Reactions of Nitrosoalkenes with Dipyrromethanes and Pyrroles: Insight into the Mechanistic Pathway

The
reactivity of nitrosoalkenes toward dipyrromethanes, pyrrole,
and 2,5-dimethylpyrrole is described. 1-(p-Bromophenyl)­nitrosoethylene
shows a different chemical behavior with these heterocycles than the
previously reported reactions of ethyl nitrosoacrylate, which proceeds
via a Diels–Alder reaction. 1-(p-Bromophenyl)­nitrosoethylene
reacts with dipyrromethanes and pyrrole to afford two isomeric oximes
via conjugate addition followed by rearomatization of the pyrrole
unit. On the other hand, this nitrosoalkene reacts with 2,5-dimethylpyrrole
through an initial conjugate addition followed by intramolecular O-
and N-nucleophilic addition with the formation of the corresponding
bicyclic oxazine and five-membered cyclic nitrone, respectively. Quantum
chemical calculations, at the DFT level of theory, indicate that the
barriers associated with the Diels–Alder reactions of ethyl
nitrosoacrylate are over 30 kJ/mol lower than those that would be
required for the cycloadditions of 1-(p-bromophenyl)­nitrosoethylene.
Thus, calculations predict that the Diels–Alder reaction is
privileged in the case of ethyl nitrosoacrylate and point to a different
reaction pathway for 1-(p-bromophenyl)­nitrosoethylene,
corroborating the experimental findings.