4'-substituted-4-biphenylyloxenium ions: reactivity and selectivity in aqueous solution.

Azide trapping shows that the 4'-substituted-4-biphenylyloxenium ions 1b-d are generated during hydrolysis of 4-aryl-4-acetoxy-2,5-cyclohexadienones, 2c and 2d, and O-(4-aryl)phenyl-N-methanesulfonylhydroxylamines, 3b and 3c. In addition, the 4'-bromo-substituted ester, 2d, undergoes a kinetically second-order reaction with N3- that accounts for a fraction of the azide adduct, 5d. Since both first-order and second-order azide trapping occurs simultaneously in 2d, the second-order reaction is not enforced by the short lifetime of 1d, which has similar azide/solvent selectivity to the unsubstituted ion, 1a. In contrast the 4'-CN and 4'-NO2 ions 1e and 1f cannot be detected by azide trapping during the hydrolysis of the dichloroacetic acid esters 2e' and 2f' even though 18O labeling experiments show that a fraction of the hydrolysis of both esters occurs through C(alkyl)-O bond cleavage. These esters exhibit only second-order trapping by azide. Correlations of the azide/solvent selectivities of 1a-d with the calculated relative driving force for hydration of the ions (DeltaE of eq 4) determined at the pBP/DN//HF/6-31G and BP/6-31G//HF/6-31G levels of theory suggest that 1e and 1f have lifetimes in the 1-100 ps range. Ions with these short lifetimes are not in diffusional equilibrium with nonsolvent nucleophiles, and must be trapped by such nucleophiles via a preassociation mechanism. The second-order trapping that is observed in these two cases is enforced by the short lifetime of the cations, and may occur by a concerted S(N)2' mechanism or by internal azide trapping of an ion sandwich produced by azide-assisted ionization. Comparison of azide/solvent selectivities of the oxenium ions 1a-c with the corresponding biphenylylnitrenium ions 8a-c shows that 4'-substituent effects on reactivity in both sets of ions are similar in magnitude, although the nitrenium ions are ca. 30-fold more stable in an aqueous environment than the corresponding oxenium ions. The magnitude of the 4'-substituent effects for electron-donating substituents suggest that both sets of ions are more accurately described as 4-aryl-1-imino-2,5-cyclohexadienyl or 4-aryl-1-oxo-2,5-cyclohexadienyl carbocations. Calculated structures of the oxenium ions are also consistent with this interpretation.