Dependence of the adsorption of chiral compounds on their enantiomeric composition

Gordon, Alexander D.; Karakalos, Stavros; Zaera, Francisco

doi:10.1016/j.susc.2014.02.003

Cited by 18 publications

(27 citation statements)

References 53 publications

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“…[18,19] Secondly,t he peak from the asymmetric deformation of the methyl moiety of the ethylamine group (1445-1460 cm À1 ) is less intense,i nr elative terms,t han the corresponding symmetric (umbrella) mode at approximately 1380 cm À1 . [18,19] Secondly,t he peak from the asymmetric deformation of the methyl moiety of the ethylamine group (1445-1460 cm À1 ) is less intense,i nr elative terms,t han the corresponding symmetric (umbrella) mode at approximately 1380 cm À1 .…”

Section: Angewandte Chemiementioning

confidence: 99%

“…Zuschriften under UHV.T he blue-shifts of both peaks suggest an additional interaction with the solvent (or ac hemical transformation, see proceeding text), and the peak split seen in some cases indicates intermolecular interactions,a sa lso observed with racemic mixtures. [18,19] Secondly,t he peak from the asymmetric deformation of the methyl moiety of the ethylamine group (1445-1460 cm À1 ) is less intense,i nr elative terms,t han the corresponding symmetric (umbrella) mode at approximately 1380 cm À1 . Moreover,t he deformation mode of the C À Hb ond next to the amine group is clearly visible in the spectra of the species adsorbed from solution, at about 1330 cm À1 .Itisthis region of the spectrum, corresponding mostly to vibrations within the ethyl moiety,w hich shows the greatest differences in peak position between the species adsorbed from solution and those dosed under UHV.All of this is consistent with amodel where NEA, when adsorbing from solution, bonds to the platinum surface through the nitrogen atom and has all of the substituents in the adjacent chiral carbon (the naphthalene and methyl groups and the extra hydrogen) pointing away from the surface.…”

Section: Angewandte Chemiementioning

confidence: 99%

“…Figure 2a lso shows that the spectra of the adsorbed species are quite different to those of the pure compounds. These differences are explored in more detail for the case of S-NEA in Figure 3, which displays IR absorption spectra for (from bottom to top): 1) pure NEA (acquired in transmission mode);2 ,3) NEA adsorbed on aPt(111) single crystal under ultra-high vacuum (UHV; acquired in single reflectionabsorption, or RAIRS,m ode) after two different exposures, namely,1 .5 L( which leads to ac overage close to monolayer saturation;1L= 1 10 À6 torr s), and 0.5 L( representative of the low-coverage limit); [18] 4) NEA adsorbed from a1m m CCl 4 solution onto apolished platinum foil [17,19] (also acquired in RAIRS mode but using adifferent setup); [20] and 5,6) NEA adsorbed from 1mm CCl 4 solutions,o n1wt %P t/Al 2 O 3 and 1wt%Pt/SiO 2 commercial catalysts,respectively (acquired in ATR-IR mode). [21] An assignment of the peaks,b ased on previous studies, [18] is provided in Table 1.…”

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Correlation between Chiral Modifier Adsorption and Enantioselectivity in Hydrogenation Catalysis

Gordon

Florisa

et al. 2017

Angew Chem Int Ed

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Infrared absorption spectroscopy performed in situ at the solid-liquid interface revealed that the adsorption on platinum supported catalysts of 1-(1-naphthyl)-ethylamine, which is used as a chiral modifier in hydrogenation catalysis, occurs through the amine group, not the aromatic ring as is widely believed. Comparisons were performed against a set of related modifier compounds with targeted substitutions to help identify the key moiety involved in the adsorption. It was determined that neither naphthalene-based modifiers without amine groups nor those with tertiary amine moieties are capable of adsorbing on the metal surface to any significant extent. A direct correlation was also found between the ability of the amines to adsorb on the platinum surface and their performance as chiral modifiers that impart enantioselectivity to the hydrogenation of α-keto esters such as ethyl pyruvate.

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Section: Angewandte Chemiementioning

confidence: 99%

Section: Angewandte Chemiementioning

confidence: 99%

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Correlation between Chiral Modifier Adsorption and Enantioselectivity in Hydrogenation Catalysis

Gordon

Florisa

et al. 2017

Angew Chem Int Ed

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“…Using a similar approach, we have recently proven that the extent of adsorption of PO depends on the enantiocomposition of the adsorbed layer. 29,30 2. EXPERIMENTAL DETAILS A three-step experimental procedure was used in the present study, as illustrated in Figure 1.…”

Section: Introductionmentioning

confidence: 99%

Amplification of Enantioselectivity on Solid Surfaces Using Nonchiral Adsorbates

Karakalos

Zaera

2015

J. Phys. Chem. C

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The amplification of enantioselectivity during the adsorption of chiral molecules on solid surfaces by using nonchiral agents was demonstrated for the case of Pt(111) seeded with a small amount of enantiopure propylene oxide and then dosed with propylene as the amplifier. Chiral chemical titration and isothermal kinetic adsorption experiments using collimated effusive molecular beams indicated the possibility of reaching enantioselectivity excesses of over 60% this way. Monte Carlo simulations provided a kinetic explanation for this effect in terms of both an adsorbate-assisted adsorption process and a bias in the chiral configuration the propylene molecules are driven to upon adsorption on the surface.

