Stabilization Effects in Phosphinyl Radicals: The Scope of the Donor, Acceptor, and Captodative Functionalization

Accessing stable phosphinyl radicals continues to pose a major synthetic challenge. To aid the effective design of such radicals and to understand their behaviors, this computational study systematically explores the possibilities and limitations of electronic and steric effects governing the stability of phosphinyl radicals, with a particular focus on captodative substitutions. To decipher the delocalization effects, the electronic structures of various radicals have been scrutinized by their spin distributions and using natural bonding orbital analyses. The best stabilization effects are primarily offered by π-donor groups; however, the most efficient π-donors are exceptions and suffer from a saturation effect. To compensate for this disadvantageous phenomenon, various captodative substitutions have been assessed, and several examples were found to benefit from extra stabilization. As electronic effects alone are insufficient to prevent the dimerization, additional protection arising from sterically encumbered substituents is also necessary, and steric effects were found to prevail over electronic effects in stabilizing the monomeric radical. Among the studied substituents, spherical groups placed at the donor and acceptor sites provide better steric protection than planar groups. Embedding the N-centers into cyclic frameworks further enhances stability. Integrating the results of our systematic investigations, we propose several potential candidates for synthetic purposes.