Two-Dimensional Self-Assembly and Catalytic Function: Conversion of Chiral Alcohols into Self-Assembled Enols on Pt(111)

Abstract

The chemical transformation and subsequent self-assembly of chiral alcohols on platinum was studied using three different pairs of prochiral ketones and their alcohol products. The ketones were chosen because they represent three different types of substrates in the asymmetric hydrogenation on chirally modified platinum catalysts. Scanning tunneling microscopy and high-resolution electron energy loss vibrational spectroscopy data were combined to show that methyl lactate transforms into the enol tautomer of methyl pyruvate on Pt(111) at room temperature. Specifically, the chiral alcohol undergoes dehydrogenation leading to the same adsorbed enol assemblies that are formed directly through the adsorption of the prochiral α-ketoester. Similarly, 1-phenylethanol transforms into assemblies of the enol tautomer of acetophenone. The interrelationship between surface reactivity and self-assembly was further explored by studying the oxidation of 1-phenyl-2,2,2-trifluoroethanol to form CH···O bonded 2,2,2-trifluoroacetophenone assemblies. In terms of catalytic function, these self-assembly processes provide insight on the optimization of the asymmetric hydrogenation of activated ketones on chirally modified platinum catalysts.

Surface‐Driven Keto–Enol Tautomerization: Atomistic Insights into Enol Formation and Stabilization Mechanisms

“…Generally,t he ketone form of neutral carbonyl compounds is more stable than the enol form, [5] therefore the formation of enol species usually requires stabilization by for example,h ydrogen bonding.T he most commonly discussed mechanism of the enol stabilization is related to diketones or ketoesters containing two carbonyl groups.I nt hese compounds,achelated ring-like keto-enol tautomer can be formed with one of the carbonyls being transformed to a À C À O À He ntity and the other carbonyl participating in the stabilization of the enol part of the molecule through hydrogen bonding between the ÀC=Oa nd HÀOÀCÀ functional groups.Numerous studies provided solid experimental proof for the mechanism leading to formation of chelated keto-enol tautomers in diketones and ketoesters. [6] While the enol stabilization is well understood for the compounds comprising two carbonyls,s ignificantly less is known about the enol formation and stabilization for monocarbonyls,i nw hich the intramolecular stabilization is not possible.I nar ecent study by Demers-Carpentier et al [7] on monocarbonyls adsorbed on Pt, an interesting observation on formation of different oligomers,c onsisting of two or three acetophenone molecules,w as reported, which were interpreted as structures comprising exclusively the enol species. As thermal activation, up to 300 Kwas needed to obtain these enol-containing oligomers,t he authors put forward ad issociative type of keto-enol tautomerization mechanism, involving dissociation of aC À Hb ond in methyl group followed by aHdiffusion through the Pt surface to the C = Ogroup.…”

Surface‐Driven Keto–Enol Tautomerization: Atomistic Insights into Enol Formation and Stabilization Mechanisms

“…It should be noted, that formation of surface dimers and trimers of acetophenone on Pt was previously observed by the group of McBreen at room temperature and they were assigned exclusively to enol–enol oligomers. Further, the dissociative mechanism of enol formation was hypothesized in this study, which is based on the assumption that a H atom of a methyl group is abstracted by the underlying metal and transfers to the carbonyl group through the Pt surface.…”

“…While the enol stabilization is well understood for the compounds comprising two carbonyls, significantly less is known about the enol formation and stabilization for monocarbonyls, in which the intramolecular stabilization is not possible. In a recent study by Demers‐Carpentier et al on monocarbonyls adsorbed on Pt, an interesting observation on formation of different oligomers, consisting of two or three acetophenone molecules, was reported, which were interpreted as structures comprising exclusively the enol species. As thermal activation, up to 300 K was needed to obtain these enol‐containing oligomers, the authors put forward a dissociative type of keto–enol tautomerization mechanism, involving dissociation of a C−H bond in methyl group followed by a H diffusion through the Pt surface to the C=O group .…”

“…In a recent study by Demers‐Carpentier et al on monocarbonyls adsorbed on Pt, an interesting observation on formation of different oligomers, consisting of two or three acetophenone molecules, was reported, which were interpreted as structures comprising exclusively the enol species. As thermal activation, up to 300 K was needed to obtain these enol‐containing oligomers, the authors put forward a dissociative type of keto–enol tautomerization mechanism, involving dissociation of a C−H bond in methyl group followed by a H diffusion through the Pt surface to the C=O group . The experimental verification of this hypothesized dissociative mechanism is not available yet.…”

“…It becomes also apparent that enol formation on pristine Pt cannot be linked merely to the availability of adsorbed H originating from H‐transfer through the surface suggested by Demers‐Carpentier et al., but rather relies on the intramolecular process involving H‐transfer to the carbonyl group to form a C−O−H entity, which is stabilized by hydrogen bonding with the neighboring ketone molecule (see the Supporting Information for detailed discussion). With this, the enol formation and stabilization mechanism of monocarbonyl compounds on surface is similar to that occurring in the intramolecular process in dicarbonyl molecules.…”