Theoretical Investigations on the Mechanism of Hetero‐Diels–Alder Reactions of Brassard’s Diene and 1, 3‐Butadiene Catalyzed by a Tridentate Schiff Base Titanium(IV) Complex

Abstract

The mechanism of the hetero-Diels-Alder reactions of Brassard's diene and 1,3-butadiene catalyzed by a titanium(IV) complex of a tridentate Schiff base was investigated by DFT and ONIOM methods. The calculations indicate that the mechanism of the reaction is closely related to the nucleophilicity-electrophilicity between diene and carbonyl substrates. A stepwise pathway is adopted for Brassard's diene, and the step corresponding to the formation of the C--C bond is predicted to be the rate-determining step with a free-energy barrier of 8.4 kcal mol(-1). For 1,3-butadiene, the reaction takes place along a one-step, two-stage pathway with a free-energy barrier of 14.9 kcal mol(-1). For Brassard's diene as substrate, the OCH(3) and OSi(CH(3))(3) substituents may play a key role in the formation of the transition state and zwitterionic intermediate by participating in charge transfer from Brassard's diene to formaldehyde. The combination of the phenyl groups at the amino alcohol moiety and the ortho-tert-butyl group of the salicylaldehyde moiety in the chiral tridentate Schiff base ligand plays an important role in the control of the stereoselectivity, which is in agreement with experimental observations.