Enantioreversal in the Sharpless Asymmetric Epoxidation Reaction Controlled by the Molecular Weight of a Covalently Appended Achiral Polymer

Polymers such as poly(ethylene glycol) (PEG) have proven use in a variety of applications including organic synthesis. We now disclose our investigations into the recently disputed report that PEG tartrate esters can reverse the enantioselectivity of the Sharpless asymmetric epoxidation reaction. Th...

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Bibliographic Details
Published in:Journal of organic chemistry 2005-03, Vol.70 (5), p.1728-1731
Main Authors: Reed, Neal N, Dickerson, Tobin J, Boldt, Grant E, Janda, Kim D
Format: Article
Language:English
Subjects:
Catalysis
Catalysts: preparations and properties
Chemistry
Epoxy Compounds - chemistry
Exact sciences and technology
General and physical chemistry
Molecular Structure
Molecular Weight
Oxidation-Reduction
Polyethylene Glycols - chemical synthesis
Polyethylene Glycols - chemistry
Stereoisomerism
Theory of reactions, general kinetics. Catalysis. Nomenclature, chemical documentation, computer chemistry
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Summary:Polymers such as poly(ethylene glycol) (PEG) have proven use in a variety of applications including organic synthesis. We now disclose our investigations into the recently disputed report that PEG tartrate esters can reverse the enantioselectivity of the Sharpless asymmetric epoxidation reaction. The results presented herein have clarified that the enantioselectivity of this reaction can be reproducibly reversed solely as a function of the molecular weight of the appended PEG. By preparing a range of tartrate ligands with varying PEG chains lengths, the reversal was found to occur within a molecular weight change of only 800. As the PEG chain did not affect the inherent chirality of the ligand, the enantioreversal was proposed to occur as a result of two Ti−ligand complexes which differ in their molecularity of ligand, one monomeric in ligand and the other dimeric. Support for this hypothesis was given through equilibrium measurements which revealed that the predominant species in Ti/PEG tartrate ester mixtures is a distinct 2:1 Ti−ligand complex, as opposed to the 2:2 Ti−ligand complex of traditional Sharpless asymmetric epoxidations. In total, these data represent an unrecognized property of PEG-supported catalysts that could open up new venues in the control of asymmetric reactions by means of achiral appended polymers.
ISSN:0022-3263
1520-6904
DOI:10.1021/jo048009x