Asymmetric transfer hydrogenation chiral amino alcohol ligand

The catalytic system of [Cp RhCl2]2 and (l/ ,25)-(+)-ciiS-l-amino-2-indanol as a chiral ligand achieved the enantioselective synthesis of a-D-benzylalcohols via asymmetric transfer hydrogenation with isopropyl alcohol as the hydrogen source (eq 27). The transformation proceeded smoothly to provide a-D-benzyl alcohols in high yield, albeit in moderate enantioselectivity. 

Norephedrine and its derivahves are efficient chiral ligands of the catalyst for asymmetric transfer hydrogenation. By using a 7 3 copolymer of a PEG ester and a hydroxyethyl ester, polymer-supported norephedrine 171 was prepared. The polymeric norephedrine/Ru(II) complex catalyzed the reduction of acetophenone in up to 95% yield and 81% ee (Scheme 3.52) [102]. Polymer-supported chiral amino alcohol 172 was also used for the same reaction [103]. 

Three classes of catalysts have been studied for the asymmetric hydrogenation of imines. One class of catalyst is generated from late transition metal precursors and bisphosphines. These catalysts have typically been generated from rhodium and iridium precursors. A second class of catalyst is based on the chiral titanocene and zirconocene systems presented in the previous section on the asymmetric hydrogenation of unfunctionalized olefins. The third class of catalyst is used for the transfer hydrogenation of imines and consists of ruthenium or rhodium complexes containing diamine, amino tosylamide, or amino alcohol ligands. " ... 

Other amino alcohols have also been used as chiral ligands in asymmetric catalytic hydrogen transfer. Scheme 6-54 depicts another example. Ruthenium complex bearing 2-azanorbornyl methanol was used as the chiral ligand, and the corresponding secondary alcohols were obtained in excellent ee.116... 

Asymmetric hydrogen transfer shows promise for use at industrial scale because ruthenium complexes that contain chiral vicinal diamino 164 or amino alcohol 165 ligands allow the reductions of substrates such as aryl ketones and imines to be achieved under mild conditions.13 207... 

The chiral ferrocenylselenium reagents were found to act as the effective ligands for Rh(I)-catalyzed asymmetric hydrosilylation and transfer hydrogenation (see Sects. 2.1 and 2.2, respectively). Fukuzawa and Tsudzuki have found that the chiral ferrocenylselenium-based amino alcohols (DASF), prepared by treatment of the chiral diferrocenyl diselenide with NaBH4 in ethanol followed by the addition of epoxides (Scheme 11), efficiently catalyzed the diethylzinc addition to aldehydes to provide the corresponding secondary alcohols with up to 99% ee... 

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