Halonitrobenzenes catalyst effect

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... 

Some of other effective platinum catalysts are sulfided platinum on carbon120 or platinum catalysts with inhibitors such as bis(2-hydroxyethyl)sulfide,121 morpholine,122 polyamines,123 phosphorous acid,105 phosphoric acid,124 and dicyandiamide.96 Dicy-andiamide was originally used as an effective inhibitor for Raney Ni, as described above (see eq. 9.50).113 Hydrogenations of halonitrobenzenes with use of these platinum catalysts are summarized in Table 9.5. In one patent, it is claimed that ethano-lamine is a better inhibitor than morpholine for Pt-C. Thus, 3,4-dichloronitrobenzene was hydrogenated over Pt-C modified with iron oxide in the presence of 1.2 mol% ethanolamine to give 3,4-dichloroaniline containing only 235 ppm of 4-chloroaniline, compared to 548 ppm with morpholine as the inhibitor.125... 

---