Dehydrocyclization of diamine borane and amine-borane alcohol catalyzed by 1-lithio-2-alkyl-1,2-dihydropyridine and its Na & K analogues: A DFT analysis of the reaction mechanism

Abstract

Reaction pathways for dehydrogenation and hence cyclization of diamine-monoborane and amine-borane alcohol have been explored theoretically using Density Functional Theory (DFT). The diamine-monoborane and amine-borane alcohol chosen for investigation in the present venture are N-(tert-butyl)(BH3)–N′-tert-butyl-ethylenediamine and α,α-Diphenyl-2-pyrrolidinemethanol-borane. Alkali metal catalysts, viz., 1-lithio-2-tert-butyl-1,2-dihydropyridine and its sodium and potassium analogues have been employed for the removal of two equivalents of H2. The optimized geometries of all the reactant complexes, intermediates, transition states and product complexes have been obtained using ωB97xd functional in conjunction with 6–31++G(d,p) basis set. The whole assisting phenomena in solvent medium has been investigated through the implementation of conductor-like screening solvation model (COSMO) primarily considering benzene as the bulk solvent medium. The investigation has further been extended to solvents tetrahydrofuran and pyridine to examine the participation of solvent molecules in bringing any change in reaction mechanism and catalytic cycle of the lithium catalyst over diamine-borane. The intramolecular and all possible intermolecular (catalytic) pathways have been studied in detail. Our study supports previous experimental studies and elucidates key role in the catalytic regime of 1,2-dihydropyridine, which is able to store and deliver Li–H on demand.