Reversible photochemical and thermal isomerization of azaboratabisnorcaradiene to azaborabenzotropilidene.

Two new tricyclic 1,2-azaboratabisnorcaradiene molecules (1 b and 2 b) generated through the photoisomerization of N-methyl-2-phenylimidazolyl-chelated dimesitylboranes (1 a and 2 a) have been found to undergo unusual photoisomerization, producing the first examples of 1,2-azaborabenzotropilidenes (1 c and 2 c), accompanied by a distinct color change, upon irradiation at 350 nm. Compounds 1 c and 2 c contain a conjugated alkylideneborane unit and can be fully reverted back to 1 b and 2 b, and subsequently to 1 a and 2 a upon heating. The mechanistic pathway of the new isomerism has been established to involve "walk" rearrangements by DFT computational studies.