ChemInform Abstract: Recent Aspects of Enantioselective Epoxidation Reactions (original) (raw)
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Enantioselective epoxidation of olefins with chiral metalloporphyrin catalysts
Chemical Society Reviews, 2005
A chiral gold(III) complex has been prepared that performs the epoxidation of olefins in the presence of O 2 , PhIO, or bleach. Catalytic experiments with 18 O show that O 2 is activated on the catalyst and can be directly incorporated into the epoxide through a non-radical mechanism that probably involves formation of gold, oxo, or peroxo species. In addition to this, there is a parallel radical mechanism operating that yields a, b-unsaturated ketones and alcohols as subproducts. Electrochemical and UV-Vis experiments confirmed the occurrence of a Au(III)/Au(I) redox cycle during the catalytic epoxidation in a mechanism sustained by molecular oxygen.
The Journal of Organic Chemistry, 1997
Three metal complexes of one particular homochiral porphyrin were investigated as catalysts for enantioselective epoxidation of unfunctionalized olefins under various reaction conditions. Much better results were obtained with the iron and ruthenium complexes than with the manganese derivative. The absence of any effect of amines on the iron porphyrin-catalyzed reaction in benzene, as well as the superior results in aromatic as opposed to both more and less polar nonaromatic solvents, suggest that specific association of aromatic molecules to the metalloporphyrin affects its solution structure. Strong evidence for the involvement of active oxidants that are more selective than trans-dioxoruthenium(VI) porphyrin is provided by the significant effect of primary oxidants on the ruthenium porphyrin-catalyzed reactions. Preliminary results with the iron complex of an only slightly modified porphyrin under the optimized reaction conditions found in this study resulted in epoxidation of styrene and 4-chlorostyrene to their epoxides with enantiomeric excesses identical to the best ever reported. These results were obtained with an unprecedented large number of catalytic turnovers, requiring only 0.01 mol % of catalyst.
Journal of Molecular Catalysis A: Chemical, 1996
The synthesis of Mn(III)-complexes of new chiral porphyrins la and lb prepared by condensation of enantiomerically pure [2.2]-p-cyclophane-4-carbaldehyde and pyrrole are described. These compounds were used as catalysts in epoxidation reactions of prochiral alkenes, carried out in the presence of aqueous NaOCl at pH = 10.0 as oxygen donor and small amounts of 4-tert-butylpyridine as axial ligand, in CH,Cl,/H,O two-phase conditions at 0°C. Results indicate a satisfactory catalytic efficiency (up to 700 overall turnovers), with enantiomeric excesses in the range 22-31%.
Journal of Inorganic Biochemistry, 2005
Counterions of manganese(III) porphyrin complexes influence diastereoselectivity in cis-stilbene epoxidation and product distribution in cyclohexene epoxidation markedly. In the epoxidation of cis-stilbene by iodosylbenzene carried out in a solvent mixture of CH 3 CN and CH 2 Cl 2 , trans-stilbene oxide is the major product in the reaction of manganese complexes bearing a ligating anion (i.e., Cl À), whereas cis-stilbene oxide is the dominant product in the reactions of manganese complexes bearing a poorly-ligating anion (i.e., CF 3 SO À 4). In cyclohexene epoxidation, the yields of allylic oxidation products such as cyclohexenol and cyclohexenone are higher when the counterion of the manganese catalysts is Cl À than when the counterion is CF 3 SO À 4. The product selectivities are also dependent on the nature of iodosylarenes and the axial and porphyrin ligands of the manganese porphyrin catalysts. The observation that product selectivities are different depending on the iodosylarenes may indicate the involvement of multiple oxidants in oxygen atom transfer reactions. These results are compared with those observed in manganese salen-catalyzed epoxidation of olefins by iodosylarenes.
Catalysis Science & Technology, 2014
New chiral manganese complexes of N 4 ligands derived from 2-acetylpyridine were prepared and used as catalysts in the enantioselective epoxidation of olefins, using H 2 O 2 as an oxidant to give epoxides, with excellent conversions (up to 99%) and enantiomeric excess (up to 88%) within 1 h at 0°C. A detailed mechanistic study was undertaken based on the information obtained by single crystal X-ray, optical rotation, UV-Vis, CD spectra and kinetic studies, and revealed that the reaction is first order with respect to the concentration of catalyst and oxidant and independent of substrate concentration. The complex (0.1 mol%) was successfully subjected to recyclability experiments over 3 cycles in the epoxidation of styrene with H 2 O 2 as an oxidant and acetic acid as an additive at 0°C with retention of performance.
Journal of Polymer Science Part A: Polymer Chemistry, 1997
Polymeric analogs of well-known chiral Mn(III)-salen complexes were synthesized and were used as recyclable catalysts for asymmetric epoxidation of olefins. For this purpose two different monomers, 2 and 3, bearing chiral Mn(III)-salen moieties were synthesized. The monomer 3 carries a bulky substituent closer to the Schiff base moiety, while monomer 2 lacks such a substituent. These metal complexed chiral monomers were subsequently copolymerized with ethylene glycol dimethacrylate producing insoluble crosslinked functional matrices that possess macroporous morphology. Chemo-and enantioselective catalytic activities of these two polymers were evaluated for epoxidation of olefins. Both polymers catalyzed the epoxidation of a variety of olefins at room temperature in the presence of iodosylbenzene (PhIO) as the terminal oxidant with yields comparable to the homogenous system. In terms of their enantioselective catalytic activity, polymer P-2 (obtained from 3) performed better than polymer P-1 (obtained from 2). Unfortunately, while the homogeneous systems are reported to offer over 80% enantioselectivity, with the present polymeric catalysts, enantioselectivity to a maximum of 30% were observed. Unlike the homogeneous system, use of an external nitrogenous donor played a very insignificant role in influencing enantioselectivity.
Asymmetric Epoxidation of Electron-Deficient Olefins
Current Organic Synthesis, 2008
This paper focuses on the latest developments in asymmetric epoxidation of electron-deficient olefins since the review by Porter and Skidmore on chiral ligand-metal peroxide systems, polyamino acid catalysed and organocatalysed epoxidations. Particular attention has been paid to the most recent advances using chiral pyrrolidines as organocatalysts.