Phosphine complexes rhodium

Asymmetric catalysis by chiral rhodium complexes in both hydrogenation and hydrosilation have been found. Kagan (42) recently reported that the chiral phosphine rhodium complex [(—)-DIOPRhCl]2 gave the most efficient asymmetric syntheses observed to date. This complex is ... 

Scheme 3.2 Asymmetric conjugate addition catalyzed by chiral phosphine-rhodium complexes [7-11].

Direct addition of VI to chlorocarbonylbis(phosphine)rhodium complexes containing more basic phosphine ligands to yield optically active complexes of the type III did not occur. An optically active benzylrho-dium complex (X) was obtained, however, by adding (S)a-trifluoro-methylbenzylchlorosulfite (IX) to chlorocarbonylbis (diethylphenylphos-... 

Industrial Applications. Several large scale industrial processes are based on some of the reactions listed above, and more are under development. Most notable among those currently in use is the already mentioned Wacker process for acetaldehyde production. Similarly, the production of vinyl acetate from ethylene and acetic acid has been commercialized. Major processes nearing commercialization are hydroformylations catalyzed by phosphine-cobalt or phosphine-rhodium complexes and the carbonylation of methanol to acetic acid catalyzed by (< 3P) 2RhCOCl. 

Metalloporphyrins have been used for epoxidation and hydroxylation [5.53] and a phosphine-rhodium complex for isomerization and hydrogenation [5.54]. Cytochrome P-450 model systems are represented by a porphyrin-bridged cyclophane [5.55a], macrobicyclic transition metal cyclidenes [5.55b] or /3-cyclodextrin-linked porphyrin complexes [5.55c] that may bind substrates and perform oxygenation reactions on them. A cyclodextrin connected to a coenzyme B12 unit forms a potential enzyme-coenzyme mimic [5.56]. Recognition directed, specific DNA cleavage... 

Figure I. Ligand exchange is a proposed key step in the mechanism of phosphine-rhodium complex catalyzed hydroformylation of olefins...

Figure 3. General mechanism for asymmetric catalytic hydrogenation using chelating phosphine rhodium complexes...

Hydrosilylation of various carbonyl compounds, enones and related functional groups catalyzed by Group VIII transition metal complexes, especially phosphine-rhodium complexes, have been extensively studied1,3, and the reactions continue to serve as useful methods in organic syntheses. 

Advances in Chemistry, Editors Alyea, E. C., and Meek, D. W., American Chemical Society, 1981, 196, 78, paper of "31p NMR Studies of Equilibria and Ligand Exchange in Triphenyl phosphine Rhodium Complex and Related Chelated Bis-Phosphine Rhodium Complex Hydroformylation Catalyst Systems," by Kastrup, R. V., Merola, J. S., and Oswald, A. A., in press. 

All complexes have shown high catalytic activity, even at room temperature (in contrast to platinum catalysts). Hydrosilylation in the presence of phosphine-rhodium complexes occurred in air, because real catalyst (active intermediate) was formed after oxygenation and/or dissociation of phosphine, as reported previously [14]. The non-phosphine complexes 1 and 4 are also very efficient catalysts for the hydrosilylation of allyl glycidyl ether. Irrespective of the starting precursor, a tetracoordinated Rh-H species, responsible for catalysis, is generated under reaction conditions, as illustrated in Scheme 3. 

A remarkable example of the cooperation of different active sites in a polyfunctional catalyst is the one-step synthesis of 2-ethylhexanol, including a combined hydroformylation, aldol condensation, and hydrogenation process [17]. The catalyst in this case is a carbonyl-phosphine-rhodium complex immobilized on to polystyrene carrying amino groups close to the metal center. Another multistep catalytic process is the cyclooligomerization of butadiene combined with a subsequent hydroformylation or hydrogenation step [24, 25] using a styrene polymer on to which a rhodium-phosphine and a nickel-phosphine complex are anchored (cf Section 3.1.5). 

