A theoretical study of the iminoxyl/oxime self-exchange reaction. A five-center, cyclic proton-coupled electron transfer

In solution, the self-exchange reactions for oxygen-centered pi-radicals, e.g., PhO center dot + PhOH reversible arrow PhOH + PhO center dot, are known to occur with low activation enthalpies (E-a approximate to 2 kcal/mol). For the PhO center dot/PhOH couple and, we conclude, for other 0-centered pi-radicals, exchange occurs by proton-coupled electron transfer (PCET) with the proton transferred between oxygen electron pairs while the electron migrates between oxygen orbitals orthogonal to the -O---H---O- transition state plane (Mayer et al. J. Am. Chem. Soc. 2002, 123, 11142). Iminoxyls, R2C=NO center dot, are sigma-radicals with substantial spin density on the nitrogen. The R2C=NO center dot/R2C=NOH self-exchange has a significant E-a (Mendenhall et al. J. Am. Chem. Soc. 1973, 95, 627). For this exchange, DFT calculations have revealed a counterintuitive cisoid transition state in which the seven atoms, > C=NO---H---ON=C <, lie in a plane (R = H, Me) or, for steric reasons, two planes twisted at 45.2 degrees (R = Me3C). The planar transition state has the two N-O dipoles close to each other and pointing in the same direction and an O---H---O angle of 165.4 degrees. A transoid transition state for R = H lies 3.4 kcal/mol higher in energy than the cisoid despite a more favorable arrangement of the dipoles and a near linear O--H---O. It is concluded that iminoxyl/oxime self-exchange reactions occur by a five-center, cyclic PCET mechanism with the proton being transferred between electron pairs on the oxygens and the electron migrating between in-plane orbitals on the two nitrogens (RN-N = 2.65 angstrom). The calculated E-a values (8.8-9.9 kcal/mol) are in satisfactory agreement with the limited experimental data.