Asymmetric carbon centers

Considerably less is known about the chemistry of palladium and platinum 1,1-dithio complexes. Of late, there has been only one report that dealt with the synthesis of a large number of palladium dithiocar-bamates 392). Twenty-five yellow palladium dithiocarbamate complexes were obtained by reaction of PdCla with NaR2dtc in methanol solution. Several other reports have appeared in which a few dithiocarbamate complexes of palladium were synthesized. Thus, the novel [Pd (OH)2dtc 2], which is soluble in water, was isolated 393). The synthesis of optically active palladium(II) complexes of AT-alkyl-a-phen-ethyldithiocarbamates, similar to (XXIV), via the reaction between the optically active amine, CS2, and PdCl2, has been described. From ORD and CD spectra, it has been established that the vicinal contribution of a remote, asymmetric carbon center could give rise to optical activity of the d—d transitions of palladium 394). Carbon disulfide has been shown to insert into the Pt-F bond of [PtF(PPh3)3]HF2, and X-ray studies indicated the structure (XXIX). 

Substituted 1-boraadamantanes contain an asymmetric carbon center C-2 and can be resolved into enantiomers. 

Over the last years, one of the most studied DCR has been the asymmetric version of the cycloaddition of nitrones with alkenes. This reaction leads to the construction of up to three contiguous asymmetric carbon centers (Scheme 4). The resulting five-membered isoxazolidine derivatives may be converted into amino alcohols, alkaloids, or p-lactams. Several chiral metal complexes have been used as catalysts for this process [13-15, 18-22]. However, the employment of iridium derivatives is very scarce. 

Figure 2.5 The two enantiomeric forms of the herbicide mecoprop. The indicates the asymmetric carbon center. Ar denotes the aromatic substituent.

Oxidative Addition of Alkyl Halides to Palladium(0). The stereochemistry of the oxidative addition (31) of alkyl halides to the transition metals of group VIII can provide information as to which of the many possible mechanisms are operative. The addition of alkyl halides to d8-iridium complexes has been reported to proceed with retention (32), inversion (33), and racemization (34, 35) via a free radical mechanism at the asymmetric carbon center. The kinetics of this reaction are consistent with nucleophilic displacement by iridium on carbon (36). Oxi-... 

Polymerization of unsymmetric vinyl monomers generates asymmetric carbon centers within the polymer chain... 

With respect to the site at which the new asymmetric carbon center is created, the catalytic enantioselective Michael reaction can be categorized into two groups (Fig. 1) in Type I an asymmetric center is generated on the Michael donor side in Type II an asymmetric center is generated on the Michael acceptor side. 

Sawamnra, M., Hamashima, H., and Ito, Y., Rhodium-catalyzed enantioselective Michael addition of (l-cyanoethyl)phosphonate. Synthesis of optically active phosphonic acid derivatives with phosphorussubstituted quaternary asymmetric carbon center, Bull. Chem. Soc. Jpn., 73, 2559, 2000. 

Methyl glyoxylate and ( )-3-hexenol produce the acctoxyalkoxyacetic acid ester 4 in a two-step sequence. Cyclization of the glyoxal derivative 4 in the presence of tin(IV) chloride gave the tetrahydropyranylcarboxylate 5 in 83% yield after 6-endo reaction and chloride ion attack27. Tetrahydropyran 5 shows defined relative stereochemistry at all three asymmetric carbon centers. 

Mefloquine 274 is a useful alternative treatment agent for Plasmodium vivax malaria and Plasmodium falciparum malaria in areas, where chloroquine is still recommended as the first-hne therapeutic agent.Mefloquine is a chiral molecule with two asymmetric carbon centers, and it exists as a mixture of diastereomers. The drug is currently manufactured and sold as a racemate of the (+1—) R,S enantiomers by Hoffman-LaRoche. The (+) enantiomer is more effective in treating malaria, and the (—) enantiomer specifically binds to adenosine receptors in the central nervous system, which may explain some of its psychotropic effects.It is not known whether mefloquine can be transformed into its stereoisomer in vivo. 

The high specificity obtained for optical resolution shows that it was in fact possible to copy the shape of the template and the arrangement of the functional groups into the polymer by this imprinting method. It should be noted that this method of resolution of racemates is a novel one. In this case no interactionsof the racemate with molecular parts carrying asymmetric carbon centers take place, but the enantiomers are bound within a chiral environment. The stability of these inclusion complexes is apparently different for the two enantiomers. This type of optical resolution is somewhat analogous to urea inclusion compounds ( ) or to silica imprinted by chiral compounds (27-29). Certain aspects of our method have been reviewed by our group (30-34) and by others (35-39). 

An -amino acid consists of an asymmetrical carbon center surrounded by four functional groups (1) -amino (2) primary-carboxyl (3) -hydrogen and (4) a side chain, R ... 

In the cyclic carbopalladation of 1,1-disubstituted alkenes, one asymmetric carbon center is generated unless the process leads to the formation of symmetrical stmctures lacking chirality. Such reactions are subject to asymmetric induction in both diastereomeric and enantiomeric senses. 

The problem was expected to be solved by chemical degradation to authentic cis-pinane, the levorotatory enantiomer of which has the absolute configuration (1S,2R,5S). Provided that the degradation exactly yields this enantiomer, as identified by value and sign of its specific rotation, then the asymmetric carbon centers of the sesquiterpenes 1 and 4 certainly possess the absolute configurations (15,55). 

Synthetic studies on aplyviolene based on stereoselective construction of continuous asymmetric carbon centers 13YGK637. 

Ring-saturated thymine lesions can exist in several stereoisomeric forms which may be energetically distinct depending on their environment. The stereochemistry of thymine glycol (Tg) and thymine hydrate are particularly complex because they have two asymmetric carbon centers. Each asymmetic carbon gives rise to two enantiomers... 

In cases where either addition or substitution proceeding via double bond migration is involved, one or more asymmetric carbon centers may be generated. Thus, face selective TT-complexation with chiral Pd complexes can, in principle, lead to asymmetric processes [15],[16] (Sect, V.3.1.1), although this possibility has not yet been extensively investigated. 

N -LC phase with the latter approach. Axially asymmetric chiral binaphthyl derivatives are known to have a larger twisting power than chiral compounds with an asymmetric carbon center [161, 162]. For our studies, we synthesized chiral (/ )- and (5)-PCH506-Binol from the optically active (R) (+) and (5) ( ) l,l -bi-2-naphtols by Williamson etherification reaction with a liquid crystalline phenylcyclohexyl (PCH) derivative (Fig. 9.34). The terminal alkyl chain was n-pentane, and the mesogen was linked to the binaphthyl with a hexamethylene spacer via ether bonds. 

When a molecule has n chiral centers, there may be up to 2" stereoisomers. Flowever, some of these may be meso-compounds, where the molecule has asymmetric carbon centers but also possesses a plane or center of symmetry and hence is not chiral. Racemic mixtures (equal amounts of the two enantiomers) may be resolved by conversion to separable diastereoisomers or by enzymatic methods. 

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