Polymer palladium-phosphine complex catalyst

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. 

Hydrodimerization is a special case of telomerization, where a (solvent) molecule A-B (the telogen, e.g., HzO) reacts with n molecules of an unsaturated molecule M (the taxogen) to yield oligomers or polymers of relatively low molecular mass (Eqs. 15 and 16). An important special case is the Kuraray 1-octanol process resulting from the products of Eq. (17) by subsequent hydrogenation. This industrially relevant reaction includes hydrodimerization of 1,3-butadiene [17]. Efficient catalysts are palladium-phosphine complexes, e.g., Pd2+/TPPMS (TPPMS = p(Qh4 -m-SO Na+)(C6H5)2). Little is as yet known on mechanisms. 

Water-soluble polymer-bound Pd(0)-phosphine catalyst has also been efficiently used in aqueous or mixed aqueous/organic media, the catalyst being recycled by solvent or thermal preparation methods [17]. Amphiphilic resin-supported palladium-phosphine complexes show high catalytic activity in allylic substitution reactions of various allylic acetates with different nucleophiles in aqueous media [18, 19]. Enantiomeric excess up to 98% is obtained using amphiphilic resin-supported MOP ligand or resin-supported P,N-chelating palladium complexes, the catalyst being recyclable [20,21]. The catalyst could be recovered by simple filtration and re-used without any loss of activity and enantioselectivity. 

Uozumi et al. developed paUadium-phosphine complex catalysts supported on amphiphilic polysterol/PEG resins. These polymer-bound amphiphiHc complexes were tested in the Heck reaction of various aryl hahdes and alkenes, giving the corresponding styrene derivahves in quantitative yields (Scheme 8.27a). Other catalytic reactions (e.g., Suzuki, Sonogashira, cycloisomerization, and hydroxycar-bonylation reactions) (Scheme 8.27b-d) were also realized with these amphiphilic palladium-phosphine complexes under mild conditions (room temperature) over long reaction periods [63]. 

New approaches to catalyst recovery and reuse have considered the use of membrane systems permeable to reactants and products but not to catalysts (370). In an attempt to overcome the problem of inaccessibility of certain catalytic sites in supported polymers, some soluble rho-dium(I), platinum(II), and palladium(II) complexes with noncross-linked phosphinated polystyrene have been used for olefin hydrogenation. The catalysts were quantitatively recovered by membrane filtration or by precipitation with hexane, but they were no more active than supported... 

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... 

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. 

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 ... 

A silica-supported polytitazane- ialladium complex (Ti-N-Pd) has been developed as a catalyst for the double carbonyladon of phenyl haUde to give a-keto amideJ" The reactivity of phenyl halide deaeases in the following order in the process where palladium-phosphine catalysts are used Hil > HiBr > HiQ. This kind of inorganic polymer catalyst was claimed to be very stable and to have great activity and selectivity in the catalytic reactions. 

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