| In this paper,density functional theory（DFT）was used to study the dehydrogenation coupling reaction from alcohols or epoxides and amines to amides catalyzed by transition metal ruthenium pincer complexes.The mechanism of dehydrogenation coupling reaction between alcohols and ammonia catalyzed by transition metal complexes and the mechanism of dehydrogenation coupling reaction between aryl epoxides and secondary amines was explored.In order to compare the aromatization-dearomatization mechanism and the metal-substrate cooperation mechanism,a new catalyst in which ligands dissociated from the metal center was designed,and it was found that the catalyst had better catalytic activity.In this paper,through the theoretical study of the reaction mechanism catalyzed by transition metal ruthenium pincer complexes,the understanding of the molecular mechanism of homogeneous catalytic reaction was further deepened,including the following two parts:1.A density functional theory（DFT）study was performed to explore the mechanism of the dehydrogenation coupling from alcohols and ammonia to amides catalyzed by the pyridine-based Ru-PNN pincer complexes.The mechanism of the catalytic dehydrogenation of alcohol and ammonia to amide by ruthenium pincer complex was investigated respectively in the ligand undissociated mode（aromatization-dearomatization mechanism）and in the partial-ligand dissociated mode（metal-substrate cooperation mechanism）.The results showed that in the ligand undissociated mode,the rate-determining step is the alcohol-assisted H2 release step,that is,the dearomatization reaction.And the energy span of the whole reaction is 26.0 kcal/mol.Whereas,in the partial-ligand dissociated mode,the rate-determining step is also the alcohol-assisted H2 release step,and the energy span of the whole reaction is 17.1 kcal/mol.The metal-substrate cooperation mechanism is more advantageous than the aromatization-dearomatization mechanism.In addition,the dissociation of N-arm of PNN ligand from metal center Ru has adverse thermodynamic effects,this was also confirmed by AIMD simulation,which results showed that the N-arm of PNN ligand could easily dissociate from the metal center Ru.This work not only could enlighten us to replace weak ligand with substrate to generate highly active difunctional ligand but also may provide some theoretical reference for designing a novel Ru-PNN transition metal catalyst for the catalytic dehydrogenation coupling reaction.2.A density functional theory（DFT）study was performed to investigate the mechanism of the formation amides via aryl epoxides and secondary amines catalyzed by the bipyridine-based Ru-PNN pincer complex.The whole reaction is divided into four steps:aryl epoxides isomerization aldehydes,t-Bu OK-assisted aldimine condensation,the release of the product of amides through the inner-sphere mechanism,and amine-assisted H2 elimination completion catalytic cycle.The calculated results showed that with the assistance of t-Bu OK,epoxides could be easily isomerized into aldehydes.In the whole catalytic cycle,aldimine condensation realized partial non-metallic catalysis.The rate-determining step of the whole reaction is the amine-assisted H2elimination with a free energy barrier of 28.0 kcal/mol,and amine could effectively reduce the energy barrier of hydrogen elimination.In addition,it is proved that epoxides would be an attractive substitute for the preparation of amides.And the bipyridine-based Ru-PNN pincer complexes could be potentially high catalytic species for synthesis of amides.This study provides a new insight and theoretical guidance for the formation of amides catalyzed by transition metal ruthenium pincer complexes. |
