Enol cation radicals in solution, 3[1] Reaction of enol cation radicals in the presence of nucleophiles

Abstract

AbstractThe one‐electron oxidation chemistry of three diaryl‐substituted enols, 1,1‐dimesityl‐1‐propen‐2‐ol (7), 1,2‐dimesityl‐1‐propen‐1‐ol (8), and 2‐mesityl‐2‐phenylethenol (9), has been examined. As a consequence of the pronounced steric shielding around the β‐carbon, 7 does not react with nucleophiles upon addition of 2 eq. of oxidant, but cyclizes to yield benzofuran 18. Under identical conditions, the less shielded enols 8 and 9 react with methanol and other nucleophiles. Several mechanistic hypotheses for nucleophile incorporation are tested, thus allowing the second part of the α‐umpolung reaction of ketones via enol cation radicals to be elucidated mechanistically. Fast‐scan cyclic voltammetry studies (2000 V · s‐1) show no reversible wave of the enol cation radicals. The corresponding methyl enol ether cation radicals, however, have a lifetime of several seconds at ‐10°C, and the kinetics of their reactions with acetonitrile and methanol was probed. The results obtained are consistent with fast deprotonation being the primary reaction of enol cation radicals. Under the given oxidative conditions the radicals formed are further oxidized. Consequently one‐electron oxidation of enols provides an interesting entry to α‐carbonyl carbocation chemistry.