摘要：

Hydroboration of unsaturated carbon-carbon bonds is a valuable synthetic method for the preparation of organoborane intermediates, which can serve as synthons for various functional groups. Using pinacolborane instead of cate-cholborane as the borane source in metal-catalyzed hydroborations has recently attracted interest owing to its stability and the ease of handling the resulting boronate ester products. While rhodium-catalyzed hydroborations have received major attention, we have recently shown that phosphine-ligated copper complexes are efficient catalysts for the highly regioselective hydroboration of styrenes with pinacolborane, and regioselective and enantioselective hydroboration is possible with chiral bisphosphine ligands such as tangphos and Me-duphos. In this study, we present a new, more efficient copper-catalytic system by using the DTBM-Segphos ligand and pinacolborane as the borane source. This catalyst is remarkably effective for reactions with challenging vinylarene substrates possessing a substituent at the beta-position or an electron-donating group on the phenyl ring, which displayed limited reactivity with our previous catalytic systems. Currently, asymmetric rhodium-catalyzed hydroboration with P,N-chelates, such as quinap (quinap velence 1-(2-diphenylphosphino-1-naphthyl)isoquinoline and its structural derivatives, is the only catalytic solution for reactions of beta-substituted styrenes with catecholborane as the borane source. However, as reported herein, a new copper-based system exclusively affords the corresponding benzylic pinacolboronate esters with high enantioselectivities up to > 99percent ee.

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