Examination of the photoreactivity of an ew class of N,C-chelate organoboron compounds, including as eries of unsymmetrically substituted boron molecules, B(naph-pyridyl)(Ar 1)(Ar 2)a nd B(naph-thiazolyl)(Ar 1)(Ar 2), led to the discovery of new and divergentp hotothermali somerization phenomena. These include the cleanand regioselectivep hotoisomerization by unsymmetrical boron,f orming borepin isomers, some of which further isomerize to the corresponding boratanorcaradiene diastereomer pairs as ar esult of the generation of two chiral centers. Significantly,t he boratanor-caradienes involving a3-thienyl substituent on boron were found to thermally convert to BN-fluoranthene annulated borapentalene via an unprecedented reversible boratacyclopropane-boratacyclopentene rearrangement. Changing the pyridyld onort oat hiazolyl donor on the boron was found to provide the B(naph-thiazolyl)(Mes) 2 compounds with a distinct new photoisomerization pathway-instead of borepin, forming new blue fluorescent polycyclic azaborinine species.T his work illustrates the richness and complexity of boron photochemistry.

Dimesitylboron-functionalized stilbene derivatives have been found to undergo an unusual regioselective photoisomerization upon irradiation at 365 nm. Using NMR to follow the photoreaction, the structures of key reaction intermediates and the final products were established. This photoisomerization occurs in four steps: trans–cis isomerization, Diels–Alder reaction, di-π-methane rearrangement, and ring opening with [1,3]-H migration. This results in the formation of a rare structure with three fused five-membered rings and a six-membered one.