Rates and equilibria of the Michael-type addition of benzenethiol to 2-cyclopenten-1-ones

The triethylamine-catalyzed addition reactions of benzenethiol to 2-cyclope nten-1-one and its 2- and 3-methyl derivatives have been found to he apprec iably reversible in chloroform solution. Rates and equilibria have been car efully measured at 25 degrees C in order to assess the negative influence o n addition exerted by methyl groups substituted on the carbon-carbon double bond. 2-Methyl-2-cyclopenten-1-one has been found to react with benzenethi ol under kinetic control to give the cis adduct as the sole detectable prod uct in a highly stereoselective anti addition process. However, on prolonge d reaction times the system slowly evolved toward a new state of equilibriu m in which the more stable trans adduct, derived from a syn addition mode, was the predominant isomer.