Structural Effects in Solvolytic Reactions. 43. Effect of Increasing Electron Demand on the 13C NMR Shifts for 2-Aryl-3-methyl-2-butyl Cations. A Static System Becoming an Equilibrating System with Electron-Withdrawing Substituents

Article abstract of DOI:10.1021/ja00341a037

A series of 2-aryl-3-methyl-2-butyl cations was prepared in SbF5/FSO3H/SO2ClF at -78 deg C and their 13C NMR shifts were measured at -80 deg C.A plot of the observed ΔδC⁺ values against the ?^C+ constants gives a straight line for substituents over range p-OCH3 to m-Cl (slope ρ^C+ = -18.2, correlation coefficient r = 0.999).However, the data points for the strongly electron-withdrawing substutients, m-CF3, 3,5-Cl2, and p-CF3, deviate upward from this straight line.The ΔδC_α/?^αC+ plot for the α-methine (CH) carbon atom exhibits a linear correlation of the data points for derivatives with substituents p-OCH3 to m-Cl, with a downward deviation of the data points for derivatives containing strongly electron-withdrawing substituents.These results are in accord with a rapid 2,3-hydride shift in derivatives containing benzylic cations with higher energies.An increase in temperature from -80 deg C to -30 deg C causes enhanced deviations, in accord with a rapid equilibration that forms more of the less stable cations at the higher temperatures.In the case of 2-aryl-2-norbornyl cations, similar upward deviations are observed in the ΔδC⁺/?^C+ plot.However, the spectra of these cations reveal no temperature dependence over the same temperature range.This indicates that equilibration is not the factor responsible for the deviation observed in the ΔδC⁺/?^C+ plot for the substituted 2-aryl-2-norbornyl cations.