The Selective Oxidation Of Primary Aromatic Alcohols Containing N-heterocycles With Molecular Oxygen Catalyzed By Vanadium Catalysts

Aromatic aldehyde with N-containing heterocycles find wide applications as intermed-iates in various fields of pharmaceuticals,agricultural chemicals and life science.Selective oxidation of heterocyclic alcohols is the important approach to produce their corresponding aldehydes.Traditionally,stoichiometric inorganic oxidants such as,potassium permanganate and chromium?VI?reagents are often employed both at the laboratory and industrial levels,which deliver considerable amounts of deleterious waste.Recently,a number of catalytic systems for oxidation of alcohols with molecular oxygen have been reported.But the great complexing actions between N-heterocycles and metal catalysts could led to catalyst deactivation.Therefore,it remain a great challenge for catalytic aerobic oxidation of the hetero-aromatic alcohols under mild reactions.In this paper,we carried out the aerobic oxidation of 2-pyridinemethanol as a model reaction.Study on the activation effects of vanadium oxides-based compounds on the oxidation of alcohols containing N-heterocyclic substituents.Research shows that 2-pyridinemethanol could be oxidized into their corresponding aldehyde with molecular oxygen catalyzed by vanadyl sulfate under mild conditions.In contrast to general expectation,the benzyl alcohol that was much easier to be oxidized theoretically could not afford benzaldehyde smoothly under the same reaction conditions.For example,when 2-pyridinemethanol was converted to2-pyridinecarboxaldehyde with 99%conversion within 4h at 80°C,0.2 MPa O₂,the oxidation of benzyl alcohol did not occur.The competitive experiment of 2-pyridinemethanol and benzyl alcohol and the additive experiment of heterocyclic compounds shows that the presence of N heteroatom of 2-pyridinemethanol could activate vanadium catalyst.The study of in situ FT-IR and UV–vis spectroscopy established the N heteroatom in situ formation of vanadium complexes not only merely inhibited by the coordination,but also activated the catalysts.Overall,this study provided a new idea for the catalytic conversion of heterocyclic compounds.The aerobic oxidation of primary N-heterocyclic substrates could be converted to corresponding aldehydes with high conversions catalyzed by VOC2O4/Zn?NO3?2 catalytic system.The effects of reaction time,temperature,pressure,solvent and catalysts were investingated.The best reaction conditions were selected through it.pyridinemethanol was smoothly oxidized to pyridinecarboxaldehyde with 99%conversion within 4 h at 80°C,0.5MPa O₂.At the same time,experiment results proved that the VOC₂O₄/Zn?NO₃?₂ catalytic system may be more suitable for the aerobic oxidation of alcohols containing N-heterocyclic substituents.Moreover,the present work established a mechanistic pathway about the aerobic oxidation of alcohols.