Hydrotelluration of acetylenic esters: structural characterization of stereoisomers of methyl/ethyl β-(aryltelluro)acrylates

Abstract

Synthesis and complete characterization of some ester functionalized vinylic tellurides bearing an aryl ligand with varying steric and electronic effects bound to tellurium is described. Hydrotelluration of methyl propiolate using Ar₂Te₂/NaBH₄ in methanol results in a mixture of stereoisomers of methyl β-(aryltelluro)acrylates, ArTeCHCHCOOMe (Ar = 4-MeOC₆H₄, 1A; 1-C₁₀H₇, 2A; 2,4,6-Me₃C₆H₂, 3A; C₅H₅FeC₅H₄, 4A; 4-Me₂NC₆H₄, 5A; and 2-C₄H₃S, 6A). The same reaction in ethanol provides isomeric mixtures of the ethyl esters ArTeCHCHCOOEt (1B–6B). However, in the reactions between methyl propiolate and Ar₂Te₂ (Ar = 2,4,6-Me₃C₆H₂, 4-Me₂NC₆H₄) in isopropanol or t-butanol, no exchange of alkyl groups between the parent ester and the solvent is observed, instead detelluration of the Ar₂Te₂ to Ar₂Te is a competing reaction along with almost exclusive formation of the (Z)-isomers (3Aa, 5Aa). The geometry of the separated stereoisomers is established in solution, with the help of ¹H, ¹³C and ¹²⁵Te NMR spectrometry. Of particular interest is the observation that ¹²⁵Te chemical shifts {deshielded in (Z) compared to (E); Δδ = 106–136 ppm} and the geminal heteronuclear coupling constants {²J(¹H–¹²⁵Te) values for (E) are more than seven times that of the corresponding (Z) isomer} can be used to distinguish between liquid isomers. Structural characterization in the solid state by single-crystal X-ray diffraction for the 2Ba, 3Aa, 3Ba, 5Aa, 8 (Z)-isomers as well as for both stereoisomers of 4-Me₂NC₆H₄TeCHCHCOOEt (5Ba and 5Bb) is also presented. The carbonyl O atom of the ester group is invariably involved, at least in the solid state, in a secondary bonding interaction with the Te(II) atom. While an intermolecular Te⋯O interaction gives rise to one-dimensional supramolecular arrays in the crystal lattice of 5Bb with (E) configuration, it is realized intramolecularly in the case of the (Z)-isomers due to the cis position of the chalcogen atoms.