Study of mechanisms of sulfonium keto ylides intramolecular cyclization

Mechanisms of monomolecular cyclization of N-maleyl protected keto stabilized sulfur ylides via three reaction pathways were investigated at the B3LYP/6-31G(d,p) level of theory. Structures and Gibbs free energies of intermediates and transition states of the reactions show that the Corey-Chaykovsky reaction occurs without trans-betaine formation and is least probable among those considered. Although methylthio pyrrolisinedione formation is thermodynamically most favorable, it has been found that an activation barrier of intramolecular 1,4-methyl shift, which is proposed to be a rate-limiting step in the reaction mentioned, is considerably high. However, the Wittig reaction, possessing lowest activation energy of the rate-limiting step, is thermodynamically disadvantageous. The presence of an alkyl substituent at the β-position of ylide and temperature influence only a new carbon-carbon bond formation at first stage of ylide cyclization.