Stable carbonium ions. XLV. Benzyl cations

Abstract

A series of ring-substituted benzyl cations (phenylcarbonium ions) were obtained by ionization of the corresponding benzyl chlorides in SbF,-SO, solution at -60°. Nmr and ultraviolet spectra of the ions were investigated . The geminal proton-proton magnetic resonance coupling constants of 2,6-disubstituted benzyl cations as well as the carbon-hydrogen couplings further prove the planar sp' hybridization nature of carbonium ions. V arious attempts to generate and observe the spectral properties in solution of the benzyl cation have thus far been unsuccessful. 1-4 Further reaction pathways open to the benzyl cation following ionization, such as intermolecular alkylation, undoubtedly conCH, Y -CH5 tribute to its apparent instability. Results and Discussion Using methods developed in our previous studies to observe stable, long-lived carbonium ions in solution' we were able to achieve the observation of the benzyl cations I-VI, VIII, XI, XII, and XIV. These benzyl cations were prepared by careful addition of the corresponding benzyl chlorides to well-stirred SbF,-S02 solutions at -75°. Introduction of alkyl substituents not only impede side reactions such as alkylation, but also further increase the stability of the benzyl cation inductively. (1) Part XLIV: G. A. Olah, M. Calin, and D. H. O'Brien, J. Am. Chem. Soc., 89, 3591 (1967); for a preliminary communication see ibid., 88, 361 (1966). (2) N. C. Deno, P. T. Groves, J. J. Jaruzelski, and M. N. Lugasch, Ibid., 82,4217 (1960). (3) J. F. A. Williams, Tetrahedron, 18, 1487 (1962). (4) I. Hanazaki and S . Magakura , ibid., 21, 2441 (1965). (5) For a summary see G . A. Olah, Chem. Eng. News, 45, 76 (March 27, 1967). In all cases the nmr spectra of the solutions of the ions at -65 ° exhibited well-resolved , substantially deshielded peaks which could be assigned from their position and their integrated areas in most cases. With the exception of ion II which slowly decomposes over a 0.5-hr period at -65°, all ions were stable indefinitely at this temperature . The thermal stability of the pentamethylbenzyl cation (I) is characteristic of the remarkable stability of the benzyl cations studied. SubBollinger, Comisarow, Cupas, Olah / Spectral Properties of Benzyl Cations