Preparation Of Electrospun Heterogeneous Pd Catalyst And Study Of The Mechanism Of Sonogashira Coupling Reaction

Palladium-catalyzed Sonogashira coupling reaction as the most straightforward and powerful method of the construction of C(sp²)-C(sp) bond,has been widely used for the synthesis of natural products,drugs and polymeric materials.Despite of high reaction rate and high turnover numbers(TON),homogeneous Pd catalysis has a number of drawbacks,for instance,high cost,complicate aftertreatment of reaction product,lack of recycling and reuse of the catalyst and so on.This leads to a loss of expensive metal and ligands and to impurities in the products.In order to address these problems,heterogeneous Pd catalysis is a promising option.Novel Pd-carrying composite carbon nanofibers based on polyacrylonitrile were prepared by electrospinning and carbonization process.The catalytic activities of the composite nanofibers were tested with a Sonogashira coupling reaction of iodobenzene and phenylacetylene.The results first showed that the catalyst not only had a high catalytic activity, but also 1D nanofibers made the catalyst easy separation from a reaction mixture and the finn bindings between the metal nanoparticles and the supporting nanofibers made the catalyst be of leaching-resistant for a good retrieval and reusability for Sonogashira coupling reaction.Since a rough surface is suitable to the catalytic activity,some methods were used to increase the surface area of nanofibers:using H₂O₂ to oxidize the nanofibers;electrospinning the mix solution of PAN and PS for the preparation of the surface-rough nanofibers.The results show that the carbon nanofibers based on the two polymers(PAN and PS) have a higher specific surface area,and the Pd catalyst composite has a higher catalytic activity.Since it is of great importance to understand the reaction,the mechanisms of the Sonogashira coupling reaction have been investigated using the quantum chemical calculations.The results show:(1) The Sonogashira coupling reaction of iodobenzene and phenylacetylene catalyzed by Pd(0) atom was studied theoretically by means of DFT calculations using the Accelrys' DMol³ program.This catalytic cycle typically occurs in three stages:oxidative addition of iodobenzene to Pd(0) atom;alkynylation of palladium(â…¡) intermediate;reductive elimination to diphenylacetylene and to regenerate the Pd(0) atom.The stage of alkynylation remands the highest activation energy in the whole catalysis cycle and is considered as the rate-determining step.(2) The mechanism of the homogeneous palladium catalysis for Sonogashira coupling reaction has been studied theoretically by DFT(density functional theory) calculations using the Gaussian03 pin,am.The model system studied consists of Pd(PH₃)₂ as the starting catalyst complex,phenyl bromide as the substrate and acetylene as the terminal alkyne, without involving co-catalyst and base.The DFT analysis shows that the catalytic cycle occurs in three stages:Oxidative addition of phenyl bromide to palladium center;Alkynylation of palladium(â…¡) intermediate;Reductive elimination to phenylacetylene.(3) On the basis of the mechanism of Pd(0) atom catalysis for Sonogashira coupling reaction,the geometry and electron structures of Pd_nclusters(n=2-8,13),and the structures of intermediates of Pd_n dusters and substrates,the mechanism of heterogeneous palladium catalysis for Sonogashira coupling reaction mechanism has been discussed.Pd metal clusters, considered as active centers in heterogeneous catalytic coupling reactions,are ready to attract the electron from the reactant and weaken the bond of the reactant molecular.Metal clusters have couple activate sites and an outstanding catalytic activity compared to the Pd(0) atom and homogeneous catalyst.