Now, Wang et al. have developed a strategy for covalently immobilizing optically active proline on well-defined metal nanoclusters, integrating organocatalysis and metal catalysis for an efficient and stereocontrolled aldol reaction. The researchers attached either l- or d-proline to Au–Ag nanoclusters by first reacting AgBF4 and the amino acid in the presence of dppfAu2(BF4)2, with dppf being a phosphorous-based ligand. The crystal structure suggested that the three-layered, inorganic core of Ag3−Au7−Ag3 is initially achiral. Thus, prolines strongly coordinate via their carboxylates to the Ag atoms, while phosphorus atoms from the dppf ligand stabilize the Au atoms. The introduced prolines transfer chirality to the metal cluster (pictured): the amino acids’ pyrrolidine moieties assume a triblade fan arrangement, which influences the organization of dppf’s phenyl groups via steric hinderance and C–H···π interactions. Consequently, the dppf adopts an (R) configuration in d-Au7Ag8 and an (S) configuration in l-Au7Ag8. The interactions were confirmed by 31P nuclear magnetic resonance of the complexes and isotopic distribution patterns in mass spectrometry. Circular dichroism confirmed the chirality of the l- or d-Au7Ag8 nanoclusters, and that the chirality of the ligands is transferred onto the metal centres through their respective Ag–O and Au–P bonds, as modelled by density functional theory studies. The authors then applied their system to the aldol reaction of acetone and p-nitrobenzaldehyde to obtain β-hydroxyketone (pictured) with similar conversions and enantiomeric excess to proline, but at a 1 mol% loading (a sixth of proline’s) and with an increased turnover number.
The combination of enamine catalysis on easier-to-access sites on the gold-stabilized silver chiral nanocluster surface is a nice example of the synergies that can be established with the integration of metal nanocluster cores and asymmetric organic ligands, for C–C bond-forming reactions.
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