Reversible Photoisomerization from Borepin to Boratanorcaradiene and Double Aryl Migration from Boron to Carbon.

B(npy)Ar2 (npy=2-(naphthalen-1-yl)pyridine) compounds bearing various nonbulky aryl groups undergo a clean and sequential two-step photoisomerization in which two aryl substituents on boron migrate to a carbon atom of the naphthyl moiety. The second isomerization step is the first example of a reversible photoisoermization between a borepin and a borirane. Both steric and electronic factors have been found to have a great impact on this photoreactivity. Furthermore, the borirane isomer reacts with oxygen, forming a rare oxaborepin dimer.