2-Aryl-selenophenes

Mercapto derivatives of furan, thiophene, selenophene (77ACS(B)198) and pyrrole (72AJC985) all exist predominantly in the thiol form. 2-Mercaptobenzothiophene is also a thiol (70JCS(C)243i) whereas 2-mercaptoindole is mainly indoline-2-thione (89) (69CPB550). The finely balanced nature of this system is indicated by the fact that a 3-aryl, but not a 3-alkyl, substituent will stabilize the 2-thiol form, whereas for 3-aryl-fV-methyl derivatives the 2-thione tautomer is preferred (71CC836). 

Ring closures which depend on the conversion of the heteroatom into an electrophile are mostly associated with the formation of thiophene, selenophene and tellurophene rings and some illustrative examples are shown in Scheme 17. The last example which concerns the conversion of reaction with isocyanides is of particular interest since it appears to entail the attack of an electrophilic nitrogen species on the aryl ring. 

H NMR, 4, 1042 ionization potentials, 4, 1046 synthesis, 4, 1066 UV spectra, 4, 1044 Selenolo[2,3 -cjthiophenes H NMR, 4, 1042 synthesis, 4, 1067 UV spectra, 4, 1044 Selenolo[3,2-6]thiophenes dipole moments, 4, 1049 H NMR, 4, 1042 ionization potentials, 4, 1046 synthesis, 4, 1066 UV spectra, 4, 1044 Selenolo[3,4-6]thiophenes synthesis, 4, 1067 Selenolo[3,4-c]thiophenes addition reactions, 4, 1062 synthesis, 4, 1076 Selenomethionine applications, 4, 970 Selenophene, 3-acetamido-reactions, 4, 953 Selenophene, 2-acetyl-mercuration, 4, 946 nitration, 4, 947 Selenophene, 2-alkyl-reactions, 4, 45 synthesis, 4, 135, 967 Selenophene, 3-alkyl-synthesis, 4, 135, 967 Selenophene, 3-aryl-synthesis, 4, 963 Selenophene, 2-benzyl-reactivity, 4, 946 Selenophene, 2-benzyl-5-ethyl-reduction, 4, 950... 

The C-Se and C-Te bonds are formed by an internal homolytic substitution of vinyl or aryl radicals at selenium or tellurium with the preparation of selenophenes and tellurophenes, respectively. An example is shown below, where (TMSIsSiH was used in the cyclization of vinyl iodide 65 that affords... 

Similarly, C—Se bonds are formed by an internal homolytic substitution of aryl radicals at selenium, with the preparation of selenophenes and benze-neselenophenes [75]. Scheme 7.7 illustrates the reaction of aryl iodides 61 with (TMS)3SiH, which afforded benzeneselenophenes in good yields. The presence of (TMS)3SiI after the reduction induced the final dehydration of the intermediate 3-hydroxyselenophenes, presumably through an intermediate silyl ether. 

In this section the reactions of selenophenes and tellurophenes with metal alkoxides, alkyls, aryls and amides will be considered. These reactions result primarily in proton abstraction and, in the monocyclic and benzo[6] fused systems, protons are removed preferentially from positions a to the heteroatom. In conformity with the general format of the reactivity sections in this work, the reactivity of the metal derivatives formed is dealt with in Section 3.16.8.8 on metalloid substituents. 

Aryl-1,2,3-selenadiazoles upon prolonged heating are converted into selenophenes. Since the latter compounds have also been obtained from the reaction of arylalkynes with selenium, 1,2,3-selenadiazoles seem to be indirect precursors of selenophenes (Scheme 49) (73JHC953, 79JHC1405). 

The UV spectra of selenazole derivatives as solvatochromic dyes were measured. The color is mostly dark red. Absorptions (A iax) of the 5-(2-thienyl)-l,3-selenazole 56 and the 5-aryl-l,3-selenazole 57 are 462 and 409 nm, respectively. The corresponding selenophenes 58, 59 show absorption at 505 and 445 nm <2000AGE556> (Figure 3). 

Aryl-l,2,3-selenadiazoles upon prolonged heating have also been converted into selenophenes 119 and 120. Since these last compounds have also been obtained from the reaction of aryl acetylenes with Se, it is probable that the selenadiazole is not a direct precursor of the selenophenes. 

Methylselenanthrenes (117) (various positions) are formed by the coupling of the appropriate sodium selenophenates (115) and 2-(methylseleno)benzenediazonium halides (114), followed by the cyclization of the intermediate aryl 2-(methylseleno)aryl selenides (116) with cone, hydrochloric acid and zinc (Scheme 30) <36YZ869, 37YZ190>. 

Side-chain reactions in which a deiocalizable positive charge is developed in the transition state are frequently considered together with electrophilic aromatic substitutions 60 the analogy between these two classes of reactions has been confirmed also in heteroaromatic chemistry.61 However, a comparison of the solvolysis data with those referring to electrophilic substitutions (Tables XVII-XIX) indicates that tellurophene behaves in a different way in the two types of reactions. As concerns the reactivity, tellurophene is less reactive than furan in electrophilic substitution but more reactive than furan in the solvolysis of 1 (2-aryl)ethyl acetates as concerns the k,tJkH reactivity ratios, tellurophene appears to be more sensitive to substituent effects than selenophene in electrophilic substitutions and less sensitive in side-chain reactions. 

A number of examples exist wherein aryl and heteroaryl moieties are interposed between two thiophene units. Oligomers that contain a furan or selenophene between two thiophenes are reported <89PS(42)i7i, 90PS(48)239> as well as the polymers shown in Table 2. Of particular interest is the incorporation of an iV-alkanethiol group to produce (154). This thiol group may serve as an anchor to gold or platinum surfaces and allows for electropolymerization directly on the metal surface. 

The arene substrates are not limited to simple benzene derivatives. A variety of het-eroarenes can also participate in alkene arylations to generate the desired coupling products. Stoichiometric oxidative coupling of aromatic heterocycles such as furan, thiophene, selenophene, A-methylpyrrole, benzofiiran and benzothiophene with a variety of alkenes, including acrylonitrile, styrene and methyl acrylate, have been extensively studied by Fu-jiwara and coworkers [8]. Furan, thiophene, selenophene and A-methylpyrrole are easily alkenylated with alkenes to give 2-alkenylated and 2,5-dialkenylated heterocycles in relatively low yields (3 6%) [8a], while the reactions of benzofuran and benzothiophene with alkenes produced a mixture of 2- and 3-alkenylated products [8b]. 

Abstract The chapter is devoted to the synthesis and application of thiophenes (selenophenes) and benzothiophenes bearing fluorine atoms, CF3 groups, and perfluorinated aryl fragments. 

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