Bidentate phosphite-phosphoramidite ligand

Figure 2.38 Chiral bidentate phosphite-phosphoramidite ligands based on carbohydrates.

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... 

Only recently did chiral phosphoramidites ligands come into the focus of asymmetric hydroformylation. Here only some typical examples will be discussed for the main part of stereoselective hydroformylation compare Section 4.3. In some cases, they could be used to replace phosphites. Reetz claimed several monoden-tate and bidentate phosphoramidite ligands for this purpose, but unfortunately no example was given [21]. 

The group of Alexakis subsequently reported a study on the use of bidentate furanoside phosphite-phosphoramidite and diphosphoramidite ligands for the Cu-catalysed AAA of cinnamyl-type substrates [29]. A range of different cinnamyl... 

Bidentate phosphorus ligands based on BINOL, such as phosphonite 23, phosphites 24 and 25, and phosphoramidite 26 (Tab. 7.2), with various bridging units were introduced by the groups of Reetz, Chan, and Waldmann [48-50]. Excellent enantioselectivities - up to 96% for ligand 23, for instance - were found. 

Besides phosphines and phosphites preferentially used as ligands in hydro-formylation, phosphoramidites have also been proposed. Phosphoramidites (sometimes also called phosphoroamidites) are a class of organic phosphorus compounds derived from phosphites in which the P-OR groups have been replaced by P-NR2 groups (Figure 2.31). Three different P-substituents produce a chiral (stereogenic) phosphorus atom. Phosphoramidites play a crucial role in the synthesis of nucleic acids [1] and have also been frequently considered as mono- or bidentate ligands in transition-metal catalysis [2]. 

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