Theoretical study of 1-methoxy-2-sulfanylethan-1-yl cation: Insight into intermediates in glycosidation reactions

Equilibrium structures and harmonic vibrational frequencies of two model compounds related to the intermediates of nucleophilic attack on 1-alkoxy-2- (arylsulfanyl)alkyl halides and 2-(arylsulfanyl)pyranosyl halides are reported. Hartree-Fock (HF) and second-order Moller-Plesset (MP2) calculations using the 6-31G(d) basis set were performed on the larger of the two model ions, 2-sulfanyltetrahydropyran-1-yl cation. The smaller model ion, 1-methoxy-2-sulfanylethan-1-yl cation, was further investigated with a triple split valence, 6-311G(d,p), basis set and with the quadratic configuration interaction, including single- and double-electron excitations (QCISD), method. Localized atomic charges in the model ions were compared to corresponding sites in oxonium and episulfonium. The studies of the model ions suggest that the intermediate is more like an oxonium ion than an episulfonium ion, although the sulfur is critical to the stereoselectivity. High-level calculations (QCISD(T)/- and CCSD(T)/6-311++G(d,p)) indicate that a previously unidentified proton-shifted second intermediate, located 8.0- 8.5 kcal/mol higher in energy, may be involved in reaction mechanisms. The unimolecular transition state corresponding to this proton shift was found to be 27-29 kcal/mol above the 1-methoxy-2-sulfanylethan-1-yl cation.