Absence of Non‐Linear Effects Despite Evidence for Catalyst Aggregation

다국어 초록 (Multilingual Abstract)

An in‐depth study of the catalytic system, consisting of the enantioselective addition of ZnEt2 to benzaldehyde with (1R,2S)‐(−)‐N‐Methylephedrine (NME) as chiral ligand, suggests the presence of dimeric and trimeric aggregates, as deduced from product ee vs. catalyst loading and NMR investigations (1H, DOSY). Formation of catalyst aggregation was excluded in earlier studies as this system displays a linear product ee vs. ligand ee‐correlation, which is usually taken as an indication for the absence of catalyst aggregation. A subsequent theoretical study, using the monomer‐dimer competition model, which we have recently developed, highlights the possible parameter configurations leading to linear product ee vs. ligand ee plots – despite the presence of catalyst dimers. It shows that, while the Kagan and Noyori models allow linearity in very specific cases only, a multitude of scenarios may lead to linearity here, especially if heterochiral dimers are catalytically active.
One might think that the absence of a non‐linear effect (NLE) might necessarily mean that only monomeric complexes catalyze a studied enantioselective reaction. Herein, a counter‐example using an ephedrine‐derived ligand in asymmetric alkylzinc additions is shown. The system is catalyzed by both monomers and dimers, although no NLE is apparent.