Aerobic Oxidation Of Benzylic C?H Bonds Via Oxynitride Species-Catalysis Or Auto-Oxidation

With the development of sustainable and green chemistry,the oxidative functionalization of C-H bonds is becoming more and more important because the C-H bonds widely exist in organic compounds.Moreover,only one hydrogen atom is wasted in the functionalization of the C-H bonds,which is consistent with the criterion of green chemistry.On the other hand,oxygen is a more ideal oxidant since not only it is inexpensive and readily available,but also the by-product of oxidation is water that has little pollution to the environment.Thus,it is of great significance to the development of green chemistry to use oxygen as the oxidant for the oxidation of the C-H bonds.In the domain of the aerobic oxidation of benzyl C-H bonds,the transition metal-catalyzed,non-transition metal-catalyzed,photo-induced and aerobic auto-oxidated strategies have been utilized to perform the reactions.Compared with the transition metal-catalyzed methods,the non-transition metal-catalyzed methods receive less attention,and the latter often requires an use of sophisticated multi-component catalyst systems,which is disadvantageous to their application.Aiming at this challenge,our research object has been set to develop various simple and effective catalysts,and to clarify the mechanisms regarding with the aerobic oxidation of benzyl C-H bonds.The main research contents and conclusion are as follows.?1?The catalysis of oxynitride species in the aerobic oxidation of benzylic C-H bonds was investigated,and it was herein found that HNO₃ had the capacity to catalyze this kind of reaction.After effects of various conditions on the reactions were examined,the optimal conditions were clarified.Subsequently,we evaluated the substrate scope of the conversion of benzylic C-H bonds to C=O bonds,and the obtained results revealed that various isochromans,xanthenes,fluorenes and their derivatives underwent smoothly this transformation to give the carbonylation products in40-93%yields.In addition,the reaction mechanism was investigated via the radical inhibition and various control experiments,then a NO₂-catalyzed mechanism pathway was proposed,while oxynitride species usually played a role of the co-catalyst in many previous literatures.The results in the present chapter not only exhibit a new method for the synthesis of carbonyl compounds,but also show a new catalytic function of nitrogen and oxygen species.?2?Based on the catalysis of oxynitride species in the aerobic oxidation of benzyl C-H bonds,the direct conversion of benzyl ethers into aryl nitriles came true via a combination of oxidation and cyanation.After various reaction conditions were optimized,preliminary studies were conducted with a variety of representative benzyl methyl ethers to explore the scope and generality of this reaction under the optimal conditions.The obtained results suggested that the presence of electron-withdrawing goups bonded to the benzene rings had a negative effect on the reaction.A series of electron-donating group-substituted benzyl methyl ethers were smoothly converted to the targeted products in 74-78%yields.According to the experimental results related to various control experiments and the detection of intermediates,we proposed a mechanism pathway that underwent the stepwise steps under the catalysis of TEMPO⁺/NO_x.The results in the present chapter are of certain academic and practical value in the preparation of aryl nitrile and the deprotection of ether-protected hydroxyl group.?3?By means of the aerobic auto-oxidation,the conversion of benzylic C-H bond to C=O group was actualized.Subsequently,effects of various conditions on the reactions were investigated,and the reaction conditions were optimized.In addition,we evaluated the substrate scope by testing the reactions of diphenylmethanes,9,10-dihydroanthracene,xanthenes,fluorenes,isochromans and their derivatives,and the obtained results indicated that the benzylic C-H bonds of these compounds underwent smoothly the aerobic oxidation,and the targeted products were obtained in 21%-94%yields.The reaction mechanism was also investigated using radical trapping,DFT calculations and ¹⁸O-labelled experiments,and a radical mechanism pathway that involved the activation of oxygen by the?bond was clarified.It was confirmed that the formations of both the OOH radical and the targeted product were the rate-determining steps.The results in the present chapter reveal a new and simple method for the synthesis of carbonyl compounds,and increase the understanding of the mechanism related to the auto-oxidation of the C-H bond.?4?The cleavage of C-C bonds in phenyl acetones was actualized via the aerobic auto-oxidation of benzylic C-H bonds,and the obtained products were benzaldehyde and benzoic acid.Moreover,effects of various conditions on the reactions were examined,and the optimal conditions were clarified.Several phenyl acetones with different substituent groups were smoothly converted to the targeted products under the optimal conditions.The ¹⁸O-labelled experiments and the detection of intermediates were also performed to gain preliminary insight into the reaction mechanism,and a rough mechanism pathway was proposed.The present method could been applied into the cleavage of C-C bonds of lignin-related?-O-4 linkage,providing some theoretical information for the aerobic auto-oxidative depolymerization of lignin.