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

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 [(pi)8(s) + (pi)2(s)] and [(sigma)2(s) + (pi)6(s) + (sigma)2(s)] 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.