Kinetic Analyses of Lewis Acid Systems

The Gagné group has reported the use of xanthyliums as thermal, green Lewis Acid (LA) catalysts for sp3 C-O bond reduction of simple acetals and ethers. These bench-stable, easy-to-synthesize, and tunable catalysts provide improvements over traditionally used LAs like tris(pentafluorophenyl)borane (BCF), but the mechanistic details of this reported reduction remain unknown. Computational studies suggest that xanthylium catalysts follow a BCF-like mechanism, participating in both hydride abstraction and donation. However, experimental evidence to support this pathway remains elusive. Recent experiments to uncover this process have used a system of THP, dimethylethylsilane, and [9-(3,5-bis(trifluoromethyl)phenyl)xanthylium][B(C6F5)4-] to observe hydride abstraction, specifically the transformation of xanthylium to xanthene. In an effort to expand upon these results, additional variations of the catalyst are needed to understand the structural and electronic effects governing this process. This research explores several xanthylium catalysts using UV-Vis spectroscopy to measure the effect of catalyst electronics on the rate of hydride abstraction. It also probes the final mechanistic step of hydride donation from the corresponding xanthene to xanthylium with NMR kinetics, completing the catalytic cycle. After using these techniques to study xanthyliums, similar experiments were conducted to understand kinetic reactivity of a tuned borane with glycosyl fluorides for nucleoside analog synthesis.