Credit: © 2008 American Chemical Society

Oxonium ions contain a positively charged oxygen atom that forms three bonds rather than the usual two it forms when it is neutral. Usually oxonium ions do not exist for long, and those that can be isolated are often highly reactive salts that can be used as alkylating agents. Now, Mark Mascal and colleagues at the University of California Davis have produced two remarkably stable oxonium ions1.

Both of the new ions have a fused tricyclic framework comprised of three rigid five-membered rings with a triple-bonded oxygen atom at the centre. The synthesis, in each case, begins with cyclononatriene, which can be transformed in three steps into an intermediate compound from which each oxonium ion can be made through a different sequence of reactions.

The first new ion, oxatriquinane, has a saturated hydrocarbon framework and was found to have longer carbon–oxygen bonds with smaller C–O–C angles between them than other examples of alkyloxonium ions previously reported. The unusual stability of the oxatriquinane ion was demonstrated by showing that it did not decompose when chromatographed on silica gel or when heated under reflux for 72 hours in water.

The second ion, oxatriquinacene, is the allyl equivalent of oxatriquinane, in which the hydrocarbon skeleton is made up of alternating single and double carbon–carbon bonds. This oxonium ion is more reactive than its saturated counterpart, but Mascal and co-workers suggest that, along with its synthetic precursors, this is the first example of a stable allyloxonium ion. In future, the researchers hope to oxidize oxatriquinacene to give so-called oxaacepentalene, which is predicted to have more aromatic character than benzene.