Rhodium complexes polymer bound

CP-MAS P NMR can be used to characterize supported complexes, for example, the polypropylene-j -styryldiphenylphosphine-bound rhodium complex [(polymer)-PPh2]Rh(acac)(CO) has been characterized in this manner. Free and complexed phosphines are clearly... 

The polymers were converted to supported catalysts corresponding to homogeneous complexes of cobalt, rhodium and titanium. The cobalt catalyst exhibited no reactivity in a Fischer-Tropsch reaction, but was effective in promoting hydroformylation, as was a rhodium analog. A polymer bound titanocene catalyst maintained as much as a 40-fold activity over homogeneous titanocene in hydrogenations. The enhanced activity indicated better site isolation even without crosslinking. 

Figure 4.42. Electrostatically polymer-bound rhodium complex.[61]...

Chloro-bridged dimeric rhodium(I) complexes, such as [Rh(CO)2Cl]2 (27, 57, 98) and [Rh(COD)Cl]a (25), react with polymeric resins to give monomeric polymer-bound complexes with phosphine and amine supports. 

The nature of the support can have a very profound influence on the catalyst activity. Thus, phosphinated polyvinyl chloride supports are fairly inactive (75), and phosphinated polystyrene catalysts are considerably more active (57), but rather less active particularly when cyclic olefins are the substrates than phosphinated silica supports (76). Silica-supported catalysts may be more active because the rhodium(I) complexes are bound to the outside of the silica surface and are, therefore, more readily available to the reactants than in the polystyrene-based catalysts where the rhodium(I) complex may be deep inside the polymer beads. If this is so, the polystyrene-based catalysts should be more valuable when it is desired to hydrogenate selectively one olefin in a mixture of olefins, whereas the silica-based catalysts should be more valuable when a rapid hydrogenation of a pure substrate is required. 

Support-bound transition metal complexes have mainly been prepared as insoluble catalysts. Table 4.1 lists representative examples of such polymer-bound complexes. Polystyrene-bound molybdenum carbonyl complexes have been prepared for the study of ligand substitution reactions and oxidative eliminations [51], Moreover, well-defined molybdenum, rhodium, and iridium phosphine complexes have been prepared on copolymers of PEG and silica [52]. Several reviews have covered the preparation and application of support-bound reagents, including transition metal complexes [53-59]. Examples of the preparation and uses of organomercury and organo-zinc compounds are discussed in Section 4.1. 

The same system was employed in the reduction of 4-phenyl-2-butanone to (S)-4-phenyl-2-butanol using HLADH as well as 5-ADH from Rhodococcus sp. with high enantioselectivity [113]. With pentamethylcyclopentadienyl-4-ethoxy-methyl-2,2 -bipyridinechloro-rhodium(III) as mediator and HLADH as catalyst, after 5 h 70% of 4-phenyl-2-butanone was reduced to (S)-4-phenyl-2-butanol with 65% ee. Using 5-ADH. 76% of the ketone was converted to the (S)-alcohol after 5 h with 77% ee. Furthermore, this system has been applied in an electrochemical EMR with a polymer bound rhodium complex as mediator. 

A library of 63 peptides, each containing one of the two phosphine-substituted amino acids 8 or 9, was prepared by Gilbertson and coworkers in a parallel synthesis on polymer beads [13]. After complexation of all peptides with rhodium, the obtained polymer-bound complexes were employed in the asymmetric hydrogenation of 10 (Fig. 5). 

The most frequently used metallic catalysts for acyldiazo- and (alkoxycarbonyl)dia-zomethanes are complexes or salts of rhodium, palladium and copper. Alkenylboronic esters A-silylated allylamines and acetylenes are successfully cyclopropanat-ed with diazocarbonyl compounds under catalysis of one of those metal derivatives. Newly developed metallic catalysts for diazoacetic esters include polymer-bound, quantitatively recoverable Rh(II) carboxylate salts ", Cu(II) supported on NATION ion exchange poly-mer ruthenacarborane clusters, Rh2(NHCOCH3)4 which produces cyclopropanes with substantially enhanced trans (anti) selectivity as shown below and (rj -CsHs)... 

The metal complexes most often studied as polymer-bound catalysts have been Rh(I) complexes, such as analogues of Wilkinson s complex. The catalytic activity of a bound metal complex is nearly the same as that of the soluble analogue. Rhodium complexes are active for alkene hydrogenation, alkene hydroformylation, and, in the presence of CH3I cocatalyst, methanol carbonylation, etc. Polymer supports thus allow the chemistry of homogeneous catalysis to take place with the benefits of an insoluble, easily separated catalyst . ... 

Diazo carbenoid chemistry, however, is not yet on the finishing line, but developments such as the synthesis of polymer-bound soluble rhodium complexes that are recoverable and, therefore, less sensitive to the exceedingly expensive rhodium (Doyle et al., 1992 a) or the use of relatively inexpensive chiral auxiliaries for enantiocontrol (Davies and Cantrell, 1991) are still energy reserves that may be important to win the race ... 

Linear polystyrene has also been used to support asymmetric hydrogenation catalysts containing chiral diphosphine rhodium(I) complexes (50). Asymmetric hydrogenations of itaconic acid were carried out, forming (R)-2-raethylbutanedioic acid with e.e. s ranging from 20-37%. None of the polymer-bound catalysts were more effective than (-)-DIOP-RhCl and the observed e.e. s were found to be dependent on the molecular weight of the polymer chain, its raicrostructure and solubility. 

Polymer-bound heterogenized homogeneous rhodium catalysts have also been successfully used in ketone reduction. Italian scientists studied ionic rhodium complexes supported on a Merrifield resin (4), but the Wilkinson-type analog proved to be active only in the presence of... 

At the start of this complex process, RhCls coordinates with PVA. An oxonium product is formed, which is further transformed via an aUcoxide intermediate into a polymer-bound hydride complex. Finally a colloid forms, resulting from the disproportionation of these rhodium hydrides and subsequent growth of particles (scheme 18). 

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. ... 

Organic Reactions Catalyzed by Polymer-Bound Palladium and Rhodium Complexes. 

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