Ammonium-Directed Olefinic Epoxidation: Kinetic and Mechanistic Insights

The ammonium-directed olefinic epoxidations of a range
of differentially N-substituted cyclic allylic and homoallylic amines
(derived from cyclopentene, cyclohexene, and cycloheptene) have been
investigated, and the reaction kinetics have been analyzed. The results
of these studies suggest that both the ring size and the identity
of the substituents on nitrogen are important in determining both
the overall rate and the stereochemical outcome of the epoxidation
reaction. In general, secondary amines or tertiary amines with nonsterically
demanding substituents on nitrogen are superior to tertiary amines
with sterically demanding substituents on nitrogen in their ability
to promote the oxidation reaction. Furthermore, in all cases examined,
the ability of the (in situ formed) ammonium substituent to direct
the stereochemical course of the epoxidation reaction is either comparable
or superior to that of the analogous hydroxyl substituent. Much slower
rates of ring-opening of the intermediate epoxides are observed in
cyclopentene-derived and cycloheptene-derived allylic amines as compared
with their cyclohexene-derived allylic and homoallylic amine counterparts,
allowing for isolation of these intermediates in both of the former
cases.