Enantiomers interconversion between

Interestingly, Peters (1988) notices that the methylene protons are enantiotopic for the light lanthanides (R = Ce-Dy) and become diastereotopic for R = Ho-Yb which implies a dynamic intramolecular interconversion between the two helical enantiomers P- R(Ll-2H)3]3 M-[7 (L1-2H)3]3 occurring at a moderate rate on the NMR time scale. The... 

Penta(isopropyl)- and penta(neopentyl)-Cp cobalticinium salts, on the other hand, have a high enough barrier to interconversion (AG% 17.1 and 19.4 kcal/mol, respectively) with coalescence temperatures of 65° and 100°C, respectively, in the NMR (14). Thus, both enantiomers can coexist at 20°C with only a slow interconversion between them. In tricarbonyl-methyl-penta(isopropyl)cyclopentadienyl molybdenum the activation energy has been estimated to 13 1 kcal/mol (211) (cf. comment in Section IV, C, 2). 

The geometry of the enolate intermediate was investigated by trapping experiments. When 23 (93% ee) was treated with potassium hydride and acetic anhydride in the presence of 18-crown-6, E-enol acetate 25 (59%) and its Z-isomer 26 (6%) were obtained together with the recovery of 23 (27%). The ee of the recovered 23 was unchanged. These observations indicate that the -enolate is the major intermediate under conditions of kinetic control. HPLC analysis of 25 with a chiral stationary phase indicated the existence of a pair of enantiomers (Figure 3.3a). Rapid interconversion between... 

Of the nine stereoisomers of inositol, the scyllo-isomer has no axial hydroxyl, the myo-isomer has one, the epi-, chiro-, and neo-isomers have two, and the alio-, cis-, and mneo-isomers have three hydroxyl groups (Figure 1). Of these, six isomers scyllo-, myo-, epi-, neo-, cis-, and mr/co-i somers) have one or more planes of symmetry in the molecule (meso compounds) and are therefore not chiral. D-chiro- and L-e/u>o-i somers do not have a plane of symmetry and are chiral molecules moreover they are enantiomers of each other. The alio-isomer is unique - the conformational isomer of (10) (Figure 2) is (11) which is also its enantiomer Since interconversion between conformational isomers is rapid, a//o-inositol exists as a 50/50 mixture of the two enantiomers at room temperature. Therefore, although alio-inositol is chiral, the compound is optically inactive at room temperature because it is a racemic mixture a chiral reagent, such as an enzyme, would be expected to preferentially react with one enantiomer and not the other. 

A simple tertiary amine cannot be resolved into its enantiomers at normal temperatures, because there is rapid interconversion between the two forms. This rapid interconversion is called the umbrella effect. A similar effect occurs in simple carbanions, and so they cannot be resolved either. This means that any stereochemical information at such a centre is lost. 

The enzyme exists in two different protonation states of the active site cysteines, each binding a different enantiomer. Conversion between enantiomers can be through the racemization path (upper manifold of Fig. 7.16) or through direct proton exchange with water (lower manifold of Fig. 7.16). Knowles and coworkers found that interconversion of enzyme protonation states was kinetically significant [82]. This was determined by measuring rates of tritiated proline washout as a function of the proline concentration. It was found that higher concentrations of proline promote slower washout of the Ca proton. Additional support for the rela-... 

Interestingly when a racemic mixture of the helical complexes M(phen)3 + [M = Fe(II), Co(II) and Zn(II)] was added to the per-C02 - S-CD host, the circular dichroism spectra showed induced Cotton effects which in the case of the iron complex indicates that the per-C02 - S-CD enriches the A-enantiomer of Fe(phen)3 +. This behaviour, known as the Pfeiffer effect, occurs when a chiral complex with labile configuration interacts with optically active species to form a pair of disatereomers with one being more stable and hence interconversion between the A- and the A-enantiomers easily occurs. 

On the other hand, it has also been shown that interconversion between A- and A-enantiomers of [Fe(phen)3] " " occurs easily and the A-isomer is enriched upon complexation of rac-[Fe(phen)3] " " with a right-handed DNA double helix.This behaviour, known as the Pfeiffer effect, was also observed when the same iron complex rac-[Fe(phen)3] " ", which is configurationally labile, was bound to the optically active species such as A-TRISPHAT (4.7) or the heptaanion modified cyclodextrin (4.12) (see Sections 4.1.3 and 4.2). 

Barriers for interconversion between BC-3 and its enantiomer, BC-7, have been calculated. Passage through either a BC-5 or a BC-1 conformation, are calculated to be 5.0 kcal mol and 6.7 kcal mol , respectively. The small barriers allow for a rapid pseudorotation between BC-3 and BC-7, even at low temperatures, and account for the symmetric spectra. NMR results on oxocane-2,2,7,7-d4 have allowed a definitive assignment for the H and C chemical shifts. Lanthanide-induced shift reagents provide compelling proof, that the lowest energy conformations are BC-3 and BC-7. 

Norton s group has demonstrated that the insertion of chiral C2-symmetric diphenylethylene carbonate into the Zr-C bond of a zirconaaziridine led to the asymmetric synthesis of an amino acid methyl ester after a subsequent treatment with NaOH in methanol. Since the zirconaaziridine enantiomers interconverted, the reaction was a DKR. It was shown that the efficiency of this process was determined by the balance between the rate of enantiomer interconversion and the rate of insertion. A slow addition of the inserting enan-tiopure carbonate was often required to maximise the stereoselectivity of the reaction, allowing a diastereoselectivity of up to 90% de combined with a quantitative yield to be obtained, as shown in Scheme 1.59. 

Scheme 5.11 Coordination polyhedra of the eight enantiomers of Ln [DTPA-bis(amide)] complexes, assuming that the geometry is a tricapped trigonal prism. Interconversions between the two columns (mirror image) correspond to the wagging process (racemization at N3), while the interconversions between rows in a column result in racem-ization at N1 and N3. Adapted with permission from H. hammers et al, Inorg. Chem. 36, 2527 (1997) [180]. Copyright (1997) American Chemical Society.

Biochemistry s hidden asymmetry was discovered by Louis Pasteur in 1857. Nearly 150 years later, its true origin remains an unsolved problem, but we can see how such a state might be realized in the framework of dissipative structures. First, we note that such an asymmetry can arise only under far-from-equilibrium conditions at equilibrium the concentrations of the two enantiomers will be equal. The maintenance of this asymmetry requires constant catalytic production of the preferred enantiomer in the face of interconversion between enantiomers, called racemization. (Racemization drives the system to the equilibrium state in which the concentrations of the two enantiomers will become equal.) Second, following the paradigm of order through fluctuations, we will presently see how, in systems with appropriate chiral autocatalysis, the thermodynamic branch, which contains equal amounts of L- and D-enantiomers, can become unstable. The instability is accompanied by the bifurcation of asymmetric states, or states of broken symmetry, in which one enantiomer dominates. Driven by random fluctuations, the system makes, a transition to one of the two possible states. 

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