Cooperative Catalysis in Stereoselective O- and N-Glycosylations with Glycosyl Trichloroacetimidates Mediated by Singly Protonated Phenanthrolinium Salt and Trichloroacetamide.

Abstract

The development of small-molecule catalysts that can effectively activate both reacting partners simultaneously represents a pivotal pursuit in advancing the field of stereoselective glycosylation reactions. We report herein the development of the singly protonated form of readily available phenanthroline as an effective cooperative catalyst that facilitates the coupling of a wide variety of aliphatic alcohols, phenols, and aromatic amines with α-glycosyl trichloroacetimidate donors. The glycosylation reaction likely proceeds via an SN2-like mechanism, generating β-selective glycoside products. The developed protocol provides access to O- and N-glycosides in good yields with excellent levels of β-selectivity and enables late-stage functionalization of O- and N-glycosides via cross-coupling reactions. Importantly, this method exhibits excellent β-selectivity that is unattainable through a C2-O-acyl neighboring group participation strategy, especially in the case of glycosyl donors already containing a C2 heteroatom or sugar unit. Kinetic studies demonstrate that the byproduct trichloroacetamide group plays a previously undiscovered pivotal role in influencing the reactivity and selectivity of the reaction. A proposed mechanism involving simultaneous activation of the glycosyl donor and acceptor by the singly protonated phenanthrolinium salt catalyst with the assistance of the trichloroacetamide group is supported by kinetic analysis and preliminary computational studies. This cooperative catalysis process involves four consecutive hydrogen bond interactions. The first interaction occurs between the carbonyl oxygen of the trichloroacetamide group and the hydroxyl group of alcohol nucleophile (C═O···HO). The second involves the trichloroacetamide-NH2 forming a hydrogen bond with the nitrogen atom of the phenanthroline (NH···N). The third involves the donor trichloroacetimidate (═NH) engaging in a hydrogen bond interaction with the phenanthrolinium-NH (NH···N═H). Lastly, the protonated trichloroacetimidate-NH2 forms a hydrogen bond with the fluorine atom of the tetrafluoroborate ion.