Insights into base-free OsO4-catalyzed aminohydroxylations employing chiral ligands

Attempts to perform the OsO4-catalyzed enantioselective base-free aminohydroxylation of β,β-disubstituted enoates are described. Low yields and racemic products were obtained in the presence of standard chiral ligands, suggesting the occurrence of a “Second Cycle” process due to slow hydrolysis of t...

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Bibliographic Details
Published in:Tetrahedron 2019-02, Vol.75 (8), p.945-948
Main Authors: Cardon, Joseph M., Coombs, James C., Ess, Daniel H., Castle, Steven L.
Format: Article
Language:English
Subjects:
Aminohydroxylation
Density functional theory
Enantioselective reaction
Osmium tetroxide
β-Hydroxy amino acids
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Summary:Attempts to perform the OsO4-catalyzed enantioselective base-free aminohydroxylation of β,β-disubstituted enoates are described. Low yields and racemic products were obtained in the presence of standard chiral ligands, suggesting the occurrence of a “Second Cycle” process due to slow hydrolysis of the amino alcohol product from the Os metal center. Support for this hypothesis was provided by the slightly improved enantioselectivity (60:40 er) obtained with an amino alcohol ligand. Based on density functional theory calculations, it is proposed that the lack of significant enantioselectivity is due to a low-energy (3 + 2) oxo/imido cycloaddition transition state without the chiral ligand in the Second Cycle that outcompetes protonolysis in the First Cycle. [Display omitted]
ISSN:0040-4020
1464-5416
DOI:10.1016/j.tet.2019.01.018