New Protocol for the Synthesis of S-Thioesters from Benzylic, Allylic and Tertiary Alcohols with Thioacetic Acid

A new one-pot solvent-less reaction to convert benzylic, allylic, ferrocenyl or tertiary alcohols into S-thioesters, bench-stable and less odorous precursors of the corresponding thiols, which is based on reactions in neat thioacetic acid in the presence of tetrafluoroboric acid, is presented. Reaction monitoring by NMR and GC of the benzyl alcohol conversion indicated the intermediate formation of benzyl acetate and benzyl thionoacetate (PhCH2OC(S)CH3) prior to the slower conversion to the final S-benzyl thioacetate product. Increasing the HBF4 concentration enhanced the reaction rate, giving good to excellent yield (up to 99 %) for a large scope of alcohols. Control experiments, with support of DFT calculations, have revealed a thermodynamically favorable, though requiring HBF4-activation, disproportionation of CH3C(O)SH to CH3C(O)OH and CH3C(S)SH, the latter immediately decomposing to H2S and (MeC)4S6 but also generating the hitherto unreported [MeC(O)C(Me)S]2(mu-S)2. Kinetic investigations demonstrated that the rate of benzyl alcohol conversion is second-order in [PhCH2OH] and second order in [HBF4], while the rate of conversion of the benzyl acetate intermediate to S-benzyl thioacetate is second order in [PhCOOMe] and fourth order in [HBF4]. The DFT calculations rationalize the need to two alcohol molecules and two protons to generate the reactive benzyl cation. Reaction of activated alcohols with neat thioacetic acid catalyzed by HBF4 provides a convenient entry into thioacetates, protected version of the corresponding thiols; the generation of intermediate acetates and thionoacetates and the mechanism of their further conversion to the final products has been fully rationalized.image