MECHANISM OF ASYMMETRIC EPOXIDATION .2. CATALYST STRUCTURE

The dominant species in equimolar mixtures of titanium tetraalkoxides and dialkyl tartrate esters is shown to be [ti(tartrate) (OR)2]2, which is identified as the active catalyst for asymmetric epoxidation of allylic alcohols by tertiary alkyl hydroperoxides. The solution-phrase structure of this species is consistent with the results of an X-ray structural determination of a titanium tartramide complex reported previously, as analyzed by IR and H-1, C-13, and O-17 NMR spectrometry. The first O-17 NMR spectra of titanium(IV) alkoxide and alkyl peroxide complexes are reported, as well as the results of a secondary deuterium isotope effect on the asymmetric epoxidation reaction. General conclusions concerning the mechanism of asymmetric epoxidation are presented.