Coupling Reactions of Aromatic Halides

Low-valent nickel complexes of bpy are also efficient electrocatalysts in the reductive coupling reaction of aromatic halides.207 Detailed investigations are in agreement with a reaction mechanism involving the oxidative addition (Equation (40)) of the organic halide to a zero valent complex.208-210 Starting from [Nin(bpy)2(X)2]0 with excess bpy, or from [Nin(bpy)3]2 +, results in the [Ni°(bpy)2]° complex (Equations (37) and (38)). However, the reactive complex is the... 

Kweon, D., Jang, Y. and Kim, H. (2003) Organic electrochemical synthesis utilizing Mg electrodes. 1. Facile reductive coupling reactions of aromatic halides. Bull. Kor. Chem. Soc., 24, 1049. 

Several chemo-selective aryl-aryl coupling reactions of aromatic halides and organometallics with transition-metal catalysts are known.16 For example, coupling reactions of arylboronic acids with aryl halides with Pd(0) catalyst,17 arylmagnesium halides with aryl halides with Ni(II) catalysts,18 and aryl trialkyl tin compounds with aryl halides with Pd(0) catalyst19 are well known. The polyhalo-aromatic substances were converted into AB -type monomers by selective metallation of one of the halo-functional groups in each monomer. 

Transition metal catalyzed coupling reactions of aromatic halides, such as Heck or Suzuki coupling, are used extensively for the synthesis of low-molecular-weight organic compounds. With bifunchonal substrates, polymers can be obtained. By contrast to all other polymerization reactions considered in this chapter, such reactions represent step-growth-type polymerizations. 

The metal-catalyzed coupling reactions of aromatic halides such as the Yamamoto, Colon or Suzuki reactions are effective in a reducing environment. Active catalytic species are low-valent metal complexes. They can be used directly or can be generated in situ by a reaction with a reducing agent such as magnesium or zinc. 

B) Cross coupling reactions of aromatic halides can sometimes be performed with copper reagents but they also take place cleanly in the presence of nickel or palladium(O) complexes through formation of arylmetal species. 

Yamamoto T, Wakabayashi S, Osakada K (1992) Mechanism of carbon-carbon coupling reactions of aromatic halides, promoted by Ni(COD)2 in the presence of 2,2 -bipyridine and triphenylphosphine, to give biaryls. J Organomet Chem 428 223-237... 

In 1974, Heck reported on the formation of amides and esters via the palladium-mediated carbonylation and coupling reactions of aromatic halides and amines, or alcohols (7). The mechanism of the reaction, as outlined in Scheme I for amide formation, involves a coordinatively unsaturated Pd(0) species oxidatively adding to the aromatic halide producing a Pd(II) complex, 1. Carbon monoxide then inserts in the aryl-palladium bond giving aroyl complex 2. This is followed by attack of the amine leading to the regeneration of the active Pd(0) catalyst and liberation of the free amide 3 (7,2). (An alternate... 

Cross-coupling reactions of aromatic or vinylic halides and olefins catalyzed by palladium. 

Catalytic formation of carbon-carbon bonds is a powerful tool for construction of complex molecular architectures, and has been developed extensively for applications in organic synthesis. Three main classes of carbon-carbon bond forming reactions have been studied in sc C02 carbonylation (with particular attention paid to the hydroformylation of a-olefins), palladium-catalyzed coupling reactions involving aromatic halides, and olefin metathesis. 

The transition metal-catalysed coupling reaction of aryl halides with alkynes (alkynylation of aryl halides) is frequently also considered as a Heck reaction [68, 108-111], Polymers containing arylacetylene and diarylacetylene units in the backbone have been obtained by the self-coupling of ethynylaryl halides [scheme (3)] and the cross-coupling of aromatic dihalides with acetylene [scheme (6)] or diethynylbenzenes [scheme (7)] [112-121]. The majority of... 

The scope and limitations of Pd(0)-mediated coupling reactions between aromatic halides linked to a polystyrene resin and boronic acid derivatives (Suzuki coupling) or arylstannanes (Stille coupling) have been reported. For all the reactions, the conditions were optimized and evaluated with various reagents. In most cases, products were obtained in excellent yields upon cleavage from the solid support (Eq. (63)) [101]. 

Domino coupling reactions of aryl halides with norbornene and its derivatives provide a simple route to PAHs. In a four component sequence, norbornene (73) is arylated with an excess of iodobenzene to the terphenyl 74, that can be converted to the benz[e]pyrenes 75 and 76 by classical aromatic conversion reactions [131]. The domino sequence is a consequence of the fact that the five-membered intermediate palladacycle 77 a is able to add a second molecule of iodobenzene (77a —> 77b), and the intermediate arylpalladium halide resulting from reductive elimination in 77 b can even add a third molecule of iodobenzene before the final elimination of the Pd(0) complex PdL2 occurs (see Scheme 38). 

Thus, this chapter focuses on the most investigated classes of metal-catalyzed coupling reactions—namely, reactions of aromatic and aliphatic electrophiles with main group carbon or nitrogen nucleophiles, reactions of aromatic halides with olefins, including enanti-oselective versions of these reactions, and direct coupling processes. The mechanisms of these reactions are presented with reference to the chapters on the stoichiometric steps of these catalytic processes. 

The intramolecular version of palladium-catalyzed decarboxylative cross-coupling reaction of aromatic carboxylic acids with organic halides also worked well. For example, 2-bromophenyl-tethered benzoic acids 116 underwent cyclization in the presence of Pd(OAc)2/PPh3 and K2CO3 in NMP to give 117 in high yields (Scheme 22.27) [51]. A mechanism that involved a sequence of oxidative addition, cyclization of carboxylate 118, decarboxylation of 119, and reductive elimination was proposed. Indole-2-carboxylic acid 121 was also applicable to this type of cyclization. 

Ullman reaction The synthesis of diaryls by the condensation of aromatic halides with themselves or other aromatic halides, with the concomitant removal of halogens by a metal, e.g. copper powder thus bromobenzene gives diphenyl. The reaction may be extended to the preparation of diaryl ethers and diaryl thio-ethers by coupling a metal phenolate with an aryl halide. 

In aqueous DMF, the reaction can be applied to the formation of C-C bonds in a solid-phase synthesis with resin-bound iodobenzoates (Eq. 6.33).80 The reaction proceeds smoothly and leads to moderate to high yield of product under mild conditions. The optimal conditions involve the use of 9 1 mixture of DMF-water. Parsons investigated the viability of the aqueous Heck reactions under superheated conditions.81 A series of aromatic halides were coupled with styrenes under these conditions. The reaction proceeded to approximately the same degree at 400°C as at 260°C. Some 1,2-substituted alkanes can be used as alkene equivalents for the high-temperature Heck-type reaction in water.82... 

Arylation of C-H bonds is achieved by coupling reactions of C-H bonds with aromatic compounds such as halides, triflates, and organometallic reagents. Early works in this field involve the reaction of aryl halides with norbornene. As shown in Scheme 5, the coupling reaction of bromobenzene with norbornene in the presence of Pd(PPh3)4 as a... 

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