Oxidative addition of aryl halides

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... 

The electrophilic character of the palladium atom in the complexes formed by oxidative addition of aryl halides and alkenyl halides to palladium(o) complexes can be exploited in useful ways. 

Carbon-carbon bond formation reactions and the CH activation of methane are another example where NHC complexes have been used successfully in catalytic applications. Palladium-catalysed reactions include Heck-type reactions, especially the Mizoroki-Heck reaction itself [171-175], and various cross-coupling reactions [176-182]. They have also been found useful for related reactions like the Sonogashira coupling [183-185] or the Buchwald-Hartwig amination [186-189]. The reactions are similar concerning the first step of the catalytic cycle, the oxidative addition of aryl halides to palladium(O) species. This is facilitated by electron-donating substituents and therefore the development of highly active catalysts has focussed on NHC complexes. 

The possible mechanism for the reactions involving stoichiometric amount of preformed Ni(0) complexes is shown in Fig. 9.8. The first step of the mechanism involves the oxidative addition of aryl halides to Ni(0) to form aryl Ni(II) halides. Disproportion of two aryl Ni(II) species leads to a diaryl Ni(II) species and a Ni(II) halide. This diaryl Ni(II) species undergoes rapid reductive elimination to form the biaryl product. The generated Ni(0) species can reenter the catalytic cycle. 

Determination of the rate constant of the oxidative addition of aryl halides with Pd°(PPh3)4 or with the Pd° complexes generated from Pd°(dba)2 and one equivalent of dppp shows that the oxidative addition is slower for ort/zo-substituted aryl halides than for the corresponding non-substituted or meta-substituted aryl halides.870... 

The present reaction may be reasonably explained by the smooth oxidative addition of aryl halides to metallic nickel to give aryl nickel halides, followed by disproportionation to bisarylnickels, which upon reductive elimination afford the dehalogenative coupled products. Providing strong support for this mechanism, the intermediates, arylnickel halide and bisarylnickel (Ar=C F ), were isolated as the phosphine complexes. 

These reactions to form aryl tin bonds could occur by initial oxidative addition of the aryl halide or the distannane. The stoichiometric reaction between [(PPh3)2Pd(Ph)(I)] and Me3SnSnMe3 in the presence of chloride generated good yields of the aryltin product. This result suggests that the reactions occur by initial oxidative addition of aryl halide. 

The reactivity order Ni>Pd>Pt has been found for the oxidative addition of aryl halides. Steric and electronic properties, and the numbers of L as well as chelate effects, play an important role [65, 194—196]. For example, Pd(0) complexes of basic chelating phosphines react substantially more easily with chlorobenzenes than their nonchelating analogues (see Section 18.2.4) [2, 100, 196]. 

In contrast to [L Pd], oxidative addition of aryl halides on [L Ni] often proceed by single electron transfer mechanism [2, 197]. SN2 and SNAr types of oxidative addition as a step of the catalytic dehalogenation have also been proposed in the literature for low-valent Ti [40, 114], Zr(II) [115], Ru(II) [20, 74, 81, 98],... 

The resting state of this catalytic system was found to be the dimer shown. The migratory insertion is the rate-determining step and not the oxidative addition of aryl halide to a palladium zero species, see Figure 13.17. These kinetics were found for phenyl iodide phenyl bromide already showed less clear-cut kinetics indicating that the oxidative addition is somewhat slower now. The system shown in Figures 13.16-17 gives at least half a million turnovers. 

BMIM]BF4 was applied to a Suzuki reaction. The active catalyst was a trico-ordinated [Pd(PPh3)2(Ar)][X] complex that formed after oxidative addition of aryl halide to [Pd(0)(PPh3)4] 211). The hydrophobic ionic liquid does not compete with the unsaturated organic substrate for the electrophilic active metal center. 

The general catalytic cycle in Heck-type polycondensation consisting of the oxidative addition of aryl halide, olefin insertion and reductive elimination is shown in Figure 8.2 [100-104]. 

Oxidative Addition of Aryl Halides to LjPd complexes (L = P(o-tolyl)3, BINAP, DPPF) and its Mechanism... 

