Heck Chemistry - a Highly Active Ligand-Free Metal Catalyst System in Situ Generated from Pd&lt;SUP&gt;II&lt;/SUP&gt; Supported on SiO&lt;SUB&gt;2&lt;/SUB&gt;

Lin Huang,Yu Wang, Zhan Wang, Fengxi Chen,

doi:10.5923/j.pc.20120201.06

Abstract

A ligand-free Pd catalyst system derived from Pd(acac)2/SiO2 and its catalytic properties toward the Heck coupling of bromobenzene and styrene have been studied. Combined studies by X-ray photoelectron spectroscopy (XPS), ultraviolet and visible absorption spectroscopy (UV-vis), thermogravimetric analysis (TGA) and powder X-ray diffraction (XRD) indicate that calcination of Pd(acac)2/SiO2 in air at 400℃ produces PdII/SiO2 that contains isolated Pd2+ ions coordinated to the lattice oxygen as the dominant component and PdO particles as the minor component. XPS and UV-vis observations together with catalytic results reveal that most of PdII is reduced to Pd0 on SiO2 by the reaction media during the Heck reaction and that the resulting catalyst system is highly active. Transmission electron microscopy measurements show that the in situ prepared Pd0/SiO2 has much finer Pd particles than conventional Pd0/SiO2. The rapid growth of the supported Pd particles during the highly efficient Heck reaction suggests that the catalysis occurs via an Ostwald ripening mechanism. Resulting in excellent catalytic properties, PdII/SiO2 possesses incomparable advantages over conventional Pd0/SiO2 as a precatalyst in catalytic activity, catalyst recycling and product contamination control.

Highlights

We have focused our research on catalysis by the Pd/SiO2 system for the Heck coupling of bromobenzene (PhBr) and styrene shown in Scheme 1.27-30 We have ever reported a preliminary study relating to a highly active
We present extended studies concerning the performances of the catalyst system in situ generated from PdII/SiO2 for the Heck reaction
Under the reaction conditions used here, most of the isolated surface Pd2+ ions and the PdO particles are reduced to the supported Pd particles by the reaction media

Summary

Introduction

Among the various ways to synthesize arylated olefins, the construction of C(sp2)-C(sp2) single Bonds through Heck coupling is probably the most attractive way.[1,2,3,4,5] It constitutes a very selective approach of forming new C-C bonds in a single operational step by coupling aryl and vinyl halides/triflates with olefins, in order to obtain a variety of substituted olefins, dienes and other unsaturated compounds.[2,3,5]. The only example of PdII/SiO2 used as a precatalyst has emerged in a letter about Heck couplings of aryl iodides and alkyl acrylates in the presence of Pd(NH3)4Cl2/SiO2.23 This supported complex was prepared by an ion-exchange method and was not subjected to any further treatments prior to Heck reactions. The effect of the supported metal particle size on Heck reactions is referred to homogeneous catalysis instead of heterogeneous catalysis

Behavior of Various Pd Catalysts in Heck Reactions

Chemistry of PdII Supported on SiO2 with Regards to Heck Reactions a

Conclusions

Experimental