Coordination complexes, chirality induction

Until recently the most popular method in asymmetric catalysis was the application of metal complexes. This is not surprising, since the use of different metals, ligands and oxidation states makes it possible to tune selectivity and perform asymmetric induction very easily. Thus, the concept of asymmetric catalysis has become almost synonymous with the use of metals coordinated by chiral ligands [1,2]. In many examples the metal is a Lewis acid [3]. 

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. 

The authors proposed an intermediate complex 150 in which the chiral induction occurs by reaction of the scandium ketone enolate with the a, S-unsaturated ketone 148 (equation 42). The absolute configuration of the Michael adduct 150a can be explained by coordination of the hydroxy groups of 149 to Sc + in a tetradentate manner and shielding of the si-facc of the scandium enolate by an adjacent f-butyl group. Therefore, the enolate attacks the Michael acceptor preferably at the re-face. ... 

In a recent fascinating development of this approach, Hirao and coworkers181 have produced chiral induction in poly(ort/zotoluidine) via the complexation of EB with chiral Pd(II) complexes bearing one labile coordination site. The chirality of the ligand moieties of the Pd complex is believed to induce a propeller twist of the PAn backbone. 

Strecker synthesis. N-Sulfimines add a cyanide ion from Et2AlCN preferentially from a coordination complex. Thus 1,3-asymmetric induction arises from the chiral sulfinyl unit. 

As mentioned above, it is reasonable to consider that the reduction by a 1,4-dihydropyridine derivative catalyzed by metal ions proceeds via a ternary complex as shown in Fig. 3. In the reduction by PNPH, the factor which determines the stereochemistry of the product is the mode of approach of the substrate to PNPH. Namely, the relative orientation between the 1,4-dihydropyridine ring and the substrate in the ternary complex is important. There are many possibilities for the molecular arrangement in the ternary complex as well as for the fashion of the coordination of Mg(II) to PNPH. Here we will discuss what kind of complexation and approach should be considered in order to understand the magnitude and direction of the induced chirality, or the mechanism of chirality induction. 

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