An Interesting Stable Oxonium Ion

The textbooks all say that hydronium ion is a strong acid, and the alkyl analog should also be very reactive. However, oxatriquinane turns out to be quite stable. One might think that it should be a powerful electrophile and react with even weak nucleophiles like water, but when boiled in water for 72 hours there was still no sign of decomposition.

Another thing I found rather satisfying is that all the chemistry used to prepare this compound is routine for a sophomore organic chemistry class. It starts with treating cyclonona-1,4,7-triene with MCPBA to form an epoxide. The epoxide is opened by reacting with LiAlH4 ( a nucleophilic hydride ion) . Reacting the resulting alcohol with iodine (I2) leads to an intramolecular halohydrin reaction. The iodine is removed with Raney Nickel. Finally, the compound is treated with HBr which protonates the remaining alkene. The oxygen bonds to the resulting carbocation to give the bromide salt of oxatriquinane.

The bromide salt could not be crystallized, so they used simple ion exchange to replace it with hexafluorophosphate ion or hexafluoroantimonate. The ion proved quite stable - they were able to measure the NMR in D2O, and even recrystallize from water. Weak nucleophiles such as water, alcohols or iodide ion did not react with oxatriquinane. On the other hand, nucleophiles like hydroxide, cyanide and azide quickly reacted.

Mark Mascal, Nema Hafezi, Nabin K Meher, James C Fettinger(2008) Oxatriquinane and Oxatriquinacene: Extraordinary Oxonium Ions J. Am. Chem. Soc., 130(41), 13532-13533. DOI: 10.1021/ja805686u