Racemization rate constants

AS -8 to +4 e.u. Even though the racemization rate constants differ slightly, their distinct dependence on the steric and to a lesser extent on the electronic effects of the substituents bonded to the sulfinyl sulfur atom was noted. It deserves adding that the activation volume for racemization of methyl p-tolyl sulfoxide 41, A F 0 ml/mol, is also consistent with the pyramidal inversion mechanism (249). 

The racemization rate constant for L — Ala coordinated to Co(III) is much larger than for free L — Ala. In addition, the larger the positive charge on the complex, the faster is the rate of racemization. These observations give credance to the idea that racemization (and exchange) takes place by an initial abstraction of an a-H by base, resulting in a carbanion. ... 

This mechanism is supported by identical dissociation and racemization rate constants. This further implies either that the bis species M(AA)2 is racemic as formed, or that it may racemize (by a cis-trans change, or by a dissociative or intramolecular path) more rapidly than it re-forms iris in the dynamic equilibrium (7.23). Identical activation parameters for the dissociation (to the bis species) and racemization in aqueous acid (Table 7.5) and other solvents of Nifphen) " and Ni(bpy)3 indicate that these ions racemize by an intermolecular mechanism. This is the only such example for an M(phen)"+ or M(bpy) + species (see Table 7.5) although recently it has been observed that Fe(bps)3 (bps is the disulfonated phenanthroline ligand shown in 13, Chap. 1) but not Fe(phen)3+ also racemizes predominantly by a dissociative mechanism in water. For the other tr/s-phenanthroline complexes (and for Fe(bps)3 in MeOH rich, MeOH/HjO mixtures ) an intramolecular mechanism pertains since the racemization rate constant is larger than that for complete dissociation of one ligand, Table 7.5. 

Tabie 12.1 Racemization rate constants (k various hydantoins [7]. 

Table 12.4-6. Racemization rate constants (trac and corresponding half-live times ti/2.rac for various hydantoins at pH 8.5 and 40 C. Values were calculated from first order rate law ...

This technique has been used in a number of studies in which the decay of has been analysed to extract the racemization rate constant. 

Measurement of CPL from racemic mixtures is not a technique that can be applied to all racemic solutions. For moderately luminescence systems values of approximately 10 are measurable. This means that the intrinsic absorption and emission dissymmetry ratios need to be on the order of 10 for this experiment to be successful. Although there have been a couple of examples using this technique in organic systems, by far the most widely studied systems are racemic lanthanide complexes, such as given in Figure 5, because of the large and g bs values that may exist for certain f o f transitions. It is also useful to perform these experiments in a time-resolved mode and as a function of temperature to determine racemization rate constants. 

Takano, Y Kobayashi, K. Ishikawa, Y Marumo, K. (2006) Emergence of the inflection point on racemization rate constants for D- and L-amino acids in the early stages of terrestrial diagenesis. Org. Geochem. 2006, 37, 334-341. 

Racemization constitutes a special case of opposing first-order reactions. The equilibrium constant is unity, and the opposing rate constants are equal to one another. Racemization can be followed by polarimetry (monitoring the angle of optical rotation) or by circular dichroism (monitoring the ellipticity). The kinetic analysis can be done by either Eq. (3-15) or (3-16). The rate constant for racemization is krac = ke/2. 

Consider the interconversion of two chiral molecules to yield ultimately the racemic mixture. This is simply the situation of opposing first-order reactions of A and P, treated in Chapter 3, for the special case of an equilibrium constant of unity. Recall that for such an equilibrating system ke = kf + kr because of that, knc is one-half the experimental rate constant. 

RCS (.sec Rate-controlling step) Racemization, 49, 95 Rate constant calculation of. 17 composite, 161-164 diffusion-controlled, 200-201 Rate-controlling step, 9, 82-86 Rate-determining step (see Ratecontrolling step)... 

In this scheme, due to the rate acceleration effect, the enantioselective hydrogenation is much faster than the two racemic hydrogenation reactions (k > kp, k > k ). Please note that the rate constants for the hydrogenation reactions of are pseudo fist order, which contains in a... 

Figure 8.9 Kinetics of a second-order reaction the racemization of glucose in aqueous mineral acid at 17 °C (a) graph of concentration (as y ) against time (as V) (b) graph drawn according to the linear form of the integrated second-order rate equation, obtained by plotting 1 / A, (as V) against time (as V). The gradient of trace (b) equals the second-order rate constant k2, and has a value of 6.00 x 10-4 dm3mol 1s 1...

J Kovacs, R Cover, G Jham, Y Hsieh, T Kalas. Application of the additivity principle for prediction of rate constants in peptide chemistry. Further studies on the problem of racemization of peptide active esters, in R Walter, J Meienhofer, eds. Peptides Chemistry, Structure and Biology. Ann Arbor, MI, 1975, pp 317-324. 

