Chiral complexing agent

Alkyl halides or alkyl sulfates, treated with the salts of sulfinic acids, give sulfones. A palladium catalyzed reaction with a chiral complexing agent led to sulfones with modest asymmetric induction. Alkyl sulfinates (R SO—OR) may be side products. Sulfonic acids themselves can be used, if DBU (p. 1337) is... 

An interesting intramolecular radical cyclization followed by enantioselective hydrogen atom transfer has recently been reported (Scheme 11) [40]. This reaction is carried out in the presence of a chiral complexing agent 38, which... 

The first resolution of [6]-helicene was achieved by Newman and Lednicer 78) by crystallization with the aid of a chiral complexing agent, 2-(2,4,5,7-tetranitro-9-fluorenylidene aminooxy)propionic acid (TAPA), which was especially designed for this purpose. (R)- and (S)-TAPA (79 a) form diastereomeric charge-transfer (CT) complexes with the enantiomers of hexahelicene. Several other helicenes could also be resolved using this reagent. 

Attempts to achieve absolute stereocontrol by means of chiral sensitizers or chiral complexing agents [46] have seen little success with cyclohexenones and other unfunctionalized enone substrates. Ester 26, for example, underwent an... 

The chiral templates that will be dicussed within this review are classified into four categories chiral solvents that provide a chiral environment without, specific binding (Sec. II), chiral Brpnsted acids that act as proton donor in the enantiodifferentiating step (Sec. Ill), chiral transition metal complexes that catalyze the enantiodifferentiating step (Sec. IV), and chiral complexing agents that accomplish face differentiation in a specific complex without chemically converting the substrate (Sec. V). 

The applicability of the developed chiral complexing agents was further demonstrated by asymmetrically directing [2 + 2]- and [4 + 4]-photocycloaddition... 

Taking into account the basic considerations outlined in the previous section, we reasoned that a chiral complexing agent, which needs to be applicable in photochemistry, requires a general binding motif. In addition, it appeared necessary to employ directed interactions in such a template rather than an unspecific complexation as encountered in chiral solvents or larger supramolecular entities. Hydrogen bonds appealed... 

While the chiral complexing agents (+)-12 and (-)-12 proved to be generally suitable for a wide range of enantioselective [2+2]-photocycloaddition reactions on the c-bond of 2(l//)-quinolones, their applicabil-... 

Radical reactions have been recognised only recently for the construction of enantiomerically pure compounds (Renaud and Sibi 2001 Zimmerman and Sibi 2006). In addition to substrate- or auxiliary-induced diastereoselective radical reactions, and in addition to the use of chiral Lewis acids, chiral hydrogen atom donors or chiral transition metal complexes, template molecules can be used to generate a chiral environment and induce chirality to the substrate. With the chiral complexing agent 12, enantioselective radical reactions were achieved with enantiomeric excesses up to 99% ee. 

Radical reactions of substrate 27 were initially conducted at low temperature in toluene with triethyl borane as initiator and tributylstannane as reducing agent. Under these conditions no conversion was detected. Increasing the temperature to ambient temperature led to 99% yield and a diastereomeric ratio (d.r.) of 47/53 in favour of the cis compound cis-29. Reactions in the presence of the chiral complexing agent... 

Bidentate base.1 The chiral lithium amide of (S)-l has been used to generate the anion of the ethyl o-toluate 2 and as a chiral complexing agent in reaction of the anion with acetaldehyde to give optically active mellein methyl ether (3) in 53% optical yield. Optical yields are markedly lower when a chiral base similar to 1 but lacking the OCH3 group is used. 

Facial differences in proton reactivity can lead to enantioselective deprotonation. A more common way to achieve enantioselective deprotonation is to use a chiral base and/or a chiral complexing agent. Enantioselective deprotonation in cyclic ketones has been studied.Enantioselective deprotonation with heterodimer bases has been studied. ... 

When the absolute structure has been determined, the result must be correlated with some physical property of the crystal, otherwise the result has no use to the chemist. The obvious correlation is with the direction of rotation of the plane of plane-polarized light, that is, whether the compound or crystal is dextrorotatory or levorotatory. Another correlation can be made with crystal appearance this was shown for zinc blende with its matte and shiny faces, and for silica and sodium ammonium tartrate crystals for the disposition of their hemihedral faces. If such data are not available, it may be necessary to list physical properties of diastereomers made with chiral complexing agents. Then, whenever this same compound is encountered by a chemist, its absolute structure is well known. 

Chiral separation using ligand-exchange chromatography involves the reversible complexation of metal ions and chiral complexing agents. The central ion, usually or forms a bis complex with bidentates ligands. 

Chiral separation in capillary electrophoresis is usually achieved by the addition of chiral complexing agents to form in situ diastereometric complexes between the enantiomers and the chiral complexing... 

Scheme 8 Photocyclization in the presence of a chiral complexing agent...

For a review of the early literature on the stereoselective reactions of chiral aldehydes, ketones, and a-keto esters, and also of the addition of Grignards and organolithiums to achiral ketones and aldehydes in the presence of a chiral complexing agent or chiral solvent, see ref. [4]. 

---