Investigation On Nitrogen Dioxide-Catalyzed Aerobic Oxidation Of Benzyl Alcohols And Benzylic Carbon-Hydrogen Bonds

The selective oxidation of alcohols and benzylic compounds are some of the most effective methods for producing aromatic carbonyl compounds.In the above-mentioned reactions,chromium trioxide,permanganate,peroxic acid and other compounds have been usually considered to be used as the oxidant.Compared with these oxidizing reagents,the oxygen?especially air?is more environment-friendly and inexpensive,and is a more attractive alternative from the practical viewpoint.However,the molecular oxygen is less reactive towards most organic molecules,thus the oxidation with molecular oxygen usually depends on the catalysis of the transition metals including palladium,copper,iron and so on.Unluckily,many transition metals have a limited stock in the earth,and their use will not only accelerate the consumption of non renewable resources,but also lead to the heavy metal pollution.So the development of nonmetallic catalysts have attracted considerable attention in the chemical and chemical industry.On the other hand,the situation of air pollution in China is severe.Among all kinds of pollutants,NOx exhaust gas has attracted much attention due to its toxicity,and their emissions would result in a significative harm to the environment,as well as the waste of resources.With more and more NOx waste gas being recovered,it is urgent to find more applications for NOx.Aiming at this problem,the aerobic oxidation of alcohols and benzylic compounds have been carried out with nitrogen dioxide?main components of nitrogen oxide waste gas?as the nonmetallic catalyst.The main research contents and conclusion are as follows.?1?Nitrogen dioxide was used as the catalyst to perform the oxidation of benzyl alcohols with oxygen as oxidizing reagents,which estabilished an effective method for the synthesis of various aldehydes or ketones.Effects of catalyst loading,temperature,solvent and time on the yields were investigated using benzyl alcohol as the substrate,and the results showed that the optimal reaction conditions were as follows:9.2 mol%NO₂,acetonitrile as the solvent,140^oC,5 h,1 atm air.Effects of various substituents on the oxidation of benzyl alcohols were investigated.The electron-withdrawing groups bonded to the benzene ring were favorable for the formation of target products,while the electron-donating substituents hindered the reaction and would result in the formation of some nitrifying by-products.In addition,the reactions of naphthalenemethanols,pyridinemethanol,hydroxymethyl furans were investigated.Naphthalene methanols were effectively converted to naphthalene formaldehyde,but pyridinemethanols underwent this transformation in lower yields.The present method could be applied into the oxidation of the secondary benzyl alcohols to give ketones.The reaction of?-carbonyl alcohol and?-acetylene benzyl alcohol could also be effectively carried out,providing a new way for the synthesis of double carbonyl compounds and acetylene ketones.The reaction mechanism was preliminarily investigated,and the results showed that the present reaction possibly proceeds through a radical pathway.Firstly,nitrogen dioxide reacts with a benzyl carbon-hydrogen bond in benzyl alcohol to give the benzyl carbon free radical.Then the carbon free radical is converted into carbonyl groups through two possible pathways:one is the reaction between the radical and oxygen,and another pathway involves the thermodynamic decomposition of nitrite intermediates to give the carbonyl.?2?Aerobic oxidation of benzyl carbon-hydrogen bonds was realized with nitrogen dioxide as the catalyst and oxygen as the oxidant,which provided an effective method for the synthesis of many kinds of ketones or esters.No any other additives except nitrogen dioxide was used.Moreover,nitrogen dioxide is a low boiling point compound,which is easily separated from the reaction system in theory.Therefore,th e present method is a potential alternative for industrial application.The oxidation of xanthene as the model reaction was used to investigate the effect of catalyst loading amount,temperature,solvent and time on the yields,and the results showed that the optimal reaction conditions were as follows:140^oC,acetonitrile,5 h,18.4 mol%NO₂,1 atm oxygen.The reaction of benzyl carbon-hydrogen bonds in a series of compounds were investigated.Among them,xanthenes and isochromans smoothly underwent the aerobic oxidation reaction,and anoxanthone or isochromanone products were obtained.These carbonyl compounds have certain applications in medicine,and have anti-tumor,antihypertensive and anti-thrombotic virtues.Effects of various substituents on the oxidation of benzyl alcohols were investigated.The electron-withdrawing groups bonded to the benzene ring were favorable for the formation of target products,while the electron-donating substituents hindered the reaction.The carbon-hydrogen bond in diphenyl methane and benzyl methyl ether could hardly undergo aerobic oxidation,which revealed that the present method seemed to be only suitable for the ring substrates.The reaction of fluorene compounds was also investigated,but both the conversion of substrates and the product yields were not high.The reaction mechanism was preliminarily investigated,and the results showed that the present reaction possibly proceeds through a radical pathway.Firstly,nitrogen dioxide abstracts a hydrogen atom from the benzyl position in isochroman,and gives benzyl free radical.At the same time,the nitrogen dioxide is converted to HNO₂.HNO₂ reacts with oxygen to give nitrogen dioxide that can continue to catalyze the aerobic oxidation.Subsequently,the carbon free radical is converted into carbonyl groups.Another pathway involves the thermodynamic decomposition of nitrite intermediates to give the carbonyl.?3?In conclusion,the aerobic oxidation of benzyl alcohols and benzyl compounds has been carried out with nitrogen dioxide?the main component of nitrogen oxide waste gas?as the catalyst,which not only promotes the utilization of nitrogen oxide waste gas,but also establishes a new method for the industrial production of a series of aldehyde or ketone compounds.