Construction And Performances Of Catalytic Oxidation Systems From Transition Metal Complexes Of Polydentate Schiff Base Ligands

Oxidation reactions, such as selective oxidation of alcohols to aldehydes and ketones,epoxidation, and oxidative kinetic resolution of secondary alcohols, are very important bothin the manufacture of bulk chemicals and the production of fine chemicals, such as drugs,pesticides and others. Many disadvantages including use of expensive oxidants, low atomeconomy, and pollution to environment are present in the traditional processes. Therefore, itis very important to build effective and green catalytic oxidation system from both thetheoretical and practical viewpoints. For these purposes, we firstly synthesized a series ofpolydentate Schiff bases and their metal complexes. Then several catalytic oxidationsystems were obtained from the complexes in combination with various oxidants. Thecatalytic systems were applied to the oxidation of alcohols, oxidative kinetic resolution ofsecondary alcohols and epoxidation of olefins, respectively.Eight Salen ligands3a-3h were prepared from the condensation of salicylaldehydeswith chiral or achiral diamines, respectively. The Salen ligands coordinated with Mn3+orCu2+to afford nine complexes4a-4h (including4c-1and4c-2), respectively. Meanwhile，other six bidentate Schiff base ligands5a-5f were synthesized from the condensation ofpicolinaldehyde with six aliphatic and aromatic amines, respectively. In addition, respectiveformylation of2-tert-butylphenol and2-methyl-butylphenol to afford two salicylaldehydederivatives, which underwent chloromethylation to give two intermediates---3-chloromethyl-6-tertbutyl-salicylaldehyde and3-chloromethyl-6-methyl-salicylaldehyde;the intermediates reacted with4-hydroxy-TEMPO respectively to give two salicylaldehydederivatives. The salicylaldehyde deriatives respectively condensed with2-aminomethylpyridine in anhydrous ethanol to afford two Schiff base ligands with a TEMPO moiety. Allthe above compounds were characterized by NMR, IR, UV and HRMS.Two catalytic systems, Salen-MnIII/NaClO/Br2and Salen-MnIII/NaClO/NBS, for theoxidative kinetic resolution of secondary alcohols were established by using chiralSalen-MnIIIcomplex (4e) as a catalyst, sodium hypochlorite as an oxidant, and catalyticamount of bromine or N-bromosuccinimide (NBS) as a cycle agent in the presence ofpotassium acetate as a buffer. The optimization conditions for the oxidative kineticresolution of secondary alcohols by the two novel catalytic systems were obtained by using1-phenylethanol as a model substrate. Under the optimized conditions, the enantiomericexcess is higher than99.9%(GC) in both cases, when the conversions of1-phenethanolwere kept62.9%and61.8%, respectively. Then, the two catalytic systems were applied tothe oxidative kinetic resolution of various secondary alcohols, and good results were received.Two catalytic systems,4e/PhI(OAc)2/KBr and4e/NaClO/PyNO, were obtained byusing chiral Salen-MnIIIcomplexe4f as a catalyst and PhI(OAc)2as an oxidant in thepresence of KBr, and NaClO as an oxidant in the presence of PyNO, respectively. Theformer was used in the oxidative kinetic resolution of1-phenylethanol, and the latter wasapplied to the asymmetric epoxidation of styrene. It was found that the small sterichindrance of the imidazole moiety at the C5(5’) position of the Salen complex compared tothe tert-butly in the classical Jacobsen’s complex, and the weaker electronic donationcapacity of the imidazole moiety lead to the low enantioselectivity in the asymetricpoxidation of alkenes. However, the introduction of quaternary ammonium imidazolemoiety enhances the catalyst’s water solubility increasing the contact opportunity of thecomplex molecule with bromine ions, and makes the complex also act as a phase transfercatalyst accelerating the transfer rate of oxidant and substrate in the two phases, whichgreatly improve the reaction rate, and make the oxidative kinetic resolution conductsmoothly at low catalyst loading.A catalytic system,4c-2/NaClO/NBS, for the selective oxidation of secondary alcoholswas established by using achiral Salen-MnIIIcomplex4c-2as a catalyst. The systemshowed high activity and selectivity in the oxidation of various secondary alcohols to theircorresponding ketones. Meanwhile, it was found that the electron-withdrawing and weakelectron-donating groups in the benzene ring of the substrates had no obvious effect on theoxidation reaction, but the strong electron-donating groups and the steric hindrance of thegroups could slow down the reaction.Two novel catalytic systems, one from the simple bidentate Schiff base ligandsysthesized in combination with CuBr2and TEMPO and the other from the bidentate Schiffbase with a TEMPO moiety in combination with CuBr2were obtained. The two catalyticsystems can catalyze molecular oxygen to oxidize primary benzyl alcohols and allylalcohols to the corresponding aldehydes in high activity and selectivity in the presence ofK2CO3. Both systems can oxidize the primary benzyl alcohols, allylic of primary alcoholsto the corresponding aldehyde in high activity, high selectivity; they are also effective in theoxidation of fused-ring aromatic alcohols, but had poor performances in the oxidation ofprimary aliphatic alcohols, all secondary alcohols and primary aromatic alcohols with ahetero atom in the ring. They were found that the electronic properties of the ligand and thesubstrates had small influence on the reaction, but the steric hindrance of the substituentshad significant effects on the reaction.The aerobic oxidations of2-methoxy-4-methylphenol,2-bromo-4-methylphenol andp-cresol to their corresponding p-hydroxy benzaldehydes catalyzed by CoCl2/NHPI wereinvestigated under basic conditions. A three-component catalytic systemCoCl2/NHPI/Salen-CuIIwas estabilished to improve the oxidation of2-bromo-4-methylphenol and p-cresol, and the yields of the correspondingIV p-hydroxybenzaldehydes respectively increased10.2%and27.5%compared to those in thecases with CoCl2/NHPI as catalyst.