Hypervalent iodine(III) oxidants in gold catalysis

Abstract

Hypervalent iodine oxidants have been used extensively in oxidative metal catalysis, especially in gold catalysis, where the redox potential of gold(I) to gold(III) is large. The reoxidation of gold(I) within an intramolecular gold-catalysed direct arylation reaction developed previously in the group occurs after the turnover-limiting step, and so cannot be studied by analysing the reaction rates. Competition reactions between ArIIII(OR)2 oxidants with different electronic parameters have been carried out to gain further understanding into the reoxidation of gold(I) under the following reaction conditions.

It was found that the more electron-rich oxidant has a higher relative rate compared to electron poor oxidants and log krel gave the best correlation with σ+ of the para-substituent. This suggests that the oxidant undergoes an initial ligand dissociation step to give an iodonium species before the gold is oxidised, as opposed to forming a π-complex with the gold(I) intermediate. Contrary to this result, the relative redox potential of these oxidants, calculated by looking at the equilibrium position between an iodine(III) oxidant and an aryl iodide showed that the more electron-deficient oxidant has a higher redox potential, and is therefore the more oxidising oxidant when compared to electron-rich oxidants. This trend has also been illustrated by looking at the electrochemical reduction of the oxidants using square-wave voltammetry.