Unexpected inversion of enantioselectivity during the hydrogenation of ethyl pyruvate using hydroquinine and hydroquinid
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Catalysis Letters Vol. 110, Nos. 1–2, August 2006 (Ó 2006) DOI: 10.1007/s10562-006-0096-9
Unexpected inversion of enantioselectivity during the hydrogenation of ethyl pyruvate using hydroquinine and hydroquinidine modified Pt/ Al2O3 Robert L. Jenkins,a Nicholas Dummer,a Xiabao Li,a Salem M. Bawaked,a Paul McMorn,a Richard P. K. Wells,a Andrew Burrows,b Christopher J. Kiely,b and Graham J. Hutchingsa,* a Department of Chemistry, Cardiff University, Cardiff CF10 3AT, UK Center for Advanced Materials and Nanotechnology, Lehigh University, Bethlehem, PA 18015-3195, USA
b
Received 28 March 2006; accepted 11 April 2006
In the enantioselective hydrogenation of ethyl pyruvate using hydroquinidine 4-chlorobenzoate modified Pt/c-Al2O3 catalyst, the sense of the enantioselectivity is a function of the modifier concentration. At low concentration (S)-ethyl lactate is preferred and at higher concentration (R)-ethyl lactate is formed; the opposite trend is observed with hydroquinine 4-chlorobenzoate. This is the first example where enantio-inversion is induced solely as a function of the chiral modifier concentration. KEY WORDS: enantioselective hydrogenation; enantio-inversion; ethyl pyruvate hydrogenation.
1. Introduction Heterogeneous asymmetric catalysis has become an increasingly researched topic and in particular enantioselective hydrogenation remains a very active area of research. A number of approaches have been adopted which have been reviewed by Davis [1,2]. In general, the chirality required within the transition state to achieve enantioselectivity in the reaction can be supplied by a chiral surface [2], by a chiral solvent [3] or by a chiral modifier associated with either an immobilised metal centre [4] or a metal surface [5,6]. For many studies it is the latter approach that has been adopted, and of the relatively few systems capable of high levels of enantioselection, the asymmetric hydrogenation of a-ketoesters using supported Pt nanoparticles modified with cinchona alkaloids has been studied extensively and is considered to be a model system [5,6]. In earlier studies [5] of the enantioselective hydrogenation of ethyl pyruvate (I), it has been well established that with cinchonidine- and quinine- modified platinum catalysts the reaction proceeds to form (R)-ethyl lactate (II-R) predominantly, whereas with cinchonine- and quinidine- modified platinum catalysts the reaction forms (S)-ethyl lactate (II-S) preferentially (Scheme 1). ConHO
O C2H5 O
(II-R)
H2
O
O C2H5 O
Pt / cinchonidine or quinine
H2
Pt /cinchonine or quinidine
(I)
HO
siderable effort has been focussed on understanding the mechanism of these reactions, and three structural features of the cinchona and related modifiers that ensure they are effective in enantio-direction have been identified, namely (a) an aromatic moiety that enables adsorption on the platinum surface (b) the absolute configuration at C(9) which controls the sense of the enantioselectivity, and (c) a basic nitrogen which is considered to interact with the substrate resulting in a 1:1
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