Historical Perspective of the Heck Reaction

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Catalysts-The reaction is performed in the presence of an organopalladium catalyst. Palladium is one of the most versatile and efficient catalyst metals in organic synthesis, be it in elemental forms (palladium black and palladium colloids in heterogeneous hydrogenation) or as palladium salts and complexes. Both the renaissance of organometallic chemistry in the 1960s and the subsequent breakthrough of homogeneous organometallic catalysis in laboratory-scale and industrial syntheses have received a major stimulus from palladium coordination chemistry. Typical catalysts are Pd°-phosphine complexes. The catalyst can be tetrakis(triphenylphosphine)palladium(0) {Pd[P(C6H5)3]4}, palladium chloride or , or in-situ catalysts such as palladium(II) acetate Pd(OAc)2 / n P(C6H5)3, with n = 2...4 (OAc = acetate). The most frequently used catalyst is an in-situ combination of Pd(OAc)2 and P(C6H5)3. Palla-dium is practically the only catalyst metal used, in the form of certain Pd° and salts or complexes; normally 1-5 mol% of catalyst is administered. Unfortunately, whilst useful these ‘classical’ catalyst systems suffer from two major limitations. Generally, they need to be used in high loadings — typically a few mol% Pd and they show little or no activity with aryl chloride substrates. For a reaction to be attractive for application in the industrial sector, such as in the fine chemical or pharmaceutical industries, then palladium contamination of the product must be in the low ppm region, often necessitating expensive product clean-up. This, coupled with the high price of not only the palladium but often the ligands, can make the whole process prohibitively expensive Ligands-The ligand is triphenylphosphine or BINAP. Heck and Spencer noticed that phosphines are necessary to somehow "stabilize" the catalysts. Phosphine ligands are expensive, toxic, and unrecoverable. In large-scale applications on industrial and semi-industrial scale, the phosphines might be a more serious economical burden than even palladium itself, which can be recovered at any stage of production or from wastes. The chemical reason is lower reactivity of fully ligated complexes of palladium, the main result of which is the need for higher loads of catalyst to achieve appropriate rates of reaction and therefore further aggravation of procedure cost. Bases-The base is triethylamine (e.g., N(C2H5)3), potassium carbonate K2CO3,or sodium acetate NaOAc 4
Solvents-Heck reactions are conducted in polar aprotic, σ-donor-type solvents such as acetonitrile, dimethyl sulfoxide, or dimethylacetamide. Reaction temperatures and times largely depend on the nature of the organic halide to be activated and on the catalyst's stability limit. Iodo derivatives are much more reactive (
Page 1 and 2: Dimitra Kovala-Demertzi Section of
Page 3: Aryl and vinyl halides- Aryl and vi
Page 7 and 8: Oxidative addition is often the rat
Page 9 and 10: Amino-Heck reaction In the amino-He
Page 11 and 12: All of the palladacycles shown in t
Page 13 and 14: The monoanionic HSal4Et ligand is c
Page 15 and 16: 3.3 Palladium nanoparticles as cata
Page 17: Figure 2. Possible pathways involve

Historical Perspective of the Heck Reaction

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