SN2 Displacement on 2-(Alkylthio)ethyl Derivatives


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 SN2 mechanism. The 2,4-dinitrophenolate derivative of specifically deuteriated 2-(methylthio)ethanol reacts by an aromatic substitution mechanism (SNAr) when exposed to amines in aprotic solvents. Use of sulfonate esters avoids competition from the SNAr 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 SN1 reaction with neighboring sulfur participation. As 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 SN2 mechanism), as shown by second-order kinetics and unrearranged product. This is the first demonstration of SN2 displacement on a 2-(alkylthio)ethyl or 2-(arylthio)ethyl derivative.

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Journal of Organic Chemistry