Rhodium catalysts phosphine-phosphite ligands

The results obtained for ligands 48 and 50, which contain only one fixed stereocentre, are interesting and very informative about the system. Ligand 48-(R,—), in which only the binaphthyl bridge has a predetermined absolute configuration R, leads to an e.e. of 83% (R-aldehyde), which is quite close to the value of 94% for (R,S)-BINAPHOS. This suggests that in the formation of the complex the binaphthyl bridge controls the conformation of the bisphenol 

Styrene 20 mmols in benzene. [Rh] = O.OlOmmol. L = 0.040 mmol. The e.e. s were determined by GLC analysis of the corresponding 2-arylpropionic acids derived by Jones oxidation of the products 

The spectroscopic data of phosphine-phosphite complexes show slight differences between the (R,S) and (S,R) complexes on the one hand and the (R,R) and (S,S) complexes on the other. In the latter compounds the data are less clear-cut, which could be due to structural deviations of the monohydride complexes from an ideal trigonal bipyramid or to equilibria between isomers. The two ea isomers cannot be distinguished by IR spectroscopy as their absorptions tend to coincide [82], 

31P NMR spectroscopy shows that the complex [RhH(CO)2(R,S-46)] is a single species at 60 °C and 25 °C in toluene-d8 under 1 bar of CO. No apical-equatorial interchange was observed at any temperature. The unique dissymmetric environment in a single catalytically active species created by the phosphine-phosphite seems to be an important factor in the high enantioselectivity obtained. 

9 Aqueous Phase Organometallic Catalysis-Concepts and Applications, eds. B. Comils and W. A. Herrmann, Wiley-VCH, Weinheim, 1998. 

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Based on the precedent of Van Leeuwen and Roobeek, livinghouse and co-workers screened a variety of electron-deficient phosphine/phosphite ligands for the rhodium-catalyzed [4-1-2] reaction, which provided an alternative catalyst system for the formation of 5,6- and 6,6-ring systems [13]. The most notable of these was the tris-(hexafluoro-2-propyl) phosphite-modified rhodium complex, which was applicable to both carbon- and oxygen-tethered substrates, and also provided the first example of a facial-directed diastereoselective intramolecular rhodium-catalyzed [4-i-2] reaction (Eq. 4). 

Figure 7 A rhodium complex of chiral phosphine-phosphite ligand (H,S)-BINAPHOS used as a catalyst for asymmetric hydroformylation.

The Nozaki groups showed that by employment of a chiral Rh catalyst based on a bidentate phosphine-phosphite ligand, 1,3-pentadiene can be selectively converted into the branched unsaturated aldehyde that was chiral (Scheme 4.13) [80]. The reaction proceeds via a symmetrical rhodium allyl complex and preferentially produces the iso-product. This is in strong contrast to the reaction with non-substituted 1,3-butadiene. In this regard, there is a possibility of running the reaction in a stereoselective manner (see Section 4.3.3.5) [80]. A similar steering effect has been observed with other 4-substituted 1,3-butadienes, such as vinyl cyclohexene, 3-methyl-1,3-butadiene, or 1-phenyl-l,3-butadiene [80, 81]. 

Related to this work, Seller and Geissler [109] advocated the use of bimetallic catalysts, one for the isomerization and the other for the hydroformylation. Indeed, with a catalytic system comprising a rhodium complex based on a chelating phosphine-phosphite ligand and Ru3(CO)j2 (0.1-0.5 mol%), almost a reversal of the regioselectivity in the reaction with -2-butene in comparison to the monometallic rhodium catalyst l/b = 42 58 TOP = 700 h ) was achieved. 

A series of chiral phosphine-phosphite ligands afforded complexes RhH(L-L)(CO)2 like 25, which showed catalytic activity toward asymmetric hydroformylation. Chiral diphosphite ligands derived from D-(+)-glucose were also coordinated to Rh(l), affording complexes rhodium-hydrido-carbonyl-diphosphite species of the type RhH(PP)(CO)2 (PP = diphosphite), which also proved to be effective asymmetric catalysts for the hydroformylation of vinyl arenes. ... 

The activity of rhodium complexes with phosphine or phosphite ligands is about three to four orders of magnitude higher than that of cobalt catalysts.21-23 [RhH(CO)(PPh3)3] preformed or prepared in situ has proved to be the active species when triphenylphosphine is used as ligand. Despite the high cost of rhodium, the mild reaction conditions and high selectivities make rhodium complexes the catalyst of choice in hydroformylation. 

The hydroformylation of olefins is one of the largest and most prominent industrial catalytic processes, producing millions of tons of aldehydes annually [102]. Initially, cobalt-carbonyl species were used as catalyst, though rhodium complexes modified by special ligands, usually phosphines, are predominantly used nowadays. Over the last two decades, continued development of new phosphine and phosphite ligands has allowed significant advances in hydroformylation chemistry, especially with respect to catalyst selectivity and stability [103]. 

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