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“…In the initial dosing stages there is aclear bias toward the adsorption of the enantiopure R-PO-d 0 .T his behavior has been reported by us before,a nd explained by ak inetic effect that relies on adsorbate-assisted adsorption with different probabilities for homo-versus hetero-enantiomeric pairs. [18,19] Theinteresting new observation here is that Figure 1. Enantiocomposition of the adsorbedP Olayers that build up on aPt(111) single-crystal surface upon exposure to increasing doses of a1:1 mixture of R-PO-d 0 + Racemic-PO-d 6 ,a sdetermined from the moleculard esorption yields in TPD experiments.…”

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confidence: 82%

Changes in the Enantiomeric Composition of Chiral Mixtures Upon Adsorption on a Non‐Chiral Surface

2016

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The adsorption of propylene oxide,achiral molecule,o naPt(111) single-crystal surface was studied as af unction of enantiomeric composition by temperature programmed desorption (TPD) and molecular beams.T wo opposing trends were observed leading to variations in the enantiomeric excess (ee) of the chemisorbed layers with respect to the composition of the gas-phase mixtures:akinetic effect dominant during the initial uptake,w ith ap reference toward the formation of enantiopure layers,a nd as teady-state effect seen after approximately monolayer half-saturation, at which point the preference is toward racemization. These effects may account for important phenomena such as the bias toward one chirality in biological systems and the selective crystallization of some chiral compounds,a nd may also be used in practical applications for chemical separations and catalysis.When molecules contain at least one carbon atom coordinated to four different moieties,t hey can exist in two chemically identical but non-superimposable mirror-image forms.T he two so-called enantiomers,d enoted Sa nd R, are said to be chiral, aproperty that is critical in the synthesis of pharmaceuticals and other biological applications because life on earth has,for reasons not fully understood to date,chosen to operate exclusively with one type of chirality. [1,2] There is an ongoing quest to find efficient ways to produce enantiopure compounds in order to avoid complex enantiopurification steps.Efficient chiral and green chemical synthesis may be achieved by using enantioselective heterogeneous catalysts. However,t hat requires ab etter understanding of the details of the adsorption of chiral molecules on solid surfaces. [3][4][5][6][7] A number of surface-science experiments have shown that the enantiocomposition of the adsorbed layers of chiral compounds may vary relative to those in the gas or liquid phases. In particular,Y ua nd Gellman have recently shown that the enantiomeric excess (ee)o fc hiral mixtures of aspartic acid can be amplified upon adsorption on achiral Cu(111) surfaces.[8] We have also shown that the ee of surface layers of chiral compounds can be amplified by adding non-chiral adsorbates. [9] In this study,w er eport interesting variations in enantiocomposition induced by adsorption of enantio mixtures of propylene oxide (PO) on aPt(111) single-crystal surface.The coverages of the adsorbates have been determined by quantifying the yields from temperature programmed desorption (TPD) experiments,a sd one in the past by us and others. [10][11][12][13][14][15][16][17] Thet wo enantiomers of PO were differentiated by using isotope labeling,m ixing regular PO-d 0 with either PO-d 2 or PO-d 6 .T ypical data on the enantiocomposition of the adsorbates as af unction of the extent of exposure of the surface to the PO gas mixture are shown in Figure 1, in this case for a1 :1 mixture of R-PO-d 0 (denoted as R-PO in the figures) + Racemic-PO-d 6 (a 1:1m ixture of the S and R enantiomers,d enoted as Rac-PO-d 6 )a nd as urface temp...

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Dependence of the adsorption of chiral compounds on their enantiomeric composition

Cited by 18 publications

References 53 publications

Correlation between Chiral Modifier Adsorption and Enantioselectivity in Hydrogenation Catalysis

Correlation between Chiral Modifier Adsorption and Enantioselectivity in Hydrogenation Catalysis

Amplification of Enantioselectivity on Solid Surfaces Using Nonchiral Adsorbates

Changes in the Enantiomeric Composition of Chiral Mixtures Upon Adsorption on a Non‐Chiral Surface

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