Carbonylation of nitroaromatics

The synthesis and characterization of a family of metallacyclic complexes (147)- (150) of Pd, were reported. The complexes were isolated during the catalytic carbonylation of nitroaromatics... 

Hydrolysis of the ester forms adipic acid, used to manufacture nylon—6. Carbonylations of nitroaromatics are used to synthesize an array of products including amines, carbamates, isocyanates, ureas and azo compounds. These reactions are catalyzed by iron, ruthenium, rhodium and palladium complexes. For example, carhonylation of nitrobenzene in the presence of methanol produces a carbamate ... 

Wang et report the carbonylation of nitroaromatics with aromatic amines in a catalytic system containing elemental sulfur in an ionic liquid, Scheme 3. 

Carbonylation of Nitroaromatics Carbonylation of nitroaro-matics to isocyanates... 

A base reaction is also made responsible for the palladium(II)-catalyzed reductive carbonylation of nitroaromatics to isocyanates. Carhon monoxide affects the reduction step Pd ---> Pd° in protic media [14] according to eq. (9). The consecutive... 

The ideal method of reductive carbonylation of nitroaromatics would employ synthesis gas according to eq. (13) CO as a carbonylation reagent is cheaper in the form of syngas than pure CO, but it is more expensive as a reducing agent (eq. (8)) than hydrogen. Unfortunately, there is as yet no catalyst for the overall conversion of eq. (13) only the stepwise reduction works cata-lytically. 

Numerous patents [3, 11-13] and other publications [3, 14-16] describe the direct carbonylation of nitroaromatic compounds to isocyanates or alternatively a modified carbonylation to urethanes in the presence of alcohol, followed by a thermal transformation to isocyanates [4, 17-19] (eq. (2)). 

In the first reported direct A -carbonylation of nitroaromatics to isocyanates, simple Pd- or Rh-based systems were used to catalyze the reaction of aromatic mononitro compounds with carbon monoxide [11, 12]. Later, it became possible to work without the drastic reaction conditions that had been required initially, by using Lewis acid co-catalysts [13], Various catalysts and catalyst mixtures, normally based on Ru, Rh, or Pd complexes with co-catalysts, were described in numerous patents and publications [1, 3, 14—16], The careful choice of the composition of the triad consisting of metal salt, co-catalysts and ligand (preferably aromatic amines) led to efficient catalyst systems [14a-e] for the direct reductive carbonylation process. A quite active Pd-phenanthroline-H system with noncoordinating carboxylic acids such as 2,4,6-trimethlybenzoic acid as proton source is worth mentioning [14 d]. 

Summarizing, from the investigations in this field it can be concluded that from an industrial viewpoint the direct carbonylation of nitroaromatics to isocyanates represents no economically feasible alternative, for the conventional phosgenation process, for the following reasons ... 

In an early publication [16] the carbonylation of nitroaromatics was described as a stepwise deoxygenation of the nitro group, generating an excited singlet nitrene (probably stabilized by coordination on a metal center). Based on this description, the formation of a metal-imido intermediate was usually assumed in most of the proposed mechanisms until the mid-1980s [5, 34-38]. 

Conceptually, the indirect carbonylation of nitroaromatics can be pictured as a direct carbonylation reaction, followed by a scavenger reaction of the highly reactive intermediate isocyanate by the alcohol in a subsequent step before by-product formation comes into play. The latter is known to occur spontaneously at ambient temperature [48,49] and is catalyzed efficiently by many compounds having... 

J. Z. Chen, G. Ling, S. Lu, Synthesis of N-phenyl-N-pyrimidylurea derivatives by selenium-or selenium dioxide-catalyzed reductive carbonylation of nitroaromatics, Eur. J. Org. Chem. 17... 

Metal-catalyzed reductive carbonylation of nitroaromatics using CO has been the subject of intensive investigation in recent years because of the commercial importance of amines, urethanes, and isocyanates (1). Biphasic operation could offer interesting horizons regarding the ease of catalyst recycling. Thus, palladium catalysts have been applied in the presence of water-soluble ligands such as TPPTS or BINAS (2) for the carbonylation of substituted nitroaromatics (Scheme 1). 

Mechanistic Study of a Catalytic Process for Carbonylation of Nitroaromatic Compounds... 

Aromatic amines have been shown to be intermediates in the metal catalyzed carbonylation of nitroaromatics to aryl carbamates. Previous research established that the novel bis(methoxycarbonyl) complex, Ru(dppe)(C0)2[C(0)0Me]2, was the most abundant species present during catalysis. In this study, the complete kinetic analysis of the reaction of p-toluidine with Ru(dppe)(C0)2[C(0)0Me]2 established that the C-N bond formed by nucleophilic attack on a metal carbonyl, and that the organic product was removed from the metal by an intramolecular elimination of aryl isocyanate. 

Although thermodynamically favorable, the direct carbonylation of nitroaromatics, equation 1, does not occur in the absence of a metal catalyst. 

Anion Source for Palladium Catalysis. The reagent serves as a source of weakly coordinating anions in the palladium-catalyzed formation of mixed phenyl ureas, a known class of commercially available herbicides, using palladium(II) acetate, copper(II) toluenesulfonate, and 2,2 -dipyridyl as the catalyst system. Other studies have suggested that use of this reagent to form palladium salts may have useful applications in the reductive carbonylation of nitroaromatic compounds to give isocyanates via initial carbamate formation (eq 2). ... 

Type (C) coordination is not known and it should correspond to a real activation of the nitro group. This type of interaction has been proposed for the intermediate formed during the catalytic carbonylation of nitroaromatics, carried out in the presence of a complex in a low oxidation state with a Lewis acid as a cocatalyst (Scheme 6) [40,41] ... 

---