Containing Neutral S-Donors

The use of phosphite-phosphoramidite ligands 168 a and b provided up to 98% ee in the hydrogenation of methyl (Z)-N-2-acelylarrii noci rinamale, but the activities were rather low when compared to 167 or to the corresponding diphosphine ligand [126]. 

In contrast to the extensive studies on phosphine-phosphites, the corresponding phosphine-phosphinites are rarely exploited. Laschat introduced this design with a bicydic chiral skeleton derived from (lS)-(+)-camphorsulfonic acid [127]. The Rh-complex based on dimesitylphosphinite 154b was found to be the most reactive catalyst, and was used to produce methyl N-2-acetamidodnnamate, with 89% ee. 

Zhang reported two new (S)-BINOL based ligands phosphine-phosphite (S,R)-o-BINAPHOS 163 and phosphine-phosphinite (S)-o-BIPNITE 164 [128]. Applications of these ligands in the Rh-catalyzed hydrogenation of methyl N-2-acetamido-cinnamate and methyl N-2-acetamidoacrylate induced very high enantioselectiv-ities ( 99% ee), and with a wide range of substrates. 

Uemura developed a water-soluble phosphine-phosphinite ligand (derived from a,a-trehalose) (166) for the Rh-catalyzed hydrogenation of enamide derivatives this induced only moderate enantioselectivity [129]. 

A number of dithioethers 169-173 (Fig. 27.16) based on the chiral skeleton of some well-known phosphines such as DIOP, Deguphos and BINAP, have been reported. The use of 1,4-dithioether ligands which lack contiguous chiral centers such as (+)-DiopsR2 169 [130], BINASR2 172 [131] and 173 [132] in the Ir- or 

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