Structural Effects in Solvolytic Reactions. 36. Effect of Increasing Electron Demand on Carbon-13 Nuclear Magnetic Resonance Shifts in the 1-Aryl-1-cyclohexyl and 1-Aryl-1-cycloheptyl Carbocations. Further Test of the Ability of the σ < sup > c+ < /sup > Constants to Correlate Carbon-13 Nuclear Magnetic Resonance Shifts

Abstract

The 13C NMR shifts of a series of meta- and para-substituted 1-phenyl-1-cyclohexyl and 1-phenyl-1-cycloheptyl carbocations were measured in SbF5/FSO3H/SO2CIF at -80 °C to provide a further test of the ability of the new set of σC+ constants to correlate such 13C NMR shifts. Indeed, the data reveal excellent linear correlation of the observed shifts, ∆δC+, in these two systems by the σC+ constants. The 1-aryl-l-cyclohexyl system yields a ρc+ value of -17.2, with a correlation coefficient of r = 0.999. The 1-aryl-1-cycloheptyl system gives ρC+ = -16.4, with r = 0.999. Previous investigation of the 1-aryl-1-cyclopentyl system yielded ρC+ = -16.8, with r = 0.999. Thus all of the systems containing five-+, six-, and seven-membered rings are correlated with satisfactory precision by the values. The variation in pc from the five-, to the six-, to the seven-membered systems parallels the changes in ρ+ observed in solvolysis of the p-nitrobenzoates. However, the variations with ring size are larger in the solvolytic studies. It is concluded that both ground-state energies and the electronic stabilization of the cations are enhanced in the five- and seven-membered rings, contributing to the change in ρ+ from that observed for the six-membered ring. On the other hand, differences in ground-state energies can play no role in the magnitude of ρC+ for the fully formed carbocations. © 1981, American Chemical Society. All rights reserved.