Unique distal size selectivity with a digold catalyst during alkyne homocoupling

Metal-catalysed chemical reactions are often controlled by steric hindrance around the metal atom and it is rare that substituents far away of the reaction site could be differentiated during reaction, particularly if they are simple alkyl groups. Here we show that a gold catalyst is able to discriminate between linear carbon alkynes with 10 or 12 atoms in the chain during the oxidative homocoupling of alkynes: the former is fully reactive and the latter is practically unreactive. We present experimental evidences, which support that the distal size selectivity occurs by the impossibility of transmetallating two long alkyl chains in an A-framed, mixed-valence digold (I, III) acetylide complex. We also show that the reductive elimination of two alkyne molecules from a single Au(III) atom occurs extremely fast, in <1 min at −78 °C (turnover frequency>0.016 s−1). Steric hindrance proximal to a metal centre is known to play a key role in reactivity during catalysis. Here, the authors study reductive elimination from gold metal centres, surprisingly showing that small changes in sterics at sites removed from the metal centre can have dramatic effects on reactivity.