Chelate complexes chiral

Among the commonest types of diketo complex are those with the stoichiometries M(dike)3 and M(dike)2. The former all have structures based on an octahedral disposition of the six oxygen atoms. The tris(chelate) molecules then actually have >3 symmetry and exist as enantiomers. When there are unsymmetrical diketo ligands (i.e., those with R R"), geometrical isomers also exist, as indicated in (11-XXVI). Such compounds have been of value in investigations of the mechanism of racemiza-tion of tris(chelate) complexes. Chiral diketonate complexes have found many... 

The [Co(phen)3]3+ complex is photoactive and a powerful oxidant in its excited state. The ion has no H-bonding groups and hence is considerably more hydrophobic1279 than hexaamine relatives. These properties have proven particularly useful. Aryl and alkyl substituted [Co(phen)3]3+ complexes have received a great deal of attention due to their ability to intercalate within the helical structure of DNA through a combination of electrostatic and hydrophobic forces. The chirality of the tris-chelate complex is crucial in determining the degree of association between the complex and... 

Reaction of the bis-chelate complex 149 and various bis(arylalkyl)barium complexes generates heteroleptic barium complexes with one chelate and one reactive arylalkyl ligand 164. The homoleptic and heteroleptic barium complexes both induce living polymerization of styrene to atactic polystyrene in cyclohexane solution. The fact that no stereocontrol is observed during polymerization despite the presence of the chiral carbanionic ligands is... 

Brunsveld, L., Chiral Discotic Molecules Expression and Amplification of Chirality, 24, 373. Buckingham, D. A., Conformational Analysis and Configurational Effects for Chelate Complexes, 6, 219. 

Chelate Complexes, Absolute Stereochemistry of (Saito) Chirality Due to the Presence of Hydrogen Isotopes at Noncyclic 10 95... 

Separation is based on the reversible chelate-complex formation between the chiral selector covalently bonded to the chromatographic support, and the chiral solute with transition metal cations. Chelation properties of both the chiral selector and the chiral solute are required. Compounds therefore need to have two polar functional groups in a favorable arrangement to each other, like a )3-amino acids, amino alcohols and a-hydroxy acids, which can form rings membered with central chelating metal ions, like Cu(II), Zn(II), Cyclic... 

The wide diversity of topologies that can today be found for NHC-precursors, together with the different efficient metallation strategies [6], have provided a large set of Ir—NHC complexes among which monodentate, bis-chelate and chiral... 

Based on preliminary results from Helfferich130, further developments by Davankov and co-workers5 131 133 turned the principle of chelation into a powerful chiral chromatographic method by the introduction of chiral-complex-forming synlhetie resins. The technique is based on the reversible chelate complex formation of the chiral selector and the selectand (analyte) molecules with transient metal cations. The technical term is chiral ligand exchange chromatography (CLEC) reliable and complete LC separation of enantiomers of free a-amino acids and other classes of chiral compounds was made as early as 1968 131. 

Figure 21, Proposed model of adsorbed chiral selector (A-alkylproline)- Cu(U)-[free amino acid] mixed chelate complex, The lipophilized proline selector is held in position via intercalation of the alkyl chain. Case A the alkyl part of the mixed chelate complex is fixed by hydrophobic interactions with stationary phase (RP-J). Case B the complex formation is stabilized by other types of hydrophobic attraction. Chiral recognition and elution order is therefore not only dependent on the simple and isolatedly viewed chelate complex stability. In general, retention and chiral recognition in chiral LC is based on mixed-mode adsorption/dcsorption processes which act synergisticallv and also antagonistically with respect to the observed chiral resolution and intermolecular complex formation.

