Racemic isomer

The study of optical isomers has shown a similar development. First it was shown that the reduction potentials of several meso and racemic isomers were different (Elving et al., 1965 Feokstistov, 1968 Zavada et al., 1963) and later, studies have been made of the ratio of dljmeso compound isolated from electrolyses which form products capable of showing optical activity. Thus the conformation of the products from the pinacolization of ketones, the reduction of double bonds, the reduction of onium ions and the oxidation of carboxylic acids have been reported by several workers (reviewed by Feokstistov, 1968). Unfortunately, in many of these studies the electrolysis conditions were not controlled and it is therefore too early to draw definite conclusions about the stereochemistry of electrode processes and the possibilities for asymmetric syntheses. 

From Eberson (1959). Values of the racemic isomer are underlined. 

Ni(00CMe)(tet-b)C104 cis Oh tet-b = 5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacydotetradecane, racemic isomer chelating MeC02 and folded macrocycle 210-211 209-216(N) 1725... 

Ni(04C2)(tet-a)-3H20 (tet = 5,7,7,12,14,14-hexamethyl-l,4,8,ll-tetraazacyclotetradecane a = meso isomer b = racemic isomer) were prepared by the reaction of a concentrated solution of sodium oxalate with an aqueous solution of the appropriate nickel(II) amine complex.1780,1781 In the dinuclear complex [Ni2(04C2)(en)4](N03)2 (234)1740-1741 the bridging oxalato group is planar and symmetrically bonded to the two nickel atoms. The same structure occurs in the complex Ni2(04C2)(0N0)2(py)6 which was obtained as a by-product in a very low yield when a pyridine solution of methanenitrosolic acid and nickel(II) were allowed to stand for several months.1741... 

The coordination chemistry of macrocyclic ligands has been extensively studied and aspects of isomerism have been considered in numerous systems.241 Methods whereby two diastereomers of complexes of tetra- N-methylcyclam may be isolated have been discussed previously.184 This, however, is a relatively simple system and it is usually necessary to consider isomerism due to the presence of asymmetric atoms in the chelate arms, as well as that due to asymmetric donor atoms that may be rendered stable to inversion by coordination. An example of a system exhibiting this level of complexity is afforded by the nickel(II) complexes of the macrocyclic ligands generated by reduction of the readily prepared macrocycle (46). These ligands contain two asymmetric carbon atoms and four asymmetric nitrogen atoms but, because AT-inversion is rapid, it is conventional to consider that only three separable stereoisomers exist. There is an enantiomeric pair, (47a) and (47b), which constitutes the racemic isomer (R, R ), and an achiral (R, S ) diastereomer (47c), the meso isomer. 

Perchlorate, racemic isomer. The three M O M angles are 85.2, 84.1, and 85.0. r Hydroxide undecahydrate. C5Me5 = fj5-pentamethylcyclopentadienyl. d Acetate tetradecahydrate. 

The isomerization between meso and racemic isomers of (OH)(en)2Cr(OH)Cr(en)2(OH)3+ has been used to synthesize the racemic isomer (34, 35). The equilibrium constant for Eq. (25) has been estimated to be K — 4 (45°C, 8 M ethylenediamine), which is significantly larger than the statistical value of unity. 

In catalysts obtained from chiral stereorigid metallocenes of class III with C2 molecular symmetry (helical), such as racemic isomers of ansa-metallocenes, e.g. rac.-(IndCH2)2MtX2 (Table 3.1) and rac.-(ThindCH2)2MtX2, the two coordination positions available for the incoming monomer and the growing... 

Corey and Zhu96 examined the sterochemistry of the oligomers (up to Si4) obtained by dehydrocoupling of PhMeSiH2 with the Cp2ZrCl2/BuLi catalyst. They used the distribution and intensities of the H NMR resonances due to the Si-Me groups in the trimer and tetramer to estimate the isomeric composition. In the case of the trimer, the 11 resonances anticipated for a statistical distribution of the 2 meso and 1 racemic isomers were all observed, including one with twice the intensity of the other 10. The case was less clear cut for the tetramer, but the cluster of Me resonances did not have the intensity distribution expected for a statistical distribution of stereoisomers.96... 

