Hydrazonyl-Quinone-Catalyzed Alcohol Dehydrogenation and One-Pot Synthesis of Azaheterocycles

The study introduces a metal-free, economical, and eco-friendly organo-catalyst featuring a hydrazonyl-quinone moiety, which can be conveniently synthesized in a one-step reaction from phenanthrenequinone and benzothiazole-hydrazine. The robustness of the catalyst is attributed to extensive π-delocalization, as ascertained through x-ray diffractometric and theoretical studies. Electrochemical analysis revealed two irreversible reductive responses at -0.68 and -1.30 V versus Ag/AgCl (cathodic peaks) in acetonitrile. Computational studies suggest that these reduction processes are primarily localized over the imino-quinone moiety. The catalyst has capable of trapping electrons in imino-quinone skeleton, facilitating redox reactions. This cost-effective organic catalyst efficiently mediates alcohol dehydrogenation and enables one-pot stepwise synthesis (OPSS) of diverse substituted triazines and pyrimidines in excellent yields under aerobic conditions, with favorable turnover numbers (TON) and turnover frequencies (TOF). The mechanism of catalysis has been proposed based on spectroscopic and control experiments and has been found to proceed via the formation of an imino-quinone radical, followed by hydrogen atom transfer (HAT) to form the aminoquinol intermediate. Due to its simple operation and workup without the need for metals, along with the cost-efficiency of the starting materials, this method offers a compelling alternative to existing synthetic approaches for substituted triazines and pyrimidines.