Solution Phase and Computational Studies on the Formation, Hydrolysis, and Dynamic Exchange of Phenyl Benzoboroles
The dynamic covalent nature of the boron-nitrogen bond of oxazaboroles and diazaboroles was investigated to gain insight into the differences in stability and the kinetics of exchange. The mechanism of hydrolysis for heteroboroles was investigated using Density Functional Theory (DFT) and Synchronous Transit-Guided Quasi-Newton (STQN) methods. This includes determining the optimized geometries, electronic and chemical properties of intermediates and transition states, and the differences in the relative stabilities of benzoxazaboroles, benzodiazaboroles, and benzodioxaboroles. In addition to the computational chemistry approach, solution-phase studies were used to explain the kinetics of exchange and the relative stabilities of heteroborole systems. The computed Gibbs energies (∆G) for the hydrolysis of benzoxazaborole and benzodiazaborole are -9.04 kJ/mol and -22.44 kJ/mol, respectively, which indicate that benzoxazaborole and benzodiazaborole are less stable than their hydrolysis products. The ∆G for the hydrolysis for benzodioxaborole was small (-0.14 kJ/mol), indicating that both the starting material and the products have similar stabilities. The computational results elucidate that the hydrolysis of benzoxazaborole is preferred when the first step involves B–N bond dissociation, which had a lower energy barrier of 109.80 kJ/mol compared to that of the B-O bond dissociation (139.64 kJ/mol) during the first step. The hydrolysis of benzodiazaborole (125.66 kJ/mol) was the second highest energy barrier out of all heteroborole hydrolysis reactions. v Further, the computational results indicate that the intermediates resulting from B N bond dissociation during borole ring-opening were relatively lower in energy (-3.44 and 2.25 kJ/mol, respectively) compared to the intermediates resulting from B-O bond dissociation (19.41 and 10.00 kJ/mol). In addition, similar percent conversions observed for 3-(alkyl)benzoxazaboroles and benzodioxaborole indicate that both have similar stabilities. Moreover, the equilibration between the benzodiazaborole and benzoxazaborole is very slow and a 39% conversion of benzodiazaborole was observed at equilibrium.