Monomers as chiral intermediates

Polyhydroxyalkanoates (PHA) is a family of structurally diverse biopolyesters accumulated by many bacteria as carbon and energy source (Figure 16.1)d PHA have been exploited with a series of applications including environmentally friendly biodegradable plastics for packaging purposes, biofuels, medical implants, and recently, smart materials. PHA monomers are also produced as chiral intermediates for medical or fine chemical applications. ... 

Polymers derived from natural sources such as proteins, DNA, and polyhy-droxyalkanoates are optically pure, making the biocatalysts responsible for their synthesis highly appealing for the preparation of chiral synthetic polymers. In recent years, enzymes have been explored successfully as catalysts for the preparation of polymers from natural or synthetic monomers. Moreover, the extraordinary enantioselectivity of lipases is exploited on an industrial scale for kinetic resolutions of secondary alcohols and amines, affording chiral intermediates for the pharmaceutical and agrochemical industry. It is therefore not surprising that more recent research has focused on the use of lipases for synthesis of chiral polymers from racemic monomers. 

For instance, Figure 1.9a plots as a function of Go the optimized energies for the model complex 5, that is, a ir-ligand with C2 symmetry with (R, R) chirality of coordination of the bridged ir-ligand. Let us recall that, in our framework, energy minima with 0O 0° or 0O 180° can correspond to preinsertion intermediates suitable for primary and secondary monomer insertions, respectively. 

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.

Figure 12 The stereochemical outcome of the deamination of chiral primary amines in micelles (a) is explained by a tight counterion-carbocationlike intermediate interaction (1) so that the nucleophile (H2O) can reach the reaction center from the same side as the leaving N2 (2). In monomers (b), the counterions are solvated by water molecules and the attack of the nucleophile is on the opposite side from the leaving group...

Chiral monomers should be exploited as intermediates for chiral synthesis... 

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