Tandem [8 + 2] Cycloaddition−[2 + 6 + 2] Dehydrogenation Reactions Involving Imidazo[1,2-a]pyridines and Imidazo[1,2-a]pyrimidines

• Published in: Journal of organic chemistry Vol. 75; no. 9; pp. 2776 - 2784
• Main Authors: Aginagalde, Maialen, Vara, Yosu, Arrieta, Ana, Zangi, Ronen, Cebolla, Vicente L, Delgado-Camón, Arantzazu, Cossío, Fernando P
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 07-05-2010
• Subjects: Benzene Derivatives - chemistry; Cyclization; Fluorenes - chemistry; Hydrogenation; Imidazoles - chemistry; Indolizines - chemistry; Microwaves; Molecular Structure; Oxidation-Reduction; Pyridines - chemistry; Pyrimidines - chemistry
• ISSN: 0022-3263; 1520-6904
• DOI: 10.1021/jo9022815

The reaction between benzynes and imidazo[1,2-a]pyridines (pyrimidines) to form benzo[a]imidazo[5,1,2-cd]indolizines and 2,3,9c-triazocyclopenta[j,k]fluorenes has been studied computationally and experimentally. It is found that these reactions take place via tandem [π8s + π2s] and [σ2s + π6s + σ2s] processes. The [8 + 2] cycloaddition steps are essentially barrierless, and the aromatization steps occur via highly synchronous aromatic transition structures. From an experimental standpoint, the reaction is feasible under microwave irradiation and using 2-(trimethylsilyl)phenyl triflates as benzyne precursors. Depending on the substitution pattern in the starting triflate a complete regiocontrol of the reaction can be achieved. The tetracyclic compounds thus prepared emitted blue light when excited at 365 nm and exhibited interesting photophysical properties.