Preparation of 3,3-bis(alkylthio)-1,1-dipiperidino-2-propenylium iodides by S-alkylation of alkyl 3,3-dipiperidinodithioacrylates

Methylation of propyl 3,3-dipiperidinodithioacrylate with iodomethane afforded a 93:7 mixture of (E)- and (Z)-3-methylthio-3-propylthio-1,1-dipiperidino-2-propenylium iodides nearly quantitatively, while propylation of methyl 3,3-dipiperidinodithioacrylate with 1-iodopropane produced a 6:94 mixture of the above isomers. Study of these iodides led to the conclusions that (a) free rotation about the C2-C3 bond is frozen for these carbenium ions, thus allowing the clearly distinguishable E- and Z-isomers to exist and (b) the alkylation takes place in such a manner that the newly formed S-alkyl group and the carbenium carbon atom become cis to each other. Such stereoselectivity was general for methylation of a series of alkyl 3,3-dipiperidinodithioacrylates and was explained by examining the preferred conformation of the starting dithioacrylates with NMR and X-ray diffraction analyses.