Asymmetric hydrogenation on platinum: nonlinear effect of coadsorbed cinchona alkaloids on enantiodifferentiation (original) (raw)
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Journal of Catalysis, 2004
The adsorption of cinchonidine on platinum has been calculated with relativistically corrected density-functional theory, by first studying the interaction of the 1(S)-(4-quinolinyl)ethanol with a platinum cluster of 31 metal atoms, and by successive addition and separate optimization of the quinuclidine moiety. The conformations of the alkaloid on the surface were analyzed and their possible interactions with a surface chemisorbed methylpyruvate and acetophenone are discussed. A chiral space that is able to selectively accommodate surface enantiomers and to promote their rapid hydrogenation in a ligand-accelerated fashion has been determined. The role of the O-alkylation of the alkaloid in the modulation of enantioselectivity has been rationalized within the new interaction model.
Catalysis Letters, 2005
The enantioselective hydrogenation of ethyl pyruvate (EtPy) was studied on Pt-alumina catalyst modified by cinchonine (CN) and for comparison by cinchonidine (CD) in toluene and in AcOH. The effects of the modifiers concentration on the reaction rate and the enantioselectivity were examined. Using the Engelhard 4759 catalyst under mild experimental conditions (room temperature, hydrogen pressure 1 bar) in the case of CN (S)-ethyl lactate (EtLt) formed in excess (ee) (in AcOH ee max $ 88%; in toluene ee max $ 72%). In the case of CD (R)-EtLt formed in excess (in AcOH ee max 9393%; in toluene ee max 93 84%). The results of H-D exchange measurements and results of modifier mixtures suggest that the compounds responsible for chiral induction are different intermediates, which structure depends mostly on the acidic or non-acidic nature of the hydrogenation medium. The proposed structure of intermediate responsible for enantioselection is an 1:1 CN-or CD-EtPy surface complex in which the quinoline skeleton of CD approximately parallel on the Pt surface while the quinoline plane of CN being tilted relative to the Pt surface under identical experimental conditions. R = Me (ethylpyruvate = EtPy), R = Ph (ethylbenzoylformate= EtBFt) Scheme 1. The structure of parent cinchona alkaloids (CD = cinchonidine, CN = cinchonine, Q = quinine, QD = quinidine).
Journal of Catalysis, 2006
Steric effects in the Pt-catalyzed asymmetric hydrogenation of nine different α-ketoesters were studied by variation of the bulkiness at the keto and ester side of the substrates, and by using cinchonidine (CD), its 6 -methoxy derivative quinine, and o-phenyl derivative PhOCD as chiral modifiers. In the presence of CD, the (R)-enantiomer always formed in good to high ee (up to 96%), independent of the steric bulkiness of the α-ketoester. None of the mechanistic models developed for ketone hydrogenation on Pt are conform to the observations. Only additional steric effects in the modifiers and replacement of toluene by acetic acid as a reaction medium enhanced the sensitivity of the catalyst system to steric effects in the substrates (ee = 0-94%). An important mechanistic consequence of the observations is that on CD-modified Pt preferred adsorption of the α-ketoester on the si-side is directed by the position of the ester group relative to the modifier, independent of the steric bulkiness on any side of the keto-carbonyl group. Ester, carboxyl, amido, carbonyl, acetal, and trifluoromethyl functions have similar directing effects, but when both trifluoromethyl and an ester or carbonyl groups are present in the molecule, the latter function is dominant. The directing effect of the electron-withdrawing (-activating) function on adsorption of the ketone is obviously related to the electronic environment provided by the chiral modifier. The critical role of electronic interactions is supported by the remarkable influence of aryl substituents in the hydrogenation of ethyl benzoylformates.
Surface Science, 2002
NEXAFS, variable temperature STM and TPR have been used to study model adsorbates of relevance to heterogeneous chiral hydrogenation on platinum surfaces. In both the presence and absence of co-adsorbed hydrogen, quinoline molecules are uniformly though randomly distributed on Ptf1 1 1g at 300 K. At 350 K, irreversible surface reactions denude the Pt terraces of modified adsorption sites. Under conditions of hydrogen starvation, extensive polymerisation of quinoline occurs. In the presence of hydrogen, polymerisation is accompanied by hydrogenolysis. In both cases, the f1 1 1g terraces are depleted of quinoid-modified adsorption sites. Quinoline lies approximately flat at 300 K ða $ 15°Þ, bonded to the surface predominantly via the aromatic p system. This adsorption geometry remains largely unaffected by heating to 360 K, although the data suggest that molecular tilt increases somewhat with surface coverage ða $ 20°Þ. Lepidine exhibits a greater tilt angle than quinoline on Ptf1 1 1g ða $ 32°Þ, the likely result of steric hindrance due to the substituent at the C4 0 position-a characteristic that is present in all known efficient chiral modifiers for the reaction of interest. These findings are discussed with respect to the mechanism of chiral induction and the origin of enantioselectivity collapse in the asymmetric hydrogenation of a-ketoesters on Pt catalysts.
