Vinylic palladium-triarylphosphine

This very brief survey of the palladium-triarylphosphine-catalyzed reaction of vinylic halides with olefins and amines is intended to show the wide applicability of the reaction to synthesizing numerous types of polyfunctional aliphatic (and aromatic) compounds. We have much more to do in several areas, but it is already clear that the reaction will be very valuable for synthesizing numerous types of organic compounds. 

R. F. Heck, Adv. Chem. Ser, 1982, 196, 213-230. Palladium-Triarylphosphine Complexes as Catalysts for Vinylic Halide Reactions. 

Dehalogenation of monochlorotoluenes can be readily effected with hydrogen and noble metal catalysts (34). Conversion of -chlorotoluene to Ncyanotoluene is accompHshed by reaction with tetraethyl ammonium cyanide and zero-valent Group (VIII) metal complexes, such as those of nickel or palladium (35). The reaction proceeds by initial oxidative addition of the aryl haHde to the zerovalent metal complex, followed by attack of cyanide ion on the metal and reductive elimination of the aryl cyanide. Methylstyrene is prepared from -chlorotoluene by a vinylation reaction using ethylene as the reagent and a catalyst derived from zinc, a triarylphosphine, and a nickel salt (36). 

The palladium-catalyzed carbonylation of isomeric vinylic halides shows the reaction to be reasonably stereospecific and proceed with retention of the original halide structure. The degree of specificity, however, depends somewhat on the reaction conditions. Low reaction temperatures and/or excess triarylphosphine favor the stereospecific reaction (11). 

Normally, the most practical vinyl substitutions are achieved by use of the oxidative additions of organic bromides, iodides, diazonium salts or triflates to palladium(0)-phosphine complexes in situ. The organic halide, diazonium salt or triflate, an alkene, a base to neutralize the acid formed and a catalytic amount of a palladium(II) salt, usually in conjunction with a triarylphosphine, are the usual reactants at about 25-100 C. This method is useful for reactions of aryl, heterocyclic and vinyl derviatives. Acid chlorides also react, usually yielding decarbonylated products, although there are a few exceptions. Likewise, arylsulfonyl chlorides lose sulfur dioxide and form arylated alkenes. Aryl chlorides have been reacted successfully in a few instances but only with the most reactive alkenes and usually under more vigorous conditions. Benzyl iodide, bromide and chloride will benzylate alkenes but other alkyl halides generally do not alkylate alkenes by this procedure. 

Triarylphosphines, which are often employed with palladium in catalysts, also can transfer aryl groups to the palladium and cause vinyl substitution reactions with alkenes.49,30 Fortunately, this reaction is usually slower than other methods for generating arylpalladium derivatives so that it usually is not a problem, but there are exceptions (equation 16). 

Palladium acetate triarylphosphine complexes catalyze the addition of vinylic groups from vinylic halides to olefinic compounds in the presence of amines. Conjugated dienes are major products from 0,/3-unsaturated acids, esters, or nitriles while unactivated olefinic compounds react best in the presence of secondary amines where allylic amines are major products. The reactions are usually regio- and stereospecific. The synthetic utility of the reaction is illustrated with a wide variety of examples. 

The Heck reaction is a palladium-catalyzed coupling of a vinyl or aryl halide with an alkene to form a more highly substituted alkene with a new C-C bond. Palladium(II) acetate [Pd(OAc)2] in the presence of a triarylphosphine [P(o-tolyl)3] is the typical catalyst, and the reaction is carried out in the presence of a base such as triethylamine. The Heck reaction is a substitution reaction in which one H atom of the alkene starting material is replaced by the R group of the vinyl or aryl halide. 

---