Chiral monodentate phosphorus ligands

A mechanistic study of rhodium catalyzed olefin hydrogenation using monodentate 

The application of a mixture of a chiral and an achiral monophosphorus ligand for the rhodium catalyzed asymmetric hydrogenation of enamides was tested by Beller and coworkers [45]. By using a mixture of a chiral monophosphine 36a and an achiral ligand tris(4 methoxyphenyl)phosphine [P(4 MeOCf,H4)3] (1 1), the N (1 phenylvi nyl)acetamide (7a) was hydrogenated to amine 8a with 88% ee, but this enantios electivity is inferior to that obtained with single monophosphine 36a (93% ee). 

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For a perspective on the use of chiral monodentate phosphorus ligands in enantioselective olefin hydrogenations see ... 

Breakthroughs that took place around the year 2000 have shown, in contrast to the common view, that indeed chiral monodentate phosphorus ligands can also lead to high enantioselectivities in a number of asymmetric hydrogenations. In the years following, monophosphines, monophos-phonites, monophosphoramidites, and monophosphites have been successfully used in the enantioselective hydrogenation of a-dehydroamino acids and itaconic acid derivatives [25],... 

Recent review about monodentate ligands, see (a) Jerphagnon, T., Renaud, J.-L. and Brrmeau, C. (2004) Chiral monodentate phosphorus ligands for rhodium-catalyzed asymmetric hydrogenation. Tetrahedron Asymmetry, 15, 2101-2111, and references cited therein, (b) de Vries, J.G. (2005)... 

For a historical review on the use of chiral monodentate phosphorus ligands in enantioselectieve olefin hydrogenations, see Komarov, I. V., Borner, A. Angew. Chem., Int. Ed 2001, 40, 1197. 

Chiral Monodentate Phosphorus Ligands (Monophosphanes and Phospho-ramidites). The initial attempts to convert Wilkinson s catalyst into a chiral system involved incorporating chiral unidentate phosphanes (Fig. 2). Historically, (S)-PMePh(nPr) (MPNPP) (5) and (S)-PMePh(iPr) (MPIPP) (6) (Horner phosphanes) were the first chiral phosphorus ligands used in enantioselec-tive catalysis. In 1968, the two independent work of Horner and Knowles and... 

Jerphagnon T, Renaud J-L, Bmneau C. Chiral monodentate phosphorus ligands for rhodium-catalyzed asymmetric hydrogenation. Tetrahedron Asymm. 2004 15(14) 2101-2111. 

After being neglected for about 30 years, monodentate phosphorus ligands [47] have recenhy begun to athact increasing amounts of attention. Chiral monoden-... 

Monodentate phosphites are another type of prominent monodentate phosphorus ligands applied in asymmetric hydrogenation of enamides for the synthesis of chiral amines. Chiral monodentate phosphites can be easily prepared from a chiral diol and an alcohol. Generally, the chiral diol was first reacted with a phosphorus trichloride to form a phosphorochloridite, followed by the reaction with an appropriate alcohol to yield a chiral monodentate phosphite [35[. The reaction of an alcohol with phos phorus trichoride to yield a phosphorodichloridite, which was then treated with a chiral diol, is also a good procedure for the synthesis of chiral monodentate phosphites [36]. 

Xie J-H, Zhou Q-L. Chiral diphosphine and monodentate phosphorus ligands on a spiro scaffold for transition-metal-catalyzed asymmetric reactions. Acc. Chem. Res. 2008 41(5) 581-593. 

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. 

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... 

Knowles next reported optical yields of up to 90% in the hydrogenation of further a-acetamidocinnamic acid derivatives, still using monodentate phosphines chiral at phosphorus. Again the catalysts were prepared in situ from the ligand and [RhCl(l,5-hexadiene)2] or a related complex. The best optical yield of 90% was obtained using cyclohexyl(o-anisyl)methylphosphine (CAMP).224... 

Chiral Bidentate and Multidentate Phosphorus Ligands (Diphosphanes and Polyphosphanes, Diphosphites, Diphosphinites, and Diphosphonites). After testing a number of monodentate phosphanes, a real breakthrough came in the area of enantioselective hydrogenation when various types of chiral diphosphanes were applied. The first two diphosphanes employed were DIPAMP and DIOP (see later) introduced by Knowles and Kagan, respectively. Their achievements stimulated research on a variety of bidentate chiral diphosphanes. 

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