Fused tellurophenes

Tellurophene aldehydes and ketones readily undergo reduction reactions typical of carbonyl compounds. The Wittig reaction of 2,5-bis(dodecyloxy)- -xylene-bis(triphenylphosphonium bromide) with tellurophene-2,5-dicarbaldehyde leads to poly(2,5-tellurophenediylvinylene) (MW 7000-29000 Da) in 65% yield <1995MM8363>. The condensation reactions of 2,5-diphenyl-3,4-diformyltellurophene 15 have been employed for the synthesis of a number of bi- and polycyclic heterocycles with a fused tellurophene ring (Scheme 8) <1976CB3886>. 

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). 

The major internal comparisons to be made within this chapter are between (13) pyrrole (1), furan (2), thiophene (3), selenophene (4) and tellurophene (5) b) pyrrole (1) and indole (6) (c) indole (6), benzo[6 Jfuran (7) and benzo[6]thiophene (8) d) indole (6), isoindole (9) and indolizine (10) and (e) benzo[6] and benzo[c] fused systems. The names of relevant heterocyclic radicals are given with the structures of the parent heterocycle. 

Like tellurophene the fused compound forms 1,1-addition products with halogens (73BSF2468), but further bromination of such a 1,1-dibromo species gave 1,1,2-tribromobenzo[/>]tellurophene, which was able to be reduced to 2-bromobenzo[b]tellurophene. The 3-chloro and -bromo compounds were isolated following reaction of triphenylphosphine and the appropriate carbon tetrahalide with 2,3-dihydro-3-oxobenzo[b]telluro-phene (80BSB763). 

The reactions of the lithio derivatives of benzo[ >]-fused systems indole, benzo[6]furan and benzo[h]thiophene are similarly diverse. Since indole and benzo[h]thiophene undergo electrophilic substitution mainly in the 3-position, the ready availability of 2-lithio derivatives by deprotonation with n-butyllithium is particularly significant and makes available a wide range of otherwise inaccessible compounds. The ready availability of 3-iodoselenophene and hence of 3-lithioselenophene (73CHE845) provides a convenient route to 3-substituted selenophenes. 2-Lithiotellurophenes are especially important precursors of tellurophene derivatives because of the restricted range of electrophilic substitution reactions which are possible on tellurophenes (77AHC(2l)ll9). 

Coupling constants for selenophene and tellurophene and their benzo[6] fused derivatives are given in Table 4. Heteroatom-proton couplings in 2-monosubstituted selenophenes and tellurophenes are in the order 2/z H(5> 3/z-h(4> > Vz-hoj and for 3-monosubstituted selenophenes 2/se-H(5) 35 2/se-H(2> 3/Se-H(4) (75CS(8)8). In selenophenes and tellurophenes of similar structure, tellurium-proton couplings are greater than selenium-proton couplings by a factor of approximately two. 

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. 

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