Enantiopure chiral side chains

Preferential Screw Sense Homopolymers with Enantiopure Chiral Side Chains... 

Another significant cooperativity effect in preferential helical screw sense optically active copolymers is the majority rule phenomenon.18bl8q In this case, the screw sense of a helical main chain with unequal proportions of opposite chirality enantiopure chiral side groups is controlled by the enantiomeric excess only. Since this phenomenon was first reported from poly-a-olefins made of vinyl co-monomers bearing nonenantiopure chiral moieties by Green et al.8b and Pino et al.,16b this majority rule has been established in... 

For example, the sergeants and soldiers experiment features a preferential screw-sense helical amplification in optical active copolymers with racemic helical structures named by Green et al. [17,18], This effect means that a small portion of enantiopure chiral side groups determines the overall screw sense (P or M) of the helical main chain bearing a majority of achiral side groups, and a population of helicity with one preferential screw-sense is nonlinearly amplified as a function of the chiral impurity. Since the first report of this phenomenon in poly-a-olefin copolymers by Pino et al. [21],... 

Preferential helical screw sense polysilanes with main-chain chirality are essentially of two types those in which PSS chirality is induced by an internal chiral field, for example, incorporation of enantiopure chiral (unichiral) side chains or end groups, and those which are inherently achiral, in which PSS chirality is induced by an external chiral field, such as a unichiral additive or solvent. 

During the two decades after this important discovery, a tremendous amount of research has been directed toward the polymerization of sterically demanding achiral monomers with chiral initiators to create enantiomerically pure helical polymers (also known as helix-sense selective or screw-sense-selective polymerization ). These polymers, known as atropisomers, are stable conformational isomers that arise from restricted rotation about the single bonds of their main chains. Key aspects of these reactions are enantiopure initiators that begin the polymerization with a one-handed helical twist, and monomers with bulky side-chains that can maintain the helical conformation due to steric repulsion. Notable examples of this fascinating class of polymers that are configurationally achiral but conformationally chiral include [8, 38, 39] poly(trityl methacrylate), polychloral, polyisocyanates, and polyisocyanides. Important advances in anionic and metal-based enantioselective polymerization methods have been reported in recent years. 

The preparation of enantiopure or enriched complexes possessing planar chirality has been accomplished either by resolution of racemic mixtures or by asymmetric syntheses. Reported methods for the resolution of planar chirality include both chemical and kinetic resolution procedures, whilst reported asymmetric syntheses of enantiomerically pure or enriched benchrotrenic complexes include enantioselective ort/io-deprotonations with chiral lithium amide bases, and the transfer of side chain chirality onto the arene ring mediated by diastereoselective orf/io-nucleophilic additions and o/tfeo-metalations. 

The Fu group has reported the first nonenzymatic DKR of secondary alcohols [41]. In this DKR, a planar-chiral DMAP derivative ((+)-Cg-Phg-DMAP) as the resolution catalyst was coupled with a Ru complex 6 as the racemization catalyst in the presence of an acyl carbonate (Scheme 5.25). The DKR of simple secondary alcohols provided good )delds but lower enantiopurities compared to the enzymatic DKRs. It is noteworthy that the DKR of sterically more demanding substrates carr3dng a branched side chain (isopropyl or cyclopentyl) also provided similarly good results (Chart 5.25). 

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