Cyclopropyl Conjugation and Ketyl Anions: When Do Things Begin to Fall Apart?

Abstract

Results pertaining to the electrochemical reduction of 1,2-diacetylcyclopropane (5), 1-acetyl-2-phenylcyclopropane (6), 1-acetyl-2-benzoylcyclopropane (7), and 1,2-dibenzoylcyclopropane (8) are reported. While 6*- exists as a discrete species, the barrier to ring opening is very small (<1 kcal/mol) and the rate constant for ring opening is >10(7) s(-1). For 7 and 8, the additional resonance stabilization afforded by the benzoyl moieties results in significantly lower rate constants for ring opening, on the order of 10(5)-10(6) s(-1). Electron transfer to 8 serves to initiate an unexpected vinylcyclopropane --> cyclopentene type rearrangement, which occurs via a radical ion chain mechanism. The results for reduction of 5 are less clear-cut: The experimental results suggest that the reduction is unexceptional, with a symmetry coefficient alpha </= 0.5, and reorganization energy consistent with a simple electron-transfer process (one electron reduction, followed by ring opening). In contrast, molecular orbital calculations suggest that 5*- has no apparent lifetime and that reduction of 5 may occur by a concerted dissociative electron transfer (DET) mechanism (i.e., electron transfer and ring opening occur simultaneously). These seemingly contradictory results can be reconciled if the increase in the internal reorganization energy associated with the onset of concerted DET is offset by a lowering of the solvent reorganization energy associated with electron transfer to a more highly delocalized LUMO.