Reactivity Studies of Boron-Nitrogen Containing Reactive Intermediates Using Matrix Isolation and Computational Tools

Abstract

Matrix isolation and computational studies were conducted to explore the reactivity of cyclic iminoboranes, specifically 1,2-azaborinine and 1-(tert-butyldimethylsilyl)-1,3,2-diazaborepine, with various molecules. These studies involved identifying reaction products and investigating potential energy surfaces. The experimental results were supported by computational data obtained by density functional theory (DFT) and ab-initio methods. The computational analysis focusing on the reactivity of 1,2-azaborinine, examined both (2 + 2) and (2 + 4) cycloaddition reactions with organic π-substrates. The results revealed that Lewis acid-base complexes form without an energy barrier, serving as key intermediates in subsequent reactions. Notably, the (2 + 4) cycloaddition was preferred over the (2 + 2) cycloaddition due to favourable symmetry considerations. Additionally, dibenzo derivatives of 1,2-azaborinine were studied to gain insights into their spectroscopic properties and reactivity. 2,4,7,9-tetra-tert-butyldibenzo[c,e][1,2]azaborinine was successfully isolated under cryogenic matrix conditions. Steric hindrance due to tert-butyl groups prevented nitrogen fixation during its generation unlike in the case of dibenzo[c,e][1,2]azaborinine. The impact of ring size on cyclic iminoborane reactivity was further investigated by isolating and characterizing 1-(tert-butyldimethylsilyl)-1,3,2-diazaborepine, a seven-membered cyclic iminoborane. Despite not interacting with dinitrogen, this compound unexpectedly underwent a (2 + 2) cycloaddition reaction with ethene.