Construction of fused BN-heterocycles via boron atom insertion: DFT insights into the Lewis acid–base (BBr3/NEt3) cooperative mechanism and selectivity

Abstract

The BBr₃-mediated N-heterocycle editing reaction through a boron atom insertion strategy reported by Song et al. (Angew. Chem., Int. Ed., 2024, 63, e202318613) has been systematically explored using density functional theory (DFT) calculations. The present results reveal that in addition to BBr₃ acting as a boron source and an electrophile, the Lewis base NEt₃ also plays a crucial role. Specifically, the Lewis acid–base cooperative interaction between BBr₃ and NEt₃ facilitates the ring-opening of the substrate 1-(2-vinylphenyl) azetidine, the rate-determining step of the overall cascade reaction. Notably, the organic base NEt₃ facilitates the formation of the real nucleophilic species BBr₄⁻, thereby promoting the progression of the reaction. Furthermore, the formation of an exceptionally stable C₄NB π-ring intermediate and the difference in distortion and exchange-repulsion energies, caused by structural characteristics of substrates, are responsible for the chemoselectivity and regioselectivity of substrates bearing typical structural motifs and functional groups, respectively. These computational findings not only provide profound mechanistic insights into the tandem reactions involved in the construction of fused BN–heterocycles, but also elucidate the underlying factors governing substrate preference.