Bimolecular reaction between two molecules of iminobenzylfuran-2,3-dione

In analogy to the unusually fast  Chapman-like rearrangement of some 5-alkoxy-2-aryl-1,3,4-oxadiazoles in the solid state which were shown to proceed in a bimolecular, double ionic fashion involving two molecules of the starting material, 6 a bimolecular mechanism is also conceivable for the present rearrangement reaction:


Nucleophilic attack of the imino nitrogen at C2 (the lacton carbonyl carbon) of a second molecule 1 would lead to a zwitterionic intermediate; cyclisation and elimination of this intermediate yields a mixture of starting material and product. Energetics as well as structural features for such a reaction sequence are summarised in Table 2 . Activation energies are fairly low; the rate determining step appears to be the formation of the primary adduct (tetrahedral intermediate).

 

 

Figure 3. Transition state for the nucleophilic attack between two molecules 1 (rate determining step).
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Based on the computed activation energies a bimolecular mechanism involving two molecules 1 seems to be a quite reasonable mechanistic possibility. However, recent experiments on highly purified 1 indicate that some catalysis is necessary to initiate the rearrangement.