Positional Isomerization of 1,2-Azaborine through BN-Benzvalene

A photochemical positional isomerization approach for converting readily accessible C5-aryl-1,2-azaborines into their C4-aryl counterparts via a BN-benzvalene intermediate is described. The oxidative radical cation pathway from BN-benzvalene to the C4-aryl products is supported by deuterium labeling experiments, transient absorption (TA) spectroscopy, and DFT calculations. The reaction's utility is demonstrated by providing the first general method to C4,C5-difunctionalized 1,2-azaborines and the regioselective synthesis of hexa-substituted derivatives.