Planarized Triarylboranes: Stabilization by Structural Constraint and Their Plane-to-Bowl Conversion

Abstract

Triphenylborane and 9,10-diphenyl-9,10-dihydro-9,10-diboraanthracene, constrained to a planar arrangement with methylene tethers, were synthesized by intramolecular multi-fold Friedel-Crafts cyclization. These compounds were stable toward air, water, and amines, despite the absence of steric protection in the vertical direction with respect to the B atoms, and showed characteristic structural, electronic, and photophysical properties. In addition, upon treatment with a fluoride ion, these compounds underwent a plane-to-bowl conversion in a controlled manner.

Recent developments in and perspectives on three-coordinate boron materials: a bright future

“…130,131 They found that planar triarylboron compounds 26 and 27 are stable to water and air without the steric protection usually required in the vertical direction (Scheme 3). The B–C bonds in 27 are much shorter than those in BPh 3 , as a result of structural constraints.…”

Stabilization of pi-conjugated organoboron scaffolds by structural constraint

“…envisaged the concept of kinetic stabilization by structural constraint by designing the planarized triphenylborane 9 (Scheme 4). 15 The synthetic route towards 9 relied on the intramolecular Lewis acid-promoted cyclization of the key precursor 10 with Sc(OTf) 3 . Quite remarkably, the methylene-bridged intermediate 11 was oxidized with CrO 3 in refluxing acetic acid to afford the planar carbonyl-bridged borane 12 in good yield, despite the harsh reaction conditions.…”

“…Consistently, the 11 B NMR signal of 9 appeared upfield shifted at 48.6 ppm compared to that of non-planarized triphenylborane (67.4 ppm). 15 Although 9 was resistant to highly Lewis basic amines, it could still be converted to its stable fluoroborate complex with tris(dimethylamino)sulfonium difluorotrimethylsilicate (TASF). The tetracoordinated anionic complex could be transformed back to neutral 9 upon addition of stronger Lewis acidic BF 3 ·Et 2 O.…”

Planarized B-phenylborataanthracene anions: structural and electronic impacts of coplanar constraint

“…6,7 We have recently synthesized a series of stable planarized triphenylborane derivatives that have three methylene bridges fixing the three benzene rings in a coplanar fashion. 8 In this study, we have demonstrated that the structural constraint is a powerful strategy for the stabilization of tri-coordinated boron compounds, an alternative to the conventional concept of the kinetic stabilization with bulky groups. We have now come up with the idea that compound 1, one of the planarized triphenylboranes with a CH 2 bridge, should be a suitable precursor for producing a new borataanthracene anion, the study of which should enable us to elucidate the structural and electronic impacts of the structural constraint.…”