Telluropyran-4-ones

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). 

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). 

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.

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>. 

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