Substituent effects on the formation and nucleophile selectivity of ring-substituted phenonium ions in aqueous solution

The reaction of 2-(4-methyphenyl)ethyl tosylate (Me-1-OTs) in 50/50 (v/v) trifluoroethanol/water at 25 °C is first order in the concentration of azide anion nucleophile. A carbon-13 NMR analysis of the products of the reactions of Me-1-[α-¹³C]OTs in 50/50 (v/v) trifluoroethanol/water at 25 °C shows the formation of Me-1-[β-¹³C]OH, Me-1-[β- ¹³C]OCH₂CF₃ and Me-1-[β-13C]N3 from the trapping of a symmetrical 4-methylphenonium ion reaction intermediate Me-2 ⁺. The formation of Me-1-[α-13C]N3 by concerted bimolecular displacement of azide ion at Me-1-[α-¹³C]OTs (k_N = 3.8 × 10^-6 M^-1 s^-1) and of Me-1-[α-¹³C]OH and Me-1-[α-¹³C]OCH ₂CF₃ by concerted bimolecular displacement of solvent (k_solv = 1.8 × 10^-8 s^-1) is also observed. An analysis of the rate and product data provides a value of k _az/k_s = 32 M^-1 for partitioning of Me-2 ⁺ between addition of azide ion and solvent that is nearly three-fold smaller than k_az/k_s = 83 M^-1 reported in an earlier study on the partitioning of MeO-2⁺ [J. Org. Chem. 2011, 76, 9568]. This change is attributed to a decrease in nucleophile selectivity with increasing electrophile reactivity for the activation-limited addition of solvent and azide anion to X-2⁺. These data set a limit of 1/k _s ≥ 10^-7 s for the lifetime of Me-2+ in aqueous solution.