Divergent enantioselective pathways in the catalytic asymmetric addition of diethylzinc to aldehydes in the presence and absence of titanium tetraisopropoxide

The stereochemical outcome of the asymmetric addition of diethylzinc to aldehydes catalyzed by (R,R)-hydrobenzoin can be influenced by the absence or presence of Ti(O-iPr)4. The enantiomeric ratios obtained in the absence of Ti(O-iPr)4 favor the (S)-enantiomer, whereas the ratios obtained from the use of Ti(O-iPr)4 favor the formation of the (R)-enantiomer. The formation of the opposite enantiomers is attributed to the different transition states mediated by either zinc or titanium.