Halonitrobenzenes, 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. 

The effect of substituents on the rate of addition of carbanions to nitroarenes and the rate of p-elimination of HL from the o adducts have also been studied [8, 30, 31]. The former effect is an important parameter, because it is, in fact, a measure of influence of substituents on electrophilic activity of nitroaromatic rings. The effect of substituents on rate of the S Ar reactions of o- and p-halonitrobenzenes has been thoroughly studied [2, 32]. However, since the S Ar of halogen is a secondary process, the obtained data cannot be used as a real measure of electrophilicity of halonitroarenes. We have determined the effects of substituents and the ring structure on the rate of the VNS reactimi of nitroarenes with the carbanion of chloromethyl phenyl sulfone by using competitive experiments under the conditions, which assure a fast p-eUmination of HL from the o adducts [30, 31]. The values of VNS rates obtained under such conditions proved to correlate with those of the addition step. Selected values of the relative rate constants in relation to nitrobenzene as the standard are shown in Fig. 1. 

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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