Pd[P(o-tolyl)3]2] underwent oxidative addition of aryl halides to provide the unusual dimeric aryl halide complexes [Pd[P(o-tolyl)3](Ar)(Br) 2 (Eq. (44)) [77,102]. It is unusual for phosphine-ligated aryl halide complexes formed by oxidative addition to be dimeric. These oxidative addition products were isolated and structurally characterized. They remain dimeric in solution, as determined by solution molecular weight measurements, but react as the monomers, as described below. 

Oxidative addition to a monophosphine palladium complex is unusual, but is a reasonable pathway if one bears in mind that reductive eliminations often occur from monophosphine palladium complexes [202,203]. These reductive eliminations from monophosphine Pdn species would form a monophosphine Pd° complex as the initial metal product, and these Pd° products are similar to the intermediate in the oxidative addition of aryl halide deduced from kinetic studies. 

In 1994, Paul, Patt, and Hartwig showed that the Pd(0) catalyst in Kosugi s process was Pd[P(o-C6H4Me)312 (3), which underwent oxidative addition of aryl halides to give dimeric aryl halide complexes (4) [91]. These aryl halide complexes reacted directly with tin amides to form arylamine products (Eq. (3)). Thus, this chemistry could formally be viewed as being roughly parallel to Stille coupling. 

Bulky ligands as above have also proved to be effective in other palladium-catalyzed reactions of aryl halides, e.g., amination [16-19], Suzuki-Miyaura reaction [20-22], Mizoroki-Heck reaction [23, 24], Migita-Kosugi-Stille reaction [25], and aryloxylation and alkoxylation [26-28] as well as the reaction with various carbon nucleophiles as described below. The ligands are considered to enhance both the initial oxidative addition of aryl halides and the reductive elimination of products [29, 30]. The effectiveness of the commercially available simple ligand, P(f-Bu)3, was first described for the amination by Nishiyama et al. [16]. 

If a Pd(II) source is used in the Heck reaction, it must be reduced to Pd(0) before entering the catalytic cycle. The initial oxidative addition of aryl halide to a Pd(0) catalyst affords... 

The oxidative addition of aryl halides to Ni(0) affords trans-arylNi(II) halides and paramagnetic Ni(I) halides. An overall second-order rate-determining step (rate = K2[Ni(0)][ArX]) involves electron transfer from Ni to aryl halide, perhaps by way of a complex . A paramagnetic ion pair is the common intermediate which is partitioned into the Ni(II) oxidation adduct and the Ni(I) adduct ... 

Stereoselective synthesis of organometallic complexes has been achieved in the oxidative addition of aryl halides to triethylphosphine nickel(O) complexes, leading to the exclusive formation of trans-2ivy nickel(II) halide complexes [383]. Electron-transfer reactions on the Fe of cis- and ra/w-[7/-C5H5Fe(CO)SR]2 occur stereospecifically with no stereoisomerization on changing the oxidation state of the Fe [384,385]. In the electrochlorination of a ligand (R) of the //-C5H5Fe(CO)R complex, the stereochemistry is retained [386]. 

Extensive mechanistic studies have been conducted on the oxidative addition of aryl halides to Pd(0) complexes ligated by PPh3 in different media and with different additives175, 176. However, palladium complexes containing these ligands are not active catalysts for amination. Instead, one must consider the addition of aryl halides to palladium complexes bound by ligands relevant to amination. Studies of the mechanism of oxidative addition to palladium(O) complexes of P(tol-o)3, DPPF, BINAP, Q-phos, P(Bu-f)3 and an /V-heterocyclic carbene ligand have been reported. 

Vinyl and aryl halides also add to the metals such as Ni(0), Pd(0) and Pt(0) regiose-lectively. In oxidative addition of aryl halide, a nucleophilic substitution mechanism is also proposed. Sometimes electron transfer processes are also considered to be involved in oxidative addition, which gives anion radicals or free radicals. However, these processes sometimes lose stereoselectivity. In general, oxidative additions take place by a variety of mechanisms. 

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 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]. 

Catellani found that the interaction of palladacycles such as 5 with aryl halides led to the incorporation of the aryl group in a manner analogous to alkyl halides [39], A similar mechanism could be proposed for the oxidative addition of aryl halides to palladacycles such as 5 to generate a palladium(TV) species, although to date there is no direct evidence for this pathway. An alternative mechanism has been put forward by Echavarren which involves transmetallation between two palladium... 

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