Halonitrobenzenes hydrogenation, selective

Iridium,204,205 together with osmium, has been not widely used in catalytic hydrogenation. Recently, however, iridium or iridium-based catalysts have been shown to be effective in various hydrogenations, such as in selective hydrogenation of a,P-unsaturated aldehydes to allylic alcohols (Section 5.2), of aromatic nitro compounds to the corresponding hydroxylamines (Section 9.3.6), of halonitrobenzenes to haloanilines without loss of halogen (Section 9.3.2), and in the stereoselective hydrogenation of carbon to carbon double bonds (see, e.g., eqs. 3.25-3.27 and Table... 

Platinum catalysts have been shown to be highly selective for the hydrogenation of halonitrobenzenes to haloanilines. A number of effective platinum catalysts or catalyst systems have been described in the literature, mostly in patents.96 Dovell and Greenfield found that the sulfides of the platinum metals and cobalt were highly selective in the hydrogenation of halo-substituted nitrobenzenes.117-119 There was no detectable dechlorination with the sulfides of palladium, platinum, rhodium, ruthenium, and cobalt no detectable debromination occurred with platinum sulfide trace debromination occurred with rhodium sulfide and cobalt sulfide and appreciable debromination occurred with palladium sulfide. Typical hydrogenations with 5% platinum sulfide on carbon catalyst are given in eqs. 9.52 and 9.53 with 2,5-dichloronitrobenzene and p-bromobenzene, respectively.118... 

Finally, the addition of modifiers can improve the performance of a catalyst. These modifiers depend largely on the reaction. Since there are currently many commercial catalysts that include modifiers, it is not possible to give a comprehensive overview in this chapter. The authors would just like to mention one example where copper is used as a modifier for the selective hydrogenation of halonitrobenzenes to the corresponding anilines without dechlorination [50]. A few examples are given in detail in Section 15.6. 

Although it may seem that homocoupling should only be useful for the synthesis of symmetrical products, useable yields of unsymmetrical products have been obtained. For instance, o-halonitrobenzenes 2.419 can be selectively coupled with a-haloenones 2.420 in the presence of copper dust (Scheme 2.133). Hydrogenation of the coupling product provides a short synthesis of indoles 2.422. 

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