Walter Reppe Revival – Identification and Genesis of Copper Acetylides Cu(2)C(2) as Active Species in Ethynylation Reactions

More than six decades after proposing copper acetylide, Cu(2)C(2), as catalytically active species in ethynylation reactions by Walter Reppe, the explosive species have been experimentally identified and investigated during catalysis in detail now. Taking into account specific safety precautions, unequivocal qualitative characterization was achieved by Raman spectroscopy and X‐ray powder diffraction of supported copper catalysts Cu/Bi/SiO(2) during and after activation and catalysis in comparison to bulk Cu(2)C(2) materials. Quantification of Cu(2)C(2) succeeded by thermal analysis and Raman spectroscopy. Its formation in aqueous suspension is studied starting from copper(II) oxide catalysts including dissolution, reduction and precipitation steps. Copper acetylide formation can be correlated with catalytic performance in the ethynylation of formaldehyde to 1,4‐butynediol.