Vertically-expanded imidazo[1,2-a]pyridines and imidazo[1,5-a]pyridine via dehydrogenative coupling.

The anion-radical coupling of structurally diverse series of aromatic compounds possessing biaryl linkages led to seven fused, polycyclic heterocycles in reasonable yields. The yield of the key step (K, toluene, O2) depends on both electronic and steric factors. The whole strategy consists of just two steps starting from an unsubstituted imidazo[1,2-a]pyridine, giving target compounds in an overall yield of 4-34%. The same strategy also works for derivatives of imidazo[1,5-a]pyridine. A new process has been discovered for such vertically-expanded imidazo[1,2-a]pyridines, consisting of a sequential Diels-Alder reaction followed by a retro-Diels-Alder reaction. The optical properties of the library of π-expanded imidazo[1,2-a]pyridines were for the first time fully characterized, showing that fluorescence quantum yields are significantly lower than for the singly-linked compounds.