Investigation On Nitrogen Oxides-Catalyzed Aerobic Oxidation Of Benzylic Carbon-Hydrogen Bonds

C-H bonds widely exist in organic compounds and their conversion provides a broad pathway for the synthesis of organic compounds.Moreover,only one hydrogen atom in the C-H bond is wasted after it is functionalized,so this kind of reaction has a characteristic of high atomic economy and plays an important role in organic chemistry.In recent years,chemists have already paid extensive attention to the selective oxidation of C-H bonds,where various oxidants such as hydrogen peroxide,t-butyl hydroperoxide,sodium hypochlorite,potassium permanganate,chromium trioxide and so on are commonly used.Compared with the above-mentioned oxidants,oxygen is more environment-friendly and easy to be obtained because it is non-toxic,rich in the nature,and the oxidative product is water,which is less pollution to the environment.But,the molecular oxygen is less reactive towards most organic molecules because it exists in the inactive triplet ground state.So it is usually depends on the catalysis transition metals such as copper,iron,ruthenium,manganese and so on.But the amount of transition metals on the earth are limited and non-renewable.Non-metallic catalysts such as 2,2,6,6-tetramethylpiperidine oxynitride and hydrogen bromide/sodium nitrite have been widely favored in recent years.Based on these current conditions,we studied the conversion of benzyl alcohol compounds to aromatic nitrile and the conversion of benzyl ethers to benzaldehydes by using nitrogen oxides as catalysts.The main research contents and conclusions are as follows:?1?The conversion of benzyl alcohol to benzonitrile was achieved by using nitric acid/2,2,6,6-tetramethylpiperidine-1-oxyl?TEMPO?as the catalyst system,air as the oxidant and ammonium acetate as the nitrogen source.We optimized the critical parameters including the catalyst,the dosage of nitric acid,temperature,nitrogen source,reaction time and other conditions,the results showed that the optimal conditions for the reaction were as follows:30 mol%nitric acid,30 mol%TEMPO,3equiv.ammonium acetate,and glacial acetic acid as solvent,reaction temperature is50°C,1 atm air,reaction time is 12 h.We set out to evaluate the scope of this strategy under the optimal reaction conditions.The results showed that the electronic nature of the substituents had less effect on the reaction,because either the electron-withdrawing substituents or the benzyl alcohol with electron-donating groups can be smoothly reacted.The yield of nitrile products was between 60%and 90%.This method can also be applied to substrates with larger steric hindrance,such as the reaction of 2,4-dichlorobenzyl alcohol and 2,6-dichlorobenzyl alcohol,the yields of the nitrile products being 85%and 75%,respectively.Naphthalene methanol can also react efficiently for the 2-naphthonitrile was obtained in 74%yield.The furfuryl alcohol and 2-thiophenemethanol also react smoothly when the amount of nitric acid was increased to 60%and the amount of TEMPO was decreased to 20%.Pyridine methanol and fatty alcohol cannot be converted to nitrile products.Preliminary mechanistic investigation suggests that the present transformation is a stepwise reaction.The true catalytic species of the reaction is NO_x/TEMPO⁺.Under the action of these catalytic species,the benzyl C-H bond in benzyl alcohol reacts with oxygen to undergo oxidative dehydrogenation to form an aldehyde.The aldehyde product is reacted with[NH₄]in ammonium acetate to form the imine.The imine is oxidatively dehydrogenated under the catalysis of NO_x/TEMPO⁺to obtain a benzonitrile target product.?2?By using nitric acid as the catalyst,oxygen as the oxidant,and hexafluoroisopropanol as the solvent,the oxidation of the C-H bond in the benzyl methyl ether compound was achieved and a benzaldehyde compound was obtained.Optimization of the reaction solvent showed that the use of hexafluoroisopropanol was the key to the efficient reaction,which allowed the reaction to proceed at room temperature.After a series of optimizations the conditions of nitrogen oxides,temperature,time,reaction atmosphere and nitric acid dosage.The results showed that the optimal conditions for the reaction were:20 mol%nitric acid,hexafluoroisopropanol as solvent,1 atm oxygen,reaction time is 30 h.We set out to evaluate the scope of this strategy under the optimal reaction conditions.The results showed that when a methyl,tert-butyl,fluorine,chlorine or bromine group was attached to the benzene,the reaction proceeds well,and the yield of the target product obtained was from 66%to 90%.When a benzyl ether substituted with a methoxy group or a cyano group was used as a substrate,only a small amount of the target production was obtained.Furthermore,benzyl 1-dodecyl ether and Benzyl cyclohexyl ether were also efficiently reacted to obtain aldehyde products in 90%and94%yields,respectively.