Arylpalladium complexes aryl halide oxidative additions

The palladation products exhibit reactivity similar to that of the arylpalladium complexes formed by oxidative addition of aryl halides to Pd(0) species, although the reactions are stoichiometric with respect to palladium. Representative examples include vinylation via an olefin insertion process (eq (88)) [119], double and single carbonylation (eq (89) and (90)) [120,121], and alkylation via a transmetallation process (eq (91)) [122]. 

B.ii. Structure of Arylpalladium(II) Complexes Formed in Oxidative Addition to Aryl Halides/Triflates as a Function of the Precursors of the Palladium(O) and the Ligand... 

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. 

Innovations in the chemistry of aromatic compounds have occurred by recent development of many novel reactions of aryl halides or pseudohalides catalysed or promoted by transition metal complexes. Pd-catalysed reactions are the most important [2,29], The first reaction step is generation of the arylpalladium halide by oxidative addition of halide to Pd(0). Formation of phenylpalladium complex 1 as an intermediate from various benzene derivatives is summarized in Scheme 3.1. 

Mechanistically, this new insertion-CI-Diels-Alder hetero domino sequence can be rationalized as follows (Scheme 64) After the oxidative addition of the aryl halide 115 or 118 to the in situ generated Pd(0) species the arylpalladium halide 120 intramolecularly coordinates and inserts into the tethered triple bond via a syn-carbopaUadation to furnish cyclized vinylpalladium species 121 with a p-acceptor substitution in a stereospecific fashion. Transmetallation of the in situ generated copper acetylide 122 gives rise to the diorganylpalladium complex 123 that readily undergoes a reductive elimination and liberates the electron poor vinylpropargylallyl ether 124. The triethylamine catalyzed propargyl-allene isomerization furnishes the... 

The synthesis of unsymmetrical biaryls 8 from two monoaryl species involves the coupling of a metallated aromatic molecule 6 with an aryl halide or triflate 4 under the action of palladium(O) catalysis. The reaction involves a catalytic cycle in which palladium(O) inserts into the C-halogen bond via an oxidative addition to generate an arylpalladium(II) species 5 (Scheme 10.18). This undergoes a trans-metallation with the metallated component, producing a biarylpalladi-um(II) complex 7. The biaryl product is formed by reductive elimination. In the process, Pd(0) is regenerated and this can then react with a second molecule of aryl halide. Pd(0) is therefore a catalyst for the reaction. 

The basic mechanism of the Heck reaction (as shown below) of aryl or alkenyl halides or triflates involves initial oxidative addition of a pal-ladium(O) species to afford a a-arylpalladium(II) complex III. The order of reactivity for the oxidative addition step is I > OTf > Br > Cl. Coordination of an alkene IV and subsequent carbon-carbon bond formation by syn addition provide a a-alkylpalladium(II) intermediate VI, which readily undergoes 3-hydride elimination to release the product VIII. A base is required for conversion of the hydridopalla-dium(II) complex IX to the active palladium(O) catalyst I to complete the catalytic cycle. 

The syntheses of trans and cis isomers of unsymmetrical diorganopalladium(II) complexes have been reported [104]. For example, the reaction of arylpalladium(II) halides, which have been prepared either by the oxidative addition of aryl halides to palladium(O) phosphine complexes or by the treatment of diarylpalladium(II) with HCl, with 1 equivalent of methyllithium provides /ra x-PdAr(Me)L2 complexes (eq (75)). On the other hand, treatment of arylpalladium(II) halides with excess methyllithium followed by methanolysis of the resulting dimethyl(aryl)palladate intermediates leads to c/.v-PdAr(Me)L2 complexes (eq (76)). 

The arylamines are generally formed in good yields (Table 1). Dehalogenation products are the only by-products observed, which pro-baly arise from base-induced )9-hydride elimination of the amido arylpalladium complex and subsequent reduction. Interestingly, the base employed has a decisive influence on the course of the reaction. In the amination of l-bromo-4-n-butylbenzene with free amines in the presence of silyl amides as base -in contrast to the coupling with tin amides -the rate-determining step in the catalytic cycle is the oxidative addition of bis(tri-o-tolylphosphine)palladium(O) to the aryl halide. However, when LiOrBu is used as base, the formation and reductive elimination of the amido arylpalladium complex is decisive for the rate of the reaction. In the presence of NaOtBu both reaction steps seem to take place at similar rates. [9]... 

