Synthetic Modifiers for Platinum in the Enantioselective Hydrogenation of Ketopantolactone: A Test for the Mechanistic Models of Ketone Hydrogenation

Abstract

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 a-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 anbetween 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-hydrogenatedstate"). 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.

Nonlinear Effects in Asymmetric Catalysis

“…Baiker and co-workers [229,230] also employed 267 and some derivatives such as 268 in the enantioselective hydrogenation of 260 in the presence of a Pt catalyst [Eq. (85)].…”

Relative Abundances of Surface Diastereomeric Complexes Formed by Two Chiral Modifiers That Differ by a Methyl Group

“…19−23 In a particularly comprehensive study, Baiker and co-workers compared the hydrogenation of ketopantolactone on Pt modified by 22 secondary amine derivatives of the synthetic modifier (R)-1-(1-naphthyl)ethylamine, (R)-NEA. 20 They found that while (R)-NEA induced an enantiomeric excess (ee) of 52%, 17 of the derivatives produced significantly lower values (ee ≤ 40%) and only two yielded values above 60%. The spread of the observed ee values underscores the challenge of relating enantioselectivity to specific structural factors.…”

“…In recognition of the initial report on cinchona alkaloid-modified Pt catalysts, the general method is referred to as the Orito reaction . In mechanistic studies of the Orito reaction, particular attention has been paid to the structural requirements of the chiral modifier and to the intermolecular interactions leading to stereodirection. − For example, there have been many investigations of the effects of introducing structural variations in chiral modifiers. − In a particularly comprehensive study, Baiker and co-workers compared the hydrogenation of ketopantolactone on Pt modified by 22 secondary amine derivatives of the synthetic modifier ( R )-1-(1-naphthyl)­ethylamine, ( R )-NEA . They found that while ( R )-NEA induced an enantiomeric excess (ee) of 52%, 17 of the derivatives produced significantly lower values (ee ≤ 40%) and only two yielded values above 60%.…”

STM Study of Ketopantolactone/(R)-1-(1-Naphthyl)ethylamine Complexes on Pt(111): Comparison of Prochiral and Enantiomeric Ratios and Examination of the Contribution of CH···OC Bonding

“…Herein, we present a comprehensive STM investigation of the formation of KPL/( R )-NEA diastereomeric complexes on Pt(111). The principal objective of the experiment is to compare STM-determined prochiral ratio (pr) values to enantiomeric ratios (er) measured under reaction conditions and reported in the literature. , We define the pr as the ratio of pro- R to pro- S complexation states of KPL in KPL/( R )-NEA/Pt­(111) complexes, measured by visually inspecting large sets of STM data. The STM experiments were carried out at various temperatures from 237 to 250 K, where in each case the surface was first exposed to ( R )-NEA and KPL, in sequence, at room temperature.…”