Radical ion probes. 2. Evidence for the reversible ring opening of arylcyclopropylketyl anions. Implications for mechanistic studies

Article abstract of DOI:10.1021/ja00031a045

Aryl cyclopropyl ketones have frequently been utilized as diagnostic probes for single electron transfer (SET) for a variety of organic transformations. The implicit assumption in these studies is that the formation of rearranged product(s) signals the intermediacy of a ketyl anion. Through a detailed examination of the mechanism, kinetics, and products arising from the decay of several arylcyclopropylketyl anions (generated electrochemically), we have demonstrated that the assumptions made in the use of these substrates as SET probes are tenuous. Radical anions generated from compounds whose only substituents on the cyclopropane ring are either H or CH₃ including phenyl cyclopropyl ketone (6), l-benzoyl-2-methylcyclopropane (7), l-benzoyl-2,2-dimethylcyclopropane (8), 1-benzoyl-l-methylcyclopropane (9), andp-tolyl cyclopropyl ketone (10) undergo slow and reversible ring opening. This preequilibrium is followed by a rate-limiting coupling of the ring-opened and ring-closed species. The direction of equilibrium overwhelmingly favors the ring-closed form. For 6*^-, the equilibrium constant for ring opening is estimated to be 2 × 10^-8 with a maximum rate constant for the forward reaction estimated to be 2 s^-1 and a minimum rate constant for the reverse direction of 8 × 10⁷ s^-1. Via deuterium labeling coupled with ²H NMR and GC/MS analysis the major product arising from the dianion is demonstrated to arise via a novel intramolecular hydride-transfer process. The radical anion generated from p-cyclopropylacetophenone (11) shows no evidence for cyclopropane ring opening. 11*^- decays with a bimolecular rate law forming a pinacol dimer. These observations are complemented by results obtained from semiempirical molecular orbital calculations at the AM1 level. Important factors in the design of radical ion probes based upon intramolecular rearrangements are identified and discussed.