Synthesis of Naphtho[1′,2′:4,5]imidazo[1,2-a]pyridines and Imidazo[5,1,2-cd]indolizines Through Pd-Catalyzed Cycloaromatization of 2-Phenylimidazo[1,2-a]pyridines with Alkynes

Abstract

In this paper, palladium-catalyzed oxidative cycloaromatization of 2-phenylimidazo[1,2-a]pyridine (PIP) with internal alkyne is studied. From this reaction, two classes of fused N-heterocycle, naphtho[1',2':4,5]imidazo[1,2-a]pyridine (NIP) and imidazo[5,1,2-cd]indolizine (IID), were formed through dehydrogenative coupling featured with cleavage of the C-H bonds located on different moiety of the PIP substrates. Moreover, when 5-methyl-2-phenylimidazo [1,2-a]pyridine or 2-mesitylimidazo[1,2-a]pyridine was used, either NIP or IID could be obtained as an exclusive product with good efficiency. Intriguingly, Pd(II) showed different action mode in promoting this reaction compared with Rh(III) and led to the formation of NIP with reversed regio-selectivity for the reaction of asymmetrical alkyne.

Recent Developments in Transition‐Metal‐Catalyzed Regioselective Functionalization of Imidazo[1, 2‐a]pyridine

“…Similar type of double functionalization of 2‐phenylimidazo[1, 2‐ a ]pyridine with diphenylalkyne to get naphtho[1′, 2′:4, 5]imidazo[1, 2‐ a ]pyridines (NIP) and imidazo[5, 1, 2‐cd]indolizines (IID), was reported by Li and his co‐workers in 2015 (Scheme 13). [43] They had carried out the reaction using Cu(OAc) 2 oxidant in O 2 atmosphere with TBAB. The mechanism of this functionalization is similar to that proposed by Ghosh et.…”

Rhodium(III)‐Catalyzed Annulation of 2‐Arylimidazo[1,2‐a]pyridines with Maleimides: Synthesis of 1H‐Benzo[e]pyrido[1′,2′:1,2]imidazo[4,5‐g]isoindole‐1,3(2H)‐Diones and their Photophysical Studies

“…[6] Later, Fan group reported palladium catalyzed annulation of 2-arylimidazo[1,2-a]pyridines with diarylalkynes to obtain a mixture of 5,6-diarylnaphtho [1',2':4,5]imidazo [1,2-a]pyridines and 2,3,4-triarylimidazo [5,1,2-cd]indolizines (Scheme 1a). [7] Cheng and Song groups independently developed the protocol to access naphtho [1',2':4,5]-imidazo [1,2-a]pyridines involving Rh(III)-catalyzed oxidative annulation of 2arylimidazo[1,2-a]pyridines and alkynes. [8] Li group developed a condition controlled synthesis of naphtho [1',2':4,5]imidazo [1,2-a]pyridines and fused isoquinolinium derivatives by Rh(III)-catalyzed annulation of 2-arylimidazo[1,2-a]pyridines with alkynes (Scheme 1b).…”

Palladium-Catalyzed Synthesis of Fluorescent Benzo[4,5]imidazo[1,2-a]pyridines through Annulation Reaction of Benzimidazoles and Alkynyl Bromides with Internal Alkynes

“…The most common approaches include transition metal-catalyzed cross-coupling reactions between (benz)­imidazoles and internal alkynes. For instances, Miura, Dong, Hua, Li, Choudhury, Fan, Wang, and Saá reported Rh or Pd-catalyzed various oxidative C–H annulations of imidazoles and alkynes to build π-extended aryl-fused imidazole scaffolds, respectively. Despite major advancements in this field, metal-catalyzed alkyne benzannulation reactions for the synthesis of multisubstituted π-expanded novel benzimidazoles are still highly desired.…”