Theoretical Study On The Reaction Mechanism Of Carbene Compounds

In recent years,as the deterioration of the environment pollution and increasing consumption of the resources on the earth,the development of new catalytic reactions and realization of the green transformations of material has become a hot and frontal topic in the current study of chemical workers.Hence,the mechanisms of catalytic reactions should be well studied and understood to realize those reactions happening more alternatively and efficiently.This dissertation,based on the theory of computational quantum chemistry,mainly studied energies and geometries of the reaction and then deepened understanding the reaction mechanism,which could help us to design new related reactions.Three specific works are as follows:(1)By using density functional theory calculations,we studied the reaction mechanisms to synthesize chiral gold(I)-aminocarbene complexes through a cyclization–rearrangement cascade.Our calculated results show that the mechanism containing a [3,3]-sigmatropic rearrangement process is more favorable compared with the 1,2-alkyl shift,which can be attributed to the steric effects.In addition,[3,3]-sigmatropic rearrangement step is the rate-determining step.By investigating the transition state structures related to the rate-determining step,we conclude that the yield of product is mainly controlled by the steric effects of the different substituent groups.(2)A density functional theory study was performed to understand detailed mechanisms for the cross-benzoin reactions catalyzed by N-heterocyclic carbene(NHC)species.Our theoretical study predicted that the first H-transfer operates with the water in solution as a mediator,and the second H-transfer undergoes a concerted mechanism rather than a stepwise one.In addition,the chemoselectivity of the reaction studied in this work has been explored.Different steric effects resulted from the catalysts is the origin leading to the chemoselectivity.(3)The reaction mechanisms on the NHC-catalyzed [4 + 3] and [4 + 1] annulations of enals with azoalkenes have been theoretically investigated with the aid of the density functional theory(DFT)calculations.It is found that the additives play an important role in promoting proton transfer and dehydration.The impacts of catalysts(NHC-A and NHC-B)and substituent group on the regioselectivity were rationalized.The origin of the regioselectivity involved in these reactions was probed by performing distortion-interaction analysis.For reaction A with NHC-A as the catalyst and Boc group as the substituent,the regioselectivity is predicted to be determined by the interaction energy of 3 with R2' in TS3 A vs TS8 A.For reaction B with NHC-B as the catalyst and Boc group as the substituent,the unstable boat-type conformation vs the stable chair-type conformation involved in the NHC-B moiety accounts for the regioselectivity.In reaction C with NHC-A as the catalyst and Ts group as the substituent,distortion energies of 3 and R2' in TS3 C vs TS6 C are found to be the major reason delivering even higher regioselectivity as compared to reaction A.