Oxidations using polymer supported catalysts

Polymer-supported catalysts often have lower activities than the soluble catalysts because of the intraparticle diffusion resistance. In this case the immobilization of the complexes on colloidal polymers can increase the catalytic activity. Catalysts bound to polymer latexes were used in oxidation reactions, such as the Cu-catalyzed oxidation of ascorbic acid,12 the Co-catalyzed oxidation of tetralin,13 and the CoPc-catalyzed oxidation of butylphenol14 and thiols.1516 Mn(III)-porphyrin bound to colloidal anion exchange resin was... 

The sequence includes several synthetic steps over polymer-supported catalysts in directly coupled commercially available Omnifit glass reaction columns [41] using a Syrris Africa microreactor system [14], Thales H-Cube flow hydrogenator [32] and a microfluidic chip. The process affords the alkaloid in 90% purity after solvent evaporation, but in a moderate 40% yield. After a closer investigation it was concluded that this is due to the poor yield of 50% in the phenolic oxidation step. On condition that this is resolved with the use of a more effective supported agent, the route would provide satisfactory yields and purities of the product. 

Table 15) highlights the stability of this system compared to the PS/MTO system (entry 6, Table 15), which shows a decrease in activity during recycling. This difference in behaviour may be due to the weaker interaction between MTO and the PS polymer, which is only accomplished by the physical envelopment of the benzene ring. The PVP/MTO combination was successfully used for other compounds of biological interest, such as ter-penes. Even highly sensitive terpenic epoxides, hke a-pinene oxide, can be obtained in excellent yields using polymer-supported MTO catalysts [73] (Scheme 20, Table 16). 

The Anelli oxidation of alcohols to aldehydes and ketones has been accomplished using polymer-supported nitroxyl radical catalysts. The practicality of removing polymer-supported reagents by filtration to simplify product purification is highlighted by these examples. Bolm and coworkers11 demonstrated that a silica-supported nitroxyl catalyst is easily filtrated after use from the reaction solution, recovered and recycled, and the residual inorganic salts present in the reaction mixture are separated from the organic product by aqueous extraction (Table II, entry 7). 

Asymmetric epoxidation of unfunctionalized aUcenes catalyzed by chiral Mn(III)(salen) complexes has proven to be a useful solution-phase reaction [88]. To simplify product isolation and to avoid degradation of the Mn(salen) complex through formation of i-oxo-manganese(lV) dimers by spatial redistribution, the polymer-supported catalyst 112 was prepared by co-polymerization of complex 113, styrene 58, and divinylbenzene as a cross-linker (Scheme 20) [89]. As a stoichiometric oxidant, a combination of meta-chlor-operbenzoic acid (mCPBA) and N-methyl-morpholine N-oxide (NMO) in acetonitrile was used. Yields and rates of conversion were satisfactory for the epoxidation of styrene 58 and of methyl styrene, but only low enantioselectivities were obtained. Nevertheless, the catalyst retained its efficiency in terms of yields and enantioselectivities after repetitive use. Similar results have been described by other researchers [90]. 

Oxidation of cyclohexane was also studied using homogeneous complex of Ru(lll) with 1,2 DAP under similar condition (Table 8). However for convenience same quantity of catalyst could not be used as the same quantity of unbound catalyst gave immeasurable oxygen uptake. Inspite of using larger amount ofRu (III), a lower reaction rate was observed as compared to polymer supported catalyst. Effect of various parameters such as concentration of substrate and catalyst, temperature, amount and nature of solvent is seen and the results are summerised in Table 8. The energy of activation was found to be 7.04 Kcal mof. 

Oxidation of toluene using polymer supported palladium (II) complex as catalyst... 

Utilisation of a polymer-bound catalyst in this reaction would allow to reduce the cost of preparation by facile recycling of the catalyst. Initial investigations in this area have been reported by Sharpless who was able to oxidise trans-stilbene in 81-87% yield and with 85-93% enantiomeric excess using the polymer-supported catalyst 76 (Figure 3.6.1), OsO and N-methylmorpholine Al-oxide (NMO). The yields were high but enantiomeric excesses were inferior to the corresponding homogeneous reactions. 

