The BN analogue of ortho-benzyne, 1,2-azaborine, is generated by flash vacuum pyroylsis, trapped under cryogenic conditions, and studied by direct spectroscopic techniques. The parent BN-aryne spontaneously binds N2 and CO2, demonstrating its highly reactive nature. The interaction with N2 is photochemically reversible. The CO2 adduct of 1,2-azaborine is a cyclic lactam that undergoes photocleavage thus resulting in overall CO2 splitting.

Formal removal of two bonding partners from boranes, BR, yields borylenes, RB, which have been inferred as reactive intermediates in a number of reactions. Phenylborylene (R = CH; 1) is accessible from phenyldiazidoborane by photochemical extrusion of dinitrogen under matrix isolation conditions. Concomitantly, the nitrene PhNBN is formed via phenyl rearrangement. Here we used a combination of UV/vis, IR, and ESR spectroscopy under cryogenic matrix isolation conditions to investigate the properties and reactivity of phenylborylene. We detected an absorption band of phenylborylene at 375 nm (S → S) and tentatively assigned the S → S transition to a very weak band at 518 nm. We also show for the first time that an electrophilic borylene such as 1 can react with N reversibly and with CO irreversibly under photochemical conditions. The corresponding photoproducts PhBNN and PhBCO have triplet electronic ground states. Their small E values are in agreement with the linear arrangements Ph-B-N-N and Ph-B-C-O obtained by density functional theory computations. The D values decrease in the series PhNBN > PhBNN > PhBCO and approach the value for phenylcarbene (PhCH). Indeed, the boron center in PhBCO is isoelectronic with the carbene center in PhCH. The compounds are the first examples of boron analogues of diazoalkanes (RCNN) and ketenes (RCCO), and their formation may serve as a demonstration of the high reactivity of phenylborylene.

The photochemistry of 1,2-dihydro-1,2-azaborinine derivatives was studied under matrix isolation conditions and in solution. Photoisomerization occurs exclusively to the Dewar valence isomers upon irradiation with UV light (>280 nm) with high quantum yield (46 %). Further photolysis with UV light (254 nm) results in the formation of cyclobutadiene and an iminoborane derivative. The thermal electrocyclic ring-opening reaction of the Dewar valence isomer back to the 1,2-dihydro-1-tert-butyldimethylsilyl-2-mesityl-1,2-azaborinine has an activation barrier of (27.0±1.2) kcal mol . In the presence of the Wilkinson catalyst, the ring opening occurs rapidly and exothermically (ΔH=(-48±1) kcal mol ) at room temperature.