Monodentate phosphonites

As described for monodentate phosphonite ligands, monodentate phosphite ligands have also been used in a monodentate ligand combination approach. 

Recently, the first report was made on the ruthenium-catalyzed enantioselective hydrogenation of aryl-methyl ketones using monodentate phosphonites (Scheme 28.18). In particular, ligand 15f induced excellent ee-values. One very early report on rhodium-catalyzed hydrogenation of ketones using the monophosphine bmpp 1 f met with a low e.e. [95]. 

It has been established that usually two monodentate ligands (phosphorami-dites, phosphites or phosphonites) are present in their rhodium-based hydrogenation catalysts. This would allow the possibility of testing catalysts based on two different monodentate ligands. Initially, this does not seem very appealing, as the suspected outcome would be the formation of a mixture of the heterocatalyst and the two homocatalysts (Scheme 36.13). 

In the studies conducted by Reetz, rhodium catalysts based on mixtures of monodentate phosphites, monodentate phosphonites and combinations of the two were screened in the enantioselective hydrogenation of a- and /9-N-acetyl-de-hydroamino acid esters, enamides and dimethyl itaconate [40], and a number of the more striking positive results are listed in Table 36.3. An enhanced ee-value was found mostly with combinations of two phosphonites, or one phosphonite and one phosphite, in particular when one of the ligands carries a bulky substituent and the other a small one. 

Table 36.3 Use of mixtures of monodentate phosphonites and or phosphites in the rhodium-catalyzed hydrogenation of substituted olefins.3 ...

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. 

The coordination chemistry of the hydrido/phosphine rhenium complexes can be extended to phosphonite and phosphite complexes. This has been demonstrated for monodentate ligands such as P(OEt)3, P(OCH2)3CEt and PPhtORE (R = Me, Et, Pri),2S8779,809,iii8 chelating phos-... 

For examples of asymmetric catalyses with BlNOL-derived monodentate phosphites, phosphonites, and phosphoramidites, see a) M. T. Reetz, G. Mehler, Angew. Chem. 2000, 112, 4047 Angew. Chem. Int. Ed. 2000, 39, 3889 ... 

Considerable success has been realized for asymmetric hydrogenations of carbon-carbon unsaturation with monodentate ligands, especially those derived from BINOL. The most popular class is the phosphoramidites (157) as used by DSM.200 20 Phosphites (158) have also been successfully used in a wide range of asymmetric hydrogenations,202 as have phosphonites (159).203 204 These ligands are discussed in depth in Chapter 14. 

Asymmetric Olefin Hydrogenation Using Monodentate BINOL- and Bisphenol-Based Ligands Phosphonites, Phosphites, and Phosphoramidites... 

Whereas bidentate phosphites and phosphonites are excellent ligands for rhodium-catalyzed asymmetric hydrogenation, bidentate phosphoramidites (not shown) gave very poor results with low reaction rates and enantioselectivities. Results of a number of other monodentate phosphoramidites in the asymmetric hydrogenation of. V-acetyl dehydrophenylalanine derivatives are shown in Table 14.7.31-35-36-39-40... 

Phosphonites. - A wide variety of chiral monodentate phosphonite ligands derived from binaphthol and biphenanthrol, e.g., (28), has been obtained from the reactions of a dichlorophosphine with the corresponding biphenol, and used... 

This procedure has also been extended to the synthesis of phosphine-phosphonites (e.g., (185),412,413 Equation (46)), and very recently (2000) an elegant series of monodentate biarylpho-sphonites (186) and (187).414... 

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