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

Aromatic hydrocarbons,as an important class of hydrocarbons,are widely found in coal chemical and petrochemical products,and are obtained easily.The selective oxidation of its primary benzylic C-H bonds is an important process in the chemical industry,especially the catalytic oxidation process under the condition of solvent-free and molecular oxygen?O₂?as oxidant.In these significant transformations,the widely available and inexpensive hydrocarbons are converted to high value-added hydroperoxides,alcohols,aldehydes,ketones,acids and esters.They also have the advantages of short reaction path,high atom economy,easy operation,and small environmental impact.All of these oxygen-containing compounds not only are essential fine chemicals and chemical intermediates,but also are indispensable structural units in the synthesis of fine chemical products such as medicine,pesticides,dyes,spices,etc.Thus in the current industrial processes,the oxidation of primary benzylic C-H bonds is usually carried out with O₂ as oxidant in the presence of homogeneous cobalt?II?acetate or manganese?II?acetate as catalyst,Br₂ or HBr as promoter and high reaction temperature above 160oC in the solvent of acetic acid.Its main problems are high reaction temperature,strong corrosiveness of solvents and co-catalysts,and poor product selectivity,making it difficult to achieve precise chemical synthesis of oxidation products.In order to solve the above-mentioned problems,a protocol for the solvent-free and additive-free oxidation of primary benzylic C-H bonds with O₂ was presented employing the combination of metalloporphyrins and NHPI as binary catalysts.The effects of reaction temperature,porphyrin structure,central metal,catalyst loading and O₂ pressure were investigated systematically.The regularity in oxidation of primary benzylic C-H bonds with O₂catalyzed by metalloporphyrins/NHPI was initially obtained,and a highly efficient and highly selective way to prepare aromatic carboxylic acids by oxidizing the primary benzylic C-H bonds was achieved.The main research contents of this dissertation are as follows:The 23 porphyrins ligands were mainly synthesized employing benzaldehyde and its derivatives,and pyrrole as raw materials in the yields of 3.4%?35.7%following the Adler-Longo method and Lindsey method,and characterized qualitatively.Then58 metalloporphyrins were synthesized using Co?II?,Mn?II?,Fe?II?,Ni?II?,Cu?II?and Zn?II?as central metals in the yields of 14.5%?83.2%,and characterized qualitatively,such as ¹H NMR,¹³C NMR,ESI-MS,UV-Vis and FT-IR.It provided a fundamental catalyst library for the study of the regularity in oxidation of primary benzylic C-H bonds with O₂ catalyzed by metalloporphyrins/NHPI.The effect of the catalyst composition on the oxidation of primary benzylic C-H bonds catalyzed by metalloporphyrins was investigated systematically employing4-methyl nitrobenzene as representative substrate.There was not any oxidation transformation of 4-methyl nitrobenzene observed before 140oC,no matter in the presence of metalloporphyrins as catalysts or not.With NHPI?5%,mol/mol?as the catalyst,the reaction was conducted at 120oC for 8.0 h,the conversion reached up to16.6%with the selectivity of 61.1%towards 4-nitrobenzoic acid.With metalloporphyrin?0.012%,mol/mol?/NHPI?5%,mol/mol?as the catalyst,the conversion reached up to 31.3%with the selectivity of 63.9%towards 4-nitrobenzoic acid.The effect of the porphyrin structure on the oxidation of primary benzylic C-H bonds catalyzed by metalloporphyrins was investigated systematically employing4-methyl nitrobenzene as representative substrate.The performance of metalloporphyrins in catalytic oxidation was influenced normally by the structure of ligands,which resulted in the excellent adjust ability in the catalytic properties.When the molecular structure of tetraphenylporphyrin had an electron-donating group in the ortho position,the metalioporphyrin/NHPI binary catalytic system had better catalytic activity,such as the catalytic activity of T?2-OCH₃?PPCo/NHPI was better than other porphyrin cobalt?II?/NHPI investigated.When the porphyrin ligands were the same,the catalytic activity of different metal ions in turn was:Co?II?>Mn?II?>Fe?II??Ni?II?>Cu?II??Zn?II?;the selectivity of 4-nitrobenzoic acid was:Co?II??Mn?II??Fe?II?>Ni?II?>Cu?II??Zn?II?;the selectivity of peroxide was:Cu?II?>Ni?II??Zn?II??Fe?II?>Mn?II?>Co?II?;the selectivity of aromatic alcohols in order was:Co?II?>Mn?II??Cu?II??Zn?II?>Ni?II??Fe?II?;the selectivity of aldehydes in turn was Zn?II??Cu?II??Fe?II??Ni?II?>Mn?II?>Co?II?,no other obvious oxidation products were detected.Based on cyclic voltammetry analysis and quantum chemistry calculations,from the perspective of metalloporphyrins,the source of the high reactivity of the T?2-OCH₃?PPCo?II?/NHPI binary catalytic system was thoroughly investigated:?1?T?2-OCH₃?PPCo?II?better electron transmission performance;?2?The superior performance of T?2-OCH₃?PPCo among metalloporphyrins involved was mainly attributed to its high efficiency in charge transfer and the lower positive electricity around central metal Co?II?which favored the adduction of O₂ to cobalt?II?forming the high-valent metal-oxo complex.Based on the further optimization on the reaction conditions,a better system of oxidation of primary benzylic C-H bonds with O₂ catalyzed by metalloporphyrins/NHPI was obtained:T?2-OCH₃?PPCo?II??0.012%,mol/mol?,NHPI?10%,mol/mol?,1.0 MPa O₂,120oC,8.0 h.For the typical substrates such as methylbenzene,4-phenyl methylbenzene,4-chloro methylbenzene,4-methyl nitrobenzene,the selectivity towards aromatic acids reached up to 92.3%,91.6%,95.1%,64.5%with the conversions of 35.1%,73.5%,88.5%,49.4%respectively.Based on the generality study of the substrate and the comparison of the literature,it was found that the preparation method of the aromatic carboxylic acid based on the oxidation rule of the C-H bond of the benzyl position of toluene and its derivatives mainly had the following advantages:?1?no solvent,no additive;?2?mild conditions;?3?higher conversion and selectivity;?4?simple operation method,easy to realize.