Structural Effects in Solvolytic Reactions. 46. Carbon-13 NMR Studies of Carbocations. 11. Anomalous Carbon-13 Chemical Shifts with Increasing Electron Demand in a Model Classical 2-Norbornyl System, the 2-Aryl-3-methylene-2-norbornyl Cations

Article abstract of DOI:10.1021/ja00320a022

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 ?-bridging in this system insignificant.Accordingly, the (13)C NMR spectra of a series of 2-aryl-3-methylene-2-norbornyl cations (4) were measured in SbF5/FSO3H/SO2ClF at -80 degC to test the proposal that deviations from linearity in plots of the (13)C NMR shifts of the cationic carbon (ΔδC⁺) in the 2-aryl-2-norbornyl cations vs. the ?^C+ constants provide evidencefor ?-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 ρ^C+= -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-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).