Reactivity of Aminyl, Thiyl, and Silyl Radicals in Addition Reactions: A Role of the Radius of an Atom with a Free Valence

Abstract

The experimental data on the addition of aminyl, thiyl, and silyl radicals to olefins as well as on the addition of silyl radicals to carbonyl compounds are analyzed within the framework of the parabolic model of the transition state. The activation energies of thermoneutral reactions E e,0 are calculated: 62.1 for (NH2 . + CH 2 =CHX), 31.4 (RS . + CH 2 =CHX), 76.6 (R 3 Si . + CH 2 =CHX), and 114.5 kJ/mol for (R 3 Si . + O=CR 1 R 2 ). The radius of the atom bearing the free valence is an important factor that affects the activation energy of these reactions. An empirical formula is derived relating the activation energy of a thermoneutral addition reaction with the strength D(C...Y) of a forming bond, its length r(C...Y), and the force constants of reacting bonds characterized by the a coefficient: ( 1 + α)(E e 0 ) 1/2 = 4.78 x 10 8 (r(C...Y)- 4.2 x 10 -11 D(C...Y)). The presence of π-bonds adjacent to the attacked double bond causes an increase in the activation energy E e, 0 . The contribution of this factor to the activation energy is evaluated. The addition of silyl radicals to olefins RCH=CHR occurs with a higher activation energy than the addition to CH 2 =CHR compounds because of the steric effect of the R substituent.