Selective reductions of haloaryl nitro compounds

10 July 2008

In a pair of separate publications two different groups report methods for achieving the reduction of haloaryl nitro compounds to the corresponding haloanilines without dehalogenation. This synthetic strategy is a very effective means to generate haloanilines and often, owing to the dehalogenation, the more classical non catalytic methods such as iron in HCl have to be employed.

One group (Y Motoyama et al, Org Lett, 2008, 10, 1601) describes the use of carbon nanofibre-supported platinum or palladium catalysts. They report <2% dechlorination for the o-chloro isomer and <0.1% for both p- and m-chloro isomers. The p-bromo and p-iodo analogues show up to 6% dehalogenation when using platinum. Other susceptible groups such as alkene, ketone or O-benzyl were essentially unreactive.

The second group (J Jiang et al, Green Chem, 2008, 10, 439) describe the use of iron in water containing carbon dioxide at up to 70bar pressure based on the observation that carbon dioxide when dissolved in water results in a pH of about 2.9 across a wide range of pressures and temperatures. In a typical procedure, they charge the nitroarene, iron (3-6 equivalents) and water to an autoclave, seal it and pressurise the unit with carbon dioxide to about 5bar. The mixture is then heated to 120°C and further pressurised with carbon dioxide to about 70bar as required. After 8h, the mixture is cooled and the pressure released slowly before the product is extracted from the aqueous mixture.

Both of the above methods provide interesting and potentially useful additions to the existing means to secure haloanilines.