Study On The Oxidation Couplong Reaction Of Primary Amines To Imines Catalyzed By Cobalt Catalysts

Imine,also known as Schiff base,widely exists in the molecular structure of natural products,bioactive compounds and drugs.The C=N double bond in imine has high reactivity and can be used as a nitrogen source in different types of reactions.Therefore,it has important applications in biology,medicine,dyes and materials.There are many preparation methods for imines,among which the condensation of alcohol and amine,primary amine coupling and secondary amine dehydrogenation have attracted much attention and made some progress in recent years because of the easy availability of raw materials and high atomic economy.However,most of the catalytic systems still have the use of environmentally unfriendly,expensive noble metal catalysts and many kinds of cocatalysts,so it is important to find cheap,environmentally friendly and efficient catalyst systems to prepare imines.In this paper,the feasibility of the oxidation coupling reaction of primary amines to imines catalyzed by cobalt catalysts has been explored:Using benzylamine as models,the catalytic effects of different kinds of catalysts were explored,including nickel catalysts(nickel chloride and nickel dichloride),iron catalysts(ferrous chloride and ferric chloride),titanium catalysts(titanium dichloride and titanium dioxide)and cobalt catalysts(cobalt chloride six hydrate)for the reaction of benzylamine with toluene as solvent and oxygen as oxidant.The results showed that the cobalt catalyst has the best catalytic effect.Then we compared the catalytic effects of several cobalt catalysts,such as cobalt monoxide,cobalt chloride hexahydrate,cobalt acetate tetrahydrate and tricobalt tetraoxide,It is found that the cobalt chloride hexahydrate is the best catalyst.Using cobalt chloride hexahydrate as catalyst to obtain better reaction conditions: when benzylamine was used as substrate,5 mol% cobalt chloride hexahydrate was used as catalyst,toluene with water content less than 50 ppm was used as solvent,in oxygen atmosphere of 110 ?for 4 hours,and the yield of imine was detected as 99% by GC-MS.Under thiscondition,the substrate scope of application of this catalytic method was investigated.Except for aliphatic amines,the other amines can get the target imines(58%-99%)in a medium to high yield.The type and position of substituents on the aromatic ring have great influence on the yield of target imine.For different types of substituents,the yield of the substrate for electron substitution is much higher than that for electron absorption.The yield of the target imine follows the rule of para > meta > ortho.For five membered heterocyclic amines(thiophene or furan),the yield of the target product was 92-93%.On the basis of catalytic synthesis of imines from benzylamine,using o-phenylenediamine and benzylamine as models,the best reaction conditions for the synthesis of benzimidazoles are as follows: 5 mol% cobalt chloride hexahydrate was used as catalyst,toluene with water content less than 50 ppm was used as solvent,in oxygen atmosphere of 110 ? for 24 hours.The yield of the target product is 13-72%,and different types of amine substrates are expanded.It was found that the yield of imidazole was greatly affected by the type and position of substituents on aromatic ring for different substituents.When the substituents were ortho substituted,the yield was 32% lower.For the same substituent in different position,the yield of the target product accords with the rule of para > meta > ortho.For the reaction of o-phenylenediamine with five membered heterocyclic amines,the yield of the target product is 64-66%.The "green" catalytic method provides a reference for the further application of primary amine coupling reaction.