Mechanistic Studies on Hydrogen Isotope Exchange Reactions and Deracemization

Abstract

Mechanistic understanding of photochemical C-H bond cleavage and reformation is crucial for the development of deracemization reactions. However, there are various challenges associated with this. As the products of a deracemization reaction are identical to the starting material, distinguishing between two or more mechanistic pathways that are all thermodynamically feasible is not trivial. Additionally, the mechanism for C-H bond reformation is often kinetically silent, rendering it challenging to study using conventional techniques.In the first part of this thesis, hydrogen/deuterium isotopic fractionation is used to elucidate mechanisms of light-driven, C-H bond cleavage and bond reformation reactions. The extent of fractionation provides a readout for the elementary steps involved in the exchange process. This method is capable of distinguishing between degenerate and non-degenerate mechanisms for isotopic exchange and can determine the mechanism of exchange even when the reaction outcome is ambiguous based on thermodynamic grounds. The second part of this thesis describes an in-depth investigation into the rate law of enantioselectivity for the deracemization of cyclic ureas. We seek to distinguish between an oxidation-deprotonation pathway and a C-H PCET pathway for the initial bond cleavage event. By varying the photocatalysts and the bases used for deracemization, we use the changes in enantioselectivity as a readout for the reaction mechanism. From the hydrogen/deuterium isotopic fractionation method described in the first part of this thesis and 13C kinetic isotope effect studies, we believe that both mechanisms for bond cleavage can be operative and is strongly dependent on the nature of the photocatalyst employed.