Structure of the radicals formed in the Mitsunobu reaction

The first step of the Mitsunobu reaction consists of the reaction between a dialkyl azodicarboxylate (1) and triphenylphosphine (2) with formation of a phosphobetaine, ROOCN(P+Ph3)N-COOR (3). It was recently observed by EPR spectroscopy that a low concentration of a radical accompanied 3, and its structure was assigned to the product of one-electron oxidation of 3, the distonic radical cation ROOCN(P+Ph3)(NCOOR)-C-. (4), based on EPR spectral evidence. [Camp. D., Hanson, G. R. and Jenkins, I. D. J. Org. Chem. 60 (1995) 2477.] A study of 3 and similar betaines by cyclic voltammetry revealed that 3 is far too difficult to oxidize (E-Pa was around 2.0 V vs. Ag/AgCl) to be a credible precursor of the radical species. In combination with an EPR spectral study of a series of 1-R3P reactions results have been obtained which suggest that the radical species instead is derived from a structure in which the phosphorus atom is connected to the carbonyl oxygen of 1, i.e. (Ph3P+O)(RO)C=N-N.COOR. The reaction between 2 and 4-phenyl-4H-1,2,4-triazolinedione (PTAD) produced similar radical species, indicating that radical cation species derived from the cyclization of 4 (P to the carbonyl oxygen) cannot be involved.