In late 1975, Enders et al.156) started a research project directed towards the development of a new synthetic method for asymmetric carbon-carbon bond formation. A new chiral auxiliary, namely the (S)-proline derivative SAMP (137), was allowed to react with aldehydes and ketones to give the hydrazones (138), which can be alkylated in the a-position in an diastereoselective manner 157,158). Lithiation 159) of the SAMP hydrazones (138), which are formed in excellent yields, leads to chelate complexes of known configuration 160). Upon treatment of the chelate complexes with alkyl halogenides the new hydrazones (139) are formed. Cleavage of the product hydrazones (139) leads to 2-alkylated carbonyl compounds (140). 

SCHEME 8. Schematic representation for the chelate-controlled addition of an organometaUic reagent (M—R) to the carbonyl group of a chiral a-alkoxy carbonyl compound (17). Two diastere-omers 18 with different orientation of R with respect to CH2R can be obtained. The syn diastereomer is obtained when the nucleophihc attack of R takes place on the same face of the plane, defined by the carbonyl group and the R-substituted carbon atom, where CH2R is located in the chelate complex... 

It was mentioned above that tris(chelate) complexes of the type (Co(en) ],+ lack an improper axis of rotation. As a result, such complexes can exist in either of two enantiomeric forms (or a racemic mixtire of the two). Figure 12.20 illustrates the complex ions (Co(en)j]3+ and (Crfoxy3-. each of which ts chiral with Di symmetry. 

In early studies of these reactions, the turnover efficiency was not always high, and stoichiometric amounts of the promoters were often necessary to obtain reasonable chemical yields (Scheme 105) (256). This problem was first solved by using chiral alkoxy Ti(IV) complexes and molecular sieves 4A for reaction between the structurally elaborated a,/3-unsaturated acid derivatives and 1,3-dienes (257). Use of alkylated benzenes as solvents might be helpiul. The A1 complex formed from tri-methylaluminum and a C2 chiral 1,2-bis-sulfonamide has proven to be an extremely efficient catalyst for this type of reaction (258). This cycloaddition is useful for preparing optically active prostaglandin intermediates. Cationic bis(oxazoline)-Fe(III) catalysts that form octahedral chelate complexes with dienophiles promote enantioselective reaction with cyclopentadiene (259). The Mg complexes are equally effective. 

Chelate complexes with two ethylenediamine rings in a cis configuration lack a plane of symmetry and thus have the potential to be separated into enantiomeric (A, A) (12) forms. Inert cis-bis(en) complexes of Co111,247 Crmn or Rh111248 can be resolved by the method of racemic modification 249 or using chromatographic techniques,35 but labile systems, such as Ni(en)2+, which occasionally crystallize in one chiral form,220 rapidly racemize in solution. 

Simple alkylation of the chiral chelate complex leads to formation of chiral dialkylacetic acids (Scheme 109).3S5 388 Simpler chiral enamines can also be used. The formation of chiral propanoic acids results from a resolution of racemic alkyl halides by the interaction of a chiral lithiooxazoline, which recognizes and reacts with one enantiomer at the expense of the other (Scheme 110).389 The above aspects of the asymmetric carbon—carbon bond formation from chiral oxazolines have been reviewed by Meyers.390... 

Upon complexation with a lanthanide ion, these complexes may form square antiprism or twisted square antiprism (TS APR) structures with a vacant coordination site in the cap position, which is assumed to be occupied by a solvent molecule. Just as in the chelated complexes described previously, two distinct types of chiral stereochemistry are present. In analogy with OC-6 species, the sense of rotation of the pendant arms is denoted as A or A depending upon if the arms rotate clockwise (A) or counterclockwise (A) as one proceeds down the direction of the C4 axis. There is also chirality (or helicity) associated with the nonplanar 12-membered ring. If one looks along the skew-line connecting the coordinated nitrogens, the carbon atoms... 

Optical activity in a molecule can occur when and only when it contains a chiral centre. This means that the molecule can exist in two diastereomeric forms, which are non-superimposable. Six-coordinate chelate complexes of the type M(bidentate ligand)3 (Fig. 3.1(a)) and ds-M(bidentate ligand)2X2 (Fig 3.1(b)) are common examples in metallic complexes. 

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