Phosphanyl Radicals From Bismuth-Mediated Reactions and Sterically Encumbered Diphosphanes

The conceptualization of radical precursors and the identification of reliable strategies for radical generation offer entries to innovative reactivity pathways and represent one of the current challenges in synthetic chemistry. More specifically, the generation of phosphanyl radicals [PR₂]^• under mild conditions and selective subsequent transformations of these species have recently been the focus of research efforts. Bi─P bond homolysis offers intriguing opportunities for creating [PR₂]^• radical species under mild conditions. Here we report the bismuth-mediated transformation of aryl- and alkyl-substituted phosphanes to the corresponding diphosphanes. Steric and electronic parameters that allow for rapid phosphanyl radical generation have been investigated. Subsequently, the thermally induced phosphanyl radical generation from diphosphanes is tested, revealing a persistent phosphanyl radical. Mechanistic pathways for the stereochemical inversion of phosphorus centers are discussed. Analytical techniques include (VT-)NMR spectroscopy, single-crystal x-ray diffraction, EPR spectroscopy, and density functional theory (DFT) calculations.