Ligands BINOL-based phosphoramidit

The hydrogenation of methyl-Z-3-acetamido-2-butenoate resulted in more surprises. Although ligand D7 based on a primary amine was known to give good results with these substrates [37], the library shows that in general all BINOL-based phosphoramidites that contain a primary amine with branching in the a-... 

Platinum catalysts are also effective for the silaboration of 1,3-dienes.234 Although almost no stereoselectivitiy is observed in the silaboration of acyclic 1,3-dienes, 1,3-cyclohexadiene undergoes the stereoselective silaboration in fair yields (Equation (87)). Enantioselective silaboration of 1,3-cyclohexadiene has been achieved with 70% ee by using a platinum catalyst bearing a binol-based optically active phosphoramidite ligand.235... 

A real breakthrough of the application of monodentate ligands has taken place recently. A family of BINOL-based monodentate phosphoramidites (11) is synthesized and is used in various homogeneous catal3d ic reactions. The amine moiety of the phosphoramidate was varied from 2-methyl-benzylamine, possessing another chiral center, to simple secondary amines such as piperidine and morpholine (12). 

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. 

Phosphoramidite ligands based on TADDOL (36) and on D-mannitol (37) [74] have also been used (Scheme 28.11). However, the enantioselectivities reported for the hydrogenation of a-dehydroamino acids and itaconates were generally lower compared to the ligands based on BINOL. A different strategy is the use of ligands 38a-g based on the achiral diol catechol, and chiral amines [75]. 

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. 

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

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. 

The MonoPhos family of ligands for the reduction of C-C double bonds, including enamides, is based on the 2,2-bisnaphthol (BINOL) backbone. These phosphoramidite ligands are comparatively inexpensive to prepare compared to bisphosphine ligands. For enamide reductions with MonoPhos (20) as ligand, it was found that the reaction is strongly solvent dependent. Very good enantioselectivities were obtained in nonprotic solvents. - ... 

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