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

The catalytic oxidation of tertiary benzylic C-H bonds is an important process in the chemical industry,in which all the oxidation products,such as peroxides,alcohols,and ketones,are important raw materials in fine chemical industry,and applied widely in the manufacture of pharmaceuticals,pesticides,dyes,perfumes,and so on.At present,the oxidation of tertiary benzylic C-H bonds in industry mainly focused on the oxidation of cumene,in which valuable cumene hydroperoxide in chemical industry usually is expected as the target product,since cumene hydroperoxide not only is the irreplaceable precursor in the production of phenol and acetone,but also is the essential oxidant in the industrial oxidation of propylene to propylene oxide.Another two main oxygenated products in the catalytic oxidation of tertiary benzylic C-H bonds are 2-benzyl-2-propanol,acetophenone,and their derivatives without enough attention being paid at present,especially 2-phenyl-2-propanol and its derivatives,which are very important products and intermediates in fine chemical industry,but could not be synthesized selectively in the direct oxidation of tertiary benzylic C-H bonds.The main reason in the low selectivity to tertiary benzylic alcohols could be ascribed to the unclear regularity in oxidation of the tertiary benzylic C-H bonds.In order to understand the regularity in oxidation of the tertiary benzylic C-H bonds and realize the precise syntheses of oxidation products,the catalytic oxidation of tertiary benzylic C-H bonds with O₂ as oxidant was studied systematically employing the chemical model compounds of cytochrome P-450 metalloporphyrins as catalysts in this dissertation.Based on the systematical investigation on the effects of reaction temperature,porphyrin ligands and metal centers,and the apparent kinetics study,not only the general regularity in oxidation of tertiary benzylic C-H bonds with O₂ catalyzed by metalloporphyrins had been obtained,but also the direct and selective oxidative syntheses of aromatic tertiary benzylic alcohols were achieved.The main research contents of this dissertation are as follows.Twenty-five porphyrin ligands were synthesized employing benzaldehyde and its derivatives such as 4-chlorobenzaldehyde,4-methoxycarbonyl benzaldehyde,3-methoxybenzaldehyde,2,6-dichlorobenzaldehyde and so on,and pyrrole as raw materials in the yields of 6%～36%following the Adler-Longo method and Lindsey method,and characterized qualitatively.Then 68 metalloporphyrins were synthesized using Co（II）,Mn（II）,Fe（II）,Ni（II）,Cu（II）and Zn（II）as central metals in the yields of88%～98%,and characterized qualitatively,which provided a fundamental catalyst library for the study of the regularity in oxidation of tertiary benzylic C-H bonds with O₂ catalyzed by metalloporphyrins.The effect of reaction temperature on the oxidation of tertiary benzylic C-H bonds catalyzed by metalloporphyrins was investigated systematically employing cumene as representative substrate.When the reaction temperature was below 90oC,no obvious autoxidation of cumene happened.And as metalloporphyrins being introduced to the oxidation of cumene as catalysts,the oxidation of cumene was promoted significantly.For example,when the oxidation of cumene was conducted at 75oC and 1 atm O₂ for8.0 h with TPFPPCo as catalyst（0.012%,mol/mol）,the conversion could reach up to56.1%with the selectivity of 56.6%towards 2-phenyl-2-propanol,21.4%towards cumene hydroperoxide,22.0%towards acetophenone,and no benzoic acid was detected.It is a very effective strategy to improve the selectivity and controllability of the reaction to carry out the catalytic oxidation of tertiary benzylic C-H bonds at the temperature without autoxidation happening.The effect of metalloporphyrin structures on the oxidation of tertiary benzylic C-H bonds was investigated systematically employing cumene as representative substrate.When the periphery of metalloporphyrins possessed electron-withdrawing groups,higher catalytic performance usually could be observed,such as T（2,3,6-tri Cl）PPCo>T（4-Cl）PPCo>T（4-OCH₃）PPCo,and for the same substituent locating at different position of the benzene rings in the periphery of metalloporphyrins,higher catalytic performance was obtained when the substituent was located at the 2-position of the benzene rings,such as T（2-Cl）PPCo>T（4-Cl）PPCo>T（3-Cl）PPCo.As for metal ions,the order of catalytic performance was as following:Mn（II）>Co（II）>Cu（II）>Zn（II）≈Ni（II）≈Fe（II）,the order of selectivity towards 2-phenyl-2-propanol was as following:Mn（II）>Co（II）>Fe（II）≈Cu（II）≈Ni（II）>Zn（II）,and the order of selectivity towards cumene hydroperoxide was as following:Zn（II）>Cu（II）≈Ni（II）≈Fe（II）>Co（II）>Mn（II）.Based on the further optimization on the reaction conditions,T（2,3,6-tri Cl）PPMn was regarded as the most promising catalyst in the direct oxidation of tertiary benzylic C-H bonds to their corresponding tertiary alcohols.When the oxidation of cumene was conducted at 70oC and 1 atm O₂ for 8.0 h employing T（2,3,6-tri Cl）PPMn（II）as catalyst（0.012%,mol/mol）,the conversion of cumene reached up to 57.6%with the selectivity of 14.0%to cumene hydroperoxide,70.5%to 2-phenyl-2-propanol,15.5%to acetophenone with no benzoic acid being detected.With Cu（II）and Zn（II）as the central metals,it was favorable to form cumene hydroperoxide in the oxidation of cumene,especially for T（4-COOH）PPCu（II）as catalyst（0.012%,mol/mol）,for which the conversion of cumene reached up to 27.3%with the selectivity of 97.2%to cumene hydroperoxide,and 2.8%to acetophenone under the reaction conditions of 80oC and8.0 h without benzoic acid formation.The source of the high performance and selectivity exhibited by T（2,3,6-tri Cl）PPMn（II）in the oxidation of tertiary benzylic C-H bonds was explored in detail through the electrochemical analysis and quantum chemical calculation,which could be concluded as:（1）The plane structure of T（2,3,6-tri Cl）PPMn（II）favoring the interaction between metal catalytic active centers and reactants.（2）The higher positive electricity around Mn in T（2,3,6-tri Cl）PPMn（II）increasing the activity of high-valence manganese-oxo complexes to abstract hydrogen atom from C-H bonds.（3）The central metal Mn（II）favoring the decomposition of oxidation intermediate products cumene hydroperoxides to alcohols.Based on the substrate tolerance study and literature comparison,the major advantages of our route which was summarized from the regularity in catalytic oxidation of tertiary benzylic C-H bonds,to oxidize tertiary benzylic C-H bonds to alcohols directly were:（1）The direct conversion of tertiary benzylic C-H bonds to alcohols with shortest reaction route.（2）Mild reaction conditions.（3）High conversion and selectivity to valuable alcohols.（4）Solvent-free and additive-free.