Hydrogenation, catalytic nitrocompounds

Though the hydrogenation of the double bond is one of the basic catalytic reactions in Pd chemistry, httle has been published on aqueous variants of this process. Hydrogenation of o ,j8-unsaturated aldehydes in recyclable biphasic systems in the presence of TPPTS complex of palladium has been studied. - Also, a biphasic hydrogenation of nitrocompounds in the presence of PdCla and TPPTS under mild conditions has been reported. Room temperature homocoupling of aryl iodides was achieved in neat water or water-acetone mixture in the presence of Pd/C or Pd(OAc)2, and Zn dust (Scheme 87). The addition of 18-crown-6 was required for the reaction in neat water. The reactions are run under aerobic conditions. The effect of phosphine is negative. - ... 

It is worth noting, however, that up until fairly recently the reduction of aromatic nitrocompounds to the corresponding amines was commonly performed on an industrial scale, e.g. in the synthesis of azo dyes, with a mixture of iron and hydrochloric acid. This so-called Bechamp reduction has an E factor of ca. 15 compared to 1 for catalytic hydrogenation. 

HTC (2) [Hydrogen transfer catalysis] A catalytic process for reducing aromatic nitrocompounds. Developed by Rohner in 1993. 

In our laboratories extensive studies on the catalytic hydrogenation of aromatic nitrocompounds, as an example of the catalytic three-phase reactions, have been carried out in reactors of different types - e g. see [8-10]. In all cited cases the time consumed for kinetic investigations had a very significant contribution to the total experimental effort [11-13]. Particularly for the hydrogenation over palladium on alumina catalyst, the experimental investigations leading to the detection and quantitative description of internal diffusion resistances in catalyst pellets have taken a lot of time. 

Silica gel-supported, cyclized polyacrylonitrile metal complexes as well as PMAA-Pd and PMAA-Pt complexes were prepared by copolymerization of MAA and a mixture of m- and p-DVB in the presence of silica gel [131]. The catalytic activity of the obtained complexes was examined in the reaction of cumene and ethylbenzene hydrogenation [131] and in the hydrogenation of aromatic and aliphatic nitrocompounds, alkenes and aliphatic aldehydes. Based on the decrease in the bond energy of Cu2p3/2 from 935 to 932.4 eV and the increase in the bond energy of Ajs from 399 to 399.6 eV, it was found, by XPS, that Cu is bound to a complex through a nitrogen atom of PAN. 

August 1997 A method is described for the catalytic hydrogenation of aromatic nitrocompounds to aromatic amines in a solvent under SCF conditions. 

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