Bromobenzenes, coupling

Homocoupling of halobenzenes. lodo- and bromobenzene couple to biphenyls in the presence of this activated Ni(0). Yields are definitely higher with the former substrate. ort/io-Substituents, particularly nitro groups, inhibit coupling. The main by-products result from reduction. This coupling reaction is generally superior to the Ullmann reaction. 

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. 

Several newer methods take advantage of the highly selective nature of organopaHadium reagents. A palladium acetate-triarylphosphine catalytic system has been employed to induce the coupling of bromobenzene with the desired acrylate ester (13). 

The Boger group synthesis commences with a Stille coupling of a stannyl acetylene with two equivalents of a highly oxygenated bromobenzene to yield a symmetrical diarylalkyne (82). The subsequent reaction of this alkyne (82) with 3,6-dicabomethoxytetrazine under Diels-Alder/retrograde Diels-Alder... 

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... 

The arylation of heteroaromatic compounds is also achieved by aryl-aryl coupling reaction. The arylation of A-methylimidazole with bromobenzene occurs under palladium catalysis (Equation (62)).72 The arylation of thiazole with aryl iodide occurs at the 2-position under PdCl2(PPh3)2/CuI catalysis.73 In this case, tetrabutylammonium fluoride improves the activity of the catalyst. Alternatively, thiazoles and benzothiazole are efficiently arylated... 

The first Pd-catalyzed C—N bond formation was described by the Migita group in 1983. They cross-coupled bromobenzene with N, N-diethylaminotributyltin to prepare N,N-... 

Martin prepared 5-trimethylstannylindole and effected coupling with bromobenzene to give 5-phenylindole [125], In a search for new cAMP phosphodiesterase inhibitors, Pearce prepared the furylindole 190 from 5-bromoindole and 5-ferr-butoxy-2-trimethylstannylfuran [196a], Benhida and co-workers explored Stille couplings of 6-bromo- and 6-iodoindole, and methyl 6-iodoindol-2-ylacetate with a variety of heteroarylstannanes and vinylstannanes [196b]. 

The atom economy of the cross-coupling reaction and the Heck reaction for making styrene from bromobenzene and vinyl bromide (cross), and bromobenzene and ethene (Heck) respectively are in favour of the Heck reaction, as that produces only one equivalent of salt. 

Lund and coworkers [131] pioneered the use of aromatic anion radicals as mediators in a study of the catalytic reduction of bromobenzene by the electrogenerated anion radical of chrysene. Other early investigations involved the catalytic reduction of 1-bromo- and 1-chlorobutane by the anion radicals of trans-stilhene and anthracene [132], of 1-chlorohexane and 6-chloro-l-hexene by the naphthalene anion radical [133], and of 1-chlorooctane by the phenanthrene anion radical [134]. Simonet and coworkers [135] pointed out that a catalytically formed alkyl radical can react with an aromatic anion radical to form an alkylated aromatic hydrocarbon. Additional, comparatively recent work has centered on electron transfer between aromatic anion radicals and l,2-dichloro-l,2-diphenylethane [136], on reductive coupling of tert-butyl bromide with azobenzene, quinoxaline, and anthracene [137], and on the reactions of aromatic anion radicals with substituted benzyl chlorides [138], with... 

Nickel(O) triphenylphosphine can be used to couple aryl halides and alkenes to synthesize substituted olefins [149], 1,2-bis[(di-2-propylphosphino)benzene]nick-el(0) can be used to couple aryl halides [150], and l,2-bis[(diphenylphos-phino)ethane]nickel(0) can be used to prepare benzoic acid from bromobenzene in the presence of carbon dioxide [151]. 

Very few transition-metal catalyzed electroreductive carbon-heteroatom bond formations have been described. The electrochemical silylation of allylic acetates was carried out in the presence of Pd-PPha [131]. The electrosynthesis of arylthioethers from thiophenol and aryl halides [132] and the coupling of bromobenzene with dichlorophenylphosphine [133] were performed with Ni-bpy as catalyst. 

The starting acetylenic substrates are prepared by the Pd +-induced coupling of the alkynes with the appropriate o-substituted bromobenzenes. ... 

As the last example of C-C bond-formation reactions catalyzed by alkaline earth hydroxides, we mention the recently reported a-arylation of diethyl malonate in the presence of a palladium catalyst and a base in a separate phase 299). The arylation of carbonyl compounds is a carbon-carbon coupling reaction between an aryl halide and an enolate, which is usually catalyzed by palladium salts in the presence of an appropriate base (300,301). The arylation of diethyl malonate with bromobenzene (Scheme 48) was performed with tetrachloropalladate as the... 

Subsequently, a copper-catalyzed cross-coupling [with substoichiometric amounts of copper(l) iodide and N,N -dimethylethylenediamine (DMEDA)] between aryl halides and sulfoximines was developed [52]. In this case, both aryl bromides and aryl iodides reacted well. For the conversion of the former substrates an in-situ copper-catalyzed aryl Finkelstein reaction [53] had to be performed first, as shown in Scheme 2.1.1.22 for the preparation of 64 starting from bromobenzene (62). 

This diazotization reaction is compatible with the presence of a wide variety of substituents on the benzene ring. Arenediazonium salts are extremely important in synthetic chemistry, because the diazonio group (N=N) can be replaced by a nucleophile in a radical substitution reaction, e.g. preparation of phenol, chlorobenzene and bromobenzene. Under proper conditions, arenediazonium salts react with certain aromatic compounds to yield products of the general formula Ar-N=N-Ar, called azo compounds. In this coupling reaction, the nitrogen of the diazonium group is retained in the product. 

The copper catalyzed formation of carbon-nitrogen bonds was exploited in the conversion of 2-(3 -aminopropyl)-bromobenzene to tetrahydro-isoquinoline (4.17.), The coupling, which proceeded readily already at 40 °C in the presence of diethyl salicylamide, was also efficient in the formation of dihydroindole derivatives.20... 

When 4-methylpyrimidine was used as the coupling partner of bromobenzene (7.87.), the attack of the phenylpalladium intermediate took place rather at the relatively acidic methyl substituent than on the electron deficient pyrimidine core, giving rise to 4-diphenylmethyl-pyrimidine.110... 

---