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.
3,3-二
哌啶二
硫代
丙烯酸丙酯与
碘甲烷的甲基化反应几乎完全产生了93:7的(E)-和(Z)-3-甲基
硫代-3-丙基
硫代-1,1-二
哌啶基-2-
丙烯鎓
碘化物混合物,而3,3-二
哌啶二
硫代
丙烯酸甲酯与1-
碘丙烷的丙基化反应则产生了6:94的上述异构体混合物。对这些
碘化物的研究得出了以下结论:(a)这些碳正离子在C2-C3键上的自由旋转被冻结,从而允许存在可清晰区分的E-和Z-异构体;(b)烷基化反应以新形成的S-烷基和碳正离子碳原子相互顺式的方式发生。这种立体选择性是一系列烷基3,3-二
哌啶二
硫代
丙烯酸酯甲基化反应的一般情况,通过核磁共振和X射线衍射分析来研究起始二
硫代
丙烯酸酯的首选构象可以解释这种选择性。