DFT Study On The Mechanisms Of The Michael Addition Reaction Of Aldehydes And Nitrostyrene

In the present paper, density functional theory (DFT) calculations were performed on the reaction mechanisms of the Michael addition of aldehydes and nitrostyrene.This paper includes the following two parts:1. The reaction mechanisms of aldehydes and nitrostyrene catalyzed by a chiral silylated pyrrolidine catalystThe asymmetric Michael reaction of aldehydes and nitrostyrene catalyzed by a new (S)-tertbutyl-diphenyl-silyl-pyrrolidine catalyst has been investigated by using density functional theory calculations. The Re face of the enamine is effectively shielded, because of the bulky2-substituent group on the pyrrolidine ring of the catalyst. For acetaldehyde, there are two different conformers of enamines. Based on the two conformers of enamines, four different reaction pathways have been considered. The calculated enantiomeric excess value is80.27%in favor of the (R)-configuration product. For propanal, eight different reaction pathways have been considered and the eight corresponding transition states have been located. The calculated enantiomeric excess value is98.96%in favor of the (2S,3R)-configuration product. These calculated results are in good agreement with the experimental observations. In addition, the calculations show that the lowest energy transition state and the second one are formed by the different enamines. This result suggests that the enamines play an important role in the Michael reactions.2. The roles of benzoic acid and water on the Michael reactions of pentanal and nitrostyrene catalyzed by diarylprolinol silyl etherThe roles of benzoic acid and water on the Michael reaction of pentanal and nitrostyrene catalyzed by diarylprolinol silyl ether are revealed by density functional theory calculations. The calculations demonstrate that the benzoic acid is ready to attack the catalysts and form a hydrogen bond between the hydrogen atom of the COOH of benzoic acid and one of the N atoms of the catalyst. The complex formed from pentanal, catalyst and benzoic acid attacks nitroalkene and forms transition states. Finally, the transition states hydrolyze and the products are formed. The calculations demonstrate that the stereoselectivity is dominated by the steric hindrance of the2-substituent groups, and the benzoic acid can increase the reaction rate evidently by decreasing the activation energies; however, H3O+or strong acid may prevent the formation of the transition states between enamines and nitroalkenes. The employed solvent can decrease the activation energies and promote the proton transfer from benzoic acid onto the catalyst2. The calculated enantiomeric excess values are in good agreement with the experimental results, These calculations also reveal that the role of benzoic acid is dependent on the sophisticated structures of the catalysts and provide a valuable index for the structural design of new catalysts and selection of additives or co-catalysts.