Addition of Methanesulfonyl Radical to Alkenes and Alkenylsilanes

Relative reactivities of a series of olefins (alkenes, cycloalkenes, and alkenylsilanes) toward the electrophilic methanesulfonyl radical (2^•) have been determined in acetonitrile at 0 °C. A kinetic treatment of the competitive photoinitiated addition of methanesulfonyl bromide (1) to olefins was developed and used to measure the relative reactivity and probe the reversibility of addition. The kinetic treatment was based on the irreversible addition of 2^• to a reference olefin serving as a standard reaction. 1,1-Dicyclopropylethene (14, a “radical clock”) was used as such a primary reference, and 1-hexene and 1-octene were employed as secondary references. 14 was 20 ± 4 times more reactive than 1-hexene, but the relative reactivities of several unactivated alkenes and alkenylsilanes vs 1-hexene were in a range from ca. 0.44 to 6.2. The absolute rate of addition of 2^• to n-alkenes was estimated to be ca. one-tenth of the diffusion-limited rate. The study showed that, even within a family of closely related alkenes, steric and electronic effects have significant influence on olefin reactivity toward 2^• as well as reversibility of the addition.