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

Abstract

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 secondorder trapping by azide. Correlations of the azide/solvent selectivities of '1a-d' with the calculated relative driving force for hydration of the ions (∆E 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 betrapped 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.