SN2 Displacement on 2-(Alkylthio)ethyl Derivatives

Article abstract of DOI:10.1021/jo00246a025

We have studied the reaction mechanism of various 2-(alkylthio)ethyl and 2-(arylthio)ethyl derivatives with strong nucleophiles in an attempt to overcome powerful neighboring sulfur participation and shift reaction to a direct displacement S_N2 mechanism.The 2,4-dinitrophenolate derivative of specifically deuteriated 2-(methylthio)ethanol reacts by an aromatic substitution mechanism (S_NAr) when exposed to amines in aprotoc solvents.Use of sulfonate esters avoids competition from the S_NAr mechanism.The rate of reaction of these esters in dimethyl sulfoxide (DMSO) or acetonitrile is independent of concentration of added methylamine, thiourea, urea, or iodide, thus indicating continued S_N1 reaction with neighboring sulfur participation.Asd would be expected on this basis, but in contrast to previous mechanistic suggestions, the product for reaction with iodide in acetone shows complete scrambling of methylene groups.In contrast, reaction with thiophenolate ions in DMSO proceeds by direct nucleophilic displacement (an S_N2 mechanism), as shown by second-order kinetics and unrearranged product.This is the first demonstration of S_N2 displacement on a 2-(alkylthio)ethyl or 2-(arylthio)ethyl derivative.