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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 CN
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