Reactions of B2 (o-tolyl)4 with Boranes: Assembly of the Pentaborane(9), HB[B(o-tolyl)(μ-H)]4

Synthesis of Electrophiles Derived from Dimeric Aminoboranes and Assessing Their Utility in the Borylation of π Nucleophiles

Dimeric aminoboranes, [H₂BNR₂]₂ (R = Me or CH₂CH₂) containing B₂N₂ cores, can be activated by I₂, HNTf₂ (NTf₂ = [N(SO₂CF₃)₂]), or [Ph₃C][B(C₆F₅)₄] to form isolable H₂B(μ-NR₂)₂BHX (for X = I or NTf₂). For X = [B(C₆F₅)₄]⁻ further reactivity, presumably between [H₂B(μ-NMe₂)₂BH][B(C₆F₅)₄] and aminoborane, forms a B₃N₃-based monocation containing a three-center two electron B-(μ-H)-B moiety. The structures of H₂B(μ-NMe₂)₂BH(I) and [(μ-NMe₂)BH(NTf₂)]₂ indicated a sterically crowded environment around boron, and this leads to the less common O-bound mode of NTf₂ binding. While the iodide congener reacted very slowly with alkynes, the NTf₂ analogues were more reactive, with hydroboration of internal alkynes forming (vinyl)₂BNR₂ species and R₂NBH(NTf₂) as the major products. Further studies indicated that the B₂N₂ core is maintained during the first hydroboration, and that it is during subsequent steps that B₂N₂ dissociation occurs. In the mono-boron systems, for example, ⁱPr₂NBH(NTf₂), NTf₂ is N-bound; thus, they have less steric crowding around boron relative to the B₂N₂ systems. Notably, the monoboron systems are much less reactive in alkyne hydroboration than the B₂N₂-based bis-boranes, despite the former being three coordinate at boron while the latter are four coordinate at boron. Finally, these B₂N₂ electrophiles are much more prone to dissociate into mono-borane species than pyrazabole [H₂B(μ-N₂C₃H₃)]₂ analogues, making them less useful for the directed diborylation of a single substrate.

Tetraaryldiborane(4) Can Emit Dual Fluorescence Responding to the Structural Change around the B-B Bond

We report the successful synthesis of tetramesityldiborane(4) (Mes₄ B₂ ) through the reductive coupling of a dimesitylborinium ion. Owing to the steric protection conferred by the mesityl groups, Mes₄ B₂ shows exceptional chemical stability and remains intact in water. Single-crystal X-ray analysis revealed that Mes₄ B₂ has an orthogonal geometry, where the B-B center is completely hidden by the mesityl groups. Remarkably, Mes₄ B₂ emits dual fluorescence at 460 and 620 nm, both in solution and in the solid state. Theoretical calculations showed that Mes₄ B₂ in the excited S₁ state adopts a twisted or planar geometry, which is responsible for the shorter- or longer-wavelength fluorescence, respectively. The intensity ratio of the dual fluorescence is sensitive to the viscosity of the medium, which suggests that Mes₄ B₂ has potential as a ratiometric viscosity sensor.

Reactions of B2 (o-tolyl)4 with Boranes: Assembly of the Pentaborane(9), HB[B(o-tolyl)(μ-H)]4

Reactions of the diborane(4) B2 (o-tolyl)4 and monohydridoboranes are shown to give B(o-tolyl)3 and (o-tolyl)BR2 (R2 =(C8 H14 ) 3, cat 4, pin 5, (C6 F5 )2 6) as the major products. The corresponding reaction with BH3 -sources gives complex mixtures, resulting from hydride/aryl exchange, dimerization and borane elimination. This led to the isolation of the first tetra-substituted pentaborane(9) HB[B(o-tolyl)(μ-H)]4 8. The reaction pathways are probed experimentally and by computations.