Kinetic evidence for the importance of solvent-separated ion pairs during the solvolyses of adamantylideneadamantyl derivatives

The aqueous ethanolysis reactions of adamantylideneadamantyl tosylate, -bromide, and -iodide (1-OTs, 1-Br and 1-I) were monitored as a function of ionic strength. Special salt effects are observed during the solvolyses of both homoallylic halides, but not in the case of the tosylate 1-OTs. The measured alpha-secondary deuterium kinetic isotope effects for the solvolysis of 1-Br in 80:20 and 60:40 v/v ethanol-water mixtures at 25 degreesC are 1.110 +/- 0.018 and 1.146 +/- 0.009, respectively. The above results are consistent with the homoallylic halides reacting via a virtual transition state in which both formation and dissociation of a solvent-separated ion pair are partially rate-determining. While the corresponding transition state for adamantylideneadamantyl tosylate involves formation of the solvent-separated ion pair.