Optically active ligand

High ee values have also been obtained with organometallics," including organotitanium compounds (methyl, aryl, allylic) in which an optically active ligand is coordinated to the titanium," allylic boron compounds, and organozinc compounds. 

Enantioselective additions of dialkylzincs and diphenylzinc to enones and related compounds were accomplished using catalytic amounts of Gu salts and optically active ligands, including biphenol-based phosphoramidites301-304... 

The idea of Hoveyda with co-workers to employ their peptide ligands (e.g., 295) as chiral inductors in allylic substitutions with dialkylzincs turned out to be very rewarding.399-401 As a result of meticulous screening of numerous optically active ligands, copper salts, and substrates under various conditions, they achieved excellent results for aliphatic alkenes. Particularly, allylic substitution products with tertiary 297 and quaternary 299 carbon centers were obtained regioselectively and with 78-96% ee (Scheme 151).401... 

The phosphetane ring is a useful synthon in the preparation of optically active ligands. Chiral l,2-bis(phosphetano)benzenes 38 are easily prepared from dilithiophenylphosphine 36 by reaction with a cyclic sulfate 37 <00T95>. 

Treatment of the optically active gem-borazirconocene alkanes with deuterium oxide followed by alkaline oxidation affords the corresponding optically active 1-deuterio primary alcohols. The enantiomeric excess of the resulting primary alcohols represents the diaster-eoselectivity of the asymmetric hydrozirconation (Scheme 7.13). Based on the cost and availability of optically active ligands, three types were explored monoterpenes, 1,2-diols, and 1,2-amino alcohols. Hydrozirconation of optically pure 1-alkenyl boranes 39 provided optically active 1,1-bimetallics 40. 

Whereas racemization is the complete loss of optical activity with time, epimerization is the reversible interconversion of diastereoisomers to an equilibrium mixture which is not necessarily optically inactive. Diastereoisomers arise from the combination of the two chiral centers in 9, namely the metal centered, R and S, and the resolved (S) optically active ligand center. The diastereoisomers (RS) and (SS) differ in their properties. 

Hoxine = 8-hydroxyquinoline) involves the ions [Cr(oxine) ] (n = 0—3). The diastereoisomers of the complexes tris[A -( of 5 )-a-benzylethyl-5-nitro-salicylaldiminato]chromium(iii) (62), involving optically active ligands, have... 

Axially chiral phosphoric acid 3 was chosen as a potential catalyst due to its unique characteristics (Fig. 2). (1) The phosphorus atom and its optically active ligand form a seven-membered ring which prevents free rotation around the P-0 bond and therefore fixes the conformation of Brpnsted acid 3. This structural feature cannot be found in analogous carboxylic or sulfonic acids. (2) Phosphate 3 with the appropriate acid ity should activate potential substrates via protonation and hence increase their electrophilicity. Subsequent attack of a nucleophile and related processes could result in the formation of enantioenriched products via steren-chemical communication between the cationic protonated substrate and the chiral phosphate anion. (3) Since the phosphoryl oxygen atom of Brpnsted acid 3 provides an additional Lewis basic site, chiral BINOL phosphate 3 might act as bifunctional catalyst. 

Optically active Schiff-base oxovana-dium(IV) complexes catalyze the asymmetric oxidation of sulfides to sulfoxides by peroxides [86]. The catalytically active species is VO(V) rather than VO(IV) and is formed in situ under the reaction conditions. A series of related complexes based on the optically active ligand shown in Eig. 15 shows linear dependence of their oxidation Ef values on the Hammett parameters of functional group X. These values ranged from 0.18 V versus Cp2Ee/DMSO for X = NO2 to —0.18 V for X = OCH3 [87]. A few complexes of planar tetradentate non-Schiff base ligands have also been investigated [88]. 

The scheme followed more recently by Moriarty et al139 again involved a racemic phosphine (64, in Scheme 7), used to synthesize a racemic phosphorane 65, but the optically active ligand was introduced only at that point, to replace NMe2. As before, two diastereoisomers, 66a and b, were obtained, as proved by the two series of signals in the 31P, lH and 13C NMR spectra, and of the S and J values which were determined for the... 

When activated by metallic catalysts, hydrogen may be transferred from the metallic center to unsaturated organic molecules. The nature and reactivity of transition metal hydrides depend on the central metals as well as on the electronic and steric properties of the ligands. Metal hydrides with optically active ligands are chiral and thus, are capable of asymmetric hydrogenation. 

NiL(H20)JCl2 Obtained from NiCl2L cis Oh optically active ligand 2690... 

Alkenes which have no symmetry planes perpendicular to the plane of the double bond such as Pmr-butene-2 or propene can coordinate to platinum in two enantiomorphous ways (77) and (78). If an optically active ligand is also bound to platinum(H), then two diastereoisomers are found which can be separated by fractional crystallization657,658 or by HPLC.659 Both cis and trans isomers of complexes PtCl(N—0)(alkene) have beenprepared, where N—O is an anion derived from an amino add (equations 235a and 235b).660-664 Epimerization cannot occur by simple rotation of the alkene about its bond axis, but only by a mechanism involving cleavage of the platinum(II)-alkene bond. 

Thus transition metal complexes capable of effecting cyanation reactions on aromatic nuclei under mild conditions have been discovered Cassar et al. describe such a catalytic system. The past few years have also seen the discovery of asymmetric catalysis. Asymmetric catalysts contain optically active ligands and, like enzymes, can promote catalytic reactions during which substantial levels of optical activity are introduced into the products. This volume contains examples of asymmetric hydrogenation and asymmetric hydroformylation catalysis in the papers, respectively, by Knowles et al. and Pino et al. 

Chiral pockets have been created, employing optically active ligands derived from l,2 -binaphthol. Asymmetric alkylations with nearly 70% optical yield have been realized using this method.438... 

An optically active cross-coupling product may also be generated by a nickel catalysts having an optically active ligand. For example, (-)-... 

In the reaction shown the (R) product was obtained in 14.8% enantiomeric excess (ee) (68). The optically active ligand, (-)-diop (58), has been used in asymmetric catalytic hydrogenation. For example, the use of the rhodium(I) complex (61) converts the alkene (59) to (R)-N-ac-etylphenylalanine (60) with an optical yield of 72% and a chemical yield of 95% (69). 

The best optical yields obtained in the hydroformylation are comparable with the highest yield obtained in hydroalkoxycarbonylation using Pd catalysts. In Table 15 the results obtained in hydroformylation with rhodium or platinum catalysts are compared with those obtained in hydroalkoxycarbonylation using identical substrates and identical optically active ligand 9). 

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