Selenophenes, review

Electrophilic substitution in furan, thiophene, selenophene and pyrrole has, up to 1970, been comprehensively reviewed by Marino.66 Italian workers have determined the relative reactivities of selenophene and thiophene as well67 relative rates are given in Table I. Including furan, the order of reactivity is furan > selenophene > thiophene. 

The potentially tautomeric side-chain thiol systems exist mainly in the thiol form in liquid solution and in the gas phase, as found by IR and NMR spectroscopy and by a study of ionization potentials.126 Upon alkylation using the ion-pair extraction method, only the S-alkylated compounds were obtained. The synthesis, reactions, and properties of some selenides of thiophene, furan, and selenophene have been reviewed.127... 

Dr. S. T. Reid has contributed the first of two sections on photochemistry of heterocycles, the present one dealing with that of /V-heterocycles. This updates part of his own earlier review The Photochemistry of Heterocycles published in 1970 in Volume 11. A further contribution, which will extend the updating to O- and 5-heterocycles, should be published shortly. Dr. A. B. Hornfeldt s review, entitled Selenophenes, also deals with a subject that was previously reviewed in Volume 12 of this series by N. N. Magdesieva. 

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%. 

This section reviews synthetic procedures for heteroaromatic systems related to thienothiophenes, viz., dithienothiophenes, selenopheno-selenophenes, and selenophenothiophenes. 

Several earlier reviews are available on selenophene (70AHC(l2)i, B-73MI31601, 82AHC(30)127> and tellurophene (77AHC(21)119, B-74MI31601), and annual summaries of new developments are provided by the Royal Society of Chemistry s Specialist Periodical Reports on Heterocyclic Chemistry. 

Several reviews are available on the synthesis and properties of selenophene, its derivatives, and compounds having a selenophene ring annulated to benzene or a heterocyclic ring [1, 14, 117-120], The methods are based on the formation of one, two or three bonds, or on ring transformations. 

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. 

This review shows that the properties and comparative reactivity of five-membered heterocycles containing oxygen, sulfur, and selenium may place selenophene differently with respect to furan and thiophene, and its position in the series depends on the reaction and the role played by the heteroatom. Selenophene is located between furan and thiophene as regards reactions in which the heterocycle displays predominantly electron-donating properties as for those principally concerned with electron-accepting properties, selenophene is placed between thiophene and benzene. 

This section of the review is concerned with reactions in the side-chain of selenophene derivatives, in which the heterocyclic nucleus exerts an influence, but is not directly attacked. 

PHC(18)126>. All other specialized review articles are cited in the appropriate section. Einally, in terms of the organization of this chapter, selenophenes and fused derivatives will be treated together in each section. 

The production of selenophene via the thermolysis of dialkyl selenides in the presence of acetylene was explored <2004RJ0290>. Thermal reactions leading to selenophenes and thiophenes have also been reviewed <2000CHE1>. 

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. 

This chapter will review novel synthetic routes to thiophenes, selenophenes, tellurophenes and some of their significant derivatives reported over the last one year (Jan-Dec 2003). The subsequent sections will review functionalization of these heterocycles, novel molecules containing these five membered rings and some of their uses. 

For the purposes of this review, heterosapphyrins are defined as being sap-phyrin-like macrocycles in which a furan, thiophene, or selenophene subunit serves to replace one or more of the pyrrole rings. Interestingly, like the pentaazasapphyr-ins, this class of macrocycles has its origins in a serendipitous discovery. It occurred during efforts directed toward the synthesis of the heteroatom analogs of corrole. Specifically, as first reported by Johnson and coworkers in 1969, reaction of... 

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