An ab initio MO study on the thiol—disulphide exchange reaction

Effect of Ring Strain on Nucleophilic Substitution at Selenium: A Computational Study of Cyclic Diselenides and Selenenyl Sulfides

Nucleophilic substitution reactions of small rings incorporating selenium are examined using computational methods. The potential energy surfaces of HS- and HSe- with 1,2-diselenirane, 1,2-diselenetane, 1,2-diselenolane, and 1,2-diselenane were computed at B3LYP/6-31+G(d) and MP2/6-31+G(d). The reactions of three-, four-, five-, and six-membered rings incorporating the S-Se bond with HS- were computed at B3LYP/6-31+G(d). The strained three- and four-membered diselenides and selenenyl sulfide rings undergo SN2 reactions, while the five- and six-membered rings react via the addition-elimination pathway, a path that invokes a hypercoordinate selenium intermediate. The strain in the small rings precludes the addition of a further ligand to either heteroatom. Substitution at selenium is both kinetically and thermodynamically favored over attack at sulfur.

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.