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 co mpounds related to the intermediates of nucleophilic attack on 1-alkoxy-2-( arylsulfanyl)alkyl halides and 2-(arylsulfanyl) pyranosyl halides are repor ted. Hartree-Fock (HF) and second-order Moller-Plesset (MP2) calculations u sing the 6-31G(d) basis set were performed on the larger of the two model i ons, 2-sulfanyltetrahydropyran-1-yl cation. The smaller model ion, 1-methox y-2-sulfanylethan-1-yl cation, was further investigated with a triple split valence, 6-311G(d,p), basis set and with the quadratic configuration inter action, including single- and double-electron excitations (QCISD), method. Localized atomic charges in the model ions were compared to corresponding s ites in oxonium and episulfonium. The studies of the model ions suggest tha t the intermediate is more like an oxonium ion than an episulfonium ion, al though the sulfur is critical to the stereoselectivity. High-level calculat ions (QCISD(T)/- and CCSD(T)/6-311++G(d,p)) indicate that a previously unid entified proton-shifted second intermediate, located 8.0-8.5 kcal/mol highe r in energy, may be involved in reaction mechanisms. The unimolecular trans ition state corresponding to this proton shift was found to be 27-29 kcal/m ol above the 1-methoxy-2-sulfanylethan-1-yl cation.