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

Abstract

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 p-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.

A Convenient Approach to Luminescent Cyclometalated Platinum(II) Complexes with Organometallic π‐Bonded Benzenedithiolate

“…Our group has shown that organometallic moieties such as Cp*M (Cp* = pentamethylcyclopentadienyl, M = Ru, Rh and Ir) can stabilize reactive intermediates by modifying their electronic properties . More recently we have reported the synthesis of organometallic π‐bonded quinones [Cp*M(η‐C 6 H 4 O 2 )] n (M = Ru, n = –1; M = Rh, Ir, n = 0) and their usage as organometallic ligands for the preparation of a unique type of supramolecular assemblies with useful luminescence properties .…”

Peltomexicanin, a Peltogynoid Quinone Methide from Peltogyne Mexicana Martínez Purple Heartwood

“…Therefore, the interest on the development of methods for quinone methides synthesis has been increased in the recent years [14], although simple quinone methides, such as, those without substituents at the exocyclic methylene group, are unstable compounds. Because of that, many efforts have been focused on their stabilization, mainly form quinone methide coordination complexes with transition metals [15,16,17]. To the best of our knowledge, however, none of the peltogynoid quinone methides have yet been characterized.…”

Planar Chiral Iridium Complexes with the Δ‐TRISPHAT Anion: Toward the First Enantiopure o‐Quinone Methide π‐Complex

“…[1][2][3][4] They usually behave as heterodienes in Diels-Alder [2 + 4] cycloadditions with olefins to provide chroman type structures. [5][6][7][8][9][10] The high reactivity of o-quinone methides is illustrated, whereby the spectroscopic characterization of the parent molecule C 7 H 6 O (1), generated in situ, occurs only at temperatures below -100 C. 2,11 In contrast, we reported some years ago the first example where this elusive molecule (1) can be stabilized and isolated as an iridium complex pR-[Cp*Ir(Z 4 -C 7 H 6 O)] (4a) and pS-[Cp*Ir(Z 4 -C 7 H 6 O)] (4a'). [12][13][14] In these compounds the o-quinone methide ligand is now p-coordinated to the metal center through the internal diene carbons.…”