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Szőllősi, György; Felföldi, Károly; Bartók, Tibor; Bartók, Mihály

doi:10.1023/a:1010390301052

Cited by 18 publications

(4 citation statements)

References 14 publications

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“…The addition of an acid does not induce inversion in these reactions when the parent cinchona alkaloids or other amine type modifiers are used as modifiers. 1,3,165 Besides, both (R)-and (S)-lactate were produced by -isocinchonine-modified Pt in toluene by changing the reaction conditions, 262,532 and there was no inversion by acid addition in the hydrogenation of various other R-ketoesters on the same catalyst. 198 It is very unlikely that the reaction mechanism would change within a substrate class.…”

Section: Inversion Of Enantioselectivitymentioning

confidence: 99%

Asymmetric Catalysis at Chiral Metal Surfaces

2007

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He continued his academic carrier at the Organic Chemical Technology Research Group of the Hungarian Academy of Sciences. In 1990, he moved to ETH Zurich, where he is currently a scientist. He was awarded a D.Sc. degree in 1994 by the Hungarian Academy of Sciences. His main scientific interests are the application of heterogeneous catalysts in the synthesis of fine chemicals, including asymmetric catalysis, oxidation, and hydrogenation reactions, and the application of electrochemical methods in catalysis.

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Section: Inversion Of Enantioselectivitymentioning

confidence: 99%

Asymmetric Catalysis at Chiral Metal Surfaces

2007

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“…In the past years, some structural isomers of CN, namely α-isocinchonine (α-iCN) and β-isocinchonine (β-iCN), obtained by thermal ethereal cyclization of CN in strong acids, have been tested as surface modifiers in asymmetric reactions. − At first, their use was restricted to the assessment of the hypothesis that open conformers (conformers where the quinuclidine N-atom points away from the quinoline anchoring moiety) of the alkaloids were implicated in the mechanism of enantiodifferentiation, because such conformers lose one rotational degree of freedom and are constrained in an open conformation. , At this stage, it seemed that their performance as modifiers (on platinum) was analogous, only with lesser efficiency, compared to the parent natural alkaloid. Later, it was recognized that β-iCN exhibits significant peculiarities with respect to CN when used as chiral surface modifier.…”

Section: Introductionmentioning

confidence: 99%

Chiral Modification of Rh and Pt Surfaces: Effect of Rotational Flexibility of Cinchona-Type Modifiers on Their Adsorption Behavior

et al. 2008

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The adsorption behavior of cinchona-type chiral modifiers (β-isocinchonine, cinchonidine, and cinchonine) on model rhodium and platinum catalysts has been studied using attenuated total reflection infrared (ATR-IR) spectroscopy and density functional theory (DFT). The ATR-IR studies performed under conditions closely resembling those encountered during enantioselective hydrogenation as well as in the absence of hydrogen revealed significant differences in the adsorption mode of the modifiers, depending on the rotational flexibility of the modifier, the platinum group metal, and the presence of hydrogen. The more rigid β-isocinchonine showed preferential tilted adsorption on both metals, independent of the presence or absence of hydrogen. Cinchonidine and cinchonine were adsorbed on Pt predominantly with the quinoline ring nearly parallel to the surface, irrespective of the presence or absence of hydrogen, whereas on Rh in the presence of hydrogen only tilted species were observed. The latter behavior is attributed to the fast hydrogenation of the aromatic anchoring group (quinoline) on Rh that interferes with the adsorption process. DFT calculations confirmed that the energy difference between tilted and adsorbed species decreases in the case of the β-isocinchonine. This combined spectroscopic and theoretical investigation indicates a greater bias toward the tilting of the anchoring group in the case of β-iCN as compared to the natural occurring alkaloids cinchonidine and cinchonine. Such behavior explains the greater stability toward quinoline ring saturation observed for β-isocinchonine on rhodium-supported catalysts during enantioselective hydrogenation. This study thus helps define the delicate equilibrium between formation of chiral surface sites and their stability in strongly reducing conditions in terms of the structure and consequent adsorption behavior of the modifier molecule, as a function of the metal catalyst of choice.

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“…This model requires the cinchona alkaloid to complex in its “closed form” (see below), a conformation in which the quinuclidine nitrogen points toward the quinoline aromatic ring, opposite to the “open form” conformation in which the quinuclidine nitrogen points away from the quinoline aromatic ring. However, rigid cinchona alkaloids that can only exist in the “open” form induce enantioselectivity similar to that of usual cinchona alkaloids . Numerous observations suggest that the “closed” form does not play an important role in the enantioselectivity. 6c,d …”

mentioning

confidence: 99%

“…However, rigid cinchona alkaloids that can only exist in the “open” form induce enantioselectivity similar to that of usual cinchona alkaloids . Numerous observations suggest that the “closed” form does not play an important role in the enantioselectivity. 6c,d …”

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confidence: 99%

Origins of Enantioselectivity in Reductions of Ketones on Cinchona Alkaloid Modified Platinum

2003

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A model to explain the stereoselectivities of reductions of activated ketones on cinchona alkaloid modified platinum is proposed and is supported by calculations by density functional and force field methods. The model involves nucleophilic catalysis by the cinchona alkaloid. The zwitterionic adduct between a cinchona alkaloid and ketone is adsorbed on Pt through the quinoline ring and two heteroatoms and is subsequently reduced with inversion. The model rationalizes the observed stereoselectivities for hydrogenation of carbonyl compounds.

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Cited by 18 publications

References 14 publications

Asymmetric Catalysis at Chiral Metal Surfaces

Asymmetric Catalysis at Chiral Metal Surfaces

Chiral Modification of Rh and Pt Surfaces: Effect of Rotational Flexibility of Cinchona-Type Modifiers on Their Adsorption Behavior

Origins of Enantioselectivity in Reductions of Ketones on Cinchona Alkaloid Modified Platinum

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