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

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·+ and 2·+ display very low reactivities toward fragmentation, consistent with the presence of groups at Cα (aryl and cyclopropyl) that after CαCβ bond cleavage would produce relatively unstable carbon-centered radicals. In basic solution, 1·+ and 2·+ display oxygen acidity, undergoing -OH-induced deprotonation from the α-OH group, leading to the corresponding 1,1-diarylalkoxyl radicals 1r· and 2r·, respectively, as directly observed by time-resolved spectroscopy. The product distributions observed in the reactions of 1·+ and 2-+ under these conditions (cyclopropyl phenyl ketone, cyclopropyl(4-methoxyphenyl) ketone, and 4-methoxybenzophenone from 1·+; cyclopropyl(4-methoxyphenyl) ketone and 4,4′-dimethoxybenzophenone from 2·+) have been rationalized in terms of a water-induced competition between O-neophyl shift and C-cyclopropyl β-scission in the intermediate 1,1-diarylalkoxyl radicals 1r· and 2r·. [ABSTRACT FROM AUTHOR]