Theoretical Understanding on the Facilitated Photoisomerization of a Carbonyl Supported Borane System

Boron compound BOMes2 containing an internal B−O bond undergoes highly efficient photoisomerization, followed by sequential structural transformations, resulting in a rare eight‐membered B, O‐heterocycle (S. Wang, et al. Org. Lett. 2019, 21, 5285–5289). In this work, the detailed reaction mechanisms of such a unique carbonyl‐supported tetracoordinate boron system in the first excited singlet (S1) state and the ground (S0) state were investigated by using the complete active space self‐consistent field and its second‐order perturbation (MS‐CASPT2//CASSCF) method combined with time‐dependent density functional theory (TD‐DFT). Moreover, an imine‐substituted tetracoordinated organic boron system (BNMes2) was selected for comparative study to explore the intrinsic reasons for the difference in reactivity between the two types of compounds. Steric factor was found to influence the photoisomerization activity of BNMes2 and BOMes2. These results rationalize the experimental observations and can provide helpful insights into understanding the excited‐state dynamics of heteroatom‐doped tetracoordinate organoboron compounds, which facilitates the rational design of boron‐based materials with superior photoresponsive performances. MS‐CASPT2//CASSCF calculations reveal detailed mechanisms for the photoisomerization of boron compounds, showing that the steric hindrance influences their photoisomerization activity.