The reaction is believed to proceed through a dimeric complex having two titanium centres (Scheme 12). The presence of the allylic hydroxyl group is necessary as it coordinates with the metal thereby allowing the transfer of oxygen atom from one face only. The absolute configuration of the epoxy alcohol can be predicted by the following mnemonic model in which the hydroxymethylene group is positioned at the lower right. The epoxidation takes place from the upper face of the allyl alcohol when (+)-( R,R )-DET is used and vice versa (Scheme 13).
Scheme 13
These reaction conditions, Ti(O i -Pr)4 , t -BuOOH and (R,R) or (S,S)-diethyltartarate, have also been extensively studied for asymmetric oxidation of aryl alkyl sulfides (Scheme 14).
Scheme 14