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Abstract
AbstractUnsupported and SiO2‐supported Ni nanoparticles (NPs) were synthesised via hot‐injection colloidal route using oleylamine (OAm) and trioctylphosphine (TOP) as reducing and protective agents, respectively. By adopting a multi‐length scale structural characterization, it was found that by changing equivalents of OAm and TOP not only the size of the nanoparticles is affected but also the Ni electronic structure. The synthetized NPs were modified with (R,R)‐tartaric acid (TA) and investigated in the asymmetric hydrogenation of methyl acetoacetate to chiral methyl‐3‐hydroxy butyrate. The comparative analysis of structure and catalytic performance for the synthetized catalysts has enabled us to identify a Ni metallic active surface, whereby the activity increases with the size of the metallic domains. Conversely, at the high conversion obtained for the unsupported NPs there was no impact of particle size on the selectivity. (R)‐selectivity was very high only on catalysts containing positively charged Ni species such as over the SiO2‐supported NiO NPs. This work shows that the chiral modification of metallic Ni NPs with TA is insufficient to maintain high selectivity towards the (R)‐enantiomer at long reaction times and provides guidance for the engineering of long‐term stable enantioselective catalysts.
Highlights
The enantioselective asymmetric hydrogenation of unsaturated molecules is broadly applied for the synthesis of pharmaceuticals and fine chemicals, where the biggest challenge is attaining total selectivity to one specific enantiomer.[1]
A series of unsupported and SiO2-supported Ni NPs were synthesized using the hot-injection method to control the size of the NPs by changing the ratio between the reducing and protecting
A sketch of this synthesis is shown in scheme 2
Summary
The enantioselective asymmetric hydrogenation of unsaturated molecules is broadly applied for the synthesis of pharmaceuticals and fine chemicals, where the biggest challenge is attaining total selectivity to one specific enantiomer.[1] One of the most studied examples of such reactions is the hydrogenation of β-ketoesters over nickel-based catalysts,[2] e. G. methyl acetoacetate (MAA) to methyl-3-hydroxybutyrate (MHB), shown in Scheme 1. MHB is an important intermediate in the synthesis of carbonic anhydrase inhibitor MK-0507, used in the Scheme 1. Hydrogenation of pro-chiral β-ketoester methyl acetoacetate (1). Gives two enantiomers, (R)- and (S)-methyl-3-hydroxybutyrate (2 and 3, respectively). When the surface of the nickel catalyst is modified with (R,R)tartaric acid ( in combination with an inorganic salt such as NaBr), the (R)-enantiomer is preferably obtained.
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