Palladium-Catalyzed Organic Reactions Supported By Imine Ligands

Our research focused on the palladium-catalyzed organic reactions supported by diimine and Schiff base ligands, including palladium-catalyzed aerobic oxidation of alcohols, Suzuki cross-coupling reactions in aqueous phase.These works were detailed as below:1. Synthesis ofα-diimine ligands and its complexesA series of water-soluble diimine ligands with different electronic property, rigidity and steric hindrance were prepared by condensation ofα,β-dione and the corresponding sodium salts of alkyl substituted sulfanilic acid. Several Pd(II) and Ni(II) complexes of diimine were prepared. These ligands and complexes are all easily diffluent and stable in polar organic solvents, such as DMF, DMA and DMSO, as well as in water.α-Diimine ligand 14 was prepared by the condensation of 3,5-bis(trifluoromethyl)aniline and acenaphthenequinone. Palladium dichloride complex and mono-methylated palladium complex of ligand 14 were synthesized. As an unforeseen product, an interesting trinuclear palladium complex with two unexpected PdCN three-membered rings was obtained in which the non-innocent diimine functions asμ²,η³-N, CN' donor ligand with radical monoanion oxidation state ([BIAN]^1-).2. Palladium-catalyzed aerobic alcohol oxidation supported byα-diimine ligandsThe new ionic sulfonatedα-diimine ligands were employed as the supporting ligands for the palladium-catalyzed aerobic alcohol oxidation. The electronic property, rigidity and steric hindrance of ligands have remarkable influence upon the catalytic activity. The best catalytic result could be achieved using ligand 9 with electron-withdrawing sulfonic substituent together with bulky groups (iso-propyl at 2, 6-position). The diimine-palladium-catalyzed system was highly efficient for the oxidation of various benzylic alcohols, and showed moderate yields for the secondary aliphatic alcohols and cyclic alcohols. An electronic effect seems to exist for the oxidation of benzylic alcohols.The mechanism of the palladium-catalyzed aerobic alcohol oxidation confirms to the palladium-oxidase pathway which is composed of two independent steps: substrate oxidation and catalyst regeneration. The reaction proceeds via the formation of a Pd(II)-alcoholate from an alcohol and a Pd(II) species followed byβ-H elimination to deliver the oxidation product and a Pd(0) species. The Pd(0) species could react with the molecular oxygen to give a Pd(II)-hydroperoxide species, and this species subsequently reacts with water to give the Pd(II) oxidant. The observation that electron-withdrawing substituents on the ligands increase the reaction rate in alcohol oxidation is a strong indication that the substrate oxidation step is slow. The bulk group on arene of ligands effectively prevent the aggregation of palladium(O) species and facilitated the reoxidation of palladium(0). This steric effect doesn't hinder approach of the substrate alcohol to catalytic species. It is believed that theβ-H elimination is the true rate-limiting step.3. Aqueous Suzuki coupling catalyzed by water-soluble diimine/Pd(II) systemsThe new water-soluble diimine ligands were applied to the high-yield Suzuki coupling reaction of aryl bromides and arylboronic acids which could be carried out in organic solvent, neat water or water/ethanol mixed solvent. Low loading of palladium catalyst (0.01 mol%) were necessary for the coupling reaction in water or water/ethanol. The coupling reaction could be smartly carried out at room temperature in water/ethanol mixed solvent. The additive TBAB greatly accelerated the rate of coupling reaction and decreased the formation of by-products in water at lower temperature. This catalytic system was found to tolerate a broad range of functional groups.4. Water-soluble palladacycles as highly efficient catalytic precursor for the Suzuki coupling in aqueous phaseSeveral new water-soluble and hydrophobic Schiff base ligands were prepared by condensation sodium salts of 3, 5-diisopropyl sulfanilic acid and corresponding aldehydes. The corresponding hydrophilic palladacycles were prepared by a traditional C-H activation reaction of the sulfonated Schiff base ligand with Li₂PdCl₄ in the presence of sodium acetate. It is believed that the weakly coordinating sulfonate anion does not strongly coordinate to palladium, allowing the simple palladacycle dimmers to be isolated. The new palladacycles were applied into the Suzuki coupling in aqueous phase. It was found that all the palladacycles showed poor catalytic activities in pure organic solvent, and the catalytic activities dramatically improved in aqueous solvent. It was shown that the palladacycles with six-membered chelate ring were much more efficient than those with five-memberred rings. The steric hindrance of the ligand has less influence on the catalysis. A quantitative conversion was observed for the coupling reaction catalyzed by palladacycle 1 with employment of water/EtOH co-solvent and 0.05 mol% loading of palladium at 15℃. It is interesting that the catalytic system using palladacycle 1 as catalyst precursor could be recycled five times in over 90% yields.The TEM analysis showed the formation of palladium particles with a diameter in the range 20-100 nm, while the palladacycyle with five-memberred chelated ring gave the particles with wider range 20-250 nm. Then it was proposed that the palladium nanoparticles might be the active catalyst for the reaction. It is believed that the reactant of phenylboronic acid could act as a stabilizer because the O^- of the deprotonated phenylboronic acid could bind to the surface of the palladium nanoparticles. As a result, the Ostwald ripening process is greatly diminished and the nanoparticles do not dramatically increase in size. The palladium nanoparticles, which are "soluble" in water because of the bound of deprotonated phenylboronic acid, does not greatly congregate and can be recyclable for several times. It could be also explained that the loss of catalytic activity is due to the thermodynamics-favored Ostwald ripening process and the overladen binding of phenylboronic acid to the free sites. The surfacants, such as TBAB and PVP, occupied the free sites of active Pd(0) species, and weakened the catalytic activites.