Stepwise Intramolecular Photoisomerization of NHC-Chelate Dimesitylboron Compounds with C–C Bond Formation and C–H Bond Insertion

C,C-chelate dimesitylboron (BMes2) compounds
containing
an N-heterocyclic carbene (NHC) donor have been obtained.
Single-crystal X-ray diffraction analyses established that the boron
atom in these compounds is bound by four carbon atoms in a distorted
tetrahedral geometry. Compared to previously reported N,C-chelate
dimesitylboron compounds, the new C,C-chelate boron compounds have
a much larger HOMO–LUMO energy gap (>3.60 eV). They do,
however,
respond to UV irradiation (300 nm) in the same manner as N,C-chelate
BMes2 compounds do, undergoing photoisomerization and converting
to an intensely colored (yellow or orange) isomer A quantitatively,
with a high quantum efficiency (0.60–0.75). NMR and single-crystal
X-ray diffraction analyses established that the structure of A is similar to the dark isomers obtained from N,C-chelate
BMes2 compounds. However, unlike the N,C-chelate dark isomers
that have the tendency to thermally reverse back to the light colored
isomers, the isomers A of the C,C-chelate BMes2 are thermally stable and no reverse isomerization was observed even
when heated to 80 °C (or 110 °C) for hours. The most unusual
finding is that isomers A undergo further photoisomerization
when irradiated at 350 nm, forming a new colorless species B nearly quantitatively. NMR and single-crystal X-ray diffraction
analyses established the structure of isomer B, which
may be considered as an intramolecular C–H insertion product
via a borylene intermediate. Mechanistic aspects of this unusual two-step
photoisomerization process have been examined by DFT computational
studies.

本研究成功合成了一类带有氮杂环卡宾（NHC）给体的C,C-螯合二均三甲苯基硼（BMes₂）化合物。经单晶X射线衍射分析证实，该类化合物中的硼原子与四个碳原子配位，呈现畸变四面体几何构型。相较于此前报道的N,C-螯合二均三甲苯基硼化合物，新型C,C-螯合硼化合物的HOMO-LUMO能隙显著更大（＞3.60 eV）。不过，这类化合物与N,C-螯合BMes₂化合物具有相同的光响应行为：在300 nm紫外光照射下可发生光异构化反应，以0.60~0.75的高量子产率定量转化为色泽浓郁的黄色或橙色异构体A。核磁共振（NMR）与单晶X射线衍射分析表明，异构体A的结构与N,C-螯合BMes₂化合物所得的深色异构体高度相似。然而，与N,C-螯合深色异构体易发生热可逆转变、回到初始浅色异构体的特性不同，C,C-螯合BMes₂的异构体A具有极佳的热稳定性，即便在80 ℃（或110 ℃）下加热数小时，也未观测到任何逆向异构化现象。最引人关注的发现是，异构体A在350 nm光照下可进一步发生光异构化，近乎定量地转化为新型无色物种B。通过核磁共振与单晶X射线衍射分析确定了异构体B的结构，其可被视为经由硼宾中间体的分子内C-H插入产物。本研究通过密度泛函理论（DFT）计算，对这一独特的两步光异构化过程的反应机理进行了系统探究。