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Synthetic Modifiers for Platinum in the Enantioselective Hydrogenation of Ketopantolactone: A Test for the Mechanistic Models of Ketone Hydrogenation
Author(s) -
Orglmeister Elisabeth,
Mallat Tamas,
Baiker Alfons
Publication year - 2005
Publication title -
advanced synthesis and catalysis
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.541
H-Index - 155
eISSN - 1615-4169
pISSN - 1615-4150
DOI - 10.1002/adsc.200404152
Subject(s) - chemistry , enantioselective synthesis , protonation , ethylamine , amine gas treating , ketone , cinchonine , medicinal chemistry , diastereomer , noyori asymmetric hydrogenation , stereochemistry , organic chemistry , catalysis , ion
Various derivatives of ( R )‐1‐(1‐naphthyl)ethylamine have been synthesized and tested as chiral modifiers of Pt/alumina in the enantioselective hydrogenation of ketopantolactone. The best modifiers (ee up to 79%) possess an ester function in the α‐position to the amino group. The modifiers performed far better in AcOH than in toluene, indicating that protonation of the N atom is important in enantioselection. The striking non‐linear behaviour of modifier mixtures with cinchonine indicates that the alkaloid adsorbs much stronger on Pt than the naphthylethylamine derivatives. Two mechanistic models are proposed for interpretation of the results, involving an NHO or N + HO bond between the amine‐type modifier and the keto carbonyl O atom of ketopantolactone, in apolar and protic media, respectively. In both cases the H atom originates from the modifier and not from the substrate (“half‐hydrogenated‐state”). The higher ee achieved in acidic medium is attributed to the better proton donor ability of the protonated amine modifiers. The models are applicable also to the hydrogenation of ethyl pyruvate.