Accessing Two-Stage Regioselective Photoisomerization in Unsymmetrical N,C-Chelate Organoboron Compounds: Reactivity of B(ppz)(Mes)Ar

A new family of unsymmetrical, N,C-chelate organoboron compounds B­(ppz)­(Mes)­Ar have been synthesized and found to undergo a rare, regioselective two-stage photoisomerization, involving the Ar group only. The initial transformation is a Zimmerman rearrangement to afford yellow azaboratabisnorcaradiene isomers that are subsequently converted to unprecedented 14aH-diazaborepins via a photochemical “walk” rearrangement. Spectroscopic and computational studies provide insight into the formation and properties of these unique systems.