Peeking More Closely At Gold Catalysis

By employing a cyclopropene rearrangement reaction, chemists have uncovered more about the types of intermediates that participate in gold catalysis. Researchers often invoke electron-poor intermediates such as carbenes or cations to explain the mechanisms of gold catalysis, but such intermediates have tended to elude characterization. Günter Seidel, Richard Mynott, and Alois Fürstner of the Max Planck Institute for Coal Research, in Mülheim, Germany, report their efforts to generate gold carbenes by opening a disubstituted cyclopropene with a gold catalyst (Angew. Chem. Int. Ed., DOI: 10.1002/anie.200806059). This type of rearrangement has been used to characterize carbene complexes of many transition metals. However, NMR spectra of the team's reactions were more consistent with a cationic species than a carbene. The authors caution that the oxygen atoms used to stabilize the intermediates could bias their system toward cationic forms. Nonetheless, these results, together with their prior reactivity data (Angew. Chem. Int. Ed. 2008, 47, 5030), lead them to conclude that evidence for cationic behavior should be considered when evaluating mechanisms for gold catalysis. The German team's results add to recent definitive observations of the two intermediate types by independent teams (J. Am. Chem. Soc. 2008, 130, 17642; Organometallics 2009, 28, 666).