Monodentate phosphoramidite ligands

The enantioselective conjugate addition of dialkylzinc to nitroalkenes using other phosphoramidite,79,79a 83a sulfonamide,84 and binaphthol-based thioether ligands65 has also been studied in the past few years. Particularly noteworthy are the efficient chiral monodentate phosphoramidite ligands (S,R,R)-29 and (A,A)-55 developed by Feringa et al. and Alexakis et al., respectively, for this reaction. (S,R,R)-29 provided excellent enantioselectivities (up to 98% ee) for acyclic nitroalkenes (Scheme 25).80 It also worked well for other nitroolefin substrates such as 3-nitrocoumarin 7068 and methyl 3-nitropropenoate 7185. 

Rhodium-Catalyzed Hydrogenation of Aromatic Enamides Using Monodentate Phosphoramidite Ligands... 

Recently, Reek et al. published the synthesis of a 9H,9 H- [4,4 ]bicarbazole-3,3r-diol (BICOL)-based chiral monodentate phosphoramidite ligand, which was functionalized with two different third-generation carbosilane dendritic wedges (Fig. 26) [57]. As reference reaction in the catalytic study, the rhodium-catalyzed asymmetric hydrogenation of Z-methyl-a-acetamido-cinnamate was chosen. Using a ligand-to-rhodium ratio of 2.2 led to enantio-selectivities which were comparable to the results obtained using the parent BINOL-derived monodentate phosphoramidite MonoPhos. 

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. 

Reek et al.73 developed a new bicarbazolediol-74 (BICOL)-based, chiral monodentate phosphoramidite ligand, in which the Ai-sites in the bicarbazole skeleton permitted the easy introduction of metal centers. As a model reaction, the Rh-catalyzed asymmetric hydrogenation of methyl 2-acetamidocinnamate was evaluated. Using a ligand to rhodium ratio of 2.2, the enantioselectivity induced by the rhodium complex (Figure 10.7) was 93% at full conversion,... 

Hydrogenation Easily prepared monodentate phosphoramidite ligands containing a BINOL residue are employed in conjunction with (cod)2RhBp4 in asymmetric hydrogenation of dehydroamino acid derivatives and a-substituted acrylic esters (ee > 99% reachable). 

Highly diastereo- and enantio-selective catalytic capture of chiral zinc enolates using nitroolefins as electrophiles has been described. The tandem y-nitro ketone products were obtained in the presence of a monodentate phosphoramidite ligand, in good yields with high diastereo- and enantio-selectivities (up to 96% ee). 

A selection of monodentate phosphite and phosphoramidite ligands used for asymmetric hydrogenation. 

Figure 2.39 Chiral monodentate phosphoramidite ligands with chiral amine residues.

Feringa prepared water-soluble analogs of his monodentate phosphoramidite ligands which served well in hydrogenation of dehydroamino acids. Thus the attachment of a PEG residue to biscarbazole-derived phosphoramidite gave ligands whose Rh(I) complexes reduce acetamidoacrylate in water with ee up to 82%. Addition of surfactants increased the rate significantly, with a moderate improvement in enantioselectivity. 

The asymmetric hydrosilylation was studied for styrene and substituted styrene derivatives. The use of 1 associated with a monodentate phosphane ligands afforded, after oxidative cleavage, the corresponding alcohol with various enantiomeric excesses (eq 91)P The best results were obtained with (5)-H-MOP and a phosphoramidite ligand. 

Palladium-catalyzed diboration can be achieved with bis(pinacolato)diboron for strained 1,2-disubstituted alkynes. ° Analogously, palladium-catalyzed addition reactions of diboron compounds provide 1,2-bisborylalkanes from l-alkenes ° or cis-l,4-bisboryl-2-alkenes from 1,3-dienes.2,3-Bisboryl-l-propenes are accessible from allenes (Scheme 5-174). The asymmetric allene diboration was examined by Burks et ai, who showed that monodentate phosphoramidite ligands gave excellent enantioselectivities (up to 97% ee) (Scheme 5-174). The (asymmetric) metal-catalyzed reactions of diboron, silylboron and disilane compounds have been reviewed.Recently, the addition across two triple bonds to give dienes has been explored by Singidi et... 

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