Hot Water-Promoted S(N)1 Solvolysis Reactions of Allylic and Benzylic Alcohols

During the studies of hydrolysis of epoxides in water, we found that the hydrolysis of (-)-alpha-pinene oxide at 20 degrees C gave enantiomerically pure trans-(-)-sobrerol, whereas the same reaction in water heated at reflux unexpectedly gave a racemic mixture of trans- and cis-sobrerol (trans/cis = 6: 4). We have examined this remarkable difference in detail and found that hot water, whose behavior is quite different compared with room-or high-temperature water, could promote S(N)1 solvolysis reactions of allylic alcohols and thus caused the racemization of trans-(-)-sobrerol. The effect of reaction temperature, the addition of organic co-solvent, and the concentration of the solute on the rate of the racemization of trans-(-)-sobrerol were further examined to understand the role that hot water played in the reaction. It was proposed that the catalytic effects of hot water are owing to its mild acidic characteristic, thermal activation, high ionizing power, and better solubility of organic reactant. Further investigation showed that the racemization of other chiral allylic/benzylic alcohols could efficiently proceed in hot water.