The Acid-Catalyzed Polymerization of Indoles

Indole itself forms a dimer or a trimer, depending on experimental conditions the dimer hydrochloride is formed in aprotic solvents with dry HCl, whereas aqueous media lead to dimer or trimer, or both. It was Schmitz-DuMont and his collaborators who beautifully cleared up the experimental confusion and discovered the simple fact that in aqueous acid the composition of the product is dictated by the relative solubilities of the dimer and trimer hydrochlorides/ -This, of course, established the very important point that there is an equilibrium in solution among indole, the dimer, the trimer, and their salts. It was furthermore demonstrated that the polymerization mechanism involves acid catalysis and that in dilute solution the rate of reaction is dependent on the concentration of acid. 

In contrast wdth pyrrole, the polymerization does not appear to go beyond the trimer stage, any amorphous material produced being the product of autoxidation. 

Skatole (3-methylindole) readily forms a dimer hydrochloride with HCl in ether or with 15% aqueous HCl, as docs 1,3-dimethyl-indole. 3-n-Propylindole is also readily dimerized in dry ether/HCl/  

Schmitz-DiiMont and B. Nicolojannis, Ber. deut. chem. Ges. 63, 323 (1930). O, Schmitz-DuMont, B. Nicolojannis, E. Sehnorrenberg, and H. H, Saenger, J. prakt. Chem. 131, 146 (1931). 

Scliinitz-DuMont s statcmcnt that 3-ethylindole does not dimerize must be in error. 3-ier -Butyl- and 3-isopropyl-indole and tryptamine do not form dimers/ neither does 2-mcthylindole.  

Schmitz-DuMont s statement43 that 3-ethylindole does not dimerize must be in error. 3-fert-Butyl- and 3-isopropyl-indole and tryptamine do not form dimers,45 neither does 2-methylindole.43 44 2-Methylindole is, however, incorporated into mixed dimers and trimers. It thus reacts with indole and with skatole to form mixed dimers, and 2 moles react with 1 mole of indole for form a mixed trimer. Likewise indole forms a dimer with 1,2-dimethylindole, 2-phenylindole, and even with 2,5-dimethylpyrrole.53 

Mainly based on the failure of 2-methylindole to form a dimer, Schmitz-DuMont proposed structure (24) for indole dimer, the argument being that a methyl group in position 2 would effectively hinder the formation of a skeleton of type (24), whereas one in position 3 would not.46 47 

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The Acid-Catalyzed Polymerization of Pyrroles and Indoles G. F. Smith... 

An aqueous Friedel-Crafts reaction has also been used in polymer synthesis. The acid-catalyzed polymerization of benzylic alcohol and fluoride functionality in monomeric and polymeric fluorenes was investigated in both organic and aqueous reaction media. Polymeric products are consistent with the generation of benzylic cations that participate in electrophilic aromatic substitution reactions. Similar reactions occurred in a water-insoluble Kraft pine lignin by treatment with aqueous acid. A Bisphenol A-type epoxy resin is readily emulsified in aqueous medium with an ethylene oxide adduct to a Friedel-Crafts reaction product of styrene and 4-(4-cumyl)phenol as emulsifier.Electrophilic substitution reaction of indoles with various aldehydes and ketones proceeded smoothly in water using the hexamethylenetetramine-bromine complex to afford the corresponding Z A(indolyl)methanes in excellent yields.InFs-catalyzed electrophilic substitution reactions of indoles with aldehydes and ketones are carried out in water.Enzymatic Friedel-Crafts-type electrophilic substitution reactions have been reported. ... 

Indole undergoes add-catalyzed dimerization the 3H-indoIium ion acts as an electrophile and attacks an unprotonated molecule to give the dimer (46). Protonation of the dimer in turn gives an electrophilic species from which a trimeric product can be derived (77CPB3122). Af-Methylisoindole undergoes acid-catalyzed polymerization, indicating that protonation at C-1 gives a reactive electrophilic intermediate. 

Indole can be nitrated with benzoyl nitrate at low temperatures to give 3-nitroindole. More vigorous conditions can be used for the nitration of 2-methylindole because of its resistance to acid-catalyzed polymerization. In nitric acid alone it is converted into the 3-nitro derivative, but in a mixture of concentrated nitric and sulfuric acids 2-methyl-5-nitroindole (47) is formed. In sulfuric acid, 2-methylindole is completely protonated. Thus it is probable that it is the conjugate acid which is undergoing nitration. 3,3-Dialkyl-3H-indolium salts similarly nitrate at the 5-position. The para directing ability of the immonium group in a benzenoid context is illustrated by the para nitration of the conjugate acid of benzylideneaniline (48). 

Indolizine is much more basic than indole (p Ta = 3.9 vs. —3.5), and the stability of the cation makes it less reactive and resistant to acid-catalyzed polymerization. Protonation occurs at C-3, although 3-methylindolizine protonates also at C-l. Introduction of methyl groups raises the basicity of indolizines. Electrophilic substitutions such as acylation, Vilsmeyer formylation, and diazo-coupling all take place at C-3. Nitration of 2-methylindolizine under mild conditions results in substitution at C-3, but under strongly acidic conditions it takes place at C-l, presumably via attack on the indolizinium cation. However, the nitration of indolizines often can provoke oxidation processes. 

The susceptibility of pyrroles to acid-catalyzed polymerization is a distinct disadvantage in the catalytic hydrogenation of pyrroles in acidic media and, similarly, the oligomerization of indoles by acids can also be a problem under such hydrogenation conditions. 

Vinylpyrroles and vinylindoles are extremely sensitive to acid-catalyzed dimerization and polymerization and it is significant that much of the early research was conducted on systems which were produced in situ. Even by this approach, the dimerization of, for example, 2-(3-indolyl)propene and l-(3-indolyl)-l-phenylethylene was difficult to prevent (see the formation of 110 and 120, Section 3.05.1.2.8). Similarly, although it is possible to isolate ethyl 2-(2- and 3-indolyl)propenoates, they appear to be extremely unstable at room temperature even in the absence of acid (81UP30500) to give [ 4 + 2] cycloadducts of the type (348) (cf. 77JCS(P1)1204>. For many years simple vinylpyrroles also eluded isolation, on account of their facile acid-catalyzed polymerization. Application of the Wittig reaction, however, permits the synthesis of vinyl-pyrroles and -indoles under relatively mild and neutral conditions (see Section 3.05.2.5). In contrast, heteroarylvinyl ketones, esters, nitriles and nitro compounds, obtained by condensation of the appropriate activated methylene compound with the heteroaryl aldehydes (see Section 3.05.2.5), are thermally stable and... 

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