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

Abstract

A series of 2-aryl-3-methyl-2-butyl cations was prepared in SbF5/FS03H/S02ClF at -78 °C and their 13C NMR shifts were measured at -80 °C. A plot of the observed δδC+ values against the sc+ constants gives a straight line for substituents over the rangep-OCH3 to m-Cl (slope pc+ = -18.2, correlation coefficient r = 0.999). However, the data points for the strongly electron-withdrawing substituents, m-CF3 3,5-Cl2, and P-CF3, deviate upward from this straight line. The δδCa/saC+ 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 °C to -30 °C causes enhanced deviations, in accord with a rapid equilibration that forms moee 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+/ sC+ 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 deviations observed in the AbC+/ac plot for the substituted 2-aryl-2-norbornyl cations. © 1983, American Chemical Society. All rights reserved.