Imidazo[1,5-a]pyridines represent an important class of heterocyclic compounds because of their largely potential application in photophysical and biological areas. These substances have been widely investigated in the context of organic light-emitting diodes (OLED) and organic thinlayer field effect transistors (FET). Furthermore, they have been explored in a wide range of potential pharmaceutical applications, including HIV-protease inhibitors, and Thromboxane A2 synthesis inhibitors. However, literature survey found that only some strategies for the synthesis of imidazo[1,5-a]pyridine derivatives were reported. Traditionally, substituted imidazo[1,5-a]pyridines were accessed via Vilsmeier-type cyclizations of N-2-pyridylmethyl amides. Recently, Murai's group reported several methods for the construction of imidazo-[1,5-a]pyridine rings. Although these methods provided useful access to substituted imidazo[1,5-a]pyridine derivatives, they suffered from some drawbacks, such as the use of strong acid and high temperature, moreover, study on the preparation of 3-arylated imidazo[1,5-a]pyridines and 1,3-diarylated imidazo[1,5-a]pyridines in a procedure is rare. Therefore, developing more efficient and practical protocols for the preparation of 3-arylated imidazo[1,5-a]pyridines and 1,3-diarylated imidazo[1,5-a]pyridines still are highly desirable. As part of a program targeting new methods for constructing some useful heterocycles probably found application in pharmaceutical and industry areas. We chose pyridin-2-ylmethanamine and aldehydes as the reaction substrates to achieve the synthesis of imidazo[1,5-a]pyridines in the presence of iodine and TBHP. Moreover, α-substituted pyridin-2-ylmethanamine used as substrate reacted with aldehydes to also provide 1,3-diarylated imidazo[1,5-a]pyridines. Therefore, 3-aryl imidazo[1,5-a]pyridines and 1,3-diaryl imidazo[1,5-a]pyridines can be obtained through this method. Herein, we report a simple and efficient iodine-catalyzed synthesis of 3-substituted and 1,3-disubstituted imidazo[1,5-a]pyridines from easily available aldehydes under mild conditions. The optimization of reaction conditions showed that N,N-dimethylformamide was the only effective solvent and 70 ℃ was the proper reaction temperature. The investigation on the scope of suitable substrates showed that the electronic effect of substituent on the aldehydes had a major impact on the success of the process. Aldehydes bearing electron-deficient substituents proven more favorable for the reaction, while electron-rich groups didn't offer good reaction result despite of prolonging the reaction time.

Key words: iodine catalyze, oxidation, tandem cyclization, heterocycles syntheses, imidazo[1,5-a]pyridines