Theoretical Investigation On The Methylation Of Amines Using CO₂ By Ruthenium???-Catalyst

Methyl-substituted amines exit frequently as bioactive compounds and have been widely utilized in many fields,such as chemical industry,medicinal,agrochemical.Previously,most common methylation reactions of amines used toxic or waste-producing methylation reagents,for example,formaldehyde,dimethyl sulfate(DMS)and phosgene(COCl2).CO2 as a cheap,non-toxic and renewable C1 feedstock is an attractive alternative to the present methylation reagents.In recent years,important developments about methylation of amines using CO2/H2 catalyzed by homogeneous ruthenium catalyst have been reported and attracted wide attention.Based on the Density Functional Theory(DFT),we investigated the reaction mechanism of the methylation of amines using CO2 catalyzed by Ru(?)complexes.The main conclusions are summarized as follows:1.The methylation reaction proceeds three sequential stages:(?)CO2 reduction to produce formaldehyde,(?)formaldehyde-amine condensation to form an imine and(?)imine reduction to afford the N-methyl product.Through the detailed calculations,we find that the TDTS(TOF-determining transition state)and TDI(TOF-determining intermediate)exist in the stage i.2.The composition of the active species has a significant effect on the reaction kinetics.Three L ligands(L=THF,PhNH2,CH3SO3-)were considered to coordinate with initial Ru complex[Ru(Triphos)H]+.Due to the stronger proton affinity of CH3SO3-,active species[Ru(TriPhos)H(?2-O3SCH3)]is more stable than the other two cases.Our calculations show that the apparent activation energy of CO2 hydrogenation catalyzed by[Ru(TriPhos)H(?2-O3SCH3)]is the lowest.We further investigated the selectivity of substrate hydrogenation.The apparent activation energy of CO2 hydrogenation is lower than the ester group hydrogenation by 5kcal mol-1.3.The role of methanesulfonic acid(MSA)is divided into two aspects:(1)MSA reacts with the precursor to generate[Ru(Triphos)H]+ unit.(2)Coordination of Methanesulfonate(CH3SO3-)with the Ru center is benefit to the kinetics of CO2 hydrogenation.