1,3-Transposition of Allylic Alcohols Catalyzed by Methyltrioxorhenium
Methyltrioxorhenium (MTO) catalyzes the 1,3-transposition of allylic alcohols to generate the more stable isomer at equilibrium. The direction of the equilibrium is largely decided by the nature of the OH group, i.e., whether it is primary, secondary, or tertiary. In the case of aliphatic allylic alcohols, tertiary is preferred to secondary which is preferred to primary. For aromatic allyl alcohols, the more conjugated isomer predominates largely at equilibrium. Oxygen-18 labeling showed that the OH groups of the parent and product are the same. The reaction is first order with respect to both allyl alcohol and MTO but strongly inhibited by traces of water. Theoretical calculations suggest the same results in the case of aliphatic allyl alcohols, although aromatic allyl alcohols do not follow the predictions. Studies of deuterium-labeled substrates show a large equilibrium isotope effect (K = 1.20 ± 0.02). For isomeric allyl alcohols differing in the position of deuterium only, the isomer with the deuterium at the sp3center predominates at equilibrium. The effect of conjugation from a phenyl group appears to be less important since calculations suggest that the phenyl group is forced out of plane of the allylic π system.
Reprinted (adapted) with permission from Organometallics 17 (1998): 1835, doi:10.1021/om971115n. Copyright 1998 American Chemical Society.