Supported phosphine catalysts

Scheme 25 Synthesis of the dendrimer-supported phosphine catalyst synthesized by Reek, Meijer and coworkCTs. ...

Scheme 26 Amination of crotyl acetate catalyzed by a dendrimer-supported phosphine catalyst. ...

The use of water-soluble reagents and catalysts allows reactions to be performed in aqueous buffered solutions. PEG-supported triarylphos-phine has been used in a Wittig reaction under mildly basic aqueous conditions (Eq. 8.115). The PEG-supported phosphine oxide byproduct can be easily recovered and reduced by alane to regenerate the starting reagent for reuse.312 The aqueous Wittig reaction has also been used in... 

Some general reviews on hydrogenation using transition metal complexes that have appeared within the last five years are listed (4-7), as well as general reviews on asymmetric hydrogenation (8-10) and some dealing specifically with chiral rhodium-phosphine catalysts (11-13). The topic of catalysis by supported transition metal complexes has also been well reviewed (6, 14-29), and reviews on molecular metal cluster systems, that include aspects of catalytic hydrogenations, have appeared (30-34). 

O2 adsorption, 28 38 surface modility, 28 34 surface structure, 28 30, 31 oxidation of CO on, 28 65 oxide-supported metal catalysts, 41 10, 11 -phosphine catalysts achiral, 25 83-85 recovery, 32 354-356, 367-369 selectivity, 30 348 platinum, 30 355 -silica catalysts... 

Y. Z. Yuan, A. P. Kozlova, K. Asakura, H. L. Wan, K. Tsai, and Y. Iwasawa, Supported Au catalysts prepared from Au phosphine complexes and As-precipitated metal hydroxides Characterization and low-temperature CO oxidation, J. Catal. 170(1), 191-199 (1997). 

Shibasaki et al. developed a polymer-supported bifunctional catalyst (33) in which aluminum was complexed to a chiral binaphtyl derivative containing also two Lewis basic phosphine oxide-functionahties. The binaphtyl unit was attached via a non-coordinating alkenyl Hnker to the Janda Jel-polymer, a polystyrene resin containing flexible tetrahydrofuran-derived cross-Hnkers and showing better swelling properties than Merifield resins (Scheme 4.19) [105]. Catalyst (33) was employed in the enantioselective Strecker-type synthesis of imines with TMSCN. 

The preparation of polymer-supported iridium catalysts (61) and (62) for the stereoselective isomerization of double bonds using polystyrene based immobilized triphenyl phosphine were recently reported by Ley and coworkers (Fig. 4.5). The immobilized catalyst is potentially useful for deprotection strategies of aUyl ethers [130]. 

In 1993 Bergbreiter prepared two soluble polymer-supported phosphines that exhibited an inverse temperature-dependent solubility in water [52]. Although PEG-supported phosphine undergoes a phase-separation from water at 95-100 °C, the PEO-poly(propylene oxide)-PEO supported catalyst was superior as it is soluble at low temperatures and phase-separates at a more practical 40-50 °C. Treatment of a diphenylphosphinoethyl-terminated PEO-PPO-PEO triblock copolymer... 

The SHB concept was expanded to chiral phosphine catalysts by de Rege et al., who reacted the trifluoromethanesulfonate (triflate) counter anion of the cationic complex [Rh(COD)((R,Rj-MeDuPhos)] with the surface hydroxyl groups of the silaceous mesoporous material MCM-41 [122]. The complex was loaded to a level of 1.03 wt% Rh. A decrease in support surface area and pore volume is consistent with the complex being located within the support pores. The counterion is very important in this process if the anion of the homogeneous catalyst precursor is altered to BArp no adsorption of the catalyst is observed. It is postulated that the mechanism of triflate binding is hydrogen bonding with the support, and that the... 

In experiments with a supported palladium catalyst, Pd/C, satisfactory yields were obtained without the use of phosphine ligands for the Heck reactions of aryl iodide with acrylonitrile, styrene, and methyl methacrylate in the ionic liquid [BMIM]PF6 (259). The addition of triethylamine improved the yields. The Pd/C remained in the ionic liquid only. The ionic liquid containing Pd/C can be reused as... 

Okubo et al. (260) reported that Pd(II)/Si02 was a more effective catalyst than Pd/C when two equivalents of triethylamine base were added to the same ionic liquid one equivalent was not sufficient. The reaction was carried out without phosphine ligands. The unreduced Pd(II)/Si02 catalyst with two equivalents of base in [BMIM]PF6 was more active than the supported palladium catalysts in DMF. Furthermore, the stability of [BMIM]PF6 also improved with the addition of triethylamine. 

The failure to observe any leaching could either indicate that the initial dissociation of the supported phosphine-rhodium(I) bond does not occur or that having been displaced from one site within the support the rhodium subsequently binds to another site. This latter possibility would not lead to any significant leaching in experiments where the catalyst was shaken with the reactants, although, if the reactants were passed through a bed or column of catalyst, leaching should then be observable. 

Organometallic complex catalysts have been removed from reaction mixtures with solid-supported phosphines as well as other ligands to facilitate reagent handling and product purification. Alternatively, Leadbeater28... 

An der Heiden, M. and Plenio, H. (2004) Homogeneous catalysts supported on soluble polymers biphasic Suzuki— Miyaura coupling of aryl chlorides using phase-tagged palladium—phosphine catalysts. Chem. Ear. ]., 10, 1789. 

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... 

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