Addition reactions 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... 

In addition to sodium, other metals have found application for the Wurtz coupling reaction, e.g. zinc, iron, copper, lithium, magnesium. The use of ultrasound can have positive effect on reactivity as well as rate and yield of this two-phase reaction aryl halides can then even undergo an aryl-aryl coupling reaction to yield biaryls. ... 

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. 

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. 

In addition to aryl halides and triflates, organometallic reagents can be utilized for the catalytic arylation reaction. The rhodium-catalyzed arylation of arylpyridines proceeds with the use of tetraarylstannanes (Equation (67)).83 The ruthenium-catalyzed reaction of aromatic ketones with arylboronates affords the ortho-arylated aromatic ketones (Equation (68)).84... 

The reaction is carried out with aryl triflates and other details such as solvent and base used are also important. Intramolecular additions of aryl halides or triflates to alkenes in a side-chain leading to cyclic compounds have been reported by Overman [24], Rather complicated ring structures can be made stereospecifically. While initially BINAP seemed the best ligand for this conversion, the number of useful ligands is increasing [25],... 

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 mechanism of the oxidative addition of aryl bromides to the bis-P(o-tolyl)3 Pd(0) complex 3 was surprising [196]. It has been well established that aryl halides undergo oxidative addition to L2Pd fragments [197 -200] thus, one would expect oxidative addition of the aryl halide to occur directly to 3 and ligand dissociation and dimerization to occur subsequently. Instead, the addition of aryl halide to [Pd[P(o-tolyl)3]2] occurs after phosphine dissociation, as shown by an inverse first-order dependence of the reaction rate on phosphine concentration and the absence of any tris-phosphine complex in solution [196]. 

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... 

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]. 

The oxidative addition to palladium reagents proceeds readily with 1-haloalkenes at room temperature. However, the oxidative addition reactions of halides other than vinyl or aryl usually are very sluggish. Moreover, alkyl-Pd(II)-X complexes in which the alkyl moiety contains an sp -bonded hydrogen at the [3-position may undergo rapid dehydropalladation by. syn- 3-hydrogen elimination, generating the hydridopalladium complex and a double bond. Thus, the substrates used for the oxidative addition reaction are usually restricted to vinyl and aryl halides and triflates. 

Amatore and coworkers have studied the addition of aryl halides to Pd(0) complexes formed by the addition of chelating phosphines to Pd2(dba)3171. Addition of these ligands to Pd2(dba)3 generates mixed phosphine/dba complexes. The kinetic behavior of the complex containing BINAP as phosphine ligand indicated that two competing pathways for addition occur, one by direct reaction of aryl halide with (BINAP)Pd(dba) and one from the (BINAP)Pd intermediate formed by dissociation of dba. 

Reductive elimination of 5 and 6 has been studied in detail in connection with the nickel-catalyzed cross-coupling reaction of aryl halides (ArX) and alkylmagnesium halides (RMgX) (eq (18)). The reaction of tram-5 is induced by addition of aryl halides (ArX) to the system, where the reductive elimination process involving para-... 

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]. 

Organopalladium compounds can be prepared by electrophilic paUadation, oxidative addition to aryl halides or reaction of Pd(II) with organometalhc reagents. These transformations are all vital for the palladium-catalysed reactions discussed later in this chapter. 

On the other hand, the arylative ring opening takes place in the presence of a Pd(0) catalyst, an aryl halide, and a base (Scheme 3, reaction b) [32-35]. Oxidative addition of aryl halides toward Pd(0) gives ArPdX species, which can readily interact with the alcohols affording arylpalladium alkoxide intermediates. Then, p-carbon elimination and subsequent reductive elimination occur to give y-arylated ketones and regenarate Pd(0) species. An example is shown in Eq. 15. 

Combination of the oxidative addition of aryl halide with olefin insertion followed by -hydrogen elimination provides a useful olefin arylation process catalyzed by a palladium complex (Mizoroki-Heck reaction) [63-65]. The essential part of the catalytic cycle is shown in Scheme 1.23. 

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