Polyhedral Cu2O to Cu pseudomorphic conversion for stereoselective alkyne semihydrogenation

Abstract

Cu₂O cubes, octahedra, and rhombic dodecahedra can be pseudomorphically converted into Cu crystals of corresponding shapes through reduction by ammonia borane in ethanol at 50 °C or below within 3 min, demonstrating the feasibility of making challenging polyhedral metal particles from metal oxide crystals. Hydrogen gas is also produced from ammonia borane in the process. The obtained Cu crystals have a slightly nanoporous interior. Addition of diphenylacetylene in the formation of Cu rhombic dodecahedra leads to complete stereoselective production of sterically hindered (Z)-stilbene. Semihydrogenation of other alkynes also gives pure (Z)-alkenes. Cu cubes and octahedra also showed considerable (Z)-stilbene selectivity along with minor formation of (E)-stilbene and bibenzyl as compared to CuCl₂ and commercial Cu₂O particles. Mechanistic studies reveal that the low binding affinity of alkenes on the rhombic dodecahedra surfaces leads to high product selectivity. These Cu crystals act as a green and low-cost catalyst for the synthesis of high-purity (Z)-alkenes.