Chromium, asymmetric epoxidation

Apart from the commonly used NaOCl, urea—H2O2 has been used/ With this reaction, simple alkenes can be epoxi-dized with high enantioselectivity. The mechanism of this reaction has been examined.Radical intermediates have been suggested for this reaction, polymer-bound Mn -salen complex, in conjunction with NaOCl, has been used for asymmetric epoxidation. Chromium-salen complexes and ruthenium-salen complexes have been used for epoxidation. Manganese porphyrin complexes have also been used. Cobalt complexes give similar results. A related epoxidation reaction used an iron complex with molecular oxygen and isopropanal. Nonracemic epoxides can be prepared from racemic epoxides with salen-cobalt(II) catalysts following a modified procedure for kinetic resolution. 

N. J. Kerrigan, H. Muller-Bunz, D. G. Gilheany, Salen ligands derived from trans-l,2-dimethyl-1,2-cycIohexanediamine preparation and application in oxo-chromium salen mediated asymmetric epoxidation of alkenes, J. Mol. Catal. A Chem. 227 (2005) 163. 

EPR Spectroscopic Study of the Intermediates of (SALEN)CHROMiUM(lll) Catalyzed Asymmetric Epoxidation of Olefins... 

Related epoxidations of olefins with PhIO in the presence of Salen and related complexes of chromium(III), manganese(III) and cobalt(III) have been reported by Kochi and coworkers [58]. The use of nickel(II) Salen in conjunction with NaOCl was also described [59]. More recently, these systems formed the basis for the development, by Jacobsen and coworkers [60], of chiral manganese(III) Salen complexes for the enantioselective epoxidation of prochiral olefins by ArlO or NaOCl. Similarly, asymmetric epoxidations with moderate to good... 

The analogous chromium complex was used in the asymmetric ring opening of meso epoxides with trimethylsilyl azide [15] (Scheme 4). In this case a strong dependence on the anion of the ionic hquid was observed. Anions leading to hydrophobic ionic hquids, such as PFe" and SbFe", led to high... 

Bruns and Haufe have described the first examples of a transition metal complex mediated asymmetric ring opening (ARO) of both meso- and racemic epoxides via formal hydro-fluorination [23]. Initial attempts with chiral Euln complexes led to very low asymmetric induction. Opening of cyclohexene oxide 30 with potassium hydrogendifluoride in the presence of 18-crown-6 and a stoichiometric amount of Jacobsens chiral chromium salen complex 29 [24a] finally yielded two products 31 and 32 in a 89 11 ratio and 92% combined yield, the desired product 31 being formed with 55% ee. Limiting 29 to a catalytic amount of 10 mol% led to an increase in the ratio of 31, however, with the enantiomeric excess dropping to 11% (Scheme 5). 

Scheme 22. Asymmetric amplification in the opening of meso epoxides with TMSN3 catalyzed by a chiral chromium-salen complex.66...

The synthesis is a catalogue of modern asymmetric catalytic methods. The epoxide 25 was resolved by a hydrolytic kinetic resolution (chapter 28) using a synthetic asymmetric cobalt complex. The asymmetric Diels-Alder reaction (chapter 26) was catalysed by a synthetic chromium... 

Besides biomimetic complexes, Jacobsen described particularly efficient bis (chromium-salen) catalyst 9 for the asymmetric ring-opening reaction of epoxides with azide (Scheme 9) [42]. The efficiency of this class of catalysts is attributed to a cooperative mechanism, both substrates being activated toward each other by their respective chromium atom. Of note, a less pronounced cooperative effect was initially demonstrated in an intermolecular manner using monomeric Cr(N3)-salen catalyst [43]. Jacobsen also showed that an analogous cooperative mechanism takes place using polymer-supported chiral Co(salen) complexes for the hydrolytic kinetic resolution of terminal epoxides [44, 45]. 

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