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Homogeneous and heterogeneous asymmetric reactions. Part 5 . Diastereoselective and enantioselective hydrogenation of cyclic β‐keto esters on modified Raney ‐nickel catalysts
Author(s) -
Wittmann Gyula,
Göndös György,
Bartók Mihály
Publication year - 1990
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/hlca.19900730312
Subject(s) - chemistry , raney nickel , enantioselective synthesis , asymmetric hydrogenation , catalysis , enantiomer , diastereomer , tartaric acid , enol , homogeneous , medicinal chemistry , ethylamine , organic chemistry , stereochemistry , physics , citric acid , thermodynamics
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 )‐C 4 H 6 O 6 / Raney ‐Ni and ( R , R )‐C 4 H 6 O 6 /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 (1 S ,2 R )‐enantiomers of the cis ‐isomers of 2 and 4 are formed in larger quantity, whereas the (l R ,2 R )‐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.