The relevant fundamental processes include (a) oxidative addition of aryl (or alkenyl) halide to a Pd(0) complex to give arylpalladium(ll) halide (A), (b) olefin coordination and insertion into the aryl-Pd bond to give arylated alkylpalladium species (B), (c) -hydrogen elimination to liberate the arylated olefin generating a hydridopalladium halide PdH(X)L2 (C), (d) removal of hydrogen halide from... 

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 arylation is explained by the following mechanism. Arylpalladium halides 28 are formed by oxidative addition of aryl halides. Then the arylpalladium eno-lates 30 are generated by transmetallation of 28 with alkali enolates of ketones 29. Finally reductive elimination of the arylpalladium enolates 30 affords a-aryl ketones. Hartwig isolated the arylpalladium enolate complexes 31 of ketones, esters and amides, and confirmed formation of or-arylated products on heating [23]. 

This section evidences that the nature and reactivity of the actnal reactive paUadium(O) species in oxidative additions strongly depend on its precnrsor. Indeed, Pd°(PPh3)2, the most commonly postulated species supposed to participate in all catalytic systems, is never present in stoichiometric amounts compared to its Pd or Pd° precnrsor. It is either generated as a minor complex when Pd°L4 and Pd°(dba)2 are precnrsors or does not exist at all when generated in the presence of anions. Instead, three-coordinate anionic Pd°L2X complexes are formed in which one halide X (or one acetate) remains ligated to the palladium(O). Consequently, the rate of oxidative additions to aryl halides ArX depends on the precursor system. This will affect the overall reactivity whenever the oxidative addition is the rate-determining step. This also affects the natnre and consequently the reactivity of the arylpalladium(II) intermediates formed by the ensuing oxidative addition to ArX. ... 

Some fundamental inorganic chenustry that is important for understanding which complexes will undergo the aromatic C—and C—O bond-forming processes will be presented before the catalytic transformations. First, the three reaction types involved in the catalytic cycle to form arylanunes are similar to those found in the catalytic cycle for C—C bond formation oxidative addition of aryl halide to Pd(0) complexes, transmetallation that converts an arylpalladium halide complex to an arylpaUadium amido complex, and reductive elimination to form a C—or C—O bond. The oxidative addition step is identical to the addition that initiates C—C bond-fomting cross-couplings,f f but the steps that form the arylpalladium amido complexes and that produce the arylamine product are different. The mechanism for these steps is discussed after presentation of the scope of the amination process. 

Palladium salts and complexes are capable of coupling aryl halides under certain reaction conditions to yield biaryls. It is well known that palladium salts (as precatalysts) can be easily reduced to metallic palladium or palladium(O) complexes if the reduction was carried out in the presence of coordinating molecules (ligands, solvents). Once again, oxidative addition of aryl halide (I) to palladium(O) species occurs to form arylpalladium(II) halide (XIII). The latter subsequently reacts with another molecule of aryl halide to produce transient diarylpalladium(IV) intermediate (XIV). It undergoes rapid reductive elimination of a biaryl molecule (II) and forms the starting palladium(II) complex which is reduced by stoichiometric reductant - closing the catalytic cycle [17], Scheme 17. 

Alternatively, under strong reductive conditions, e.g. electrochemical reduction, arylpalladium(II) halide (XIII) is reduced to arylpalladium(0) which, upon the second oxidative addition, gives diarylpalladium(II) complex, also capable of undergoing reductive elimination of biaryl similarly to XIV [10]. Isopropanol [50-52], amines [4,17,52-54], hydrazine [55], tetrakis(dimethylamino)ethylene [18], hydroquinone [56], tetrabutylammonium ftuoride [57], formates [58-60], zinc [61-64], and molecular hydrogen [65] have been used as stoichiometric reductants. Electro-reductive homocoupling of aryl halides was also reported [10]. The most important methods for... 

The oxidative addition step probably proceeds through the charge-transfer complex, Ar-X Pd(PPh3)4, to produce the relatively stable arylpalladium(II) halides (or pseudohalides), see Chapter 3. The formation of a stable ionic metal-salt, e.g. MgCl2, is the driving force for the further reaction step, transmetallation of an aryl-group from the arylmetallics to arylnickel(II) or arylpalladium(II) complex. Generally, all... 

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