Catalytic and Stoichiometric Enantioselective Addition of Diethylzinc to Aldehydes Using a Novel Chiral Spirotitanate

Simple variation of the reaction conditions in the addition of Et2Zn to aldehydes in the presence of spirotitanate 1 can lead to a reversal in the configuration of the product. Stoichiometric amounts of the spirotitanate result in Re addition of Et2Zn to the aldehyde (up to 98% ee). In the presence of equimolar amounts of Ti(OCHMe2)4, catalytic amounts of 1 lead to Si addition of Et2Zn (up to 99% ee).