Development And Catalytic Performance Of Palladium Catalyzed Systems Stabilized By Micelles In Aqueous

The development and application of green reaction media is an essential research direction in green chemistry.Organic solvents account for more than 80 percent of chemical waste and have a huge impact on the environment.Water is considered an ideal reaction medium to replace organic solvents due to its abundant reserves,excellent environmental compatibility,non-toxic,and non-volatile properties.However,the poor water solubility of the organic substrate affects the mass transfer and diffusion during the reaction.Therefore,the use of surfactants to construct micellar systems in water for solubilization and enrichment of insoluble substrates has become an influential method for realizing organic reactions in water.For organic reactions involving transition metals in the fine chemical industry,there are still some problems with micellar catalysis:a large number of organometrical ligands need to be added;the preparation of the surfactants used is complex and has toxic molecular migration phenomena;the micelles constructed with surfactants are less versatile.In this paper,we have prepared sugar-based surfactants with ligand function by using sugar and amine compounds as raw materials.The micellar system constructed in water not only increases the solubility of the substrate,but also stabilizes and disperses palladium nanoparticles.The Suzuki C-C coupling reactions,hydrogenation reactions,and hydrogen borrowing reactions have been efficiently catalyzed in the aqueous phase.The details of the paper are as follows and will be presented in three sections:1.In this chapter,a series of sugar-based surfactants with Schiff base or amide groups（AGAn,ALAn,Cn NG）were prepared by using sugar rings as hydrophilic groups and alkyl amines with different chain lengths as hydrophobic groups.The micelle system was found to be capable of achieving aqueous Suzuki C-C coupling reactions.The optimal reaction conditions were selected as follows:using glucosamide type surfactant C8NG aqueous solution(0.01 mol·L^-1)as the reaction medium,Pd（OAc）₂（2 mol%）as the catalyst,K₂CO₃ as the base,reacting at 25^oC for 40 min,and the maximum yield of the reaction was 95%.The C8NG micellar palladium catalyzed system had good substrate applicability and recycling performance.In addition,it was found that C8NG exhibited a variety of micellar morphologies in different alkaline environments,such as worm-like,spherical,and three-dimensional tubular morphologies.The study showed that the micellar morphologies were related to the p H of solution,and the spherical micelles obtained in K₂CO₃ aqueous solution had the best substrate solubilization ability.Through characterization methods such as in-situ IR,TEM,XRD,and XPS,it was verified that the average particle size of micelles and palladium nanoparticles in the formed micellar palladium catalyst system were 25 nm and 2.0 nm,respectively.The amide group of C8NG could chelate the palladium catalyst,thereby avoiding the use of external ligands.The study of reaction kinetics showed that the Suzuki reaction had zero-order kinetics for the reactants and a first order kinetics for the catalyst palladium.The oxidation addition process of iodobenzene with palladium was rate determining step,and the reaction activation energy was 46.7 k J·mol^-1.2.However,the above-mentioned sugar-based surfactants have problems with poor water solubility in pure water and different micelle morphologies in different p H solutions,which limit the selection of reaction systems.Therefore,in this chapter,a group of sugar-based polyether surfactants were prepared by introducing sugar rings onto the polyether surfactant M2070 with excellent water solubility and stable structure.The micellar stabilized Pd nanoparticle solution was prepared in situ by adding reductants.It could catalyze the hydrogenation of nitrobenzene efficiently under mild conditions.The optimal reaction conditions were as follows:Pd nanoparticles with Glu M micellar stability were used as catalyst in 25^oC,hydrogen atmosphere for 4 h,and the reaction yield was as high as 99%.Substrate expansion experiments showed that Glu M micellar palladium catalyzed system had a good range of substrate application.For aromatic nitro compounds with electron absorption,electron pushing and steric hindrance,16 aromatic amino compounds have been obtained at 80%to 99%yield.The reaction yield of the palladium catalyzed micellar system reached 92%after 5 reuses,which was better than that of commercial Pd/C catalyst.By means of TEM,XRD,XPS and UV-vis,it was proved that the Schiff base structure of Glu M can act as palladium ligand and avoid the agglomeration of palladium nanoparticles.Moreover,palladium nanoparticles with different sizes could be prepared by selecting different reducing agents.In the palladium catalytic system of Glu M micellar with L-ascorbic acid as reducing agent,the average particle size of palladium nanoparticles was the smallest（2.3 nm）,and the distribution was the most uniform.Reaction kinetics showed that the reaction was a quasi-first-order reaction with a low activation energy of 20.35 k J·mol^-1 for nitrobenzene.Through exploring the mechanism experiment,it was found that the reaction followed the Haber mechanism.The kinetic isotope experiment also verified that water as a protic solvent can promote the hetero-cracking of hydrogen on the metal surface.3.In this chapter,the Glu M micellar palladium catalytic system has been successfully extended to the alkylation of alcohol amines using a hydrogen borrowing strategy.The optimal conditions for screening were as follows:Glu M micellar aqueous solution(0.005 mol·L^-1)as the reaction medium,Na₂Pd Cl₄（1 mol%）as the catalyst,KOH as the base,and reacting at100^oC for 12 h,the maximum yield of the template reaction could reach 88%.The substrate expansion under the optimal reaction conditions proved that the reaction system had excellent applicability.By means of TEM,XRD,XPS,and other characterization,it has been proved that Glu M micelles can effectively solubilize organic substrates,and the reducing groups on the sugar chain can be reduced in situ to obtain palladium nanoparticles with an average particle size of 1.9 nm.Finally,combined with validation experiments,it was confirmed that aniline and benzyl alcohol in micelle media follow a hydrogen borrowing reaction route,which was that alcohol dehydrogenated into carbonyl compounds under alkaline conditions,then aldehyde reacted with aniline to form unsaturated imine intermediate,finally the unsaturated imine intermediate reduced to produce the final target product.