sigma-pi-Diauration as an alternative binding mode for digold intermediates in gold(I) catalysis

While investigating the gold(I)-catalyzed intramolecular hydroarylation of allenes, the structure of a digold-vinyl intermediate was verified. Instead of the previously proposed geminally diaurated binding mode for the digold when L = PPh3, an alternative sigma-pi-diauration mode was observed with the bulkier ligand L = P(o-Tol)(3). Reactivity studies indicate the sigma-pi-mode has a disproportionate effect on protonolysis reactivity.