Reactions coupling, aromatic rings

The Scholl reaction involves an overall oxidation of the coupled aromatic rings, yet there is no obvious oxidizing agent. This poses the question of what happens to the two hydrogen atoms that are produced in this reaction. It has been suggested that oxygen (air) may act as the oxidant, but this currently lacks confirmation [18]. 

An application of this reaction to aromatic ring extension is noteworthy. As shown in Eq. 2.43, zirconacydopentadienes couple with diiodobenzene (63) to afford naphthalenes 64. When tetraiodobenzene (65) is used, octasubstituted anthracene derivatives 66 are obtained [7c],... 

The synthetic utility of anodic reactions that couple aromatic rings activated by alkoxy groups has been explored for the formation of natural products. Thus laudanosine (78 R1 = R2 = R3 = R4 = CH3) has been oxidized... 

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 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],... 

Arenediazonium salts are also used for the couplina[563], (Z)-Stilbene was obtained unexpectedly by the reaction of the ti-stannylstyrene 694 by addition-elimination. This is a good preparative method for cu-stilbene[564]. The rather inactive aryl chloride 695 can be used for coupling with organostannanes by the coordination of Cr(CO)3 on aromatic rings[3.565]. 

A reaction of aryl diazonium salts that does not involve loss of nitrogen takes place when they react with phenols and arylamines Aryl diazonium ions are relatively weak elec trophiles but have sufficient reactivity to attack strongly activated aromatic rings The reaction is known as azo coupling two aryl groups are joined together by an azo (—N=N—) function... 

With a substituted aromatic ring compound 2, mixtures of isomeric coupling products may be formed the ort/zo-product usually predominates. The rules for regiochemical preferences as known from electrophilic aromatic substitution reactions (see for example Friedel-Crafts acylation), do not apply here. 

In the first of these, the key step in the synthetic sequence involves an oxidative phenol coupling reaction patterned after the biosynthesis of the natural product. Preparation of the moiety that is to become the aromatic ring starts by methyla-tion of phloroglucinol (5) with methanolic hydrogen chloride to give the dimethyl ether (6). Treatment of that intermediate with sulfuryl chloride introduces the chlorine atom needed in the final product (7). 

One of the first reactions to be carried out in a molten salt (albeit at 270 °C) was the Scholl reaction. This involves the inter- or intramolecular coupling of two aromatic rings. A example of this reaction, in which 1-phenylpyrene was cyclized to indeno[l,2,3-cd]pyrene [26] is given in Scheme 5.1-7. A more elaborate version of the Scholl reaction is shown in Scheme 5.1-8 and involves bicyclization of an aromatic cumulene [27]. 

Arenediazonium salts undergo a coupling reaction with activated aromatic rings such as phenols and arylamines to yield brightly colored azo compounds, Ar—N=N—Ar. ... 

The Suzuki reaction97 allows tire coupling of two aromatic rings by reaction of an arylboronic compound with a iodo or bromo aryl derivative. The tetrakis (U iphenylphosphine) Pd is the catalyst working in the basic medium. This reaction was recently used98 in aqueous media for the preparation of different isomers of diphenyldicarboxylic acids (Fig. 5.21) but also for the synthesis of soluble rodlike polyimides99 by coupling the 3,6-diphenyl- V, V,-bis(4-bromo-... 

ArSnRs, and with arylmercury compounds. Aryl triflates react with arylbo-ronic acids ArB(OH)2, or with organoboranes, in the presence of a palladium catalyst, to give the arene in what is called Suzuki couplingCyclopropyl groups can be attached to aromatic rings by this reaction. Even hindered boronic acids give good yields of the coupled product. 

In these reactions a new carbon-carbon bond is formed, and they may be given the collective title coupling reactions. In each case, an aUcyl or aryl radical is generated and then combines with another radical (a termination process) or attacks an aromatic ring or alkene to give the coupling product. ... 

It can also be mentioned that polyphosphazenes substituted with aromatic groups, such as phenols or naphthols, can form inter- and intra- molecular excimers by coupling reaction of the planar aromatic rings of the substituents under illumination [467-471,473,725]. These species disappear as soon as the light is switched off. 

Rehahn et al. [35] recently presented the synthesis of constitutionally homogeneous oligophenylenes, 22a/b, with 2,5-alkyl substituents located on the central aromatic ring, generated via the cross-coupling reaction of Suzuki. They are exclusively linked in the para-positions and composed of 3 -15 benzene rings. 

So far only Pd-based systems have been highlighted in this section however, the use of other metals such as Ni has clear economic advantages. In this regard, Chiu and co-workers have used a bis-carbene tetradentate ligand to catalyse the coupling of aryl bromides and chlorides with both electron rich and electron poor aromatic rings however, the reaction with electron poor aryl bromides lead to superior yields (Scheme 6.30) [113]. 

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