title: Reactions of nitrosoalkenes with dipyrromethanes and pyrroles: insight into the mechanistic pathway.
abstract: 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.
authors.affiliation: Department of Chemistry, University of Coimbra , 3004-535 Coimbra, Portugal.
authors.forename: Sandra C C
authors.initials: SC
authors.lastname: Nunes
authors.forename: Susana M M
authors.initials: SM
authors.lastname: Lopes
authors.forename: Clara S B
authors.initials: CS
authors.lastname: Gomes
authors.forename: Américo
authors.initials: A
authors.lastname: Lemos
authors.forename: Alberto A C C
authors.initials: AA
authors.lastname: Pais
authors.forename: Teresa M V D
authors.initials: TM
authors.lastname: Pinho E Melo