Coordinated ligands, chirality polymerization

Figure 1.25 Minimum-energy diastereoisomeric monomer free intermediates for butadiene polymerization catalyzed by titanium complexes presenting Cp group as ancillary ligand. Chiralities of coordination of allyl groups (assumed to be si) and back-biting double bonds (si or re) are indicated, in order to easily visualize possible stereoregularity (iso or syndio) of model chains. In fact, like and unlike chiralities would possibly lead to isotactic and syndiotactic enchainments, respectively.

The combination of organolithiums with chiral coordination ligands such as sparteine allows the asymmetric polymerization to furnish optically active polymers with very high enantioselectivity. especially when trityl methacrylate is utilized as a monomer [13, 14]. 

The binding of the diuretic amiloride 95 to DNA has been studied <06CC1185>. The chiral camphor-derived pyrazine ligand 96 showed monomeric coordination to Cu and Zn, in contrast to the bidentate polymeric behaviour of related earlier compounds <06ARK218>. 

The models considered in this section refer to catalytic systems for which the polymerization reaction occurs by primary insertion of the 1-alkene and neither chirality of coordination of the aromatic ligands nor chirality at the... 

Heat treatment of carbon black impregnated with a salt of [Fe(phen)3] gives an oj gen reduction catalyst. i The compounds [Fc2(88)Cl4], [Fe(89)Cl2] and [Fe(90)Cl2] are potential ethene polymerization catalysts they all contain five-coordinated iron(II). The chiral terpy ligands (91) (R R = all three combinations of H, Pr ) and the Schiff base analogue (92), also... 

The driving force for isoselective propagation results from steric and electrostatic interactions between the substituent of the incoming monomer and the ligands of the transition metal. The chirality of the active site dictates that monomer coordinate to the transition metal vacancy primarily through one of the two enantiofaces. Actives sites XXI and XXII each yield isotactic polymer molecules through nearly exclusive coordination with the re and si monomer enantioface, respectively, or vice versa. That is, we may not know which enantio-face will coordinate with XXI and which enantioface with XXII, but it is clear that only one of the enantiofaces will coordinate with XXI while the opposite enantioface will coordinate with XXn. This is the catalyst (initiator) site control or enantiomorphic site control model for isoselective polymerization. 

Okuda and Arnold have reported a series of 5- and 6-coordinate chiral indium complexes (e.g. 12), analogous to those previously discussed with yttrium (Table 4), which show varying activities for the polymerization of rac-lactide [71]. The homoleptic species was fluxional, interconverting between a 5-coordinate and fac- and mer-isomers of a 6-coordinate complex. It resulted in controlled polymerization and some stereoselectivity (max. P, = 0.63 and max. Ps = 0.64). MALDI-ToF mass spectrometry confirmed that the chiral alkoxide ligand was an initiating group. 

The molecular imprinting method can be used to synthesize enantioselective solid materials for asymmetric organic synthesis. The first attempt to use a metal complex with an attached chiral ligand as a template was attempted by Lemaire [52]. The Rh complex, ((15,25)-V,V -dimethyl-l,2-diphenylethane diamine)-[Rh(CgHj2)Cl]2 coordinated with optically pure l-(5)-phenylethoxide or phenylethoxide (Rh 1-phenylethanolate) (template) was polymerized in the presence of isocyanate, and the polyurea-supported Rh complex is reacted with isopropanol to extract the template from the polymer backbone. They reported the influence of molecular imprinting on catalytic performance (conversion and enantiomeric excess) for the asymmetric transfer hydrogenation (Table 22.2). The imprinted polymer exhibited higher enantioselectivity compared to a nonimprinted... 

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