Electrophilic addition/substitution of an alkynyl methyl ketone with tellurium tetrahalides: a novel synthetic approach to telluracyclopentenones

The interaction of tellurium tetrahalides, TeX 4 (X = Cl. Br) with 4-(trimethylsilyl)but-3-yn-2-one under dry nitrogen gives novel crystalline 1,1,3-trihalo-2-(trimethylsilyl)telluracyclopent-2-en-4-ones (X = Cl, 1a and X = Br, 2b) as the major products. In a tandem reaction, the trisubstituted vinyltellurium trihalide formed initially by the anti addition of a tellurium tetrahalide across the polar triple bond of the 1,2-disubstituted acetylene, instantly undergoes intramolecular electrophilic substitution of the carbonyl activated methyl protons to eliminate a molecule of hydrogen halide. These tellurium(IV) derivatives can be reduced safely with aqueous sodium meta bisulphite to give tellurium(II) heterocyles, 3-halo-2-(trimethylsilyl)telluracyclopent-2-en-4-ones as fairly stable viscous oils. Molecular geometry of 1a as substantiated from the single-crystal X-ray diffraction data has Cs symmetry and resembles a see-saw with a pentagonal stand that supports the Cl─Te─Cl beam at the Te atom as its fulcrum.