The effects of cationic and zwitterionic micelles on the keto–enol interconversion of 2-phenylacetylfuran and 2-phenylacetylthiophene

Abstract

The presence of micelles from cationic and zwitterionic surfactants increases the apparent acidity of either the keto and the enol forms of 2-phenylacetylfuran ( 2PAF ) and 2-phenylacetylthiophene ( 2PAT ). This effect can be attributed to the affinity of the surfactant micelles for the enolate of the two substrates. Although the equilibrium constants for keto–enol tautomerism of 2PAF and 2PAT , K T=[enol]/[ketone]=p K a KH−p K a EH, do not change much, the presence of micelles provides an efficient method for producing appreciable quantities of the enolates under mild experimental conditions and in aqueous solutions. The obtained rate-profiles for the ketonisation reactions and the consistency of the kinetic rate constants over a wide range of ‘pH’ in several overlapping buffers indicate that the pH of the aqueous pseudophase (but not that at the micellar surface) can be controlled by buffers. Moreover, the increase of the acidity and the decrease of the ‘water’ rate of ketonisation of the enols of 2PAF and 2PAT upon addition of surfactants allow the uncovery of a metal ion catalysed pathway that cannot be observed in absence of surfactants.