Catalytic reductive carbonylation nitrobenzenes

Transition metal alkoxycarbonyl compounds are key intermediates in a number of metal-catalyzed processes involving alkanols and carbon monoxide. The square-planar palladium(ii) complex, [Pd(phen)(G02GH3)2] 63, was determined in this context.A compound isolated as a plausible intermediate in the catalytic reductive carbonylation of nitrobenzene (viz. PhN02 + 3GO PhNGO + 2GO2) using palladium-phen-based catalysts was established by... 

The intercalated catalysts can often be regarded as biomimetic oxidation catalysts. The intercalation of cationic metal complexes in the interlamellar space of clays often leads to increased catalytic activity and selectivity, due to the limited orientations by which the molecules are forced to accommodate themselves between sheets. The clays have electrostatic fields in their interlayer therefore, the intercalated metal complexes are more positively charged. Such complexes may show different behavior. For example, cationic Rh complexes catalyze the regioselective hydrogenation of carbonyl groups, whereas neutral complexes are not active.149 Cis-Alkenes are hydrogenated preferentially on bipyridyl-Pd(II) acetate intercalated in montmorillonite.150 The same catalyst was also used for the reduction of nitrobenzene.151... 

Recent mechanistic studies were conducted by Ragaini et al. using tetracarbonyl rhodate complexes. Several potential intermediates such as (152) have been isolated.584 585 The catalytic system has been optimized and the influence of solvent was examined.586 2-Hydroxypyridine has a large activating effect on the [PPN][Rh(CO)4]-based catalytic system for the reductive carbonylation of nitrobenzene to Me phenylcarbamate 587... 

Terminal alkenes can be hydrogenated selectively in the presence of PdCI2 [63] or RhCl(PPhj)3 [64] and heteropoly compounds. The catalytic system is also highly active for the production of urethane or isocyanate compounds by the reductive carbonylation of nitrobenzene. It is considered that polyoxometalate coordinating with Pd2+ in the reduced form is the active species, since easily reducible heteropolyanions are more active [63]. 

Each of the catalysts listed earlier in the hydroformylation reaction are found to be catalysts for the reduction of nitrobenzene to aniline the rhodium, osmium, and ruthenium species are particularly eflFective although with osmium higher temperatures must be used. (A. F. M. Igbal previously has reported that several derivatives of rhodium, including Rh6(CO)i6 can act as catalysts for the reduction of nitrobenzene to aniline using CO + H2O (19).) The catalysts listed are also much sturdier than was found in the case of iron carbonyl, and in the experiments listed in Table III the total amount of nitrobenzene and catalyst in a 1000 molar ratio was added to the reaction at the outset. In no case was there observed any precipitate of metal oxides or carbonates, and presumably much higher catalytic turnover numbers could be realized if the reaction were run in a continuous-type reactor. 

A more selective palladium catalyst for the reductive carbonylation of 2-nitrostyrenes to the corresponding indoles have been more recently reported [36]. The catalytic system, Pd(TMB)2/TMPhen (TMBH = 2,4,6-trimethylbenzoic acid, TMPhen = 3,4,7,8-tetramethyl-l,10-phenanthroline) is the same used for the carbonylation of nitrobenzene to phenylisocyanate (Chapter 2.2.) [37]. However, while in the latter case the presence of an excess of free TMBH was a requisite, in the synthesis of indoles it was not necessary, thus making the catalytic system rather simple. 

To the best of our knowledge, this is the first report of a dealkylation of tliis type mediated by a transition metal cluster carbonyl. The formation of this hydride could explain in part the large amount of amine 57 formed during the catalytic reactions. This compound has in fact been considered a key intermediate in the catalytic hydrogenation of nitrobenzene to aniline [97]. However, as has been discussed in Chapters 4 and 6, the participation of hydride derivatives in this type of reduction of the nitro to amino group is doubtful. 

The feet that part of the starting perdeuteronitrobenzene is found at the end of the reaction as perdeuteroaniline, coupled with the other selectivity data, indicates that the nitrobenzene acting as an oxidant is at least in large part transformed into aniline (which may possibly reenter the catalytic cycle only at a later stage) and is not directly converted to carbonylated products (Some of the perdeuteroaniline may derive from a direct reduction of nitrobenzene by CO and the water formed in eq. 22 [51], a reaction which has not been considered in the... 

O-CNTs Reduction of nitrobenzene Carbonyl species groups The advantages of lower environmental impact and corrosion problems due to using green oxidized HjOj Better catalytic performance than that for HNO3 oxidized [129]... 

Ragaini and colleagues recently studied the influences of acid additives [20-22]. Using the palladium-phenanthroline catalyst system for the carbonylation of nitrobenzene to methyl phenylcarbamate, the addition of anthraniUc acid [20] or phosphorus acids [21, 22] can accelerate the reaction. Anthranilic acid produced higher activity compared with the use of simple benzoic acid. The 4-amino isomer does not show the same increased activity. Later on, they established an improved catalytic system for the carbonylation of nitrobenzene by adding phosphoms acids as an additive, for the first time yielding activities and catalyst fife in the range necessary for industrial applications. By pafladium-phenanthroline complexes and phosphorus acids as promoters, nitrobenzene was carbonylated to methyl phenylcarbamate with unprecedented reaction rates (TOP up to 6,000/h) and catalyst sta-bUity (TON up to 10 ). The best promoter was phosphoric acid, which is very cheap, nontoxic and easily separable from the reaction products. The catalyst system was also applied to the economically very important dinitrotoluenes reduction. 

Catalytic systems based on rhodium clusters or [Rh(CO)4] for the carbonylation of nitro compounds to carbamates have been described in Chapter 3. The application of the same or similar systems to the reduction of nitro compounds to anilines have been described in Chapter 4. Initial work has shown that rhodium clusters Rh4(CO)i2 and Rh<5(CO)i6, but even several mononuclear compounds such as Rh(CO)2(acac) (acac = acetylacetonate), are active catalyst precursor for the carbonylation of nitrobenzene to carbamates, when promoted by an heterocyclic nitrogen base [56, 140, 187, 188]. Later, Liu and Cheng and us independently reported that even higher catalytic activities could be obtained by the use of preformed [PPN][Rh(CO)4] (PPN = (PPh3)2N ) [189, 190]. We have also conducted a mechanistic study of the catalytic cycle using this last complex [1, 192] and the initial stage of the reaction has also been reinvestigated by Liu et al. [193]. Since no mechanistic study has been yet reported on the cluster-based systems, we will first discuss the [PPN][Rh(CO)4] system and then draw a comparison with the other systems. 

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