Solvent Effects in the Regio- and Diastereoselective Epoxidations of Acyclic Allylic Alcohols by Dimethyldioxirane:  Hydrogen Bonding as Evidence for a Dipolar Transition State

A mechanistically significant solvent effect is observed in the regioselectivity of the geraniol epoxidation by dimethyldioxirane. In hydrogen-bonding solvents (MeOH), the 6,7-epoxide is preferred over the 2,3-epoxide (74:26), which reveals that the more nucleophilic 6,7 double bond (the 2,3 double bond is inductively deactivated by the allylic hydroxy group) is preferentially attacked by the electrophilic dimethyldioxirane. In MeOH, both regioisomeric dipolar transition states are equally well stabilized by interaction through intermolecular hydrogen bonding with solvent molecules. In the nonpolar CCl4, intramolecular hydrogen bonding with the allylic hydroxy functionality favors attack at the 2,3 double bond and proportionally more 2,3-epoxide is formed. Similarly, also the π-facial selectivity in the dimethyldioxirane epoxidation of methyl-substituted chiral acyclic allylic alcohols is controlled by intermolecular versus intramolecular hydrogen bonding. Thus, higher threo selectivities are obtained in the nonpolar CCl4 by stabilization of the diastereomeric transition state with minimal allylic strain through intramolecular hydrogen bonding with the allylic hydroxy group. The geometry of the dipolar transition state for the dimethyldioxirane epoxidations is similar to that of m-CPBA, but with apparently a slightly larger (ca. 130°) dihedral angle α to relieve 1,2-allylic strain.