Structural Effects in Solvolytic Reactions. 40. Carbon-13 Nuclear Magnetic Resonance Studies of Carbocations. 8. Anomalous Carbon-13 Chemical Shifts with Increasing Electron Demand in the 2-Aryl-2-norbornyl Cations and Related Systems. Evidence That These Anomalous Shifts Are Not Diagnostic for the Onset of Nonclassical σ Bridging

Abstract

13C NMR spectra of an extended range of 2-aryl-2-norbornyl (11), 2-aryl-exo-5,6-trimethylene-2-norbornyl (12), 2-aryl-endo-5,6-trimethylene-2-norbornyl (13), 3-aryl-3-nortricyclyl (14), 1-aryl-1-cyclopropylethyl (15), 1-aryl-1-phenylethyl (28), and 1-aryl-1-phenylmethyl (29) carbocations have been obtained under standard conditions. When the cationic carbon substituent chemical shifts Δδ C+ are plotted against the new δ C+ constants, the points for the substituents more electron donating than H (Z = 3′4′-CH2CH2O to Z = H) yield excellent linear correlations, whereas those for substituents more electron demanding than H (Z = H to Z = 3′,5′-(CF3)2) curve away from this correlation line. These deviations from linearity are in contrast to the plots for a large range of static, classical acyclic (1–4), cyclic (5–7), and polycyclic (8–10) carbocations, which yield excellent correlations of Δδ C+ with σ C+. The deviations in the plots for 11 and 12 have been rationalized previously in terms of the onset of σ bridging, for 14 in terms of the onset of increased cyclopropyl conjugation and for 28 in terms of steric inhibition of resonance. However, examination of our data shows that for each of the three different molecular structures, 2-norbornyl (11–13), cyclopropylcarbinyl (14, 15), and phenylalkyl (28, 29), the same, in fact, almost identical, deviations are observed. In addition, the aryl derivatives of other cationic systems for which σ -bridged structures have been proposed, cyclobutyl (19), 2-bicyclo[2.1.1]hexyl (20), 2-bicyclo[2.2.2]octyl (21), and 6-bicyclo[3.2.1]octyl (22), fail to exhibit such deviations with the data available. Thus we conclude that the anomalous 13C shifts, which give rise to the deviations from linearity in the plots of Δδ C+ against σ C+ (or Δδ C+, cyclopentyl), are not diagnostic for the onset of nonclassical σ bridging. Possible explanations for these anomalous C+ shifts are considered. © 1982, American Chemical Society. All rights reserved.