Tellurocarbonyls

Tellurocarbonyl derivatives R C(=Te)OR and telluroamides, e.g. PhC(= Te)NMe2 (mp 73°) have been prepared and shown to be similar to, though more reactive than, the corresponding seleno derivatives. 

By using flash vacuum thermolysis, 12 was obtained from 1,3,4-telluradiazo-line as green crystals.25,26 The visible spectrum (CHC13 at 80°C) showed kmax 825 nm assigned to the transition of the C=Te bond. In the 13C NMR spectrum, tellurocarbonyl carbon resonates at 301 ppm, which is the most deshielded carbon observed in a neutral molecule. Similarly, the 125Te-NMR shift at 2858 ppm is in the extreme downfield shift range.245... 

The most straightforward route to heteroaldehyde and heteroketone complexes is the substitution of a heterocarbonyl compound for another coordinated ligand. This method is naturally restricted to heteroaldehydes and heteroketones stable in the uncoordinated form, i.e., is usually restricted to thioketones and a few stable seleno- and tellurocarbonyl compounds.141718,27118 In most cases, metal carbonyls or solvent complexes of metal carbonyls were used as the complex precursors. The photochemically or thermally induced loss of the ligand to be replaced is followed by coordination of the heterocarbonyl compound [Eq. (3)]. 

The first tellurocarbonyl complex, which is also the only osmium complex of an organotellurium ligand, is the recently reported112 OsCl2(PPh3)2(CO)CTe species. 

The [2+4] cycloaddition of selenocarbonyl (selenoxo) or tellurocarbonyl (telluroxo) compounds to diene systems remains one of the most widely used reactions for the preparation of various six-membered rings containing selenium or tellurium (see CHEC-II(1996), Section 5.11.7.2.1). The extremely sterically hindered selenal 154 has been isolated in monomeric form in solution and reacts with 2,3-dimethylbutadiene to afford the corresponding dihydro-selenin cycloadduct 155 (Equation 62) C1996AGE660, 1997T12167, 1998PS633>. 

As expected, similar cycloaddition reactions have been used to prepare analogous tellurium-containing heterocycles. In ///-generated tellurocarbonyl difluoride 156 can be trapped as the difluorodihydro-2//-tellurin 157 using 2,3-dimethylbutadiene (Equation 63) <1993JCD2547>. 

The perfluorinated tellurocarbonyl fluorides 158 have been trapped similarly (Equation 64) <1996JCD4463, 1997PS413> as has been perfluorotelluroacetone 159 (Equation 65) <2000JCD11>. 

The preparation of the very reactive perfluorinated tellurocarbonyls such as F2C=Te and CF3(F)C=Te opened another totally new way into tellurium heterocyclic chemistry. These tellurocarbonyls dimerize at room temperature to stable perfluorinated 1,3-ditelluretanes, which at elevated temperatures undergo [4 + 2]-cycloadditions with 1,3-dimethylbutadiene, forming fluorinated l-telluracyclohex-3-enes. This type of reaction can be extended to other ene synthons. Meanwhile similar reactions have been reported for 2,2,4,4-tetrakis(trifluoromethyl)-l,3-ditelluraetane, which was synthesized by the pyrolysis of (CH3)3SnTeCF(CF3)2 (96UP2). 

Complexes with tellurocarbonyl ligands such as telluroureas can be formulated as tellurolate complexes. 

Tellurocarbonyl compounds, formed from aldehydes or ketones and bis[dimethylalumino] tellurium in refluxing dioxane, added to conjugated dienes to give cyclic tellurium compounds in good yields1. 

G. R. Clark, K. Marsden, W. R. Roper, and L. J. Wright, Carbonyl, Thiocarbonyl, Selenocarbonyl, and Tellurocarbonyl Complexes Derived from a Dihalocarbene Complex of Osmium, J. Am. Chem. Soc. 102, 1206-1207 (1980). 

The organotelluride salt (28) readily dehalogenates a variety of a-halocarbonyl compounds, including ketones, acids and esters. It is not clear whether the first step is displacement of halide to afford an a-tellurocarbonyl compound, followed by attack upon the latter by (28), or whether (28) attacks the a-sub-stituent directly. Reduction can be effected by NaBH4 containing a catalytic quantity of (28). ... 

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