The potential use of polymer supports in electrocatalysis has been one of the major ctors driving die development of polymer mtKlified electrodes over the past 20 years (1,2). The early focus was on the use of molecular catalysts such as porphyrins and other macrocyclic complexes. However, such systems provide limited activity and durability and are therefore not suitable for application in fuel cells. There has therefore been an increasing focus on the use of bulk metals, alloys, and oxides on polymer supports. 

The relatively high operating potentials of methmiol anodes, and slowness and mechanistic complexity of the medianol oxidation reaction provide considerable incentive to develop polymer supported catalysts, and this has resulted in much research activity. Polypyrrole has been most widely used as a support, presumably because its conductivity extends to lower potentials than for most other conducting polymers. Polyaniline has also attracted significant attention, and some polythiophenes are attractive for their enhanced stability. 

All report studies of methanol oxidation at polypyrrole supported catalysts have involved the use of electrochemically prepared films of the polymer on a solid electrode. Strike et al. (20), who electrochemically deposited Pt onto such electrodes from a H2PtCl5 solution, reported current densities for methanol oxidation that were enhanced by factors of 10-100 over those at bulk Pt and platinized gold electrodes. Furthermore, a less rapid decay of the current with time... 

These comparisons show that our PEDOT/PSS supported catalysts are as active for methanol oxidation as the best polymer supported catalysts r >orted in the literature. However, a question that must be answered is vdiether polymer supported catalysts can provide superior performance to commercially available carbon supported catalysts. To answer this, the PEDOT/PSS supported catalyst used for the experiments in Fig. 5, was compared with a commercial 0 -TEK) carbon supported Pt-Ru alloy catalyst Fig. 6 shows normal pulse voltammograms at 60° C, while Fig. 7 shows the results of constant potential experiments (at 22 °C) over a much longer time period. In both experiments, and over all timescales studied, the carbon supported catalyst delivers currents that are as much as 10 times higher than for the polymer supported catalyst. The only conditions under which the polymer supported catalyst is superior are at short times and high potentials, which are not relevant to fuel cell operation. 

Polyquiaolines have been used as polymer supports for transition-metal cataly2ed reactions. The coordinatkig abiUty of polyqukioline ligands for specific transition metals has allowed thek use as catalysts ki hydroformylation reactions (99) and for the electrochemical oxidation of primary alcohols (100). 

The pyromellitic dianhydride is itself obtained by vapour phase oxidation of durene (1,2,4,5-tetramethylbenzene), using a supported vanadium oxide catalyst. A number of amines have been investigated and it has been found that certain aromatic amines give polymers with a high degree of oxidative and thermal stability. Such amines include m-phenylenediamine, benzidine and di-(4-amino-phenyl) ether, the last of these being employed in the manufacture of Kapton (Du Pont). The structure of this material is shown in Figure 18.36. 

The presence of redox catalysts in the electrode coatings is not essential in the c s cited alx)ve because the entrapped redox species are of sufficient quantity to provide redox conductivity. However, the presence of an additional redox catalyst may be useful to support redox conductivity or when specific chemical redox catalysis is used. An excellent example of the latter is an analytical electrode for the low level detection of alkylating agents using a vitamin 8,2 epoxy polymer on basal plane pyrolytic graphite The preconcentration step involves irreversible oxidative addition of R-X to the Co complex (see Scheme 8, Sect. 4.4). The detection by reductive voltammetry, in a two electron step, releases R that can be protonated in the medium. Simultaneously the original Co complex is restored and the electrode can be re-used. Reproducible relations between preconcentration times as well as R-X concentrations in the test solutions and voltammetric peak currents were established. The detection limit for methyl iodide is in the submicromolar range. 

Oxidative cleavage of alkenes using sodium periodate proceeds effectively in a monophasic solution of acetic acid, water, and THF with very low osmium content or osmium-free. The orders of reactivity of alkenes are as follows monosubstituted trisubstituted >1,2 disub-stituted > 1,1-disubstituted > tetrasubstituted alkynes.100 Cleavage with polymer-supported OSO4 catalyst combined with NaI04 allows the reuse of the catalyst.101... 

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