Oxidative addition aryl halides

With aryl halides oxidative addition is often the rate-determining step this is promoted by appropriate ligands such as triarylphosphines. Finely divided palladium metal or ligand-free palladium, prepared in situ by reduction of palladium(II) species, can also be used in weak donor media. Palladium(0) with a triarylphosphine as ligand reacts with aryl halides in the order Arl > ArBr> ArCl. Electron-withdrawing groups on the aryl halide increase the reactivity... 

Systems for which suggestions of intermediate Pd(IV) species formed by aryl halide oxidative addition have been proposed include reactions such as that of Scheme 9 developed by Catellani, Chinsoli and and de Meijere... 

Systems for which Pd(II)-Pd(IV) cycles are consistent with model reactions involve alkyl halide oxidative addition or oxidation by strong inorganic oxidants. Thus, the reaction of Scheme 10 may be explained by a process very similar to that for Scheme 9 but involving alkyl halide (RT) addition modeled by formation of 27 and its decomposition to 28, cyclization and repetition of this process, and subsequent elimination of norbomene.t In this proposal the sequence for the alkylation of the ortho positions of the aryl group in the intermediate Pd(QH4—C7Hio-C,C )L,2, involving... 

No studies on the mechanism of the complete catalytic cycle for formation of aryl ethers from aryl halides have been conducted, but the elementary reactions in the cycle are likely to be similar to those in the catalytic cycle for formation of aryl amines. Oxidative addition, formation of an alkoxo complex, and reductive elimination of ether are likely to occur. It is the slow reductive elimination of ether that allows for amide complexes to form and undergo reductive elimination in the presence of t-BuO" instead of undergoing reductive elimination of ether. 

The oxidative addition of disilanes occurs to palladium complexes of isonitrile ligands and platinum complexes of trialkylphosphine ligands as part of tiie catalytic silylation of alkynes and aryl halides. The addition of stannylboranes to Pd(0) complexes has also been reported,and the addition of diboron compounds to many metal systems, such as Pt(0) complexes (Equation 6.67), is now common. These reactions all occur with metal complexes that do not undergo intermolecular reactions with alkane C-H bonds, let alone C-C bonds. Thus, the Lewis acidic character of these reagents must accelerate the coordination of substrate and cleavage of the E-E bonds. 

The Reactions of Alkyl, Alkenyl, and Aryl Halides Oxidation, Reduction, Substitution, Addition, Elimination, and Rearrangement... 

The reactions of the second class are carried out by the reaction of oxidized forms[l] of alkenes and aromatic compounds (typically their halides) with Pd(0) complexes, and the reactions proceed catalytically. The oxidative addition of alkenyl and aryl halides to Pd(0) generates Pd(II)—C a-hondi (27 and 28), which undergo several further transformations. 

In Grignard reactions, Mg(0) metal reacts with organic halides of. sp carbons (alkyl halides) more easily than halides of sp carbons (aryl and alkenyl halides). On the other hand. Pd(0) complexes react more easily with halides of carbons. In other words, alkenyl and aryl halides undergo facile oxidative additions to Pd(0) to form complexes 1 which have a Pd—C tr-bond as an initial step. Then mainly two transformations of these intermediate complexes are possible insertion and transmetallation. Unsaturated compounds such as alkenes. conjugated dienes, alkynes, and CO insert into the Pd—C bond. The final step of the reactions is reductive elimination or elimination of /J-hydro-gen. At the same time, the Pd(0) catalytic species is regenerated to start a new catalytic cycle. The transmetallation takes place with organometallic compounds of Li, Mg, Zn, B, Al, Sn, Si, Hg, etc., and the reaction terminates by reductive elimination. 

Success of the reactions depends considerably on the substrates and reaction Conditions. Rate enhancement in the coupling reaction was observed under high pressure (10 kbar)[l 1[. The oxidative addition of aryl halides to Pd(0) is a highly disfavored step when powerful electron donors such as OH and NHt reside on aromatic rings. Iodides react smoothly even in the absence of a... 

Lithiation at C2 can also be the starting point for 2-arylatioii or vinylation. The lithiated indoles can be converted to stannanes or zinc reagents which can undergo Pd-catalysed coupling with aryl, vinyl, benzyl and allyl halides or sulfonates. The mechanism of the coupling reaction involves formation of a disubstituted palladium intermediate by a combination of ligand exchange and oxidative addition. Phosphine catalysts and salts are often important reaction components. 

Perfluoroalkyl or -aryl halides undergo oxidative addition with metal vapors to form nonsolvated fluonnated organometallic halides and this topic has been die subject of a review [289] Pentafluorophenyl halides react with Rieke nickel, cobalt, and iron to give bispentafluorophenylmetal compounds, which can be isolated in good yields as liquid complexes [290] Rieke nickel can also be used to promote the reaction of pentafluorophenyl halides with acid halides [297] (equation 193)... 

In the arylations of enamines with very reactive aryl halides (352,370) such as 2,4-dinitrochlorobenzene, the closely related mechanistic pathway of addition of the enamine to the aromatic system, followed by elimination of halide ion, can be assumed. The use of n-nitroarylhalides furnishes compounds which can be converted to indolic products by reductive cycliza-tion. Less reactive aryl halides, such as p-nitrochlorobenzene, lead only to N-arylation or oxidation products of the enamines under more vigorous conditions. 

This reaction is not a bona fide Heck reaction per se for two reasons (a) the starting material underwent a Hg Pd transmetallation first rather than the oxidative addition of an aryl halide or triflate to palladium(O) (b) instead of undergoing a elimination step to give an enone, transformation 134 136... 

The mechanism of action of the cyanation reaction is considered to progress as follows an oxidative addition reaction occurs between the aryl halide and a palladium(O) species to form an arylpalladium halide complex which then undergoes a ligand exchange reaction with CuCN thus transforming to an arylpalladium cyanide. Reductive elimination of the arylpalladium cyanide then gives the aryl cyanide. 

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