Steric trends and kinetic parameters for radical reductions involving alkyldiphenyltin hydrides

Absolute rate constants and Arrhenius parameters for hydrogen atom abstract ion by primary alkyl radicals from methyldiphenyl-, ethyldiphenyl-, butyldi phenyl-, isopropyldiphenyl-, cyclohexyldiphenyl- and (trimethylsilyl)methyl diphenyltin hydride were determined in tert-butylbenzene through utilizatio n of the '5-hexenyl radical clock' reaction. At 80 degrees C, rate constant s (k(H)) for all hydrides were found to lie in the range (8.2-11.5) x 10(6) lmol(-1) s(-1), with similar Arrhenius expressions for all reactions studi ed [viz. log k(H) = (8.92-8.97)-(3.03-3.24)/2.3RT]. The nature of the alkyl substituent appears to have a subtle effect on the function of the hydride such that the order or reactivity of stannanes (RPh2SnH) is Me > Et > Bu > i-Pr > c-Hex greater than or equal to Me3SiCH2; this trend can be directly traced to steric effects operating in the transition states for hydrogen t ransfer from tin to carbon. The implications of these observations are disc ussed. Copyright (C) 1999 John Wiley & Sons, Ltd.