Palladate salts from ionic liquids as catalysts in the Heck reaction

Florence Gayet,Jean-Daniel Marty,Nancy Lauth-De Viguerie

doi:10.3998/ark.5550190.0009.h06

Abstract

Palladate salts from imidazolium and pyridinium ionic liquids (ILs) were used as catalysts in Heck reaction. When compared to traditional precatalysts, they demonstrated to be more efficient catalysts. These results are attributed to the presence of nanoparticles (NPs) which served as a reservoir of palladium species. Indeed the use of palladate salts from ILs, that produce no byproducts, provide better control of the formation and surface composition of the NPs leading to a constant recyclability.

Highlights

The Heck reaction is one of the most important methods in synthetic organic chemistry for the formation of C-C bonds.[1,2] The reaction is frequently catalysed by palladium catalysts in the presence of a phosphine and a base at high temperatures
Certain cases, this enabled the regioselectivity of the products obtained to be controlled;[16] (iii) The involvement of Pd nanoparticles (Pd NPs) as a reservoir of molecular palladium species, as proposed by Dupont[17] and in agreement with the results obtained by de Vries 18 has received abundant experimental support especially in the case of ligand-free palladium catalysts.[17,19]
Bis(1-butyl-3-methylimidazolium) tetrachloropalladate noted [Bmim]2[PdCl4] was synthesized according to a procedure described by Dupont et al.[22] and each pure stereoisomers and racemate of bis(1-phenylethylpyridinium) tetrachloropalladate [Pyr]2[PdCl4] were prepared as previously described in quantitative yield by reacting palladium chloride with a 2-molar excess of the (R), (S)- and-1-phenylethylpyridinium chloride respectively in acetonitrile for 24 hours under reflux.13,23 [Pyr]2[PdCl4] and [Bmim]2[PdCl4] are obtained as light brown and yellow-orange monocrystals respectively with crystal X-ray structures described in literature.[22,23]

Summary

Introduction

The Heck reaction is one of the most important methods in synthetic organic chemistry for the formation of C-C bonds.[1,2] The reaction is frequently catalysed by palladium catalysts in the presence of a phosphine and a base at high temperatures. A large variety of Pd(II) and Pd(0) complexes, among which Pd(OAc)[2], Pd2(dba)[3] are the most commonly used, can serve as effective precatalysts, or precursors to the active Pd(0) catalyst. Polar aprotic solvents such as DMF and CH3CN are the most commonly used solvents in this reaction. In order to develop systems that minimize the formation of chemical by-products, the use of metalcontaining ILs that only generate IL moieties in the media, could be of special interest.[20] such precursors should permit, especially when NPs are involved, a better control of the surface chemistry of the active species with IL species bonded to the NP surfaces. Itoh et al.[21] have shown that when gold NPs were bound to ILs, the hydrophobic and hydrophilic properties of the NPs can be tuned by exchanging the anions of the IL moiety

Methods

Results

Conclusion