The nucleophilic silyl radical: Dual-parameter correlation analysis of the relative rates of bromine-atom abstraction reactions as measured by a rigorous methodology

The relative rates [k(R)(Y) values] of bromine-atom abstraction reactions of 13 p-Y-substituted benzyl bromides (1-Y's : Y = H, Et, tBu, Me3Si, MeS, Ph, F, Cl, Br, CF3, CN, CO2Me, SO2Me) by tris(trimethylsilyl) silyl radicals [(Me3Si)(3)Si-.] in cyclohexane at 80 degrees C have been measured by a rigorous methodology. Correlation analysis of the kinetic data by the dual-parameter equation (log k(y)/k(H) = rho(x) sigma(X) + rho(.)sigma(.)) shows that the silyl radical is distinctly nucleophilic and the transition states of the bromine-atom abstraction reactions are affected by both the polar and spin-delocalization effects of the Y-substituents. Comparison of the /rho(p)/rho(JJ)(.)/ values suggests that the contribution of the spin-delocalization effects in this Br-atom abstraction reaction may be greater than the spin-delocalization effects in some H-atom abstraction reactions. (C) 1997 Elsevier Science Ltd.