Ligand optical activity

A molecule is optically active when it cannot be superimposed on its mirror image. Although this condition is met by an octahedral complex such as MLaLbLgLdLgLf it is rare indeed to be able to resolve such a complex. In practice, optical activity is largely confined to octahedral complexes of chelating ligands. Optical activity has also been observed for chelated tetrahedral and square planar complexes but only rarely. It is necessary for the chelated complex to be stable kinetically to permit resolution, it must retain its configuration for at least a matter of minutes. This confines attention to complexes of a few ions, of which cobalt(III), chromium(III)... 

Phosphine oxides are prepared similarly[644]. Selective monophosphiny-lation of 2,2 -bis[(lrifluoromethanesulfonyl)oxy]-l,1 -binaphthyl (784) with diphenylphosphine oxide using dppb or dppp as a ligand takes place to give optically active 2-(diarylphosphino)-1,1 -binaphthyl (785). No bis-substitution is observed[645,646]. 

Efficient enantioselective asymmetric hydrogenation of prochiral ketones and olefins has been accompHshed under mild reaction conditions at low (0.01— 0.001 mol %) catalyst concentrations using rhodium catalysts containing chiral ligands (140,141). Practical synthesis of several optically active natural... 

The chemistry of complexes having achiral ligands is based solely on the geometrical arrangement on titanium. Optically active alcohols are the most favored monodentate ligands. Cyclopentadienyl is also well suited for chiral modification of titanium complexes. 

Synthesis of novel optically active cyclic phosphine ligands and their use in catalytic asymmetric reactions 98YGK511. 

Lipophilic ligands, e.g. 45-49 having chiral L-2-pyrrolidine-methanol moieties, have been prepared and examined for their catalytic activities in reactions with optically active esters (50-52). 

In an extension of this work, the Shibasaki group developed the novel transformation 48—>51 shown in Scheme 10.25c To rationalize this interesting structural change, it was proposed that oxidative addition of the vinyl triflate moiety in 48 to an asymmetric palladium ) catalyst generated under the indicated conditions affords the 16-electron Pd+ complex 49. Since the weakly bound triflate ligand can easily dissociate from the metal center, a silver salt is not needed. Insertion of the coordinated alkene into the vinyl C-Pd bond then affords a transitory 7t-allylpalladium complex 50 which is captured in a regio- and stereocontrolled fashion by acetate ion to give the optically active bicyclic diene 51 in 80% ee (89% yield). This catalytic asymmetric synthesis by a Heck cyclization/ anion capture process is the first of its kind. 

Optically active alkynyl alcohols can, however, be conveniently prepared by the addition of dialkylzinc reagents of alkynyl aldehydes catalyzed by the chiral ligand (S)-l-methyl-a,a-diphenyl-2-pyrrolidine methanol33-34. 

Combination of nickel bromide (or nickel acetylacetonate) and A. A -dibutylnorephcdrinc catalyzed the enantioselective conjugate addition of dialkylzincs to a./Tunsaturated ketones to afford optically active //-substituted ketones in up to ca. 50% ee53. Use of the nickel(II) bipyridyl-chiral ligand complex in acetonitrile/toluenc as an in situ prepared catalyst system afforded the //-substituted ketones 2, from aryl-substituted enones 1, in up to 90% ee54. 

Cyclopolymerization of the bis-methacrylates (10, ll)6" 6j or bis-styrene derivatives (12)64 has been used to produce heterotactic polymers and optically active atactic polymers. Cyclopolymcrization of racemic 13 by ATRP with a catalyst based on a chiral ligand (Scheme 8.12) gave preferential conversion of the (S, )-enantiomer. 66... 

Optically Active Ring-Annulated Cyclopentadienyl Ligands. . . . 135... 

Optically Active Ring-Annulated Cyclopentadienyl Ligands... 

A certain jr-facial selectivity was achieved when MCpCl2 (M = Ti, Zr) fragments were coordinated to the optically active fused cyclopentadienyl ligands. For instance, reaction of ZrCpCl3 with the lithium derivative of 126 at —78 °C gave predominantly 133 which was characterized by X-ray structural analysis [152]. 

Optical activity of cobalt(III), chromium(III) and rhodium III) complexes with aminopolycarboxy-late, edta-type and related ligands. D. J. Radanovic, Coord. Chem. Rev., 1984, 54, 159-261 (195). 

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