Synthesis and site selective C-H functionalization of imidazo-[1,2-a]pyridines

Imidazo[1,2-a]pyridine has attracted much interest in drug development because of its potent medicinal properties, therefore the discovery of novel methods for its synthesis and functionalization continues to be an exciting area of research. Although transition metal catalysis has fuelled the most significant developments, extremely beneficial metal-free approaches have also been identified. Even though pertinent reviews focused on imidazo[1,2-a]pyridine synthesis, properties (physicochemical and medicinal), and functionalization at the C3 position have been published, none of these reviews has focused on the outcomes obtained in the field of global ring functionalization. We wish here to describe a brief synthesis and an overview of all the functionalization reactions at each carbon atom, viz, C2, C3, C5, C6, C7 and C8 of this scaffold, divided into sections based on site-selectivity and the type of functionalization methods used.

Electrochemically Enabled Direct C3-Formylation of Imidazopyridines with Me3 N as a Carbonyl Source

A metal-free and oxidant-free electrochemically enabled strategy for C-3 formylation of imidazopyridines using trimethylamine as a one-carbon source has been established. This conversion has high functional group compatibility under mild conditions and ensures late-stage functionalization of pharmaceutical molecules. Furthermore, unexpected hexafluoroisopropoxylation products have been observed in some cases. Mechanistic studies using cyclic voltammetry and control experiments reveal the key intermediate of the formylation process.

Halogen Bond-Activated Visible-Light-Mediated Regioselective C-H Arylation of 2-Phenylimidazo-[1,2-a]pyridines

An efficient method for transition metal-free halogen bond-assisted regioselective C-H arylation of 2-phenylimidazo-[1,2-a]pyridines under visible-light condition has been developed. The halogen bond between an aryl halide and base KO^tBu initiates an electron transfer process and generates an aryl radical, which catalyzes its coupling with 2-phenylimidazo-[1,2-a]pyridines to give arylated products in good yield. Several control experiments, density functional theory calculations, and ultraviolet-visible analysis indicate the presence of a halogen bond between an aryl halide and KO^tBu. This methodology has been successfully utilized to synthesize antileishmanial agents.