fig. 4.31) [191, 192]. The ketene is then hydrolyzed to yield the carboxylic acid as the observed metabolite. By analogy to the oxidation of olefins, the immediate product shotrld be the unsaturated epoxide (oxirene), but oxirenes are extremely un-... 

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