Styrene/substituted styrenes chirality

Section 4 will deal with catalytic systems whose stereospecificity is controlled principally by the chirality of the closest tertiary carbon atom of the growing chain (chain-end stereocontrol). In Section 4.1 possible mechanisms for chain-end controlled isospecific and syndiospecific propene polymerizations will be reviewed. In Section 4.2 informations relative to the mechanism of chain-end controlled syndiospecific polymerization of styrene and substituted styrenes will be reviewed. In Section 4.3 chain-end controlled mechanisms for the isospecific and syndiospecific cis-1,4 and 1,2 polymerizations of dienes will be presented. 

A chiral D4-manganese(III) porphyrin catalyst, Mn(P )(MeOH)(OH) [H2P = 5, 10, 15,20-tetrakis(l,2,3,4,5,6,7,8-octahydro-l,4 5,8-dimethanoanthracene-9-yl)porphyrin], has been shown to catalyse the asymmetric aziridination of substituted styrenes (105) with enantiomeric excess of 43-68% (Scheme 40). ... 

Wullf and Hohn recently described several new stereochemical results (93). They reported the synthesis of a copolymer between a substituted styrene (M ) and methyl methaciylate (M2) having, at least in part, regular. . . M,M M2M MiM2. . . sequences. Polymerization involves the use of a chiral template to which the styrene monomer is loosely bound. After elimination of the template, the polymer shows notable optical activity that must be ascribed to the presence of a chiral stmcture similar to that shown in 53 (here and in other formulas methylene groups are omitted when unnecessaiy for stereochemical information). This constitutes the first stereoregular macromolecular compound having a three monomer unit periodicity. 

As is the case with non-chiral epoxidations model systems are much used, and styrene (and substituted styrenes), stilbenes and 1,2-dihydronaphthalene are the preferred models. 

The hydrogenation of arylalkenes such as the substituted styrene shown below has in these recent reports been exclusively investigated. This choice of substrate is more determined by analytical problems than by reactivity issues [2]. Very often a 4-methoxyphenyl substituent is included as a non-coordinating aid for the chiral HPLC and GC enantiomeric purity measurements. 

When / -substituted styrenes are used as substrates, the reaction proceeds regioselectively (>99 1) and gives enantiomerically enriched 1-aryl-l-silylalkanes in high yield and with high enantiomeric excesses the products have been converted into chiral alcohols31. 

Naruta et al. [225, 226] designed the twin-coronet porphyrin ligands (62) and (63) with binaphthyl derivatives as chiral substituents (Figure 13). Each face of the macrocycle is occupied by two binaphthyl units and the ligand has C2 symmetry. Iron complexes of these compounds can be very effective catalysts in the epoxidation of electron-deficient alkenes. Thus, nitro-substituted styrenes are readily epoxidized in 76-96% ee [226]. The degree of enantioselectivity can be explained on the basis of electronic interactions between the substrate aromatic ring and the chiral substituents rather than on the basis of steric interactions. 

New inq)etus for asymmetric hydroformylations came primarily from Takayas phosphine-phosphinite ligand (BINAPO) 4 which constitutes an enormous breakthrough l In combination with rhodium the BINAPO ligand gave enantioselectivities up to 95% and i/n ratios > 86/14 in the hydroformylation of substituted styrene derivatives. Conversions are > 99% at substrate/catalyst ratios between 300 and 2000. Shortly afterwards, similar catalytic results were reported by Union Carbide with chiral diphosphinite ligands, e.g. 5 . After many years of stagnation these new catalysts now point the way towards fixture developments in asymmetric hydroformylation. 

Another very good chiral Ru(II) catalyst for the asymmetric cyclopropanation of styrene with ethyl diazoacetate was recently reported [50,51 ]. The chiral ruthenium-porphyrin 37 was found to provide optically active phenylcyclopro-pane derivatives with a very high catalyst turnover number. A 96 4 transxis ratio of products was obtained with styrene and in the presence of only 0.05 mol % of the catalyst 37. The ee of the trans isomer was 91%, Eq. (13). Similar results with high transxis selectivities and high enantioselectivities for the trans product have also been found with other substituted styrenes. 

