Carbon-13 NMR Studies of Carbocations. 12. < sup > 1 < /sup > Effect of Increasing Electron Demand on the < sup > 13 < /sup > C Chemical Shifts of 1-Aryl-1-cycloalkyl-1-ethyl and 1-Aryl-2-methyl-1-cycloalkyl Cations

Abstract

Four different series of 1-aryl-1-cycloalkyl carbocations (8, 9,11,14) have been prepared in SbF5/FSO3H/SO2ClF solutions and their 13C NMR spectra measured at -80 °C. From the plots of ΔδC+ vs. σc+ for 1-aryl-1-cyclopentyl-1-ethyl (8) and 1-aryl-1-cyclohexyl-1-ethyl cations (9) the ρc+ values were obtained and, when compared with data available for 2-aryl-3-methyl-2-butyl (18) and 1-aryl-1-cyclopropyl-1-ethyl cations (6), provided evidence for the stabilizing abilities of adjacent groups: cyclopropyl > cyclopentyl > cyclohexyl > isopropyl (ρc+ -10.5, -17.1, -17.6, -18.2, respectively). This parallels the order determined from solvolysis of the corresponding p-nitrobenzoates. When cation 8 is substituted with electron-withdrawing groups, an irreversible rearrangement occurs to give a 1-aryl-2-methyl-1-cyclohexyl cation (14). Ionization of the 1-aryl-2-methyl-1-cyclohexanols (15) also yields 14, and 1-aryl-2-methyl-1-cyclopentanols (12) yield 1-aryl-2-methyl-1-cyclopentyl cations (11). The plots of ΔδC+ vs. σc+ and of ΔδCα vs. σαC+ for cations 9, 11, and 14 bearing electron-withdrawing substituents show upward (ΔδC+) and downward (ΔδCα) deviations consistent with the incursion of equilibration. © 1985, American Chemical Society. All rights reserved.