Hydrogen-bonded imide protons

Since the second solvent pair fall within the poor hydrogen bonding group of solvents, increased basicity of the organic base in these solvents would be consistent with the observed behavior. Based on the model compound studies, indications are that the base-catalyzed imidization process may involve a two-step mechanism, Jee Scheme 23. The first step corresponds to the complete or partial proton abstraction from the amide group with the formation of an iminolate anion. Since this iminolate anion has two possible tautomers, the reaction can proceed in a split reaction path to either an isoimide- or imide-type intermediate. Although isoimide model reactions indicate an extremely fast isomerization to the imide under the conditions employed for base-catalysis, all indications to date are that it is not an intermediate in the base-catalyzed imidization of amic alkyl esters. 

Fig. 2). It was originally envisioned that, as shown in Fig. 3c, when the Zn complex 8 interacts with the imide functionalities, Zn might first dissociate the imide proton to form a Zn -N bond, whereupon the imide carbonyls with developing negative charges would become better acceptors for the acidic NH hydrogens at the complementary positions. 

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