Asymmetric catalytic epoxidations of simple olefins
Zhang, Wei
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Permalink
https://hdl.handle.net/2142/20625
Description
Title
Asymmetric catalytic epoxidations of simple olefins
Author(s)
Zhang, Wei
Issue Date
1991
Doctoral Committee Chair(s)
Jacobsen, Eric N.
Department of Study
Chemistry
Discipline
Chemistry
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Chemistry, Organic
Language
eng
Abstract
Two types of chiral (salen)Mn(III) complexes, either cationic or neutral, were synthesized by complexation of the corresponding chiral salen ligands with manganese salt. Neutral Mn(III) salen complexes were prepared from the ligands and $\rm Mn(OAc)\sb2{\cdot}4H\sb2O$ via in situ aerial oxidation of the corresponding (salen)Mn(II) species. This approach circumvents the manipulation of highly air sensitive Mn(II) salen intermediates. The use of commercially available bleach solution as the oxidant greatly increases the practical value of this asymmetric epoxidation process.
High enantioselectivities in alkene epoxidation, especially for cis-disubstituted olefins, has been achieved through a rational sequence of ligand modifications. A mechanism involving side-on approach of alkene to a Mn(V)-oxo intermediate accounts for the sense and degree of asymmetric induction with various types of olefins, and it provides a guideline for improving catalyst design.
Electronic environment of the catalyst can dramatically affect the enantioselectivity of olefin epoxidation. Sterically non-biasing substituents were shown in the epoxidation of 2,2-dimethylchromene to effect a difference in $\Delta\Delta{\rm G}{\sp\ne}$ of 2.0 Kcal/mol, corresponding to a range in ee's from 22% to 96%. Increases in selectivity due to electron donating substituents are attributed to a more product-like transition structures in the ee-determining olefin oxidation step.
Different reaction mechanisms are proposed for alkyl-substituted and aryl-substituted alkenes. Epoxidation of alkyl-substituted olefins is very likely to be a concerted oxygen transfer process, while a stepwise reaction mechanism is indicated with aryl-substituted olefins.
A convenient and very inexpensive catalyst synthesis, a simple and practical epoxidation procedure, good catalyst turnovers, and high levels of asymmetric induction render the (salen)Mn(III) epoxidation system superior to any other catalytic or stoichiometric asymmetric processes for unfunctionalized olefins reported thus far. Widespread application of these catalysts may be foreseen.
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