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1.12.4.2 Oxidation

DOI: 10.1055/sos-SD-234-00262

Bietti, M.; Dénès, F.Science of Synthesis: Free Radicals: Fundamentals and Applications in Organic Synthesis, (20211539.

In the 1950s, Kharasch, Sosnovsky, and Yang reported a general method to achieve the allylic oxidation of terminal and internal alkenes (used in excess) to furnish the corresponding allylic esters.[‌162‌‌164‌] The reaction is carried out by heating a mixture of an alkene and a tert-butyl peroxy ester R1CO2Ot-Bu (typically tert-butyl peroxybenzoate[‌162‌] or tert-butyl peroxyacetate),[‌164‌] in the presence of trace amounts (typically <0.1 mol%) of a transition-metal catalyst such as a copper or a cobalt salt. For instance, the oxidation of cyclohexene (0.5-mol scale) into cyclohex-1-en-3-yl benzoate can be achieved in 73% yield (based on tert-butyl peroxybenzoate) upon heating at 80 °C in the presence of copper(I) bromide (Scheme 62). Likewise, hex-1-ene leads to hex-1-en-3-yl benzoate as the major product. The initial report by Kharasch and Sosnovsky indicated the exclusive formation of the terminal alkene, but this was later proved wrong by others.[‌165‌] The alkene is usually used in excess (typically 4–5 equivalents with respect to the peroxy ester) to prevent the activation of multiple CH bonds. Although generally less efficient, oxidation at benzylic positions is also possible, as illustrated by the oxidation of cumene to give 2-phenylpropan-2-yl benzoate.[‌164‌] Interestingly, the reaction can be carried out in a carboxylic acid solvent if the OC(O)R1 substituent needed cannot be introduced easily via the tert-butyl peroxy ester. This topic was reviewed by Rawlinson and Sosnovsky in 1972[‌163‌] and more recently by Andrus and Lashley.[‌166‌]

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References


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