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Published in Joseph C. Salamone, Polymeric Materials Encyclopedia, 2020
The Heck reaction is a palladium-catalyzed vinylation of organic halides (Scheme I), forming a carbon–carbon bond. Various organic halides, such as aryl, heterocyclic, benzyl, or vinyl bromides or iodides, can be utilized in this reaction. Organic halides with β-hydrogen atoms can not be used because the β-elimination will lead to the formation of an olefin under the normal Heck reaction conditions. A base is necessary to remove the hydrogen halide so that the catalytic cycle can be completed. When organic bromide is used, a ligand such as triarylphosphine or a secondary amine is required.
Interfacial Catalysis at Oil/Water Interfaces
Published in Alexander G. Vdlkdv, Interfacial Catalysis, 2002
Of great value for organic synthesis are the Pd-catalyzed processes of vinylation of aryl halides (Heck reaction), and ethynylation of aryl or vinyl halides, etc. Although these reactions are successfully executed in a homogeneous medium, in many cases use of PTC in heterogeneous systems give significant advantages as far as yields, selectivities and procedures are concerned:
Dendrimers in Supramolecular Catalysis
Published in Jubaraj Bikash Baruah, Principles and Advances in Supramolecular Catalysis, 2019
Dendrimers are used as ligands to prepare metal complexes of different nuclearities. Conventionally, mononuclear complexes are formed when the pivot part acts as a binding site to occupy the coordination site of metal ions. Such complexes are used to provide hydrophobic confinement to metal complexes of prepared heterogeneous metal complexes. The nickel complex 5.3b shown in Figure 5.3 is formed through a pincer-type ligand which is anchored through an amide bond to a dendrimer polyether. The complex is a useful catalyst of C─C bond formation between ethylene with styrene to form but-3-en-2-ylbenzene as a major product. The dendrimer 5.3a has nickel complexes bound at its exterior. Preparing these complexes with uniform distributions of metal complexes is difficult due to steric and electronic factors. Carbosilane dendrimers functionalised with pincer-nickel(II) groups 5.3b are created through the binding of NCN sites to the nickel(II) ion. Such complexes are used in regioselective catalysts in the reaction of alkenes with halogenated compounds under photochemical conditions to form polyhalogenated alkanes. In this catalysis, passivation of the catalyst occurs due to the irreversible formation of inactive nickel (III) species. Dendrimers with denser peripheries in metal sites are deactivated easily. Such an effect is called the proximity effect occurring between nickel centres. The addition of carbon tetrachloride to methyl methacrylate is catalysed by the dendrimer by these uniformly decorated nickel complexes at the outer periphery. The catalytic activity of the mononuclear NCN-pincer nickel complex is similar to the dendritic complex. The reaction passes through homogeneous catalytic paths, as revealed by a study on the reactivity of a third-generation nickel dendrimer by compartmentalising the reactant and catalyst via membrane. The dendrimer also catalyses hydro-vinylation of styrene and ethylene to form 3-phenyl-2-butene as a major product. Depending on the catalyst, concentration and reaction conditions, the dendrimer catalyses the racemization reaction of the chiral 2-phenyl-2-butene. Hence, there is a necessity to optimise catalyst performance to achieve stereo-selective products.
Palladium nanoparticles immobilized on EDTA-modified Fe3O4@SiO2: a highly stable and efficient magnetically recoverable catalyst for the Heck–Mizoroki coupling reactions
Published in Inorganic and Nano-Metal Chemistry, 2019
Mohsen Esmaeilpour, Saeed Zahmatkesh
Carbon–carbon bond construction is the basis of organic chemistry, and transition metals catalysts represent the most useful tools to obtain cross-coupling reactions.[13] The palladium-catalyzed arylation and vinylation of olefins, known as the Heck reaction,[14] is a powerful and modern method for generation of C−C bonds in organic synthesis, from both the industrial and research points of view.[15–17] In the past few years, the development of ligands has provided highly active homogeneous palladium catalysts for Heck reactions.[18–20] However, the separation and recovery of homogeneous catalysts are not easy and so it is still significant to prepare more active heterogeneous catalysts and to find effective ways of heterogenizing homogeneous catalysts for industrial reaction processes.[21–24] In this term, nanoparticles have recently generated a new research line in the catalytic activity of the supported catalysts because of their large surface area, higher loading capacity and higher dispersion.[25–27] Furthermore, the use of moisture sensitive expensive palladium complexes, additives, tedious workup procedure, and longer reaction time draw a line of limitation over the previous methods.[28,29] Therefore, in recent years, numerous papers have been reported concerning improvements to the Heck reaction,[30–33] but it still is an attractive synthetic goal in academia as well as in industry. Thus, in continuation of our interest on the application of supported catalysts,[34–36] we focused our attention on an efficient method for Mizoroki–Heck cross-coupling reactions in the presence of Fe3O4@SiO2-EDTA-Pd NPs (Figure 1; Scheme 1).