Photo- and Thermal-Induced Multistructural Transformation of 2‑Phenylazolyl Chelate Boron Compounds

The new N,C-chelate
boron compounds B­(2-phenylazolyl)­Mes2 [Mes = mesityl; azolyl
= benzothiazolyl (1a), 4-methylthiazolyl
(2a), benzoxazolyl (3a), benzimidazolyl
(4a)] undergo an unprecedented multistructural transformation
upon light irradiation or heating, sequentially producing isomers b, c, d, and e. The
dark isomers b generated by photoisomerization of a undergo a rare thermal intramolecular H-atom transfer (HAT),
reducing the azole ring and generating new isomers c,
which are further transformed into isomers d. Remarkably,
isomers d can be converted to their diastereomers e quantitatively by heating, and e can be converted
back to d by irradiation at 300 nm. The structures of
isomers 1d and 1e were established by X-ray
diffraction. The unusual HAT reactivity can be attributed to the geometry
of the highly energetic isomers b and the relatively
low aromaticity of the azole rings. The boryl unit plays a key role
in the reversible interconversion of d and e, as shown by mechanistic pathways established through DFT and TD-DFT
calculations.