Molecular weight catalysts

For a possible quantitative description of typical polymer effects we made the assumption that the values of AH and AS found for the low molecular weight catalysts stand for the activation process of the naked catalyst-substrate complex and are independent of cx. So, after subtracting these values the separate polymer effects are found. Then we have to explain why more entropy is gained and more enthalpy is needed for adaptation of the intermediate chains to... 

Retention of homogeneous catalysts can be achieved by binding the low-molecular-weight catalyst to a dendrimer, to an already formed polymeric backbone, or to a polymerizable monomeric unit which is polymerized subsequently. Currently, the disadvantages concern the durability of the nanofiltration or ultrafiltration membranes, which, after all, in most cases have not been slated for use in... 

Catalytic kinetic resolution of amines has been a typical domain of enzymatic transformations. Attempts to use low-molecular-weight catalysts have notoriously been frustrated by the rapid uncatalyzed background reaction of the amine substrate with the acyl donor [40]. The first solution to this problem was recently developed by Fu, who used the planar chiral catalyst 21d and O-acyl azlactone 40 as the acyl donor (Scheme 12.19) [41]. In this process, the acyl transfer from the azlactone 40 to the nucleophilic catalyst 21d is rapid relative to both direct transfer to the substrate and to the transfer from the acylated catalyst to the substrate amine. Under these conditions, which implies use of low reaction temperatures, selectivity factors as high as 27 were achieved (Scheme 12.19) [41]. 

Roska, A., Klavins, M., and Ziemanis, A.. High-molecular-weight catalysts in organic-synthesis. XIX. New method of synthesis of polymer supported crown ethers, iMtv. PSR Zinat. Akad Ve.sti.s Kim. Ser., 458, 1988 Chem. Ahstr., 110, 192.545, 1989. 

The first report of a polymer-supported approach to this reaction appeared in 1987 [48]. Enantiopure amino alcohols such as ephedrine, prolinol, and 3-exo-amino-isoborneol were attached to Merrifield polymer. The use of polymer-supported 3-exo-aminoisoborneol 40 resulted in quite high enantioselectivity ( 95 % ee) in the ethylation of aldehydes with diethylzinc (Eq. 15), a result comparable with those obtained from the corresponding low-molecular-weight catalyst system (Eq. 16). A similar system was also reported in 1989, this time using ephedrine derivatives (41,42) and prolinol derivative (43) [49]. A methylene spacer was introduced between the polymer and the amino alcohol to improve activity [50]. Despite this the selectivity was always somewhat lower than that obtained from the low-molecular-weight catalyst (44). These chiral polymers were all prepared by the chemical modification method using Merrifield polymer. 

Hyperbranched and dendritic macromolecules have recently been the subject of considerable interest. Bolm developed chiral hyperbranched macromolecules 57 that catalyzed the enantioselective addition of diethylzinc to benzaldehyde [75]. The enan-tiocontrol of the hyperbranched chiral catalysts was slightly lower than for the low-molecular-weight catalyst. TADDOLs linked with dendritic molecules have been synthesized [59]. For example, use of the first generation dendrimer 58 with six terminal TADDOL units resulted in high enantioselectivity. 

One of the most powerful catalysts of the Mukaiyama aldol reaction is a chiral Ti(IV)-Schiff base complex 91 prepared from Ti(0 Pr)4 and enantiomerically pure salicylaldimine reported by Carreira [103-105]. This catalyst furnished aldol adducts in good yields and with excellent enantioselectivity. The Ti(IV)-Schiff base catalyst system is unique among the aldol catalysts yet reported in terms of operational simplicity, catalyst efficiency, chirality transfer, and substrate generality. Because the Ti(IV)-Schiff base complexes are remarkably efficient catalysts for the addition of ketene acetals to a wide variety of aldehydes, the polymeric version of catalyst 92 was prepared [106]. The activity and enantioselectivity of the polymer-supported chiral Ti(IV)-Schiff base complex were, however, much lower than were obtained from the low-molecular-weight catalyst (Eq. 28). 

