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

Abstract

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.

Heterocyclic Nanographenes and Other Polycyclic Heteroaromatic Compounds: Synthetic Routes, Properties, and Applications

“… a Reagents and conditions: (a) ArBr or ArI, Pd­(OAc) 2 , KOAc, DMA, 150 °C, 50–83%; (b) K, toluene, air, 95 °C; (c) dimethyl acetylenedicarboxylate, mesitylene, 150 °C; (d) imidazole, 140–160 °C; (e) Zn­(OAc) 2 , quinoline, 180 °C; (f) AcOH, Ac 2 O, malononitrile, reflux, 31%. …”

“…478 A synthesis of "vertically expanded" imidazo[1,2-a]pyridines, which contain the 2-azaphenalene motif, was recently reported by Gryko and co-workers (Scheme 130). 749 Using Heck-type coupling, the unsubstituted 130.1 was converted into 3-aryl derivatives 130.2, which were subjected to the anion−radical coupling with potassium metal to yield the fused products 130.3a−e. A sequential cycloaddition−cycloreversion performed with DMAD was used for further modification of the ring system, producing fused indolizines such as 130.4.…”

Why vertically π-expanded imidazo[1,2-a]pyridines are weak fluorescence emitters: experimental and computational studies

“…Compounds 2 and 3 were prepared as previously described. 13 Absorption and fluorescence spectra of 2 and 3 in liquid solutions of n-hexane and n-heptane (spectroscopic grade) were recorded at room temperature with the aid of a PerkinElmer UV/VIS Spectrometer Lambda 35, and a Perkin-Elmer 512 Fluorescence Spectrometer, respectively. Fluorescence quantum yield (F fl ) was determined by using perylene in cyclohexane as a standard (F fl = 0.96).…”

“…Compounds 2 and 3, whose synthesis was already described in ref. 13, are composed of the imidazo[1,2-a]pyridine moiety connected in two different fashions with a phenanthrene moiety.…”

3,5-Diarylimidazo[1,2-a]pyridines as Color-Tunable Fluorophores

“…Imidazo­[1,2- a ]­pyridines are also crucial in the optoelectronics and organometallic chemistry, where they serve as excited-state intramolecular proton transfer (ESIPT) fluorophores, for example. Whereas the preparation of 2- or 3-arylimidazo­[1,2- a ]­pyridines has received considerable attention, modifications of the pyridine skeleton of imidazo­[1,2- a ]­pyridines have been less explored . In particular, straightforward methods to access diarylimidazo­[1,2- a ]­pyridines are rare.…”