Vinyl halides carbon-transition metal bonds

Pd-catalyzed metallation of carbon electrophiles is also applicable to carbon-transition metal bond formation. Beletskaya and colleagues have reported that, in the presence of a Pd(II) catalyst, zinc salts of metal (Fe, Re, W, and Mn) carbonylates smoothly react with aryl and vinyl halides to afford the corresponding rj -aryl and Tj -vinyl complexes (Table 7 and Scheme 16). " The metallation of vinyl halides proceeds with stereochemical retention as in the case with other metal nucleophiles. The use of alkali salts of the metal carbonylates considerably reduces the yields of coupling products. 

The transition metal catalyzed carbon-carbon bond formation between organomagnesium reagents and aryl (vinyl) halides has been one of the pioneering entries into cross-coupling chemistry. The reaction has been widely utilized since than in azine chemistry,22 with the limitation that the functional group tolerance of Grignard reagents is only moderate. Here only some of the more recent developments will be mentioned. 

Among common carbon-carbon bond formation reactions involving carbanionic species, the nucleophilic substitution of alkyl halides with active methylene compounds in the presence of a base, e. g., malonic and acetoacetic ester syntheses, is one of the most well documented important methods in organic synthesis. Ketone enolates and protected ones such as vinyl silyl ethers are also versatile nucleophiles for the reaction with various electrophiles including alkyl halides. On the other hand, for the reaction of aryl halides with such nucleophiles to proceed, photostimulation or addition of transition metal catalysts or promoters is usually required, unless the halides are activated by strong electron-withdrawing substituents [7]. Of the metal species, palladium has proved to be especially useful, while copper may also be used in some reactions [81. Thus, aryl halides can react with a variety of substrates having acidic C-H bonds under palladium catalysis. 

Transfer of organic groups from tin to carbon electrophiles, e.g. alkyl halides and acyl hahdes, can occur in the presence of a transition metal (e.g. Pd) catalyst (equation 40). Reactivity sequences for elecfrophihc carbon-tin bond cleavages are generally allyl > phenyl > benzyl > vinyl > methyl > higher alkyl. The precise sequence is somewhat dependent on the solvent and electrophile. 

The 1,2-insertion of alkenes into transition metal-carbon o-bond leads to C-C bond formation under mild conditions, as described in Chapter 6. This reaction is considered to be a crucial step in the coordination polymerization and carbometalation of alkenes catalyzed by transition metal complexes. A common and important carbometalation is the Heck-type arylation or vinylation of alkene catalyzed by Pd complexes [118], The arylation of alkene, most typically, involves the formation of arylpalladium species and insertion of alkene into the Pd-aryl bond as shown in Scheme 5.20. The arylpalladium species is formed by the oxidative addition of aryl halides to Pd(0) complexes or the transmetalation of aryl compounds of main group metals with Pd(II) complexes. Insertion of alkene into the Pd-aryl bond produces 2-arylalkylpalladium species whose y6-hydrogen elimination leads to the arylalkene. Oxidative chlorination of the 2-arylalkylpalladium intermediate forms chloroalkanes as the product. 

The generation of ethers through the formation of oxygen-carbon(sp ) bonds (Williamson approach) is rare due to the sluggish reactivity of vinyl and aryl halides in nucleophilic substitution. Furthermore, while nucleophihc substitution chemistry tends to work well for primary systems, significant amounts of elimination products are often observed with secondary alkyl hahdes, and tertiary substrates are typically unresponsive. As a result, a number of transition metal-catalyzed routes to the formation of ethers through the formation of oxygen-carbon(sp ) bonds have been developed. 

Due to sluggish reactivity of aryl and vinyl halides in nucleophiUc substitution reactions, the formation of sulfur-carbon(sp ) bonds is typically carried out using transition metal catalysis [22-27]. While the field is dominated by the use of palladium, copper, and nickel catalysts, considerable advances have been made using more abundant metal catalysts such as iron. Additionally, a number of transition metal-fiee approaches have been developed for the formation of sulfur-carbon(sp ) bonds. The following sections will highlight representative examples of C—S bond forming reactions. 

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