Isolation molecular catalysts, dimerization

A modification of this system was also used in intramolecular MBH reactions (also called as aldol cycloisomerization) [71, 74]. In this reaction, optically active pipecolinic acid 61 was found to be a better co-catalyst than proline, and allowed ee-values of up to 80% to be obtained, without a peptide catalyst. The inter-molecular aldol dimerization, which is an important competing side-reaction of the basic amine-mediated intramolecular MBH reaction, was efficiently suppressed in a THF H20 (3 1) mixture at room temperature, allowing the formation of six-membered carbocycles (Scheme 5.14). The enantioselectivity of the reaction could be improved via a kinetic resolution quench by adding acetic anhydride as an acylating agent to the reaction mixture and a peptide-based asymmetric catalyst such as 64 that mediates a subsequent asymmetric acylation reaction. The non-acylated product 65 was recovered in 50% isolated yield with ee >98%. 

In addition to the enhanced cooperative activation effect of the nanoreactor, the isolation effect could also be expected in the confined nanospace if the diameter of nanopore is similar to the size of the molecular catalysts, because the limited nanospace could restrict the free movement of the molecular catalysts. Two issues relevant to the isolation effect of the nanoreactor, namely selectivity control in organic reactions and inhibition dimerization of the molecular catalysts, will be discussed. 

To overcome this issue Kureshy et al. [55, 56] reported dimeric form of Jacobsen s catalysts 3, 4. They used the concept of solubility modification by altering the molecular weight of the catalyst so that in a post catalytic work-up procedure the catalyst is precipitated, filtered and used for subsequent catalytic runs. The complexes 3, 4 (0.2 mol % of Co(lll)-salen unit) (Figure 2) were effectively used for HKR of racemic epoxides, e.g., styrene oxide, epichlorohydrin, 1,2-epoxypropane, 1,2-epoxyhexane, 1,2-epoxyoctane, and 1,2-epoxydodecane to achieve corresponding epoxides and 1,2-diols in high optical purity and isolated yields. In this process, once the catalytic reaction is complete the product epoxides were collected by reduced pressure distillation. Addition of diethylether to the residue precipitated the catalyst which was removed by filtration. However, the recovered catalyst was required to be reactivated by its treatment with acetic acid in air. The catalysts were reused 4 times with complete retention of its performance. 

Generally, in conclusion, it is worth noting that the molecular and immobilized complexes show very similar catalytic activity in terms of the initial TOP in olefin metathesis. However, the supported catalyst has a longer lifetime under catalytic conditions, which indicates that the effect of active-site isolation prevents some deactivation pathways such as dimerization of reachve intermediates [30]. 

This chapter focuses on several recent topics of novel catalyst design with metal complexes on oxide surfaces for selective catalysis, such as stQbene epoxidation, asymmetric BINOL synthesis, shape-selective aUcene hydrogenation and selective benzene-to-phenol synthesis, which have been achieved by novel strategies for the creation of active structures at oxide surfaces such as surface isolation and creation of unsaturated Ru complexes, chiral self-dimerization of supported V complexes, molecular imprinting of supported Rh complexes, and in situ synthesis of Re clusters in zeolite pores (Figure 10.1). 

No other cofactors are required for enzymatic catalysts of this reaction. Amino acid analysis on isolated isoenzymes I and III indicate that the lower molecular weight monomers from I are not proteolytic degradation products of the monomers from III [68], When dimers of I (a ) are mixed with dimers of III ()8y8) an equilibrium concentration of II (afi) is formed. 

One approach to creating heterogeneous oxidation catalysts with novel activities and selectivities is to incorporate redox metals, by isomorphous substitution, into the lattice framework of zeolites and related molecular sieves. Site-isolation of redox metals in inorganic lattices prevents the dimerization or oligomerization of active oxometal species which is characteristic of many homogeneous oxometal complexes and leads to their deactivation in solution. We coined the term redox molecular sieves to describe such catalysts . The first and most well-known example is titanium silicalite (TS-1) which has been shown to catalyze a variety of systhetically useful oxidations with H202. ... 

The polarity of chlorinated solvents can also play a role in affecting the product distribution of an olefin metathesis reaction. Clark and Ghadiri [8] observed that the macrocyclic peptide 10 self assembles by inter molecular H-bonding in nonpolar solvents. The cylindrical conformation that resulted did not allow for successful dimerization to occur between macrocycles. When the cyclization of the cyclic peptide 10 was conducted with Ru catalyst 12 in chloroform (Scheme 12.5), the chloroform was proposed to disrupt the H-bonding within molecules. The new conformation produced in solution with the CHClj proved conducive to ring closure. The cyclic dimer 11 was obtained in 65% isolated yield as a mixture of cisicis, transitrans, and cisitrans isomers. 

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