The group 11 metals have an important place in organic chemistry, not least since the discovery of Cu-catalysed couplings of aryl halides with diverse nucleophiles. Yet, Cu aryls are labile and prone to redox, so we know little about oxidative addition or reductive elimination involving these intermediates. The intractable nature of some first- and second-row transition metal catalysts is often overcome by studying third-row congeners, and indeed Au complexes are informative models as well as being catalysts in their own right. In search of mechanistic clues, Qilong Shen, Yu Lan and colleagues prepared Auiii aryls and describe in Organometallics how reductive elimination of aryl halides depends on the halide and supporting ligands.
The first clues to the reactivity of [(RtBu2P)Auiii(ArF)ClX] came when attempting substitution of [(tBu3P)Auiii(ArF)Cl2] with I−, whereupon the putative mixed halido product [(tBu3P)Auiii(ArF)ClI] rapidly eliminated ArFI to afford [(tBu3P)AuiCl]. Given that the other mixed halide complexes are isolable, this shows how heavy halogens (I versus Cl or Br) and phosphine bulk (tBu3P versus ArtBu2P) can accelerate reductive elimination. Thus, the dichloro [(tBu3P)Auiii(ArF)Cl2] smoothly eliminates ArFCl at 100 °C, while the less bulky species [(ArtBu2P)Auiii(ArF)Cl2] is robust even at 150 °C. Replacing one Cl− ligand with Br− or I− gives more reactive complexes, to the point where [(RtBu2P)Auiii(ArF)ClI] eliminates ArFI (not ArFCl) cleanly even when R = Ar — electron-poor Ar groups render the Auiii centre particularly hungry for electrons.
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