4- telluropyranes

The first ( ) and second (E ) oxidation potentials (versus saturated calomel electrode (SCE)) have been determined by voltammetry. The electrochemistry of the 4-(tel-luropyranyl)-4//-telluropyran has been examinated and compared to the O, S and Se analogues. 

As shown in Fig. 5, the initial fast reaction is the association of bromine with the chalcogen atom to form the -complex of bromine with diphenylselenide, dicyanomethylidene telluropyran 20, and telluropyranone 22. These structures are analogous to the 1,4-diselenane iodine complex 15 (Fig. 3), whose structure has been characterized by X-ray crystallography. The second-order rate constants are on the... 

Fig. 5 Plausible mechanistic path for the oxidative addition of bromine to diphenylselenide, dicyanomethylidene telluropyran 20, and 2,6-diphenyltelluropyran-4-one (22) based on stopped-flow kinetics. An initial fast reaction followed second-order kinetics (first order in both bromine and substrate) while a second, slow reaction followed first-order kinetics. For diphenylselenide, a third very-slow reaction was observed.

For diphenylselenide and dicyanomethylidene telluropyran 20, a third, very slow, first-order reaction was discerned. At 284 K, rate constants, k, for this process are... 

TELLURIN AND DERIVATIVES OF 4-H-TELLURINS AND 4-OXO-4-H-TELLURINS (TELLUROPYRAN-4-ONES)... 

Diphenyl telluropyran-4-one (typicalprocedure)7° 120 mL (0.12 mol) of a 1.0 M solution of lithium triethylborohydride in tetrahydrofuran are added to 7.65 g (60 mmol) of powdered tellurium under nitrogen, and the mixture stirred at 20°C for 4 h. A solution of sodium ethoxide (prepared from 5.52 g (0.24 mol) of sodium and 240 mL of absolute alcohol) is added to the dilithium telluride, 13.8 g (60 mmol) of bis(phenylethynyl) ketone are dissolved in a mixture of 150 mL of tetrahydrofuran and 150 mL of 1 M sodium ethoxide in ethanol this solution is poured as quickly as possible into the deep-purple-coloured dilithium telluride soluhon. The flask containing the reaction mixture is immediately placed in a water bath at 50°C and the temperature slowly increased over 30 min until ethanol begins to condense on the side of the flask. The water bath is removed and the mixture is stirred overnight at 20°C. Dichloromethane (400 mL) is then added, the resultant mixture is washed with 800 mL of water, and the organic phase is separated and concentrated to an oil. The oil is dissolved in 600 mL of dichloromethane, and the solution is filtered through a pad of sand. The filtrate is washed with 200 mL of 2% aqueous sodium chloride soluhon, dried with anhydrous sodium sulphate, filtered and evaporated. The brownish solid residue is triturated with 20 mL of butanenitrile and the fine yellow solid is collected by filtration yield 10.9 g (51%) m.p. 126-129°C (from acetonitrile). 

Telluropyran-4-thiones are prepared by treating the pyranones with Lawesson reagent. ... 

The thiones, obtained as depicted above undergo copper-assisted dimerization giving (tel-luropyrany l)telluropyranes ... 

Kuthan, J., Sebek, P., Bohm, S., Developments to the Chemistry of Thiopyrans, Selenopyrans, and Telluropyrans, 59, 179. 

Dehydroacetic acid, triacetic acid lactone, and related pyrones, 53, 1 Developments in the chemistry of furans (1952-1963), 7, 377 of Reissert compounds (1968-1978), 24, 187 of thiopyrans, selenopyrans, and telluropyrans, 59, 179... 

The chemistry of six-membered heterocycles containing one selenium or tellurium atom was initially covered in CHEC-11(1996) (chapter 5.11). This updated chapter primarily deals with material dating from 1993 and is complete to 2005 with occasional references from 2006. The structures and nomenclature of the selenium- and tellurium-containing heterocycles discussed in this chapter are included in Figure 1. One major review of tellurium-containing heterocycles has appeared . The chemistry of thiopyrans, selenopyrans, and telluropyrans has also been reviewed <1994AHC180>. 

Methylideneseleno- and telluropyrans 53a and 53b are air oxidized to the corresponding bipyranylidenes 54a and 54b (Equation 18) <1995JOC6631 >. This reaction is involved in the chalcogen scrambling observed in the preparation of unsymmetrical pyrylium trimethine dyes (see Section 7.11.5.6). 

Treatment of the conjugated telluropyran aldehyde 82 with the carbanion of the tungsten carbene 83 afforded the conjugated push-pull Fischer-type carbene complexes with extended conjugation 84 (Equation 34) <2005JOM4982> (cf. Equation 10). These complexes exhibit interesting nonlinear optical properties (see Section 7.11.8.2). 

Detty et al. reported a XPS analysis of several series of telluropyran, telluropyranone, and telluropyrylium compounds in both the Te(II) and the Te(IV) oxidation states (89MI2). Two linear correlations were found between Te NMR shifts and Te(3rf,/2) binding energies for the neutral and cationic Te(II) compounds, respectively, whereas the Te(IV) compounds showed no apparent correlation (Section II,C,2,c). 

Ethoxy-2,6-diphenylteUuropyrylium underwent dealkylation, instead of the expected substitution of the alkoxy group, by reaction with diethyl-amine in ethanol to yield the corresponding telluropyran-4-one [140, Z = Te, R = Ph] (82JOC5235). 

Disubstituted telluropyrylium cations 20 and 28 in pyridine with triphenylphosphine under aerobic conditions gave an oxidative dimerization to produce l,l-dioxo(telluropyranylidene)telluropyrans 272 and 273, respectively (87JOC2123). The reaction is also successful with the exclusion of oxygen if triphenylphosphine oxide is substituted for triphenylphosphine. The oxidative dimerization could not be extended to thiopyrylium cation 26 and selenopyrylium cation 27, which gave the corresponding bipyranylidenes 14 (Z = S, Se, R = Bu ). 

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