Chiral ligands bidentate phosphine

Finally, the participation of the Ar,P-bidentated phosphinous amides 61-65 (Scheme 45) as chiral ligands in the catalytic enantioselective hydrosilylation of acetophenone with diphenylsilane is worth mentioning, despite low ee (2-20%) [52]. 

It thus came as a surprise that in the year 2000, three groups independently reported the use of three new classes of monodentate ligands (Scheme 28.2) [12], The ligands induced remarkably high enantioselectivities, comparable to those obtained using the best bidentate phosphines, in the rhodium-catalyzed enantioselective alkene hydrogenation. All three being based on a BINOL backbone, and devoid of chirality on phosphorus, these monophosphonites [13], monophosphites [14] and monophosphoramidites [15] are very easy to prepare and are equipped with a variable alkyl, alkoxy, or amine functionality, respectively. 

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. 

The use of copper catalysts based on chiral phosphorus ligands to assist 1,4-additions of dialkylzinc reagents has in recent years produced major breakthroughs, with excellent enantioselectivities. A number of monodentate and bidentate phos-phoramidites, phosphites, phosphonites, and phosphines are now available as chiral ligands for alkyl transfer to a variety of cyclic and acyclic enones. So far. 

This NOE idea was then extended to Pd(ii) allyl complexes with bidentate phosphine auxiliaries [99-111], with the ortho P-phenyl protons acting as the reporters (see 81). Figure 1.17 shows a section of the H, H NOESY for [Pd(P-pinene allyl) (Chiraphos)](OTf) (Chiraphos = Ph2PCH(CH3)CH(CH3)PPh2), 81 [129], and reveals the numerous contacts from the chiral phenyl array to the allyl ligand. 

Styrene characteristically yields the branched acid in the presence of palladium and monodentate phosphine ligands,132 142 and in the [Fe(CO)5]-promoted process.143 Palladium with certain bidentate phosphines, in turn, produces more linear acid.142 Asymmetric hydrocarboxylations with palladium complexes and chiral ligands with enantiomeric excesses up to 84% have been reported.144 145... 

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