Of nitro arenes

Scheme 1.10 Catalytic hydrogenation of nitro-arenes to give arylamines.

The use of nitro arenes as the nitrogen fragment donor in combination with CO under catalytic, and reductive, conditions has also been presented by Nicholas et al. who have found that [CpFe(CO)2]2 can be used as the catalyst [69]. A variety of different alkenes were tested, and a-methyl styrene gave the highest yield of the allyl amine 113 when nitrobenzene was used. The yield of the allyl amine depends markedly on the structure of both the alkene and the nitro compounds. 

Recently, it has been demonstrated that this anion is the active species in the reduction of the carbon-nitrogen triple bond in acetonitrile10 and it is proposed to be a key intermediate in the reduction of nitro arenes with triiron dodeca-carbonyl.11... 

The carbonylation of nitro arenes can lead to amines, isocyanates, carbamates, azo compounds, or ureas.80 Iron, ruthenium, rhodium, or palladium complexes have been used as catalysts. The carbonylation to give isocyanates, for example, involves the steps... 

FIGURE 33.15 Reductive biotransformation of nitro arene compounds. 

The aim of the work presented here was to examine the influence of additional substituents on the rate of the catal5dic reduction with hydrazine hydrate of nitro-benzenes and of a variety of nitro arenes carrying phenyl azo substituents. Particular interest was focused on the influence of substituents in nitroazo compounds on the selectivity for the reduction of the nitro group (1) vis-a-vis that of the azo bridge in reaction (2) or (3) (Scheme 1). 

Influence of substituents on the catalytic reduction of nitro arenes... 

The influence of substituents on the reduction rate of nitro arenes is most conveniently represented by the following Hammett ap-relationship, where ko = 1 mole/(hmin). 

Kinetic investigations have been reported for the hydrogenation of a variety of substrates [29,30]. Typically, the reaction is between zero- and first-order for hydrogen and, especially at higher concentrations of nitro arene, zero-order in substrate. A Langmuir-Hinshelwood approach (reversible adsorption of reactants and intermediates on the metal surface) is usually chosen for kinetic analysis, quite often with good agreement. 

As described below, modifiers and promoters can play an important lole in achieving highly selective hydrogenation of nitro arenes. For this reason, some ideas relating to their mode of action will briefly be discussed here. 

The catalytic hydrogenation of nitro arenes nevertheless has some peculiarities, especially process safety, that require thorough understanding of the technology. We strongly recommend that the development of production processes is performed by teams with good experience of catalysis or by service companies that offer the development of such catalytic reactions [32]. 

Although in all of Geoffroy s papers it was always remarked that no firm evidence existed that the reactions reported were of any relevance to the actual catalytic cycle, this prudent statement is not always found in other authors papers and seems anyway to have been overlooked by many readers, so that the involvement of imido clusters as intermediates in the catalytic carbonylation reactions of nitro-arenes catalyzed by Ru3(CO)i2 seems now to be taken for granted by many researchers in the field. 

Other isocyanate syntheses that have recently been reported include several well-known reactions. One area which has attracted considerable attention is that of the direct production of isocyanates by the carbonylation of nitro-arenes. Both mono- and di-isocyanates are claimed to have been produced using various catalysts palladium, rhodium, and iron compounds often being cited. Other preparative reactions for isocyanates which have appeared in the literature include the acid catalysed hydrolysis of isocyanide dihalides and the reaction between alkyl halides and alkali-metal cyanates, although the latter has been given a modern flavour by the use of a polymer-supported reagent. ... 

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