Asymmetric Ligand Environments

In order to generate more structurally relevant biomimetics for dinuclear metallohydrolases much effort has been devoted to the synthesis of asymmetric ligands. These ligands are considered to be more suitable models for the asymmetric coordination environment found in enzymatic systems. Nordlander et al. proposed that asymmetric complexes are not only more appropriate functional models for the active site of phosphoesterase enzymes, but also that they exhibit enhanced catalytic rates compared with their symmetric counterparts [1-3]. A selection of ligands used to generate purple acid phosphatase [1, 4, 5, 6-10], phosphoesterase [11], urease [12, 13], catechol oxidase [14] and manganese catalase biomimetics [15, 16] is displayed in Fig. 7.1. 

Some of these ligands generate a hard and a soft coordination site for the generation of heterodinuclear model complexes for purple acid phosphatase metalloenzymes [6, 8]. In some cases the ligands have one stmcmral variation in one arm [6], in others one donor arm has been omitted [1, 14]. Often the vacant coordination site is found to be occupied by water or solvent molecules in the complex [15, 16]. 

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The use of asymmetric catalysts in chiral syntheses is taking on increasing importance. Asymmetric ligands or asymmetric metal complexes used in these transformations are quite expensive and need to be efficiently separated from reaction mixtures and recycled. Scheme 16 shows the preparation of a polymer-anchored dibenzophosphole-DIOP platinum-tin catalysts system. The asymmetric ligand places the Pt-SnClj system in a chiral environment. This catalyst has given the highest enantiometric excesses ever observed in catalytic hydroformylation. The initially achieved 70-83% e.e. values were improved to >95% by the use of triethylorthoformate (TEOF) as the solvent. ... 

Chirality [17] can be introduced into organometallic arene complexes by modifications such as asymmetric substitution of the arene, introduction of chiral substituents into the ligands, the asymmetric coordination environment of the metal ion, or conformational arrangements of the ligands. Chirality in arene complexes can be classified depending on the components that generate the asymmetry in the molecule (for examples, see Figure 3.3). 

Two ligands, mimicking the asymmetric coordination environment of GpdQ more accurately were synthesized and complexed with Zn(II). While in the crystal structure a 5,6-coordination sphere was retained, this is not the case for the solution structure. MCD measurements of the Co(II) derivative and mass spectral studies of the di-Zn(II) derivative underline this. The complexes were shown to be less active than substances based on symmetric ligands. 

The gx and gy values are degenerate (axial symmetry) in a ligand field with C4v symmetry but are expected to become unequal in the [NiFe] hydrogenase, due to the asymmetric protein environment. A rhombic g tensor is observed for all hydrogenases. 

Compounds in which one or more carbon atoms have four nonidentical substituents are the largest class of chiral molecules. Carbon atoms with four nonidentical ligands are referred to as asymmetric carbon atoms because the molecular environment at such a carbon atom possesses no element of symmetry. Asymmetric carbons are a specific example of a stereogenic center. A stereogenic center is any structural feature that gives rise to chirality in a molecule. 2-Butanol is an example of a chiral molecule and exists as two nonsuperimposable mirror images. Carbon-2 is a stereogenic center. 

The ferrocene moiety is not just an innocent steric element to create a three-dimensional chiral catalyst environment. Instead, the Fe center can influence a catalytic asymmetric process by electronic interaction with the catalytic site, if the latter is directly coimected to the sandwich core. This interaction is often comparable to the stabilization of a-ferrocenylcarbocations 3 (see Sect. 1) making use of the electron-donating character of the Cp2Fe moiety, but can also be reversed by the formation of feirocenium systems thereby increasing the acidity of a directly attached Lewis acid. Alternative applications in asymmetric catalysis, for which the interaction of the Fe center and the catalytic center is less distinct, have recently been summarized in excellent extensive reviews and are outside the scope of this chapter [48, 49], Moreover, related complexes in which one Cp ring has been replaced with an ri -arene ligand, and which have, for example, been utilized as catalysts for nitrate or nitrite reduction in water [50], are not covered in this chapter. 

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