Novel SN2 ring-opening reactions of 2- and 2,2-substituted thiiranes with thiols using Na+-exchanged X-type zeolite or triethylamine in methanol
A regiospecific ring-opening reaction occurs at the C-3 of 2-methyl- and 2,2-dimethyl-thiiranes 1a and 1b upon treatment of 1a and 1b with thiols 2 using Na+-exchanged X-type zeolite (Na+-zeolite X) or Et3N in MeOH to yield alkylthioethanethiols 3. However, the reaction of 2-phenylthiirane 1c is no longer regiospecific, giving 3 and 4 by the ring-openings at C-3 and C-2. The yield of the products is influenced by the catalyst, the solvent polarity, the concentration and nucleophilicity of 2, the reactivity of 1 towards 2 and so on. This suggests that the reactions using Na+-zeolite X or Et3N proceed via a zwitterion or an ion-pair intermediate, respectively, by an SN2 attack on 1 of 2, having enhanced nucleophilicity due to the basic catalysts. The reactions using Na+-zeolite A-3 or H+-zeolite X in MeOH or benzene and that using H2SO4 in benzene are also investigated.