Phosphoramidites bidentate

A range of bis(tetrachlorocatecholato)spirophosphoranides 91 and 92 have been prepared from diones <2002EJ03580>. Careful fine-tuning of the polarity of the solvent mixture was required for the precipitation of the dimethylammonium salts of the spirophosphoranides. A Cj-symmetric spirophosphoranide 93 with three different bidentate ligands was also prepared. The synthesis required the addition of the dione to a phosphoramidite generated... 

Phosphoramidites are probably the most versatile ligands in this series as in amidites the substituents at the nitrogen atom are in close proximity to the metal centre and also the substituents could carry chiral centres. In Figure 4.22 we have depicted the simplest derivative, named Monophos , which is highly efficient for asymmetric hydrogenation but for a variety of other reactions as well. The ligand is much easier to make than most, if not all, chiral bidentate phosphine ligands and surely commercial applications will appear. 

The presence of two ligands in the active catalyst is proposed on the basis of the optimum ligand-to-copper ratio of 2 and the nearly identical selectivities of monodentate and bidentate phosphoramidites in the 1,4-addition of Et2Zn to 2-cydohexenone [45]. 

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. 

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... 

Finally, a chiral N linked C2 symmetric bidentate phosphoramidite was developed by Miyaura for the arylation of N sulfonyl imines [115]. Over 38 different chiral N tosyl amines were prepared in high yields and enantiomeric excesses using this novel ligand. These conditions were also quite effective (72 99% yield and 93 98% ee) for the arylation of N p nitrobenzenesulfonyl imines, which can be cleaved under mild conditions to liberate the free amine. 

Scheme 9.11 Rh catalyzed asymmetric hydrogenation of ortho substituted arylenamides with a triphosphorus bidentate phosphine phosphoramidite ligand 49.

Although the polymerized or linked monodentate phosphoramidites are no longer monodentate phosphorus ligands, the nature of their coordination to the central metal of the catalysts resembles to that of the monodentated phosphoramidites. The catalysts are not chelate complexes like those with bidentate ligands. So it is suitable to discuss the polymerized or linked phosphoramidites as monodentate phosphorus ligands. 

Hydroxamic acids and the cyclic phosphoramidite (64) in the presence of tri-ethylamine gave the salts (65). The first six-co-ordinate phosphate anions (66) containing three different bidentate oxygen ligands were obtained as shown. ... 

The intramolecular asymmetric Heck reaction has featured in the synthesis of complex heterocyclic compounds. Bidentate ligands, such as diphosphines (especially BINAP) and phosphine—oxazolines, have been used as a chiral ligand of the asymmetric Heck reaction. Imbos et al. demonstrated that the mono-dentate phosphoramidite 513 was an effective ligand for the asymmetric Heck reaction of the prochiral cyclohexadienone 511 (Scheme 153).227 The reaction of 511 in the presence of catalytic amounts of Pd-(OAc)2 and the chiral phosphoramidite 513 gave the 4a-methoxy-4a//-benzo[c]chromen-2(6E/)-one 512 in 71% yield with 96% ee. By contrast, the reaction of... 

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