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

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.