Structural Effects in Solvolytic Reactions. 41. The Effect of Coplanarity on the Carbon-13 Nuclear Magnetic Resonance Shifts of Substituted o-Methyl-tert-cumyl Cations

Abstract

The effect of steric inhibition on coplanarity was examined by preparing a series of 4/5-substituted o-methyl-tert-cumyl carbocations in SbF6/FSO3H/SO2ClF at −78 °C and measuring their 13C NMR shifts at −80 °C. The ΔδC+ shifts correlate linearly against the σC+ constants with a ρC+ value of −18.3 and a correlation coefficient of 0.9995. The excellent correlation observed for this system indicates that the deviations in the σC+-ΔδC+ plots observed for certain systems of benzylic carbocations are probably not the result of steric hindrance to coplanarity in these systems. The ρC+ value of −18.3 is essentially the same as the value of −18.2 observed for the substituted tert-cumyl cations. This indicates that the fully developed dimethylcarbinyl cationic center must be able to overcome the steric difficulties in achieving coplanarity with the aromatic ring with only minor costs in energy, in contrast to the situation in the solvolysis of related derivatives. © 1982, American Chemical Society. All rights reserved.