ANODIC BEHAVIOR OF CROWDED TRIARYLPHOSPHINES - ESR STUDY OF TRIARYLPHOSPHONIUMYL RADICALS, AR3P.+

A large number of triarylphosphines exhibiting different steric hindrance has been prepared. The pyramidalization angle alpha 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 alpha) phosphines, the oxidation potential correlates with the sum of the sigma+ 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 easily 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.