Ti-IV-BINOLate-catalyzed highly enantioselective additions of beta-substituted allylstannanes to aldehydes

Enantiomerically pure homoallyl alcohols were prepared from aldehydes R(1)-CH=O (R(1)=Ph, pentyl, Ph-CH=CH-, iPr) and beta-substituted allylstannanes H2C=CR(2)-CH2-SnBu(3) (R(2)=pentyl, tBuPh(2)SiO-CH2-, tBuPh(2)SiO-CH2-CH2-, PhS-CH2-CH2-). These reactions were catalyzed by the same additives-Ti(OR)(4) (10 mol%) and (R)BINOL (20 mol%)-that Keck et al. used in analogous reactions with methallyl-and allyltributylstannane. To attain optimum ee values (96.4-99.2% in the reaction with hexanal) these additives had to be premixed for 2 h at room temperature. Ti(OEt)(4) and Ti(OiPr)(4) gave equally good results, while Ti(OMe)(4), Ti(OCHEt(2))(4), and Ti(OtBu)(4) were inferior. Our proce dure works in the absence of molecular sieves [which were previously found to give rather unreliable results in reactions catalyzed by Ti(OiPr)(4)/(R)-BINOL] and can be extended to enantioselective addition reactions with allyl- and methallyltributylstannane, too (+hexanal: 97.4 and 97.0% ee, respectively).