Pfeiffer effect, equilibrium shift

The Pfeiffer effect (the shift in a chiral equilibrium on the addition of an optical isomer of a different compound) of racemic [Cr(ox)3]3- has been examined using for the first time optically stable metal complexes cis-[MXY(AA)2]"+ (where M = Cr3+ or Co, AA = en or tmd and X and Y = anionic monodentate ligand). It was found that the chiral equilibrium of [Cr(ox)3]3-was always displaced in favour of its A enantiomer in the presence of A enantiomers of the cis complexes, and it is proposed that the absolute configurations of cis complexes could be inferred from the equilibrium shift induced in [Cr(ox)3], 438 Laser irradiation of an aqueous solution of racemic [Cr(ox)2(phen)] or [Cr(ox)(phen)2]+ in the presence of ( + )- or (- )-cinchonine hydrochloride rapidly shifts the chiral equilibrium in a direction opposite to that induced by the usual Pfeiffer effect in the dark.439... 

Dwyer and co-workers (2) and Kirschner and co-workers (3) have proposed that the enantiomers of racemic mixtures of optically labile, dissymmetric complexes are in equilibrium in solutions containing no environment substance, and that the equilibrium constant in such systems is equal to 1. However, in the presence of one enantiomer of an optically active environment substance, this equilibrium is shifted, with a consequent enrichment of one of the enantiomers of the complex, thereby changing the equilibrium constant to something greater or less than 1. An equation which represents a typical Pfeiffer Effect equilibrium is ... 

The Pfeiffer effect is a term used to describe changes in the optical activity of solutions containing a chiral compound (the environmental substance ) on the addition of a racemic dissymmetric complex. The effect is generally attributed to a shift in the position of the equilibrium between d and l isomers for the racemic complex. The exact mechanism involved in mediating the chiral interaction is unknown. Perhaps surprisingly, both environmental substance and complex may simultaneously be cations. Studies of the Pfeiffer effect usually involve a moderately labile racemic complex [Cr(ox)3]3 is a popular choice for such studies, summarized in Table 82. Other studies of the optical activity of tris oxalates include work on photoinduced optical activity,898 photoracemization899 and the solid-state racemization of K3[Cr(ox)3]. 900 901... 

Paul Pfeiffer discovered a very interesting stereochemical phenomenon, which now bears his name — the Pfeiffer effect this has received a good deal of attention.30 When an optically active substance which is stable in solution is added to a solution of a labile chiral substance, the optical rotation of the solution changes, reaching a new level in some hours. Several theories have been advanced to explain the phenomenon, the most satisfactory based on the supposition that the optically active ion or molecule forms an association with one isomer of the racemic pair of the labile substance and thus shifts the dextro—levo equilibrium. In general it is not possible to use this as a means of resolution, for when the added optically active substance is removed from the labile material, the latter immediately racemizes. 

Source of the Pfeiffer Effect. No completely satisfactory explanation has yet been set forth which accounts for all of the observations associated with the Pfeiffer effect. Dwyer and co-workers (7) have proposed a configurational activity explanation which states that the dextro and levo enantiomers of optically active, labile complexes in solution are in equilibrium (with Keq. = 1), but that in the presence of an optically active environment the equilibrium shifts in favor of one of the enantiomers, resulting in a change in optical rotation. However, this proposal does not account for the fact that the effect is observed for some labile complexes and not for others. 

The experiments described above have led to the conclusion that the chiral-induced equilibrium shift could be induced in the lanthanide (III) complex by a combination of electrostatic and hydrophobic interactions. Hydrogen bonding effects appear to be less important as suggested by experiments carried out under variable pH, temperature, concentration, solvent type, and solution dielectric constant conditions. By analogy to the associated/dissociated equilibrium shift models of Schipper (1978) in which the source of the equilibrium perturbation is attributed to a free energy of mixing, Brittain (1981) and Wu et al. (1989) attempted to ascertain the complex mechanism responsible for this type of Pfeiffer effect. However, their conclusions were opposite as they concluded that the [Tb(DPA)3] complex interacted with the... 

It was noted (Table T) that the equilibrium constants calculated according to equation 2 (for the enantiomeric shift described by equation 1) were not, in fact, constant. Therefore, it is proposed that, while the equilibrium described by equation 1 can hold for solutions of racemic mixtures of optic y labile complexes themselves, it does not accurately describe Pfeiffer Effect equilibria for such systems that also contain optically active environment substances. 

KIRSCHNER ET AL. Equilibrium Shift Mechanism for the Pfeiffer Effect 305... 

Data derived by Dwyer on the ratio of the rate constants (kjki) has been criticized by Harris since the values used by Dwyer for the rotation at the end of the racemization of d-[Ni(phen)3] were those obtained immediately after adding the optically active ion to the racemic mixture of the complex in solution. The ratio thus was not the equilibrium rotation. It was not reasonable, according to Harris, to use the data in support of an equilibrium theory for the Pfeiffer effect. Contradictory results are reported by Craddock and Jones in that no difference was found for the racemization rates for either isomer (e.g., d- or /-[Ni(phen)3] ) if the complex is in the presence of an optically active species. These authors point out that another environmental factor, temperature, could have accounted for unusual or anomalous rotations previously found. It is evident that something more than an equilibrium shift or a configurational activity is needed to explain Pfeiffer rotations. 

The configurational activity concept is not in keeping with the time-rotation study of the present authors as well as Pfeiffer s own studies on the system [Ni(phen)3](BCS)2. Since the racemization rate of the nickel(II) complex is slow, any shift in the equilibrium of the d- and /-isomers of the complex should also be slow. The Pfeiffer effect for this complex would be expected to increase from a zero value to a maximum. The observed data shows an instantaneous increase (vide supra) that is about 46% of the final rotation. Although from this study an equilibrium shift may be operative it cannot be the only effect. 

The Pfeiffer effect is not adequately explained by the concept of an equilibrium shift or configurational activity. There is strong evidence for interaction between the resolvable complex and the optically active species by a bonding force that is proposed herein to be hydrophobic in character. Ion pair interaction (when oppositely charged species exist), equilibrium shift, differential association and the hydrophobic bonding are suggested as contributing to the Pfeiffer activity. 

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