Of tellurophene

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. 

A convenient procedure for the synthesis of tellurophene employs the reaction of Na2Te with l,4-bis(trimethylsilyl)-l,3-butadiene. The crude product is isolated as the corresponding dibromide, which is then reduced to tellurophene. ... 

Tellurophene. A mixture of tellurium (4.0 g, 31 mmol), sodium formaldehyde sulphoxylate of 85% (28 g, 200 mmol), sodium hydroxide (17 g, 425 mmol) in 150 mL water is heated at reflux, under N2 atmosphere for 15 min, and then cooled at 20°C. A solution of 1,4-bis(trimethylsilyl)-l,3-butadiene (8.2 g, 42 mmol) in 100 mL of ethanol is slowly added to the stirred sodium telluride solution, the mixture is heated at reflux for 15 min, then stirred at 20°C for 3 h and extracted with ether. The extract is dried (Na2S04), filtered, and 10 mL (200 mmol) of bromine are added dropwise until the bromine colour persists. This solution is concentrated in a water bath under aspiration vacuum to a volume of 50 mL, and the red precipitate of tellurophene dibromide is collected 8.9 g (84%), m.p. 120°C dec. 

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). 

Tellurophene forms an easily isolated 1,1-dibromide (66AG(E)896). This is readily reduced to the parent heterocycle and it has been used in the isolation of tellurophene from reaction mixtures in which it is present. 

Tellurienyllithium, obtained by metallation of tellurophene with butyllithium, on treatment with dimethyl sulfate, JV-methylformanilide, acetaldehyde, hexachloroethane and hexabromoethane yields 2-methyltellurophene, tellurophene-2-carbaldehyde, 2-(l -hydroxyethyl)tellurophene, 2-chlorotellurophene and 2-bromotellurophene, respectively... 

Diarylselenophenes (212) are conveniently obtained by the reaction of acetophenone anils with selenium (54JCS2720). Attempts to extend this process to the obtention of tellurophenes were unsuccessful. 

The nature of the complexes of tellurophene and its benzo analogs depends on the type of metal carbonyl. Thus, interaction of tellurophene with (MeCN)3Cr(CO)3 yields the -complex [2 E = Te M = Cr(CO)3], whereas the reaction with Fe3(CO)12 is a complicated transformation [96JCS(D) 1545]. A similar reaction is observed for dibenzotellurophene. 

A synthesis which is more convenient for the small-scale preparation of tellurophene employs sodium telluride, formed by the reduction of tellurium with sodium formalde-hydesulfoxylate in aqueous sodium hydroxide, and 1,4-bis[trimethylsilyl]-l, 3-butadiyne2... 

LithioteIlurophene2 A 500 ml. three-neckcd flask is fitted with a reflux condenser, a stirrer, and a nitrogen inlet tube and is flushed with nitrogen. 20 g (0.11 mol) of tellurophene and 150 ml of dry diethyl ether are placed in the flask. 60 ml (0.12 mol) of a 20% solution of bulyl lithium in hexane are slowly added dropwise to the stirred solution of tellurophene. After the exothermic reaction has subsided, the mixture is stirred for 45 min at 20°. The resultant solution of 2-lithiotellurophene is suitable for use in subsequent reactions. 

Treatment of tellurophenes with phosgene and dimethylformamide gave 2-formyl-tellurophenes4,5. The kinetics of this Vilsmeier-Haack formylation of tellurophenes and the other chalcogenophenes were investigated4,5. 

Treatment of tellurophenes with acetic anhydride in the presence of tin (IV) chloride gave 2-acetyltellurophenes6. 

Refluxing a solution of tellurophene and mercury (II) acetate in ethanol yielded 2,5-bis[ucetoxymercurio]tellurophene2. 

Tellurophene was oligomerized by the dropwise addition of a suspension of iron(III) chloride to a solution of tellurophene in chloroform at 20°. The electrically-conducting oligomer had approximately four tellurophene units per molecule3. 

Finally, a rare example of tellurophene chemistry has to be mentioned, namely the preparation and spectroscopic studies of a platinum complex containing two ethynyl(tellurophen-2-yl)units <07IC7232>. 

The scope of tellurophenes was well defined at the beginning of Chapter 3.16 in CHEC(1984) <1984CHEC(4)935> and a number of general reviews on this subject published between 1986 and 1991 were indicated in the introduction to Chapter 2.14 of CHEC-II(1996) <1996CHEC-II(2)749>. A comprehensive review of the chemistry, structural and physical properties of tellurophene, dihydro- and tetrahydrotellurophenes and their benzoanalogues that covered... 

The geometry of tellurophene in the gas phase was determined based on its microwave (MW) spectrum <1973CPH217>. Tellurophene is a liquid at room temperature and X-ray crystallography has not been applied to the parent compound. The molecular structure of the tellurophene nucleus was derived from the X-ray crystallographic analysis of its crystalline derivatives <1977AHC119, 2002CHE763>. X-Ray structures have been reported... 

Data on H NMR spectral parameters of tellurophene <1972J(P1)199, 1974MP257>, 2-substituted tellurophenes <1974ACB175, 1976ACB605> and, for comparison, furan, thiophene and selenophene <1965SA85> are listed in Table 3. 

Table 4 C NMR chemical shifts of tellurophene and congenerous rings and 2-substituted tellurophenes ((CD3)2CO), 2-RC4H3X...

Table 5. Te chemical shifts of tellurophenes correlate well (r=0.98) with Se chemical shifts of similar ...

The direction of the dipole moment vector of tellurophene is from the center of the ring to the heteroatom <1973CC342, 1973GR(C)203, 1974JHC827>. The difference between the dipole moments of a heteroaromatic five-membered heterocycle and its tetrahydro derivative is considered as the mesomeric moment of the conjugated heterocycle and is directed opposite to the vector of its dipole moment. From the values indicated above and the dipole moment of tetrahydrotellurophene (1.63Z)) <1977J(P2)775, 1973CR(C)203>, the mesomeric moment of tellurophene is evaluated to be 1.17Z). 

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