Quantitative aspects of electrophilic

Two excellent reviews <71AHC(13)235, 72IJS(C)(7)6l) have dealt with quantitative aspects of electrophilic substitution on thiophenes. Electrophilic substitution in the thiophene ring appears to proceed in most cases by a mechanism similar to that for the homocyclic benzene substrates. The first step involves the formation of a cr-complex, which is rate determining in most reactions in a few cases the decomposition of this intermediate may be rate determining. Evidence for the similarity of mechanism in the thiophene and benzene series stems from detailed kinetic studies. Thus in protodetritiation of thiophene derivatives in aqueous sulfuric and perchloric acids, a linear correlation between log k and —Ho has been established the slopes are very close to those reported for hydrogen exchanges in benzene derivatives. Likewise, the kinetic profile of the reaction of thiophene derivatives with bromine in acetic acid in the dark is the same as for bromination of benzene derivatives. The activation enthalpies and entropies for bromination of thiophene and mesitylene are very similar. 

Volume 47 of Advances in Heterocyclic Chemistry is, unlike most volumes, a monograph and deals with the quantitative aspects of electrophilic substitution of heterocycles. It is written by Roger Taylor of the University of Sussex, Brighton, England, and your editor with one chapter contributed by Ross Grimmett of the University of Otago in New Zealand. It is hoped that this survey of the whole area of electrophilic substitution of heterocycles, covering as it does semiqualitative as well as completely quantitative aspects, will be of considerable help to workers in the field. 

Quantitative aspects of electrophilic substitution in the series 73KGS579. 

Volume 13 of this serial publication comprises six chapters of which four deal with general accounts of compound classes 1-azirines (F. W. Fowler), phenanthridines (B. R. T. Keene and P. Tissington), tri-thiapentalenes (N. Lozac h), and heterocyclic ferrocenes (F. D. Popp and E. B. Moynahan). The other two chapters are concerned with particular aspects of the chemistry of groups of heterocyeles the tautomerism of purines (B. Pullman and A. Pullman) and quantitative aspects of the electrophilic substitution reactions of five-membered rings (G. Marino). 

In this review we have gathered the important work on quantitative and mechanistic aspects of electrophilic aromatic reactivity of heterocycles. We have concentrated in particular on acid-catalyzed hydrogen exchange, nitration, and gas-phase elimination, these being the major efforts of our own research groups. However all other electrophilic substitution reactions are covered for completeness. 

The fact that none of these reports has emphasized the physical aspects of electrophilic substitution in the series reflects the paucity of quantitative studies, and the low reactivity of these compounds in the presence of electrophiles. Few kinetic studies have been reported and the regio-chemical effects of substituents have seldom been quoted in quantitative form. The present chapter brings together those quantitative results that are available, and collates data on substituent effects. One worthwhile field of study would appear to be the application to the azines of Taylor s method involving thermolysis of esters [75JCS(P2)277, 75JCS(P2) 1783]. 

Electrophilic Substitution.—Quantitative aspects of the electrophilic substitution of furan, thiophen, pyrrole, and other five-membered heteroaromatic systems are treated in a review by Marino. Gol dfarb and his school are continuing their extensive work on electrophilic substitution in simple thiophens. The action of benzoyl chloride on 2-acetylthiophen in an excess of aluminium chloride at 110—115 °C without solvent gave a complex mixture of 2-benzoylthiophen,... 

For monographs, see Taylor, R. Electrophilic Aromatic Substitution Wiley NY, 1990 Katritzky, A.R. Taylor, R. Electrophilic Substitution of Heterocycles Quantitative Aspects (Vol. 47 of Adv. Heterocycl. Chem.) Academic Press NY, 1990. For a review, see Taylor, R. in Bamford Tipper Comprehensive Chemical Kinetics, vol. 13 Elsevier NY, 1972, p. 1. 

Katritzky, A. R, Taylor, R., Electrophilic Substitution of Heterocycles Quantitative Aspects, 47, 1. 

Katritzky AR, Taylor R (1990) Electrophilic substitution of heterocycles quantitative aspects. In Katritzky AR (ed) Advances in heterocyclic chemistry. Academic Press, San Diego... 

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