Revisiting the Aufbau Reaction with Acetylene: Further Insights from Experiment and Theory

The first steps of acetylene chain growth at AlEt₃, via migratory insertion, have been investigated both experimentally and theoretically. The first insertion into the Al−Et bond occurs readily, leading to the stable alkenyl-bridged dimer [{Et₂Al(μ-CHCHEt)}₂] (1). The alkenyl bridging mode has been observed through isolation and structural analysis of Al₂Et₂(OC₆H₃Ph₂)₂(μ-CHCHEt)(μ-OC₆H₃Ph₂) (2), synthesized by way of controlled reaction of 1 with 2,6-Ph₂C₆H₃OH. This stable binding mode increases the barrier to a second insertion of acetylene, as insertion proceeds through monomeric Al-acetylene adducts. The energetics of further chain growth, dimer formation, and chain termination via hydrogenolysis were investigated theoretically. The results provide further insight and explanation for previous experimental findings in relation to the Aufbau reaction with acetylene.