Oligomerization asymmetric

Peroxidases have been used very frequently during the last ten years as biocatalysts in asymmetric synthesis. The transformation of a broad spectrum of substrates by these enzymes leads to valuable compounds for the asymmetric synthesis of natural products and biologically active molecules. Peroxidases catalyze regioselective hydroxylation of phenols and halogenation of olefins. Furthermore, they catalyze the epoxidation of olefins and the sulfoxidation of alkyl aryl sulfides in high enantioselectivities, as well as the asymmetric reduction of racemic hydroperoxides. The less selective oxidative coupHng of various phenols and aromatic amines by peroxidases provides a convenient access to dimeric, oligomeric and polymeric products for industrial applications. 

Ready, J. M. Jacobsen, E. N. (2001) Highly active oligomeric (salen)Co catalysts for asymmetric epoxide ring opening Reaction, J. Am. Chem. Soc., 123 2687-2688. 

Ferrocenylphosphine-gold complexes, 225 Formaldehyde oligomerization, 332 Formic acid, transfer hydrogenation, 123 Formulas, chiral compounds, xiii-xvii Four-dimensional chemistty. See Asymmetric catalysis... 

Chiral binaphthol(BINAP)-titanates (59 X = OR) have been used as asymmetric catalysts of additions to aldehydes, and show evidence of oligomeric Ti—O—Ti... 

The alkylation of substrates 16 or 34 by catalyst 33 was found to exhibit a pronounced positive non-linear effect [34]. In addition, there was an optimal concentration of catalyst 33 in these reactions, with higher or lower catalyst concentrations being detrimental to the asymmetric induction [41]. These results are consistent with a dimeric form of catalyst 33 being catalytically active and in equilibrium with catalytically inactive monomeric and oligomeric species. On the basis ofthese results,... 

Empirical experimentation has revealed that the catalyst formed from a 1 1-1 1.4 ratio of Ln(0-i-Pr)3 (Ln = La or Yb) and 17 provides the maximum enantiomeric excesses for epoxidation (Figure 6). The 13C NMR spectrum of La-17 was not interpretable, suggesting that both the chiral Yb-17 catalyst and the La-17 catalyst exist as oligomers. Moreover, the catalytic epoxidation of 39 with Yb-17 displays a pronounced nonlinear effect (Figure 7). (For a treatment of nonlinear effects, see Chapter 5 in this volume.) Thus the oligomeric structure of these lanthanoid-BINOL catalysts may play a key role in the catalytic asymmetric... 

Chirogenic supramolecular systems based on dimeric/oligomeric porphyrinoids can also be classified according to their complexity and structural features. First, the assemblies consisting of two or more monomeric porphyrins fixed in an asymmetrical fashion by chiral bridges via supramolecular binding will be examined. 

Fig. 24. Asymmetric oligomerization of propylene at various temperatures gas-chromatographic separation of 2,4-dimethyl-2-heptene (trimer) in a capillary column coated with octakis(6-0-methyl-2,3-di-0-pentyl)cyclodextrin) (204).

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