Supported Palladium Catalysts in Heck Coupling Reactions - Problems,Potential and Recent Advances

Abstract

The review reports the general approach, problems, potential and recent advances in heterogeneously catalyzed Heck reactions. It demonstrates the advantages and limitations for practical applications at present. A literature overview is given separately for substrates that are easier to activate (aryl iodides, activated aryl bromides) and for non-/deactivated aryl bromides and aryl chlorides during the period between 2001 and 2005. New approaches and strategies for the activation of aryl bromides and especially chlorides by heterogeneous catalysts are discussed. The argumentations clearly exceed simple separation and reuse arguments. Particular attention is given to the relation between homogeneous and heterogeneous catalysis from the mechanistic point of view. Very recent reports show clear success in the understanding of corresponding reaction mechanisms. Palladium species dissolved from the support are proven to be responsible for high activity and selectivity in Heck reactions by supported catalysts (Pd on activated carbon, oxides, polymers and in zeolites). The careful choice of optimum catalyst and reaction conditions are crucial and allowed the development of simple heterogeneous catalysts that activate even deactivated aryl chlorides with high yields within few hours of reaction time.

Heck Reaction—State of the Art

“…The problems, potential and recent advances with general approach in heterogeneously catalyzed Heck reactions especially with supported palladium catalyst such as Pd on activated carbon, oxides, polymers and in zeolites has been reviewed by Kohler et al [98]. The advantages and limitations for practical applications are considered.…”

Pd/MOx Materials Synthesized by Sol‐Gel Coprecipitation as Catalysts for Carbon–Carbon Coupling Reactions of Aryl Bromides and Chlorides

“…Dissolution is then induced by an oxidative addition of an aryl halide molecule. [4,15] Another conclusion that can be drawn from this comparison is that higher calcination temperatures also lead to higher catalytic performance. Calcination at high temperatures causes a stronger interaction between Pd and the catalyst support.…”

“…In the case of Pd/SiO 2 catalysts a different procedure had to be applied. A preformed gel, which was obtained by addition of water to a solution of Si(OEt) 4 in ethanol, was dissolved again in 1-butanol. Coprecipitation of Pd/SiO 2 ("Pd-Si") was then achieved by addition of a solution of PdCl 2 in hydrochloric acid.…”

“…In spite of this extended reaction time, only poor conversion (14 %; Table 3, entry 1) was obtained. It is widely accepted that addition of salts like tetra-n-butylammonium bromide N(C 4 H 9 ) 4 Br exhibits a promoting effect in Heck reactions. [2,15,20,21] The influence of different tetran-alkylammonium halides was studied and the results obtained are presented in Table 3.…”

Polyacrylonitrile fiber mat‐supported palladium catalyst for Mizoroki–Heck reaction in aqueous solution

“…[5,6,10] Thus, it is tedious and time-consuming to separate and recycle these particulate heterogeneous catalysts by filtration. Clearly, it is much easier to separate and recycle the Pd/PAN fiber mats owing to the large fiber mat structure.…”