Targeted Hydrogenations

Magic bullets havea knack for zeroing in on just the right target even when the target area is crowded. Chemical reagents, in contrast, tend to have less perfect aim and sometimes hit multiple targets, thereby forming unwanted by-products.

One way to improve a reagent's chances of hitting the bull's-eye every time is formulating a catalyst that facilitates reactions selectively. That's just what a team of scientists in Spain has done in the case of nitro-group hydrogenation reactions. The researchers report that gold nanoparticles supported on titania or iron oxide selectively convert nitro groups to amino groups in organic molecules featuring more than one reducible functional group (Science 2006, 313, 332).

Compounds in the aniline family serve as key intermediates in the production of pharmaceutical agents, agrochemicals, dyes, and other materials. Typically, aromatic amines are produced from nitro compounds through commercial-scale catalytic hydrogenation reactions. For simple compounds, the transformation is straightforward. But for molecules containing C=C, C≡C, C=O, C=N, or C≡N groups in addition, hydrogenation reactions tend to be nonselective and can lead to large amounts of waste.

Now, Avelino Corma, a professor at the Institute of Chemical Technology, Polytechnic University of Valencia, in Spain, and graduate student Pedro Serna have demonstrated that even with multifunctional starting materials, selective hydrogenations are possible.

Structure (right)

Choosy
Hydrogenating multifunctional molecules in the presence of gold nanoparticles only affects nitro groups.

For example, the Valencia team treated 3-nitrostyrene with hydrogen under mild conditions (9 bar and 120 °C) in the presence of solid catalysts containing up to a few percent gold by weight. On the basis of gas chromatography/mass spectrometry measurements, they report that the reaction produced 3-vinylaniline with 96% selectivity and just a few percent 3-ethylaniline. The team measured an overall conversion of more than 98% of the starting material and only trace quantities of hydroxylamines, which are common problematic by-products in these types of reactions. Similar results were observed in the hydrogenation of other multifunctional nitro compounds, including 4-nitrobenzaldehyde, 4-nitrobenzonitrile (shown at left), 4-nitrobenzamide, and 1-nitro-1-cyclohexene.

In a commentary published in the same issue of Science, Hans-Ulrich Blaser, chief technology officer at Solvias in Basel, Switzerland, describes the advance as "promising," and finds it "remarkable" that by-products are formed at such low levels.

The gold catalyst is a welcome addition to the chemist's toolbox for selective reduction of nitro compounds containing multiple reducible groups, Blaser says. He notes that the mechanism needs to be established but adds that the study "should give strong stimulus to this area of catalytic chemistry."