Cobalt/Nitroxyl Radical Catalytic Systems:Preparations,Mechanism And Applications

Direct oxidation of hydrocarbons functionalization is the most basic and important reaction in organic chemistry.It has good application prospects in industrial catalysis and involves the synthesis of many basic chemicals,natural products,fine chemicals and pharmaceutical products.In recent years,the direct esterification of aldehydes/alkanols and alkanols to prepare organic esters has attracted much attention due to its outstanding atom economy.Some notable achievements have been made in related methodologies,but there are some common problems still remain that need to be solved.For instance,stoichiometric amount of organic/inorganic oxidant are normally required,and most reactions need to be conducted under an inert atmosphere.Consequently,development of an efficient homogeneous catalytic system for direct hydrocarbon C-H oxidation,especially a catalytic system with oxygen as the terminal oxidant,enables the desired selective oxidation of hydrocarbons and cross-coupling reactions thereafter under milder conditions,which therefore has a highly academic value and practical significance.This dissertation focuses on the research of homogeneous non-noble metal cobalt catalysis.Firstly,Co(Ⅱ)-catalyzed oxidative cross-coupling of aldehydes/alcohols and alcohol were explored with tBuOOH as the oxidant.Secondly,Co(Ⅱ)/nitroxide aerobic oxidation catalytic system was studied.In this section,the coordination behavior of TEMPO to Co(Ⅱ)and the performance of the complexes thereof in the single electron transfer reaction were investigated.Additionally,a solvent-free ternary system of Co(Ⅱ)/NHPI/IL was achieved for aerobic catalytic oxidation of hydrocarbons.The details as the following:(1)A catalytic system of Co(Ⅱ)/CF3COOH(TFA)was developed by using tBuOOH as oxidant,thereby the oxidative cross-coupling of aldehydes/alkanols with alkanols was achieved.Mechanistic studies indicated that the oxidative esterification of aldehydes with alkanols involves three primary steps.The first is the formation of hemiacetal HOCH(OR1)R2 upon nucleophilic addition of alkanol to aldehyde.The second is the condensation of hemiacetal with Co(Ⅲ)-OH to form Co(Ⅲ)-O(OR1)R2 intermediate.Final oxidation of α-H therein by the in-situ formed tBuO radical offers the desired ester.Noteworthily,the second step of condensation reaction is the rate-determining step,of which the rate was significantly accelerated in the presence of a strong acid TFA as a co-catalyst,Besides,the reduction of Co(Ⅲ)to Co(Ⅱ)was favored in a strong acidic environment.Consequently,it resulted a high selectivity in the desired cross-coupling reaction in the presence of TFA.In the case of oxidative esterification of alkanols with alkanols,it preceded oxidative dehydrogenation of alkanols with active α-H bond to the corresponding aldehydes,such as benzyl alkanol,as the first step,followed by the general steps of oxidative esterification of aldehydes.(2)With TEMPO as nitroxide,the coordination behavior of TEMPO towards CoX2(X = Cl,I)and the performance of complexes thereof in the one-electron oxidation with HX(X = Cl,I)were studied.The complex of CoX2(η1-TEMPO)2(X = Cl,I)was synthesized in weakly coordinating solvent THF and CH2Cl2 respectively.X-ray analysis showed that the coordination configuration of Co(Ⅱ)in complex[CoCl2(η1-TEMPO)2]and[CoI2(η1-TEMPO)2]was pseudo tetrahedron.Both SQUID magnetic and ESR paramagnetic resonance analysis indicated that both of them were paramagnetic.Importantly,different halogen ligation has significant impact on zero field splitting parameter(D).The complexes[CoCl2(η’-TEMPO)2]and[CoI2(η’-TEMPO)2]were subsequently reacted with HX(X = Cl,I)to form[CoCl4(TEMPOH2)2]and[COI2(TEMPOH)2I2],respectively.In sharp contrast,the reaction of TEMPO with CoCl2 in strong coordination solvent CH3CN resulted in the formation of form[TEMPO][CoCl3].Independently,the complex of CH3CN and CoCl2,[Co(CH3CN)6][CoCl3(CH3CN)]2[CH3CN],was isolated and characterized.(3)The effect of imidazolium-based ionic liquid(IL)as an additive on the catalytic efficiency of Co(Ⅱ)/NHPI-catalyzed hydrocarbons oxidation was explored,thereby a ternary system of Co(Ⅱ)/NHPI/IL was achieved.Studies have shown that IL can self-assemble at the interface of reactant(organic phase)and traces of water(polar phase:including catalyst Co(Ⅱ)salt and NHPI)to form micro-reaction environment composed by reversed multilamellar vesicles.The initial oxidation rate is affected by the composition of the reversed vesicles and the structural variation of IL launched as well.For instance,a suitable water/IL ratio is essential to attain an optimized reaction efficiency.In addition,the interface consisting of C2-alkylated imidazolium-based ILs such as[bdmim]SbF6 and[C12dmim]SbF6 not only contributes to the formation of higher stability ternary systems(hydrocarbons/IL/H2O)but,also increases the diffusion rate of O2 and increases the oxygen concentration,thereby enabling oxidation of hydrocarbons more efficient.