Hydrogen Peroxide and Tin Catalyst Combine to Minimise Waste in Baeyer-Villiger Reactions

Scientists in Spain have developed a catalyst that puts an environment friendly face on an important but prodigiously waste-producing industrial process.

The century-old Baeyer-Villiger reaction, in which ketones are oxidized to lactones or esters, typically via a peroxycarboxylic acid, is used to manufacture many compounds needed in everything from plastics manufacture to pharmaceutical syntheses. However, the reaction often leaves in its wake more reduced acid waste than product.

But now, scientists at University of Valencia have performed the reaction using hydrogen peroxide as the oxidizer. H2O2 has long been eyed as a potential benign industrial reagent, because its waste product is water. The key to the tamed Baeyer-Villiger reaction, though, is the catalyst, which is composed of a zeolite infused with tin, developed by the scientists. They used the catalyst peroxide combination to oxidizenumerous compounds - including damantanone, cyclohexanone, and dihydrocarvone-into their corresponding lactones with extraordinary selectivity.

In the Baeyer-Villiger reaction, a peracid attacks the carbonyl carbon of the ketone. For a number of years, scientists have known that H2O2 should be able to do the reaction with the help of a catalyst, but they have been frustrated in their attempts to find a suitable catalytic candidate.

A number of potential catalysts, including metal complexes, have not worked well. They are slow, or they tend to activate the hydrogen peroxide rather than the ketone. This leaves any oxidizable group on the ketone vulnerable to attack, producing a host of unwanted compounds.

The scientists selected tin, knowing that, as a Lewis acid, it would likely activate the ketone's carbonyl group. And the porous zeolite -tetraethylorthosilicate-provided an ideal framework for evenly distributing the tin.

Source: C & EN, July 30, 2001