Polymer-supported phosphine palladium catalyst

Supported catalysts involving palladium on carbon and dendrimer-encapsulated palladium and a polymer-supported phosphine palladium catalyst have facilitated C-C coupling reactions in SCCO2. Polymer-tethered substrates or amine bases have also been successfully used for the Mizoroki-Heck and Suzuki-Miyaura reactions in SCCO2. For example, REM resin underwent a Mizoroki-Heck reaction with iodobenzene to yield, after cleavage, ( )-methyl cinnamate 48 (74%) (Scheme 88). It is assumed that SCCO2 acts as a good solvent that swells the polymers and exposes reactive sites. 

The peptide-based phosphine ligand 105 was identified from a polymer-supported phosphine library of 75 members [154]. Enantioposition-selective desymmetrization of the meso-cyclopentenediol derivative 100 was promoted by a palladium complex of 105 to afford the cyclic carbamate 101 with 76% ee. This result demonstrated that the combinatorial approach is effective in the lead-generation stage of stereoselective catalyst development [155, 156]. The resin-supported palladium complex of Ac-D-Phg-Pro-D-Val-Pps-D-Leu-NH resin 106, which has also been developed through the combinatorial approach. 

The counter-ions of some of the quaternary onium groups were exchanged with an anionic phosphine compound, which was then used to complex palladium. Thus, a polymer material containing phase transfer catalyst and transition-metal catalyst groups was obtained (Fig. 20). The Heck-type vinyla-tion reaction [137] was used to examine the catalytic activity of the heterogeneous system. The polymer-supported catalyst was found to compare favourably with the homogeneous system (Fig. 21). 

Entries 2 and 5 in Table 15 exonplify the use of a polymer-supported palladium(0)-phosphine catalyst which may offer practical advantages for carrying out p ladium-catalyz ene-type cycliz ons. As to the stereochemistry, dienes (229) containing a (Z)-enophile gave, under kinetic control, predominantly... 

Phosphine free catalysts and halogen-free reactions are known for the Heck reaction. Improvements on the palladium catalyst system are constantly being reported, including polymer-supported catalysts." °° The influence of the ligand has been examined." Efforts have been made to produce a homogeneous catalyst for the Heck reaction." The Heck reaction can be done in aq. media," ° in perfluori-nated solvents," in polyethylene glycol," ° in neat tricaprylmethylammonium... 

An insightful discussion on an alternahve approach is provided in Chapter 6 of this Handbook, where insoluble polymer-supported chiral catalysts (chiral palladium phosphine complexes supported on TentaGel-type amphiphilic polymer bearing PEG chains) are used for heterogeneous asymmetric processes in water. 

Synthesis of polymer-supported Pd catalyst [44] For the synthesis of polymer 41, see refs. [46-48]. All solvents vs ere degassed by ultrasonication and argon purging prior to use. To a -well-stirred solution of 44 (0.36 mmol in phosphine) in THF (72 mb) was added a solution of 45 (0.12 mmol) in H2O (30 mb) and the mixture was again degassed. After the mixture had been stirred for 62 h at room temperature, a yellow precipitate formed. Water (30 mb) was added to the suspension, and THF was removed at 80 °C by distillation for 4 h in an apparatus fitted with a Dean-Stark head to leave a reddish precipitate. This precipitate was stirred at 100 °C, first in H2O (100 mb) for 12 h, then in THF (100 mb) for 3 h, and finally in further H2O (100 mb) for 12 h to wash away the unreacted palladium species and polymers. After drying in vacuo (ca. 0.1 mmHg), 41 was obtained as a dark-red solid in almost quantitative yield. 

Recently, Plenio et al. [51] have described a new polymer-supported palladium phosphine catalyst This catalyst can also be used for Sonogashira coupling. 

An early report by Konig deals with rapid parallel Suzuki reactions in water with phase-transfer catalysts. The solid support used in these reactions was PEG, and a variety of aryl palladium precursors were evaluated aryl halides as well as aryl tri-flates and nonaflates (Scheme 15.8). The inclusion of PEG is appealing, because it not only helps to solubilize the reagents but is also suggested to stabilize the palladium catalyst in the absence of phosphine ligands. Both the polymers and the esters were reported to withstand 10 min of 900 W multimode MW irradiation whereas the thermal conditions induced substantial ester cleavage (up to 45%). Nonaflates were found to be associated with lower yields and diminished product purity [32]. 

In a related study, polymer-supported triphenyl phosphine was used in palladium-catalyzed cyanations [136]. Commercially available resin-bound triphe-nylphosphine was mixed with palladium(II) acetate in N,N-dimethylformamide to generate the heterogeneous catalytic system under a nitrogen atmosphere. The reagents were then added to the activated catalyst and the mixture was irradiated at 140 °C for 30-50 min (Scheme 16.89). Finally, the resin was removed by filtration and evaporation of the solvent furnished the desired benzonitriles in high yields and excellent purity. 

Since the researcher normally looks to the chemistry of soluble complexes in designing polymer-bound catalysts, it is notable that some areas that have proven fruitful in homogeneous catalysis have been omitted from investigations using polymer-bound catalysts. One of these areas concerns the reactions of arenes. Benzene, for example, may be hydrogenated with homogeneous cobalt phosphite and ruthenium phosphine complexes, but the corresponding supported versions are not reported. Aryl halides may be carboxylated in the presence of a soluble palladium catalyst ... 

Alternatively, sterically highly congested phosphines such as tBujP [38], diadamantylalkylphosphines [39], bulky secondary phosphines [40], or (bulky) bidentate phosphines (e.g., dippb (l,4-bis(diisopropylphosphino)butane) or even dppe (l,2-bis(diphenylphosphino)ethane)) [41a-d] are suitable for coupling of chloroarenes as well. In particular, polymer-bound ligands [42, 43] or inorganic solid-supported palladium metal [42] have been frequently used for the ease of regeneration of the catalyst. With solid-supported palladium catalysts, the reaction proceeds presumably in the Hquid phase [44-46]. 

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