Gold catalysis is one of the hot topics in current organic synthesis. The “gold-rush” started with the discovery that seemingly inert gold can efficiently catalyze the addition of nucleophiles, such as water or methanol, to alkynes.[1, 2] Nowadays is gold efficiently used for the construction of complex structures through reaction cascades based on π activations of multiply unsaturated molecules.[3–11] The key steps are often based on couplings between a CÀC triple bond and a CÀC double bond. Couplings between two CÀC triple bonds are usually initiated by the addition of an alcohol to one of the triple bonds.[12–14] The generally accepted reaction mechanism for the gold (I)-mediated addition of either water or methanol to an alkyne involves coordination of cationic gold to the CÀC triple bond, which promotes the addition of the nucleophile.[15–18] The addition is completed by the migration of a proton bound to the oxygen atom of the incoming nucleophile to the second carbon atom of the multiple bond. The proton migration is assisted by solvent molecules.[15] Herein, we report an investigation of the gold (I)-mediated addition of methanol to an alkyne by means of electrospray ionization mass spectrometry (ESI-MS),[19] NMR experiments, and theoretical calculations. Furthermore, the key reaction intermediate is characterized by IR multiphoton dissociation (IRMPD) spectroscopy. The reaction was investigated for 1-phenylpropyne, PhÀC CÀMe, with [AuCl (PMe3)] as the catalyst.[20]

Electrospray ionization of a methanolic solution of 1-phenylpropyne and [AuCl (PMe3)] leads to the [Au (PMe3) 2]+ cation and its cluster bound by the chlorine counterion,[Au2Cl (PMe3) 2]+(Figure1a; for an assignment of minor peaks, see the Supporting Information). Furthermore, two complexes containing the alkyne can be detected:[Au-(PhCCMe)(PMe3)]+ and a complex X with m/z 693. Collision-induced dissociation experiments with mass-selected X (Supporting Information, FigureS1) as well as analogous experiments in [D4] methanol (Supporting Information, FigureS4) show that X contains 1-phenylpropyne, two gold atoms with two trimethylphosphine groups, and a methoxy group. If this ion were a simple cluster of two [Au (PhCCMe)-(PMe3)]+ cations bound by a methanolate counterion, a cluster bound by chloride would also be expected, which is however not present in the spectrum. Therefore, the complex X may correspond to a possible intermediate. In minor abundance, a complex between the (trimethylphosphino) gold cation and the product of the addition of methanol to 1-phenylpropyne is detected, which is denoted as Y (m/z 421; addition of methanol to 1-phenylpropyne can lead to (E)/(Z)-2-methoxy-1-phenylpropene or (E)/(Z)-1-methoxy-1-phenyl-