Anodic Bahavior of Crowded Triarylphosphines. ESR Study of Triarylphosphoniumyl Radicals, Ar3P.+

Article abstract of DOI:10.1021/jo00011a018

A large number of triarylphosphines exhibiting different steric hindrance has been prepared.The pyramidalization angle α of these compounds was calculated with use of the MM2 force field and was shown to depend almost exclusively on the number of ortho substituents on the phenyl rings.In a series of isosteric (same α) phosphines, the oxidation potential correlates with the sum of the ?⁺ Hammett parameters of the phenyl substituents.In the absence of oxygen, anodic oxidation of all the triarylphosphines bearing two o-methyl substituents on each phenyl ring is reversible and yields very persistent phosphoniumyl radicals.These radicals are easly detected by ESR in liquid solution and were shown to retain a pyramidal geometry that is significantly flattened compared to that of the parent phosphine.