I₂/TBPB Mediated Oxidative Reaction to Construct of Imidazo[1,5‐α]pyridines under Metal‐Free Conditions

Abstract

A new method for the synthesis of imidazo[1,5‐α]pyridines, one of the most important N‐heterocyclic compounds due to their wide application, was reported in this study. Specifically, using I2 as a non‐metal catalyst and TBPB as an oxidant can efficiently facilitate the synthesis and avoid the use of a metal catalyst. The reaction between ethyl (E)‐2‐(benzylideneamino)acetate and pyridine represents an unprecedented 1,3‐diolar cycloaddition in highly straightforward process with great value.

Luminescent Imidazo[1,5‐a]pyridine Scaffold: Synthetic Heterocyclization Strategies‐Overview and Promising Applications

“…[76] Typically these catalysts require further reagents or co-catalysts such as Lewis acids (BF 3 ), dehydrating agents (e. g. molecular sieves [63] or propane phosphoric acid anhydride [42] ) or oxidant agents (e. g. oxygen, iodine, [72] selenium dioxide, [51] elemental sulphur [76] or tert-butylperoxybenzoate). [75] These problems hindered the practical application of these products and their corresponding transformation or functionalization. Therefore, the development of new strategies under metal-and oxidant-free conditions from readily available materials remains a great challenge to design new luminescent products.…”

“…In the same year a new and different method for the synthesis of impy was reported (last reaction in Scheme 11). [75] This synthetic approach between ethyl ( E )‐2‐(benzylidene amino)acetate and pyridine employed I 2 as the non‐metal catalyst and tert ‐butylperoxybenzoate as the oxidant. This reaction represents an unprecedented 1,3‐dipolar cycloaddition in a highly straightforward process with great value.…”

“…The general synthetic routes involve different catalysts such as CuCl 2 , [55–57] copper/iodine, [70] boron trifluoride etherate, [73] CBr 4 [43] or MnO 2 [76] . Typically these catalysts require further reagents or co‐catalysts such as Lewis acids (BF 3 ), dehydrating agents (e. g. molecular sieves [63] or propane phosphoric acid anhydride [42] ) or oxidant agents (e. g. oxygen, iodine, [72] selenium dioxide, [51] elemental sulphur [76] or tert ‐butylperoxybenzoate) [75] …”

Recent Developments in the Synthesis of Nitrogen-Containing Heterocycles through C–H/N–H Bond Functionalizations and Oxidative Cyclization

“…In 2018, we synthesized ethyl 3-phenylimidazo[1,5a]pyridine-1-carboxylates by the oxidative cyclization of ethyl (E)-2-(benzylideneamino)acetates with pyridines by using I 2 as a catalyst and tert-butyl peroxybenzoate (TBPB) as an oxidant (Scheme 21). 48 This reaction is an extension of that described in Scheme 4, permitting the syntheses of various heterocycles with complementary diversification.…”

Recent synthetic methodologies for imidazo[1,5-a]pyridines and related heterocycles

“…Qin et al 45 reported a straightforward method for synthesizing imidazo[1,5- a ]pyridines 120 from ethyl ( E )-2-(benzylideneamino)acetates 118 and pyridines 119 via 1,3-dipolar cycloaddition. Specifically, I 2 was used as a nonmetal catalyst and tert -butyl peroxybenzoate (TBPB) was used as an oxidant to facilitate the synthesis without requiring metal catalysts.…”