Preparation Of Microcapsule-Supported Palladium Catalysts Using SPG (Shirasu Porous Glass) Emulsification Technique And Their Catalytic Activities

Homogenous coordination catalysis has many attractive properties, such as high efficiency and well selectivities. And therefore, homogenous coordination catalysis has been widely employed in the modern organic synthesis for construction of the new carbon-carbon bonds. However, homogeneous systems often suffered from some drawbacks that it is difficult to separate the product from the catalyst in any organic solvents, and the expensive transition metal catalyst cannot be recycled. To resolve the above mentioned problems, MCM-41, SBA-15 and polymer have been applied to immobilize the transition metal catalysts by chemists. In the supported catalysts, the main problem is that the weak bonds between metal and ligand are often broken and reformed during catalytic reactions. This may leads to the loss of catalytic activity when they are recovered by filtration and recycled due to the leaching of active species from inorganic or polymeric support. In addition, usually prolonged reaction time is needed in the supported metal catalyzed reaction due to the solid catalyst can not be dispersed in the reaction medium. Microcapsule-supported transition metal catalysts are a kind of novel polymer-supported catalysts, which can be dispersed in the full reaction medium. The multi-coordinative effects of ligands inside of the microcapsule could restrain the leaching of active species.In this paper, the monomer 4-styryldiphenylphosphine (SDPP) 45 is synthesized by a improved method, and the highly monodispersed cross-linked polystyrene microcapsules containing phosphine ligand have been prepared by using 4-styryldiphenylphosphine as a monomer, divinylbenzene as cross-linking agent and AIBN as an initiator within the oil-in-water (O/W) emulsion droplets prepared by the hirasu porous glass (SPG) emulsification technique. The optimized conditions for the preparation of microcapsule are found that the mixture solution of 4-styryldiphenylphosphine (1.20 g),20 wt% of DVB, AIBN (3 wt% of monomers) and toluene (1.2 mL) as the dispersed phase permeats into the continuous phase containing of PVA (1.0 wt%), SDS (0.3 g/L) and deionized through the pore of SPG membrane to form emulsion droplets, then the polymerization is carried out at 70℃for 24 h. The microcapsule-supported palladium catalyst has been obtained by ligand-exchange reaction. The content of palladium in the catalyst can be detected by inductively coupled plasma emission spectrometry (ICP). The microcapsule-supported palladium catalyst can be also characterized by these instruments, such as the optical microscope, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) and so on.Suzuki-type cross-coupling reaction between aryl bromide and phenylboronic acid is chosen to evaluate the catalytic activity or stability of the microcapsule-supported palladium catalyst. Under the optimized conditions (1.0 mol% of Pd,0.25 mmol of aryl halide,0.38 mmol of aryl boronic acid,0.38 mmol of K2CO3,2 mL of IP A,80℃), Suzuki cross-coupling reaction of aryl bormides with phenyl boronic acid proceeds smoothly to give corresponding coupling product in more than 80% yield, and the highest yield is 99%. By simple filtration, the supported catalyst can be recovered and reused eleven times, and the coupling reaction of 4-bromoacetophenone with phenyl boronic acid gives the corresponding product in more than 96% yield.Our research demonstrated that the microcapsule-supported palladium catalyst is also an effective catalyst for Heck reaction of aryl iodides with methyl acrylate. The yields of Heck reactions range from 51% to 99%. The supported catalyst can be reused five times by simple filtration, and the reaction of iodobenzene with methyl acrylate can give more than 91% yield.Three-phase experiment shows that the microcapsule-supported palladium catalyst actes as a microreactor, in which Suzuki cross-coupling reaction between aryl bromides and phenyl boronic acid occurs. It can be considered that the substrates enter the microcapsule through their pores, and transform to their corresponding products after the catalytic active species encountered within the microcapsule. Then the products come back out through the pores again.