Ligand site environment, influence complexation process

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. 

Competitive effects for the interactions of cisplatin with various active sites in the cellular environment are discussed in papers of Deubel.55,56 In the earlier paper, energetic and structural data of complexes with the different substituted ligands were explored. The more recent work deals with kinetic factors in the relation to the transition state (TS) for water replacement of the semihydrated cisplatin complex (cis-[Pt(NH3)2(H20)Cl]+ ) with either an N- or S-containing ligand (thiopheneimidazol, dimethyl sulphide, or methanethiolate which serve as amino acid models). Deubel concluded the kinetic preference of N-sites over S-nucleophiles where the important role is played by the electrostatic terms. In addition, the aliphatic/aromatic character of the substituent as well as the influence of different dielectric constants of the environment are very important. A more realistic model for the aqua-ligand replacement with adenine and guanine was studied in works of Chval et al.53,57 and Eriksson and coworkers.58 They performed independently the estimation of the thermodynamic and kinetic parameters of this process. 

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