Tungsten Breaks Tough Bond

A strong aromatic carbon-carbon bond can be cleaved with ease by a tungsten complex that inserts the metal between the two carbon atoms, report chemists at Columbia University (Nature 2010, 463, 523). The mechanism of this unusual bond breaking, which was observed in quinoxaline under mild conditions, could be extended to other systems, say the report’s authors, opening new avenues for functionalizing aromatic molecules.

Aaron Sattler and Gerard Parkin discovered the tungsten complex’s bond-breaking ability while searching for a compound that would cleave C–N aromatic bonds. They had been working with molybdenum complexes but decided to switch to tungsten, which is a more aggressive metal. Sattler and Parkin were surprised to find that in the presence of the N-heterocyclic molecule quinoxaline, the tungsten complex breaks the aromatic C–C bond adjacent to the aromatic C–N bond, even though the C–N bond is typically more reactive.

“Carbon-carbon bond cleavage reactions are uncommon and are typically only observed when the C–C bond is held in close proximity to the metal center, or when the cleavage is accompanied by relief of strain energy or the formation of an aromatic system,” Parkin explains. “The most promising aspect of the study is, therefore, that this type of cleavage could be extended to other transition-metal compounds and other substrates, and thereby ultimately leads to a new way of functionalizing organic molecules.” The researchers have examined the reactivity of the tungsten complex with a few other aromatic compounds but have not observed the same C–C bond cleavage.

“Sattler and Parkin’s reaction is a molecular tour de force, although it is probably a long way from seeing any useful applications,” writes Rutgers University chemistry professor Alan Goldman in a commentary that accompanies the paper. Even so, he notes, “the unusual mechanism may inspire new routes to a more general cleavage of C–C bonds, applications of which can be envisaged, ranging from the syntheses of pharmaceutical compounds to the development of new catalysts for the hydrodenitrogenation of fossil fuels.”