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. 2Alkenylthiiranes can undergo an S(N)2' ring opening reaction in some cases via the attack on the beta-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.