“…In the absence of firm experimental and theoretical proofs, the generally accepted plausible mechanism is based on the catalytic Cu(I) acetylide as the reactive species, which undergoes cycloaddition, followed by protonation of the cuprated cycloadduct. 22,24,28,29,31,[33][34][35]37,38,42,45,46 This reaction pathway is supported by experiments performed with copper(I) acetylides, deuterated acetylenes, and/or proton sources, such as D 2 O and AcOD. 29,[33][34][35]38 Copper(I) was, either used directly, 29,33 or formed in situ by reduction of Cu(II) 34,35 or by oxidation of Cu(0).…”