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 S_N2 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.