Hydrogenation diastereoselective reactions, chiral

BINAP-Ru is effective for the diastereoselective hydrogenation of some chiral yS-keto esters (Fig. 32.13). Reaction of N-Boc-protected (S)-y-amino / -keto esters 13A catalyzed by the (R)-BINAP-Ru complex results in the syn alcohols 13B exclusively [52]. The stereocenter at the / -position is controlled by the chirality of the catalyst therefore, use of the S catalyst affords the anti isomer, as predicted. Derivatives of statine, a key component of the aspartic proteinase inhibitor pep-... 

Diastereoselective transfer hydrogenation of the chiral ketone (S)-A with (R,/ )-43 in 2-propanol gives (3S.4S)-B in >97% yield (Scheme 1.87) [327). Reaction with (S,5)-43 affords the 3R.4S alcohol predominantly. The degree and sense of diastereoface differentiation are mostly controlled by the chirality of the Ru catalyst. 

Figure 32.22 shows the diastereoselective hydrogenation of (R)-/ -keto sulfoxides with Meo-BIPHEP-Ru catalysts [72]. The R chiral center of the substrate matches with the S catalyst, giving the S,R alcohols in >99 1 selectivity, whereas reactions with the R catalyst affords a 6 94 to 10 90 mixture of the S,R and R,R diastereomeric alcohols. The diastereoselection is controlled mainly by the configuration of the catalyst. 

Chiral amines.1 These reagents also add to imines and this reaction can be used for synthesis of optically active amines. Thus RCeCl2 adds to SAMP-hydra-zones (12, 30) to form hydrazines in good yield and high diastereoselectivity. These are reduced to optically active amines by hydrogenation catalyzed by Raney nickel. The hydrazines are prone to oxidation, but can be isolated as the stable carbamates. 

The chiral auxiliaries anchored to the substrate, which is subjected to diastereoselective catalysis, is another factor that can control these reactions. These chiral auxiliaries should be easily removed after reduction without damaging the hydrogenated substrate. A representative example in this sense is given by Gallezot and coworkers [268], They used (-)mentoxyacetic acid and various (S)-proline derivates as chiral auxiliaries for the reduction of o-cresol and o-toluic acid on Rh/C. A successful use of proline derivates in asymmetric catalysis has also been reported by Harada and coworkers [269,270], The nature of the solvent only has a slight influence on the d.e. [271],... 

Hydrogen atom transfer implies the transfer of hydrogen atoms from the chain carrier, which is the stereo-determining step in enantioselective hydrogen atom transfer reactions. These reactions are often employed as a functional group interconversion step in the synthesis of many natural products wherein an alkyl iodide or alkyl bromide is converted into an alkane, which, in simple terms, is defined as reduction [ 19,20 ]. Most of these reactions can be classified as diastereoselective in that the selectivity arises from the substrate. Enantioselective H-atom transfer reactions can be performed in two distinct ways (1) by H-atom transfer from an achiral reductant to a radical complexed to a chiral source or alternatively (2) by H-atom transfer from a chiral reductant to a radical. 

---