Diphosphines rhodium complexes

The last example of a dendritic effect discussed in this chapter is the use of core-functionalized dendritic mono- and diphosphine rhodium complexes by Van Leeuwen el al. [45] Carbosilane dendrimers were functionalized in the core with Xantphos, bis(diphenylphosphino)ferrocene (dppf) and triphenylphosphine (Figures 4.22, 4.32 and 4.33). 

Monsanto (2) A catalytic process for synthesizing the drag L-DOPA. The catalyst is a chiral diphosphine-rhodium complex. Invented in the early 1970s. 

Fig. 16.8 Schematic of a diphosphine rhodium complex covalently tethered to a cross-linked styrene/divinylbenzene matrix, used for the hydrogenation of quinoline.

With the objective of increasing the stability of the catalysts, we considered the possibility of preparing diphosphine rhodium complexes supported on ROX-N (mainly by ion-exchange). Apart fi-om the stabilization effect of the diphosphines, this kind of ligands does not favour the isomerization reaction and the complexes are more selective to linear products [19]. 

The bis-diphosphine rhodium complex [Rh(DPPB)2]BF4 supported on activated carbon ROX-N (entry 6) shows lower activity (but better selectivity) in the st run and fester deactivation. 

Preliminary examples of catalytic enantioselective aldol additions using rhodium complexes with chiral phosphine ligands attached, e.g. (42), have been disclosed by Reetz and Vougioukas (equation 15) 32 although the low level of asymmetric induction so far obtained in the silylated aldol adduct (43) requires substantial enhancement before this reaction has any synthetic value. However, there is ample scope for future improvement by the use of more effective chiral diphosphine-rhodium complexes. 

Four generations of dendrimers beating (72-symmetrical diphosphine rhodium complexes have been prepared and used in asymmetric hydrogenation (Scheme Acid-functionalized analog 146 of the easily immobilized... 

The addition of prochiral substrates, such as esters of a-acetamidocin-namic acids, to a solution of the solvate of a chiral diphosphine rhodium complex allowed the observation of at least one of the two diastereomeric complexes. Equation 49. 

Rhodium-Diphosphine Catalysts. The mechanism of rhodium-catalyzed asymmetric hydrogenation is one of the most intensively investigated and best understood. Reaction pathways have been accurately studied both experimentally and theoretically (138,162,213-221). In early studies, Halpern (222) and Brown (214) established that the hydrogenation proceeds according to the reaction sequence presented in Figure 51 for the hydrogenation of a dehydroamino acid with a chiral diphosphine-rhodium complex. Many variants on both catalyst and reactant have been described. Stereoselectivity takes place via the difference in reactivity of the involved diastereomeric square-planar... 

Modified Rh systems show considerable activity in the hydroformylation of styrene to the branched aldehydes. Chiral diphosphine rhodium complexes that lead to high activity and high regioselectivity in branched aldehyde have been studied. These systems do not provide high enantioselectivity [2, 3]. Stanley reported that a tetraphosphine hgand can be used to form a bimetallic complex [8] and provide ee s up to 85% using vinyl esters. 

Glaser, R., and S. Geresh Structural Requirements in Chiral Diphosphine-Rhodium-Complexes. VII. Use of Z-Methyl-a-acylaminocinnamates as Structural Probes for DIOP-Rhodium(I) Complexes. Tetrahedron Letters 1977, 2527. 

Glaser, R., S. Geresh, and J. Blumenfeld Structural Requirements in Chiral Diphosphine-Rhodium Complexes. III. Small Scale Method For Fresh Preparation of Cationic DIOP-Rhodium Complexes and Comparison with Neutral DIOP-Rhodium Complexes. J. Organomet. Chem. 112, 355 (1976). 

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