New Approach for Controllable Synthesis of N-MnOx Microflowers and Their Superior Catalytic Performance for Benzoxazole Synthesis
Condensation of 2-amino-phenols with carboxylic acid derivatives has been practically used for the production of benzoxazoles, wherein the homogeneous catalytic system including strong acids and dangerous peroxides is generally employed. Herein, we report the novel approach for controlled synthesis of nitrogen-doped MnOx (denoted as N-MnO2, N-Mn5O8, and N-Mn3O4), by adjusting the heteroatom nitrogen amount, and uncover that the N-MnO2 catalyst as a sustainable and cost-effective heterogeneous catalyst exhibits high catalytic performance for condensation of 2-amino-phenols and o-phenylenediamine with alcohols into the corresponding benzoxazoles and benzimidazole, respectively. The N-MnO2 catalyst displays >99.9% yield for benzoxazole synthesis under room temperature and no decay (10 cycles), compared with the neglect activity (∼0%) for MnO2 catalysts. X-ray absorption spectroscopies and experimental studies uncovered that oxygen vacancies generated by heteroatom N doping play a key role for promoting intramolecular oxidative dehydrogenation of alcohol and 2-aminophenol derivatives to directly yield desired products. N-MnO2 catalyst rapidly oxidative dehydrogenated the reactants into corresponding benzoxazoles for 10 examples with >87.6% yields.
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