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 (kN = 3.8 × 10-6 M-1 s-1) and of Me-1-[α-13C]OH and Me-1-[α-13C]OCH2CF3 by concerted bimolecular displacement of solvent (ksolv = 1.8 × 10-8 s-1) is also observed. An analysis of the rate and product data provides a value of kaz/ks = 32 M-1 for partitioning of Me-2+ between addition of azide ion and solvent that is nearly 3-fold smaller than kaz/ks = 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/ks ≥ 10-7 s for the lifetime of Me-2+ in aqueous solution.