Theoretical Studies on the Mechanistic Insertions of Singlet Methylene and Halomethylene into Polar S?H Bonds of Methanethiol,Ethanethiol, 1-Propanethiol and 2-Propanethiol

Abstract

The insertion of ¹CH₂, ¹CHF, ¹CF₂,¹CHCl, and ¹CCl₂ into primary and secondary polar S?H bonds of methanethiol, ethanethiol, 1-propanethiol, and 2-propanethiol have been investigated at HF (Hartree–Fock), MP2 (Møller–Plesset Perturbation Theory), and DFT (density functional theory) levels using 6-31G (d, p) basis set. The insertions follow a two-step mechanism. The potential energy surface exploration identifies ylide-like structures undergoing the 1,2-sigmatropic hydrogen shift concertedly giving the thioether product. But ¹CF₂ forms weak complexes involving 1,2-hydrogen shift. The barrier height in the concerted mechanism varies with the type of carbene moiety and S?H bond. The initial interaction seems to be a function of HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) energies of thiols and carbenes, respectively. The natural bond orbital charge analysis showing a net charge flow from the substrate thiol to the reagent carbene at the transition states corroborates this fact.