Kinetics of racemization

Recently, Lacour, Sauvage and coworkers were able to show that the association of chiral [CuL2] complexes (L=2-R-phen,6-R-bpy and2-iminopyridine) with TRISPHAT 8 leads to an NMR enantiodifferentiation, which allows the determination of the kinetics of racemization of the complexes (bpy=2,2 -bipyri-dine phen=l,10-phenanthroline) [119]. This type of application has recently been reported in conjunction with chiral sandwich-shaped trinuclear silver(l) complexes [122]. Several reports, independent from Lacour s group,have confirmed the efficiency of these chiral shift agents [123-127]. Finally, TRISPHAT can be used to determine the enantiomeric purity of (r] -arene)chromium complexes. These results broaden the field of application of 8 to chiral neutral, and not just cationic, species [114,128,129]. 

A number of systematic structural analyses have been described for families of saturated oxazolones. First, as mentioned previously, detailed smdies of NMR long-range coupling in 2,4-disubstimted-5(47/)-oxazolones and in 5(27/)-oxazo-lones have been reported." Similarly, detailed NMR studies of the kinetics of racemization of 2,4-disubstimted-5(47/)-oxazolones have been performed. A theoretical study of the spectral-luminescence properties of some 4-alkyl-2-phenyl-5(47/)-oxazolones has been reported and an investigation of the infrared (IR) and Raman spectra of 5(4//)-oxazolones, particularly of the carbonyl group vibration, has been reported. Electron impact mass spectra of saturated 5(47/)-oxazolones have been published. More recently this technique has been used to distinguish between the stereoisomers of some spirocyclopropane oxazolones 352 (Fig. 7.36). Finally, several studies of the HPLC behavior of 5(47/)-oxazolones complete a general view for this family of compounds. " " ... 

The reaction was studied theoretically by Hoffmann who concluded that it should involve a double rotation of the methylene groups.81 A study of the kinetics of racemization vs. isomerization of optically active trans-l, 2-dideuteriocyclopropane provided evidence for this process.82 Further experimental studies have made the interpretation of the details of this reaction more difficult.83 The most recent calculations84 have concluded that the conrotatory double rotation is slightly preferred. It is likely that substituents could have a major effect on the course of the reaction. 

The complexes [Cr(C204)(N—N)2]C1, K[Cr(C204)2N—N] (N—N = bipy or phen) and [Cr(C204)(bipy)phen]Cl have been resolved.538,54 45 The kinetics of racemization of a number of chelates described in this section have been studied and evidence for both intermolecular... 

A comprehensive study662 of the kinetics of racemization and aquation of [Rh(ox)3]3 showed that both racemization and aquation proceed by two-term rate laws ... 

An alternate mathematical treatment of the first-order kinetics of racemization gives equation 3. However, equation 2 is operationally more efficient than equation 3 because we can measure D/L ratios with a 1-3% error compared to a 10-15% error when concentrations of D only are measured. 

One of the possible explanations for the kinetics of racemic III fluoresceme (in vviiich dual exponential decay in the monomer r ion is ( served) is shown in Scheme 4 Fig. 32, in x4iich the estabMunent of an equilibrium between GG and TT ground-state and excited state conformation precedes formation of a TG excimer from TT . Intramolecular excimer formation in dinaphthyl alkanes has been dmHarly own to be related to the conformation of the ground-state molecule and the facility for conformational rdaxation to the excimer state Thus koM (see Scheme 1) for meso 2,4-dinaphthyl pentane was much greater than that for racemic 2,4-dinaph-thyl pentane, as in lU above. 

The first experimental determination of the inversion barrier of a tertiary arsine was reported in 1971 ". The kinetics of racemization of (/ )-(—)- and (S)-( + )-ll, resolved by the metal complexation method, at 217.6 +0.3 °C in decalin (sealed tube) was determined polarimetrically in the 310-350 nm region. From the kinetic data, by substitution into the Eyring equation, the free energy of activation, AG was calculated to be 175 + 2kJmol at 217.6°C. This energy value corresponds to a half-life for racemization of the arsine of ca 740 h at 200 °C. It had been reported previously that resolved ethylmethylphenylarsine and methyl(n-propyl)phenylarsine showed no detectable loss of optical activity over 10 h at 200 On the basis of photoracemization studies... 

A number of reagents convert an optically pure 2-phenylhydantoic acid to an optically active 5-phenylhydantoin. Trifluoroacetic anhydride and dilute hydrochloric acid are the reagents of choice. The temperature dependence of the kinetics of racemization of 5-phenylhydantoin in dilute hydrochloric acid supports the conclusion that the resultant products are nearly, if not entirely, optically uniform.8... 

Amino acids are racemized by concentrated acid (50, 51, 52) and base (52, 53, 54) at elevated temperatures, and some preliminary experiments have shown that at 116° aspartic acid is racemized slowly at neutral pH values (38, 55). Also, the amino acids in fossil shells are partially racemized, with the amounts of racemization increasing with the age of the shell (56, 57, 58) racemization is essentially complete in shells of Miocene age. Since the kinetics of racemization of amino acids have not been investigated in detail at any pH, I have recently carried out a detailed study of the kinetics of racemization of aspartic acid between pH 0 and 13 and also the kinetics of racemization of phenylalanine, alanine, and isoleucine at pH 7.6. The results of these investigations are reported herein. 

Kinetics of Racemization of Amino Acids. Phosphate was used to buffer solutions of the L-amino acids at pH 7.6. Solutions of L-aspartic acid were buffered at the various pH values by either hydrochloric acid, oxalate, succinate, phosphate, borate, or NaOH. The pH values of the buffered solutions at the elevated temperatures were estimated as described previously (38). Sodium chloride was added to the solutions to adjust the final ionic strength to 0.5. The solutions were degassed and sealed under vacuum in borosilicate glass ampules. The ampules were sterilized immediately after being sealed by heating at 100 °C for 15 to 20 minutes. The rates of racemization were determined from measurements of the rate of change of optical rotation (a) of the solutions. The measurements were made on a Perkin-Elmer 141 polarimeter at 365 nm. With the exception of the aspartic acid solutions at pH values less than 2 and the phenylalanine solutions, all samples were diluted in 1M HC1 before measuring the rotation. 

The kinetics of racemization of hyoscyamine to atropine in different alcohols have been studied.Optimum yields were obtained in Mc2CHCH20H. 

For the tris(oxalato)rhodium(III) ion in water, Damrauer and Milbum have studied the kinetics of racemization, aquation and exchange of the oxalate oxygens. The racemization and aquation have acid-dependent pseudo-first-order rate constants given by... 

The kinetics of racemization of siIicon(ivj complexes of four types of / -diketonate... 

The kinetics of racemization of (+)s89-[Co(bipy)2(paox)]N03 (21) (paox is the dianion of 2-ketopropionic acid oxime) has been studied in 0.1 M NaNOs with 0.05 M pH 7 phosphate buffer over the temperature... 

The kinetics of racemization of the pseudo-tetrahedral compound (13) have been studied by n.m.r. and polarimetric techniques. Racemization... 

The kinetics of racemization of the [Co(ox)3] anion in the presence of perchloric, hydrochloric, or sulphuric acid are consistent with an Al mechanism. Some features of this acid catalysis do not correspond with metal-ion-catalysed racemization of this complex, which may mean that different mechanisms are involved. An obvious difference would be between an intramolecular twist involving no metal-ligand bond breaking on the one hand, and a mechanism with a unidentate oxalato-complex as intermediate on the other. ... 

---