The mechanism of copper-mediated Sonogashira couplings (so-called Stephens–Castro and Miura couplings) is not well understood and lacks clear comprehension. In this work, the reactivity of a well-defined aryl-CuIII species (1) with p-R-phenylacetylenes (R=NO2, CF3, H) is reported and it is found that facile reductive elimination from a putative aryl-CuIII-acetylide species occurs at room temperature to afford the CarylCsp coupling species (IR), which in turn undergo an intramolecular reorganisation to afford final heterocyclic products containing 2H-isoindole (P, P, PHa) or 1,2-dihydroisoquinoline (PHb) substructures. Density Functional Theory (DFT) studies support the postulated reductive elimination pathway that leads to the formation of CCsp bonds and provide the clue to understand the divergent intramolecular reorganisation when p-H-phenylacetylene is used. Mechanistic insights and the very mild experimental conditions to effect CarylCsp coupling in these model systems provide important insights for developing milder copper-catalysed CarylCsp coupling reactions with standard substrates in the future.