Acid-Catalyzed Olefin-Alcohol Interconversion in the 1-Methylcyclooctyl System. Strain-Relief Acceleration of the Hydration of 1-Methyl-trans-cyclooctene

Article abstract of DOI:10.1021/jo00200a041

Rates of reaction with aqueous perchloric acid at 25 deg C have been measured for 1-methyl-cis-cyclooctene, 1-methyl-trans-cyclooctene, and their exo isomer, methylenecyclooctane.The only products formed to any significant extent in each of these reactions are 1-methylcyclooctanol and the cis-cycloalkene, but both the trans- and exo-olefins give these products in initially noneequilibrium proportions: / = 1.27 +/- 0.05 at equilibrium, 15.1 +/- 0.9 from the trans-olefin and 0.44 +/- 0.05 from the exo-olefin.This is taken as evidence for the existence of two conformationally different 1-methylcyclooctyl carbocationic intermediates in these reactions, an unstable crown (or twist) cation formed by protonation of the trans-olefin and a (conformationally) stable boat-chair cation formed from the other two olefins and the alcohol.The barrier for interconversion of these two cations is estimated at ΔG(excit.) = 5.3 +/- 0.6 kcal mol^-1.The barrier for hydration of either cation to the alcohol is also estimated, by two different methods, at ΔG(excit.) = 3.6 +/- 0.6 kcal mol^-1 or 4.3 +/- 0.3 kcal mol^-1.The smallness of these barriers suggests that the reaction of tertiary aliphatic carbocations with water in aqueous solution is an essentially unactivated process.These estimates plus the rate and equilibrium constants measured here allow nearly complete free energy characterization of the 1-methylcyclooctyl system.The rates of reaction of the cis- and exo-olefin with the hydronium ion are normal, but strain relief accelerates the rate of the trans-cycloalkene by a factor of 18500.