α-fluoro decalones as chiral epoxidation catalysts:: fluorine effect

Three rigid monofluorinated trans-decalones 4a, 5e, and 6e (90% ee) have been synthesized from commercially available (-)-(R)-methyl naphthalenone (90% ee). Their structures have been fully characterized (NMR, X-ray): ketones 4a and 5e are Me,F-disubstituted alpha to the carbonyl with the fluorine axial and equatorial, respectively, while ketone 6e is F-monosubstituted alpha to the carbonyl with the fluorine equatorial. The use of these ketones as chiral catalysts for the epoxidation of trans-olefins (such as stilbene, beta-methylstyrene and p-methoxy cinnamate) through the formation of dioxiranes shows (i) that dioxiranes with an equatorial fluorine alpha to the dioxirane ring are less reactive and provide lower ee's than dioxiranes with an axial fluorine and having the same chirality and (ii) that an axial methyl alpha to the dioxirane ring is significantly less efficient than a fluorine. The results corroborate Armstrong and Houk's theoretical model and our first hypothesis to rationalize the inverted enantioselectivities observed using alpha-fluorinated cyclohexanones having the same chirality, i.e.: rapid ring inversion (Curtin-Hammett principle) allows the dioxirane conformation to have the fluorine axial (even if less populated than the other) to contribute significantly to the epoxidation reaction. (C) 2004 Published by Elsevier Ltd.