Selenophens and Tellurophens

General.—During the past two years a marked increase in interest in selenophen chemistry can be noticed, and especially many differently fused 

Meth-Cohn, J.C.S. Perkin I, 1973, 1104. W. Ried and E. Nyiondi-bonguen, Annolen, 1973, 134. 

Reagents i, NajTe (from Te + NaOH + HOCHjSOjNa, in water) ii, Br Hi, Na SOj, KjCOj, HjO 

3-diyne (58) as the organic moiety/ Compounds in which selenophen rings are fused on to benzene or naphthalene/ on to benzofurans/ or on to quinolines or carbazoles have been prepared. The last-named compounds are the selenophen analogues of known carcinogens. 

The benzoseleninium salt (61) has been made by the reaction of selenoflavone with phenylmagnesium bromide, followed by perchloric acid.  

Marechal, L. Christiaens, M. Renson, and P. Jacquignon, Collect. Czech. Chem. Commun., 1978, 

Nishino, K. Nakasuji, and I. Murata, Chem. Lett., 1978, 723 K. Nishino, S. Yano, Y. 

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The much less sophisticated PPP approximation has been shown to well reproduce the electronic spectral features not only of the monocyclic furan, pyrrole, thiophene, selenophene and tellurophene but also many of the benzo fused derivatives as well (79MI30101, 68JPC3975, 68MI30100). 

Halogens attack the ring heteroatom in selenophene and tellurophene. Thus the selenienyl bromide (89) is among the bromination products of benzo[6]selenophene. Tellurophene reacts with halogens to give 1,1-dihalogeno derivatives e.g. 90). 

The first proton to be removed from iV-methylpyrrole by w-butyllithium is from an a-position a second deprotonation occurs to give a mixture of 2,4- and 2,5-dilithiated derivatives. The formation of a 2,4-dilithio derivative is noteworthy since in the case of both furan and thiophene initial abstraction of a proton at C-2 is followed by proton abstraction from C-5 (77JCS(P1)887). iV-Methylindole, benzo[6]furan and benzo[6]thiophene are also deprotonated at C-2. Selenophene and benzo[6]selenophene and tellurophene and benzo[6]tellurophene similarly yield 2-lithio derivatives (77AHC(21)119). 

Table 4 pKa Values of Pyrrole-, Furan-, Thiophene-, Selenophene- and Tellurophene-earboxylie Aeids... 

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. 

Selenophene and tellurophene were obtained in 15-20% yield by the reaction of selenium and tellurium with diacetylene in the KOH/DMSO/N2H4-H2O/H2O system at 0-20°C (90MI1). 

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... 

Thiophenes continue to play a major role in commercial applications as well as basic research. In addition to its aromatic properties that make it a useful replacement for benzene in small molecule syntheses, thiophene is a key element in superconductors, photochemical switches and polymers. The presence of sulfur-containing components (especially thiophene and benzothiophene) in crude petroleum requires development of new catalysts to promote their removal (hydrodesulfurization, HDS) at refineries. Interspersed with these commercial applications, basic research on thiophene has continued to study its role in electrocyclic reactions, newer routes for its formation and substitution and new derivatives of therapeutic potential. New reports of selenophenes and tellurophenes continue to be modest in number. 

This chapter follows the organization used in the past. A summary of the electronic properties leads into reports of electrocyclic chemistry. Recent reports of studies of HDS processes and catalysts are then summarized. Thiophene ring substitution reactions, ring-forming reactions, the formation of ring-annelated derivatives, and the use of thiophene molecules as intermediates are then reported. Applications of thiophene and its derivatives in polymers and in other small molecules of interest are highlighted. Finally, the few examples of selenophenes and tellurophenes reported in the past year are noted. 

