Tetra(o-tolyl)diborane(4), 1, was synthesized and characterized experimentally as well as theoretically by density functional theory (DFT) calculations. Exposure of 1 to H (1 bar) at room temperature afforded the corresponding di(o-tolyl)hydroborane through cleavage of the H-H and B-B bonds. DFT calculations suggested a diarylboryl anion character for the transition state.

Transition metal reagents and catalysts are generally effective to cleave all three bonds (one σ and two π) in a triple bond despite its high bonding energy. Recently, chemistry of single-bond cleavage by using main-group element compounds is rapidly being developed in the absence of transition metals. However, the cleavage of a triple bond using non-transition-metal compounds is less explored. Here we report that an unsymmetrical diborane(4) compound could react with carbon monoxide and tert-butyl isonitrile at room temperature. In the latter case, the carbon–nitrogen triple bond was completely cleaved in the absence of transition metal as confirmed by X-ray crystallographic analysis, 13C NMR spectroscopy with 13C labelling and DFT calculations. The DFT calculations also revealed the detailed reaction mechanism and indicated that the key for the carbon–nitrogen triple-bond cleavage could be attributed to the presence of nucleophilic nitrogen atom in one of the intermediates.

We have clarified and observed the high electron affinity of pinB-BMes2 (1; Mes = mesityl, pin = pinacolato). By using electrochemistry, it was shown that 1 has a higher electron affinity than those of B2pin2 and Mes3B. One-electron reduction of 1 gave the corresponding radical anion. The ESR spectroscopy and DFT calculation revealed the unsymmetrical distribution of electron density over the B-B bond. UV/Vis spectroscopy showed that the SOMO-related absorption supports the deep purple color of the radical anion. DFT studies on the torsion angle dependency of the LUMO levels and relative energies revealed the reason why 1 has high electron affinity as a result of the substituent effect of the Bpin group.

An unsymmetrical diborane(4), pinB-BMes 2 , reacted with 2,6dimethylphenyl isocyanide to give a spirocyclic 1,2-oxaboretane or isocyanidecoordinated boraalkene. The former product formed via ring contraction of the pinacolatoboryl group. DFT calculations revealed the ring contraction proceeded via a carbocationic intermediate. This new degradation pathway from the Bpin group would provide important information in Bpin-related chemistry.

The reaction of pinB-BMes2 with Xyl-NC and pyridine results in the formation of a pyridine-coordinated boraalkene that exhibits an intense color. In the presence of an excess of pyridine, the ortho C–H bond of pyridine was selectively functionalized.