Synthesis Of Imines And Quinoline Over Manganese Oxide Catalysts Through Green Catalytic Oxidation

Imines and quinoline are common intermediates for the synthesis of pharmaceutically and biologically active compounds,and fine chemicals.Traditionally,imines are synthesized by the condensation of amines with carbonyl compounds.This process especially requires unstable aldehydes,and dehydrating agents as well as Lewis acid catalysts in many situations.From the perspective of green chemistry,the scale application of the method is subject to many restrictions.During the past decade,three new synthetic approaches had drawn more attention to?1?the oxidative cross-coupling of alcohols with amines,?2?the oxidative self-coupling of primary amines,?3?the oxidative dehydrogenation of secondary amines.Aromatic nitrogen-containing heterocyclic compounds,such as quinoline,possess-?R¹?C=N-CH?R²?-?R¹,R²=-H,-alkyl?structure similar to imines.Therefore,they can also be obtained by oxidative dehydrogenation of saturated nitrogen-containing heterocyclic compounds similar to secondary amines such as 1,2,3,4-tetrahydroquinoline.Among the many oxidants,air is economical,green,and sustainable,and has been widely concerned in recent years.Among three synthetic routes,the development of the efficient catalysts is the key technology for the entire routes when air is used as the oxidant.Among solid catalysts,manganese oxides,with low price,low toxicity,adjustable redox,and various crystal structures,have wide applications in the field of catalytic oxidation.In this thesis,manganese oxides with different structures were synthesized by oxalate route,and applied to the synthesis of imines and quinoline.The microstructures and physicochemical properties of the catalysts were investigated by TG-DSC,XRD,N₂ sorption,SEM,TEM,XPS,H₂-TPR,ICP-OES,and AAS technologies.Meanwhile,the structure-activity relationships were investigated in the aerobic oxidation of imines and quinoline.?1?The oxidative cross-coupling of alcohols with amines catalyzed by manganese oxides calcined at low temperatureIn order to overcome the lack of activity of traditional solid catalysts in cross-oxidative coupling of alcohols and amines,four different crystal forms of manganese oxide,such as amorphous manganese oxide?AMO?,Mn₅O₈,Mn₃O₄,MnO₂,were first synthesized at low temperature?the narrow temperature range from 350?to 400??by controlling the calcination conditions of manganese oxalate.It was investigated that four kinds of manganese oxide were employed as aerobic oxidation coupling catalysts,and found that AMO was the most effective catalyst using air as an oxidant.The conversion of benzyl alcohol and the selectivity of imine reached as high as 100%and97.1%respectively at 80?,in toluene for 1 h.And it had the excellent adaptability to different substrates?26 substrates?.By investigating structure-activity relationship,it was found that amorphous structure,highest surface area,higher(Mn³⁺+Mn⁴⁺)/Mn²⁺ratio,and easy reducibility were the key factors for achieving high activity.Mn³⁺and Mn⁴⁺in manganese oxide surfaces might be catalytic active sites for alcohols oxidation,and the catalytic activity of Mn⁴⁺was the higher than that of Mn³⁺.?2?The oxidative self-coupling of primary amines catalyzed by manganese oxides calcined under high temperatureIn view of the low selectivity of the traditional catalysts in the oxidation self-coupling of primary amines,different structures of Mn₂O₃ were successfully prepared by thermal decomposition of manganese oxalate at high temperature.It was found that Mn₂O₃ obtained at 450?showed higher catalytic activity and higher selectivity in the oxidation self-coupling of primary amines.Using air as oxidant,it gave 100%conversion of benzylamine,96.2%selectivity of imine at 110?,toluene as solvent for 2 h,and it also had good substrate adaptability.The imine selectivity of Mn₂O₃calcined at high temperature was much higher than that of amorphous manganese oxide calcined at low temperature.By investigating structure-activity relationship,It was found that Mn³⁺and Mn⁴⁺in catalyst surfaces were the catalytic active sites of primary amine oxidation,but higher content of Mn⁴⁺was easy to cause excessive oxidation of primary amine to nitrile.The Mn³⁺/Mn⁴⁺ratio was positively correlated with the selectivity of the imine.In addition,surface area,reductibility,content of surface water,etc.,were also important factors to influence the catalytic activity and the selectivity of imine.?3?The oxidative dehydrogenation of 1,2,3,4-tetrahydroquinoline catalyzed by doped manganese oxidesThe saturated nitrogen-containing heterocyclic compounds similar to secondary amines,such as 1,2,3,4-tetrahydroquinoline,are relatively difficult to undergo oxidative dehydrogenation due to large steric hindrance,easy overoxidation.CuO-doped MnO_x catalysts?MnO_x,Cu1-MnO_x,Cu2-MnO_x,Cu3-MnO_x,Cu4-MnO_x,Cu2-450 and Cu2-550?were prepared by oxalate route,and applied to the oxidative dehydrogenation aromatization of 1,2,3,4-tetrahydroquinoline.A small amount of CuO greatly improved the catalytic performance of MnO_x.Cu2-MnO_x showed the 99.1%conversion of1,2,3,4-tetrahydroquinoline,the 97.2%selectivity of quinoline.The doping of CuO increased content of surface Mn³⁺,content of surface adsorbed oxygen and lattice oxygen mobility in amorphous manganese oxide,which were the key factors for the enhancing of the catalytic performance.Mn³⁺acting as the key catalytic active site played an important role in the oxidative dehydrogenation of 1,2,3,4-tetrahydroq-uinoline.Mn⁴⁺was also the catalytic active site,but higher content of Mn⁴⁺easily caused excessive oxidation of quinoline.In addition,it was found that the content of adsorbed oxygen was positively correlated with the catalytic activity.The results showed the absence of precious metals,the simple catalyst preparation process,the cheap air as the sole oxidant,no alkali and ligand,the mild reaction conditions,higher catalytic efficiency,along with catalyst reusability and easy isolation of the products made our catalytic protocol green and environmentally benign.These explorations had important referential significance for the green synthesis of imines and quinoline.