Tartrate-like Ligands in the Asymmetric Epoxidation

Article abstract of DOI:10.1021/jo00273a010

Three "tartrate-like" ligands, (3R,4R)-diisopropyl 3,4-dihydroxyadipate (3), (2S,4S)-diisopropyl 2,4-dihydroxyglutarate (4), and (2S,5S)-diisopropyl 2,5-dihydroxyadipate (5) were synthesized with the aim of gaining further insight into the nature and composition of the asymmetric epoxidation catalyst.Both 3 and 4 appear able to from and maintain a dimetric (2:2) Ti-ligant complex analogous to Ti-tartrate in the presence of competitive alcohol binders, but 5 seems to give a mixture of compexes.In reactivity screens, Ti-3 is a very sluggish and nonselective epoxidation catalyst; Ti-4 mimics Ti-tartrate in the epoxidation of primary allylic alcohols (more than 93percent ee), but is ineffective as a kinetic resolution catalyst; Ti-5 is only weakly selective, implying the presence of more than one active epoxidation catalyst.The relationships between catalyst intramolecular fluxional equilibration, torsional ring strain, alkoxide exchange rate, reaction rate, and face selectivity are explored.