REVERSIBILITY IN FREE-RADICAL REACTIONS OF ARYLTELLURIDES WITH TRIBUTYLSTANNYL, TRIBUTYLGERMYL AND TRIS(TRIMETHYLSILYL)SILYL RADICALS

H-1, C-13, Si-29, Se-77, Sn-119 and Te-125 NMR spectroscopies reveal t hat methyl, primary and secondary allkyl radicals, generated through t he reaction of aryltelluroalkanes (4-9) with tributyltin hydride, trib utylgermanium hydride or tris(trimethylsilyl)silane) under standard ra dical conditions (benzene, AIBN) are capable of displacing tributylsta nnyl, tributylgermyl and tris(trimethylsilyl)silyl radicals from arylt ellurotributylstannanes (1, 2), aryltellurotributylgermanes (10, 11) a nd aryltellurotris(trimethylsilyl)silanes (13, 14) respectively. These observations are in agreement with high-level ab initio molecular orb ital studies. Calculations using a (valence) double-zeta pseudopotenti al basis set supplemented with polarization functions and with the inc lusion of electron correlation (MP2-/DZP) predict energy barriers for the displacement of stannyl (SnH3), germyl (GeH3) and trisilylsilyl (( H3Si)(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 10 kJ mol(-1). Consequently, the use of aryltellurides as allkyl radic al precursors together with (standard) chain-carrying reagents such as tributyltin hydride, tributylgermanium hydride and tris(trimethylsily l)silane may be complicated with equilibria which may result in dimini shed reaction yields. (C) 1998 Elsevier Science S.A.