Scholl coupling reaction

Other coupling reactions were also employed to prepare poly(arylene etherjs. Polymerization of bis(aryloxy) monomers was demonstrated to occur in the presence of an Fe(III) chloride catalyst via a cation radical mechanism (Scholl reaction).134 This reaction also involves carbon-carbon bond formation and has been used to prepare soluble poly(ether sulfone)s, poly(ether ketone)s, and aromatic polyethers. 

Various carbon-carbon cross-coupling reactions are employed to arrive at oligomers from fluorene monomers, including nickel-cataylzed Yamamoto coupling between aryl halides [48], palladium(0)-catalyzed Suzuki coupling between boronic acids/esters and halides [49], and ferric-chloride-catalyzed Scholl reaction, also called Friedel-Crafts arylation, of bare fluorenes [50,51]. 

Over the past century, several methods have been developed for the synthesis of biaryl compounds [7]. Among these are the Ullmann-type coupling [8,9], the Scholl reaction [10], the Gomberg-Bachmann reaction [11], and recently transition-metal-catalyzed cross-coupling reactions [12], In particular, palladium-catalyzed crosscoupling reactions have been successfully applied to the synthesis of biaryls due to their generally high yields and excellent selectivities. 

Nitronium salts are capable to induce the OCA reaction via generation of aryl radical-cations, even with relatively electron-poor arenes, such as benzene [105]. Namely, NOBF4 in catalytic amount (1-5 mol%) efficiently catalysed the oxidative coupling reactions of polyalkoxybenzenes in dichloromethane (containing 20% trifluoroacetic acid) under exposure to air affording the corresponding biaryls in almost quantitative yields [105]. Finally, oxidative couplings of arenes can be accomplished under the classical Scholl reaction conditions [106]. For example, 1-ethoxynaphthalene (419) was coupled to binaphthyl 420 by reaction with anhydrous aluminum chloride and nitrobenzene (as oxidant) in 65% yield, Scheme 28. 

A nice feature of reductive as well as oxidative coupling reactions has been presented recently in the synthesis of quateranthene 132 (Scheme 32) [82]. After addition of the lithiated species of 128 into the bianthrone and dehydrogenation, base induced cyclization gave the intermediate 130. Grignard addition to the ketone and subsequent reduction delivers 131, which was then fully aromatized by a Scholl-reaction involving DDQ as an oxidant in the presence of Sc(OTf)3. 

In the early 1990s, an increased interest in molecular electronics has led to the desire to obtain functionally and geometrically interesting planar organic species via facile synthetic routes. As such, the Scholl reaction has become increasingly useful due to its ability to effect intramolecular aryl-aryl coupling reactions in a controlled manner. To meet the needs in this area, milder reaction conditions have been developed, making the Scholl reaction more applicable to the synthesis of compounds with sensitive functionality. 

The Scholl reaction is one of the oldest and most useful C-C coupling reactions, and is often used in the synthesis of polycyclic aromatic hydrocarbons (PAHs). In general, a large amount of metal oxidant (such as FeCls) is employed in this reaction, and chlorinated by-products are produced in many cases. Rathore found that DDQ and protic acid effectively promote the Scholl reaction of various 1,2-diarylbenzenes (86) to give triphenylenes (87) in excellent yields (Scheme 8.40). The advantages of this reaction over typical metal oxidant promoted Scholl reactions are (l) an excess of DDQ is not required (2) no chlorinated by-products are produced and (3) recovery and re-use of reduced hydroquinone DDQ-H2 is possible. The reaction of other diaiyls tethered with methylene and propylene, and the intermolecular reaction also proceeds effectively. Furthermore, the treatment of hexaatylbenzene (88) with DDQ quantitatively affords collonene (89). 

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]. 

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]. 

Friedel-Crafts Arylation. The Scholl Reaction De-hydrogen-coupling... 

Oxidative coupling of aromatic compounds via the Scholl reaction has been applied successfully to synthesize a polyarylethersulfone (18). High molecular weight polymer was obtained upon treating 4,4,-di(l-naphthoxy)diphenylsuLfone and 4,4,-di(l-naphthoxy)benzophenone with ferric chloride. Equimolar amounts of the Lewis acid are required and the method is limited to naphthoxy-based monomers and other systems that can undergo the Scholl reaction. 

