Polymer-supported phosphine palladium

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. 

In a related study, Srivastava and Collibee employed polymer-supported triphenyl-phosphine in palladium-catalyzed cyanations [142]. Commercially available resin-bound triphenylphosphine was admixed with palladium(II) acetate in N,N-dimethyl-formamide in order to generate the heterogeneous catalytic system. The mixture was stirred for 2 h under nitrogen atmosphere in a sealed microwave reaction vessel, to achieve complete formation of the active palladium-phosphine complex. The septum was then removed and equimolar amounts of zinc(II) cyanide and the requisite aryl halide were added. After purging with nitrogen and resealing, the vessel was transferred to the microwave reactor and irradiated at 140 °C for 30-50 min... 

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). 

In the last few years numerous reports have been published in the field of microwave-promoted aryl halide cyanation, utilizing nickel [71], palladium [72,73] and copper [74,75] catalysis. Even water [75] and ionic liquids [76] have proven useful as solvents in these processes. Srivastava and Collibee have exemplified a swift and dynamic procedure using polymer-supported triphenyl phosphine to enable easy subsequent removal through filtration [72]. As shown in Scheme 19, both bromides and iodides could be activated using palladium catalysis in DMF. Even without optimization of the individual reaction times, the overall process time involving simple filtration and extraction for compound isolation appears to be short. 

Scheme 19 Palladium-catalyzed aryl cyanation using a triaryl phosphine on polymer-support...

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. 

Scheme 1S.S3. Aryl cyanation using palladium and polymer-supported triphenyl phosphine.

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