N-Boron-pyrrole: A negative charge stabilizing group

Abstract

We show that the N-boron-pyrrole (B-pyrrole) group stabilizes the negatively charged compounds, despite not being an electron withdrawing group (EWG). Density functional theory (DFT) calculations revealed that this distinctive property of the B-pyrrole group arises from enhanced π-electron delocalization within the pyrrole ring. The effect of B-pyrrole group on the Brønsted and Lewis acidity of various compound classes was examined and compared with common EWGs including F, Cl, Br, CN, NO₂. Substitution of pyrrole into the boracyclohexadiene derivatives enhances their acidity by about 1–6 kcal mol⁻¹ while the acidity enhancement due to pyrrole substitution into boric acid was about 17 kcal mol⁻¹. Interestingly, B(Pyrrole)₃ exhibited a fluoride affinity comparable to BF₃ (∼78 kcal mol⁻¹) and approximately 15 kcal mol⁻¹ higher than that of BH₃.