Studies On The Mechanism Of Nhc-carbene Catalyzed Areobic Oxidation Of Aldehyde And Rh(?)-catalyzed Annulation Reaction

The oxidation reaction of aldehydes catalyzed by nitrogen heterocyclic carbene in air atmosphere is theoretically a kind of oxidation method with the advantages of green chemistry and sustainable development.The catalyst is an organic compound,which avoids the use of heavy metals such as palladium,ruthenium,and osmium;it uses air oxidation;the reaction is usually carried out at room temperature,which is very mild and has high functional group compatibility.However,in actual research,it was found that despite the above-mentioned advantages,the practical applicability of preparing corresponding carboxylic acids from aldehydes by this method is very low.First,the reaction rate is usually low.At a catalyst concentration of 5 mol%,the reaction is usually takes more than 24 hours to complete the conversion;at the same time,the abnormality is that the reaction rate is inversely proportional to the substrate concentration,that is,the higher the concentration,the lower the reaction rate,so the reaction is usually carried out at a concentration of 0.1 M,and the solvents used are mostly environmentally unfriendly DMSO and DMF,these defects greatly limit their application in organic synthesis.On the other hand,although the reaction has been reported in many studies,the mechanism of the reaction is still largely unclear.In this subject,a green and efficient catalytic mode has been developed to quickly convert aldehydes into carboxylic acids in aqueous solutions.And through systematic reaction kinetics research,it is the first time to observe a self-accelerating reaction kinetic curve similar to that in free radical polymerization.This experimental evidence proves that at least one key free radical intermediate is contained in the catalytic cycle and that the reaction is catalyzed in the form of free radicals.At the same time,according to the kinetic research results,a series of polymeric nitrogen heterocyclic carbene catalysts were designed and prepared,which achieved very efficient aldehyde catalytic oxidation reaction,by recycling the polymerized nitrogen heterocyclic carbene catalyst,the cyclic catalysis of the aldehyde oxidation reaction is realized.The C-H bond functionalization reaction catalyzed by Rh(?)has the advantages of mild reaction conditions,good functional group tolerance,high reaction efficiency and regioselectivity.In recent years,this synthetic strategy has received extensive attention and has been applied to synthesize various heterocycles,such as indole,isoquinoline,isoquinolone,pyrrole,pyridine and polyheterocycles.These heterocyclic compounds are important components in biomolecules,natural products and pesticides.Although Rh(?)catalyzes the cycloaddition reaction of aromatic imines and alkynes,considerable progress has been made,but from the perspective of synthesis and application,or from the perspective of the cognition of the reaction mechanism,this research field is still in its infancy.At present,a large number of methodological researches still belong to the design orientation of the substrate/electrophile,that is,the reactivity and selectivity are largely derived from the design of the substrate.Therefore,through more extensive methodological research,it provides important theoretical guidance value for the realization of product-oriented synthetic application research in the future.In the early stage of our research group,reported on[Cp*Rh Cl₂]₂to catalyze the[3+2]cycloaddition reaction of aromatic imines with maleimides and alkynes.Through in-depth research,we found that on the same type of substrate,because of the subtle changes in the structure of alkynes,the cycloaddition methods are different.Through further research,we expanded the original redox conservation[3+2]cycloaddition reaction into a new type of[4+2]cycloaddition reaction strategy in which oxygen participates in the oxidation of the Rh-Cu bimetal catalytic cycle,and obtained polycyclic aza compounds.Under the condition of isolating oxygen,the original[4+2]cycloaddition reaction can still be carried out in the manner of[3+2]cycloaddition,which expands the controllability of the method and the potential for synthetic applications.At the same time,through the combination of experiment and theoretical calculation,the mechanism of the reaction has been studied in depth.