Carbonylation of nitroarenes

The reductive carbonylation of nitroarenes with transition metal catalysts is a very important process in industry, as the development of a phosgene-free method for preparing isocyanate is required. Ruthenium, rhodium, and palladium complex catalysts have all been well studied, and ruthenium catalysts have been shown to be both highly active and attractive. The reduction of nitroarene with CO in the presence of alcohol and amine gives urethanes and ureas [95], respectively, both of which can be easily converted into isocyanates [3,96]. 

Ragaini and associates found that fluorides can promote the palladium-phe-nanthroline catalyzed carbonylation of nitroarenes to carbamates. The effect was more evident on the rate of the reaction at short reaction times, but a positive effect on selectivity is also observed under certain conditions. The effect was observed even under conditions in which chloride inhibits the reaction. The reason for these results might be that fluoride can avoid the formation of inactivating byproducts that deactivate the catalyst. Additionally, tetraethylammonium fluoride was found to be better than sodium fluoride. 

Besides bipyridine and phenanthroline, cheaper pyridine derivatives were also applied in the carbonylation of nitroarenes [26, 27]. The researcher synthesized a series of palladium chloride complexes with pyridines and applied the carbonylation of nitrobenzene and nitrobenzene/aniline and found that a more basic pyridine ligand gave better results. 

Ruthenium catalysts was explored and applied in the reduction of nitro compounds as well. But because of its high price, their large scale applications are limited. Cenini and colleagues reported a Ru3(CO)i2 and Ru(CO)3(PPh3>2 catalyzed carbonylation of nitroarenes [44, 45]. The corresponding carbamates are produced in high selectivity in the presence of NEt4Cl under 82 bar of CO. 

The effect of the chloride countercation on the mechanism of the Ru3(CO)i2/ chloride catalyzed carbonylation of nitroarenes to carbamates was investigated by them afterwards. The reason for the higher activity and selectivity obtained with... 

Moreover, selenium was also applied in the carbonylation of nitro compounds, even its toxic. Yu, Lu and colleagues found that selenium can catalyze the carbonylation of nitroarenes to symmetrical 1,3-diarylureas under atmospheric pressure of carbon monoxide [66]. In the presence of KOH or NaOAc as base, various... 

A number of studies of the important (non-phosgene) route to aryl isocyanates via carbonylation of nitro compounds have appeared. In a comparative study of Rh(I), Ir(I), Pd(I) and Pd(II) catalysts, Bu,N+[RhX2(CO)2] (X Cl, Br, I) was most effective giving 83-89% PhNCO with 100% PhN02 conversion at 125 C under 80 atm CO pressure . The kinetics of carbonylation by [PdCl2py2l at 170-230 C and 23-94 atm CO pressure are first order in p[CO] and catalyst and zero order in PhNOj . The reductive N-carbonylation of nitroarenes to the carbamates is catalysed by tPtCl2 (PPh3 >2] in ethanol, promoted by Lewis acids (SnCli,... 

In 1984, one of us first reported on the use of a Ru3(CO)i2-based catalytic system for the carbonylation of nitroarenes to carbamates [170]. This report was followed by several others, mostly from our and Grate s groups [171-176]. Since the catalytic systems are very similar, they are discussed together. Note that, although Ru3(CO)i2 is always used as a catalyst precursor, it is known that, under the reaction conditions, it is in equilibrium with the mononuclear Ru(CO)5 and it has become now clear that it is this last compound which is mainly responsible for catalytic activity [177]. However, Ru(CO)s is an unstable volatile liquid, which tends to aggregate to Ru3(CO)i2 within a few hours at room temperature and is thus an unsuitable starting catalyst for most applications. Anyway, in the only experiments we performed by using Ru(CO)s as catalyst, we obtained results essentially identical to those obtained under the same conditions... 

The chemistry involved in several selenium-based processes have been partly reviewed [218], but this review does not deal in detail with reductive carbonylation of nitroarenes. A more specific review on the use of selenium in... 

The reaction described in eq. 8 is promoted by bases, with tertiary amines being often used [220]. Since some base is always added in all catalytic reactions, it has also been suggested that H2Se is in equilibrium with the deprotonated forms HSe and Se and that these last species may be the ones responsible for nitroarene reduction [213], Since no mechanistic study has been conducted on the selenium-catalysed reductive carbonylation of nitroarenes, we will not discuss the details of the mechanistic pathway. However, the occurrence of the reactions described by eqs. 7-10 appears to be well based. The reduction of nitroarenes by CO/H2O (see Chapter 4) and the oxidative carbonylation of amines by CO/O2 [218] have both been reported to be catalysed by selenium under conditions similar to those employed in the reactions here discussed. 

The use of sulphur or sulphur compounds, even in the presence of a metal compound (most often vanadium), as a catalyst for the reductive carbonylation of nitroarenes have also attracted some attention [222-235]. Sulphur compounds are isoelectronic to their selenium analogues and some similarities in the reactivity of COS and H2S with respect to SeCO and H2Se is to be expected. Sulphur has the very important advantage of having a much lower toxicity than selenium, but the reactivity of its compounds is also much lower. As a consequence, several of the processes reported employ a stoichiometric amount of sulphur or, in the catalytic processes, the catalytic efficiency is extremely poor. Catalytic ratios around 2-4 are conunon and this number is never higher than 10. This implies than a very large amount of sulphur is always necessary and needs to be separated from the products and recycled. The form in which sulphur is present at the end of the process has never been reported. If oxygenated products are formed at least in part, as it is likely, their elimination may be costly or environmentally problematic. 

Evidence for the intermediate formation of nitrone species during the carbonylation of nitroarenes in e s-cyclooctene as solvent catalysed by Ru3(CO)i2 have been obtained [14], Moreover, zerovalent palladium species with nitrogen donor ligands have been shown to be active catalysts in the reductive carbonylation of organic nitro derivatives [41]. The hypothesis that an intermediate having the olefinic double bond coordinated to the metal is formed during the catalytic cycle is supported by the steric effect that has been observed in the case of p,p -dimethyl-o-nitrostyrene (7f) as substrate. Moreover, such an intermediate could explain why a pentaatomic indole nucleus is preferentially formed, even when a conjugated double bond is present in the olefinic chain ... 

During the reductive carbonylation of nitroarenes to carbamates catalysed by PdCb-Lewis acids, the nitroarene is first reduced to the corresponding aniline that is then carbonylated. Depending upon the conditions, more amine may be consumed than nitro compound. 

Some years ago, research in our group showed that Ru3(CO)i2, when activated by a tetralkylammonium halide, is a very active catalyst for the carbonylation of nitroarenes in the presence of alcohols to afford the corresponding carbamates [144, 173]. If no alcohol was present, only a very... 

Gasperini M, Ragaini F, Remondini C, (Taselli A, Cenini S (2005) The Palladium-phenanthroline Catalyzed Carbonylation of Nitroarenes to Diarylureas effect of Chloride and Diphenylphosphinic Acid. J Organomet Chem 690(20) 4517 529... 

---