TiIV‐BINOLate‐Catalyzed Highly Enantioselective Additions of β‐Substituted Allylstannanes to Aldehydes

Enantiomerically pure homoallyl alcohols were prepared from aldehydes R 1 ‐CHO (R 1 = Ph, pentyl, Ph‐CHCH‐, i Pr) and β‐substituted allyl‐stannanes H 2 CCR 2 ‐CH 2 ‐SnBu 3 (R 2 = pentyl, t BuPh 2 SiO‐CH 2 ‐, t BuPh 2 SiO‐CH 2 ‐CH 2 ‐, PhS‐CH 2 ‐CH 2 ‐). 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(O i Pr) 4 gave equally good results, while Ti(OMe) 4 , Ti(OCHEt 2 )4, and Ti(O t Bu) 4 were inferior. Our procedure works in the absence of molecular sieves [which were previously found to give rather unreliable results in reactions catalyzed by Ti(O i Pr) 4 /( R )‐BINOL] and can be extended to enantioselective addition reactions with allyl‐ and methallyltributylstannane, too (+ hexanal: 97.4 and 97.0% ee , respectively).