Polymer resin rhodium

Primary ureas 1365 are good substrates in rhodium-catalyzed N-H insertion reactions with an array of 2-diazo-l,3-keto esters 1364. The products from the insertion reaction cyclize with the aid of acid to yield imidazolones 1366. This chemistry has been translated onto insoluble polymer resins and utilized to prepare a small array of imidazolones (Scheme 351) <20030L511, 2004JOC8829>. 

Methanol Carbonylation. Some researchers have described the possibility of supporting a rhodium compound on an ionic resin such as a copolymer of styrene and 4-vinylpyridine alkylated with methyl iodide forming a methylpyridinium-functionalized polymer (16). They have concluded that their ionic polymer-supported rhodium catalyst for methanol carbonylation in liquid phase is approximately equal in catalytic activity to the dissolved complex and that leaching of the complex could be minimized by suitable choice of solvent and by selecting high resin-to-rhodium ratios. However, experiments carried out only at low temperature (120°C) and low pressure were reported. 

An insoluble chiral polymer-supported rhodium complex (XXV) was prepared by Dumont et al [88], using a Merrifield resin. This system catalyzes the asymmetric hydrogenation of a-ethylstyrene and methyl atropate with low optical yields. Optical purities... 

A mechanistic study by Haynes et al. demonstrated that the same basic reaction cycle operates for rhodium-catalysed methanol carbonylation in both homogeneous and supported systems [59]. The catalytically active complex [Rh(CO)2l2] was supported on an ion exchange resin based on poly(4-vinylpyridine-co-styrene-co-divinylbenzene) in which the pendant pyridyl groups had been quaternised by reaction with Mel. Heterogenisation of the Rh(I) complex was achieved by reaction of the quaternised polymer with the dimer, [Rh(CO)2l]2 (Scheme 11). Infrared spectroscopy revealed i (CO) bands for the supported [Rh(CO)2l2] anions at frequencies very similar to those observed in solution spectra. The structure of the supported complex was confirmed by EXAFS measurements, which revealed a square planar geometry comparable to that found in solution and the solid state. The first X-ray crystal structures of salts of [Rh(CO)2l2]" were also reported in this study. 

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. 

Polymer beads serve as the solid phase, and synthesis is set up in such a way that eventually every bead carries just one compound (about 100 pmol w 10 molecules). As a consequence the obtained polymer-bound products are spatially separated. In each cycle, one reagent (A-F, e. g. an amino acid) and an encoding tag molecule (M, -M4) are attached to separate samples of resin beads. After this, the beads are thoroughly mixed and split up into equal quantities, and this is followed by the next reaction cycle. Diazoketones (1), which can be selectively incorporated into the excess polymer backbone using the corresponding acylcarbenes, being activated by rhodium catalysis, proved to be good tag molecules. 

Another approach, developed by Chiyoda/UOP, uses a rhodium catalyst heterogenized on a polymeric cation exchange resin. This takes advantage of the fact that the rhodium catalyzed carbonylation involves anionic complexes (see Section 4.2.5 below). The Chiyoda/UOP Acetica process employs a cross-linked polyvinylpyridine which is quaternized by methyl iodide to generate cationic pyridinium sites and which hold the anionic rhodium complexes by electrostatic interactions. The polymer support is tolerant of elevated temperatures and the ionic attachment of the catalyst is quite robust, resulting in only... 

An approach to imidazolones started from polymer-bound a-diazo-p-ketoester 33, which was transformed to intermediate 35 by treatment with urea 34 in the presence of a rhodium carboxylate catalyst (Scheme 10) [76]. Treatment of the resin-bound insertion product 35 with 10% TFA at room temperature afforded the resin-bound imidazolone 36 within 1 h. The polymer-bound imidazolone could then be cleaved by transesterification to give esters 37 or by a diversity building amidation reaction to provide amides 38. After preparative TLC, the products were obtained in yields of 19-84% (14 examples). 

The DlOP-rhodium(I) complex attached to organic polymers , e.g., polystyrene resin and poly(methyl vinyl alcohol), exhibits good catalytic activity as a chiral catalyst comparable to the corresponding homogeneous catalyst. In contrast, the rhodium(I) complexes anchored on inorganic supports display only a low efficiency . Studies show that the steric requirements for a match of the chiral ligand, a hydrosilane and a ketone are of definite importance in bringing about effective asymmetric induction. 

A solution phase chiral auxiliary for 1,3-dipolar cycloaddition of isomunchnones with vinyl ethers has been adapted for solid phase synthesis by attaching both enantiomers of the precursor a-hydroxyvaline to benzhydrylamine resin (Scheme 12.12) [13,19]. The auxiliary 22 was then functionalized by acylation and diazotiza-tion to provide diazoimide resin 23. Rhodium(II)-catalyzed nitrogen extrusion and cycloaddition in the presence of different vinyl ethers afforded, after detachment from the polymer, various bicydic molecules (24) in 49-65% yield and provided high degrees of selectivity (93-95% ee). 

U sing polymers was one of the very first methods in order to heterogenize the catalyst in a homogeneous catalytic reaction. Thus, thanks to these supports, the catalyst acquires the property of insolubility while maintaining its catalytic performance [39-42]. Some authors synthesized phosphonated resins, such as polystyrene, and used them as a ligand in several rhodium and platinum complexes. Thus, hydrogenation [43, 44], hydrosilylation [45], and hydroformylation of olefins were catalyzed. 

The rate ratio analysis must also account for entropy effects associated with the limited mobility of the substrate relative to the active site within the polymer. Roucls and Ekerdt (16) measured the relative rates for hydrogenation of cyclooctene and cyclohexene over an immobilized rhodium complex and found that the rate for cyclooctene was less than that for cyclohexene for the homogeneous rhodium complex, RhCl(PPh3)3, and in 1, 2, and 3 X dlvlnylbenzene polystyrene resins. Furthermore, they found that the relative rate differed with dlvlnylbenzene content. The relative rates were shown to be intrinsic rate ratios (16). Roucls and Ekerdt proposed that sterlc constraints due to the presence of the polysier support in the vicinity of active rhodium affected activity. All the rhodium sites were exposed to the same substrate concentration the sites displayed a different Inherent activity toward the two substrates. 

Phosphinated polystyrene resins have been functionalized with both bisphosphine nickel carbonyl and tristriphenylphosphinehydridocarbonylrhodium. Using this system, butadiene can be cyclodimerized to vinylcyclohexane and selectively hydroformylated at the terminal double bond in sequence. Similarly, the use of a NiBr2 chelate, reduced by NaBHj, together with the rhodium polymer permitted the linear oligomerization-selective hydroformylation of butadiene. These were both one-pot processes. 

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... 

It has also been shown that Rh 80 C in the presence o-f N,N-dimethyl benzyl ami ne, using any one o-f the -following reducing gases H2/CO, H2 or C0/H20[99. The reaction is highly selective and can be also carried out with Amberlyst A-21 resin beads which contained polymer bound N,N-dimethylbenzyl amine moieties which, in turn, imobilized the rhodium cluster. In this latter case an air sensitive catalytic system... 

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