DFT Mechanistic Probe Of Hydrogen Transfer Reactions In C=O And C=N Bonds Catalyzed By Transition Metal Complexes

Hydrogen is a very special element,which has extremely rich reactivity and has many important uses in science and technology,industrial production and modern social life.Hydrogen transfer reaction(HTR)is one of the most common reactions in chemistry.Transition metal catalyzed HTR to hydrogenate carbon polar double bonds and back to dehydrogenate the respective single bonds is an important chemistry to access useful synthons.These transformations offer a powerful mean to alcohols and carbonyls,and amine and imines via hydrogen transfer reactions in C=O and C=N bonds,respectively.Mechanistically,the two processes are the reverse of each other following the same mechanism and using the same catalysts.This microscopic reversibility offers a gateway to the reaction insights more rationally and lead to design more effective catalysts.In this paper,we use density functional theory(DFT)to study these two hydrogen transfer reactions.The reaction mechanism and stereoselectivity source of asymmetric hydrogenation of oxime to hydroxylamine catalyzed by chiral substituted monocyclopentadienyl iridium(III)metal complexes under acidic conditions,and the cross-coupling reaction of methanol and benzyl alcohol catalyzed by manganese-PNN pincer complexes to prepare methyl benzoate.1.The reaction mechanism and the origin of stereoselectivity of asymmetric hydrogenation of oximes to hydroxylamines catalyzed by the cyclometalated iridium(III)complexes with chiral substituted single cyclopentadienyl ligands under acidic condition were unveiled using DFT calculations.The catalytic cycle for this reaction consists of the dihydrogen activation step and the hydride transfer step.The calculated results indicate that the hydride transfer step is the chirality-determining step and the involvement of methanesulfonate anion(Ms O~-)in this reaction is of importance in the asymmetric hydrogenation of oximes catalyzed by A1 and B1.The calculated energy barriers for the hydride transfer steps without an Ms O~-anion are higher than those with an Ms O~-anion.This is due to the non-covalent interactions among the protonated substrate,Ms O~-anion and catalytic species.The hydrogen bond could not only stabilize the catalytic species,but also change the preference of stereoselectivity of this reaction.2.The density functional theory(DFT)method was employed to investigate the cross-coupling of methanol with benzyl alcohol to afford methyl benzoate catalyzed by Mn-PNN pincer complex.The whole reaction process mainly includes three stages:the dehydrogenation of benzyl alcohol to benzaldehyde,the coupling of benzaldehyde with methanol to hemiacetal and the dehydrogenation of hemiacetal to methyl benzoate.The calculated results indicated that two dehydrogenation processes are influenced by two competitive mechanisms of inner and outer spheres.Dehydrogenation of benzyl alcohol to benzaldehyde is the rate-determining step of the whole reaction,with the energy barrier of 22.1 kcal/mol.In addition,the regeneration of catalyst is also extremely important.Compared with direct dehydrogenation,the dehydrogenation mode assisted by formic acid is more advantageous.This work might provide theoretical insights and shed light on the design of cheap transition-metal catalysts for the dehydrogenation reaction.