Catalyst supports carboxylated resins

Palladium metal catalysts supported on organic resins containing tertiary amino, cyano, carboxyl, and pyridyl groups have been recently investigated in some Heck reactions, such as the coupling of iodobenzene with methyl acrylate and methyl vinyl ether (Scheme 11) [31]. 

Inhibition of the coordinating molybdenum centre by chemisorpiion of the reaction products normally accounts for the observed evolution of the reaction rates. The rates of deactivation of Mo-fixed catalysts have been varied considerably by varying the density and the acid strength of OH groups ol the support surface. The fixed catalysts have been synthesized molecularly by taking advantage of the ready reaction between molybdic acid and differently functionalized polymeric matrixes such as boronic,phosphonic and carboxylic resins. 

Amberlyst 15 has also found use as an efficient acid catalyst in the synthesis of low molecular weight esters such as 3-methylbutyl acetate (3) by using acetic acid as the carboxylic acid component (eq 14). This reaction was performed under microwave-mediated conditions, which Illustrates the thermal stability of Amberlyst 15 under these reaction conditions. A related sulfonic acid resin, Amberlite IR-100, has been used as an acid catalyst for the hydrolysis of ethyl acetate to give acetic acid and ethanol by conducting the reaction in water (eq 15). It is interesting to note that the use of a less acidic polymeric supported carboxylic acid resin (Wofatit C) was at least 20 times less efficient than Amberlite IR-100 Itself. 

Schreiber and co-workers (436) prepared a library calculated to contain 2.18 million polycyclic compounds through the 1,3-dipolar cycloaddition of a number of nitrones with alkenes supported on TentaGel S NH2 resin (Scheme 1.83). (—)-Shikimic acid was converted into the polymer bound epoxycyclohexenol carboxylic acid 376 (or its enantiomer), coupled to the resin via a photolabile linker developed by Geysen and co-workers (437) to allow release of the products from the resin in the presence of live cells by ultraviolet (UV)-irradiation. A range of iodoaromatic nitrones (377) was then reacted with the ot,p-unsaturation of the polymer-bound amide in the presence of an organotin catalyst, using the tandem esterification/ dipolar cycloaddition methodology developed by Tamura et al. (84,85) Simultaneous cyclization by PyBrop-mediated condensation of the acid with the alcohol... 

In the same way, dipyridyl amide-functionalized supports suitable for the SPE of metal ions from aqueous solutions can be prepared. The resins are synthesized via the copolymerization of the functional monomer endo-norbornene-5-yl-N,N-di-2-pyridyl carboxylic amide with a molybdenum-based catalyst (42). Essentially no loss of performance was observed after extensive use over more than twenty cycles. After exposure to air for at least 2 months, a change in color from bright white to yellow was observed. However, this change in color did not influence the characteristic properties of the resins. 

An example of a polymer-supported catalyst was produced from a tailor-made resin based on N,N-dimelhylacrylamide with 4 mol% methylene bis(acrylamide) as the cross-linker and 12 mol% methacrylic acid as the functional, metal binding comonomer. Treatment of the resin with a solution of Cu(OAc)2 in methanol resulted in a ligand exchange reaction with partial substitution of the acetates with polymer-bound carboxylate groups (Scheme 11.3) [4], The use of the catalyst is discussed further below. 

In a similar vein, a series of papers published between 2002 and 2008 contains spectacular claims of highly enantioselective asymmetric additions of water to styrenes, unsaturated carboxylic acids, or simple terminal alkenes [34-Al]. The catalysts used are of the heterogeneous type and based on chiral biopolymers such as wool, gelatin, or chitosan as solid supports (sometimes in combination with silica or ion-exchange resins) that are doped with transition metal salts. This series of papers contains spectacular claims, insufficient experimental data, and erroneous chemical structures for the biopolymers used. As earlier work from the same group of authors on asymmetric catalysis on bio-polymeric supports is irreproducible [42], one is well advised to await independent confirmation of those results. 

Polystyrene-supported iodosylbenzene (22) (loading of -lO groups up to 1.50 mmol g" ) has been prepared by a solvent-free reaction of poly[(diacetoxyiodo)styrene] (4) with sodium hydroxide (Scheme 5.11) [22]. Elemental analysis of polymer 22 indicates that the -lO groups are partially hydrated as shown in structure 23. This resin has been successfully used for efficient oxidation of a diverse collection of alcohols to aldehydes and ketones in the presence of BF3-Et20. Reagent 22 can also be employed as efficient co-catalyst in combination with RuCls in the catalytic oxidation of alcohols and aromatic hydrocarbons, respectively, to the corresponding carboxylic acids and ketones using Oxone as the stoichiometric oxidant [22]. 

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