Aerobic oxidation of benzyl alcohols catalyzed by aryl substituted N-hydroxyphthalimides.: Possible involvement of a charge-transfer complex

A series of aryl-substituted N-hydroxyphthalimides (X-NHPIs) containing either electron-withdrawing groups (4-CH3OCO, 3-F) or electron-donating groups (4-CH3, 4-CH3O, 3-CH3O, 3,6-(CH3O)(2)) have been used as catalysts in the aerobic oxidation of primary and secondary benzylic alcohols. The selective formation of aromatic aldehydes was observed in the oxidation of primary alcohols; aromatic ketones were the exclusive products in the oxidation of secondary alcohols. O-H bond dissociation enthalpies (BDEs) of X-NHPIs have been determined by using the EPR radical equilibration technique. BDEs increase with increasing the electron-withdrawing properties of the aryl substituent. Kinetic isotope effect studies and the increase of the substrate oxidation rate by increasing the electron-withdrawing power of the NHPI aryl substituent indicate a rate-determining benzylic hydrogen atom transfer (HAT) from the alcohol to the aryl-substituted phthalimide-N-oxyl radical (X-PINO). Besides enthalpic effects, polar effects also play a role in the HAT process, as shown by the negative rho values of the Hammett correlation with sigma(+) and by the decrease of the rho values (from -0.54 to -0.70) by increasing the electron-withdrawing properties of the NHPI aryl substituent. The relative reactivity of 3-CH3O-C6H4CH2OH and 3,4-(CH3O)(2)-C6H3CH2OH, which is higher than expected on the basis of the sigma(+) values, the small values of relative reactivity of primary vs secondary benzylic alcohols, and the decrease of the rho values by increasing the electron-withdrawing properties of the NHPI aryl substituent, suggest that the HAT process takes place inside a charge-transfer (CT) complex formed by the X-PINO and the benzylic alcohol.