Reversible Photochemical and Thermal Isomerization of Azaboratabisnorcaradiene to Azaborabenzotropilidene

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