Shape-Selective Enantioselective Hydrogenation on Pt Nanoparticles

Abstract

The structure sensitivity of enantioselective hydrogenations on chirally modified metals was investigated using Pt nanoparticles of different shapes. All three samples had an average particle size of 10 nm, but the fraction of dominantly cubic, cubooctahedral, and octahedral particles varied with decreasing {100} and increasing {111} faces in the same order. In the absence of chiral modifier the hydrogenation of ethyl pyruvate was independent of the shape of the Pt nanoparticles; variation of the specific reaction rates did not exceed the experimental error on all self-prepared catalysts and on a commercial Pt/Al(2)O(3) used as reference. Addition of cinchonidine or quinine induced a significant rate enhancement by a factor of 4-15, and the rate was always higher with quinine. Also, 72-92% ees were achieved, and the reaction was shape selective: both the rate and the ee increased with increasing Pt{111}/Pt{100} ratio. A similar correlation in the hydrogenation of ketopantolactone confirmed that decarbonylation or aldol-type side reactions of ethyl pyruvate were not the reason for structure sensitivity. A combined catalytic and theoretical study revealed that the probable origin of the particle shape dependency of enantioselective hydrogenation is the adsorption behavior of the cinchona alkaloid. DFT studies of cinchonidine interacting with Pt(100) and Pt(111) terraces indicated a remarkably stronger interaction on the former crystallographic face by ca. 155 kJ/mol. The higher adsorption strength on Pt(100) was corroborated experimentally by the faster hydrogenation of the homoaromatic ring of the alkaloid, which fragment interacts the strongest with Pt during its adsorption. Thus, an ideal catalyst for the hydrogenation of activated ketones contains dominantly Pt{111} terraces, which crystallographic face is more active and affords higher enantioselectivity, combined with the higher stability of the modifier.

Asymmetric Hydrogenation and Transfer Hydrogenation of Ketones

“…Furthermore, it has been shown that the rate of hydrogenation and enantioselectivity outcome depends on the shape and terrace sites (Pt{100}or {111}) of the nanoparticles. Both the rate and the ee increased in the hydrogenation of ethyl pyruvate and ketopantolactone when Pt {111}nanoparticles were modified using CD or QN as the chiral modifiers (Schmidt et al, 2009). …”

P-Stereogenic Phosphines for the Stabilisation of Metal Nanoparticles. A Surface State Study

“…The enantioselective catalysis on surfaces is commonly explained by three main mechanisms: the presence of chiral crystalline atomic structures at the surface, chiral adsorbates which modify the surface and chiral modifiers which control the approach of the substrate on the surface [11,12]. The stereoselective reactions mainly concern hydrogenations and C-C bond formation reactions based on the third mechanism [13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31]. In this context, the optically pure ligand should be able to generate both a strong enough coordination at the metallic surface to stabilize MNPs and an asymmetric environment to efficiently lead to one of the expected stereoisomers during the organic transformation.…”

“…Ligands coordinate at the metallic surface by different ways, such as dative donor bonds between heteroatoms of the ligands and the metal surface, and also by π-interactions through aromatic fragments of the ligands and the nanoparticle. The nature of the chiral stabilisers reported in the synthesis of MNPs is wide-ranging, including dienes [15], cinchona alkaloids [16][17][18], mono or bidentate amines [19][20][21], phosphorus-based ligands (phosphites [22][23][24][25][26], diphosphines [27][28][29][30], secondary phosphine oxide [31]). P-stereogenic phosphines are well-known in catalysis using organometallic complexes as catalytic precursors, finding applications in hydrogenation, transfer hydrogenation, cyclopropanation, hydrovinylation, Diels Alder reactions and allylic substitutions [32][33][34].…”

Solid State Morphology and Size Tuning of Nanostructured Platinum Using Macromolecular Complexes

“…Pt acts as catalyst in hydrogenations [2], in the reduction of polluting gases expelled during the combustion of gasoline in automobiles [3], in the industrial synthesis of nitric acid (from decomposition of petroleum) [4] and in proton-exchange fuel cell [5]. All of these applications use Pt finely divided [6].…”