High Relative Reactivities of Aromatic Cation Radicals Compared to Related Carbocations. Observation of an Unexpected Deuterium Kinetic Isotope Effect

Article abstract of DOI:10.1021/ja00204a019

The reaction between 9-phenylanthracene (PAH) cation radical and acetate ion was studied to probe for an electron-transfer pathway.The decomposition of acetoxy radical to methyl radical and carbon dioxide, accompanied by the regeneration of PAH, is the expected result of an initial electron transfer.Rate-determining attack of acetate ion on PAH^.+ followed by rapid product-forming reactions could lead to a secondary α deuterium kinetic isotope effect.Neither of these results were observed.Attack by acetate ion on PAH^.+ in acetonitrile at 293 K was observed to take place at nearly diffusion control.The slow step in the disappearance of acetate ion was observed to be the attack of acetate ion on PA(+)(H)(OAc), resulting from oxidation of the initial adduct: PA(+)(H)(OAc) + AcO(-) -> PA-OAc + HOAc (i).The proton abstraction reaction (i) resulted in a large (25 or greater) deuterium kinetic isotope effect for the reaction of the cation radical derived from 9-phenylanthracene-10-d.These observations show that nucleophilic attack on PAH^.+ is more than 1 order of magnitude faster than reaction i and much faster than the attack of acetate ion on the carbocationic center of PA(+)(H)(OAc).This result is unexpected according to recent discussions of cation-radical reactivity.