Study On The Regularity In Oxidation Of Secondary Benzylic C-H Bonds With O₂ Catalyzed By Metalloporphyrins

The catalytic oxidation of aromatic secondary benzyl C-H bonds?ethylbenzene and its derivative benzyl C-H bonds?was an important transformation process in chemical industry,its oxidation products such as aromatic ketones,were the basis of a series of important fine chemical products and fundamental chemical raw materials,widely applied in the synthesis of chemicals such as spices,essences,medicines,pesticides,resins,dyes,and so on.In the current industrial process of aromatic secondary benzylic C-H bonds oxidation was mainly used O₂ as oxidant in the presence of homogeneous Co?OAc?₂ or Mn?OAc?₂ as catalyst and acetic acid as solvent.The main problems encountered in this process were heavy solvent-corrosiveness,high catalyst-loading,difficult catalyst-recovery,poor product-selectivity,and difficult to achieve the precise chemical synthesis of oxidation products.In view of the above problems,this dissertation employed metalloporphyrin,a model compound of cytochrome P-450,as catalyst,systematically studied the regularity of O₂ oxidation of secondary benzylic C-H bonds under solvent-free and additive-free conditions,and deeply explored the influence of reaction temperature,structure of porphyrin ligand,central metal,amount of catalyst,oxygen pressure and other factors on the reaction.The regularity of catalytic oxidation of aromatic secondary benzylic C-H bonds was preliminarily obtained,and a solvent-free catalytic oxidation route of secondary benzylic C-H bonds with solvent-free,additive-free,mild reaction conditions and high selectivity of aromatic ketones had been concluded.The main contents of this dissertation were as follows.1.Employing aromatic aldehyde,pyrrole and metal acetates such as Co?OAc?₂,Mn?OAc?₂,Fe?OAc?₂,Ni?OAc?₂,Cu?OAc?₂ and Zn?OAc?₂ as raw materials,propionic acid and dichloromethane as solvents to synthesize 67 metal porphyrins.Based on aromatic aldehydes,the yield of metal porphyrins ranged from 1.22%to 20.6%.The synthesized metalloporphyrins were characterized qualitatively by ¹H NMR,¹³C NMR,ESI-MS and OEA,which provided a catalyst basis for the study of the metalloporphyrins catalyzed the O₂ oxidation of aromatic secondary benzylic C-H bonds.2.The effect of reaction temperature on the benzyl secondary C-H bonds of aromatic hydrocarbons catalyzed by metalloporphyrins was systematically studied employing 4-ethylnitrobenzene as model compound.The study found that when the reaction temperature was below 120oC,the secondary C-H bonds of aromatic benzyl had no obvious autoxidation,and the conversion of 4-ethylnitrobenzene was only10.8%;metal porphyrin as catalyst could significantly promote the reaction.Employing T?4-Cl?PPCo as catalyst(2.4×10^-3%,mol/mol),reaction for 8.0 h at 120oC,the conversion of 4-ethylnitrobenzene was reached 36.6%,the selectivity of4-nitroacetophenone was 77.4%,the selectivity of 1-?4-nitrophenyl?ethanol was 6.4%,the selectivity of peroxide was 12.9%,and the selectivity of 4-nitrobenzoic acid was3.3%.Carrying out catalytic oxidation at a temperature without obvious autoxidation was beneficial to the selective oxidation of the secondary C-H bonds at the benzyl position of aromatic hydrocarbons.3.The effect of metalloporphyrins structure on the oxidation of secondary benzylic C-H bonds by O₂ was systematically studied employing 4-ethyl nitrobenzene as model compound.The study found that when the metalloporphyrins molecular structure contains electron-withdrawing groups,especially when the metalloporphyrins had an electron-withdrawing substituent group with a larger molecular volume,it was conducive to the improvement of reactivity,such as 120oC,1.0 MPa O₂ pressure for 8.0 h,the conversion of 4-ethylnitrobenzene was:T?2,3,6-tri Cl?PPCo?47.6%?>T?4-COOH?PPCo?41.6%?>T?4-Cl?PPCo?36.6%?>T?4-F?PPCo?28.0%?>T?4-CH₃?PPCo?<2%?;For metal ions,under the same reaction conditions,the conversion of 4-ethylnitrobenzene was:T?2,3,6-tri Cl?PPCo?47.6%?>T?2,3,6-tri Cl?PPMn?18.9%??T?2,3,6-tri Cl?PPCu?10.2%??T?2,3,6-tri Cl?PPNi?5.4%??T?2,3,6-tri Cl?PPFe?3.7%??T?2,3,6-tri Cl?PPZn?3.6%?;the selectivity of4-nitroacetophenone was:T?2,3,6-tri Cl?PPCo?87.5%?>T?2,3,6-tri Cl?PPMn?59.9%?>T?2,3,6-tri Cl?PPCu?41.0%?>T?2,3,6-tri Cl?PPFe?30.6%?>T?2,3,6-tri Cl?PPZn?27.8%?>T?2,3,6-tri Cl?PPNi?18.5%?.4.Employing 4-ethylnitrobenzene as the model compound,the reaction conditions were further optimized to obtain a better catalytic system based on the metalporphyrin in O₂ oxidation of secondary benzylic C-H bonds:T?2,3,6-tri Cl?PPCo(2.4×10^-3%,mol/mol)as catalyst,1.0 MPa O₂ pressure,120oC reaction for 8.0 h,the conversion of 4-ethylnitrobenzene reaching 47.6%,the selectivity of4-nitroacetophenone was 87.5%,1-?4-nitrophenyl?ethanol selectivity was 4.1%,the selectivity of hydroperoxides was 3.8%,the selectivity of 4-nitrobenzoic acid was4.6%.5.Based on the analysis of electrochemical properties and quantum chemical calculation,the origin of high activity and selectivity in the catalytic oxidation of secondary benzylic C-H bonds with T?2,3,6-tri Cl?PPCo as catalyst was investigated deeply.?1?T?2,3,6-tri Cl?PPCo molecular structure had good planarity,which was conducive to the contact between the substrate and the catalytic active center;?2?In the molecular structure of T?2,3,6-tri Cl?PPCo,the lower positive charge of Co?II?was favorable for its output of electrons with O₂,forming the high valence cobalt oxygen complex of catalytic active species and promoting the occurrence of hydrogen extraction;?3?T?2,3,6-tri Cl?PPCo had better electron transmission performance.6.Through the universality study of the substrate and the literature comparison,it was found that the catalytic oxidation method of the aromatic benzyl secondary C-H bonds based on the metal porphyrin-catalyzed O₂ oxidation of the aromatic benzyl secondary C-H bonds had the following advantages:?1?solvent-free and additive-free;?2?high aromatic ketone selectivity;?3?low catalyst loading;?4?mild reaction conditions and low reaction temperature;?5?simple catalyst structure.A relatively delicate balance had been achieved between high selectivity,low catalyst usage,simple catalyst,and mild reaction conditions.