Nucleophilic Attack at Selenium in Diselenides and Selenosulfides. A Computational Study

Nucelophilic substitution at selenium is examined using the B3LYP and MP2 methods. Various nucleophiles (HS -, CH 3 S -, HSe -, and CH 3 Se - ) and substrates (R 1 SSeR 2 and R 1 SeSeR 2 with R 1 and R 2 = H or Me) are used to model substitution at selenium in diselenides and selenosulfides. In all cases, the mechanism is addition−elimination. A stable hypercoordinate selenium intermediate lies in a well that is 8−14 kcal mol -1 deep. Nucleophilic attack at selenium is both kinetically and thermodynamically more favorable than at sulfur.