Cyclization of nonterminal alkynic β-Keto esters catalyzed by gold(I) complex with a semihollow, end-capped triethynylphosphine ligand

(Chemical Equation Presented) A cationic gold(I) complex with a semihollow-shaped trialkynylphosphine catalyzed 5-exo-dig and 6-endo-dig cyclizations of various internal alkynic β-keto esters, showing a marked advantage over a gold(I) - PPh₃ complex with respect to the rates of the reactions and the product yields. It is proposed that the gold-bound alkynic substrate in a catalytic pocket must be somewhat folded and that such a steric effect makes the carbon-carbon bond formation entropically more favorable.