Pulse radiolytic kinetic study of the decay of .alpha.-methyl-substituted benzoquinone radical anions: a possible mechanistic model for bioreductive alkylation

Abstract

Pulse radiolysis and transient absorption spectroscopy were used to kinetically characterize the decay of several α-methyl-substituted 1,4-benzoquinone radical anions. Electron attachment rates, absorption spectra, extinction coefficients, and pK a ' s for the reduced quinones are very similar regardless of substituent, but the decay kinetics are dependent on both substituent and pH. The semiquinone radical anions disproportionate at pH 7 regardless of substituent. At pH 3, the radical anion is readily protonated, and in the phenoxy- and methoxy-substituted models reductive elimination becomes competitive with disproportionation. The involvement of an intramolecular H-bonded intermediate is proposed in the formation of the quinone methide. The existence of such an intermediate is supported by theoretical results (AM1 calculations)