Donor–acceptor interactions in the enantioselective hydrogenation of α-ketoesters
Journal of Molecular Catalysis A: Chemical, 2005
The origin of enantiodiscrimination in the hydrogenation of methyl pyruvate (MP) on cinchona alkaloid modified by Pt has been mainly ascribed to interactions between the modifier and the substrate. In the present work, the role of these substrate-modifier interactions on the stabilization of intermediate complexes is discussed on the basis of ab initio MP2/6-31G(d) and MP2/6-31G(d,p) calculations. The amines ammonia, trimethylamine and quinuclidine are employed as model for the cinchona alkaloid. Our results show that MP interacts with the amines via a donor-acceptor complex with a stabilization energy that increases from ammonia, to trimethylamine and to quinuclidine, being in the last case on the order of 4.0 kcal mol −1 after correction for BSSE and inclusion of solvent effects. NBO analysis of the interacting orbitals confirms the nitrogen lone pair of the amines as a donor and the antibonding (C O) * orbital of the ␣-keto carbonyl as the acceptor. These results give support for experimental observations that interactions between the basic quinuclidine moiety of cinchonidine and the MP molecule may control the stereoselectivity of the catalytic process.
Hydrogenation of cinchona alkaloids over supported Pt catalyst
Chirality, 2003
The heterogeneous catalytic hydrogenation of two isomeric cinchona alkaloids, cinchonidine and cinchonine, was studied over Pt/Al 2 O 3 in 1N H 2 SO 4 solution under 100 bar H 2 at 25°C. Cinchonidine was transformed into two diastereomeric hexahydroderivatives by hydrogenation of ring A (with N) of the quinoline moiety (yield over 95%, diastereomeric ratio 2/3), whereas hydrogenation of cinchonine resulted in the formation of three products, the major one being formed by the hydrogenation of ring B (without N) (yield 60%). The isolated hexahydroderivatives were investigated by 1 H-, 13 C-NMR, 1 H-1 H COSY, 1 H-13 C HetCOSY, and NOESY spectroscopy and were used as modifiers in the heterogeneous enantioselective hydrogenation of ethyl pyruvate and as catalysts in the Michael addition of ethyl 2-oxocyclopentanecarboxylate to methyl vinyl ketone. Chirality 15:S82-S89, 2003.
The Journal of Physical Chemistry C, 2009
ABSTRACT The main orientations of methyl pyruvate in the force field of cinchonia alkaloids have been fully mapped and analyzed using molecular mechanics building techniques and density functional theory methods. Beside the known “open (3), (4)” and “closed (1), (2)” forms of cinchonidine, its group of “open (10), (11)” conformation is also analyzed, as well as the potential surface of isocinchonines. In this way an origin of different mechanistic models by different authors for the hydrogenation of activated ketones on cinchona alkaloid modified platinum is revealed: All models can be originated from the conformational properties of the internal rotation of the two-ring system in cinchona alkaloids (particularly cinchonidine, α and β-isocinchonine) with and without methyl pyruvate adduct formation.
Time-Lapse STM Studies of Diastereomeric Cinchona Alkaloids on Platinum Metals
The Journal of Physical Chemistry B, 2006
The adsorption of cinchonidine (CD) and cinchonine (CN) on Pt(111) and Pd(111) single crystals has been investigated by means of scanning tunneling microscopy (STM) in an ultrahigh vacuum system. In timelapse series the mobilities of different adsorption species have been determined on a single molecule basis and with varying hydrogen background pressures in the system. The diastereomeric cinchona alkaloids, CD and CN, which are widely used as chiral modifiers of platinum group metals in catalytic enantioselective hydrogenation, showed similar adsorption modes and diffusion behavior on Pt(111), except that the flatly adsorbed CN molecules which were free (not in a dimer/cluster) were significantly more mobile than their CD analogues. CD adsorbed on Pd(111) showed similar adsorption modes as observed on Pt(111) but at considerably higher mobility of the flatly absorbed species already in the low-pressure region. The observed adsorption behaviors are discussed in the context of independent ATR-IR measurements and theoretical calculations. Special emphasis is put on the nonlinear effect observed in hydrogenation reactions with CD/ CN mixtures. Our observations corroborate that this effect is mainly a consequence of the different adsorption strengths of CD and CN on Pt.