Racemization activation parameters

The activation parameters of Table 22 suggest that the energy, the charge distribution, and the location of the relevant transition structures (TS) along the reaction coordinate depend significantly on the nature of the moving AOH moiety. When A = H, the racemization and isomerization TS are located early on the reaction coordinate, whereas they are late with A = CH3 and are characterized by comparatively stronger H-bond interactions. 

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... 

An extraordinarily easy thermal racemization was observed for aryl arenethiosulfinates (256). It occurs at a convenient rate at about 50°C. The following activation parameters were estimated for the racemization of p-tolyl p-toluenethiosulfinate 218 A// = 23 kcal/mol A5 = -4 e.u. Thus, the rate of racemization is about 10" times greater than that of diaryl sulfoxides. An internal displacement of sulfenyl sulfur rather than pyramidal inversion was proposed as the mechanism. Recent studies on the chemistry and stereochemistry... 

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. 

Octahedral Complexes 345 Tkble 7.5 Activation Parameters for Racemization and Ligand Dissociation of MCAA)" Ions at 25 °C... 

The racemization of Diels-Alder adducts in the solid state appears to proceed via diradical or complete cycloreversion. For example, (-l-)-96 racemizes in the solid state from 130 to 155 °C (AH =40.0 kcal mol AS =14 cal mol K ) to give (-)-96 (Scheme 9), whereas the melt reaction (eutectic temperature is 165 °C) from 176 to 194 °C has much lower activation parameters (AfJ =... 

Activation Parameters of Nucleophile-Induced Racemization of Triorganosilanes... 

The activation parameters of the pyramidal inversion have been determined for various dialkyl, diaryl, and alkyl aryl sulfoxides. They vary between 35 and 42 keal/mol for AH, and -8 and +4 cal/(mol-K) for AS. These values indicate that, in most cases, the thermal stereomutation of sulfoxides occurs at a significant rate only at about 200 °C. There are a few exceptions, such as benzyl and allyl sulfoxides, whose racemization is raised at 130-150 and 50-70 °C, respectively. 

Activation parameters at coalescence temperature show that the coordinate interaction in these compounds is not a function of the electronegativity of X but is controlled by the ability of the nitrogen atom to stretch the Si—X bond. The tendency of the silicon atom to increase its valency decreases in the order X = OCOR, Br, Cl > SR F > OR, H This sequence corresponds directly to the rate of racemization of halosilanes and to the substitution of R3SiX with inversion of configuration Although no intramolecular coordination was observed in solutions of acetoxysilanes (CH3) Si(OCOR)4 by the Si NMR method the shape of the H NMR spectra of these compounds with chiral silicon atom points to Si -0 interaction... 

Spontaneous racemization is very slow in nonpolar solvents at ambient temperature, but is greatly accelerated by protonic and Lewis acids. Racemization is first order in both 1-phenylethyl chloride and acid. Racemization catalyzed by SnCL, in CC14 at 25° C proceeds with a rate constant ka = 1.5 x 10-2 mol, -L-sec [48]. Because styrene and 1-phenylethyl chloride consumption is 90 times slower than racemization, the rate of racemization is not affected by adding styrene to the system. That is, the efficiency of ion capture by styrene is low, whereas the ion pair collapse must be very fast. Racemization of 1-phenylethyl chloride with SnCl4 is nearly 100 times faster in benzene than in CCL, ka = 1.3 mol-, L sec- at 25° C [49], with activation parameters AHt = 35 kJ-rnol- and A St = 120/mor K-. ... 

In the case of 16a and 16b, it has been demonstrated that the rate constants are almost independent of concentration and solvent polarity, which indicates that the racemization is a unimolecular reaction and does not involve ionic species in the rate-controlling step. The value of k2 is typically about 4.2 x 10 s at 25 °C. Activation parameters have been calculated from the rate constants measured in a temperature range from 20.4 to 39.8°C, revealing the values for A// = 24.3 kcalmol , and AA = —2.0calK . The same authors also reported thermal isomerization in solution between diastereomers of 20 accompanied by decomposition. Gradual isomerization in solution of m-9 into trans- obeying the first-order kinetics (k = 4.0 x 10 s , = 0.966) has also been reported <2005T6693>. As in the case of 16... 

In the mid-1960s Mislow started a research program on the mechanism of the thermal racemization of sulfoxides [97-99]. In the course of these efforts he recognized an enormous racemization rate acceleration for (R)-allyl-p-tolyl sulfoxide ((R)-151) as compared to benzyl or, even more pronounced, to alkyl sulfoxides (Scheme 42). For this compound, prepared by Andersen synthesis [100,101], he found a racemization rate exceeding that of the phenyl-substituted sulfoxide by a factor of 560 000. Based on kinetic measurements Mislow et al. determined the activation parameters to be AH = 22 kcal mor ... 

Racemizations in MeOH between 10 and 45°C resulted in the activation parameters presented in Table 7. For their detailed understanding, theoretical calculations will be required. 

A kinetic study of the racemization equilibrium (eq. [109]) was carried out by a visual fit of observed and calculated 19 F NMR spectra. The rate constant so determined was shown to be first order with respect to the nucleophile for both tetrahydropyran and benzaldehyde. The activation parameters were AZJ = 6.9 kcal/mol and A S = -27.9 e.u. 

The authors have also reported kinetic studies of the thermal racemization of optically active spirosulfuranes 45,46, and the oxide 48. Activation parameters... 

Table 6. Activation parameters for thermal racemization of spirosulfurane 46 in different solvents...

TABLE 10. Activation parameters for nucleophile-catalysed racemization... 

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