Bases as ligands

Scheme 2.55 The combinatorial preparation of schiff bases as ligands for asymmetric catalysis.

The selected examples by Cole et al. [120] and Shimizu et al. [121] reported the parallel synthesis of a small library of solid supported dipeptide Schiff bases as ligands for the Ti-catalyzed enantioselective addition of trimethylsilyl cyanide to meso epoxides, and the determination of their catalytic activity on different substrates. The catalyzed addition reaction and the general structure of the dipeptide ligands are shown in Figure 7.15. 

Purine and pyrimidine bases as ligands in enzymes, estimation of protein-ligand binding by enzyme inhibitors 91ACR209. 

From a practical viewpoint the recently discovered vanadium-based and iron-based asymmetric sulfoxidation with hydrogen peroxide is worth mentioning [305, 306]. For vanadium, in principle as little as 0.01 mol% of catalyst can be employed (Fig. 4.111). With tridentate Schiff-bases as ligands, formed from readily available salicylaldehydes and (S)-tert-leucinol, ees of 59-70% were obtained for thioanisole [305], 85% ee for 2-phenyl-l,3-dithiane [305] and 82-91% ee for tert-butyl disulfide [307]. For iron, similar results were obtained using 4 mol% of an iron catalyst, synthesized in situ from Fe(acac)3 and the same type of Schiff base ligands as in Fig. 4.111 (see Ref. [306] for details). 

A remarkable stabilization of selenium dihalides is achieved if the central Se atom is coordinated with anionic (see Section 4.2) or neutral Lewis bases as ligands. 

The macrocyclic Cu(II)-Cu(I) complex with two Schiffs bases as ligands consists of [Cu2(Ci2Hi3N20)2] islands (Fig. 43), approximately tetrahedral CIOJ groups and disordered CH3OH molecules. The nearly planar islands in the crystal are arranged parallelly (Fig. 44). The distance between the least-squares planes of the nitrogen atoms from the two neighbouring islands is 3.37 A. The Cu(2) atoms are disordered in two different... 

The nickel complex 11, with a fluorous Schiff base as ligand, was prepared from salicylaldehyde and 4-perfluorodecylaniline (Scheme 15). This fluorous Ni complex was used as a catalyst for the conjugate addition of -diketones to diethyl azodicarboxylate (DEAD). The mixture was stirred at 60 C in a mixture of... 

Structural characterization, recognition patterns, and theoretical calculations of long-chain N-alkyl substituted purine and pyrimidine bases as ligands On the importance of anion—tt interactions 13CCR2705. Structure, functionalization, and apphcations of giant hollow spher-... 

Abundant literature also exists about Schiff base ligands linked, by the N atoms, to the p positions of Th or Py rings, for the development of particular type 2 metallopolymers. Thanks to the versatility of the Schiff bases as ligands, different metal ions can be coordinated according to the scheme in Fig. 3.27a. Alternatively, the phenyl residue of the outermost portion of the Schiff base is bound to a-positions of the heterocycle (see Fig. 3.27b). By polymerization of monomers like that reported in Fig. 3.27a, metallopolymers are obtained. They show a structure that classifies them as type 2 metallopolymers, with possible interesting coupling of electronic and redox conduction, by redox matching. On the other hand, type 3 metallopolymers are obtained from monomers reported in Fig. 3.27b. 

However, in the presence of a metallic cation also the oligonucleotides can be formed from non-preferential pairs and thus, the detected arrangement appears under a symmetrical geometric form with the cation as interstice ( ) and the coordination bases as ligands, Figure 2.47. 

Transition metal complexes with Schiff bases as ligands attracted a great interest in this respect. These metal complexes can easily be immobilized on electrode surfaces via electropolymerizatiOTi generating an electrically conductive polymeric film. The reactivity of metal complexes depends on the nature of the metal as well as on the ligand attached. 

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