Relationship between the configurations of 2-phenyltetrahydrothiophenium 1-methylides and their rearrangement products

trans-2-Phenyltetrahydrothiophenium 1-methylide (trans-3), which is generat ed by fluoride ion-induced desilylation of trans-2-phenyl-1-[(trimethylsily l)methyl]tetrahydrothiophenium salt (trans-2), gave a mixture of 1,4,5,10a- tetrahydro-3H-2-benzothiocine (4) ([2,3]sigmatropic rearrangement product) and 4-methylsulfanyl-1-phenyl-1-butene (5) (Hofmann elimination product); Y lide trans-3 cannot undergo [2,3]sigmatropic rearrangement because the ylid e-carbon is too far from the phenyl group, and trans-3 would instead isomer ize to cis-3. In this paper, we discuss the mechanism of the isomerization of trans-3 to cis-3.