structural Effects in Solvolytic Reactions. 47. Effects of p-Alkyl and p-Cycloalkyl Groups on the Carbon-13 NMR Shifts of the Cationic Carbon Center in p-Alkyl-tert-cumyl Cations

Abstract

A series of p-alkyl- and p-cycloalkyl-tert-cumyl cations were prepared in SbF5/FSo3H/So2ClF at −78 °C, and their 13C NMR shifts were measured at ™80 °C. The C+ carbon in the p-alkyl derivatives is increasingly deshielded from p-methyl through p-tert-butyl. This observation is in accord with the Baker-Nathan order of stabilization (methyl > ethyl > isopropyl > tert-butyl) and in agreement with the rate data for the solvolysis of p-alkyl-tert-cumyl chlorides but in contrast to the gas-phase stability order for the para-protonated alkylbenzenes (RC6H6+). Possible reasons for this difference in the apparent effects of these four alkyl groups are discussed. The stability order achieved by cycloalkyl groups based on the C+ shifts is p-cyclopropyl ≫ p-cyclobutyl ≃ p-cyclopentyl > p-cyclohexyl. In the case of p-cyclopropyl-tert-cumyl cation, the C+ signal appears relatively shielded (231.6 ppm) compared to the C+ shift (244.0 ppm) for the p-isopropyl-tert-cumyl cation. This observation supports the ability of the cyclopropyl group to supply electrons (through C-C hyperconjugation) to the electron-deficient center without the intervention of σ-bridging through space. For other cycloalkyl derivatives, the C+ signals of the p-cyclobutyl and p-cyclopentyl derivatives appear slightly upfield relative to the p-cyclohexyl derivatives. This may again be attributed to the greater hyperconjugating (CH and CC) ability of the strained cyclobutyl and cyclopentyl bonds, as compared with the nonstrained cyclohexyl bonds. For the p-norbomyl-tert-cumyl derivatives examined (exo-2- and endo-2-norbomyl and exo-5, 6-trimethylene-exo- and -endo-2-norbomyl), the C+ for the exo derivatives is relatively shielded compared to the C+ of the endo derivatives. Although it may be attractive to interpret this observation in terms of a greater electron supply by the exo derivatives, the small effects may very well arise from the steric factors in the endo derivatives. Accordingly, the apparent stabilizing ability of the norbornyl moiety, as indicated by the relatively less negative pc+ value (-14.0) observed in the Δδc+/σc+ plot of the 2-aryl-2-norbomyl cations, cannot be confirmed by this direct examination of the comparative electron-releasing characteristics of these two exo- and endo-norbornyl substituents. © 1984, American Chemical Society. All rights reserved.