Chiral ligand phosphorus-based

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 ability to efficiently synthesize enantiomerically enriched materials is of key importance to the pharmaceutical, flavor and fragrance, animal health, agrochemicals, and functional materials industries [1]. An enantiomeric catalytic approach potentially offers a cost-effective and environmentally responsible solution, and the assessment of chiral technologies applied to date shows enantioselective hydrogenation to be one of the most industrially applicable [2]. This is not least due to the ability to systematically modify chiral ligands, within an appropriate catalyst system, to obtain the desired reactivity and selectivity. With respect to this, phosphorus(III)-based ligands have proven to be the most effective. 

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. 

Very recently, Reetz, Ma and Goddard reported phosphoramidites based on BINOL bearing a single ortho-substituent (Scheme 28.10) [69]. These ligands are also chiral on phosphorus, such that the synthesis results mostly in diastereo-mers which have to be separated. In several cases, however, one of the diaster-eomers was formed exclusively. Some of the ligands afford high ee-values in the hydrogenation of methyl N-acyl dehydroalanine and dimethyl itaconate. 

Alexakis, employing various chiral trivalent phosphorus ligands, has recently described Cu(OTf)2-catalyzed 1,4-additions of Et2Zn to a number of nitroalkenes (Scheme 7.22) [77]. TADDOL-based phosphonite 82 gave the highest ees for ar-ylnitroalkenes (up to 86%), whereas phosphoramidite 18 is the ligand of choice for alkylnitroalkenes [ees of up to 94%). 

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. 

Fumaric and maleic compounds are also suitable substrates in the rhodium-catalyzed asymmetric 1,4-addition reactions. While phosphorus-based chiral ligands, such as (/f)-binap, provide low enantioselectivity for these substrates (<51% ee), chiral diene ligands 38 and 45 are particularly effective for achieving high ee (Figures 3.39 and 3.40). 

In recent years we have developed a new type of planar-chiral ligand system which is based on a phosphaferrocene skeleton equipped with an additional donor function Y [8]. This structure is closely related to the well-known ferrocene-type ligands in that a CH unit of the latter has been replaced by a P atom. The phosphaferrocene moiety serves as both a chiral metallocene-type backbone and as a donor group via the phosphorus atom lone pair. These new ligands are unique in their topological architecture and show interesting ligand properties. 

Figure 8E.3. Some chiral ligands based on phosphorus.

Chiral cyclic phosphines have useful properties as ligands in transition metal asymmetric catalytic systems. The most impressive example is the five-membered ring phosphorus (phospholane)-based chiral ligand DuPFIOS <2000ACR363>. 

In this chapter, we review a growing family of modular phosphorus heterocycles that have been found broadly useful as chiral ligands in asymmetric catalysis.4 Specifically, the utility of catalysts based on 4-membered phosphetane and 5-membered phospholane ligands will be the focus herein. A description of other types of chiral phosphorus heterocycles can be found in recent review articles4 as well as in Chapters 12 and 15. 

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