Vertically π‐Expanded Imidazo[1,2‐a]pyridine: The Missing Link of the Puzzle

Abstract

The dehydrogenative coupling of imidazo[1,2-a]pyridine derivative has been achieved for the first time. In cases in which the most-electron-rich position of the electron-excessive heterocycle was blocked by a naphthalen-1-yl substituent, neither oxidative aromatic coupling nor reaction under Scholl conditions enabled the fusion of the rings. The only method that converted the substrate into the corresponding imidazo[5,1,2-de]naphtho[1,8-ab]quinolizine was coupling in the presence of potassium in anhydrous toluene. Moreover, we discovered new, excellent conditions for this anion-radical coupling reaction, which employed dry O2 from the start in the reaction mixture. This method afforded vertically fused imidazo[1,2-a]pyridine in 63% yield. Interestingly, whereas the fluorescence quantum yield (Φ(fl)) of compound 3, despite the freedom of rotation, was close to 50%, the Φ(fl) value of flat naphthalene-imidazo[1,2-a]pyridine was only 5%. Detailed analysis of this compound by using DFT calculations and a low-temperature Shpol'skii matrix revealed phosphorescence emission, thus indicating that efficient intersystem-crossing from the lowest-excited S1 level to the triplet manifold was the competing process with fluorescence.

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

“…Thus, the spectra of both compounds are located in a similar spectral range, although the onset of the spectra of 3 is located at slightly higher energies than that of 2. Interestingly, the onset of the spectra of the reference compound 1 (19 050 cm À1 in absorption and 20 400 cm À1 in fluorescence) 12 is close to that of compounds 2 and 3. It can thus be concluded that p-expansion of imidazo[1,2-a]pyridine upon attaching a phenanthrene moiety, when compared with attaching a naphthalene moiety, does not contribute considerably to the energy separation between the ground and the lowest excited electronic singlet states.…”

“…All of these show that prediction of properties of p-expanded imidazo[1,2-a]pyridines is not a trivial problem, and needs careful investigation, as has previously been done for hydrocarbons. 16 Suggestions for future studies A characteristic feature of both of the investigated compounds, as well as of imidazo[5,1,2-de]naphtho[1,8-ab]quinolizine, 12 is very low fluorescence quantum yield. Nonradiative depopulation of the fluorescence emitting S 1 state proceeds either by internal conversion, S 1 -S 0 , and/or by intersystem crossing, Fig.…”

“…11 However, the usefulness of these new systems depends on the efficiency of the system-specific radiative and non-radiative excitation energy dissipation pathways. In our previous paper 12 we studied imidazo-[5,1,2-de]naphtho [1,8-ab]quinolizine (compound 1 in Fig. 1) -an example of vertically p-expanded imidazo[1,2-a]pyridine.…”

Polycyclic imidazo[1,2-a]pyridine analogs – synthesis via oxidative intramolecular C–H amination and optical properties

“…In particular, they typically possess high fluorescence quantum yields. [22][23][24][25][26][27][28] With the advent of interest in ladder-type aromatic heterocycles, 29,30 we reasoned that π-expanded imidazo[1,2-a]pyridines being analogues of recently explored systems such as indolo [3,2-b]indoles 31,32 can offer new opportunities once an efficient synthetic methodology is developed. Indolo[3,2-b]indoles (1), benzofuroindoles (2) and benzothioindoles (3) were recently reported as highly active sex steroid hormone receptor modulators 33 and anticancer agents ( Fig.…”

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

“…9 Recently we revealed that 3-(naphthalen-1-yl)-imidazo[1,2-a]pyridine can be synthesized from a singly-linked precursor by anion-radical coupling 10 in 63% yield. 11 Since anion-radical coupling is a relatively weakly studied reaction (Scheme 1), we wondered if this methodology can be expanded to its analogs possessing other aromatic units instead of naphthalene. This would allow us to study the relationship between the structure of substrates and the efficiency of this reaction.…”