Boron-based Lewis Acids: Towards Intramolecular Frustrated Lewis Pairs and Enantioselective Catalysis

Abstract

A rapid development of metal-free, main group compounds for small molecule activation and as hydrogenation catalysts was triggered after the disclosure of H2 activation by sterically hindered Lewis acids and bases over a decade ago. The combination of a Lewis acid and base with unquenched reactivity was later coined as a “frustrated Lewis pair” (FLP). In this research, boron-based Lewis acids were studied for their application as catalysts in FLP chemistry, with an emphasis on exploring the production of chiral FLPs and intramolecular FLPs. In the exploration towards enantioselective catalysis, 3,5-bicyclic aryl piperidines were synthetically modified to produce B/N FLPs and were shown to activate dihydrogen, demonstrating the potential for expanding chiral FLP templates beyond the typical chiral ligands used in transition metal complexes. Chiral borenium cations were generated from different families of carbene-borane adducts, in which their chirality resides on either the carbene or the borane. Catalytic studies found that they were able to effect imine reduction without epimerisation of the resulting chiral amine, and the reactivity and enantioselectivity of these cations were found to be inversely proportional to steric demands. In the exploration towards a cationic intramolecular FLP, a borenium cation with a pendant phosphine was synthesized through hydroboration of a phosphinoalkene with an isolable B-H borenium cation, and its FLP reactivity was investigated. Finally, initial efforts towards the production of a bisborane-carbene as a tri-functionalized FLP were documented.