Introduction Catalytic hydrogenation with soluble catalysts Hydrogenation with C2-Symmetrical ftis-Phosphine Rhodium Complexes C2 symmetric ligands (DIPAMP, DIOP, PNNP)... 

Hydrogenation with C2-Symmetrical bis-Phosphine Rhodium Complexes... 

Fig. 24-B-2. Catalytic cycle for the hydroformylation of alkenes involving triphenyl-—phosphine-rhodium-complex specics -Note that the-eoniigtiratieiis of the-complexes arc...

The hydrido phosphine rhodium complexes possess a relatively large negative charge on the hydrido ligand, and as such these compounds are not good catalysts for isomerization. Also, the rate of this reaction is influenced negatively by steric repulsion between phosphine molecules and the isoalkyl ligand in the intermediate compound [structure (13.59)]. 

ASYMMETRIC HYDROSILYLATION OF a,j3-UNSATURATED KETONES PhMeC=CHCOR WITH HSiMe R" CATALYZED BY CHIRAL PHOSPHINE-RHODIUM COMPLEXES [50]... 

In the preceding Sections it was described that chiral phosphine-rhodium complexes are effective in causing stereoselective addition of a hydrosilane to a variety of prochiral carbonyl compounds to give silyl ethers of the corresponding alkanols with fairly high enantiomeric bias at the carbon atom. The present section describes an application of the catalytic asymmetric hydrosilylation of ketones to the preparation of some new asymmetric bifunctional organosilanes. 

A considerable variation of optical yields on changing ketone structure should also be mentioned (Table 21) benzophenone gave appreciably higher optical yield than other ketones in the reaction with both XIII and XIV. It seems reasonable that the difference in bulkiness of the ketone, which is coordinated to the chiral phosphine-rhodium complex like solvent, must influence the stereoselectivity to some extent because of modifying the effective bulk of the rhodium complex. Then, benzophenone is of advantage under the given conditions to attain higher asymmetric induction over the less bulky ketones. 

An impressive application of rhodium is in asymmetric synthesis. A feix-phosphine rhodium complex using an asymmetric phosphine is used to prepare l-DOPA (di-hydroxyphenylalanine), a chiral compound used to treat Parkinson s disease, in high optical yield. Asymmetric synthesis is a very active and promising area of research. Rhodium has been used as a dimerization catalyst in... 

Another very useful alternative is to promote the deprotonation of the imida-zohum salt by the addition of external bases, usually under refluxing conditions [32]. Once the carbene is generated, it reacts with the metal complex to produce the corresponding metal-carbene complex. In general, polar aprotic solvents, for example, THF, are used, and the reaction is run at room temperature. Carbenes can also displace a phosphine ligand in the Wilkinson complex [33]. Other phosphine-rhodium complexes can be likewise employed [34]. 

Zhang, H. Hu, M. Cai, M. Reductive coupling of disulfides and diselenides with alkyl halides catalyzed by a silica-supported phosphine rhodium complex using hydrogen as a reducing agent. J. Chem. Res. 2013, 645-647. 

Notable is also the high selective hydrogenation of alkynes to alkenes achieved by a polysiloxane-bound (ether-phosphine) rhodium complex (Scheme 24-5) but not by the non-entrapped catalyst (Lindner, 1997). Likewise, remarkable is the fact that various entrapped alkene hydroformylation catalysis lead often to a much higher ratio of linear branched products than the homogeneous complex (see e.g., Lindner, 2000). 

In 1978, J. K. Stille and his group proposed an interesting extension of the concept of asymmetric synthesis via rhodium complexation by attaching the metallic site to an insoluble polymer (15). The main advantage of this modification is the possibility of recovering the optically active phosphine-rhodium complex catalyst. 

Achiwa, K. Asymmetric Hydrogenation with New Chiral Functionalized Bis-phosphine-Rhodium Complexes. J. Amer. Chem. Soc. 98, 8265 (1976). 

Fisher, C., and H. S. Mosher Asymmetric Homogenous Hydrogenation with Phosphine-Rhodium-Complexes Chiral Both at Phosphorus and Carbon. Tetrahedron Letters 1977,2487. 

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