Historical Perspective of the Heck Reaction

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
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