Investigations into Enantioselective Boro-Ion Catalysis
1,2,3-Triazolylidene carbene stabilized borenium ions were investigated as frustrated Lewis pair (FLP) type hydrogenation catalysts and were found to be able to effect the hydrogenation of imines and N-heterocycles at ambient and near ambient pressures and temperatures. The unprecedented activity of our MIC-borenium ion catalysts was attributed to the high σ-donating ability of 1,2,3-Triazolylidene-based carbenes producing an exceptionally stable and active borenium ion catalyst. Several modifications to the original catalyst structure were employed in an attempt to develop an enantioselective variant of our MIC-borenium ion catalyst. One of the modification to impart asymmetry into the reaction manifold that was investigated in this thesis was the use of a chiral borane, specifically the 9-borobicyclo[3.3.2]decanes (BBDs), in place of the achiral 9-BBN present in the original catalyst design. Disappointingly catalyst stability proved to be highly problematic leading to a difficulty in synthesizing the necessary carbene-borane and resulting in a highly unstable and ineffective catalyst. During the course of our studies into enantioselective borenium ion catalyzed hydrogenations we became concerned with the facility of hydride abstraction by MIC-borenium ions and whether or not this would result in the racemization of enantioenriched α-N sp3 C–H centres. NMR studies revealed that our MIC-borenium ion catalyst was not able to abstract hydrides from α-N sp3 C–H’s at ambient temperatures and only appreciable rates of hydride abstraction were observed at elevated temperatures of 60 °C. This led us to conclude that the reversibility of the hydride addition could not account for poor enantioselectivity observed for chiral borenium ion catalyzed hydrogenations and showed that an enantioselective MIC-borenium ion catalysts should be possible. The MIC-borenium ions were also investigated for their ability to activate Si–H bonds, and if they could be used to catalyze the hydrosilylation of C=N functionalities. The MIC-borenium ions were found to be highly active hydrosilylation catalysts, able to tolerate a variety of functional groups and able to effect the rapid reduction of imines under ambient conditions. Finally BBD based borohydrides were also investigated as enantioselective nucleophilic hydrosilylation catalysts. Although they were able to effect the hydrosilylation of ketones, the BBD’s were only able to impart minor enantioselectivity into the catalytic manifold.
URI for this recordhttp://hdl.handle.net/1974/15581
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