Theoretical investigation on regioselectivities of Diels‐Alder reactions by conjugated effect

The Summary of main observation and conclusion: The regioselectivities of 76 Diels-Alder reactions between unsymmetrical dienes and dienophiles were investigated by local softness and generalized local softness at atomic and chemical bond view, respectively, where the reactivity descriptors were calculated by finite difference approximation and ABEEM sigma pi model. Based on local HSAB principle and matching criterion of local softness, reasonable explanation and prediction on regioselectivities of all reactions through ABEEM sigma pi model have been obtained, which are in agreement with the experimental results. Especially, at pi-bond level, while the 1-substituents of dienes were NH2, NMe2, and NEt2, because of the p-pi conjugated effect between the lone pairs of N atom and the conjugated pi-bond of the four double-bonded carbon atoms in diene, it made the dienophile not easy to approach diene by Manner A, hence, Manner B can forecast the ortho regioisomer as main product well, and Manner A cannot. As to the other reactions, both Manner A and Manner B can predict their regioselectivities. It can be seen that the ABEEM sigma pi model can take the effect of the chemical environment on the reaction reactivity into account richly, especially the system includes pi-bond and lone pairs. However, the results through the finite difference approximation are not reasonable.