Observing Initial Steps in Gold‐Catalyzed Alkyne Transformations by Utilizing Bodipy‐Tagged Phosphine–Gold Complexes

The Pd‐catalyzed reactions of 3‐chloro‐bodipy with R₂PH (R=Ph, Cy) provide nonfluorescent bodipy–phosphines 3‐PR₂–bodipy 3 a (R=Ph) and 3 b (R=Cy; quantum yield Φ<0.001). Metal complexes such as AgCl( 3 b )] and AuCl( 3 b )] were prepared and shown to display much higher fluorescence (Φ=0.073 and 0.096). In the gold complexes, the level of fluorescence was found to be qualitatively correlated with the electron density at gold. Consequently, the fluorescence brightness of AuCl( 3 b )] increases when the chloro ligand is replaced by a weakly coordinating anion, whereas upon formation of the electron‐rich complex Au(SR)( 3 b )] the fluorescence is almost quenched. Related reactions of AuCl( 3 b )] with Ag]ONf)] (Nf= nonaflate) and phenyl acetylenes enable the tracking of initial steps in gold‐catalyzed reactions by using fluorescence spectroscopy. Treatment of AuCl( 3 b )] with Ag(ONf)] gave the respective Au(ONf)( 3 b )] only when employing more than 2.5 equivalents of silver salt. The reaction of the “cationic” gold complex with phenyl acetylenes leads to the formation of the respective dinuclear cationic {( 3 b )Au}₂(CCPh)]⁺ and an increase in the level of fluorescence. The rate of the reaction of Au(ONf)( 3 b )] with PhCCH depends on the amount of silver salt in the reaction mixture; a large excess of silver salt accelerates this transformation. In situ fluorescence spectroscopy thus provides valuable information on the association of gold complexes with acetylenes.