Diphosphine homogeneous

Immobilization of homogeneous catalysts for hydrogenation reactions concerns essentially enan-tioselective hydrogenations, important for the synthesis of fine chemicals (see Chapter 9.2). The pioneering work of Pugin et al.131 concerns the synthesis of a rhodium-based catalyst, with a diphosphine-pyrrolidine-based ligand for the hydrogenation of methylacetamide cinnamate (Equation(8)). 

It is interesting to note that using the sol-gel procedure (I) the pre-formation of the rhodium diphosphine complex suppressed the formation of ligand free rhodium-cations on the silica surface. This approach gave rise to a well-defined, very selective hydroformylation catalyst. All immobilised catalysts were 10 to 40 times slower than the homogeneous catalyst under the same conditions, the sol-gel procedure yielding the fastest catalyst of this series. 

The synthesis of cationic rhodium complexes constitutes another important contribution of the late 1960s. The preparation of cationic complexes of formula [Rh(diene)(PR3)2]+ was reported by several laboratories in the period 1968-1970 [17, 18]. Osborn and coworkers made the important discovery that these complexes, when treated with molecular hydrogen, yield [RhH2(PR3)2(S)2]+ (S = sol-vent). These rhodium(III) complexes function as homogeneous hydrogenation catalysts under mild conditions for the reduction of alkenes, dienes, alkynes, and ketones [17, 19]. Related complexes with chiral diphosphines have been very important in modern enantioselective catalytic hydrogenations (see Section 1.1.6). 

This achievement was unique in two respects 1) it was the first example of industrial application of a homogeneous enantioselective catalysis methodology and 2) it represented a rare example of very quick convergence of basic knowledge into commercial application. The monophosphine ligand CAMP was shortly replaced by the related diphosphine ligand DIPAMP which improved the selectivity for the I-DOPA system up to 95% ee [45]. 

Recently Togni et al. [19] focussed on the preparation of asymmetric dendrimer catalysts derived from ferrocenyl diphosphine ligands anchored to dendritic backbones constructed from benzene-1,3,5-tricarboxylic acid trichloride and adamantane-l,3,5,7-tetracarboxylic acid tetrachloride (e.g. 11, Scheme 11). In situ catalyst preparation by treatment of the dendritic ligands with [Rh(COD)2]BF4 afforded the cationic Rh-dendrimer, which was then used as a homogeneous catalyst in the hydrogenation reaction of, for example, dimethyl itaconate in MeOH. In all cases the measured enantioselectivity (98.0-98.7%) was nearly the same as observed for the ferrocenyl diphosphine (Josiphos) model compound (see Scheme 11). 

In principle, the mechanism of homogeneous hydrogenation, in the chiral as well as in the achiral case, can follow two pathways (Figure 9.5). These involve either dihydrogen addition, followed by olefin association ( hydride route , as described in detail for Wilkinson s catalyst, vide supra) or initial association of the olefin to the rhodium center, which is then followed by dihydrogen addition ( unsaturate route ). As a rule of thumb, the hydride route is typical for neutral, Wilkinson-type catalysts whereas the catalytic mechanism for cationic complexes containing diphosphine chelate ligands seems to be dominated by the unsaturate route [1]. 

The complexes were prepared from [CODRhCl]2 and 1.1 equiv of chiral diphosphine in dichloromethane as solvent. The chiral complex was added to a suspension of the support in dichloromethane. After being stirred for 24 h, the solid was filtered, washed with dichloromethane until the solvent showed no color, and afterward dried at room temperature for 16 h. In order to remove the excess of Rh complex not fixed to the solid carrier, the catalysts were extracted with methanol in a Soxhlet apparatus under reflux for 24h (Scheme 2.1.6.1). Both ICP-AES analysis and FTIR spectra of the remaining solvent indicated no content of homogeneous complex. The resulting catalysts had a pale yellow color similar to that of the homogeneous complex. 

An optically active, supported chelating diphosphine has been prepared from the Merrifield resin using reaction (15) to give a polymer-supported analog of the homogeneous diop catalysts (28, 94). 

Hanson et a/.149 hydrogenated the prochiral olefin methyl a-acetamidocinna-mate using rhodium catalysts modified with the tenside chiral sulfonated diphosphine 34 (Table 2) in an ethylacetate/H20 micellar system at 25° C and 1 bar H2. The yield (100%) and enantiomeric excess (69%) were considerably higher than with the tetrasulfonated diphosphine 31 (Table 2 m=0, n=0) which gave 32% yield and 20% e.e. and the reaction time was shorter (1.5 versus 20h). Rh/34 and Rh/31 (m=0, n=0) gave nearly the same results (100% yield and 72-75% e.e. within < lh) in homogeneous methanol solutions.149... 

A simple method for the hydrosolubilization of chiral diphosphines is the introduction of a functional group such as sulfonate. Such chiral-sulfonated diphosphines can be used to prepare homogeneous hydrogenation catalysts in situ for the asymmetric reduction of prochiral substrates.1... 

The Effect of Chelating Diphosphine Ligands on Homogeneous Catalytic Decarbonylation Reactions Using Cationic Rhodium Catalysts... 

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