Oxygen Acidity of Ring Methoxylated 1,1-Diarylalkanol Radical Cations Bearing alpha-Cyclopropyl Groups.The Competition between O-Neophyl Shift and C-Cyclopropyl beta-Scission in the Intermediate 1,1-Diarylalkoxyl Radicals

摘要

A product and time-resolved kinetic study on the reactivity of the radical cations generated from cyclopropyl(4-methoxyphenyl)phenylmethanol(1)and cyclopropyl[bis(4-methoxyphenyl)]methanol(2)has been carried out in aqueous solution.In acidic solution,1~(centre dot+)and 2~(centre dot+)display very low reactivities toward fragmentation,consistent with the presence of groups at C_alpha(aryl and cyclopropyl)that after C_alpha-C_beta bond cleavage would produce relatively unstable carbon-centered radicals.In basic solution,1~(centre dot+)and 2~(centre dot+)display oxygen acidity,undergoing ~-OH-induced deprotonation from the alpha-OH group,leading to the corresponding 1,1-diarylalkoxyl radicals 1r~(centre dot)and 2r~(centre dot),respectively,as directly observed by time-resolved spectroscopy.The product distributions observed in the reactions of 1~(centre dot+)and 2~(centre dot+)under these conditions(cyclopropyl phenyl ketone,cyclopropyl(4-methoxyphenyl)ketone,and 4-methoxybenzophe-none from 1~(centre dot+);cyclopropyl(4-methoxyphenyl)ketone and 4,4'-dimethoxybenzophenone from 2~(centre dot+))have been rationalized in terms of a water-induced competition between O-neophyl shift and C-cyclopropyl beta-scission in the intermediate 1,1-diarylalkoxyl radicals 1~(centre dot)and 2r~(centre dot).