Stereoselectivity, ligand coordination

Scheme 1.2 jt-ligands coordinated to the metal atom for the catalytic models whose calculated stereoselectivities have been reported in Table 1.1. 

Racemate separation by stereoselective ligand exchange occurs when a chiral matrix complex has additional coordination sites which are capable of readily exchanging a racemic substrate ligand. The chiral induction, i. e., the efficiency of die matrix complex, is related to the product distribution, which depends on the stability of the complexes with the two enantiomers of the racemic substrate (Fig. 7.1). The problem to be solved in the design of effective chiral matrix complexes for specific racemic substrates is therefore related to conformational analyses of the type discussed above. 

If one wants to estimate the ee via/lF g 9 (cf Regioselectivity), it is necessary to consider all possible ligand coordination modes (for example, all possible coordination modes (axial-equatorial ae/equatorial-equatorial ee) are shown in transition states 8,9 for a C2-symmetric bidentate phosphine ligand (e.g., LIO). In the case of Cl symmetry, each coordination mode is duplicated). Each coordination mode contributes with its asymmetric induction to the total stereoselectivity. To achieve high ee values, at least one of the following requirements should be met [7, 37]. 

In the first part of this chapter, we will present chiral supramolecular architectures formed from achiral building units that are linked via metal-ligand coordination, where chirality of the assembly results only from the asymmetric arrangement of the molecular components. In the second section, we will deal with chiral assemblies that result from resolved chiral bridging ligands, where chiral information can be transferred to the metal centre and hence a predetermination of the absolute configuration at the metal centre can be achieved and, eventually, the control of the chirality of the supramolecular architecture which can also be defined as stereoselective asymmetric supramolecular assembly . 

The hydroxide-promoted exchange of the a-proton of amino acids is catalyzed by metal ions [326]. If the amino acid is coordinated to a chiral complex fragment, the ratio of the two enantiomeric forms of the amino acid is controlled by a chiral intermediate [3]. One of the chiral fragments presented above in the discussion of the stereoselective ligand exchange reactions, [Co(S),(S)-ppm)j (see Table 8.2), was used to promote the epimerization of optically pure a-alanine [327]. The proposed mechanism is shown in Figure 8.10. 

Different stereochemical situations can arise during the synthesis of metal coordination compounds depending on the coordination geometries and the types of donor atoms or ligands coordinated to the metal center [22]. An enantiopure metal complex can be obtained through a stereoselective synthesis or by resolution of a racemic mixture. The first examples of tetracoordmated organometallic arene complexes that could be isolated as pure isomers were cyclopentadienyl half-sandwich diastereoisomers reported by Brunner and Schmidt [86-88]. 

A systematic study of a series of chiral diphosphites with C2 symmetry and various bridges between the two phosphorus atoms on the structure of the penta-coordinate rhodium complex [RhH(diphosphite)(CO)2] and the stereoselectivity in styrene hydroformylation revealed that the highest enantioselectivities are obtained with ligands coordinating in an equatorial-equatorial fashion to the rhodium (273). 

The diimine of benzaldehyde with // j-(9-l,2-cyclohexanediamine reacts with [PtMe2(//-SMe2)]2 to form a methyl complex with the cyclometallated 6 ,A, iV-tridentate ligand 636. The complex exists as a mixture of racemic stereoisomers. Protonation by [H(OEt)2] in the presence of ethylene produces a cationic complex with 7r-coordinated ethylene 637 (Equation (136)). Complexes with styrene and with propene 638 exhibit high stereoselectivity toward coordination of enantiofaces of the prochiral olefms. ... 

Information about the stability and stereoselectivity of coordinately unsaturated metal carbene complexes is important for understanding a number of reactions, including olefin metathesis and cyclopropanation. A study of CO exchange in [(CO)5MC(OMe)(Ph)] (M = Cr, Mo, or W) shows that the carbene ligand is cis labilizing compared to CO. Ratedetermining CO loss is inferred from the similar rates of "CO exchange and cis-CO substitution by phosphines. Measurements of the ratio cis- CO/frans- CO as a function of percent CO incorporation imply that... 

A model for the mechanism of the highly enantioselective AlMe-BINOL-cata-lyzed 1,3-dipolar cycloaddition reaction was proposed as illustrated in Scheme 6.13. In the first step nitrone la coordinates to the catalyst 11b to form intermediate 12. In intermediate 13, which is proposed to account for the absolute stereoselectivity of this reaction, it is apparent that one of the faces of the nitrone, the si face, is shielded by the ligand whereas the re face remains available... 

Agbossou E., Carpentier J. E. Hapiot E., Suisse I., Mortreux A. The Aminophos-phine-Phosphinites and Related Ligands Synthesis, Coordination Chemistry and Enantioselective Catalysis Coord. Chem. Rev. 1998 I78-I80 1615-1645 Keywords stereoselective Diels-Alder reaction catalysts, aminophosphine-phosphinites, enantioselective catalysts... 

In 2004, Dieguez et al. reported the development of novel C2-symmetric dithioether ligands derived from the corresponding binaphthyl or biphenanthryl diols. Thus, various (i )-binaphthyl dithiols substituted by alkyl groups on the sulfur atom in order to increase the steric bulk were synthesised, and the corresponding mononuclear cationic Ir(I) -cyclooctadiene complexes were prepared and characterised (Scheme 8.20). NMR studies demonstrated that, in all cases, the coordination of the ligands proceeded with complete stereoselectivity at the... 

Cp2Zr(H)(Cl) (8). The apparent record for catalyzed double bond movement is on 9-decene-l-ol to decanal (nine positions) using Fe3(CO)i2 (9). However, 30 mol % was required, which means that nearly a mole of metal was used per mole of alkenol. Herein we expand upon our initial report (10) of a very active catalyst (1) which has been shown to move a double bond over 30 positions. Catalyst 1 appears to have an intriguing and useful mode of action, in which the pendant base ligand performs proton transfer on coordinated alkene and Ti-allyl intermediates in a stereoselective fashion. 

Presumably, the stereoselectivity in these cases is the result of coordination of iridium by the functional group. The crucial property required for a catalyst to be stereodirective is that it be able to coordinate with both the directive group and the double bond and still accommodate the metal hydride bonds necessary for hydrogenation. In the iridium catalyst illustrated above, the cyclooctadiene ligand (COD) in the catalysts is released by hydrogenation, permitting coordination of the reactant and reaction with hydrogen. 

Racemic amino acids have been resolved via stereoselective coordination to the square planar chiral matrix complex (178).584 The bisamidobispyridyl ligand (179) forms a square planar Ni11 complex with considerable tetrahedral twist due to repulsion of the ortho protons of the pyridyl rings.585... 

N-donor ligand. The reaction appears to proceed via an acyclic iminoplatinum(II) intermediate that undergoes a subsequent intramolecular cyclization. Some mechanistic aspects of this versatile reaction have been elucidated.225,226 A4-l,2,4-oxadiazolines have been prepared by the [2+3] cycloaddition of various nitrones to coordinated benzonitrile in m-[PtCl2( D M SO)(PhCN)] precursors.227,228 Racemic and chiral [PtCl2(PhMeSO)(PhCN)] complexes have also been used in order to introduce a degree of stereoselectivity into the reaction, resulting in the first enantioselective synthesis of A4-l,2,4-oxadiazolines, which can be liberated from the complexes by the addition of excess ethane-1,2-diamine. 

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