Metal-Stabilized o-Quinone Methides, Thioquinones, and Selenoquinones: Trapping Important Reactive Intermediates and Beyond

Quinone methides (quinomethanes) act as important intermediates in organic syntheses, as well as in chemical and biological processes. However, examples of such isolated species are scarce owing to their high reactivity. Other important reactive intermediates are thio- and selenoquinones. However, unlike quinones, they are highly unstable and consequently do not exist in nature, hence their chemical and biological properties remain unknown. In this account, we describe the first isolation of the above important reactive intermediates, by pi-coordination to a (pentamethylcyclopentadienyl) iridium (Cp*Ir) moiety, and their X-ray molecular structures. Further, the reactivity of such trapped intermediates is reported. For instance, the metalated o-quinone methide underwent an unusual cycloaddition reaction compared with the free o-quinone methide, while the metalated thioquinones were successfully used as organometallic linkers (OM-linkers) to construct luminescent coordination assemblies. The anticancer activity of the selenoquinone metalated complex was studied, together with related oxygen and sulfur analogues, and remarkably it was shown to be the only cytotoxic compound with a comparable activity to cisplatin.