Transition metal cations catalyze 16 O/ 18 O exchange of catechol motifs with H 2 18 O.

Catechol motifs are ubiquitous in nature, as part of plant, animal and microbial metabolites, and are known to form complexes with various metal cations. Here, we report for the first time that complexation with transition metal cations, especially Fe(III), results in rapid O/O exchange of the catecholic hydroxyl groups with HO. We discuss the implications of this finding for mechanistic studies using HO and potential relevance for production of O-labeled catechol derivatives.