Polymer-bounded catalysts complexes

Although the lariat ethers (29-31) were conceived on principles related to biological activity, they are interesting candidates for study as either free phase transfer catalysts, or as polymer-bound catalysts. In the latter case, the sidearm could serve both a complexing function and as a mechanical link between macroring and polymer. Polymeric phase transfer catalyst systems have been prepared... 

Soluble polymer-bound catalysts for epoxidation reactions have also been explored, with a complete study into the nature of the polymeric backbone performed by Janda [70]. Chiral (salen)-Mn complexes were appended to MeO-PEG, NCPS, Jan-daJeF and Merrifield resin via a glutarate spacer. It was found that for the Jacobsen epoxidation of ds-/ -mefhylstyrene, the enantioselectivities for each polymer-supported catalyst were comparable (86-90%) to commercially available Jacobsen catalyst (88%). Both soluble polymer-supported catalysts could be used twice before a decline in yield and enantioselectivity was observed. However, neither soluble polymer support proved as suitable as the insoluble JandaJel-supported (salen)-Mn complex for the epoxidation because residual impurities during precipitation and leaching of Mn from the complex, resulted in lowered yields. 

The alternative strategy for heterogenization has been pursued by Blechert and co-workers, for a polymer-supported olefin metathesis catalyst. A polymer-anchored carbene precursor was prepared by coupling an alkoxide to a cross-linked polystyrene Merrifield-type resin. Subsequently, the desired polymer-bound carbene complex was formed by thermolytically induced elimination of ferf-butanol while heating the precursor resin in the presence of the desired transition metal fragment (Scheme 8.30). 

Important features of macromolecule-metal complexes as polymer bound catalysts are greater selectivity, recoverability, good facility for treatment, and, especially, appearence of novel catalytic activities. 

Hydroformylation of 1-hexene was studied at 80 °C under 30 bar of CO/H2 (1 1) with the in-situ generated catalyst 7. The reaction was monitored via the CO/H2-uptake which was measured with mass flow meters. The polymer-bound catalysts possessed moderate activities in methanol (200-400 TO/h), and the activity was approx. 4-times higher when non polymer-bound TPPMS was used as the ligand. The activity of the catalyst 7 decreased in recycling experiments (entries 2a-2b in Table 2), which can presumably be attributed to a partial oxidation of the phosphine ligands. Moreover, the activity of the complex was not significantly affected by the change of P Rh... 

Complexation of the three silver salts Ag(CBnH12), Ag(CBnH6Br6), and Ag(OTf) to polymer bound triphenylphosphine also yielded active catalyst systems. The polymer-bound catalyst could be recycled 3 times with no loss of activity. Dimeric complexes [e.g., [Ag(PPh3)2(CBnH12)]2] were significantly poorer catalysts. 

Polymer-bound Organometallic Complexes as Catalysts for Use in Organic Synthesis... 

The same system was employed in the reduction of 4-phenyl-2-butanone to (S)-4-phenyl-2-butanol using HLADH as well as 5-ADH from Rhodococcus sp. with high enantioselectivity [113]. With pentamethylcyclopentadienyl-4-ethoxy-methyl-2,2 -bipyridinechloro-rhodium(III) as mediator and HLADH as catalyst, after 5 h 70% of 4-phenyl-2-butanone was reduced to (S)-4-phenyl-2-butanol with 65% ee. Using 5-ADH. 76% of the ketone was converted to the (S)-alcohol after 5 h with 77% ee. Furthermore, this system has been applied in an electrochemical EMR with a polymer bound rhodium complex as mediator. 

Different conditions (including additives and solvent) for the reaction have been reported,often focusing on the palladium catalyst itself," or the ligand." Catalysts have been developed for deactivated aryl chlorides," and nickel catalysts have been used." Modifications to the basic procedure include tethering the aryl triflate or the boronic acid to a polymer, allowing a polymer-supported Suzuki reaction. Polymer-bound palladium complexes have also been used." " The reaction has been done neat on alumina," and on alumina with microwave irradiation." Suzuki coupling has also been done in ionic liquids," in supercritical... 

The functionalized ligands were tested for various hydrogenation reactions (see Scheme 12.17). Ir-Josiphos bound to silica gel as well as to a water-soluble complex produced TONs in excess of 100000 and TOFs up to 20000h for the Ir-catalyzed hydrogenation of 2-methyl-6-ethyl aniline (MEA) imine to give an intermediate for (SJ-metolachlor [45a]. The polymer-bound Ir complex was much less active, and in all cases the ee-values were comparable to those for the homogeneous catalyst Selected results are summarized in Table 12.3. However, no immobilized system could compete with the homogeneous catalyst which is used to produce >10000 tonsy of enantioenriched (. S J-metolachlor and which, under optimized condi-... 

