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Taming Alkyl Oxonium Ions

Fused tricyclic structure stabilizes famously reactive alkylating agents

by Rachel Petkewich
September 29, 2008 | A version of this story appeared in Volume 86, Issue 39

A TEAM OF CHEMISTS from the University of California, Davis, has synthesized and isolated oxatriquinane and oxatriquinacene, the first water-stable versions of normally highly reactive alkyl oxonium ions (J. Am. Chem. Soc., DOI: 10.1021/ja805686u). The achievement could lead to a deeper understanding of chemical bonding and reactivity.

Alkyl oxonium ions are widely known as reactive intermediates in organic reactions. The few oxonium ions that have been isolated and purified are paired with inert counterions and considered among the most powerful alkylating agents in the laboratory. For example, the commercial reagent trimethyloxonium tetrafluoroborate is so reactive it quickly degrades in water and most organic solvents, even at low temperatures.

Associate professor of chemistry Mark Mascal and colleagues at UC Davis synthesized the two water-stable alkyl oxonium ions from 1,4,7-cyclononatriene. They found that oxatriquinane does not react with hot water or room-temperature alcohols, alkyl thiols, halide ions, or amine bases. They successfully recorded an NMR spectrum of the oxatriquinane ion in D2O, recrystallized it from water, and purified salts of the ion by column chromatography.

Oxatriquinacene, the first stable allylic oxonium ion, is more reactive than oxatriquinane yet significantly more stable than simple alkyl oxonium salts. Mascal attributes the stability of the two ions to their rigid trefoil construction of five-membered rings.

"This lack of reactivity would be unheard of for any other oxonium ion," says Michael M. Haley, a chemistry professor at the University of Oregon. "What we learn at this basic level of science helps us refine our understanding of bonding and reactivity in chemistry."

Thomas W. Bell, a chemistry professor at the University of Nevada, Reno, says, "The discovery is not only a landmark in organic chemistry, it is very significant to many subfields of chemistry and biochemistry." He adds that the method used in the new study may stabilize other reactive intermediates that cannot be carefully investigated and understood in any other way.

Mascal says his group now plans to try stabilizing other reactive molecules such as oxonium ylides and possibly a novel aromatic oxaacepentalene.



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