An axially chiral, electron-deficient borane: Synthesis, coordination chemistry, Lewis acidity, and reactivity

Abstract

An axially chiral dihydroborepine with a binaphthyl backbone and a C 6F5 substituent at the boron atom was prepared by transmetalation from the corresponding tin precursor. This novel motif was structurally characterized by X-ray diffraction analysis as its THF and its PhCN Lewis acid/base complex. 1H NMR measurements at variable temperatures of the former adduct revealed a remarkable dynamic behavior in solution. Several more Lewis pairs with oxygen, nitrogen, carbon, and phosphorus σ-donors were synthesized and analyzed by multinuclear NMR spectroscopy. The determination of the borane's Lewis acidity with the Gutmann-Beckett method attests its substantial Lewis acidity [85% with Et3PO as well as 74% with Ph3PO relative to the parent B(C6F5) 3]. Representative examples of SiH bond activation (carbonyl reduction and dehydrogenative SiO coupling) are included, demonstrating the chemical stability and the synthetic potential of the new chiral boron-based Lewis acid. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.