Hydration of bis(pentamethylphenyl)- and bismesityl-ketenes leading to ene-1,1-diols (enols of carboxylic acids)

Abstract

The reaction of the sterically hindered diarylketenes (Ar = Me₅C₆ or 2,4,6-Me₃C₂H₂) with water is pH independent over the range 1–9 and is not subject to strong buffer catalysis. The primary products formed are the corresponding ene-1,1-diols which result from addition across the CO (rather than the CC) of the ketene. These ‘enols of carboxylic acids’ are relatively long lived owing to slow protonation of the β-carbon (because of steric hindrance caused by the o-Me groups) and evidence for their structures in solution is presented. On attempted isolation of the ene-1,1-diols facile oxidation to a stable free radical, as well as ketonisation to the corresponding acids, occurs. Evidence is presented in terms of the large negative entropy of activation (ca. 200 J K^–1 mol^–1), solvent isotope effects and the absence of significant general-base catalysis, that (despite the sterically hindered nature of these ketenes) the characteristics of their hydration are typical of other ketenes and that this most likely involves an initial concerted reaction with a water oligomer. Updated ab initio calculations are also presented, which are consistent with this view.