Temperature‐dependent nonlinear effects and catalyst evolution in the asymmetric addition of diethylzinc to benzaldehyde

Abstract

AbstractSeveral efficient and highly enantioselective titanium‐based catalysts promote the asymmetric addition of alkyl groups to aldehydes. The catalysts involved in these reactions have been found not to exhibit nonlinear behavior. We have been able to identify a series of chiral bis(sulfonamide)‐based titanium catalysts that do show nonlinear behavior. Monitoring of the product ee's (eep's) at different conversions and different temperatures has led us to propose that nonlinear behavior is caused by an autoinduction process rather than by catalyst aggregation. Evidence to support this hypothesis includes eep's that change with the extent of conversion. We have found that when enantiopure ligand was used, the eep increases with conversion from 72% to 80%. However, use of non‐enantiopure ligand results in eep's that decrease with conversion. These results are rationalized by different rates of alkoxide exchange processes between like and unlike titanium‐alkoxide containing complexes.