Racemic complexes

In 1996, Fu et al. reported the S3mthesis of the planar chiral heterocycles 64, formally DMAP fused with a ferrocene core [82]. While the original synthesis provided racemic 64a in only 2% overall yield requiring a subsequent resolution by preparative HPLC on a chiral stationary phase, a recently improved synthesis furnished the racemic complexes 64 in 32-40% yield over seven steps. A subsequent resolution with di-p-toluoyltartaric or dibenzoyltartaric acid gave access to the enantiomers with >99% ee (28 14% yield for each isomer in this step) [83]. 

As typical examples of crystal-to-crystal thermal reactions, the cyclization of allene derivatives to four-membered ring compounds and the transformation of a racemic complex into a conglomerate complex are described. 

Table 10 shows the results of polymerization of oc-olefins catalyzed with trivalent complexes. When a bulkiler tBuMe2Si group instead of the Me3Si group was introduced into the yttrium complex, the racemic complex was formed exclusively [74c]. However, this alkyl complex did not react with olefins, and hence it was converted to a hydride complex by reaction with H2. The complex obtained was reactive to various olefins and produced polymers at... 

Racemic complex (racemic PBG), composed of PBLG and its enantiomer poly(y-benzyl D-glutamate) (PBDG), takes an a-helical conformation in the solid state and has different physical properties from those of PBLG or... 

Iodine may attack both sides of the planar intermediate to yield a racemic complex. 

Deprotonation of complex 1 with butyllithium at — 78 °C generates the enolate species 2 (described in Section 1.1.1.3.4.1.1.), which reacts with electrophiles while in the anti conformation (acyl oxygen anti to carbon monoxide oxygen). Enolate 2 is inert to 1,2-epoxypropane (3a) at — 78 °C, but in the presence of a Lewis acid, rapid reaction ensues leading to preferred alkylation of the least hindered site of the epoxide13. Reaction of the enolate 2, derived from the racemic complex 1, with racemic monosubstituted epoxides results in preferential formation of one of two possible diastereomers this can be termed a double enantiomer-differentiating reaction. 

The Pfeiffer effect is a term used to describe changes in the optical activity of solutions containing a chiral compound (the environmental substance ) on the addition of a racemic dissymmetric complex. The effect is generally attributed to a shift in the position of the equilibrium between d and l isomers for the racemic complex. The exact mechanism involved in mediating the chiral interaction is unknown. Perhaps surprisingly, both environmental substance and complex may simultaneously be cations. Studies of the Pfeiffer effect usually involve a moderately labile racemic complex [Cr(ox)3]3 is a popular choice for such studies, summarized in Table 82. Other studies of the optical activity of tris oxalates include work on photoinduced optical activity,898 photoracemization899 and the solid-state racemization of K3[Cr(ox)3]. 900 901... 

The racemic complex oxalate has been resolved by the fractional eiystallization of the cations of optically active bases As the product itself is self-resolving, it may be separated into its d and l forms by simply crystallizing above the transition temperature of 13.2°C the two isomers settle out independently,... 

Cr(CO)3 coordinates from either the top or bottom side of aromatic rings, bearing two different substituents in ortho or meta position, so that the enantiomers 285 and 286 are obtained. Optical resolution of the enantiomers is carried out by recrystallization, or column chromatography. The racemic complex of benzyl alcohol derivative 287 was separated to 288 and 289 by lipase-catalysed acetylation [68]. Enzymes recognize Cr(CO)3 as a bulky group. Chiral Cr(CO)3-arene complexes are used for asymmetric synthesis [68a]. 

The racemic complex 4 containing 50% of NCN-pincer platinum(II) and 50% of tosylate groups has been tested in catalytic double Michael addition of methyl vinyl ketone and ethyl cyanoacetate using (1 mol %) of catalyst. 

At first, reaction of 1,2-dibromethane with an excess of indcnyl magnesium bromide leads to the ethano-bridged bis-tetrahydroindenyl ligand 32 (mixture of isomers), from which the racemic complex (rac-31) is obtained. The separation of the enantiomers is finally achieved by fractionated crystallization of the derivative 33 (R OH = (R)-O-acetylmandelic acid). 

The a-protons of iron acyl complexes are acidic and these can be deprotonated with Lithium diisopropylamide (LDA) or with n-butyllithimn. Thus the corresponding enolates are readily functionalized and undergo reaction with alkyl halides, aldehydes, disulfides, trimethylsilyl chloride, and epoxides to afford the corresponding a-derivatized products. " Early work on racemic complexes revealed that these transformations occur in a highly diastereoselective fashion,... 

Cince Werner s early paper 19, 20) on the optical activity he postulated and discovered in coordination compounds, there have been several discoveries closely related to this work. One of these is the observation by Pfeiffer and Quehl 15) that the optical rotation of an aqueous solution containing an optically active substance (e.g., ammonium d-a-bromocamphor-TT-sulfonate, later referred to as the optically active en-vironment O be changed by adding solutions of racemic mixtures of certain coordination compounds (e.g., D,L-[Zn(o-phen)3](N03)2, where o-phen = or /io-phenanthroline). This effect has been referred to as the Tfeiffer effect in honor of its discoverer 3, 4) who first observed it during an attempted resolution of the racemic complex mentioned above 15). 

Reaction of racemic allyl alcohol and 10 mol of racemic complex 88 affords the produet with little selectivity. 

The Ti-Cp bond is cleaved by reaction of Cp2TiCl2 with the lithium salt of Schiff base to give racemic complexes of titanium.1188... 

An X-ray crystallographic analysis revealed that 64 is a 2 -metallacryptand ( = triple helicate) (Figure 29). Each of the two iron centers is octahedrally surrounded by six oxygen atoms. In the chiral, racemic complex 64, both iron centers are identically coordinated. Therefore, the 2 -metallacryptand is either a (A,A)- c or A,A)-fac triple helicate. The crystals obtained are composed of the homochiral 2 -metallacryptand 64. The donating power within the interior of the complex is insufficient for the complexation of alkali cations. Above all, the three phenyl hydrogen atoms in 64 are directed inward towards the empty cavity in the center. 

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