Vertical ionization energies of the two highest molecular orbitals and of orbitals mainly localized on the substituent of a-substituted derivatives of furans, thiophenes, selenophenes, and tellurophenes have been determined.25 Assignments of some of the bands other than the first two in the photoelectron spectra of tellurophene and selenophene are proposed, and the effect of the ring on the orbitals mainly localized on the substituent is briefly discussed. 

Selenophene and tellurophene have an additional fragmentation as shown in Scheme 2. 

There are several systematic nuclear magnetic resonance studies of the interaction between the substituents and the protons and ring atoms of five-membered heterocycles. In some 2-substituted furans, thiophenes, selenophenes, and tellurophenes there is a linear correlation between the electronegativity of the chalcogen and several of the NMR parameters.28 As there also is a good correlation between the shifts of the corresponding protons and carbons in the four heterocycles, the shifts of unknown selenophene and tellurophene derivatives can be predicted when those of thiophene are known. This is of special interest for the tellurophene derivatives, since they are difficult to synthesize. In the selenophene series, where a representative set of substituents can be introduced in the 2- as well as in the 3-position, the correlation between the H and 13C shifts and the reactivity parameters according to Swain and Lupton s two-parameter equation... 

However, the rate of substitution of pyrrole is too high and that of benzene too low to be followed by standard techniques, and consequently a kinetic study was limited to furan, thiophene, selenophene, and tellurophene. Activation entropies are constant for all four members of the series, indicating that the arrangement of the atoms around the reaction center is similar, i.e., the transition states of all four rings occur at similar positions along the reaction coordinate. The relative rates for the formylation are thus controlled by the activation enthalpies. At 30UC relative rates are furan (107), thiophene (1), selenophene (3.64), and tellurophene (36.8).68... 

Computer simulation of lanthanide-induced shifts in the 2-formyl and 2-acetyl derivatives of furan, thiophene, selenophene, and tellurophene108 indicate a nearly equipopulated mixture of trans and cis conformers of the furan, and a preponderance of the cis for the thiophene, selenophene, and tellurophene derivatives. This difference is due to an interaction between the ring heteroatom and the carbonyl oxygen lone pair electrons. 

At present, the chemisty of selenophenes and tellurophenes is a relatively scantily studied area. Nevertheless, a number of new valuable contributions dealing with their chemistry have emerged. Electrophilic cyclization of l-(l-alkynyl)-2-(methylseleno)arenes provides a route to a variety of 2,3-disubstituted benzo[fe]selenophenes, as illustrated by the preparation of the system 88. Other useful electrophiles for similar reactions are E or NBS <06JOC2307>. Similar chemistry has also been employed in preparation of 2,3-disubstituted benzo[f>]selenophenes on solid phase <06JCC163>. In addition, syntheses of 2,3-dihydroselenolo[2,3- >]pyridines have been achieved using radical chemistry <06OBC466>. 

Higher Chalcogenophenes Selenophene and Tellurophene and their Polymers... 

Similar correlations have been observed (75CS(7)2ll) between the 13C NMR spectra of mono-substituted furans, thiophenes, selenophenes and tellurophenes as with their H NMR spectra (cf. Section 2.3.3.3). Thus, for the 2-substituted compounds the A(C-3)/A(C-5) ratios decrease systematically in the series furan (2.58) > thiophene (1.34) = selenophene (1.34) > tellurophene (0.91). Extensive quantitative correlations have been established between the shifts of the corresponding carbon atoms in the different heterocycles (75CS(7)21l). In most cases Tsubstituted heterocycles can be linearly correlated with the electronegativity of the heteroatom, with the couplings being largest for the furans. 

Increasing tendency towards electrophilic attack at the ring heteroatom is shown in thiophenes, selenophenes and tellurophenes. 

Pyrrole-3-carboxylic acid (358) is appreciably weaker than benzoic acid and this is attributed to the stabilization of the undissociated acid by electron release from nitrogen. The 2-carboxylic acids of furan, thiophene, selenophene and tellurophene are all stronger acids than benzoic acid... 

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