Alkenes haloarenes, reaction with

The palladium(II)-catalyzed reaction of haloarenes with alkenes and alkynes (Heck-type reactions) in aqueous media has become known only in the preceding few years [Eq. (14)]. 

Palladium(0)-catalysed coupling reactions of haloarenes with alkenes, leading to carbon-carbon bond formation between unsaturated species containing sp2-hybridised carbon atoms, follow a similar mechanistic scheme as already stated, the general features of the catalytic cycle involve an oxidative addition-alkene insertion-reductive elimination sequence. The reaction is initiated by the oxidative addition of electrophile to the zero-valent metal [86], The most widely used are diverse Pd(0) complexes, usually with weak donor ligands such as tertiary phosphines. A coordinatively unsaturated Pd(0) complex with a formally d° 14-electron structure has meanwhile been proven to be a catalytically active species. This complex is most often generated in situ [87-91],... 

An important feature of the Suzuki reaction is that it allows cr/Z Z-alkenyl or alkyl-aryl coupling of fi-a//cy/-9-borabicyclo[3.3.3]nonane derivatives with haloalkenes and haloarenes without concomitant p-hydride elimination. The required organoboranes are obtained by hydroboration of the appropriate alkenes or functionally substituted alkenes with 9-BBN. The coupling reaction tolerates the presence of functional groups in both reaction partners, thus circumventing a requirement of their prior pro-tection. ... 

The Heck reaction can be used to couple alkenyl, aryl, allyl (the intramolecular Heck reaction on aUyUc substrates is called a palladaene reaction),f ° t f benzyl, methyl, alkoxycarbonyhnethyl, alkynyl, certain alkyl, and silyl fragments to a variety of alkenes. The nature of the leaving group greatly affects the reaction rate aryl iodides react faster than bromides, and aryl chlorides are notoriously unreactive unless special catalysts or ligands and elevated temperatures are used to enhance the reaction rate. This has been taken to indicate that the oxidative addition of the haloarene (haloalkene) to pal-ladium(O) is the rate-determining step. " It has been shown that the Heck reaction can be performed with aryldiazonium salts, A -nitroso-Af-arylacetamides, and hy-pervalent iodo compounds " at room temperature. 

The o-complexes generated by oxidative addition of haloarenes and haloalkenes to paUadium(O) are electrophilic at the metal-substituted center and can therefore react with nucleophiles other than alkenes, especially with enolate and homoenolate ions to form new C-C bonds [331, 332]. This reaction mode has been termed anion capture [333]. 

The cross-coupling reaction ofB-R-9-BBN occurs readily with 1-haloalkenes or haloarenes in the presence of catalytic amount of PdCljfdppf) and bases such as sodium hydroxide, potassium carbonate, and potassium phosphate and affords the corresponding alkenes or arenes. The reaction does not take place in the absence of base due to poor nucleophilic character of R of B-R-9-BBN, and the reaction also fails with weak base such as sodium acetate. 

The direct photolysis of alkyl or aryl halides in solution to form carbon-centered radicals is rarely used in organic synthesis." Alkyl iodides usually afford mixtures of radical and ionic products, while alkyl bromides can produce radical-derived products but in low yield. A notable exception is the photocycliza-tion of haloarenes, which has been shown to produce carbon-centered radicals that can add to aromatic rings. A similar reaction has recently been observed on irradiation of iodoheterocycles, with substituted benzenes or electron-poor alkenes, to form arylated or alkylated heterocycles in good yield. Related reactions have also been reported on irradiation of 4-chloroanilines in the presence of (electron-rich) alkenes, although in this case, the alkylations appear to involve the formation of a phenyl cation. An alternative approach to form carbon-centered radicals is to irradiate the alkyl iodide or bromide in the presence of triethylamine this is proposed to form an amine-haHde exciplex, which cleanly breaks down to give a carbon-centered radical and a halide anion. Cossy and co-workers have shown this to be a fast, convenient, and chemoselective method of radical generation, which has recently been used to prepare the bicyclic core of ( )-bisabolangelone (Scheme 1). ... 

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