Phosphine ligands amino phosphines

In the same context, Kang et al. have examined several bidentate thiol derivatives as ligands in the same reaction to that described above. A nickel complex derived from the p-amino thiol depicted in Scheme 2.31 catalysed the reaction in a useful asymmetric level of 74% ee, whereas the use of the thiol phosphine ligand depicted in Scheme 2.31 gave a lower enantioselectivity of 27% ee. In this study, the authors found that the enantioselectivity was strongly dependent on the ligand concentration and on the nickel-to-ligand ratio. 

Scheme 10.91 Grignard crosscoupling of 1-phenylethylmagnesium chloride and vinyl bromide with S-alkylated amino phosphine ligands.

Recent developments in chirality induced by ligands, and in efficient synthetic methods for ligands such as diphosphines, chiral diamines, amino-phosphine-phosphinites, phosphino-oxazolines and diimino-diphosphines, are available in the literature.105-107... 

Rh complexes with ChiraPhos, PyrPhos, or ferrocenyl phosphines lacking amino alkyl side chains (such as BPPFA) are much less active toward tetrasubstituted olefins. Table 6-1 shows that in asymmetric hydrogenations catalyzed by 5a-d, the coordinated Rh complex exerts high selectivity on various substrates. It is postulated that the terminal amino group in the ligand forms an ammonium carboxylate with the olefinic substrates and attracts the substrate to the coordination site of the catalyst to facilitate the hydrogenation. 

The phosphoranes (81),64 (82),65 and (83),68 each containing a hydrogen ligand, have been prepared by the now standard procedure from amino-phosphines and amino-alcohols. A similar reaction of the amino-alcohol (84) gave oxazaphosphol-idine derivatives (85).6 7... 

That the second option can also be successfully used has recently been revealed by our synthesis of 2-methoxyphenazine (117) [90]. The reduction of o-bromo-o -nitrodiphenylamine 132 accessible via intermolecular Pd-catalyzed JV-arylation provides the o-amino-o -bromodiphenylamine 133, which can then be cyclized to give 117 in a Pd-catalyzed intramolecular AT-arylation by employing Pd2(dba)3 as the Pd complex and 134 as the phosphine ligand. It should be noted that the outcome of both the intermolecular and the intramolecular AT-arylations heavily depends on the appropriate choice of the Pd complex as well as the phosphine. Ether cleavage leads to 2-hydroxyphenazine (9). 

It appears from these early studies that modest to rather high yields and enan-tioselectivities can be achieved with structurally very diverse chiral ligands. Furthermore, both relatively hard (amino alcohols) and soft (thiols, phosphines) ligands... 

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