Reactivity of tris(trimethylsilyl)silane toward diarylaminyl radicals

Absolute rate constants for the reaction of tri-tert-butylphenoxyl radical (ArO.) with (TMS)(3)SiH were measured spectrophotometrically in the tempera ture range 321-383 K. Rate constants for the hydrogen abstraction from (TMS )(3)SiH by diarylaminyl radicals of type (4-X-C6H4)(2)N-. were determined b y using a method in which the corresponding amines catalyze the reaction of ArO. with (TMS)3SiH. At 364.2 K, rate constants are in the range of 2-50 M -1 s(-1) for X = H, CH3, CH3O, and Br, whereas the corresponding value for ArO. is 3 orders of magnitude lower. A common feature of these reactions is the low preexponential factor [log(A/M-1 s(-1)) of 4.4 and 5.2 for ArO. an d Ph2N., respectively], which reflects high steric demand in the transition state. A semiempirical approach based on intersecting parabolas suggests t hat the observed reactivity is mainly related to the enthalpy of the reacti on and allowed to estimate activation energies for the reaction of (4-X-C6H 4)(2)N-. and ArO. radicals with a variety of silicon hydrides.