Rearrangement by an electrocyclic ring closure - ring opening sequence
The main features of this mechanistic possibility consists in an (hetero)electrocyclic
ring closure of the iminobenzyl group and the endocyclic double bond
in 1 to form a bicyclic product
which then might rearrange an isomeric bicyclus.
Subsequent (hetero)electrocyclic ring opening then could yield the pyrrole-2,3-dione
2.
The semi-empirical calculations reveal a rather complex reaction sequence
for this mechanism (energetic as well as structural details are provided
in Table 1). The calculated activation energies
for this reaction sequence are rather high - considerably higher than
those for elimination of carbon monoxide to give a-oxo
ketenes !
Figure 2. Transition state for electrocyclic ring
closure of 1 (rate determining step)
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Formation of compound 2 by this mechanism,
therefore, would require prohibetively large activation energies and, thus,
is highly unlikely.