Chiral bidentate phosphine

The use of chiral bidentate phosphines (such as BINAP) in enantioselective cross-coupling reactions has been described, though only modest stereoselectivity has been achieved.431... 

Phosphoramidites are probably the most versatile ligands in this series as in amidites the substituents at the nitrogen atom are in close proximity to the metal centre and also the substituents could carry chiral centres. In Figure 4.22 we have depicted the simplest derivative, named Monophos , which is highly efficient for asymmetric hydrogenation but for a variety of other reactions as well. The ligand is much easier to make than most, if not all, chiral bidentate phosphine ligands and surely commercial applications will appear. 

An almost isotactic material has been obtained in CH2CI2 with the palladium catalyst [Pd(Me)(NCMe)((R,S)-BINAPHOS)]BAr4 (Ar = 3,5-bis(trifluoromethyl)phe-nyl) containing the unsymmetrical chiral bidentate phosphine-phosphite (i, S)-BI-NAPHOS = [(R)-2-(diphenylphosphino)-(S)-l,l -binaphthalene-2,2 -diyl phosphite]... 

AsPh3, SbPh3).1388 A partial resolution has been achieved for complexes PtMe(P)(P—P) (P is a chiral phosphine PPhR R" P—P is a chiral bidentate phosphine). The resolution can be monitored by 31P NMR and ORD, and the configuration of the preferentially bound enantiomer obtained by X-ray crystallography.1389... 

Hamada, Y., Matsuura, F., Oku, M., Hatano, K., and Shioiri, T. Synthesis Mid application of new chiral bidentate phosphine, 2,7-di-tert-butyl-9,9-dimethyl-4,5-bis(methylphenylphosphino)xanthene, Tetrahedron Lett. 1997, 38, 8961-8964. 

Phosphines that contain another one or more phosphorus atoms are particularly useful as ligands for transition metals. This stems from the fact that they can form a stronger bond to the metal with both phosphorus atoms donating and hence placing substituents in a well-defined position. (This has been exploited very successfully in the case of chiral bidentate phosphines.)... 

QCfim] [various] [CgCdmHBFJ Rh(ndb)2 + chiral bidentate phosphines Enamides 1 bar, 20°C methanol, /PrOH or water as co-solvent. [40]... 

Subsequently, some commercially available chiral bidentate phosphine ligands... 

Molecular mechanics modeling of the asymmetric hydrogenation must begin with the mechanism of the reaction. When the prochiral olefin binds to the catalyst containing chiral bidentate phosphine, two possible diastereomers result one with the re face and one with the si face of the olefin coordinated to the metal (Fig. 3). Work in the Halpern and Brown laboratories has shown that the observed enantiomeric product cannot result from the diastereomer observed in solution (17-20). Thus, the minor diastereomer, which cannot be observed, must be responsible for the dominant chiral product. Any molecular mechanics model of the asymmetric hydrogenation reaction must explain how the minor diastereomer reacts faster than the major. 

The Pd(II)-catalyzed carbocyclization reactions of 1,6-enynes have generally been performed by Pd(OAc)2 or by combined use of Pd(0) species, [Pd2(dba)3] CHCI3, and a weak acid such as acetic acid or trifluoroacetic acid. However, these typical palladium catalysts were ineffective for the 1,6-enyne system (8) in the presence of a chiral bidentate phosphine ligand, such as BINAP, exhibit-... 

Many of the chiral bidentate phosphines synthesized in the last years have also been tested for enantioselective ketone reduction. Some of the results achieved are compiled in Table 2. The influence of phosphine structure on optical selectivity and catalytic activity is considerable, but a reliable correlation could not yet be found. It seems that chiral bidentate 6w(diphenyl)phosphines, like prophos forming 5-membered chelate rings with the rhodium atom and used with great success for the hydrogenation of dehydroaminoacids, are not suitable for ketone reduction because of very low reaction rates. 

Although free rotation of the acetylene in the active intermediate Ij (Fig. 9) is possible, the orientation and the steric bulk of the substituents on the chiral bidentate phosphine ligands may restrict the rotation. Thus, asymmetrical induction of the chiral phosphine ligands on the reactive intermediate Ij leads to a preference for forming one enantiomer of deltacyclene. Figure 12 shows the proposed asymmetrical complex with S,5-Chiraphos. The prediction of the absolute configuration of the product from this asymmetrical complex agrees with the experimental results. 

The use of OH nucleophiles for cyclization was first reported by Ban [9] and Negishi [10]. Subsequently, Shibasaki and coworkers demonstrated that desymme-trization of alkenyl halides could yield chiral a-methylene y and 6-lactones with chiral bidentate phosphine ligands (Scheme 2.3). The presence of silver salts was crucial to the observed enantioselectivity (up to 53%). It is postulated that without these salts, the bidentate ligand can only bind through one arm, thus leading to poor selectivity however, the use of silver salts generates a cationic palladium species which can offer two coordination sites to the ligand [11]. In some cases, over-oxidation of the 2° alcohol led to ketone formation. 

Wacker oxidation of olefins by palladium complexes involves water as the nucleophilic reagent. Depending on conditions, the oxidation of olefins by Pd(II) salts in the presence of water gives ketones or chlorohydrins. The enantioselective procedure leading to the latter involves chiral bidentate phosphines, either sulfonated or nonsulfonated (Scheme 83, L = sulfonated (f )-TolBINAP).f ... 

Scheme 15.23 Copper-catalysed as mmetric allylic allq lation with allg l boranes employing a chiral bidentate phosphine.

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