Zinc Perchlorate Hexahydrate Catalysed Conjugate Addition of Thiols to (alpha),(beta)-Unsaturated Ketones

Zn(II) perchlorate hexahydrate has been found to be a new and efficient catalyst for conjugate addition of thiols to (alpha),(beta)-unsaturated ketones under solvent-free conditions at room temperature. The reaction of aryl, arylalkyl and alkyl thiols with cyclic and acyclic (alpha),(beta)-unsaturated ketones takes place affording excellent yields after five minutes to six hours. The compatibility of Zn(ClO4)2·6H2O with different solvents provides a means to carry out the reaction under versatile experimental conditions. The rate of thiol addition was dependent on the electronic and steric factors of the enones and the thiols. The substituent at the (beta)-carbon of the (alpha),(beta)-unsaturated ketone substrate caused steric hindrance during conjugate addition and required longer reaction times. The rate of reaction for alkane thiols e.g. ethanethiol was sluggish compared to that of aryl thiols.