Racemization rate

Stereochemical constraints in cyclic sulfones and sulfoxides impart increased weight to strain and conformational factors in the generation of carbanions and their stability, causing distinct differences between the behavior of cyclic and open-chain systems233, due primarily to the prevention of extensive rotation about the C —S bond, which is the major way that achiral carbanions racemize. Study of the a-H/D exchange rate fce and the racemization rate ka may provide information concerning the acidity-stereochemical relationships in optically active cyclic sulfone and sulfoxide systems. 

The DKRs of (J-azido alcohols and p-hydroxy nitriles were also accomplished hy employing 1 and CALB with PCPA as the acyl donor. The DKRs of p-azido alcohols were performed at 60°C while those of (3-hydroxy nitriles required higher temperature (100°C) primarily to enhance the racemization rate. The optical purities of products were satisfactory in all cases. In the case of p-hydroxy nitriles, dehydrogenation lowered the yield. 

The small increase in racemization rate observed when an aqueous solution of L-pro-line was heated under reflux on a MW oven at atmospheric pressure could be attributed to localized superheating or a generalized superheating of the solvent. It is known that water superheats by 4—10 °C when boiled in a MW oven [39, 40]. 

The study of the decomposition of optically active 1-methyl-2,2-diphenylcyclopropanoyl peroxide proved the retention (37%) of the product of the geminate radical pair recombination [90]. The radical center in the formed cyclopropyl radical is so strained that the racemization rate is unusually slow. 

Bada, J. L., Shou, M., Effects of various environmental parameters on amino acid racemization rates in fossil bones, Geological Soc. America, Abstracts with Programs, 8, 762-763... 

Because at ambient temperatures the racemization rates of all amino acids are slow, it is usually found that aspartic acid is most useful archaeologically, but over much longer geological timescales aspartic acid may become racemic... 

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... 

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. 

Polarimetric Determination of Intrinsic Racemization Rates Typical Procedure 29 ... 

Fujiwara and Yamamoto have studied the racemization of anhydrous compounds of the type [M(phen)3]X2 and [M(bipy)3]X2 where M = Fe or Ni and X = Cl-, Br-, I- or C104-.26 The rates of racemization of the nickel compounds were found to depend on the identity of the anion racemization rates for the iron compounds were anion-independent. However, the kinetic analysis has been called into question.27 28 If valid, the kinetic anion effect for the nickel compounds follows an order that is opposite that found by Kutal and Bailar23 for the racemization of [Co(en)3]X3 salts C104 < I- < Br- < Cl . It was proposed25 that the iron compounds racemize by a twist mechanism and that the nickel compounds racemize by nucleophilic attack by lattice anions to form a seven-coordinate transition state. 

Steric isotope effects are well documented in the literature of organic chemistry, and have been measured in several reactions in which severe crowding occurs. Mislow and co-workers first demonstrated such effects by determining racemization rates of I, where R = H or D [92], The inverse... 

It may also be surprising how easily this racemization may occur. Friedman and Liardon (126) studied the racemization kinetics for various amino acid residues in alkali-treated soybean proteins. They report that the racemization of serine, when exposed to 0.1M NaOH at 75°C, is nearly complete after just 60 minutes. However, caution must be used when examining apparent racemization rates for protein-bound amino acids. Liardon et al. (127) have also reported that the classic acid hydrolysis, employed to liberate constituent amino acids, causes amino acids to racemize to various degrees. This will necessarily result in D-isomer determinations that are biased high. Widely applicable correction factors are not possible since the racemization behavior of free amino acids is different from that of amino acid residues in proteins (which can be further affected by sequence). Of course, this is not a problem for free amino acid isomer determinations since the acid hydrolysis is unnecessary. Liardon et al. also describe an isotopic labeling/mass spectrometric method for determining true racemization rates unbiased by the acid hydrolysis. For an extensive and excellent review of the nutritional implications of the racemization of amino acids in foods, the reader is directed to a review article written by Man and Bada (128). 

Know the meaning of SN2 mechanism, inversion of configuration, SN1 mechanism, racemization, rate-determining step, E2 and E1 mechanisms. 

Fig. 26 A Chiral memory effect applied to the construction of a chiral softball homo-dimeric capsule made of achiral units. B Racemic softball binds a chiral guest leading preferentially to one diastereoisomer. Fast exchange, compared to capsule s racemization rate, with a better achiral guest allows preparation of enantioenriched capsule...

Fig. 8 Series of chelating ligands 1, 3 and 5, forming the three complexes 2 (mononuclear), 4 (dinuclear) and 6 (tetranuclear), respectively, illustrating the concept of mechanical coupling by slowing racemization rates of individual metal centers from 10 s 1 (2) [118] to 0.1 s 1 (4) [117] and no observed racemization (6) [119]...

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