Dissymmetric complexes

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... 

The idea that the instantaneous equilibrium between the different conformations of dissymmetric complex ions could be influenced by ion-pair formation finds some support in recent experiments of Mason and coworkers with diastereoisomeric cobalt(III) complexes. Thus, Mason and Norman (31) have shown that the circular dichroism spectra of D-Co(d-pn)3" and L-Co(d-pn)3" (pn = propylenediamine) are differently changed by oxo-anions such as P04, S04 and 8203 . According to these authors the oxo-anion and D-Co(d-pn)3+ (or D-Coen3" ), but not L-Co(d-pn)3 should have a preferred mutual orientation in the ion pairs. At low concentrations the effect of sulfate and thiosulfate ions on the circular dichroism of D-Co(d-pn)3" is similar to that of phosphate but with these anions at concentrations > 0.2M the previous changes are reversed— probably due to breaking down the specific orientation of the ion pair in the denser ionic atmosphere. It is also interesting that Mason and Norman (32) have found that Co(NH3)6 associated with d-tartaric acid produces a pronounced Cotton effect. These results show that some... 

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 asymmetric compound [(pico)Cl2(pico)-Tc-0-TcCl(pico)3Cl ° is an isomer to the just described dissymmetric compound. The two types of complexes differ in their arrangement of equatorial ligands around each Tc atom. Both complexes were prepared in the same route. The asymmetric species was suggested to be the precursor of the dissymmetric complex, lire dark reddish-brown [(pico)Cl3(pico)-Tc-0-TcCl (pico)3Cl]° crystallizes in the monoclinic space group P2jln with n=]2.421(1), 6=15.471(1). c-18.764(1) A. /1=93.174(5)", and Z=4. The geometry around each Tc atom is essentially octahedral, lire Tc atoms are linked by a p-oxo bridge with the... 

Development of nonracemic intermolecular Pauson-Khand reactions has followed two directions. The first efforts addressed the generation of dissymmetric complexes of the form Co2(CO)5L alkyne, in which L is an enantiomerically pure ligand. Using (/ )-... 

The first well-defined, stable d°-alkylidene species having catalytic activity in alkene metathesis was Ta(=CHtBu)(OtBu)2Cl(PMe3) (5) [16]. However, research efforts moved rapidly to Mo and/or W alkylidenes, first in the form of 0X0 complexes (6) [17, 26], and then with the more easily accessible, imido bisalkoxy alkylidene complexes of general formula M(=NR )(=CHR )(OR )2 (7) [18, 19, 27], as well as the isoelectronic Re-alkylidyne-alkylidene complexes Re(=CR )(=CHR )(OR )2 (8) [28]. In 2001, it was shown that the silica-supported, Re-alkylidyne-alkylidene complex (=SiO)Re(=CtBu)(=CHtBu)(CH2tBu) (12) was unexpectedly very active [21,29]. According to computational studies, which will be described later in this chapter, the remarkable reactivity of this silica-supported catalyst arises from the different nature of the X and Y ligands (referred to as dissymmetric complexes) [7]. In addition, new molecular precursors based on bis-alkyl and bis-pyrrolyl M(=NR )(=CHR )(X)2 complexes became available [30]. Consequently, from 2005, considerable research effort has been devoted toward generating silica-supported (14 to 16 in Scheme 6.4) [23, 31, 32] and molecular (17 and 18 in Scheme 6.4) [22, 33, 34] Mo- and W-imido complexes... 

A novel chiral dissymmetric chelating Hgand, the non-stabiUzed phosphonium ylide of (R)-BINAP 44, allowed in presence of [Rh(cod)Cl]2 the synthesis of a new type of eight-membered metallacycle, the stable rhodium(I) complex 45, interesting for its potential catalytic properties (Scheme 19) [81]. In contrast to the reactions of stabihzed ylides with cyclooctadienyl palladium or platinum complexes (see Scheme 20), the cyclooctadiene is not attacked by the carbanionic center. Notice that the reactions of ester-stabilized phosphonium ylides of BINAP with rhodium(I) (and also with palladium(II)) complexes lead to the formation of the corresponding chelated compounds but this time with an equilibrium be-... 

Tridentate amines come in a variety of forms. Diethylenetriamine (dien) (129) and its bis-trimethylene homolog ditn (130) may adopt either facial or meridional coordination modes. Furthermore, the /ao[Co(dien)2]3+ complex may exist as either the centrosymmetric meso or dissymmetric rac isomer where the secondary amines are trans or cis respectively.662 However,... 

D. H. Metcalf, S. W. Snyder, J. N. Demas, andF. S. Richardson, Chiral discrimination in electronic energy-transfer processes between dissymmetric metal complexes in solutions, 1. Am. Chem. Soc. 112, 5681-5895(1990). 

P NMR spectroscopy shows that the complex [RhH(CO)2(R,S-46)] is a single species at 60 °C and 25 °C in toluene-d8 under 1 bar of CO. No apical-equatorial interchange was observed at any temperature. The unique dissymmetric environment in a single catalytically active species created by the phosphine-phosphite seems to be an important factor in the high enantioselectivity obtained. 

When only one heteroatom of the dinucleophile possesses a hydrogen substituent, the reactions lead instead to alkenyl complexes rather than carbene compounds. Effectively, treatment of diphenylallenylidenes 1 and 6 with pyrazoles yields the heterocyclic derivatives 61 (Scheme 2.25) [76]. Interestingly, the dissymmetric 3-methylpyrazole (R=H, R = Me) provides only one regioisomer, in which the methyl group points towards the metal. This process, which formally corresponds to the addition of two nitrogen nuclei at C and Cy and a hydrogen atom at Cp, is assumed to take place through an initial nucleophilic attack at the Ca position. 

This dissymmetric phosphine is prepared by standard reactions from ( + )-l,T-binaphthyl and is then resolved as a chiral Pd(II) complex. 

These are extremely common and can be a problem when the synthesis of the related tris complexes is being attempted, particularly as they are of rather limited solubility. They have been known for many years.1148,1160,1161 The isomers possible for these species are profuse.1149 For the general formula [(aa)2Cr-( U-OH)2Cr(aa)2] there are three possible geometric configurations about each metal ion tram N cis O, cis N tram O and cis N cis O. In addition, each metal ion is a dissymmetric centre bringing the total number of geometric and optical isomers to 21. 

It is interesting to note that whereas four different unidentate ligands are required for asymmetry in a tetrahedral complex, only three are needed for the octahedron. An asymmetric or dissymmetric tetrahedral complex involving a transition metal ion has not yet been resolved (see, however, ref. ... 

Lateral macrobicycles are dissymmetric by design thus, monoelectronic reduction of the Cu(ll) ion bound to the [12]-N2S2 macrocyclic subunit in the bis-Cu(ll) cryptate 45, gives a mixed valence Cu(i)-Cu(ll) complex [4.6]. Macrotricycle 46 forms a dinuclear Cu(ll) cryptate that acts as a dielectronic receptor and exchanges two electrons in a single electrochemical wave [4.7]. Complexes of type 47 combine a redox centre and a Lewis acid centre for the potential activation of a bound substrate [4.8]. 

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