Reactivity of tris(trimethylsilyl)silane toward diarylaminyl radicals

Absolute rate constants for the reaction of tri-tert-butylphenoxyl radical (ArO•) with (TMS)3SiH were measured spectrophotometrically in the temperature range 321-383 K. Rate constants for the hydrogen abstraction from (TMS)3SiH by diarylaminyl radicals of type (4-X-C6H4)2N• were determined by 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• and Ph2N•, respectively], which reflects high steric demand in the transition state. A semiempirical approach based on intersecting parabolas suggests that the observed reactivity is mainly related to the enthalpy of the reaction and allowed to estimate activation energies for the reaction of(4-X-C6H4)2N• and ArO• radicals with a variety of silicon hydrides.