DFT studies on cinchona alkaloid-catalyzed Diels-Alder reactions

초록

The mechanism and origin of the selectivity of asymmetric Diels-Alder reactions of 2,4-dienones by trienamine catalysis were investigated at the B3LYP/6-311++G**(SMD, toluene)//B3LYP/6-311G** level. The acidic additive (salicylic acid or trifluoroacetic acid) promotes the isomerization of the ketoiminium ion into the key reactive cistrienamine catalytic species with a preferred conformation by constructing a suitable hydrogen-bond bridge for a H shift. One-step cycloaddition was adopted for Nphenylmaleimide (R2), while a stepwise process was used for benzylidenecyanoacetate (R3) dienophile. The -CN and -CO2Et substituents in R3 may play an important role in forming a zwitterionic intermediate by participating in charge transfer by trienamine catalysis. The combination of a hydrogen bond from the protonated N atom of the tertiary amine and steric repulsion from the α-aryl group of trienamine makes the dienophile substrates approach the trienamine from the one preferred face. The orbital factors define the most favored alignment of the trienamine and dienophile to realize the endoselectivity of the product.