Benzo--tellurophenes

Benzo-[c]-tellurophenes have been synthesized starting from o-bishalomethyl benzenes, as depicted in the following scheme.  

In the case of R = H and MeO, aU the attempts to convert the diiodide into the teUurophene by direct elimination of HI with a base, failed. Such conversion was attained via the bistriflu-oroacetate. However, as expected, the nitro derivative (R = NOj) undergoes facile dehy-droiodination upon treatment with EtjN. 

Benzotellurophene (R = H) is stable only in benzene solution at low temperature, but the diester and disulphonyl derivatives (obtained by the bis-lithiation and subsequent reaction with alkyl chloroformates and p-toluenesulphonic anhydride) as well as the nitroderivative are more stable. 

Synthesis of l,3-dihydro-2,2-bis(trifluoroacetoxy)-benzo[c]tellurophenef A mixture of 2,2-diiodo-l,3-dihydrobenzo[c]tellurophene (4.86 g, 10 mmol) and silver trifluoroacetate (4.42 g, 20 mmol) in benzene (200 mL) was stirred at room temperature for 2 h. After filtration, the fdtrate was concentrated to give l,3-dihydro-2,2-bis(trifluoroacetoxy)-benzo[c]tellnrophene (4.17 g, 91%), m. p. 160°C (dec.). 

Benzo[c]telluropheneA mixtnre of l,3-dihydro-2,2-bis(triflnoroacetoxy)-benzo[c]tel-Inrophene (0.916 g, 2 mmol), triethylamine (2.02 g, 20 mmol) in degassed benzene (200 mL) was refluxed under argon for 15 min. The resulting solntion was washed with deionized water (2 X 200 mL) and dried over sodium sulphate. Removal of solvent in vacuo at room temperature resulted in a mixtnre (0.36 g) of benzo[c]tellnrophene and 1,3-dihydro-benzo[c]tellurophene (78%, 89% of 2-benzotellurophene). 

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Z-fl-Acetoxyethyb-henzotellurophene1 A solution of 3.9 g (14.2 mmol) of 2-(l-hydroxyethyl)-benzo-tellurophene and 3.6 g (35 mmol) of acetic anhydride in 10 m/ of dry pyridine is kept at 20° for 48 h and is then poured into ice/ water. The mixture is extracted with diethyl ether, the organic phase is separated, washed with ice-cold, 10% aqueous sulfuric acid and then with aqueous sodium carbonate solution. The organic phase is dried with anhydrous sodium sulfate, filtered, and the filtrate evaporated under reduced pressure. The residue is purified by column chromatography on silica gel using petroleum ether (b.p. 40-60°)/diethyl ether mixtures (49 1, 47 1, 43 1) as mobile phases yield 3.2 g (70%) m.p. 30°. 

The first proton to be removed from A-methylpyrrole by -butyllithium is from an -position a second deprotonation occurs to give a mixture of 2,4- and 2,5-dilithiated derivatives. In both furan and thiophene, initial abstraction of a proton at C(2) is followed by proton abstraction from C(5). A-Methylindole, benzo[ ]furan, benzo[ ]thiophene, selenophene, benzo[ ]selenophene, tellurophene, and benzo[ ]tellurophene similarly yield 2-lithio derivatives. 

Te COOH H pyridine, acetic anhydride 2-carboxy-l-benzo-tellurophene 1 754... 

Tellurium-containing heterocyclic compounds also react with methyl halides, ethyl halides, allyl halides, benzyl bromide, and benzoylmethyl bromides, with or without a solvent, to produce telluronium halides. Tetrahydrotellurophene, l-methyltetrahydrotellurophene, 2,7-dihydrobenzo[c] tellurophene, 1-benzotellurophene 2-oxo-2,3-dihydro-l-benzo-tellurophene, dibenzotellurophene , 1,4-oxatellurane l,4-thiatellurane °, 3,4-dihydro-l/f-2-benzotellurin, and 4-oxo-2,3-dihydro-47/-l-benzotellurin" reacted to produce telluronium halides. 

Similar behaviour is observed with benzo[(>)tellurophene. When reacted at 20 °C with -butyl-lithium in hexane, the heterocycle loses tellurium and gives the o-lithiostyryllithium (7) which can be quenched by various electrophiles <92AG(E)1339>. In THF at 0°C, the same reagent effects a metallation reaction which gives rise to 2-substituted benzo[Z ]tellurophenes (8). 

The zirconium-mediated syntheses of pinacolboronate (BPin) appended benzo[ ]tellurophenes and two phenyl/BPin-substituted tellurophene isomers with different colors of emission were achieved in the year 2014 (14MI-001).These compounds are new additions to this emerging class of inorganic heterocycles that display visible phosphorescence in the sohd state under ambient conditions. 

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. 

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). 

Benzo[h]tellurienylethyl acetate solvolysis, 4, 952 Benzo[h]tellurophene, 3-bromo-synthesis, 4, 954 Benzo[h]tellurophene, 3-chloro-synthesis, 4, 954... 

Benzo[h]tellurophene, 2,3-dihydro-3-oxo-halogenation, 4, 948 Benzo[h]tellurophene, 3-halo-synthesis, 4, 116, 966 Benzo[b]tellutophene, 3-hydroxy-tautomerism, 4, 37, 945 Benzo[h]tellurophene, 3-lithio-reactions, 4, 956 Benzo[h]tellurophene, 2-methyl-synthesis, 4, 952... 

Benzo[6]tellurophene-2-carboxylic acids reactions, 4, 952 Benzotellurophenes, 4, 935-971 mass spectra, 4, 22 synthesis, 4, 103 Benzo[fa]tellurophenes acetylation, 4, 946, 948... 

Benzo[h]selenophene is more reactive than the oxygen and sulfur analogues, but less so than benzo[b]tellurophene, although substituents may modify this to some extent [90AHC(47)181]. 

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>. 

Selenation of the 1,4-dicarbonyl precursor 89 using the reagent (Me2Al)2Se gave the benzo[c]selenophene derivative 90 in good yield <06TL2887>. The synthesis and structural studies of 4,7-dimethoxybenzo[c]tellurophene have also been reported <06AG(E)5666>. 

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