From Other Tellurophenes

The reactivity sequence furan > tellurophene > selenophene > thiophene is thus the same for all three reactions and is in the reverse order of the aromaticities of the ring systems assessed by a number of different criteria. The relative rate for the trifluoroacetylation of pyrrole is 5.3 x lo . It is interesting to note that AT-methylpyrrole is approximately twice as reactive to trifluoroacetylation as pyrrole itself. The enhanced reactivity of pyrrole compared with the other monocyclic systems is also demonstrated by the relative rates of bromination of the 2-methoxycarbonyl derivatives, which gave the reactivity sequence pyrrole>furan > selenophene > thiophene, and by the rate data on the reaction of the iron tricarbonyl-complexed carbocation [C6H7Fe(CO)3] (35) with a further selection of heteroaromatic substrates (Scheme 5). The comparative rates of reaction from this substitution were 2-methylindole == AT-methylindole>indole > pyrrole > furan > thiophene (73CC540). 

Dipole moment studies of 2-methylmercapto-selenophene and -tellurophene indicate that in both cases the cis conformer (26 Z = Se, 59% Z = Te, 65%) is preferred to the trans one (27) (73BSF1924). The conformational preferences, (28) versus (29), of a variety of other 2-substituted compounds as deduced from dipole moment or lanthanide induced NMR shift measurements are shown in Table 5. Selenophene-3-carbaldehyde exists mainly as the trans conformer (30) according to dipole moment measurements, while the selenophenedicarbaldehydes (31)-(34) exist preferentially in the conformations depicted (73BSF1924). 

The absence of any derivatives of selenophene or tellurophene from the chemical catalogues makes their laboratory synthesis an unavoidable point of departure for any study of their chemistry or other properties. As the range of potential organic selenium or tellurium precursors is also extremely restricted, there has been particular emphasis on routes starting from simple inorganic species such as the elements themselves, the dioxides, the halides, the dihydrides and, in the case of selenium, the selenocyanate anion. 

Apart from uses based upon their biological properties, a number of other practical applications of selenophenes and, to a lesser extent, tellurophenes have attracted attention. 

The difference between the dipole moments of the aromatic compound and the corresponding tetrahydro derivative is called the mesomeric moment and represents a measure of the -electron delocalization. The mesomeric dipole moment is directed, in all the congener systems, from the heteroatom toward the ring. The values obtained (furan 1.03 D, thiophene 1.35 D, selenophene 1.29 D, tellurophene 1.17 D) are in excellent quantitative agreement with other aromaticity indices based on structural and magnetic properties12 (see discussion in Section II,C). 

There are two different basic methods for obtaining the five-membered heterocycles. In the first approach, the ring is constructed directly from aliphatic compounds the other and less common approach makes use of congener rings or other heterocyclic systems as starting materials. All the syntheses reported for the tellurophene system have been based on the first principle. 

Two major applications have emerged between 1983 and 1993 the first is the use of tellurophene derivatives for the synthesis of electoconducting polymers, and the second is the thermolysis of organotellurium compounds for the production of thin films of cationic tellurides. Semiconductors derived from 3,4-disubstituted tellurophenes have been patented for photosensors <90JAP(K)0241317>. Electrochemical polymerization of benzo[c]tellurophene derivatives provides useful semiconductors <90JAP(K)02263823>, applicable among other uses, for solar cells <88eup273643>. 

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