Ipso-substitution--the hidden gate to xenobiotic degradation pathways

Screening for metabolites of environmental pollutants, the focus is often on transformation products based on well-known pathways. Hydroxylation at unsubstituted positions of the aromatic ring or side chain modifications, followed by meta ring-cleavage pathways are usually considered, whereas less obvious mechanisms are often ignored. Here, a glimpse of the multitude of transformations involving ipso-substitution events, which are often overlooked as such, is presented. These reactions can be catalyzed by a variety of enzymes, proceed via several mechanisms and will often result in metabolites that are not expected to arise from generally known pathways. Hence, there is a future need when looking into transformations of emerging pollutants, to stray from the 'beaten pathways' and explore the possibilities of less obvious mechanisms.