The soluble polymer-supported catalysts have also been used for asymmetrically catalyzed reactions Following a procedure for the preparation of insoluble polymeric chiral catalysts a soluble linear polystyrene-supported chiral rhodium catalyst has been prepared. This catalyst displays high enantiomeric selectivity compared to the low molecular weight catalyst. Thus, hydroformylation of styrene using this catalyst produces aldehydes in high yields. The branched chiral hy drotropaldehy de is formed in 95% selectivity. 

The oxidative coupling reaction of terminal alkynes is critically dependent on the water concentration in the reaction mixture (see Section 2.5.2). Since water is produced during the reaction, careful elimination of it may be required. Challa and Meinders have demonstrated that the polymer catalyst derived from copper(II) chloride and either N,/V-dimethylbenzylamine or N,/V-dimethylaminomethylated atactic polystyrene (37) provides an extra protection of the catalytic copper complexes against water in the coupling reaction of phenylacetylene (equation 23), resulting in a higher reaction rate than the low molecular weight catalyst. 

Low-molecular-weight catalysts which mimic a natural enzymic function syn-Zymes) have potential utility for the treatment of diseases characterized by the overproduction of a potentially deleterious metabolic by-product or foreign gene product. The discovery and development of pentaaza macrocyclic ligand complexes of manganese(II) as functional mimics of superoxide dismutase (SOD) enzymes and their potential utility as human pharmaceutical agents is described. 

Compounds that catalyze polyamldation but not degradation will increase the attainable molecular weight. Catalysts for polyamldation include boric acid, hypophosphorous acid and its salts, and phosphoric acid (13). 

The separation of catalysts and products in a liquid/liquid biphasic system is a scheme that has the advantage of demonstrated practicality. Aqueous biphasic catalysis is used in hydroformylation and biphasic separations are important parts of several other commercialized processes [86,142,143]. Recent reports using low molecular weight catalysts in aqueous biphasic systems, in fluorous systems, and in ionic liquids are indicative of the growing interest in this general area - chemistry that has been summarized in a number of recent reviews [8,144-150]. 

It can be seen that the local concentration of amino groups in the macromolecular coil is several orders of magnitude higher than the concentration of the respective low-molecular weight analogue, dimethylbenzylamine (0.0033 mole/dm at the same initial concentration of high- and low-molecular weight catalysts in the solution. 

Polymeric sulfones obtained by radical copolymerization of monomeric sulfones with styrene are active interfacial catalysts. Their activity was studied in the reaction of n-CgH]7Br with MI (M = Li, Na, K) in a toluene-water system [191]. Reaction of n-CgHiyBr with Nal at 100 °C for 48 hours produced only traces of n-CigH]7l when catalysts, including such low-molecular weight catalysts as DMSO, methyl phenyl sulfoxide and methyl benzyl sulfoxide, were absent. When this reaction was catalyzed by polymeric sulfone, the yield of n-CigH,7l was 43%. However, in the presence of polymeric catalyst an 82% yield was obtained after a reaction time of 160h. 

References concerning research studies of enyzme-like polymers conducted during the period of 1978-1982, catalytic properties of water-soluble imidazole-containing polymers during ester hydrolysis, as well as the latest achievements in the manufacture and application of high-molecular weight catalysts containing thiocrown-ethers or crown-sulfides, can be found in previous reviews [199-201]. 

The same catalyst precursor PPI(G2)-[(diphosphine)PdMe2)i5 was employed by Reetz and co-workers for Heck reactions [9]. By addition of diethyl ether the polymer-bound catalyst could be precipitated and isolated by filtration. Upon its repeated use for catalysis, a slight decrease in activity was observed. By contrast to analogous low molecular-weight catalysts that were not polymer-bound, no formation of palladium black was observed with the dendrimer-bound catalyst. 

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