Scheme 1.

Mechanistic proposal presenting combined chemical and ultrasonic reduction of chloroaurate. A) Ligand replacement: negatively charged oxygen atom from carboxylate group in citric acid, substitutes a chloride ligand of tetrachloroaurate complex anion (with formal loss of a molecule of chlorhydric acid equivalent). B) Au (III) coordination-sphere reorganization (adduct I → II): it is a two-step representation of the necessary assembly about electron arrangement to allow the reductive elimination step. C) Reductive elimination: the two electrons are constituting the O-Au (III) bond will be transferred to the core–shell of the gold atom, passing from +3 → +1 formal oxidation state in adduct II. D) Oxidative elimination: the carbon dioxide loss from adduct II generate 3-oxoglutaric acid as a citric acid byproduct. Homolysis: homolytic cleavage of solvent generates a local accumulation of high-energy species, such as hydroxyl and hydrogen radicals capable of reacting in the periphery of cavitation bubbles.