Alumina-Supported Gold Nanoparticles as a Bifunctional Catalyst for the Synthesis of 2‑Amino-3-arylimidazo[1,2‑a]pyridines

The bifunctional catalytic efficacy of alumina-supported gold nanoparticles (Au/Al₂O₃) was investigated for the synthesis of a series of 2-amino-3-aryl-imidazopyridines through the chemoselective reduction of the corresponding 2-nitro-3-aryl-imidazo­[1,2-a]­pyridines in high isolated yields. This highly efficient protocol was initially applied for the synthesis of 2-nitro-3-aryl imidazo­[1,2-a]­pyridines via the reaction between 2-aminopyridine and nitroalkenes catalyzed by the present catalytic system Au/Al₂O₃. Moreover, the heterogeneous surface γ-Al₂O₃ was also found to catalyze this pathway in a comparable manner. However, only Au/Al₂O₃ was further proved as the appropriate catalytic system for the selective transfer hydrogenation of the synthesized 2-nitro imidazopyridine derivatives into the corresponding 2-amino-3-aryl imidazo­[1,2-a]­pyridines using NaBH₄ as a hydrogen-donor molecule. In addition, the one-pot two-step reaction between nitroalkenes and aminopyridines in the presence of Au/Al₂O₃–NaBH₄ provided directly the fast and facile synthesis of 2-amino-3-aryl imidazopyridines, highlighting a useful synthetic application of the catalytic protocol.