Chiral bipyridine-copper complexes have also been recommended as catalysts in cyclopropanations of styrene and E-substituted styrenes by diazoesters, but this process leads to mixtures of stereoisomers [966, 967], The same limitations occur with bomanethione-derived chiral complexes [968],... 

Similar applications of chiral titanium Lewis acid catalysts to asymmetric [2 + 2] cycloadditions, with up to nearly quantitative asymmetric induction, have employed 4-benzoquinones as additions and substituted styrene-type substrates11. In all of these asymmetric [2 + 2]-cvcload-dition reactions, the Lewis acid catalyst presumably is attached to peripheral functional groups and thus, similar to Lewis acid catalyzed Diels Alder reactions (see Section 1.5.8.3.5.4.), is only indirectly involved in the reaction course7. 

Such a principle was effectively employed in the synthesis of both achiral and chiral heterocyclic compounds. For instance, O Shea et al. [8] have demonstrated the benefit of intermolecular carbolithiations of o-substituted styrenes 9a in a one-pot synthesis of substituted indoles 11a and have extended this principle to the carbolithiation of 3-vinylpyridin-2-ylamines 9b eventually resulting in 7-azaindoles 11b (Scheme 10.5). 

Tg measurements have been performed on many other polymers and copolymers including phenol bark resins [71], PS [72-74], p-nitrobenzene substituted polymethacrylates [75], PC [76], polyimines [77], polyurethanes (PU) [78], Novolac resins [71], polyisoprene, polybutadiene, polychloroprene, nitrile rubber, ethylene-propylene-diene terpolymer and butyl rubber [79], bisphenol-A epoxy diacrylate-trimethylolpropane triacrylate [80], mono and dipolyphosphazenes [81], polyethylene glycol-polylactic acid entrapment polymers [82], polyether nitrile copolymers [83], polyacrylate-polyoxyethylene grafts [84], Novolak type thermosets [71], polyester carbonates [85], polyethylene naphthalene, 2,6, dicarboxylate [86], PET-polyethylene 2,6-naphthalone carboxylate blends [87], a-phenyl substituted aromatic-aliphatic polyamides [88], sodium acrylate-methyl methacrylate multiblock copolymers [89], telechelic sulfonate polyester ionomers [90], aromatic polyamides [91], polyimides [91], 4,4"-bis(4-oxyphenoxy)benzophenone diglycidyl ether - 3,4 epoxycyclohexyl methyl 3,4 epoxy cyclohexane carboxylate blends [92], PET [93], polyhydroxybutyrate [94], polyetherimides [95], macrocyclic aromatic disulfide oligomers [96], acrylics [97], PU urea elastomers [97], glass reinforced epoxy resin composites [98], PVOH [99], polymethyl methacrylate-N-phenyl maleimide, styrene copolymers [100], chiral... 

For hydrogenation of styrene and its derivatives over several cationic Rh complexes, in addition to the hyperpolarized multiplets of ethylbenzene the H NMR spectra contained similar polarized multiplets but shifted to a higher field [41,42]. These signals were attributed to the product molecules that have not yet detached from the metal center after the hydrogen-transfer stage was over (e.g., with the aromatic moiety r -coordinated to the Rh(I) center). The results demonstrate that the detachment process can be fairly slow on the NMR timescale. The use of chiral catalysts and/or asymmetrically substituted styrenes led to more complicated spectral patterns. Kinetic studies can be used to measure the rates of formation and decay of such catalyst-product complexes [43]. The fact that the observed product remains coordinated to the catalyst was confirmed [44] in experiments with polarization transfer from the product to the hydrogens of other ligands of the catalyst induced by cross relaxation. 

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