Structural Effects in Solvolytic Reactions. 46. Carbon-13 NMR Studies of Carbocations. II. < sup > 1 < /sup > Anomalous Carbon-13 Chemical Shifts with Increasing Electron Demand in a Model Classical 2-Norbornyl System, the 2-Aryl-3-methylene-2-norbornyl Cations

Abstract

The 3-methylene-2-norbornyl cation has long been accepted as a model “classical” 2-norbornyl system. Allylic resonance involving the 3-methylene group stabilizes the positive charge, rendering σ-bridging in this system insignificant. Accordingly, the 13C NMR spectra of a series of 2-aryl-3-methylene-2-norbornyl cations (4) were measured in SbF5/FSO3H/SO2ClF at −80 °C to test the proposal that deviations from linearity in plots of the 13C NMR shifts of the cationic carbon (ΔδC+) in the 2-aryl-2-norbornyl cations vs. the σc+ constants provide evidence for (σ-bridging in the parent cation. In fact, the observed shifts (ΔδC+) give a linear correlation for the electron-donating substituents, with r = 0.998 and pc+ = −10.0. On the other hand, the points for the electron-withdrawing substituents deviate upward from this line. No correlation has been observed for the presence (D) or absence of such deviations (ND) in the 2-aryl derivatives of parent systems that are accepted as being classical cations (C) or those that are postulated to be nonclassical cations (NC): 1-aryl-1-cyclobutyl (NC, ND), 2-aryl-2-bicyclo[2.1.1]hexyl (NC, ND), 3-aryl-3-nortricyclyl (C, D), 2-aryl-exo-5,6-trimethylene-2-norbornyl (C, D), 2-aryl-endo-5,6-trimethylene-2-norbornyl (NC, D), 2-aryl-2-bicyclo[2.2.2]octyl (NC, ND), 2-aryl-2-bicyclo[3.2.1]octyl (NC, ND), 2-aryl-2-norbornyl (NC, D), 2-aryl-3-methylene-2-norbornyl (C, D). © 1984, American Chemical Society. All rights reserved.