Complexation, effects optical activity

A review8 with more than 186 references discusses the synthesis of Rh and Pd complexes with optically active P,N-bidentate ligands and their applications in homogeneous asymmetric catalysis. The effect of the nature of the P,N-bidentate compounds on the structure of the metal complexes and on enantioselectivity in catalysis was examined. Allylic substitution, cross-coup-ling, hydroboration and hydrosilylation catalyzed by Rh or Pd complexes with optically active P,N-bidentate ligands are considered. 

By the formation of complex tacticities. To identity the simplest of these stmctures the cyclic model reported earlier has been found quite useful (41). By analogy with c/ ro-inositol, 80, it was predicted that a polymer constituted of a succession of six homosubstituted tertiary atoms of the type. . . , R, S, S", S", S, R,. . . , 81, would be chiral. Until now this stmcture has neither been realized, nor have calculations been made to ascertain if it would be effectively optically active or simply crypto-chiral. 

M(A-A)3 complexes are optically active, and the problem can be remedied if a circular dichroism spectrum of one enantiomer can be measured and the sharp electronic lines identified. The bite and twist angles have very different effects on the rotational strengths. The twist angle has a marked effect that is quite plausible when one considers that a 60° twist causes conversion to the opposite enantiomer. Sign changes in the rotational strength can also occur at twist angles near / = 0°. 

At about the time of Wilkinson s discovery, new schemes were developed by others for the preparation and configurational correlation of chiral phosphines (lOa-e). The combination of advances in homogeneous catalysis and chiral phosphine technology prompted research on chiral phosphine complexes. Horner et al. (11) were the first to hypothesize in print that rhodium complexes containing optically active tertiary phosphine ligands should effect the asymmetric hydrogenation of unsymmetrically substituted olefins. 

Stereospecific Effects. Early studies of Co (III) complexes of optically active propylenediamine 62, 65, 66), 1,2-cyclopentanediamine 28, 29, 30, 31), and 1,2-cyclohexanediamine 32) indicated that one did not get all combinations of +) and (—) ligand configurations with the two configurations of chelate rings in the tris complexes. The composition of the cyclopentanediamine complexes reported by Jaeger have been found... 

XAES) shows that CuLj chelates (LH = glutamine or asparagine) have distorted octahedral structures. The oxidative decarboxylation product of Gly-Gly-L-His and Cu(OH)2 is four-co-ordinate and square planar decarboxylation occurs at C-5 with deprotonation at C-4 to give a C==C system. Amine adducts of bis(ben-zoyl-/5-alaninato)copper(ii) have been isolated and characterized. Equilibrium constants have been reported for copper(ii) complexes of histidylhistidine from pH-titration data. " Qualitative analysis of the Cotton effect of d-d transitions of copper(ii) complexes with optically active acids has been obtained. ... 

Most CD spectra have been studied to investigate various sources of optical activity such as distribution of chelate rings, conformation of chelate rings, vicinal effect due to asymmetric carbon in an optically active ligand and vicinal effect due to an asymmetric donor atom Much less studies have been done on complexes whose optical activity results from the arrangement of unidentate li nds b au such chiral complexes could not be synthesized. 

The c.d. spectra of chromophores of symmetry Cj, including trans-cyclo-octene, are discussed, and the sign of the Cotton effect has been related to the chirality of a series of Jt-complexes of optically active ligands, including trans-cyclo-octene, with tetracyanoethylene. Rotatory strengths for trans-... 

Chiral glyoxylates have been used to effect of/z o-hydroxyalkylation of phenols via coordinative complexes. In this way, optically active 2-hydroxymandehc esters have been obtained with up to 94% diastereoselectivity (36). 

In a catalytic asymmetric reaction, a small amount of an enantio-merically pure catalyst, either an enzyme or a synthetic, soluble transition metal complex, is used to produce large quantities of an optically active compound from a precursor that may be chiral or achiral. In recent years, synthetic chemists have developed numerous catalytic asymmetric reaction processes that transform prochiral substrates into chiral products with impressive margins of enantio-selectivity, feats that were once the exclusive domain of enzymes.56 These developments have had an enormous impact on academic and industrial organic synthesis. In the pharmaceutical industry, where there is a great emphasis on the production of enantiomeri-cally pure compounds, effective catalytic asymmetric reactions are particularly valuable because one molecule of an enantiomerically pure catalyst can, in principle, direct the stereoselective formation of millions of chiral product molecules. Such reactions are thus highly productive and economical, and, when applicable, they make the wasteful practice of racemate resolution obsolete. 

Utilization of the Pfeiffer effect and outer-sphere complexation for the prediction of absolute configurations of optically active metal complexes. S. Kirschner and I. Bakkar, Coord. Chem. Rev., 1982,43, 325-335 (27). 

For example, the racemic thioester 57 was placed in contact with a certain optically active amide. After 28 days the solution contained 89% of one enantiomer and 11% of the other. To effect the deracemization two conditions are necessary (1) the enantiomers must complex differently with... 

Optically active (—)-(8R)-methylcanadine was stereoselectively synthesized through selective monocomplexation of (—)-canadine (26) to chromium tricarbonyl (240). Heating of chromium hexacarbonyl with 26 effected regioselective complexation of the D ring to give the diastereomeric complexes, which were treated with n-butyllithium and trimethylsilyl chloride to give the 11-trimethylsilyl derivative 475 (Scheme 97). Methylation of this complex with methyl iodide gave stereoselectively the 8-methyl derivative 476 by preferential alkylation from the opposite face to the bulky chromium... 

The preparation of helically well-ordered polymers with stable screw-sense, which is able to be transmitted to newly formed polymer main-chains effectively, is highly desired for the development of new methodology for the synthesis of optically active helical polymers. An aromatizing polymerization of 1,2-diisocyanobenzenes is promoted by methylpalladium(II) complexes, producing poly(quinoxaline-2,3-diyl)s.146-148 The polymerization proceeds with successive insertion of the two isocyano groups of the diisocyanobenzene to the carbon palladium bond of... 

Optical activity in metal complexes may also arise either if one of the ligands bound to the metal in the first co-ordination sphere is itself optically active or if the complex as a whole lacks a centre of inversion and a plane of symmetry. Thus all octahedral cts-complexes of the tris-or bis-chelate type have two isomeric forms related by a mirror plane, the d- and /-forms. These species have circular dichroism spectra of identical intensities but opposite in sign. The bands in the circular dichroism spectrum are, of course, modified if ligand exchange occurs but they are also exceedingly sensitive to the environment beyond the first co-ordination sphere. This effect has been used to obtain association constants for ion-pair formation. There also exists the possibility that, if such compounds display anti-tumour activity, only one of the mirror isomers will be effective. 

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