Theoretical studies on intramolecular methyl transfer in the S-2-alkenyl-dimethylsulfonium ion

It is well known that the intramolecular methyl transfer plays an important role in chemistry. In this paper, a novel intramolecular methyl-to-double bond transfer in the model compound S-2-alkenyl-dimethylsulfonium ion has been given systematic theoretical study with density functional theory (DFT) method. Three possible methyl transfer pathways have been assumed: (i) [1,4]-methyl-transfer, (ii) [1,5]-methyl-transfer and (iii) formation of the bridged methyl-onium ion. It is found that the type of substituent and its site have a great influence over the methyl transfer pathway. Moreover, whichever pathway the methyl transfer proceeds in, the advantageous transition state always exhibits methyl retention-chair mode.