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

• Published in: Journal of organic chemistry Vol. 79; no. 21; pp. 10456 - 10465
• Main Authors: Nunes, Sandra C. C, Lopes, Susana M. M, Gomes, Clara S. B, Lemos, Américo, Pais, Alberto A. C. C, Pinho e Melo, Teresa M. V. D
• Format: Journal Article
• Language: English
• Published: WASHINGTON American Chemical Society 07.11.2014; Amer Chemical Soc
• Subjects: Chemistry; Chemistry, Organic; Physical Sciences; Science & Technology; BODIPY DYES; CHEMISTRY; CYCLIC NITRONES; 1,2-OXAZINES; ALKENYL OXIMES; DIELS-ALDER REACTION
• ISSN: 0022-3263; 1520-6904
• DOI: 10.1021/jo502095k

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.