The Reaction of Alkynes with Model Gold Catalysts: Generation of Acetylides, Self-coupling and Surface Decomposition

Alkynes are one of the most important groups of building blocks in organic chemistry as they work as nucleophiles upon deprotonation of its acidic hydrogen. On the surface of a gold catalyst the reactivity of an alkyne towards dehydrogenation depends on the stability of the resulting acetylide species and on its Brønsted acidic behavior with respect to other species in the system. Herein, we study the reaction of alkynes on an oxygen-covered gold surface, hereafter described as O/Au, with and without species able to build acid–base pairs on the surface. As expected, alkynes undergo a facile dehydrogenation process, forming acetylide species and coupling products such as CH₃–CC–CC–CH₃ from propyne (CH₃–CC–H). However, temperature-programmed reactions show that the catalytic intermediates are strongly attached to the surface, leading to parallel decomposition pathways that result in the carbon poisoning of the catalyst. The strong attachment suggests that the alkynes can displace species known to undergo facile dehydrogenation on gold, such as adsorbed alkoxy, as demonstrated by experiments using selected alcohols and alkynes. Moreover, alkynes can even displace alcohols of similar molecular weight (acetylene vs. methanol; phenylacetylene vs. phenol). The results indicate that a gold catalyst facilitates the formation of acetylide species and coupling products, even though their strong interaction with the surface may be counterproductive, as it results in their decomposition. Graphical Abstract: [Figure not available: see fulltext.].