Scholl reaction

The elimination of two aryl-bound hydrogens accompanied by the formation of an aryl-aryl bond under the influence of Friedel-Crafts catalysts. Cf. Friedel-Crafts reaction. 

Name Reactions A Collection of Detailed Mechanisms and Synthetic Applications, DOI 10.1007/978-3-319-03979-4 246, Springer International Publishing Switzerland 2014 

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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]. 

Scheme 5.1-9 The Scholl reactions of two helicines. (a = NaCI/AICb (X(AICl3) = 0.69) at 140 °C).

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. 

As depicted in Scheme 2, homopolymerization of 1 to form the aromatic poly(aryl ether) 6FNE utilized the Scholl reaction in nitrobenzene with anhydrous ferric chloride at room temperature. A 100-ml round-bottom flask was charged... 

The homopolymerization ofl consists of a room-temperature reaction of the monomer dissolved in nitrobenzene in the presence of anhydrous ferric chloride. Polymerizations were carried out under a stream of dry nitrogen. As depicted in Scheme 2, the homopolymerization of 1 to form 6FNE takes place by means of the Scholl reaction. The mechanism of the Scholl reaction was assumed to proceed through a radical-cation intermediate derived from the single-electron oxidation of the monomer and its subsequent electrophilic addition to the nucleophilic monomer. The reaction releases two hydrogens, both as protons, to form the... 

Scholl reaction, 12 171 Schotten-Baumann procedure, 10 485 Schotten-Baumann reaction, 19 798 Schottky barrier, 19 137, 138 Schottky barrier arrays, platinum silicide, 19 157-158... 

It was established in 1929 by Liittringhaus and Neresheimer that 4,4,-bibenzanthronyl (6.77) is an intermediate in the formation of violanthrone. Thus, compound 6.77 results when an Ullmann reaction is carried out on 4-chlorobenzanthrone the same product is obtained when benzanthrone reacts under relatively mild conditions (approximately 110 °C) with a mixture of potassium hydroxide and potassium acetate in 2-methylpropan-l-ol (isobutanol). Alkali fusion at a higher temperature then converts 4,4,-bibenzanthronyl into violanthrone. Use of aluminium chloride also leads to ring closure (Scholl reaction). 

An alternative route to pyranthrone, involving baking 1,6-dibenzoylpyrene (6.96) with aluminium chloride, was also devised by Scholl (Scheme 6.18). The Scholl reaction is a key step in the synthesis of several polycyclic quinones the cyclisation of 1-benzoylnaphthalene to give benzanthrone (6.73) has already been mentioned. The mechanism of this cyclodehydrogenation reaction may involve an initial protonation step, if traces of water are present, or complexation with aluminium chloride. Electrophilic substitution is thereby... 

Yields are frequently moderate for Scholl reactions. Aldoximes are not usually compatible with these harsh reaction conditions and are very sensitive to factors such as temperature and reaction time. Consequently, oxidation to the corresponding carboxylic acid is a major side-reaction. However, both the ketoxime (51) and the aldoxime (53) are reported to give good yields of the corresponding m-dinitro compounds, (52) and (54) respectively, on treatment with absolute nitric acid in methylene chloride followed by hydrogen peroxide. 

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. 

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]. 

Figure 43 Types of groups incorporated into bis(l,l -binaphthoxy) monomers polymerized by the Scholl reaction.

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