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(6)F(5) 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. (1)H 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 Et(3) PO as well as 74% with Ph(3) PO relative to the parent B(C(6)F(5))(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.

Hydrogenations at Room Temperature and Atmospheric Pressure with Mesoionic Carbene‐Stabilized Borenium Catalysts

“…[6,17,19] However, reacting MIC-borohydride 2 a with B(C 6 F 5 ) 3 led to clean, quantitative hydride transfer to B(C 6 F 5 ) 3 , thereby producing 3 a' and demonstrating the greater hydride affinity of B(C 6 F 5 ) 3 . At extended reaction times, however, reduction of the second heterocycle was observed.…”

“…[6] An interesting alternative is the use of borenium ions, which have made sporadic appearances in the literature in other contexts. [6] An interesting alternative is the use of borenium ions, which have made sporadic appearances in the literature in other contexts.…”

Frustrated Lewis Pairs

“…49 It is noteworthy that chiral borane catalysts have also been used to effect stereoselective hydrosilylation reactions with enantiomeric excesses as high as 85%. [63][64] Finally, in a very recent development Repo and coworkers 65 achieved up to 99% ee in the asymmetric hydrogenation of imines and enamines using a N/B FLP with a binaphthyl backbone ( Figure 2). Mechanism: The reactions of intermolecular FLPs with H 2 presented a conceptual quandary as these reactions involve three components and yet any two components of these mixtures do not react.…”

Frustrated Lewis Pair Chemistry: Development and Perspectives

“…[68] Similarly, Oestreich et al employed a chiral silane as the reductant either with B(C 6 F 5 ) 3 or a binaphthyl derived axially chiral borane. [69] Liu and Du [70] have used a chiral bis-borane which was generated in situ by hydroboration of a substituted divinylbinaphthyl derivative using HB(C 6 F 5 ) 2 . The best system (81) resulted in asymmetric hydrogenation of a series of 19 imines with asymmetric inductions between 74 % and 88 % ee (Scheme 32).…”