Selective Carbonyl Hydroboration via Homogeneous Lanthanide Catalysis

Barger, Christopher

doi:https://doi.org/10.21985/n2-nz1w-bh91

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Selective Carbonyl Hydroboration via Homogeneous Lanthanide Catalysis

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The lanthanides, with their limited orbital effects and high oxophilicity, represent a class of catalytic metals highly distinguished from more commonly-utilized transition metals. Homogeneous lanthanide catalysts often afford high catalytic rates and impressive selectivity. However, challenges regarding the synthesis and utilization of highly air- and water-sensitive organo-lanthanide complexes have limited widespread adoption throughout the catalytic and synthetic chemistry communities. The focus of the work presented herein is on the discovery of more accessible methodologies utilizing lanthanide catalysis for the reduction of carbonyl-containing functional groups. Throughout this work, the ability of commercially-available lanthanide complex La[N(SiMe3)2]3 to catalyze the hydroboration of carbonyl groups is explored. First, ketones and aldehydes are shown to be reduced by pinacolborane with catalyst loadings as low as 0.01% and turnover frequencies as high as 40,000 h-1. Second, La[N(SiMe3)2]3 is shown to also catalyze the hydroboration of esters with high activity and selectivity over a variety of functional groups. Third, secondary and tertiary amides are cleanly reduced with pinacolborane to their corresponding amines, again using La[N(SiMe3)2]3 as a catalyst. Throughout, detailed discussions on the mechanisms of these reactions are presented, supported by robust experimental and computational findings. Notably, it is shown that La[N(SiMe3)2]3 does not act as a simple Lewis acid activating the carbonyl for reduction, but rather it catalyzes complex, multi-step reactions involving unusual and highly reactive hemiacetal/hemiaminal intermediates.

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