非对称环硫乙烷的区域选择性亲核开环反应

环硫乙烷与它的氧类似物环氧乙烷和氮类似物氮杂环丙烷一样,是一类重要的有机合成中间体,在医药和农用化学品工业领域也得到广泛应用。通过开环和异构化反应,还广泛用于制备硫醇和硫醚等含硫化合物。本文总结了常用亲核试剂对非对称环硫乙烷的亲核开环反应及其区域选择性。环硫乙烷的亲核开环反应通常只受空间效应影响,亲核试剂进攻非对称环硫乙烷位阻小的碳原子,对于烯基取代的环硫乙烷有时可以进攻烯基的β碳原子发生SN2’开环反应。强亲核性的亲核试剂容易致使环硫乙烷脱硫生成烯烃,而亲核性相对较弱的亲核试剂容易发生多聚反应生成多硫醚。在Lewis酸存在下,电子效应会对开环反应的区域选择性产生影响,甚至起主导作用。虽然烷基取代环硫乙烷在Lewis酸存在下的开环仍然主要发生在其取代基少的碳原子上(位阻控制),但受电子效应影响,芳基和烯基取代环硫乙烷的亲核开环,其亲核试剂一般倾向于进攻环硫乙烷的芳甲位和烯丙位碳原子(电子效应控制)。

Regioselective Nucleophilic Ring Opening Reactions of Unsymmetric Thiiranes

Thiiranes are a class of important intermediates in organic synthesis,as well in pharmaceutical and agrochemical industries as their oxygen analogs oxiranes and nitrogen analogs aziridines.They have been widely applied in the preparation of sulfur-containing compounds,such as diverse thiols and thioethers and so on,via ring opening reaction and isomerization.Nucleophilic ring opening reactions of unsymmetric thiiranes and their regioselectivity with various widely used nucleophiles are reviewed.Generally,nucleophilic ring opening reactions of unsymmetric thiiranes occur on their less substituted carbon atom,controlled by the steric hindrance.2-Alkenylthiiranes can undergo an SN2′ ring opening reaction in some cases via the attack on the β-carbon atom of their alkenyl group.Strong nucleophiles easily lead to desulfurization of thiiranes to afford the corresponding olefins,while the relatively weak nucleophiles readily result in polymerization of thiiranes,affording polythioethers.In the presence of Lewis acid,the electronic effect affects the regioselectivity in ring opening reactions,even as the prominent effect in the reactions.Although aliphatic thiiranes are still attacked predominantly on their less substituted carbon atom with nucleophiles(the steric hindrance control),aromatic thiiranes and 2-alkenylthiiranes are attacked on their more substituted carbon atom with nucleophiles(the electronic effect control).The current review provides a general rule for the regioselectivity of the nucleophilic ring opening reaction of unsymmetric thiiranes.