Reversibility in free-radical reactions of aryltellurides with tributylstannyl, tributylgermyl and tris(trimethylsilyl)silyl radicals

Abstract

1H, 13C, 29Si, 77Se, 119Sn and 125Te NMR spectroscopies reveal that methyl, primary and secondary alkyl radicals, generated through the reaction of aryltelluroalkanes ( 4– 9) with tributyltin hydride, tributylgermanium hydride or tris(trimethylsilyl)silane) under standard radical conditions (benzene, AIBN) are capable of displacing tributylstannyl, tributylgermyl and tris(trimethylsilyl)silyl radicals from aryltellurotributylstannanes ( 1, 2), aryltellurotributylgermanes ( 10, 11) and aryltellurotris(trimethylsilyl)silanes ( 13, 14) respectively. These observations are in agreement with high-level ab initio molecular orbital studies. Calculations using a (valence) double- ζ pseudopotential basis set supplemented with polarization functions and with the inclusion of electron correlation (MP2/DZP) predict energy barriers for the displacement of stannyl (SnH 3), germyl (GeH 3) and trisilylsilyl ((H 3Si) 3)Si) radicals by methyl, ethyl and iso-propyl radicals to lie between 22 and 39 kJ mol −1, with reverse barriers of between 12 and 40 kJ mol −1. Consequently, the use of aryltellurides as alkyl radical precursors together with (standard) chain-carrying reagents such as tributyltin hydride, tributylgermanium hydride and tris(trimethylsilyl)silane may be complicated with equilibria which may result in diminished reaction yields.