LIGAND-EXCHANGE PROCESSES IN TETRAALKOXYTELLURANES

Alkoxy groups in acyclic tetraalkoxytelluranes interchange quickly on the NMR time scale at room temperature, both intramolecularly (axial/e quatorial positional interconversion) and intermolecularly (the reacti on between telluranes and free alcohols). In the latter case, no therm odynamic preference is found for the binding of simple primary or seco ndary alcohols, but tert-butyl alcohol is significantly less favored a s a ligand. The Te-125 NMR signals for Te(OEt)4 and Te(O(i)Pr)4 are sh own to be broad and very solvent- and concentration-dependent, probabl y due to associative processes in solution. In contrast, cyclic tellur anes such as Te(OCH2CH2O)2 and Te(OCMe2CMe2O)2 are thermodynamically f avored and give sharper Te-125 lines. For the latter substance, three ligand reorganization processes were defined and measured by NMR line- shape analysis: (a) a low-barrier (DELTAG(double dagger) = 7.0 kcal/mo l) axial/equatorial interchange (such as a Berry pseudorotation) that still leaves two distinct methyl signals in the spectrum; (b) a high-b arrier (AG(double dagger) = 20.9 kcal/mol) mechanism that averages all the methyl groups in the molecule to one line; (c) an acid-catalyzed process, probably involving tellurium inversion, that has the same res ult as process b.