An Axially Chiral, Electron-Deficient Borane: Synthesis, Coordination Chemistry, Lewis Acidity, and Reactivity

• Published in: Chemistry : a European journal Vol. 17; no. 34; pp. 9406 - 9414
• Main Authors: Mewald, Marius, Fröhlich, Roland, Oestreich, Martin
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 16-08-2011; WILEY‐VCH Verlag; Wiley Subscription Services, Inc
• Subjects: boron; Chemistry; chirality; electron-deficient compounds; Lewis acids; silanes
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201100724

An axially chiral dihydroborepine with a binaphthyl backbone and a C6F5 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 SiH bond activation (carbonyl reduction and dehydrogenative SiO coupling) are included, demonstrating the chemical stability and the synthetic potential of the new chiral boron‐based Lewis acid.
3‥2‥1 electron‐withdrawing C6F5 group is enough to equip the novel axially chiral borane with sufficient Lewis acidity (see picture). The rigidified variant of the strong Lewis acid B(C6F5)3 catalyzes SiH bond activation reactions and opens the door to asymmetric B(C6F5)3 chemistry. Lewis pairs with several donors are reported, and the Lewis acidity was measured by the Gutmann–Beckett method.