Halides, aryl, arylation alkenes

Another important type of reactivity of palladium, namely oxidative addition to Pd(0), is the foundation for several methods of forming carbon-carbon bonds. Aryl126 and alkenyl127 halides react with alkenes in the presence of catalytic amounts of palladium to give net substitution of the halide by the alkenyl group. The reaction, known as the Heck reaction,128 is quite general and has been observed for simple alkenes, aryl-substituted alkenes, and substituted alkenes such as acrylate esters, vinyl ethers, and A-vinylamides.129... 

Intramolecular arylation of alkenes. The intramolecular palladium-catalyzed condensation of aryl halides with an alkene group (Heck arylation5) can actually proceed more readily than the original bimolecular version. These intramolecular cyclizations typically proceed in acetonitrile at room temperature, particularly when catalyzed by Ag2C03, which also inhibits isomerization of the double bond of the product. They can be used to obtain spiro, bridged, and fused systems. Even tetrasubstituted alkenes can participate, with formation of quaternary centers.6... 

This reaction involves the two reactants carbon monoxide and alcohol and produces esters, or lactones. The starting material, which will be considered here, is an alkene or an alkyne but it is also possible to start from activated halides (aryl- or allyl- iodides and bromides) to produce the same kind of organic products. 

C-C bond formation using the Heck reaction allows the introduction of functional groups to obtain new organic structures on solid supports. This reaction between an alkene with an alkenyl or an aryl halide has been widely employed in various in-tra- and inter-molecular versions on solid-phase because of the readily accessibility of starting materials. The Heck reaction was performed on immobilized aryl or alkenyl halides with soluble alkenes and vice versa (Scheme 3.11). 

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. 

Aryldiazonium salts react with bis(dibenzylideneacetone)palladium to form arylpalladium salts and nitrogen. Therefore, diazonium salts may be employed to catalytically arylate alkenes under mild conditions. Since many aryl halides are made from diazonium salts this variation could even be more convenient than using aryl halides. The reaction proceeds in good to excellent yields in nonaqueous solvents, using sodium acetate as the base at room temperature with terminal alkenes and cyclopentene." Internal alkenes usually give poor yields, however. 

The palladium-catalyzed C-C coupling between aryl halides or vinyl halides and activated alkenes in the presence of a base is referred as the Heck Reaction . Recent developments in the catalysts and reaction conditions have resulted in a much broader range of donors and acceptors being amenable to the Heck Reaction. 

Kondolff, I. Doucet, H. Santelli, M. Tetra-phosphine/Pd-catalyzed Heck reactions of aryl halides with disubstituted alkenes. Tetrahedron Lett. 2003, 44, 8487-8491. 

The Heck reaction is the palladium-catalyzed coupling of an aryl or vinyl halide with an alkene to give a new C—C bond at the less substituted end of the alkene, usually with trans stereochemistry. 

Oxidative addition to Pd (0) is the oxidation of choice used in several methods to form C— C bonds. Aryl and alkenyl halides react with alkenes in the presence of catalytic amounts of Pd and substitute the halide with the alkenyl group. This is called the Heck reaction (see Welton, 1999) ... 

Treatment of alkali metal alkyl or aryltellurolates (Sections) with alkyl halides, aryl halides, alkenes, or alkynes have been nsed to prepare unsymmetrical tellurides, as exemplified by the reaction of sodium phenyl teUurolate or paramethylphenyl tellnrolate with PhC CHCOPh. ... 

The direct coupling of an aryl halide with an alkene to produce a phenylethene is known as the Heck reaction (Scheme 10.23). The mechanism involves coordination of the alkene to the palladium to form a k-complex 9 with which the arene ligand can react. A variety of substituents on the alkene is compatible with the reaction. 

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 triaiylphosphine [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. 

The palladium-catalyzed Heck reaction of a vinyl or aryl halide with an alkene... 

Heck reaction (Section 26.3) The palladium-catalyzed coupling of a vinyl or aryl halide with an alkene to form a more highly substituted alkene with a new carbon-carbon bond. 

The carbon-halogen bond is stronger in vinyl halides than in alkyl halides, and Ni complexes with slightly lower reduction potentials have to be used to catalyze the reductive cyclization of vinyl halides on to alkene functions. Using conditions similar to those for the alkyl and aryl halides, 5-c.vo-cyclized products were obtained from 88 [237]. In a single case (methyl substitution at the position of S- .vc-attack), only the 6- /o-product was obtained [Eq. (53)] [237]. 

Negishi, E., Takahashi, T., Baba, S., Van Horn, D. E., Okukado, N. Nickel- or palladium-catalyzed cross coupling. 31. Palladium- or nickel-catalyzed reactions of alkenylmetals with unsaturated organic halides as a selective route to arylated alkenes and conjugated dienes scope, limitations, and mechanism. J. Am. Chem. Soc. 1987,109, 2393-2401. 

The rate of reaction and regioselectivity of Heck olefination are sensitive to steric hindrance about the C=C bond of the vinylic partner. For simple aryl halides reacting with alkenes, the rate of reaction as a function of alkene substitution varies95 according to... 

Regioselectivity of Addition of an Aryl Halide to Various Alkenes during Heck Olefl nation... 

Some authors have described the generation of boronates and stannanes bound to a support starting from the corresponding aryl haHdes [372, 6]. Boranes, boronic esters, and stannanes can furthermore be readily obtained from vinyl halides or from alkenes or alkynes by means of hydroboration or hydrostannylation (see Section 4.2). Boronates and silanes or stannanes can act as carbanion equivalents. Thus, support-bound boronates can release aryl alkenyl groups upon transmetala-tion to Rh. The intermediately formed Rh species can act as nucleophiles, and react with aldehydes to give alcohols (618) or can perform Michael additions (621, 622) [437, 438, 439] (Scheme 129) (see also Sections 4.7.15 and 4.2). 

Conjugate additions. Cobalt bromide catalyzes electrochemical addition of aryl halides to activated alkenes. 

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