Alkali α-MnO2/NaxMnO2 collaboratively catalyzed ammoxidation–Pinner tandem reaction of aldehydes

Abstract

The tandem reaction is a growing field to yield important advances toward green and sustainable chemistry. Herein, we report a bifunctional manganese oxide catalyst with an interface binding redox phase (α-MnO₂) and a basic phase (Na_xMnO₂). The molar ratio of NaOH/Mn plays a great role in the formation of α-MnO₂/Na_xMnO₂. The sodium cation is essential for the formation of a basic Na_xMnO₂ phase while the potassium cation promotes the formation of a redox-active α-MnO₂ phase. The interface structure of α-MnO₂/Na_xMnO₂ geometrically favors the ammoxidation–Pinner tandem reaction to synthesize imidates in a 58–96% yield from aldehydes. Thus a phase collaborative effect is observed. In the ammoxidation process, the redox cycle of Mn^IV/Mn^III is involved and the lattice oxygen in the α-MnO₂ phase acts as an active oxygen species. The O–H in methanol is activated and dissociated on the basic sites of Na_xMnO₂ to the adsorbed methoxyl species to facilitate the Pinner synthesis. This approach bypasses the conventional synthesis of imidates, which suffer from harsh reaction conditions and the requirement for multiple steps.