Polymer-bound palladium catalysts

A polyethylene glycol-polystyrene graft copolymer palladium catalyst has been used in allylic substitution reactions of allyl acetates with various nucleophiles in aqueous media.58 Another polymer-bound palladium catalyst 40 was developed and used in a Heck coupling of allylic alcohols with hypervalent iodonium salts to afford the substituted allylic alcohols as the sole products under mild conditions with high catalytic efficiency.59 The same polymer-bound palladium catalyst has also been used for Suzuki cross-coupling reactions.60... 

The semihydrogenation of the carbon-carbon triple bond is a particularly valuable and frequently used application of heterogeneous catalysis to synthetic chemistry, and is the subject of several recent re-views. > Catalysts prepared from palladium and nickel are most commonly used, but the form of the catalyst and the conditions of use affect the results (see Section 3.1.1.2). A polymer-bound palladium catalyst, PdCh with poly-4-diphenylphosphinomethylstyrene, is intended to combine the selective properties of mononuclear transition metal complexes with the ease of separating the product from a solid. Whether catalysts of this type will replace the more traditional heterogeneous catalysts remains to be seen. 

Generally, the activities of polymer-bound palladium catalysts are less than those of unsupported ones, but here too there are exceptions. Kaneda et al reported that PdCl2, on phosphinated polystyrene was more active than the homogeneous situation.Rates were very solvent dependent— hydrogenation of styrene was slow in dimethyl sulfoxide optimum activity was obtained in solvents of moderate coordinating ability (see Table 6). 

Classical Heck reactions can also be conducted with polymer-bound palladium catalysts, for example, with diarylphosphinylated polystyrene. ... 

A unique class of polymer, polyquinone-bearing sulfonic acid functional groups, when combined with Pd(II), displayed activities for the oxidation of aUcenes to aldehyde, without the need for a cocatalyst such as CuCl2. ° Here the polymer matrix serves both as a catalyst carrier and a redox mediator. A remarkable solvent effect on the selectivities of a polymer-bound palladium catalyst was observed with the oxidation of 2-octene, affording 97% 2-octayne in Et0H-H20 and 89% 2-octanone when EtOH was replaced with dixoane. Oxidation of 2-methylnaphthalene to 2-methyl-1,4-naphthoquinone (vitamin K3) can be effected by palladium absorbed on ion exchanged resin with aqueous H2O2. 

Scheme 21 A synthetic applicatirai of polymer-bound palladium catalyst to hydrophosphorylation...

Allylic acetoxy groups can be substituted by amines in the presence of Pd(0) catalysts. At substituted cyclohexene derivatives the diastereoselectivity depends largely on the structure of the palladium catalyst. Polymer-bound palladium often leads to amination at the same face as the aoetoxy leaving group with regioselective attack at the sterically less hindered site of the intermediate ri -allyl complex (B.M. Trost, 1978). 

Palladium catalysts have been found which are effective in the Suzuki coupling reaction of arylboronic acids with aryl chlorides carrying electron-withdrawing groups.73 Biaryls may also be synthesized by cross-coupling of arylboronic acids with arenediazonium salts.74,75 There has been a report of the polymer-bound palladium-catalysed Suzuki coupling of aryl triflates with organoboron compounds.76 Arylbor-onates may themselves be synthesized by the palladium-catalysed reactions of... 

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 inverse temperature-dependent solubility in aqueous media of polymer-bound palladium(0)-phosphine catalysts, based on the water-soluble polymer poly(Wisopropyl)acrylamide (PNIPAM) 28, was also used to recycle and reuse these catalysts in nucleophilic allylic substitutions (Equation (8)) and cross-coupling reactions between aryl iodides and terminal alkynes (Equation (9)). The catalyst was highly active in both reactions, and it was recycled 10 times with an average yield of 93% in the allylic nucleophilic substitution by precipitation with hexane. ... 

Rapid and efficient Suzuki coupling of protoxygencol has been developed using polymer-supported palladium catalysts under microwave conditions at 110 °C within 10 min. in the synthesis of various benzolidines [15]. Radical cyclization of resin-bound N-(2-bromophenyl)acrylamides using Bu3SnH proceeded smoothly in DMF or toluene as solvent under the action of microwave irradiation for preparation of 2-oxindoles as illustrated in Scheme 10.9. This method is superior to conventional solution synthesis [16]. 

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. 

The selective hydrogenation of alkynes to alkenes may he achieved with polymer-bound palladium(n) complexes, particularly in solvents such as dimethyl formamide, dimethyl sulphoxide, and ethanol. The catalytic activity appears to depend on the acidity of the alkyne rather than steric factors. This polymer-bound palladium complex is siniilar in selectivity to cationic rhodium and the lindlar catalysts. ... 

Acid derivatives that can be converted to amides include thiol acids (RCOSH), thiol esters (RCOSR), ° acyloxyboranes [RCOB(OR )2]. silicic esters [(RCOO)4Si], 1,1,1-trihalo ketones (RCOCXa), a-keto nitriles, acyl azides, and non-enolizable ketones (see the Haller-Bauer reaction 12-31). A polymer-bound acyl derivative was converted to an amide using tributylvinyl tin, trifluoroacetic acid, AsPh3, and a palladium catalyst. The source of amine in this reaction was the polymer itself, which was an amide resin. 

Several combinatorial approaches to the discovery of transition metal based catalysts for olefin polymerization have been described. In one study Brookhart-type polymer-bound Ni- and Pd-(l,2-diimine) complexes were prepared and used in ethylene polymerization (Scheme 3).60,61 A resin-bound diketone was condensed with 48 commercially available aminoarenes having different steric properties. The library was then split into 48 nickel and 48 palladium complexes by reaction with [NiBr2(dme)] and [PdClMe(COD)], respectively, all 96 pre-catalysts being spatially addressable. 

Several microwave-assisted protocols for soluble polymer-supported syntheses have been described. Among the first examples of so-called liquid-phase synthesis were aqueous Suzuki couplings. Schotten and coworkers presented the use of polyethylene glycol (PEG)-bound aryl halides and sulfonates in these palladium-catalyzed cross-couplings [70]. The authors demonstrated that no additional phase-transfer catalyst (PTC) is needed when the PEG-bound electrophiles are coupled with appropriate aryl boronic acids. The polymer-bound substrates were coupled with 1.2 equivalents of the boronic acids in water under short-term microwave irradiation in sealed vessels in a domestic microwave oven (Scheme 7.62). Work-up involved precipitation of the polymer-bound biaryl from a suitable organic solvent with diethyl ether. Water and insoluble impurities need to be removed prior to precipitation in order to achieve high recoveries of the products. 

A number of modified reaction conditions have been developed. One involves addition of silver salts, which activate the halide toward displacement.94 Use of sodium bicarbonate or sodium carbonate in the presence of a phase-transfer catalyst permits especially mild conditions to be used for many systems.95 Tetraalkylammonium salts often accelerate reaction.96 Solid-phase catalysts in which the palladium is complexed by polymer-bound phosphine groups have also been developed.97 Aryl chlorides are not very reactive under normal Heck reaction conditions, but reaction can be achieved by inclusion of triphenylphosphonium salts with Pd(OAc)2 or PdCl2 as the catalyst.98... 

---