~1H DOSY Spectra of Ligands for Highly Enantioselective Reactions- A Fast and Simple NMR Method to Optimize Catalytic Reaction Conditions

摘要

The demand for enantiopure chemicals, for example, natural products, pharmaceuticals, or materials, has been increasing rapidly for years, and the global market is continually expanding. In the field of asymmetric catalysis, transition-metal catalysts using chiral ligands represent one of the most effective and versatile approaches. However, the development of highly efficient catalysts is often an unpredictable, challenging, and time-consuming process. Accordingly, every method that shortens this laborious procedure or allows an assessment of selectivity contributions is highly valued. In this context, rational models were developed to predict asymmetry in resulting products, for example, Cram's rule and the Felkin-Anh model, and quadrant models. Furthermore, combinatorial libraries provide empirical strategies for ligand selection. With regard to temperature optimization, the isoinversion principle provides a general model for reactions with two or more selectivity steps. At present, the rational models have to address more complex issues because of the importance of noncovalent interligand interactions in organ-ometallic complexes. Even weak π -π interactions were found to influence complex structures, for example, a cis coordination of the ligands was found for a bis(phosphonite) Pt complex and a bis(phosphoramidite) Pd complex; this coordination was explained by weak intermolecular interactions.