RuCl2 -dpen

S)-Xylyl-BINAP-RuCl2-(S.S)-DPEN] [CTH-(R)-Xylyl-P-Phos-RuCl2 (/. / -DPEN]... 

S)-fixed-4/(S,S)-DPEN Figure 7-14. Stereomutation of bipheps-RuCl2/dpen complexes. 

The BINAP/l,2-diamine-RuCl2 complexes require the addition of alkaline base to form reactive RuH2 species for the hydrogenation of simple ketones to neutralize releasing HC1. trans-RuH( /1-BH4)(xylbinap)(dpen) (31G) produces the active species without an additional base, while an even higher rate is obtainable in the presence of an alkaline base (Fig. 32.31) [102]. The base-free procedure can be applied to the hydrogenation of several base-sensitive ketones. For example, the reaction of ethyl-4-acetylbenzoate catalyzed by (S,SS)-31G quantitatively yields ethyl (R)-4-(l-hydroxyethyl)benzoate in 99% ee [102]. No transesterification is observed. 

DM-BIPHEP, a conformationally flexible diphosphine, exists as an equilibrium mixture of the R and S isomers (Fig. 32.50) [141]. Addition of an equimolar amount of (S,S)-DPEN to RuCl2(dm-biphep)(dmf) forms a 3 1 mixture of (S)-DM-BIPHEP/(S,S)-DPEN-RuC12 complex and the R,SS diastereoisomer. The major S,SS complex gives a more active and enantioselective catalyst for the hydro-... 

The complex TolBINAP-Ru dichloride existing in aggregate form is incapable of catalyzing the hydrogenation reaction. In the presence of (S,S)-DPEN, however, a monomeric diphosphine/diamine complex is readily formed to facilitate catalytic hydrogenation. In other words, under the hydrogenation conditions, one of the precatalysts, RuCl2[(-R)-Tolbinap)(DMF)n], is selectively... 

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