The hydrogenations of methyl 2‐oxoeyclopentanecarboxylate (1), ethyl 2‐oxocyelohexanecarboxylate (3), and 2‐methylcyclohexanone (5) on unmodified Raney‐Ni catalyst lead predominately to the formation of the cis‐hydroxy diastereoisomers of 2, 4, and 6, respectively (Scheme 2). In the asymmetric hydrogenations on catalysts modified with chiral tartaric acid ((R, R )‐C4H6O6/Raney‐Ni and (R, R)‐C4H6O6/NaBr/Raney‐Ni), the predominance of the cis‐isomer increases significantly. The hydrogenations of β‐keto esters 1 and 3 proceed with an enantioselectivity of 10–15% on the modified catalysts, while the similar hydrogenation of 5 yields optically inactive 6. The (1S,2R)‐enantiomers of the cis‐isomers of 2 and 4 are formed in larger quantity, whereas the (lR,2R)‐enantiomers of the corresponding trans‐isomers predominate (Scheme 1). The enantioselective formation of trans‐2 and trans‐4 can be interpreted mainly in terms of the asymmetric hydrogenation of cyclic β‐keto esters through the keto form, while that of the corresponding cis‐hydroxy esters proceeds through the enol form.
134ChemInform Abstract The asymmetric hydrogenation of the prochiral double bond in α-phenylcinnamic acid derivatives such as (I) on a modified Raney Ni catalyst is investigated. High enantioselectivity is observed in the conversion of compound (Ia) to (IIa).
081ChemInform Abstract Hydrogenation of the ketones (I) with an unmodified Raney-Ni catalyst leads to the cis-and trans-alcohols (II) and (III) with only low diastereoselectivities. In the presence of (R,R)-tartaric acid (RRTA), high diastereoselectivities and 10-15% enantioselectivity are observed for the ester ketones (Ia) and (Ib), while no change in diastereoselectivity and no enantioselectivity are observed for (Ic). The ester group in the β-position to the carbonyl group has a definite role in the formation of a H-bond between the substrate and the adsorbed tartaric acid, and hence in the enantioselectivity.
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