Enantioselective reactions high enantioselectivity

Many chiral metal complexes with Lewis acid properties have been developed and applied to the asymmetric Diels-Alder reaction. High enantioselectivity is, of course, one of the goals in the development of these catalysts. Enantioselectivity is not, however, the only factor important in their design. Other important considerations are ... 

In the same context, these authors have also studied the catalytic activity of the p-amino thiol and its corresponding disulfide depicted in Scheme 3.8 for similar reactions.High levels of enantioselectivity were obtained for both these ligands but, however, in the case of the disulfide ligand the reaction rate was slower than that observed with the corresponding thiol, while the disulfide proved to be more chemically stable under the reaction conditions. 

Bandini, M. Cozzi, P. G. Melchiorre, P. Umani-Ronchi, A. (2004) Kinetie resolution of epoxides by a C-C bond-forming reaction Highly enantioselective addition of indoles to cis, trans, and meso aromatic epoxides eatalyzed by [Cr(salen)] eomplexes., Angew. Chem. Int. Ed., 43 84-87. 

Cycloheptadiene (340) is obtained by the Cope rearrangement of cis-divinylcyclopropane (339.) Based on this reaction, highly diastereoselective and enantioselective construction of the 1,4-cycloheptadiene 343 (98% ee) was achieved by domino asymmetric cyclopropanation to generate cA-divinylcyclopropane... 

Chiral silver complexes bearing bidentate NHC ligands (24) have been synthesized. They are used in alkene metathesis and allylic alkylation reactions high diastereos- (g) electivity is observed induced by the chiral backbone on the prochiral biphenyl.27 Ruthenium-based complexes obtained from transmetalation with a Grubbs-Hoveyda complex exhibited high activities and enantioselectivities in ring-opening metathesis/ ... 

Enantioselective Diels-Alder reaction. Highly stereoselective Diels-Alder reactions can be achieved by use of the 4,4 -diphenylbis(oxazoline) 2b, prepared from (+)-phenylglycinol, as a chiral, bidentate ligand for iron salts. Thus reaction of Fel3 with 2b and I2 in CH3CN forms a complex presumed to be I-Fel3, which can catalyze reaction of 3-acryloyl-l,3-oxazolidin-2-one with cyclopentadiene at —50° to give the endo-adduct in 95% yield. The product is the 2R-enantiomer (82% ee). 

Lu, X. Zhang, Q. Effect of Ligands on the Divalent Palladium-Catalyzed Carbon-Carbon Coupling Reactions. Highly Enantioselective Synthesis of Optically Active y-Butyrolactones, Pure Appl. Chem. 2001, 73, 247-250. 

Paterson et have prepared the enolate of 3-pentanone, an achiral ketone, with (-( )- or (-)-IpcaBOTf and have found that its aldol reactions with various aldehydes proceed with high syn.anti ratios (>9 1) and respectable enantioselectivities (5 1-20 1) (Scheme 44). High degrees of asymmetric induction are noted with unhindered aldehydes, llie combination of the chiral ethyl ketone (104) and (-t-)-Ipc2BOTf constitutes a matched pair, which enhances the diastereofacial selectivity of the resulting enolate (compared to that obtained with an achiral boron reagent), and provides via aldol reactions high... 

Hetero-Diels-Alder reaction. High enantioselectivity is observed for this process using chiral cationic Pd(II) complexes. ... 

The final definition concerns formation of enantiomers, and the terms enantioselective and enantiospecific are used. If a reaction produces an unequal mixture of enantiomers it is enantioselective. If it generates only one enantiomer of two possibilities, it is enantiospecific. The baker s yeast reduction (see sec. 4.10.F) of 134 gave 135 with >99% ee (S). (Here % ee means percent of enantiomeric excess.) A 0% ee means a 50 50 mixture (racemic mixture), 50% ee means a 75 25 mixture and 90% ee means a 95 5 mixture. The predominance of the (S) enantiomer makes this reaction highly enantioselective. 

Amongst the other metal catalysts employed in this reaction, high levels of enantioselectivity in the addition of aromatic thiols to conjugated oxazolidinones such as (11.37) have been achieved using the nickel complex of DBFOX ligand (11.77), which functions as a Lewis acid activator. 

Various chiral ligands or counter-anions, or complexes of other than copper transition metals, have been evaluated in these reactions. High enantioselectivity in the copper-catalyzed aziridination of styrene derivatives was observed in the presence of chiral biaryldiamines [782], chiral C2-symmetric bisferrocenyldiamines... 

The progress of any given lipase-catalysed reaction is monitored using two main indicators. The enantiomeric excess of either the starting material or product gives a measure of the enantioselectivity of the reaction. High enantioselectivity... 

Cooperative catalysis in multi-component reactions highly enantioselective synthesis of y-hydroxyketones with a quaternary carbon stereocenter, (b) X. Han, M. Gan, H. Qin, J. Ji, X. Zhang, L. Jiang, W. Hu, Synlett 2011, 1717-1722. Trapping of oxonium ylides with Michael acceptors highly diastereoselective three-component reactions of diazo compounds with alcohols and ben-zylidene Meldrum s acids/4-oxo-enoates. 

The hydride tram to the phosphine migrates to the olefin and this tram destabilisation probably contributes to the rapid rate of reaction. High enantioselectivity (<90%) has been restricted to hydrogenation of prochiral o-acetylaminocinnaniic derivatives and a variety of chiral phosphines have been used. The highest enantioselectivity was obtained with the dppe-analogues (-)DIPAMP (7) and (S,S)-Chiraphos (8)... 

Axtell AT, Klosin J, Abboud KA. Evaluation of asymmetric hydrogenation ligands in asymmetric hydroformylation reactions. Highly enantioselective ligands based on bis-phosphacycles. Organometallics 2006 25 5003-5009. 

Here we will illustrate the method using a single example. The aldol reaction between an enol boronate and an aldehyde can lead to four possible stereoisomers (Figure 11.32). Many of these reactions proceed with a high degree of diastereoselectivity (i.e. syn anti) and/or enantioselectivity (syn-l syn-Tl and anti-l anti-lT). Bernardi, Capelli, Gennari,... 

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