Scholl reaction mechanism

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. 

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... 

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... 

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]. 

Although a mechanism involving a radical cation has been proposed for the Scholl reaction, as indicated by the paramagnetic properties of many polycyclic aromatic hydrocarbons (PAHs) when they are treated with Lewis acids or concentrated sulfuric acid, it is assumed that the Scholl reaction occurs in a manner similar to the Friedel-Crafts Alkylation, involving an arenium cation instead of a radical cation. In detail, the Scholl reaction of hexaphenylbenzene involves the complexation between a Lewis acid and aromatic nucleus, electrophilic addition, and deprotonation,as illustrated here. In the presence of NaCl or HCl, chloride is beneficial for the elimination of aryl hydrogen by the formation of hydrogen chloride, as indicated by the bold chloride. 

Ultimately, however, it was admitted that the involvement of a parallel arenium cation mechanism could not be ruled out, owing to the coexistence and interchangeability of both species (radical cation versus arenium cation) under the conditions of the Scholl reaction. This fact clearly illustrates the continuing dichotomy that is characteristic of attempts to assign these processes to discrete mechanistic categories. Apparently, the extent to which one mechanism operates at the expense of the other mechanism depends on the chemical structure of the particular precursor, and the reaction conditions applied. 

As pointed out later by Balaban, protonation of an alkene is energetically more favorable than protonation of carbon monoxide, according to the conjugated acid/base (A/B) character, as indicated in Scheme 1 (always only the weaker acid-base pairs are involved in reactions). Balaban s Ph.D. thesis also advanced new mechanisms for the Scholl reactions (dehydrogenating condensations of aromatics) and other reactions. ... 

Rempala P, Kroulik J, King BT (2006) Investigation of the mechanism of the intramolecular Scholl reaction of contiguous phenylbenzenes. J Org Chem 71 5067-5081. doi 10.1021/... 

Neither the synthetic scope nor mechanism was significantly investigated until 1935, when Baddeley and co-workers provided new insights into the Scholl reaction. Their work provided the first mechanistic interpretation, suggesting that the homocoupling reaction of naphthalene proceeds via an arenium cation intermediate. Based on experiments in... 

There are two plausible mechanistic pathways under debate for the Scholl reaction. The mechanistic uncertainty revolves around whether the reaction proceeds through cationic or radical cation intermediates. Both of these mechanisms are outlined below. 

The arenium cation mechanistic pathway was first proposed in 1935 by Baddeley and was later reinforced by Balaban and Nenitzescu. Computational calculations performed by King and co-workers have produced several important conclusions. First, it was determined that the arenium cation mechanistic pathway was thermodynamically favored in studies under both vacuum and solvated conditions due to lower energy transition states than those found in the radical cation mechanistic pathway. Second, due to the increasingly exergonic nature of the reaction and the observed nonaccumulation of intermediates, C-C bond formation, in the case of hexaphenylbenzene, was found to occur slowest for the first bond and fastest for the last bond. The arenium cation mechanistic pathway is further supported by evidence that the Scholl reaction can proceed in acidie solutions that do not promote radical formation, such as anhydrous HF. The cationic mechanism has been shown to predominate in strongly protic conditions. ESR and EPR studies have concluded that the radieal cations previously observed during the Scholl reaction are not part of the actual reaction, but... 

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. ... 

Figure 44 Mechanism of the Scholl polymerization reaction of bis(l,r-binaph-thoxy) monomers.

The Scholl synthesis of biaryls proceeds mainly via the radical-cation mechanism, while in certain instances some evidences indicate, at least partially, cationic reaction pathway [106],... 

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