Collins and coworkers have developed two synthetic routes for the preparation of chiral [l,2-ethylenebis(jj -3-alkylcyclopentadienyl)]titanium dichlorides (R = Me, Et, i-Pr, r-Bu) in 80 85% yields as a nuxture of racemic and meso-titanocene dichlorides. In more recent work, the addition of a methyl group to the cyclopentadienyl rings has allowed Collins and coworkers to prepare a series of [1,2-ethylene-l,T-bis(4-R-2-methylcyclopentadienyl)] titanium dichlorides (R = Me, j-Pr, r-Bu) stereoselectively to give the racemic isomers. ... 

Even better regio- and enantioselectivities were observed when 1-naphthyl-substituted allylic acetates (10b and 11b) were used. The regio- and enantioselectivities were essentially the same using either the achiral substrates (10) or the racemic isomers (11) (eq 3, Table 1). 

At the same time, to our surprise, P8 M appears visually transparent and nonbirefringent, both in bulk samples and in thin films. This is illustrated by Figure 5.3, where the spot of chiral polymer can hardly be recognized between two glasses in contrast to its racemic isomer, P8 M. We should stress here that it is impossible to induce any birefringence in P8 M using shear flow, electric field up to 15 V/pm, or magnetic field up to 2.5 T. 

To 500 mL of methanol in a 2-L beaker is added 100 g (0,21 mole) of 5,7,7,12,14,14-Me6[14]4,ll-diene-l,4,8,ll-N4-2HC104 [Sec. 1-B]. The solution is stirred and 19 g (0.63 mole) of sodium tetrahydroborate(l-) and 16.5 g (0.42 mole) of sodium hydroxide are added in alternate small portions over a 1-hour period. The addition should be conducted in a well-ventilated fume hood (hydrogen is evolved) and in a cold-water bath to moderate the temperature, if necessary. After the addition is complete, the solution is stirred for 1 hour at room temperature and then heated to reflux for 15 minutes. After it is cooled, 50 g of sodium hydroxide in 1 L of water is added and the solution is stirred until precipitation of the product is complete (usually about 1 hr) and then filtered. The white product is washed with cold water and air-dried overnight. Additional material can be recovered from the filtrate by volume reduction. The yield is 52 g (88%). The product is a 50 50 mixture of the meso and racemic isomers that can be separated by fractional crystallization from methanol as detailed below. ... 

To the filtrate remaining after removal of the meso isomer is added a further 200 mL of water. The solution is stirred rapidly for about 30 minutes during which time a precipitate forms. This material (about 8 g or less) is a mixture of both meso and racemic isomers and is removed by filtration. The filtrate that remains... 

The designations meso and racemic refer to the asymmetric carbon atoms at positions 7 and 14. Chiralities for these carbon atoms in the meso ligand ate R and S while for the racemic isomer they ate R,R or S,S. 

Racemic isomers of a-cyano-3-phenoxybenzyl cis-2,2-dimethyl-3-(2,2,2-trichloroethyl)cyclopropanecarboxylate (64) were isomerized to 65 at the a-position by heating in /-propanol in the presence of DBU (81GEP3008986). 

For some applications of metallocenes, especially for making i-PP, only the racemic isomers formed in the transmetallation step are of interest. In some cases the meso isomer formed with the rac isomer in a 50 50 mixture is not active in propene polymerization, while in other cases it produces undesired a-PP causing unacceptably high soluble polymer fractions. 

Even though the Paal-Knorr pyrrole synthesis has been around for 120 years, its precise mechanism was the subject of debate. In 1991, V. Amarnath et al. investigated the intermediates of the reaction and determined the most likely mechanistic pathway. The formation of pyrroles was studied on various racemic and meso-3,4-diethyl-2,5-hexanediones. The authors found that the rate of cyclization was different for the racemic and meso compounds and the racemic isomers reacted considerably faster than the meso isomers. There were two crucial observations 1) the stereoisomers did not interconvert under the reaction conditions and 2) there was no primary kinetic isotope effect for the hydrogen atoms at the C3 and C4 positions. These observations led to the conclusion that the cyclization of the hemiaminal intermediate is the rate-determining (slow) step. 

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