September 29, 2008

Volume 86, Number 39

p. 10

Organic Chemistry

Taming Alkyl Oxonium Ions

Fused tricyclic structure stabilizes famously reactive alkylating agents

Rachel Petkewich

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