Palladium‐Catalyzed Vinylation of Organic Halides

Abstract The palladium‐catalyzed vinylation of organic halides provides a very convenient method for forming carboncarbon bonds at unsubstituted vinylic positions. Generally the reaction does not require anhydrous or anaerobic conditions although it is advisable to limit access of oxygen when arylphosphines are used as a component of the catalyst. The transformation is valuable because it cannot be carried out in a single step by any other known method (except in certain Meerwein reactions). The organic halide employed is limited to aryl, heterocyclic, benzyl, or vinyl types, with bromides and iodides seen most often. Halides with an easily eliminated beta‐hydrogen atom ( i.e. , alkyl derivatives) cannot be used since they form only olefins by elimination under the normal reaction conditions. The base needed may be a secondary or tertiary amine, sodium or potassium acetate, carbonate, or bicarbonate. When nucleophilic secondary amines are used as coreactants with most vinylic halides, a variation occurs that often produces tertiary allylic amines as major products. The catalyst is commonly palladium acetate, although palladium chloride or preformed triarylphosphine palladium complexes, as well as palladium on charcoal, have been used. A reactant, product, or solvent may serve as the ligand in reactions involving organic iodides, but generally a triarylphosphine or a secondary amine is required when organic bromides are used. The reaction, which occurs between ca. 50° and 160° proceeds homogeneously. Solvents such as acetonitrile, dimethylformamide, hexamethylphosphoramide, N‐methylpyrrolidinone, and methanol have been used, but are often not necessary. The procedure is applicable to a very wide range of reactants and yields are generally good to excellent. Several variations of the reaction are known in which the organic halide is replaced by other reagents such as organometallics, diazonium salts, or aromatic hydrocarbons. These reactions are not discussed in detail, but are only briefly compared with the halide reaction. Other related reactions such as the palladium‐catalyzed replacement of allylic substituents with carbanionic reagents, the palladium‐promoted nucleophilic substitutions at olefinic carbons, and the numerous palladium‐catalyzed coupling reactions of halides and organometallics are also beyond the scope of this review. The palladium‐catalyzed vinylic substitution reaction has not yet received much attention from organic chemists, but its broad scope and simplicity demonstrate that it is a useful method for the synthesis of a variety of olefinic compounds.