Theoretical studies on the reactivity of phenylthiocyanates

Abstract

Molecular orbital calculations are reported on the electronic structure of phenylthiocyanate and its 4-nitro and 4-amino derivatives using the semi-empirical AM1 and PM3 methods. The reactivity of these systems at the cyano carbon, sulfur atom and aryl carbon have been explored with the methoxide ion in methanol using the AM1/COSMO method. In terms of overall stability, attack at the aryl carbon with the displacement of the thiocyanate anion is the preferred process in all three cases followed by the alternative attack at the cyano carbon resulting in the displacement of the phenylthiolate anion: the corresponding attack at the sulfur atom with the displacement of the cyanide ion is strongly disfavoured. Kinetically, the reaction selectivity is highly dependent on the substituent present though all three molecules readily form reversible complexes at the sulfur atom. The activation energies suggest that 4-nitrophenylthiocyanate will react at both the aryl carbon or cyano carbon, but phenylthiocyanate and 4-aminophenylthiocyanate would be expected to react preferentially at the cyano carbon only.