Chiral bidentate phosphorus ligands

P -P = chiral bidentate phosphorus ligand) all show the diene twisted in the same sense relative to the chirality at the... 

The Rh-catalyzed asymmetric hydrogenation of prochiral p-A-acet-ylamino-vinylphosphonates (145) allowed preparation of chiral p-A-acet-ylaminoethyl-phosphonates (146) with excellent yields (up to 100%) and high enantioselectivities (up to 92% ee) (Scheme 48). The reaction was strongly dependent on the structure of a chiral bidentate phosphorus ligand (PL ) and the solvent employed (THF, CH2CI2). In several cases an inversion of the induced chirality was observed by using the corresponding E- or Z-isomeric substrates. 

The monocyclopentadienyliron complexes with chiral bidentate phosphorus ligand such as 177 and 178 are suitable Lewis acid catalysts for Diels-Alder reaction of dienes with acryl aldehydes. The reaction proceeds under mild conditions with high yields and ees. In some cases the complex 178 gave better results than 177. Some representative examples of enantioselective Diels-Alder reaction of various acroleins with dienes are given in Table 35 [ 116]. Similar results can be achieved with the ruthenium complexes 179 [117]. 

Bidentate Phosphorus Ligands Supported on Atropisomeric Biaryl Backbones. Most ligands of this group contain binaphthyl or biphenyl moieties-providing an axial element of chirality. The flagship of this class of compounds is the atropisomeric diphosphane BINAP (36), whose complexes (primarily Ru(II) complexes (37)) play a pivotal role in asymmetric scale-up hydrogenations and isomerizations (Fig. 5). 

Fig. 6. Most widely used bidentate phosphorus ligands with planar and mixed planar-central chirality.

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. 

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. 

Scheme 1.4 Bidentate chiral phosphorus ligands usually used in catalysis. The four P-phenyl or alkyl substituents are useful NOE reporters.

Assemblies based on 8 and pyridine phosphorus ligands 5-7 were used as supramolecular ligands in the rhodium-catalyzed hydroformylation and showed typical bidentate behavior. The chelating bidentate assembly exhibited lower activities (a factor of three) than the monodentate analogue. Only a slightly higher selectivity for the linear aldehyde was observed. The chiral ligand assemblies based... 

The best results were obtained with the bisoxazole class of ligands such as 35, described by Kanemasa and coworkers [64—66] and for the derivative 36 reported by Corey and coworkers [67, 68] and Sibi et al. [69], who applied heterocyclic additives to the iron-catalyzed reaction. The bidentate sulfoxide ligand 37 introduced by Khiar et al. [70] and the phosphorus oxide ligand 38 reported by Imamoto s group [71] were less effective with respect to chiral induction. 

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.)... 

---