Catalyst noncovalently-bound

Schwab E, Mecking S (2005) Recoverable catalysts noncovalently bound to a hyperbranched polyelectrolyte. Organometallics 24 3758-3763... 

Additive-Induced Stereoselectivity Stereoselectivity in a reaction between achiral reactants is additive induced when (a) stereogenic unit(s) is (are) stereoselectively generated under the influence of a noncovalently bound chiral compound (e.g., solvent, catalyst). 

There are several different approaches to fixing a molecular catalyst into a host material, some of these methods have been reviewed recently by On et al.[881 in 2001 and by De Vos et af.[89] Reviews from the perspective of chiral catalysis appeared in 2002 by Song and Lee,[90] and in 2004 by Li,[91] and noncovalently bound catalytic species on solid supports have been reviewed in 2004 by Horn et al.[92] This section is intended to complement these recent reviews and highlight as well as define the approaches encountered and to update some of the latest developments in this field. 

In DNA-based asymmetric catalysis, DNAs role is that of a chiral scaffold, i.e., as the source of chirality for the catalyzed reaction. In this concept, it is assumed that if the catalyzed reaction takes place close to the DNA double helix, the chirality of DNA can be transferred to the reaction product, resulting in an enantiomeric excess. This can be achieved by binding of the catalyst to the DNA. Analogously to hybrid enzymes [145], two general approaches can be followed (i) the supramo-lecular approach, in which the catalyst is bound using noncovalent interactions, and (ii) the covalent approach, in which the catalyst is attached to the DNA via a covalent linkage. 

It was Swain [15] who pointed out more than half a century ago that when a reaction between a nucleophile N and a substrate S is catalyzed by an electrophile E (case a), enhanced catalysis is expected if two of the components, either N and S (case b), S and E (case c), or N and E (case d) are bound together in the same molecule or in the same complex by covalent bonding or noncovalent interactions, respectively. An additional possibility is given by case e, where the two reactants and catalyst are held together in a ternary complex. 

These noncovalently anchored catalysts in general exhibit a behavior similar to their covalently bound analogues, but can now be separated from the support after the reactions by a simple filtration step. So far, these immobilized systems have not been used in continuous flow reactors. 

Schweb and Mecking have reported one of the first examples of noncovalent anchoring of catalysts to soluble polymeric supports in 2001. This noncovalently anchored catalyst featured phosphine ligands that were bound by multiple sulfonate groups to soluble polyelectrolytes using electrostatic interactions. The catalyst system was employed in the hydroformylation of 1-hexene and exhibited typical selectivity for a bis-triphenylphosphine-bound rhodium catalyst. The complex was readily recovered and recycled by ultrafiltration. Independently, Reek et al. described similar systems, but these systems made use of a soluble... 

In Eq. (8.75), Z is the adsorption site on palladium on alumina catalyst. On this site, acetophenone (A) is adsorbed and hydrogenated to J4-l-phenyl ethyl alcohol (B) and to S-1-phenyl ethyl alcohol (C). This is followed by acylation of B to J4-1-phenyl ethyl acetate (P) over an immobihsed Hpase. First the acyl donor, ethyl acetate (Q), is bound with the free enzyme (E) forming a noncovalent enzyme-acyl complex (EQ), releasing ethanol (I). The substrate in the acylation step, J4-1-phenyl ethyl alcohol (B), is thereafter combined with EQ, which subsequendy relinquished J4-phenyl ethyl acetate (P) and E. Styrene (S) is obtained by dehydration of B and C over Pd/AI2O3. The side product ethyl benzene (F) is formed as a result of fast hydrogenation of S (step 7 ) and de-acylation of P in the presence of H2 releasing acetic acid (AcOH) to the media. 

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