Mechanistic study of double boron-silicon exchange reactions between dibenzosiloles and boron tribromide

This paper describes a mechanistic study on double boron-silicon exchange reactions between dibenzosiloles and boron tribromide. This type of reaction presents a safe and environmentally benign approach to convert electron-rich siloles into corresponding electron-deficient boroles and hence shows intriguing potential for the synthesis of boron-doped π-conjugated molecular materials. However, the detailed mechanisms for such reactions have not yet been established in the current literature. In this work, we performed density functional theory (DFT) calculations in conjunction with NMR analysis to unravel the reaction pathways and energy profiles for relevant boron-silicon exchange reactions where two dibenzolesiloles and a phenanthrosilole were employed as reactants, respectively. Our results have shown that excess boron tribromide provides beneficial microsolvation effects on key transition states and reactive intermediates to lower the activation energy barrier for the overall reaction. In the meantime, the substitution pattern at the bay region of the dibenzosilole framework exerts significant impacts on the potential energy surface of the reaction; increased steric hindrance at the bay region decelerates the second boron-silicon exchange step, while extended π-conjugation at the bay region makes the first boron-silicon exchange more challenging. Overall, our computational and experimental results provide an in-depth understanding of the reactivity and scope of this type of reaction, which in turn serves as useful guidance for ongoing research in the field of borole-based organic optoelectronic materials.