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.
ASJC Scopus subject areas
- Drug Discovery
- Physical and Theoretical Chemistry
- Organic Chemistry
- Inorganic Chemistry