Mechanistic Insight into the Thermal Ring Opening of the Dewar Isomer of 1,2-Dihydro-1,2-azaborinines

In contrast to the multichannel photochemistry of benzene, 1,2-dihydro-1,2-azaborinines generally undergo a selective electrocyclic ring-closure reaction, yielding the corresponding Dewar (2-aza-3-borabicyclo[2.2.0]-hex-5-ene) isomers. The Dewar isomers react back to the dihydroazaborinines in unimolecular reactions with long half-lives at room temperature, making this isomer pair of relevance as a molecular solar thermal (MOST) energy storage system. A systematic investigation of the influence of C-backbone functionalization on the photoisomerization or the thermal ring opening has not yet been conducted. We report here a study of the late-stage C3 functionalization of 1,2-dihydroazaborinines by cross-coupling and discuss the electronic impact of aryl substitution at C3 on the properties of 1,2-dihydro-1,2-azaborinines itself, as well as on the photoreaction and the thermal back reaction employing a Hammett analysis for the latter. 3-Aryl-substituted dihydroazaborinines react selectively and reversibly in almost quantitative fashion to the Dewar isomer. The Woodward-Hoffmann forbidden thermal ring-opening reaction proceeds by interaction with the adjacent boron center involving a three-center-two-electron interaction and is accelerated by both electron-withdrawing and electron-donating groups. Through the targeted selection of C3 substituents, both the electronic absorption characteristics of the dihydroazaborinine and the half-life of its Dewar isomer can be addressed.