A Theoretical Study of the Iminoxyl/Oxime Self-Exchange Reaction. A Five-Center, Cyclic Proton-Coupled Electron Transfer

摘要

In solution, the self-exchange reactions for oxygen-centered π-radicals, e.g., PhO· + PhOH →← PhOH + PhO·, are known to occur with low activation enthalpies (E_a ≈ 2 kcal/mol). For the PhO·/PhOH couple and, we conclude, for other O-centered π-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, R_2C=NO·, are σ-radicals with substantial spin density on the nitrogen. The R_2C=NO·/R_2C=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° (R = Me_3C). The planar transition state has the two N-O dipoles close to each other and pointing in the same direction and an O—H—0 angle of 165.4°. 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 (R_(N-N) = 2.65 A). The calculated E_a values (8.8-9.9 kcal/mol) are in satisfactory agreement with the limited experimental data.