Of furan, thiophene, and their

Volume 78 of Advances in Heterocyclic Chemistry contains four contributions. A. P. Sadimenko of Fort Hare University of South Africa has covered organometal-lic compounds of furan, thiophene, and their benzannulated derivatives. This constitutes the first installment of a projected series on the organometallic chemistry of heteroaromatic ligands, a subject of great fundamental and technical importance that has exploded in the 1990s. 

Compounds of Furan, Thiophene, and Their Benzannulated Derivatives, 78, 1. 

P. Sadimenko, Organometallic Compounds of Furan, Thiophene, and Their Benzannulated Derivatives, Adv. Heterocycl. Chem. 2001, 78, 1-64. 

The electronic structures of furan, thiophene, and selenophene, their protonated complexes, and their anions have been calculated by the extended Hiickel method.6 The results of these calculations have been used to determine the influence of the heteroatom on the degree of aromaticity and electron density. 

Recently, the hydroxy derivatives of furan, thiophene, and selenophene have been studied with regard to their physical properties and reactions. These compounds are tautomeric and if the oxygen function is placed in the 2-position they exist as unsaturated lactones and undergo carbon-carbon rearrangement, whereas the 3-hydroxy derivatives form oxo-enol tautomeric systems. By NMR the structures of the different tautomeric forms have been determined as well as the position of the tautomeric equilibrium and the rate of isomerization. 

And 3-vinyl derivatives of pyrrole, furan, thiophene and their benzologues behave as outer-inner dienes and react with ir-electron-deficient alkenes and alkynes to produce the corresponding Diels-Alder adducts. Thus, 3-vinylpyrrole (307) in combination with methyl propiolate gives dihydroindoles (308) or (309) (80JOC4515, 90TL4613, 91H(32)1199). 

A summary of bond lengths and angles from X-ray structures of heterocyclic compounds studied up to 1970 has been published <1972PMH(5)1>. This compilation contains many examples, particularly of furans, thiophenes, and pyrroles and their benzo derivatives further examples are contained in the appropriate chapters of the three editions of Comprehensive Heterocyclic Chemistry. The Cambridge Structural Database (CSD) maintains a comprehensive collection of heterocyclic structures. 

Chloromethyl-2,5-dimethyl derivatives of furan, thiophene, and selenophene are solvolyzed faster than their 2-chloromethyl derivatives. For both the 2-chloromethyl and the 3-chloromethyl-3,5-dimethyl series the solvolysis rate increases in the order thiophene < selenophene < furan. 

Many features of reactions of pyrrole, furan, thiophene and their derivatives bearing electron-releasing substituents with electrophiles are governed by their ready formation and the high stability of corresponding 2H-hetarenium ions, mainly a-C-protonation products like A (E = H). The reversible formation of such cr-complexes (Scheme 2, reaction 1) competes with acid-catalyzed oligomerization (reaction 2), which is practically irreversible and starts from the attack of an hetarenium cation on the neutral five-membered heterocycle analogously to the first step of electrophilic substitution. 

Partial and Complete Reduction of Pyrroles, Furans, Thiophenes and their Benzo Analogs... 

A fundamental chemical tactic in the synthetic manipulation of heterocycies by organic chemists is the reduction of the heterocyclic ring. This chapter will review advances in the partial and complete reduction of pyrroles, furans, thiophenes and their benzo analogs, indoles, benzo[Z>]furans and benzo[Z>]thio-phenes. As appropriate, coverage will include the reduction of those benzo analogs of lesser interest isoindole, carbazole, benzo[c]furan (isobenzofuran), dibenzofuran and dibenzothiophene. 

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