Monocyclic pyrroles, reviews

Electrophilic substitution is an important type of reactions for five-membered heterocycles with one heteroatom and enables compounds with various substituents to be obtained. The present work is devoted to certain features of substrate and positional selectivities in electrophilic substitution reactions of derivatives of pyrrole, furan, thiophene and selenophene, and also the corresponding benzannulated systems, which had not been explained until recently. In a recent review (05RKZ(6)59), these problems were mainly discussed for thiophenes, while in a previous review (94H(37)2029) only monocyclic pyrrole, furan and thiophene derivatives were considered. 

Preparations of isopyrroles by oxidation generally give rise to 2-hydroxy-2//-pyrrole derivatives and have been carried out almost exclusively using 2,3,4,5-tetraarylpyrroles. Exceptions have been the formation of 2,2 -bis-2H-pyrroles39 and a 3f/-pyrrole.40 Oxidation has been attempted using a wide range of chemical oxidants, by photooxidation, and by electrochemical oxidation. The oxidation of monocyclic pyrroles was reviewed in 1973.3... 

Structures and nomenclature for the most important five-membered monocycles with one or more heteroatoms are depicted in Scheme 1. The aromaticity scale in five-membered heterocycles has been long debated.97-101 The decreasing order of aromaticity based on various criteria is (DRE values in kcal/ mol) benzene (22.6) > thiophene (6.5) > selenophene > pyrrole (5.3) > tellurophene > fur an (4.3). Pyrrole and furan have comparable ring strains (Scheme 38). The aromaticity of furan is still controversial 100 the NMR shielding by ring current estimated it at about 60% of the aromaticity of benzene, and other methods reviewed earlier102 estimated it at less than 20%. 

Gronowitz first applied the modern electronic theories of organic chemistry to elucidating the directing effects of substituents in the thiophene ring259 and has summarized concepts and experimental data in a recent review.2 The same rules can be extended without appreciable modifications to the other monocyclic systems fur an, selenophene, and pyrrole. 

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