Schiff base addition to cyclic dicarboxylic anhydrides: an unusual concerted reaction. An MO and DFT theoretical study.

The existing controversy as to whether dicarboxylic anhydride iminolysis by Schiff bases is a concerted [4 + 2] addition or a stepwise reaction following either a Perkin-like route or occurs as iminolysis via zwitterionic intermediates is solved computationally by DFT and MO theory. Both types of theory favor a concerted mechanism starting as bimolecular addition of imine to the carbonyl carbon of anhydride monoenol, accompanied by simultaneous elimination of carboxylate. The reaction proceeds further without forming any intermediate and completes as intramolecular charge transfer from enol HOMO to imine LUMO with ring closure.