The mechanistic details of this reaction have been extensively explored by Crich’s laboratories. Low-temperature H, C, and F NMR spectroscopic investigations revealed that anomeric triflate 3 derived from 1 is the intermediate glycosyl donor. Moreover, the mechanism of glycosidic bond forming reaction (3→2) was examined thoroughly by the determination of kinetic isotopic effects (KIEs) and NMR spectroscopy. Consequently, the magnitude of KIEs indicated that the displacement of the triflate from 3 proceeded with the development of significant oxacarbenium ion character at the anomeric position. This might be rationalized either by (1) a dissociative mechanism involving the intermediacy of either a transient contact ion pair (CIP) 4 or a solvent-separated ion pair (SSIP) 5, or (2) a mechanistically variant transition state 7 (Scheme 2).
For the intermediate CIP 4, the triflate anion is closely associated with face where it just departed thus shields that side against nucleophilic attack. For the alternative intermediate SSIP 5 which is in equilibrium with an initial CIP, the anomeric center could presumably be attacked by incoming alcohol from either face, giving β-mannoside 2 along with the undesired α-anomer 6. Along these lines, the presence of the 4,6-O-benzylidene protecting group, which serves to rigidify the pyranoside against rehybridization at the anomeric carbon, is essential in shifting the equilibrium toward the covalent triflate, thus reducing α-glycoside formation. Additionally, the only intermediate observed by NMR spectroscopy is the covalent triflate 3, indicating that the complete set of equilibria between 3, the CIP 4, and SSIP 5 set is very heavily biased towards 3.