Rhodium catalysts polystyrene-supported

TABLE 3.1. Asymmetric hydroformylation of styrene using polystyrene supported rhodium catalysts based... 

Electron spin resonance (ESR) signals, detected from phosphinated polystyrene-supported cationic rhodium catalysts both before and after use (for olefinic and ketonic substrates), have been attributed to the presence of rhodium(II) species (348). The extent of catalysis by such species generally is uncertain, although the activity of one system involving RhCls /phosphinated polystyrene has been attributed to rho-dium(II) (349). Rhodium(II) phosphine complexes have been stabilized by steric effects (350), which could pertain to the polymer alternatively (351), disproportionation of rhodium(I) could lead to rhodium(II) [Eq. (61)]. The accompanying isolated metal atoms in this case offer a potential source of ESR signals as well as the catalysis. 

The soluble polymer-supported catalysts have also been used for asymmetrically catalyzed reactions Following a procedure for the preparation of insoluble polymeric chiral catalysts a soluble linear polystyrene-supported chiral rhodium catalyst has been prepared. This catalyst displays high enantiomeric selectivity compared to the low molecular weight catalyst. Thus, hydroformylation of styrene using this catalyst produces aldehydes in high yields. The branched chiral hy drotropaldehy de is formed in 95% selectivity. 

Chiral ligands can be varied with rhodium catalysts modifications with Diop, Diop-DBP and other variants of Diop, Chiraphos, BPPM, BnCH3PhP, CH3PrPhP, NMDPP, and AMPP type ligands are reported (for abbreviations and literature see Section 1.5.8.2.2.2.). Polymer-supported catalysts are mainly used as non-cross-linked polystyrene with attached phosphane ligands, among others Diop and BPPM9 62,153-155. Aspects of these variations are discussed below under platinum catalysts). 

A polymer-supported rhodium catalyst modified with Diop attached to non-cross-linked polystyrene, first used in the asymmetric hydroformylation of styrene, gives 95 % branched aldehyde, however with only 2% ee9. Further developments in the preparation and use of cross-linked polymers with attached chiral phosphane ligands (Diop, DIPHOL, BPPM) in rhodium- and platinum-catalyzed asymmetric hydroformylation have led to good to excellent results with respect to the asymmetric induction62-124 157,159 and arc described in Section 1.5.8 2.2.3.2. The results arc integrated in Table 4. 

Phosphorus donor ligands have also been used to activate Ru3(CO)i2 in the catalytic reduction of nitrobenzene by CO/H2O in the presence of sodium hydroxide [32], Reaction conditions are mild (room temperature and one atmosphere of CO). In a three hours reaction, a turnover of 95 was observed by adding l,2-bis(diphenylphosphino)ethane (DPPE) to Ru3(CO)i2 in a 0.5 molar ratio. By adding PPh3 or in the absence of any phosphorus ligand, the observed turnover was 43 and 51 respectively. However, better results have been obtained with rhodium catalysts (see later) [32]. Preformed phosphine-substituted clusters of the type Ru3(CO)9L3 (L = arylphosphine) have also been used as catalysts for the same reaction [33] and the same complex was also supported on a polystyrene-divinylbenzene copolymer. 

Cyclopropanation Reactions. Davies and Nagashima reported the first example of a catalytic asymmetric cyclopropanation of alkenes on a solid support. Carbene dimerization represents a limitation in solution phase, lowering yields and necessitating additional purification steps. Immobilization of the olefin 97 on a polystyrene diethylsilyl resin followed by reaction with various diazoacetates in the presence of a rhodium catalyst generated the cyclopropanes 98 and 99 in high yield and enabled the removal of dimerization products 102 through a simple wash step (Scheme 6.23). The products 100 and 101 were cleaved as a mixture of diastereomers from the resin under mild conditions. The stereoselectivity of the reaction was not influenced by the solid support, but rather by the catalyst selection most important, >90% ee was observed under these conditions. 

Bidentate ligands were used by Alper and coworkers. The bis(diphenylphospino-methyl)amine ligands were prepared on primary amine-terminated PAMAM dendrons on silica as well as polyamido dendrons on polystyrene via the double Mannich-like reaction with formaldehyde and diphenylphosphine (Scheme 15.36a). " Subsequently, Alper and coworkers subjected the dendronized ligand-decorated supports to complexation with rhodium and palladium precursors in order to prepare active catalysts for a number of important chemical transformations (Scheme 15.36b). Initially, the dendronized rhodium catalysts were tested in the hydroformylation reaction and carbonylative ring expansion of... 

Dygl996 Dygutsch, D.P. and Eilbracht, P, Synthesis of Cyclopen-tanone Derivatives with Polystyrene-Supported Cyclopentadi-enyl Rhodium Catalysts, Tetrahedron, 52 (1996) 5461-5468. 

Figure 3.8. An early example of a hybrid support applied in the rhodium catalysed hydroformylation operated in a continuous flow reactor. Polystyrene containing phosphite ligands were grafted on inorganic silica, such that the catalyst will behave as a homogeneous catalyst when using a compatible solvent...

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. 

There have been multiple efforts toward supported catalysts for asymmetric transfer hydrogenation, and the 4 position on the aryl sulfonate group of 26 has proven a convenient site for functionalization. Thus far, this ligand has been supported on dendrimers [181,182], polystyrenes [183], silica gel [184], mesoporous siliceous foam [185], and mesoporous siliceous foam modified with magnetic particles [186]. The resulting modified ligands have been used in combination with ruthenium, rhodium, and iridium to catalyze the asymmetric transfer of imines and, more commonly, ketones. 

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. 

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