Catalysts chirality

A closely related asymmetric synthesis of chiral sulphoxides, which involves a direct oxidation of the parent sulphides by t-butylhydroperoxide in the presence of metal catalyst and diethyl tartrate, was also reported by Modena and Di Furia and their coworkers-28-7,288 jjje effect 0f the reaction parameters such as metal catalyst, chiral tartrate and solvent on the optical yield does not follow a simple pattern. Generally, the highest optical purities (up to 88%) were observed when reactions were carried out using Ti(OPr-i)4 as a metal catalyst in 1,2-dichloroethane. 

This regioselectivity is opposite to the one observed by the non-catalysed additions of BH3 THF or 9-BBN to the same alkene, or those catalysed by Rh and Ir catalysts. Chiral NHC ligands (generated from 84) on Cu under the same conditions proceed with high enantioselectivity (enantiomeric ratio 99 1) [71] (Scheme 2.12). 

Effect of the Oxidizing Agent and Catalyst Chirality on the Diastereoselective Epoxidation of... 

The steric factor results from physical interactions between atoms. From this factor comes the basic chirality. For the cases of solid metal catalysts, chirality may come from the support, the arrangement of active sites on the metal surface (Ogston concept1), or from an adsorbed or attached chiral entity. 

Dimethylchromene has also proven to be a useful substrate for the assessment of various transition metal complexes as epoxidation catalysts. Chiral Mn(III)-salen complexes are efficient <00CC615 00T417> and can be recycled when used in an ionic liquid <00CC837>. The enantioselective aziridination of a chromene has been achieved using a chiral biaryldiamine-derived catalyst (Scheme 22) <00JA7132>. 

The ligand synthesis requires only two steps from simple starting materials. As with the PHOX type catalysts, chirality is built in through the use of a chiral amino alcohol (Scheme 29.5). 

Year Substrate Catalyst Chiral auxiliary3 ee [%i Comment Refer- ence... 

Theoretical calculations proved that the reaction intermediate leading to R-ethyl lactate on cinchonidine-modified Pt(lll) is energetically more stable than the intermediate leading to the S-ethyl lactate [147], However, the catalytic system is complex and the formation and breaking of intermediates are transient, so it is certainly difficult to obtain direct information spectroscopically. It is therefore advisable to use simplified model systems and investigate each possible pairwise interaction among reactants, products, catalyst, chiral modifier, and solvent separately [147, 148]. In order to constitute these model systems, it is important to get initial inputs from specific catalytic phenomena. 

Only a few cases of nonlinear effects have been reported some are listed in Table 7.2. Most of the examples involve proline as the catalyst. Chiral phosphora-mides and some phase transfer catalysts were also reported to give NLEs. 

There has been great interest in the area of chiral acid catalysts in organic synthesis over the past few decades. This topic has been the subject of several previous reviews. For example, the book Lewis Acids in Organic Synthesis (edited by Hisashi Yamamoto) was published by Wiley-VCH in 2000. In this chapter, successful and significant chiral Brpnsted acid catalysts, chiral Lewis acid catalysts [typical Lewis acidic elements main group elements, B(III) and Al(III), and early transition metal, Ti(IV)], and Lewis acid-assisted chiral Brpnsted acid catalysts developed after 2000 are discussed. Chiral acid/base catalysts wdl be discussed in Chapter 13 by Shibasaki and Kanai. 

The hypothesis of stereochemical control linked to catalyst chirality was recently confirmed by Ewen (410) who used a soluble chiral catalyst of known configuration. Ethylenebis(l-indenyl)titanium dichloride exists in two diaste-reoisomeric forms with (meso, 103) and C2 (104) symmetry, both active as catalysts in the presence of methylalumoxanes and trimethylaluminum. Polymerization was carried out with a mixture of the two isomers in a 44/56 ratio. The polymer consists of two fractions, their formation being ascribed to the two catalysts a pentane-soluble fraction, which is atactic and derives from the meso catalyst, and an insoluble crystalline fraction, obtained from the racemic catalyst, which is isotactic and contains a defect distribution analogous to that observed in conventional polypropylenes obtained with heterogeneous catalysts. The failure of the meso catalyst in controlling the polymer stereochemistry was attributed to its mirror symmetry in its turn, the racemic compound is able to exert an asymmetric induction on the growing chains due to its intrinsic chirality. 

If an asymmetric hydrogenation of C=C bonds is desired in the presence of achiral catalysts, chiral information is required to be present in the substrate. Peptides and cyclopeptides containing dehydroaminos acid units are very good substrates achieving quite high stereoselectivities upon asymmetric hydogenation on 10% Pd-C or other achiral catalysts 49 841. 

Asymmetric epoxidation with hydrogen peroxide as the oxidizer promoted by chiral phase-transfer catalysts (chiral PTCs, Figure 6.7) can be performed under mild... 

Molecular triangle lOd contains chiral dihydroxy functionalities and has been used for highly enantioselective catalytic diethylzinc additions to aromatic aldehydes, affording chiral secondary alcohols upon hydrolytic work-up, as shown in Eq. (4.2) (Table 4.2) [22]. With Ti(IV) complexes of lOd as catalyst, chiral secondary alcohols were obtained in >95% yield and 89-92% ee for a wide range of aromatic aldehydes with varying steric demands and electronic properties (Table 4.2). In comparison, when the free ligand 6,6 -dichloro-4,4 -diethynyl-2,2 -binaphthol was used instead of... 

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