N‐Heterocyclic Silylenes in Boron Chemistry: Facile Formation of Silylboranes and Silaborinines

Abstract

Reaction of a N-heterocyclic silylene (NHSi) with PhBX (X=Cl, Br) readily afforded six-membered silaborinines through an insertion/ring expansion sequence. Increasing the sterics of the borane from phenyl to duryl enabled the selective generation and isolation of the highly colored silylborane intermediates. Theoretical studies on the mechanism and energetics of the silaborinine formation were fully consistent with the experimental observations.

Reaction of Dihalodiboranes(4) with a N‐Heterocyclic Silylene: Facile Construction of 1‐Aryl‐2‐Silyl‐1,2‐Diboraindanes

“…Thus, X‐ray crystallographic data of 3 a – c provide, for the first time, structural details of NHSi–borane adducts. The dative B−Si bonds are slightly shorter than the covalent B−Si bond in the same molecule [1.982(3) vs. 2.003(3) Å ( 3 b ), 1.969(2) vs. 1.998(3) Å ( 3 a ), and 1.947(3) vs. 1.991(3) Å ( 3 c )], both of which represent shorter B−Si bonds than those reported previously (2.007–2.054 Å), and this difference increases when the halogen substituent becomes smaller: Br ( 3 b )<Cl ( 3 a )<F ( 3 c ). Although this finding appears counterintuitive, the shorter dative bonds are most likely due to the lower coordination number of the associated Si center.…”

“…Dihalodiboranes(4) were found to react readily with three equivalents of an NHSi to give the NHSi adducts of 1‐aryl‐2‐silyl‐1,2‐diboraindanes and the formal HX (X=halogen) silylene insertion products (Scheme ) . Apparently, beyond the simple Lewis acid/base reaction, transformations including inorganic Wagner–Meerwein rearrangement, B−X insertion, and both sp 3 ‐ and sp 2 ‐C−H borylation are involved in this process. Notably, the reaction of Mes 2 B−BF 2 with NHSi represents the first B−F bond activation by silylene insertion .…”

“…According to our previous study of the reaction of NHSi 1 with monoboranes, MesClB−BClMes ( 2 a ) was initially treated with one equivalent of 1 in benzene at room temperature (Figure ). The reaction solution immediately turned dark red upon mixing and then decolorized to light red after stirring for approximately 30 minutes.…”

An Isolable Potassium Salt of a Borasilene–Chloride Adduct

“…Tr eating silane 4,w hich was obtained in 86 %y ield from the reaction of dialkylsilylene 3 [11] with MesBCl 2 [12] in hexane (Scheme 1), [5,13] with KC 8 and 18-c-6 in 1,2-dimethoxyethane (DME) at room temperature induced an immediate color change to orange.A fter stirring for 3h, [ 2][K(18-c-6)] was obtained in 73 %yield as air-sensitive and thermally unstable orange crystals, [14] which were characterized by multinuclear NMR spectroscopy,h igh-resolution mass spectrometry,e lemental analysis,a nd single-crystal X-ray diffraction (XRD). Thec oordination of Cl À to the unsaturated boron center suggests that the Lewis acidity of borasilene 1 is very high.…”

“…Molecular structure of[2][K(18-c-6)] [35]. Thermale llipsoids set at 30 %probability,h ydrogen atoms omitted for clarity.S elected bond lengths []and angles [8 8]: Si1-B1 1.859(2), B1-Cl1 1.879(2), Cl1···K1 3.0150(6); Si1-B1-C5 132.69(13), C5-B1-Cl1 113.03(12),C l1-B1-Si1 114.25(10).…”