Racemic complex concentration

The decline in optical rotation of the esters 2a and 2 b as a function of time is not affected by the addition of triphenylphosphine (Fig. 1). P(CgHs)3 in concentrations up to 10-fold excess with respect to complex concentration had no effect at all on the rate of racemization of 2a and 267. Deuteration experiments indicated that triphenylphosphine is indeed the ligand which dissociates. When the racemization of the manganese esters 2 is carried out in the presence of deuterated triphenylphosphine, P(C6D5)3 is incorporated into the complexes 16 [Eq. (14)]. A kinetic study showed that the rate of triphenylphosphine exchange is exactly equal to the rate of racemization8. ... 

Fig. 2. Effect of p-substituents Xin the complexes CsHsMntCO-p-CglLiXHNOlPtp-CglfyXJa 18a-32a on the half-lives r,p of the first-order racemization reaction in toluene at 20 °C (complex concentration 2 mg/ml)...

Functionalization at C-9 (see Scheme 10) in camphor illustrates not only the complexity of the pathways when the possibility of Nametkin (2,3-methyl shift) and 2,6-hydride shifts are taken into account, but also one of the solutions to the simplification of the rearrangement.The product is a mixture of (-) )-and (-)-camphor-9-sulfonic acids, i.e. the pathway allows for racemization. Indeed camphor itself can be racemized in concentrated sulfuric acid by a similar pathway involving both Wagner-Meerwein and... 

The equilibria that have been proposed to exist (and to change) during the occurrence of the Pfeiffer Effect are described, along with proposals for the nature of these equilibria. In particular, the effects on the equilibria of changing concentrations and concentrations ratios of the environment substance to the racemic complex have been studied, in an effort to identify the equilibria that actually exist during the appearance of the Pfeiffer Effect. 

Table I shows the effects on the equilibrium constant calculated from equations 1 and 2. It should be noted that, not only does die "equilibrium constant" not remain constant as the concentrations of both the environment substance and the racemic complex increase (while keeping their ratio constant), but this "constant" also increases when the ratio of the concentrations of the environment substance to the complex increases. This implies that it may be possible for more than one molecule of the environment substance to undergo hydrogen-bonding simultaneously to one molecule of the complex, which is a matter that is currently undergoing careful scrutiny in this laboratory.

Extractions of aqueous solutions of racemic amino-acid ester salts with solutions of / -6/s(dinaphthyl)-22-crown-6 [284] in chloroform revealed the dependence of the enantiomeric distribution constant on the structure of the amino acid ester (Table 64). In order to limit the concentrations of complex in the aqueous phase, inorganic salts were added. In the case of tyrosine, serine and alanine no extraction of salt was observed obviously these salts form very hydrophilic complexes. The highest degree of chiral recognition was found with [284] and p-hydroxyphenylglycine methyl ester hexafluorophosphate [A(AG°)... 

Step (7.41) leads to hydrogen exchange, = k. Only step (7.42) leads to racemization and is considered pH-independent. Most of the time the amide complex becomes reprotonated with retention of configuration. If a steady-state concentration for the amide form is assumed. 

In order to assess whether intramolecular cooperativity could occur within the dendrimeric [Co(salen)]catalyst the HKR of racemic l-cyclohexyl-l,2-ethenoxide was studied at low catalyst concentrations (2xl0 " M). Under these conditions the monomeric [Co(salen)] complex showed no conversion at all, while the dendritic [G2]-[Co(salen)]catalyst gave an impressive enantiomeric excess of 98% ee of the epoxide at 50% conversion. Further catalytic studies for the HKR with 1,2-hexen-oxide revealed that the dendritic catalysts are significantly more active than a dimeric model compound. However, the [Gl]-complex represents already the maximum (100%) in relative rate per Go-salen unit, which was lower for higher generations [G2] (66%) and [G3] (45%). 

Quite a few complexes with the bidentate pentasulfido ligand are also known. The first reported was the homoleptic and optically active complex [Pt(85)3] (15) (53, 64, 65, 68, 69, 176). Brick-red (NH4)2[Pt(85)3] 2H20 is formed from the reaction of K2[PtCl6] with aqueous (NH4)28 solution. Addition of concentrated HCl results in the separation of maroon (NH4)2[Pt8i7] 2H20 (54). The [Pt(85)3] ion crystallizes from the solution as a racemate, which can be resolved by forming diastereoisomers. Upon crystallization, [Pt8,7] undergoes a second-order asymmetric transformation, so that the solid contains an excess of the (—) enantiomer (54). 

The confusion generated in the initial report185 on photoracemization of sulfoxides has recently been removed with the postulate that naphthalene singlet forms an excited complex with sulfoxides.186 Thus, despite the fact that the singlet state of 36 lies at 113 kcal, some 23 kcal above that of naphthalene, 36 quenches the fluorescence of this hydrocarbon with kes = 3.2 x 107Af -1 sec-1. From the dependence of the quantum yield of racemization on sulfoxide concentration (Eq. 34), a value of kes = 2.3 x 107Af-1 sec-1 was deduced. Since these values are the same within experimental error, it follows that the singlet state of naphthalene is responsible for photoracemization. 

The high yields and enantioselectivities recorded in the Pd(0)/BPA-catalyzed reaction of rac-27a-27c with water and KHCO3 show that not only a highly enan-tioselective alkylation but also an efficient dynamic kinetic resolution either via racemization of the more slowly reacting enantiomer of the substrate or isomerization of the diastereomeric jt-allyl-Pd complexes (see Scheme 2.1.4.29) had occurred. Experiments with other racemic unsymmetrical allylic carbonates revealed a dependence of the enantioselectivity on the concentration of the Pd(0)/... 

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... 

A recently uncovered mode of racemization involves attack of the ir-allylpalladium complex by an un-complexed PdL2 species present in the solution. This mode of racemization can be significant if high concentrations of PdL4 catalyst are employed in the reaction (equation 350). 

Reacting gases may be in excess if they react with solids and do not condense in liquid phases, but supercritical media are clearly not the subject of solvent-free chemistry and deserve their own treatment. For practical reasons, this book does not deal with homogeneous or contact-catalyzed gas-phase reactions. Furthermore, very common polymerizations (except for solid-state polymerizations), protonations, solvations, complexations, racemizations, and other stereo-isomerizations are not covered, to concentrate on more complex chemical con-... 

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