On the Reaction of Diaminocarbenes with Aroylimines

Several possible reaction pathways are analyzed for the recently studied experimental reaction of diaminocarbenes with aroylimines, where the carbene acted as an oxygen-abstracting agent. A number of structures corresponding to local minima and transition states are located by geometry optimization. In contrast to the more recent interpretation of the mechanism of this process, the reaction does not proceed via the direct formation of the corresponding carbonyl ylide resulted from the electrophilic addition of diaminocarbene to the carbonyl oxygen atom. Two other, more favorable pathways were predicted instead: the nucleophilic attack of the carbene lone pair on the imino nitrogen (pathway "a") or on the carbon atom in the C═N moiety of aroylimine (pathway "b"), in agreement with predictions of the frontier molecular orbital (FMO) theory. Both intermediate adducts undergo a subsequent decomposition onto nitrile ylide and urea. Which of the two pathways becomes preferential depends on the nature of the substituents: pathway "a" is more favored for the experimentally studied species, whereas pathway "b" is thermodynamically preferable for the small-sized model structures.