Benzene derivatives, coupling with

Enolate Alkylations with Transition Metal Coordinated Electrophiles. Coordination of various transition metals to dienes and aromatic compounds sufficiently activates these compounds to nucleophilic addition, resulting in high asymmetric induction at the a-center. However, the manganese complexes of various benzene derivatives couple with lithium enolates in low selectivity at the nascent stereogenic center on the ring (eq 15). ... 

For the preparation of biphenyl compounds with substituents in only one ring, it is usually advisable to employ a substituted aniline and couple with benzene rather than to use aniline and couple with a benzene derivative, since with benzene only a single biaryl compound is produced. Thus, 4-methylbiphenyl can be isolated readily in pure form from the reaction between diazotized p-toluidine and benzene, but not from the mixture of 2- and 4-methylbiphenyl formed from diazotized aniline and toluene. 

Reaction that can be carried out by the oxidative coupling of radicals may also be initiated by irradiation with UV light. This procedure is especially useful if the educt contains oleflnic double bonds since they are vulnerable to the oxidants used in the usual phenol coupling reactions. Photochemically excited benzene derivatives may even attack ester carbon atoms which is generally not observed with phenol radicals (I. Ninoraiya, 1973 N.C. Yang, 1966). 

The benzene derivative 409 is synthesized by the Pd-catalyzed reaction of the haloenyne 407 with alkynes. The intramolecular insertion of the internal alkyne, followed by the intermolecular coupling of the terminal alkyne using Pd(OAc)2, Ph3P, and Cul, affords the dienyne system 408, which cyclizes to the aromatic ring 409[281]. A similar cyclization of 410 with the terminal alkyne 411 to form benzene derivatives 412 and 413 without using Cul is explained by the successive intermolecular and intramolecuar insertions of the two triple bonds and the double bond[282]. The angularly bisannulated benzene derivative 415 is formed in one step by a totally intramolecular version of polycycli-zation of bromoenediyne 414[283,284],... 

Katsuya et al. [5 published the oxidative coupling (agent copper(II) chloride/ aluminum chloride) of electron-rich benzene derivatives such as 2,5-dimethoxy-benzene to poly(2,5-dimethoxy-1,4-phenylene) (2). The resulting polymer is only soluble in concentrated sulfuric acid, and is fusible at 320r C. Ueda et al. 16] described the coupling of the same monomer with iron(III) chloride/aluminum chloride. The polymers obtained by the authors were not thoroughly para-linked. 

Of considerable interest is a reaction of monoarylazo derivatives of B10Hio containing electron-withdrawing substituents in the benzene ring (e.g., 2,4,6-Br3, 4-N02), which was subsequently found by Leyden and Hawthorne (1973, 1975). If the azo coupling system mentioned above (acetonitrile, - 35 °C) is allowed to come to room temperature and is treated with excess sodium propionate, the 1-diazonio-B10H9 anion and the corresponding benzene derivative are formed in considerable yield (52% with the 2,4,6-tribromo derivative as shown in Scheme 12-85). 

Hydroxyl elimination is necessary for the formation of benzaldehyde and benzoic acid derivatives and, ultimately, benzene and toluene (Fig. 7.46).2 It is proposed that a cleavage between the hydroxyl group and aromatic ring leads to benzenoid species which undergo further cleavage coupled with oxidation to give various decomposition products. 

The second synthetic route consists of the coupling of hexa(4-iodophenyl)ben-zene (34) with an alkylated oligophenylboronic acid to produce a hexa(oligo-phenyl)benzene by extending the aromatic chain [52]. This route is illustrated by the reaction of hexa(4-iodophenyl)benzene (34) with an alkylated terphenyl boronic acid with formation of the hexa(quaterphenyl)benzene derivative 33. Once again, the aliphatic substituents serve to guarantee sufficient solubility. 

The synthetic route represents a classical ladder polymer synthesis a suitably substituted, open-chain precursor polymer is cyclized to a band structure in a polymer-analogous fashion. The first step here, formation of the polymeric, open-chain precursor structure, is AA-type coupling of a 2,5-dibromo-1,4-dibenzoyl-benzene derivative, by a Yamamoto-type aryl-aryl coupling. The reagent employed for dehalogenation, the nickel(0)/l,5-cyclooctadiene complex (Ni(COD)2), was used in stoichiometric amounts with co-reagents (2,2 -bipyridine and 1,5-cyclooctadiene), in dimethylacetamide or dimethylformamide as solvent. 

Transition metal mediated or catalyzed benzene formation reactions have been reported using various metals. However, the use of three different alkynes is difficult [38], In many cases, a mixture of several benzene derivatives is obtained. In 1974, Wakatsuki and Yamazaki used three different alkynes with Co complexes [27b], but isomers were formed and a tedious chromatographic separation was necessary. The first selective coupling of three different alkynes in high yields was reported in 1995 using a combination of unsymmetrical zirconacydopentadienes and CuCl, as shown in Eq. 2.52 [7k]. 

The chlorinated intermediate 255 is eliminated and cycloadds to Cjq, yielding pyrazo-linofullerenes of the structure 257 (Scheme 4.42). The 4-nitrophenyl-group can be replaced by a 4-methoxyphenyl- or a phenyl substituent. In this reaction various aromatics and substituted aromatics are tolerated as residues R (e.g. furan, ferrocene, pyrazole or benzene and substituted benzene). The nitro group of the nitrophenyl residue can be reduced with Sn-HCl to the aniline derivative, which can be further functionalized by amide coupling with acid chlorides [311]. 

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