0-Aryl complexes from metal halides

Palladium(II) is one of the most important transition metals in catalytic oxidations of allenes [1], Scheme 17.1 shows the most common reactions. Transformations involving oxidative addition of palladium(O) to aryl and vinyl halides do not afford an oxidized product and are discussed in previous chapters. The mechanistically very similar reactions, initiated by nucleophilic attack by bromide ion on a (jt-allene)pal-ladium(II) complex, do afford products with higher oxidation state and are discussed below. These reactions proceed via a fairly stable (jt-allyl)palladium intermediate. Mechanistically, the reaction involves three discrete steps (1) generation of the jt-allyl complex from allene, halide ion and palladium(II) [2] (2) occasional isomeriza-... 

In this context it is interesting to note that benzonitrile, Ph—C=N, trimerizes to a triazine on a Raney nickel surface. It was assumed that Jt-bonded nitriles were involved in the reaction mechanism.10 This reaction resembles the well-known template synthesis of phthalocyanine complexes from phthalodinitrile. Formation of linear polymers [—C(R)—N—] occurs on heating aryl or alkyl cyanides with metal halides.11... 

A three-component reaction based on the umpolung of re-allylpalladium (II) complexes indium metal was reported by Grigg and co-workers (Scheme 8.31) [74]. In this reaction, the electrophilic nature of the n-allyl palladium species generated from aryl halides and allenes is reversed by transmetallation with indium metal. The resultant nucleophilic allylindium reagent subsequently adds to the third component - aldehyde [75] or imine [76] - to give the corresponding homo-allylic alcohol 64 or amine 65 respectively. 

In the original process using tin amides, transmetallation formed the amido intermediate. However, this synthetic method is outdated and the transfer of amides from tin to palladium will not be discussed. In the tin-free processes, reaction of palladium aryl halide complexes with amine and base generates palladium amide intermediates. One pathway for generation of the amido complex from amine and base would be reaction of the metal complex with the small concentration of amide that is present in the reaction mixtures. This pathway seems unlikely considering the two directly observed alternative pathways discussed below and the absence of benzyne and radical nucleophilic aromatic substitution products that would be generated from the reaction of alkali amide with aryl halides. 

Synthesis of aryl- or vinyl-metal compounds is best accomplished in two steps. Initially an acyl-metal complex can be formed by nucleophilic displacement of halide from an acyl halide viz. ... 

Organophosphorus compounds find wide use in the chemical industry as catalysts, intermediates, complexes, and end-use products. Arylphosphines and phosphine oxides are often produced by the reaction of a preformed Grignard reagent with a halophosphine or phosphine oxide. Yields are reduced by the production of unwanted side-reaction products such as biaryls. These unwanted products are reduced when the reaction is conducted under Barbier conditions. When alkyl and aryl halides are reacted with magnesium metal, a trihalophosphine or phosphine oxide, a metal halide or amine catalyst, in THE benzene mixtures, at reflux, good yields of phosphines or phosphine oxides are obtained [74]. For example, triphenylphosphine can be prepared in a 97.2% yield from the reaction of bromobenzene, trichlorophosphine, magnesium metal, aluminum chloride, and sodium chloride in THF-benzene at 70 80 C. 

A number of other chiral ligands are available, but have been studied much less extensively. Hie chiral diamines (106) and (107) are reported to mediate the reactions between aryl Grignards and aldehydes (Bgure 22 equation 26). Hie alcohols range in optical purity from 40 to 75% ee selectivities increase with the bulkiness of the aldehyde substituent (see Table 28). Hie use of an aiyloxy metal halide to complex the aldehyde moiety enhances the observed enantioselectivities. °... 

The term Heck reaction summarizes catalytic C-C-coupling processes, such that a vinylic hydrogen is replaced by a vinyl, aryl, or benzyl group, with the latter being introduced from a halide or related precursor compound (cf. eq. (3)) [9 a, 14-16]. Therefore, the final step of product formation is the elimination of a hydrogen halide, and a base is thus required to bind the acid. The olefinic (vinylic) double bond is retained throughout the Heck reaction. Palladium is practically the only catalyst metal used, in the form of certain Pd and Pd" salts or complexes normally 1-5 mol % of catalyst is administered. 

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