Mechanistic Studies On The Selective Aerobic Oxidation Of C-H Bond

The selective aerobic oxidation of C-H bond not only is a crucial process in the body,but also is of great importance in the industrial productions.As a product of phenol oxygenation,the 2,3,5-trimethyl-1,4-benzoquinone is the key internediate during the industrial process of vitamin E,while the poly(2,6-dimethyl-1,4-phenylene oxide)as a product of oxidative phenol polymerization,is a high-temperature thermoplastic.Different from the C(sp2)-H bond at phenols,the a-C(sp3)-H bond at carbonyl compounds has higher bond strength,and its hydroxylation products are not only commonly found in bioactive molecules,but also commonly used as the precursors for chiral syntheses.To develop highly selective oxidation methods for these two types of C-H bonds,it is crucial to investigate their reaction mechanisms.During the first part,the CuCl-LiCl catalyst system was oxidized by O2 in 1-hexanol,producing an activated catalyst species(2),which was suggested to be a bi-or multinuclear copper(?)species with antiferromagnetically coupled Cu(?)centers,on the basis of cyclic voltammetry,XAFS and the electron paramagnetic resonance.This activated catalyst 2 showed highly effective dehydrogenating activity on phenols,in which the phenoxyl radicals were detected by EPR.Its reaction with 2,3,6-trimethylphenol under N2 could only generate polymers,however,the quinone was produced when the reaction was carried out under O2 atmosphere.The main pathway to produce benzoquinones was suggested to be the attack of another activated catalyst,superoxo-copper(?)complex,on phenoxyl radicals,and the phenoxyl radical with higher spin density at its para-carbon preferred this pathway rather than the competitive dimerization.Moreover,a positive correlation was found between the para-hydrogen hyperfine coupling constant and the yield of benzoquinone catalyzed by CuCl-LiCl.The coorperation of coppers was observed during the oxidation of phenols to benzoquinones,therefore,during the second part,the restricting effect of solvent aggregates on the coorperation among coppers was studied in 3-hexanol,Based on spectroscopies and electrical conductivity studies,the catalyst CuC12 was found to be restricted at polar regions of the solvent aggregates in 3-hexanol.As a result,CuCl2 exists as mononuclear species at lower concentration,and could not coorperate during the reaction,leading to the production of polymers.At higher concentration,however,CuC12 exists as bi-or multinuclear species,and could well coorperate during the reaction,leading to the production of benzoquinones.The third part revolved around the selective hydroxylation of C(sp3)-H bond of carbonyl compounds by O2.With a catalytic amount of 1,5,7-triazabicyclo[4.4.0]dec-5-ene(TBD),which is one of the guanidine derivatives,the hydroxylation was achieved in absence of phospine reductants in DMSO.Suggested by a series of control experiments,spectroscopies and DFT calculations,the C-H bond at the substrate and the O-O bond at the peroxide intermediate were both activated by TBD through the double hydrogen bonds.A detailed mechanism was proposed on this basis.In summary,this work studied the activation of O2 by copper catalysts and the activation of C-H bond by TBD,contributing to the selective oxidation of different types of C-H bonds.Moreover,a new type of solvent effect was suggested in this work,namely the restricting effect of solvent aggregates on the catalyst,providing a new idea for the study of catalysis mechanism and solution properties.