Oxidation of alcohols [52] The appropriate alcohol (3 mmol), tBuOH (70% solution in water, 6 mmol), and 64 (25.6 mg, 1 mol% Co complex) in dichloromethane (20 mL) were refluxed for 4 h, the reaction mixture being agitated by means of slow nitrogen bubbling. After coohng, the polymer-bound catalyst was removed by filtration and the product mixture was quenched with sodium sulfite. The purity of the crude product mixture was found to be high, and final purification was achieved by recrystaUization in the case of soHds and by flash chromatography in the case of Hquids. 

Polymer bound catalyst systems have become highly sophisticated. For supports, commercial resins have in cases given way to custom tailored polymers designed to optimize a supported catalyst s performance. This progression from simple to complex systems is illustrated by advances in supported catalysts used for the asymmetric hydrogenation of enamides. 

The metal complexes most often studied as polymer-bound catalysts have been Rh(I) complexes, such as analogues of Wilkinson s complex. The catalytic activity of a bound metal complex is nearly the same as that of the soluble analogue. Rhodium complexes are active for alkene hydrogenation, alkene hydroformylation, and, in the presence of CH3I cocatalyst, methanol carbonylation, etc. Polymer supports thus allow the chemistry of homogeneous catalysis to take place with the benefits of an insoluble, easily separated catalyst . ... 

Soluble polymer-bound catalysts can be expected to receive continued attention as they offer specific advantages. By comparison to aqueous two-phase catalysis, a range of substrates much broader with respect to their solubility can be employed. By comparison to heterogenization on solid supports, the selectivity and activity of homogeneous complexes can be retained better. However, it must also be noted that to date no system has been unambiguously proven to meet the stability and recovery efficiency required for industrial applications. 

Metathesis reactions have been known for many years and can be carried out using both homogeneous and heterogeneous341 catalysts. Although other catalysts have been used, 42 ruthenium complexes s j ve been the most common. With the development of the Grubbs catalyst (428) the reaction has taken on increa-ed synthetic utility. The Grubbs catalyst is also stable to Lewis acidic conditions.Catalysts have been developed that allow the reaction to take place in aqueous and alcohol solution and polymer-bound catalysts have been used.346... 

Microwave-assisted Suzuki coupling using a reusable polymer-supported palladium complex has been achieved in a more recent study [135]. The reaction mixture was treated with the polystyrene-bound palladium catalyst and irradiated in an open flask for 10 min in a domestic microwave oven (Scheme 16.88). After cooling, the mixture was filtered and the catalyst extracted with toluene and dried. The recycled polymer-bound catalyst can be reused five times without loss of efficiency. 

Polymer-Bound Metal Complexes as Catalysts for C-C and C-N Coupling... 

Allylic substitutions were also carried out with Pd(0) complexes as catalyst precursors with phosphine ligands bound covalently to polar polymers, namely random copolymers of acrylamide and N-isopropylacrylamide (PNIPAM) (la, = 500 000, determined by viscometry). After the catalytic reaction, the polymer-bound catalysts were precipitated by adding the reaction mixture to an apolar solvent and could be re-used for further cycles (with an average of 93% conversion in ten consecutive reaction cycles, each 6 h at 50 °C in THF/H2O 1 1,1 mol% palladium) [2],... 

The aforementioned PNIPAM-bound palladium(O) complexes (la) were employed as catalyst precursors for Heck and Suzuki coupling under thermomorphic conditions in heptane/DMAc or heptane/EtOH/water, respectively [Eqs. (2) and (3) DMAc = dimethylacetamide] [3], After cooling to room temperature, the heptane-rich phase formed was separated from the polar phase containing the polymer-bound catalyst. New heptane solution of the substrate was added to the catalyst phase. In three consecutive cycles, no loss in activity was observed. 

With supported Mn(III)-salen complexes [27], the use of polymer-bound catalysts for the asymmetric epoxidation of olefins is possible, allowing once more the easy recovery of the catalyst by precipitation with a suitable solvent [28]. Poly(ethylene... 

Linear polystyrene has also been used to support asymmetric hydrogenation catalysts containing chiral diphosphine rhodium(I) complexes (50). Asymmetric hydrogenations of itaconic acid were carried out, forming (R)-2-raethylbutanedioic acid with e.e. s ranging from 20-37%. None of the polymer-bound catalysts were more effective than (-)-DIOP-RhCl and the observed e.e. s were found to be dependent on the molecular weight of the polymer chain, its raicrostructure and solubility. 

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