Addition Reactions of Phosphorus-Centered Radicals Onto Olefins: Interplay Between Stability and Reactivity

Phosphorus-centered radicals are important intermediates in various chemical transformations, enabling wide-ranging applications. Among these, the addition of radicals onto the carbon centers of olefins is the common key step in two distinct synthetic approaches: the P-H addition reactions catalyzed by radical additives, as well as the radical-induced polymerization. To gain a comprehensive understanding of reactivity trends, we explored computationally how the chemical nature of radicals affects the thermodynamics and kinetics of their reaction toward the parent ethylene and a set of variously substituted olefins. As a result, the fundamental differences between the main types of phosphinyl and phosphonyl radicals have been clarified. In general, the reaction energies of additions correlate with the radical stabilization energies obtained for the attacking radical: Less stable radicals lead to more exothermic reactions, and their reactions are also characterized by lower activation barriers. Consequently, the relative reactivity of a P-centered radical, in terms of both kinetic and thermodynamic parameters of the addition, can be well predicted using the easily accessible radical stabilization energies prior to experiments. The predictive value of this methodology was verified using explicitly calculated energies on a set of radicals suitable for experimental investigations.