Chiral Phosphinooxazolidine Ligands for Palladium- and Platinum-Catalyzed Asymmetric Diels−Alder Reactions

Reuse of chiral cationic Pd–phosphinooxazolidine catalysts in ionic liquids: highly efficient catalytic asymmetric Diels–Alder reactions

Chiral cationic palladium-phosphinooxazolidine catalysts in ionic liquid afforded excellent enantioselectivity in Diels-Alder reactions and the catalyst was easily recycled eight times without any significant decrease in chemical yields or enantioselectivity (89-99%, 88-99% ee).

The Role of Achiral Pyrazolidinone Templates in Enantioselective Diels−Alder Reactions: Scope, Limitations, and Conformational Insights

We have evaluated the role of achiral pyrazolidinone templates in conjunction with chiral Lewis acids in room temperature, enantioselective Diels-Alder cycloadditions. The role of the fluxional N(1) substituent was examined, with the bulky 1-naphthylmethyl group providing enantioselectivities up to 99% ee, while templates with smaller fluxional groups gave lower selectivities. High selectivities were also observed in reactions of 7d with chiral Lewis acids derived from relatively small chiral ligands, suggesting the pyrazolidinone templates are capable of relaying stereochemical information from the ligand to the reaction center. Lewis acids capable of adapting square planar geometries, such as Cu(OTf)2, Cu(ClO4)2, and Pd(ClO4)2, were found to be particularly effective at providing high selectivities. Additionally, substitution at the C-5 position of the pyrazolidinone templates has been shown to be critical for optimal selectivity. Reactions of the optimal pyrazolidinone appended with a number of common dienophiles and various dienes demonstrate the utility of this achiral template. Furthermore, catalytic loadings could be lowered to 2.5 mol % with essentially no loss in selectivity. Pi-Pi interactions were evaluated as a means to explain the unusually high selectivity observed at room temperature. Finally, non-C2-symmetric ligands were employed as a test to determine if chiral relay was operative.

Pyridinyloxazolidines: versatile scaffolds for chiral catalyst construction

Pyridinyloxazolidines are structurally versatile chiral ligand frameworks that are easily made and coordinated to Pd(II); the resulting complexes, which show significant steric and electronic differences from their oxazoline counterparts, as well as intermolecular H-bonding in the solid state, can be isolated in diastereomerically pure form and are remarkably resistant to hydrolysis.