The alternative route to synthesis of poly(p-phenylene), which has been widely used, is the Scholl reaction5A), which involves direct oxidative elimination of two aryl hydrogen atoms with concomitant formation of a new carbon-carbon bond. This reaction occurs under Friedel-Crafts conditions and requires the presence of an appropriate oxidant to remove the hydrogen liberated in the coupling process. This route is typified by the Kovadk polymerization of benzene55) induced by aluminium chloride in the presence of stoichiometric amounts of Cu(II) chloride ... 

Aromatic polyethers, including poly(ether sulfone)s and poly(ether ke-tone)s, have been synthesized by the Scholl reaction. In the Scholl reaction a Friedel-Crafts catalysts is used to effectuate the coupling of two aromatic groups to form an aryl-aryl bond, accompanied by the elimination of two aromatic hydrogens [Eq. (58)] [188-190]. This reaction proceeds under oxidative reaction conditions by a cation-radical mechanism [191,192]. 

The coupling of two aromatic molecules by treatment with a Lewis acid and a proton acid is called the Scholl reaction. Yields are low and the synthesis is seldom useful. High temperatures and strong-acid catalysts are required, and the reaction fails for substrates that are destroyed by these conditions. Because the reaction... 

Scholl reaction. Coupling of aromatic molecules by treatment with Lewis acid catalysts. 

O. Beck, A. Amann, E. Scholl, J. E. S. Socolar, and W. Just Comparison of time-delayed feedback schemes for spatio-temporal control of chaos in a reaction-diffusion system with global coupling, Phys. Rev. E 66, 016213 (2002). 

This oxidative cross-coupling was reminiscent of the Scholl reaction and of the work of van Helden, Verberg, Itahara, and Fujiwara, who used stoichiometric Pd(II) salts for the synthesis of biaryls and styrenes. At the time that we were acquiring these data, mie example of a palladium-catalyzed oxidative cross-coupUng had was reported for the synthesis of a biaryl (the synthesis of 1-phenylnapthalene), so we decided to explore this new chemical territory. 

This reaction was first reported by Scholl in 1910. It is a Lewis acid-catalyzed coupling of aromatic compounds while eliminating two of the aryl-bound hydrogens. Therefore, it is generally known as the Scholl reaction,or Scholl condensation. " Occasionally, this reaction is also referred to as the Scholl oxidation. The commonly used Lewis acid is because some other Lewis acids may lead to the formation of... 

Comparison of oxidative aromatic coupling and the Scholl reaction 13AG(E)9900. 

Figure 4.8 A general strategy for coupling various building blocks to form networks, (a) The typical Scholl reaction, (b) A model structure of the SMP-1 network, (c) Other monomers or co-monomers.

The Scholl reaction, formally representing a Friedel-Crafts arylation reaction, is the aryl-aiyl coupling of two aromatic species 1 via dehydrogenation under Lewis acid and protic acid conditions. 

The intermolecular version of the Scholl reaction is much less prevalent in the literature due to the difficulty in controlling the formation of the desired products. Bushby and co-workers have used an intermolecular version of the Scholl reaction in efforts toward the synthesis of chirally discotic liquid crystals featuring the triphenylene nucleus. The fluorobenzene derivative 20 was coupled with the symmetric biphenyl 19 to... 

There is a significant number of references " describing efforts to make high polymers via aromatic carbon-carbon bond formation. These efforts were unsuccessful. In one case, however, high polymer was prepared by an elegant oxidative coupling (the Scholl reaction). High... 

A review on the oxidative coupling of electron-rich aromatic substances (Scholl reaction) and the synthesis of various biaryl substances by double C-H activation suggests that some of these reactions proceed only in the presence of non-oxidizing Lewis acids and some only in the presence of certain oxidants because there are two possible mechanisms that depend on the electronic structure of the substrates and the nature of the catalyst . One of the mechanisms involves the intermediacy of a radical cation while in the other the formation of a sigma complex between the acid and the substrate is suggested. ... 

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