Five-, six-and eight-membered tellurium-containing heterocycles with vicinal Te, O and Te, N heteroatoms

Previously undescribed tellurium-containing heterocycles (5H-oxatelluroles-1,2 and their benzoderivatives, 1,2,3-telluradiazines and 1,2,6-oxatellurazocines) were prepared by dehydrohalogenation of the aromatic or alkene compounds containing vicinal Te(R)Hal 2 or TeHal and hydroxy or NH groups. The same procedure was used for the synthesis of isotellurazoles. By oxidation of β -aryltelluroallylic alcohols and o -aryltellurobenzoic acids 5 H - 1,2-oxatelluroles and 3 H -2,1-benzoxatellurol-3-ones have been prepared in good yields.


Introduction
Derivatives of 1,2-telluroles represent the most thoroughly studied group of five-membered tellurium-containing heterocycles with two vicinal heteroatoms. 1 The interest in these compounds has been mostly because of their use as the donor components of conducting chargetransfer complexes and radical-ion salts.The heterocyclic compounds with Te, O centers in the positions 1,2 of a five-membered ring have been first described in our papers.Of Te, Ncontaining heterocycles of this type only benzisotellurazole 2 was known at the beginning of our work and some of the 3-substituted and 3,5-disubstituted isotellurazoles 3 had been prepared in very low yields.This paper gives a brief survey of our study in the area and contains also some recently obtained results.
Oxatelluroles 1 and benzoxatelluroles 7 are susceptible to readily occurring exchange of the halogen atoms at the tellurium centers for other anionic groups 9 .When treated with potassium iodide in an acetone solution, the compounds 1d,f are converted to the corresponding iodo derivatives (the yields are 80-85%).1-Acetoxy-1-butyl-2,1-benzoxatellurole 7n was obtained by the reaction of its bromo analogue with silver acetate.Contrary to expectations, the action of an ethanol solution of NaOH on 7b leads not to 1-butyl-1-hydroxybenzotellurole-2,1, but to a product of its dehydration 7o 9 .
The halogen exchange in the compounds 7 may also be realized through their reduction with zinc powder followed by a treatment of the reaction mixture with an equivalent amount of halogen 9 .Thus, the compound 7c was prepared in almost quantitative yield from the bromo derivative 7b.
o-Carbonyl derivatives of Te,Te-dibromophenylbutyl telluride 14a-c are formed in 73-95% yields when chloroform or carbon tetrachloride solutions of 1-bromo-1-butyl-2,1benzoxatelluroles 7b,f,i and an equimolar amounts of bromine are heated at reflux 9,12,13  Treatment of 7b with excess bromine in acetic acid solution results in formation of 2tribromotellurobenzaldehyde 17 in 63% yield 9,12,13 .The initially formed aldehyde 14a eliminates a molecule of butyl bromide to give the tellurenyl bromide 18 which is readily oxidized by bromine affording the tellurium tribromide 17 as the final product.With 1-bromo-1-butyl-3-methyl-2,1-benzoxatellurole 7l, this reaction leads not only to the ketone 19a, but also 19b 9,12 .When two moles of bromine are used, the ω,ωdibromoacetophenone 19a has also been found among the products of the reaction.

2,1-Oxatellurol-3-ones
The methods developed for the synthesis of oxatelluroles and benzoxatelluroles have been found to be applicable for the synthesis of their oxo-derivatives 5 9 , 6 19 .Thus, 1-aryl-1-chloro-2,1benzoxatellurole-ones-3 5 were prepared in 75-90% yields by oxidation of 2-carboxydiphenyl telluride with tert-butylhypochlorite and the dehydrohalogenation of 2-(dichlorotelluroaryl)benzoic acids 20 under the action of triethylamine 9 .The dihydro derivatives of the oxatellurolones were obtained in high yields (more than 90%) by oxidation of β-aryltelluropropionic acids 21 with tert-butylhypochlorite in chloroform solution.The bromo analogues of 6 are accessible through dehydrobromination of aryl(βcarboxyethyl)tellurium dibromides in toluene solution 19 .Benzoxatellurol-3-ones 5 are isoelectronic to 2-phenyliodonium carboxylates.One may expect that by this analogy thermolysis of 5 would lead to the formation of dehydrobenzene and aryltellurenyl chlorides, which in their turn would disproportinate into diaryltellurium dichlorides and Te.However, no dehydrobenzene was generated in this reaction were performed under various conditions.

Isotellurazoles and N-arylisotellurazolium cations
Isotellurazoles 2 were first prepared by the reaction between α-acetylenic ketones, potassium telluride and hydroxylamine-O-sulfonic acid occurring in an aqueous solution of potassium acetate 3 .The compounds 2 are formed in very low yields (less than 10%).The method cannot be applied for the synthesis of 3H-isotellurazoles because when α-acetylenic aldehydes are used in this reactions instead of the ketones, only di(β-cyanovinyl)tellurides are formed as the final products 3 .
We have found that β-bromotellurenylvinylaldehydes 7,8 20 may serve as the precursors of isotellurazoles when reacted with ammonia in a benzene solution.In this way, 4,5tetramethyleneisotellurazole 2a was obtained in 70% yield 22  Preparation of the compounds 20 is difficult, 7,8 however the readily accessible βmethyldibromotellurovinylaldehydes 22 have been found to be useful precursors of isotellurazoles.
By bubbling ammonia through benzene solutions of the tellurium dibromides 22 isotellurazoles 2a,b were obtained in about 70% yields.In accordance with these findings 23 , the following mechanism has been suggested for this reaction.An analogous route has been previously used for the synthesis of isoselenazoles 24 .However, due to low thermal stability of the tetracoordinated selenium analogues of 22 the reaction must be carried out at very low temperature (-70˚ C).Likewise benzoisotellurazole, isotellurazoles 2 react with methyl iodide under mild conditions to give the quaternary salts, e.g. 24, in good yields 22 .N-Arylisotellurazolium perchlorates 25 22 and their benzoderivatives 25 (R 1 +R 2 = (CH=CH) 2 ) 23,25 were prepared in high yields by treating solutions of βbromotellurenylvinylaldimines 26 (X = Br) 22 and 2-chlorotellurenylbenzaldimines 26 (26, R 1 + R 2 = (CH=CH) 2 , X = Cl) 25 with silver perchlorate in acetone or dimethylformamide.