1,4-Diphosphines chiral catalysts

A chiral diphosphine ligand was bound to silica via carbamate links and was used for enantioselective hydrogenation.178 The activity of the neutral catalyst decreased when the loading was increased. It clearly indicates the formation of catalytically inactive chlorine-bridged dimers. At the same time, the cationic diphosphine-Rh catalysts had no tendency to interact with each other (site isolation).179 New cross-linked chiral transition-metal-complexing polymers were used for the chemo- and enantioselective epoxidation of olefins.180... 

The development of chiral catalysts for use in enantioselective rhodium-catalyzed hydroborations was pioneered by Burgess9, Suzuki,77 and Hayashi.78 The chiral diphosphine ligands employed in their preliminary investigations 23-26 (Figures 2(a) and 2(b)), had previously been successfully applied in other catalytic asymmetric transformations. 

The mechanism of the enantioselective 1,4-addition of Grignard reagents to a,j3-unsaturated carbonyl compounds (Scheme 5 R1 = alkyl R2 = alkyl, OR3), promoted by copper complexes of chiral ferrocenyl diphosphines (180), has been explored using kinetic, spectroscopic, and electrochemical analysis. The roles of the solvent, copper halide, and the Grignard reagent have been thoroughly examined. Kinetic studies support a reductive elimination as the rate-limiting step, in which the chiral catalyst,... 

The catalyst is a cationic complex of rhodium with another diphosphine, DIPAMP. DIPAMP s chirality resides in the two stereogcnic phosphorus atoms unlike amines, phosphines are configurationally stable, rather like sulfoxides (which we will discuss in the next chapter). The catalyst imposes chirality on the hydrogenation by coordinating to both the amide group and the double bond of the substrate. Two diastereoisomeric complexes result, since the chiral catalyst can coordinate to either of the enantiotopic faces of the double bond. 

The general synthetic scheme outlined in Fig. 2.1 allows the facile preparation of a series of chiral catalysts using commercial chiral diphosphines as ligands. The use of complexes of the type [(P-P)Pt(C6F5)(H2O)]OTf represents the most versatile, active. 

Inspired by the seminal report of Nagel [34], which described a very active and selective Rh-pyrphos catalyst attached covalently to sihca gel, Pugin and colleagues have developed the modular toolbox which his depicted schematically in Figure 12.4. The main elements of their system are functionalized chiral diphosphines, where three different hnkers are based on isocyanate chemistry and various carriers [37, 45, 58]. This approach allows for a systematic and rapid access to a variety of immobilized chiral catalysts, with the possibility of adapting their catalytic and technical properties to specific needs. 

Genet, J.P, Pinel, C., Ratovelomanana-Vidala, V., MaUarL S., Pfister, X., Cano De Andrade, M.C., and Laffitte, J.A., Novel, general synthesis of the chiral catalysts diphosphine-ruthenium (II) diallyl complexes and a new practical in situ preparation of chiral ruthenium (II) catalysts. Tetrahedron Asymmetry, 5, 665, 1994. 

N-acyldehydrodipeptides were readily prepared either by the condensation of N -acyldehydro-a-amino acids with a-amino acid esters or by the reaction of the azlactones of dehydro-a-amino acid with a-amino acid esters (eq. 1). Asymmetric hydrogenation of the N-acyldehydrodipeptides thus obtained (eq. 2) was carried out by using rhodium complexes with a variety of chiral diphosphines such as -Br-Phenyl-CAPP (3), Ph-CAPP (3), (-)BPPM (4), (+)BPPM (4), (-)DIOP ( ), (+)DIOP ( ), diPAMP (6), Chiraphos (7), Prophos (S), BPPFA (9) and CBZ-Phe-PPM (Fig. 1)(10). The chiral catalysts were prepared in situ from chiral diphosphine ligand with [Rh(NBD)2l -CIO4 (NBD = norbomadiene). Typical results are summarized in Tables I-V. 

In this communication we report a study of the stereochemical features of this catalytic process and the use of chiral diphosphine modified catalysts for the enantioselective synthesis of certain naturally ocurring lactones through kinetic resolution of a racemic mixture of the starting ketones. 

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