Investigation of the interaction between a novel unnatural chiral ligand and reactant on palladium for asymmetric hydrogenation

Abstract

We report about the mechanistic studies of the reaction between a newly synthesized (S)-2-((R)-3H-dinaphtho[2,1-c:1′,2′-e]azepin-4(5H)-yl)-2-phenylethanol based on the binaphthyl skeleton and (E)-2-methyl-5-phenylpent-2-enoic acid for the asymmetric hydrogenation of α,β-unsaturated acids with heterogeneous palladium catalysts. The specific interactions between the chiral ligand and reactant were investigated in solution with palladium nanoparticles, as well as under ultrahigh vacuum (UHV) conditions on the palladium metal surface in the absence of hydrogen. The reactions were explored using nuclear magnetic resonance (NMR) spectroscopy, scanning tunneling microscopy (STM), and high-resolution photoemission spectroscopy (HRPES) combined with density functional theory (DFT) calculations. A NMR study identified the interaction between both molecules with palladium nanoparticles in solution. In addition, STM and HRPES studies revealed the spatial distribution and configuration of both compounds on the palladium metal surface under UHV conditions. The theoretical results support the experimental results with respect to the interaction energy value. It was found that the reaction between the ligand and reactant occurs with hydrogen bonding on palladium surface, simultaneously. The present study provides mechanistic details of the asymmetric hydrogenation reaction, which bears a correlation between the ligand, reactant, and catalyst during the reaction.