Carbonylative kinetic resolution of a racemic mixture of trans-2,3-epoxybutane was also investigated by using the enantiomerically pure cobalt complex [(J ,J )-salcy]Al(thf)2 [Co(CO)4] (4) [28]. The carbonylation of the substrate at 30 °C for 4h (49% conversion) gave the corresponding cis-/3-lactone in 44% enantiomeric excess, and the relative ratio (kre ) of the rate constants for the consumption of the two enantiomers was estimated to be 3.8, whereas at 0 °C, kte = 4.1 (Scheme 6). This successful kinetic resolution reaction supports the proposed mechanism where cationic chiral Lewis acid coordinates and activates an epoxide. 

In a variation of this method, Tencer and Stein (1978), mixed the isotopic quasi-racemate to near, but not exactly, zero rotation so that at a certain time, tz, the observed optical rotation of the reaction mixture was zero. The equations for this type of kinetic experiment enable one to calculate the difference between the individual isotopic rate constants from tz and the ratio of rate constants (the KIE) from te and tz provided that the ratio of the initial rotations for the two isotopic substrates is known. Usually it is preferable to... 

A similar situation can occur when the chiral complex (containing a chiral bidentate for instance) can form dimeric species. These dimers may either be the catalyst or just an inactive resting state. Since free energies and rate constants for meso and racemic dimeric species may differ, non-linear behaviour of e.e. of ligand versus e.e. of product can result. 

Racemization of chiral a-methyl benzyl cation/methanol adducts. The rate of exchange between water and the chiral labeled alcohols as a function of racemization has been extensively used as a criterion for discriminating the Sn2 from the SnI solvolytic mechanisms in solution. The expected ratio of exchange vs. racemization rate is 0.5 for the Sn2 mechanism and 1.0 for a pure SnI process. With chiral 0-enriched 1-phenylethanol in aqueous acids, this ratio is found to be equal to 0.84 0.05. This value has been interpreted in terms of the kinetic pattern of Scheme 22 involving the reversible dissociation of the oxonium ion (5 )-40 (XOH = H2 0) to the chiral intimate ion-dipole pair (5 )-41 k-i > In (5 )-41, the leaving H2 0 molecule does not equilibrate immediately with the solvent (i.e., H2 0), but remains closely associated with the ion. This means that A inv is of the same order of magnitude of In contrast, the rate constant ratio of... 

Another historically important reaction is the reorganization of chiral ion pair intermediates of solvolysis of a chiral substrate that leads to racemization of substrate during solvolysis. This reorganization competes with other reactions of the ion pair intermediate of solvolysis of a chiral substrate, so that the relative rate constant for ion-pair racemization can be obtained by determining the relative rates of formation of products from partitioning of the ion pair reaction intermediate, including the enantiomer of substrate (Scheme 14). 

Equation (16) gives the relationship between the rate constant ratio iso = 0.53 and the rate constants from Scheme 15 for partitioning of the ion pair reaction intermediate. Racemization and isomerization will proceed at similar rates, if inversion of the ion pair is much faster than the other reactions... 

The situation is different for solvolysis reactions in most other solvents, where the intermolecular interactions between ions at an ion pair are stronger than the compensating interactions with solvent that develop when the ion pair separates to free ions. This favors the observation of racemization during solvolysis. There are numerous reports from studies on solvolysis in solvents with relatively low dielectric constant such as acetic acid, of polarimetric rate constants (fe , s ) for racemization of chiral substrates that greatly exceed the titrimetric rate constant (fet, s ) for formation of acid from the solvolysis reaction. ... 

All diaryl sulfoxides, some alkyl aryl sulfoxides (e.g., methyl p-tolyl sulfoxide, 41), and some dialkyl sulfoxides (e.g., methyl 1-adamantyl sulfoxide), were found to undergo racemization according to the pyramidal inversion mechanism. Mislow and co-workers (248) have found that for most of the compounds investigated, irrespective of the nature of the substituents attached to sulfur, the first-order rate constant for racemization in p-xylene at 210 C is about 3 X 10" sec", corresponding to a half-life of about 6 hr. Moreover, the activation parameters do not show significant differences and their values were contained in a narrow range 35 to 42 kcal/mol and... 

Oae (251,252) as well as Darwish and Datta (253) investigated the process of thermal racemization of chiral alkylarylsulfimides and diarylsulfimides. It was found to proceed at temperatures as low as 65 to 100°C with a rate constant of the order 1 to 10 X 10" sec" , which corresponds to an activation energy of about 23 to 30 kcal/mol. These data indicate that the thermal racemization of sulfimides is much faster than that of analogous sulfoxide systems. The racemization of sulfimides is a unimolecular reaction practically independent of the polarity of the solvent this property, coupled with the absence of decomposition products, supports the view that racemization of sulfimides occurs by pyramidal inversion. 

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