Asymmetric Hydroboration of Ketones by Cooperative Lewis Acid-Onium Salt Catalysis: A Quantum Chemical and Microkinetic Study to Combine Theory and Experiment

We apply microkinetic modeling in homogeneous catalysis and show how it can be used to reveal important details of a complex mechanism and how this can lead to a direct comparison between theory and experiment. While regularly used in heterogeneous catalysis, its applications to organic chemistry or homogeneous catalysis are still comparatively scarce. This approach is exemplarily applied to the mechanism of the asymmetric hydroboration of acetophenone with a highly active cooperative Lewis acid-ammonium salt catalyst. In combination with density functional theory, it is a gateway to shed light into important mechanistic details. In our study, it reveals that the counterion of the ammonium salt of the catalyst facilitates the hydride transfer step of the cycle. Chloride replacing iodide speeds up the main reaction but simultaneously has the same effect on a side reaction that consumes the product. This observation is confirmed by experimental measurements of both the main catalytic cycle and the side reaction. A sensitivity analysis showed that the transition from the product complex to the hydride transfer is rate-limiting and that it determines the enantioselectivity. Based on this insight, an enantioselective kinetic model was applied, from which the difference of the Gibbs free energy barriers of the two pathways forming the two enantiomers can be extracted. The barriers are in fairly good agreement with the ones calculated by DFT, which reveal that the asymmetric backbone interacts with the reactant sterically to favor asymmetric product formation.