Breaking C–F Bonds

BRANDEIS UNIVERSITY researchers report that a robust silylium-carborane catalyst can transform notoriously strong, unreactive carbon-fluorine bonds into carbon-hydrogen bonds under mild conditions (Science 2008, 321, 1188).

The stable, inert C–F bonds that coat nonstick frying pans are what allow fried eggs to slip off, but the bonds are also why fluorocarbon-containing molecules cause concern as greenhouse gases and persist in the environment.

The team's results could lead to a strategy for disposal or recycling of fluorocarbons. Most of the prior work in C–F bond activation has used transition-metal catalysts or strong reducing agents with limited success.

"The key difference in our approach is in how we set out to break a C–F bond," says Oleg V. Ozerov, an associate professor of chemistry at Brandeis. Rather than delivering electrons to the C–F bond through either simple electron transfer or oxidative addition to a metal center, Ozerov and colleagues use an extremely strong silylium Lewis acid to abstract the fluoride.

The researchers previously showed that a silylium catalyst could break C–F bonds, but the catalyst didn't last very long. In the new work the catalyst turnover is "orders of magnitude higher," Ozerov says.

In the longer lived catalyst that he and postdoc Christos Douvris used, the silylium cation is coupled to a weakly coordinating carborane anion. The catalyst was first created by Douvris' former adviser, Christopher Reed, a chemistry professor at the University of California, Riverside.

"Normally, the counteranion in these catalysts would get in the way by binding to the Lewis acid and quenching its reactivity," says chemist Robert H. Crabtree of Yale University. But incorporating this particular carborane counteranion sustains high Lewis acidity and prevents the catalyst from degrading, so it turns over more than a thousand times, says Russell P. Hughes, a chemistry professor at Dartmouth College.

The catalyst pulls a fluoride from the fluorocarbon and swaps it for a hydride from a trialkylsilane, making C–H and Si–F bonds. That the silane can regenerate the catalyst in a single step by delivering hydride is a neat idea, says Robin N. Perutz, professor of chemistry at the University of York, in England.

Douvris and Ozerov tested their reaction on three fluorocarbons and found that it is completely selective for aliphatic C–F bonds.

Ozerov says his team has had success with converting partially fluorinated alkanes to hydrocarbons but acknowledges that the process so far fails to break C–F bonds in fully fluorinated alkanes, which have high global warming potential.

"In addition to prospects for more satisfactory disposal of fluorocarbons, there are exciting possibilities for applying these principles in synthesis," Perutz writes in a commentary accompanying the Brandeis team's report.