Diethylzinc chiral modified

Enders and coworkers <96AG(E)1725> have developed an interesting general one-pot method for the asymmetric epoxidation of enones with oxygen in the presence of diethylzinc and (l ,/ )-N-methylpseudoephedrine (30), which provides a, P - epoxyketones in very high yield and high enantiomeric excess (e.g., 33 —> 34). The actual reactive species is believed to be the chirally modified alkoxy(ethylperoxy)zinc 31, which attacks the si face of the s-cis conformation of the (E) enones (cf. 32). 

Further optimization of this reaction was carried out with TFE as an achiral adduct, since reaction with TFE is much faster than that with neopentyl alcohol. We found that dimethyl- and diethylzinc were equally effective, and the chiral zinc reagent could be prepared by mixing the chiral modifier, the achiral alcohol and dialkylzinc reagent in any order without affecting the conversion and selectivity of the reaction. However, the ratio of chiral to achiral modifier does affect the efficiency of the reaction. Less than 1 equiv of the chiral modifier lowered the ee %. For example with 0.8 equiv of 46 the enantiomeric excess of 53 was only 58.8% but with 1 equiv of 46 it was increased to 95.6%. Reaction temperature has a little effect on the enantiomeric excess. Reactions with zinc alkoxide derived for 46 and TFE gave 53 with 99.2% ee at 0°C and 94.0% ee at 40°C. 

Asymmetric epoxidation using chiral modified diethylzinc. 61... 

Enantioselective alkylations have been achieved using modified MCM-41 materials.135 Mesoporous templated MCM-41s with covalently linked chiral ephedrine are active heterogeneous chiral auxiliaries in the enantioslective alkylation of benzaldehyde by diethylzinc. Lower rates, selectivities and enantioselectivities are obtained under heterogeneous conditions compared to homogeneous catalysis. This can be explained either by the participation of the uncovered surface to the racemic alkyl transfer or by a restricted accessibility to the catalytic sites in the heterogeneous reactions. 

Cho, B.T. and Chun, Y.S., Catalytic enantioselective reactions. Part 12. Enantioselective addition of diethylzinc to aldehydes catalyzed by zinc complexes modified with chiral P-sulfonamido alcohols. Synthetic Commun., 29, 521,1999. 

In the same year, Roland and coworkers described a chiral silver-NHC complex with a tert-butyl substituted backbone for copper-catalyzed addition of Et2Zn to 2-cyclohexanone. However, the addition product was isolated in low enantioselectivity (23% ee) [77]. Later on, Alexakis et al. modified the carbene structure to improve the enantioselectivity (Scheme 3.51) [78]. By using the chiral silver-NHC salt 90 to transmetallate and generate the Cu catalyst, the asymmetric conjugate addition of diethylzinc to 2-cycloheptanone was achieved in good yield (95%) and enantioselectivity (93% ee). 

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