In this paper, a metal-free catalytic method for synthesis of 2-picolinamide derivatives is reported. Under the promotion of proton acid, simple pyridines react with isocyanides to provide 2-picolinamides by means of the aminocarbonylation of the aryl C-H bond in the C2 position of pyridines. The product formation involves in the electrophilic addition of isocyanide to pyridine ring, hydrolysis and the oxidative aromatization regenerating pyridine ring in the presence of Di-t-butyl peroxide (DTBP) and oxalic acid dihydrate. Control experiments in the optimization section disclose the fact that the proton acid and oxidant are both indispensable for this C-H bond aminocarbonylation reaction. Generally, the synthetic reactions run smoothly under air atmosphere by heating all the substrates and reagents in one-pot at 100℃. The pyridine substrates containing methyl, t-butyl, cyclic dialkyl, methoxyl, halogen substituents at different site of the pyridine ring have displayed fine tolerance to the synthesis of corresponding products with diverse substructures in the pyridine ring. On the other hand, both alkyl and aryl functionalized isocyanides have also been found applicable to this synthetic protocol to provide 2-picolinamides containing correspondingly various N-alkyl and N-aryl fragment. The primary results indicate that the stability of the isocyanide substrate evidently influence the reaction result. The reactions employing relatively more stable 2,6-dimethylphenyl isocyanide give corresponding products with higher yield than those ones using other isocyanides. Comparing with those reported methods employing transition metal catalyst such as silver or palladium salt to activate the C2-H bond in pyridines for the synthesis of analogous products, the present method benefits from the distinctive features of totally metal-free catalysis, broad substrate tolerance, specific regioselectivity in transforming C2-H bond, and high atom economy. Therefore, such a synthetic method will reasonably be a practical approach in complementing those already known strategies for the synthesis of structurally diverse and useful 2-picolinamide scaffolds.
王昱赟, 刘云云. 无金属催化的吡啶C2位碳-氢键胺甲酰化反应合成吡啶甲酰胺[J]. 化学学报, 2019, 77(5): 418-421.
Wang Yuyun, Liu Yunyun. Metal-Free C2-H Aminocarbonylation of Pyridines for the Synthesis of Picolinamides. Acta Chim. Sinica, 2019, 77(5): 418-421.