Novel Evidence on the Role of the Nucleophilic Intermediate Complex in the Orito-Reaction: Unexpected Inversion in the Enantioselective Hydrogenation of 2,2,2-Trifluoroacetophenone on Pt-Cinchona Chiral Catalyst Using Continuous-Flow Fixed-Bed Reactor

Abstract

The enantioselective hydrogenation of 2,2,2,-trifluoroacetophenone over Pt/Al 2 O 3 catalysts modified by cinchona alkaloids was investigated for the first time using continuous-flow fixed-bed reactor system in toluene/AcOH 9/1 solvent mixture in absence and presence of 0.

Disrupting Aryl-CH···O Interactions on Pt(111) Through the Coadsorption of Trifluoroacetic Acid and 2,2,2-Trifluoroacetophenone (TFAP): Inhibition of Competing Processes in Heterogeneous Asymmetric Catalysis

“…The STM data provide additional insight into reports in the catalysis literature [13][14][15][16][17] that the presence of small amounts of TFA in toluene gives rise to an increase in the ee and a decrease in the reaction rate in the hydrogenation of TFAP over CD modified Pt. We propose that these changes are caused, at least in part, by the disruption by TFA of aryl-CHÁÁÁO interactions revealed by the STM images in Fig.…”

“…As part of a set of systematic tests, they used toluene with added TFA as the solvent and cinchonidine (CD) as the modifier. Among many intriguing results, they confirmed that (i) addition of small amounts of TFA significantly increased the ee and (ii) they found that the conversion rates were lower in the presence of TFA [15][16][17]. Our study was performed by taking scanning tunneling microscopy (STM) measurements for the coadsorption of TFA and TFAP on Pt(111) under ultrahigh vacuum conditions.…”

“…In this article, aryl-CHÁÁÁO interactions are considered within the context of literature reports on the effect of trifluoroacetic acid (TFA) as an additive in the enantioselective hydrogenation of TFAP on cinchona modified platinum [13][14][15][16][17]. In an early report, TFA was used as an additive to achieve 93% ee in the hydrogenation of TFAP [13].…”

Heterogeneous Catalytic Hydrogenation Reactions in Continuous‐Flow Reactors

“…[112][113][114] Several other examples describing enantioselective hydrogenations carried out in commercial and non-commercial continuous flow reactors have been reported in the literature. Among them, the hydrogenation of methyl/ethyl pyruvate [38,[115][116][117][118][119][120][121][122][123][124][125][126][127][128][129][130] ketopantolactone, [116,117,119] 2,2,2-trifluoroacetophenone, [121,122,131,132] isopropyl 4,4,4-trifluoroacetoacetate, [133] 1-phenyl-1,2-propanedione, [134] methyl/ethyl benzoylformate, [116,117,125,135] ethyl-2-oxo-4-phenylbutyrate [136] and 4-methoxy-6-methyl-2-pyrone, [137] should be mentioned. Bartok and co-workers investigated the Pt/Al 2 O 3 -cinchonidine catalyzed enantioselective hydrogenation of ethyl pyruvate (Scheme 10 b), ketopantolactone and methyl benzoylformate using a continuous flow fixed-bed reactor system.…”

“…Supposedly, in the hydrogenation of 2,2,2-trifluoroacetophenone the reaction route involves an equilibrium of electrophilic and nucleophilic intermediate complexes, depending on the acid strength and concentration. [131,132] Ketopantolactone and isopropyl 4,4,4-trifluoroacetoacetate (Scheme 11 a) were hydrogenated in a fixed-bed reactor over cinchonidine -modi- www.chemsuschem.org fied Pt/Al 2 O 3 catalyst resulting in 83 % and 90 % ee respectively. [119,133] During the enantioselective hydrogenation of 1-phenyl-1,2-propanedione, the enantioselectivity of the resulting (R)-1-hydroxy-1-phenylpropanone proved to be independent of the reactant concentration and gave comparable enantiomeric excess values (60 %) for both the continuous-flow and batch experiments.…”