Rearrangement of a Homoallylic Alcohol via an Acid-Catalyzed 1,4-Hydride Shift Yields a Saturated Ketone

The homoallylically substituted alcohols (1-OH and 3-OH) of the sterically congested alkene adamanylideneadamantane (1-H) react with acid catalysis in a solvent of 50% v/v acetic acid:50% v/v aqueous sulfuric acid at 110 degrees C to give a saturated ketone 5, the structure of which has been characterized by (1)H and (13)C NMR spectroscopy and by single-crystal X-ray diffraction. Isotopic labeling studies demonstrate that the reaction involves a stereospecific protonation of the double bond and an intramolecular 1,4-hydride transfer from the secondary alcohol C-H to the other carbon of the olefin. The rearrangement reaction exhibits a kinetic isotope effect (KIE) of 2.34 +/- 0.17 for the intramolecular hydride transfer reaction and a competitive isotope effect of 1.2 for protonation of the olefin. These results are consistent with a two-step reaction in which the protonation of the double bond, a potentially reversible process, is followed by the rate-determining, intramolecular 1,4-hydride transfer.