Effect of ring strain on the thiolate-disulfide exchange. A computational study

B3LYP/aug-cc-pVDZ and MP2/6-31+G calculations of the reactions of HS(-) with small cyclic disulfides (dithiirane, 1,2-dithietane, 1,2-dithiolane, and 1,2-dithiane) were performed to determine the reaction mechanism. For the five- and six-membered rings, the reaction proceeds via the addition-elimination pathway, consistent with acyclic analogues. The smaller, more strained three- and four-membered rings react by the S(N)2 mechanism. Addition of the nucleophile cannot be accommodated by the small rings without concomitant ring cleavage.