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Halide-Dependent Mechanisms of Reductive Elimination from Gold(III)

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posted on 2015-06-24, 00:00 authored by Matthew S. Winston, William J. Wolf, F. Dean Toste
Two unique organometallic halide series (Ph3P)­Au­(4-Me-C6H4)­(CF3)­(X) and (Cy3P)­Au­(4-F-C6H4)­(CF3)­(X) (X = I, Br, Cl, F) have been synthesized. The PPh3-supported complexes can undergo both Caryl–X and Caryl–CF3 reductive elimination. Mechanistic studies of thermolysis at 122 °C reveal a dramatic reactivity and kinetic selectivity dependence on halide ligand. For X = I or F, zero-order kinetic behavior is observed, while for X = Cl or Br, kinetic studies implicate product catalysis. The selectivity for Caryl–CF3 bond formation increases in the order X = I < Br < Cl < F, with exclusively Caryl–I bond formation when X = I, and exclusively Caryl–CF3 bond formation when X = F. Thermodynamic measurements show that Au­(III)–X bond dissociation energies increase in the order X = I < Br < Cl, and that ground state Au­(III)–X bond strength ultimately dictates selectivities for Caryl–X and Caryl–CF3 reductive elimination.