Chiral diphosphites

Several types of chiral diphosphite ligand have been synthesized and tested in Pt-catalyzed asymmetric hydroformylation.338-341... 

The first reports on asymmetric hydroformylation using diphosphite ligands revealed no asymmetric induction. In 1992, Takaya et al. published the results of the asymmetric hydroformylation of vinyl acetate (ee = 50%) with chiral diphosphites.358... 

Chiral diphosphites based on (2R,3R)-butane-2,3-diol, (2R,4R)-pentane-2,4-diol, (25, 5S)-hexane-2,5-diol, (lS -diphenylpropane-hS-diol, and tV-benzyltartarimide as chiral bridges have been used in the Rh-catalyzed asymmetric hydroformylation of styrene. Enantioselectivities up to 76%, at 50% conversion, have been obtained with stable hydridorhodium diphosphite catalysts. The solution structures of [RhH(L)(CO)2] complexes have been studied NMR and IR spectroscopic data revealed fluxional behavior. Depending on the structure of the bridge, the diphosphite adopts equatorial-equatorial or equatorial-axial coordination to the rhodium. The structure and the stability of the catalysts play a role in the asymmetric induction.218... 

A chiral diphosphite based on binaphthol, coordinated with rhodium (I) forming a nine-member ed ring, led to an efficient hydroformylation of vinylarenes, although moderate ees were obtained (up to 46%) at mild pressure and temperature reaction conditions.364 Chiral diphosphites and phosphinite-phosphites derived from spiro[4.4]nonane-l,6-diol were synthesized. Using these catalysts in the asymmetric hydroformylation of styrene, high regioselectivity (97%) and... 

After the discovery of the high ee provided by rhodium/diphosphite and rhodium/phosphine-phosphite complexes, with total conversion in aldehydes and high regioselectivities, rhodium systems became the catalysts of choice for asymmetric hydroformylation. Important breakthroughs in this area have been the use of rhodium systems with chiral diphosphites derived from... 

The first reports on asymmetric hydroformylation using diphosphite ligands revealed no asymmetric induction [71], In 1992, Takaya et al. published the results of the asymmetric hydroformylation of vinyl acetate (e.e.=50%) with chiral diphosphites [72], In 1992, an important breakthrough appeared in the patent literature when Babin and Whiteker at Union Carbide reported the asymmetric hydroformylation of various alkenes with e.e. s up to 90%, using bulky diphosphites derived from homochiral (2R,4R)-pentane-2, 4-diol (see Figure 8.20). Van Leeuwen et al. studied the influence of the bridge length, bulky substituents and cooperativity of chiral centres on the performance of the catalyst [73,74],... 

Optically active diols are useful building blocks for the synthesis of chiral diphosphite ligands. Chiral diphosphites based on commercially available optically active 1,2 and 1,4-diols, l,2 5,5-diisopropylidene-D-mannitol, L-a,a,a,a-tetramethyl-l,3-dioxalan-4,5-dimethanol and L-diethyl tartrate, were first used in the asymmetric hydroformylation of styrene [75],... 

Since the discovery and development of highly efficient Rh catalysts with chiral diphosphites and phosphine-phosphites in the 1990s, the enantioselectivity of asymmetric hydroformylation has reached the equivalent level to that of asymmetric hydrogenation for several substrates. Nevertheless, there still exist substrates that require even further development of more efficient chiral ligands, catalyst systems, and reaction conditions. Diastereoselective hydroformylation is expected to find many applications in the total synthesis of complex natural products as well as the syntheses of biologically active compounds of medicinal and agrochemical interests in the near future. Advances in asymmetric hydrocarboxylation has been much slower than that of asymmetric hydroformylation in spite of its high potential in the syntheses of fine chemicals. 

The asymmetric catalytic Pauson-Khand reaction met success in the late 1990s. Not only the conventional Co catalyst but also other metal complexes, such as Ti, Rh, and Ir, are applicable to the reaction. Asymmetric hydrocyanation of vinylar-enes is accomplished using Ni complex of chiral diphosphite. Further studies on the scope and limitation are expected. 

Chart 6.5 Chiral diphosphites forming exclusively ea, 14, or ee, 15, rhodium complexes. 

Ramies, O., Net, G., Ruiz, A. and Claver, C. (2000) Chiral diphosphites as ligands for the rhodium- and iridium-catalysed asymmetric hydrogenation Precatalyst complexes, intermediates and kinetics of the reaction. Eur. J. Inorg. Chem., 1287-1294. 

A modified 14 with perfluoralkylphenyl groups increased the selectivity up to 96%.280 Both high enantioselectivity (91%) and high regioselectivity (98.8%) were achieved under mild conditions with a new chiral diphosphite ligand derived from D-(+)-glucose.281... 

Asymmetric hydrogenation of alkenes is efficiently catalysed by rhodium complexes with chiral diphosphite and diphosphoramidite ligands derived from BINOL or diphenylprolinol. Choice of a proper achiral backbone is crucial.341 Highly enantioselective hydrogenation of A-protected indoles was successfully achieved by use of the rhodium catalyst generated in situ from [Rh(nbd)2]SbF6 (nbd = norborna-2,5-diene)... 

Until recently rhodium catalysts gave lower enantioselectivity, but higher chemoselectivity and activity, than the platinum-based catalysts. However, in the past few years rhodium complexes of a few chiral diphosphites and phos-phinophosphito ligands have been reported. These complexes have excellent activities and high chemo-, regio-, and enantioselectivities. 

As represented in Equation (122), a rhodium-catalyzed hydroformylation of 2,3- and 2,5-dihydrofuran using furanoside-derived chiral diphosphite ligands, for example, 237, provided 3-formyltetrahydrofuran as the major product with ee up to 75% <2005CC1221>. 

Effective control of chemo and steric selectivity of a chemical reaction lies in our ability to manipulate nano-environment of active sites. Due to difiGculty in manipulating the active site on the nanoscale and lack of fxmdamental nderstanding of the reaction mechanism, development of chemo- and stereoselective catalysts has relied heavily upon empirical studies. One successful xample of fine-tuning steric environment of the active site is the use of chiral diphosphite ligands to control the selectivity of styrene hydroformylation on Rh omplex catalysts (1-3). 

The Rh complex with the chiral diphosphite ligand 92 derived from (R,R)-pentane-2,4-diol has shown enantioselectivity up to 90% ee with 98% branched... 

A systematic study of a series of chiral diphosphites with C2 symmetry and various bridges between the two phosphorus atoms on the structure of the penta-coordinate rhodium complex [RhH(diphosphite)(CO)2] and the stereoselectivity in styrene hydroformylation revealed that the highest enantioselectivities are obtained with ligands coordinating in an equatorial-equatorial fashion to the rhodium (273). 

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