Pyrroles 2- alkyl-, polymerization

As can be seen from the survey, only a limited number of alkyl pyrroles are polymerized, actually dimerized, by acid in ether or in water. The dimerizable pyrroles include all those which have free adjacent a- and jS-positions, that is 2- and 3-methyl-, 2-isopropyl-, and 2,3-dimethyl-pyrrole. It must be repeated that no homogeneous product has yet been obtained from 3-methylpyrrole, and this reaction deserves more study. 

The ease with which pyrroles are polymerized by acids depends on the substituents present. This can be seen from their behaviour with ethereal hydrogen chloride. Pyrrole is trimerized sa, 309-io 2-phenyl- and some 2-alkyl- and 2,3-dialkyl-pyrroles give hydrochlorides of their dimers (Table 4.6), 3-methylpyrrole forms amorphous polymeric materiaU and... 

Pyrrole itself is very easily converted by acid into intractable and readily autoxidized polymers. In this case and in the case of the alkyl pyrroles, it is important to distinguish between anaerobic acid-catalyzed reactions and autoxidative polymerizations the decomposition of pyrrole and its alkyl derivatives on standing in air belongs to the latter type, this review is concerned only with the former. 

The range of preparatively useful electrophilic substitution reactions is often limited by the acid sensitivity of the substrates. Whereas thiophene can be successfully sulfonated in 95% sulfuric acid at room temperature, such strongly acidic conditions cannot be used for the sulfonation of furan or pyrrole. Attempts to nitrate thiophene, furan or pyrrole under conditions used to nitrate benzene and its derivatives invariably result in failure. In the case of sulfonation and nitration milder reagents can be employed, i.e. the pyridine-sulfur trioxide complex and acetyl nitrate, respectively. Attempts to carry out the Friedel-Crafts alkylation of furan are often unsuccessful because the catalysts required cause polymerization. 

Pyrrole and its simple alkyl derivatives are polymerized by many of the typical acidic electrophiles. Nitration with acetyl nitrate (Scheme 4.31) under mild conditions gives the 2-nitro compound, together with small amounts of 3-nitropyrrole. Halogenation is rapid but it gives a complex mixture of polyhalogeno compounds. 

Formation of a nitrogen radical by N-H deprotonation must be faster than a-deprotonation of the pendant alkyl group. Similarly, pyrrole could be photo-polymerized [129] and picolines could be photocatalytically degraded [130]. 

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