| Free N-heterocyclic carbenes were first isolated in 1991.Since the separation of carbenes by arduengo and his colleagues,N-heterocyclic carbenes have developed rapidly in the field of organic chemistry in the past two decades,especially the metal complexes bonded by carbenes and metals.N-heterocyclic carbene is a strong σ-donor with similar reactivity to traditional phosphine.Therefore,N-heterocyclic carbene is often regarded as the substitution and extension of organic phosphine.N-heterocyclic carbenes have strong reaction activity.In recent years,the direct use of N-heterocyclic carbenes as catalysts has been greatly developed,especially when there is no metal complex,it also has strong catalytic activity,relatively low price,green and mild reaction system,and even can be purchased directly in the market,which makes it widely concerned in various fields.In this paper,a series of aldehydes were used as starting materials,N-heterocyclic carbenes(mainly imidazolium salts)were used as catalyst,oxygen concentration in air was used as oxidant,and alkali was added into solvent under appropriate temperature.In order to find the optimal reaction conditions,a series of screening studies were carried out in this paper,including the type of catalyst N-heterocyclic carbene,the optimal reaction temperature,the type and concentration of reaction base,reaction time,the type and concentration of reaction substrate.After screening a series of reaction conditions,it was surprised to find that under a wide range of substrate aldehydes,the optimal catalyst was 1,3-dicyclohexyl imidazole chloride,when the reaction temperature is set at 80 ℃,the conversion can be maximized.After that,the effects of substrate concentration and catalyst concentration on the reaction rate were studied to explore the change of reaction rate under different concentration conditions and fit the reaction kinetics curve.The method introduced in this paper is a new method for the oxidation of aldehydes to carboxylic acids catalyzed by N-heterocyclic carbene.This method is simple,green and mild,and avoids the use of high cost and toxic organic solvents.The corresponding carboxylic acids are obtained from a wide range of aldehydes by using non-toxic deionized water or tap water and air as oxidant under high substrate concentration. |
