Enatiomeric excess

Rhodococcus erythropolis NCIMB 11540 has been employed as biocatalyst for the conversion of (R)- or (.S )-cyanohydrins to the corresponding (R)- or (S)-a-hydroxycarboxylic acids with an optical purity of up to >99% enatiomeric excess (ee) [27-29] the chiral cyanohydrins can separately be produced using hydroxynitrile lyase from Hevea braziliensis or from Prunus anygdalis [30]. Using the combined NHase-amidase enzyme system of the Rhodococcus erythropolis NCIMB 11 540, the chiral cyanohydrins were first hydrolyzed to the... 

Wilson and Pasternak reported the first asymmetric Staudinger reaction (Scheme 4) where the chiral phosphane (7) reacts with the racemic azide (8) to afford diastereomeric iminophosphoranes in different quantities, giving after hydrolysis an amine in slight enatiomeric excess. Separation of the racemate seems to be kinetically controlled, but needs optimization (90MI1). 

Enantioselective reduction of oxime ethers promoted by chiral spiroborate esters (10) with an O3BN framework is reported. In the presence of (R,S)-10, aralkyloxime ethers are reduced by borane-THF at give (S)-l-aralkylamine in high yield and excellent enatiomeric excess (up to 98% ee). A possible mechanism (Scheme 13) of the catalytic reduction is suggested.310... 

Keywords Asymmetry Chemistry Chirality Enatiomeric excess Spin-polarized... 

In the asymmetric hydrosilylation of prochiral diaUcylketones and aryUcetones [98] Cesar et al. achieved excellent yields (90-99%) and enatiomeric excess (ee) values (88-91%) with arylalkylketones after abstraction of the halide substituent from the rhodium precatalyst using AgBF. ... 

There has been considerable academic and industrial effort to produce chiral, nonsteroidal anti-inflammatory agents such as Naproxen and Ibuprofen via the stereoselective hydroformyiation of vinyl aromatics. Such a process requires not only a high enatiomeric excess (i.e. optical yield) of the branched isomer but also a high branched to normal (b/n) regioselectivity ... 

When insertion of the coordinated prochiral olefin to a metal alkyl or a metal hydride takes place, the stereochemistry of the substituted carbon atom is determined as either R or S enantiomer. When the chiral alkyl group is reductively eliminated with the hydrido ligand, asymmetric hydrogenation of an olefin producing enatiomeric excess of one of the optical isomers can be achieved. 

In this study, Ochsner et al. also showed the positive effect of IL on the catalyst. Without any IL, no catalyst activity was observed and higher amounts of IL enhanced the catalyst activity, while effects to the enatiomeric excess were only small [21]. 

The use of aniline is also mandatory, as very low enatiomeric excess was obtained with aliphatic amines, most probably due to the problem of catalyst turnover in the later case. 

In 2002, Strukul et al. [18] reported the synthesis of a series of Pd(II) and Pt(II) cationic complexes of the types [(P-P)M-(solv)](Y)2, [(P-P)M(h2-Y)](Y), and [(P-P)M(m-Cl)]2(Y)2 (P-P = various diphosphines including chiral diphosphines M = Pd, Pt Y = TfO , Cl04, Bp4 ) (Figure 16.2). The variety of cationic complexes of Pd(II) (57) and Pt(II) (58) were found to be active catalysts for the Diels-Alder reaction between cyclopentadiene or cyclohexadiene and a number of simple dienophiles such as acrolein, methacrolein, and methylvinyl ketone. Some of the complexes modified with chiral diphosphines promoted enantioselective Diels-Alder reaction with moderate enatiomeric excesses up to 49% ee. 

Monoisopinocamphenylborane (2) gives excellent chiral selectivity during hydroboration of phenyl substituted tertiary alkenes. For example, hydroboration of 1-phenylcyclopentene with reagent made from (+)-a-pinene, followed by oxidation, gives the alcohol (3) in optically pure form (100% enatiomeric excess). ... 

Ghorai and co-workers reported on the synthesis of non-racemic amino aziridines 177 vmmemory of chirality (MOC) concept. The precursor 175 obtained from the addition of axially chiral enolate of amino acid ester to A7-sulfo-nyl imine following the concept of MOC. Reduction of 175 to the corresponding amino alcohol 176 followed by C—N cyclization produced the chiral aziridines 177 in good yields with excellent enatiomeric excess (Scheme 40.35). ... 

Monoisopinocampheylborane (IpcBH2) can be prepared in enantiomerically pure form by purification of a TMEDA adduct.158 When this monoalkylborane reacts with a prochiral alkene, one of the diastereomeric products is normally formed in excess and can be obtained in high enatiomeric purity by an appropriate separation.159 Oxidation of the borane then provides the corresponding alcohol in the same enantiomeric purity achieved for the borane. 

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