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.新型N,C-螯合硼化合物（N,C-chelate boron compounds）B(2-苯基唑基)Mes₂ [其中Mes为均三甲苯基(mesityl)，azolyl分别指代苯并噻唑基(1a)、4-甲基噻唑基(2a)、苯并恶唑基(3a)与苯并咪唑基(4a)]在光照或加热条件下会发生前所未有的多结构转变，依次生成异构体b、c、d与e。由a经光异构化生成的暗态异构体b会经历罕见的热诱导分子内氢原子转移（intramolecular H-atom transfer, HAT）反应，还原唑环并得到新的异构体c，后者可进一步转化为异构体d。值得注意的是，异构体d可通过加热定量转化为其非对映异构体e，而e经300 nm光照后可逆向转化为d。异构体1d与1e的结构已通过X射线衍射（X-ray diffraction）得到确证。这种非同寻常的氢原子转移反应活性可归因于高能异构体b的空间构型以及唑环相对较低的芳香性。如通过密度泛函理论（Density Functional Theory, DFT）与含时密度泛函理论（Time-Dependent Density Functional Theory, TD-DFT）计算确定的反应机理路径所示，硼基单元在d与e的可逆相互转化过程中发挥着关键作用。