Aniline, oxidative carbonylation

The process associated with the second equation is favoured by the use of an high aniline concentration. The occurrence of a process that can be described as an aniline oxidative carbonylation using nitrobenzene as an oxidant closely parallels many reactions described in paragraph 3.2.1. and the Pd(OAc)2/DPPP/AcOH-catalysed synthesis of carbamates [125] described in paragraph 3.2.2. The mechanistic implications of these competing stoichiometries are discussed in detail in Chapter 6. 

It is worth mentioning that both the carboxylation of epoxides and anilines are acid-base reactions, which do not entail redox processes. Therefore a catalyst active in these reactions must provide acid-base functionality. In this perspective, positively charged gold could be the real player, although a co-catalytic or promotion effect of ze-rovalent gold could also be important. Therefore the catalysts for the oxidative carbonylation of aniline, supported on Merck Ion-exchanger IV, could be actually bifunctional. On one side, Au could catalyze the oxidation of CO with O2 to CO2, a reaction for which it is... 

The starting reactant is aniline for the oxidative carbonylation and nitrobenzene for the reductive carbonylation. The major advantage of the oxidative carbonylation is that the oxidative carbonylation is more thermodynamically favorable than the reductive carbonylation. The former can occur at a significantly milder condition than the latter (11-16). However, nitrobenzene is the feedstock for the production of aniline ... 

Carbamates are produced by the oxidative carbonylation of amines in alcohol, and active research for commercial production of carbamates 235 as precursors of isoyanates 236 based on this reaction has been carried out. As one example, ethyl phenylcarbamate is produced in a high yield (95%) with a selectivity higher than 97% by the reaction of aniline with CO in EtOH at 150 °C and 50 atm. Pd on carbon is the catalyst and KI is added as a promoter [141], The reaction proceeds even at room temperature and 1 atm of CO [142],... 

The oxidative carbonylation of aniline (Asahi Chemicals) produces alkylphe-nylcarbamate ... 

This process was elaborated as a heterogeneously catalyzed variation by Asahi Chemicals (Japan) in order to open a new route to diisocyanates, not depending on the use of phosgene [120, 134]. Ethyl phenylcarbamate, which in a first step is obtained by catalytic oxidative carbonylation of aniline, CO, oxygen, and ethanol (eq. (17)), is condensed with aqueous formaldehyde to yield methylene diphenyl diurethane. Thermal decomposition leads to methylene diphenyl diisocyanate (MDI), which is one of the most important intermediates for the industrial manufacture of polyurethanes (eq. (18)). The yields and selectivities of the last reaction step seem to be the main reasons why this process is still inferior to the existing ones. 

The release of phosgene and toxic solvent has stimulated the development of novel processes. The BASF process for MDI, that employs high temperatures and pressures to increase the rate of reaction, does not release phosgene. Catalytic routes, via oxidative carbonylation of aniline to methyl A-phenyl carbamate (12), using palladium metal... 

The properties of a- or y- zirconium hydrogen phosphate, a synthetic inorganic ion-exchanger material with layered structure, as "heterogenizing" support for carbonylation reactions are still to be studied. Our first studies in this field showed that the Pd(II)-2,2 -bipyridyl complex intercalated in these materials catalyzes the oxidative carbonylation of aniline even though its activity falls with time, because the support slowly loses palladium and deactivates during the catalysis process [2]. 

So, also the selectivity based on the CO consumed was very high (100%). However, when the aniline was carbonylated in the presence of methanol at T>100 °C, relevant amounts of CO2, increasing upon increase of temperature, were obtained. The CO2 formation only under these conditions suggests that it is not formed by direct oxidation of CO, but likely by hydrolysis of DPU (reaetion 3) followed by decomposition of resulting carbamic acid (CO(NHPh)(OH)) in CO2 and aniline. 

It is well known that the oxidative carbonylation of aniline and the reductive carbonylation of nitrocompounds to give DPU or MPC occur according to the stoichiometry of reactions (1-2) and (4-5). Alkoxycarbonyl complexes (M-COOR 1) and carbamoyl complexes (M-CONHR 2) which then evolve into the final products, are believed to be key intermediates for these reactions. The two accepted different mechanisms for the formation of 1 and 2 along with their catalytic cycles are illustrated in the schemes 1 and 2 for the oxidative carbonylation of amines catalyzed by noble metals. Both the cycles involve a two electron redox process. 

However, an alternative mechanism similar to that described in scheme 2, that considers the oxidative addition of aniline to the Rh° finely dispersed on the support, cannot be completely excluded. The evolution of carbamoyl intermediate to DPU should occur still via iodoformamide. The last mechanism could be also operative in the reductive carbonylation of nitrobenzene, when aniline is necessary for its conversion. In this case, the reaction could be better considered as an oxidative carbonylation process in which the nitrobenzene is playing the role of the oxidant in place of the oxygen. It has been ascertained that under these conditions the carbonylation occurs with the stoichiometry of reaction (11) [14], different from the one reported in reaction (4). 

Another manufacturing process [9l] for 4,4 -diphenylmethyl diisocyanate (MDI) was introduced by Asahi Chemical. In contrast to the ARCO route, aniline is used for the carbonylation to N-phenylethyl urethane otherwise, the same steps are followed. The oxidative carbonylation of aniline is done in the presence of metallic palladium and an alkali iodide promoter at 150-180°C and 50-80 bar. The selectivity is more than 95% with a 95% aniline conversion ... 

The first step is the oxidative carbonylation of aniline to form the intermediate diphenyl urea shown in Eq. (38). This reaction is carried out with a noble metal catalyst and an iodine promoter. A palladium catalyst with a sodium iodide promoter has been used successfully. The intermediate, diphenyl urea is oxidatively carbonylated in ethanol in the presence of the palladium catalyst to form ethyl phenyl carbamate (EPC) as shown in Eq. (39). Reactor conditions are 160°C (320°F) and 80 bars (1175 psig). 

Pd-bipy(l l)-a-ZP was employed in the oxidative carbonylation of aniline to diphenylurea and carbamic esters at 80 (30). After a first induction... 

This means that, on reduction, Pd can migrate from the bulk to the surface of the solid, and from there it is released in the solution, where it is reoxidized and able (but now as a homogeneous catalyst) to promote the oxidative carbonylation of aniline present in the solution. 

The synthesis o-f MDI (methyl ened I phenyl di i socyanate, one o-f the most important monomer -for the production o-f pol yurethanes together with TDI, 2,4-toluenediisocyanate) without phosgene, has been carried out v i a the catalytic oxidative carbonyl at i on o-f aniline 73 ... 

However we like also to remind that ureas can be intermediates in this reaction (4.2.6.3. and 4.2.7.). This is what actually happens in the oxidative carbonyl ation of aniline[7l <1). On the other hand carbamates can be prepared by treatement of ureas and alcohols, with catalysts such as Pd<0Ac) and Cureacts with CH3OH in toluene to give PhNHC02Me, without the presence of any catalyst. 

Pd bipy(l l)-a-ZP was employed in the oxidative carbonylation of aniline to diphenylurea and carbamic esters at SO (30). After a first induction period, the catalyst reaches a level of good activity. In correspondence, the XPS Pd 2p spectra reveal that Pd is present on the surface of the solid, together with N-Pd coordinated species and an oxidized Pd form (337 eV). After several cycles, the catalyst becomes exhausted and XPS spectra show the presence of little Pd and a small amount of Pd-O species (336,1 eV), whilst the N-Pd species is no longer present. 

Scheme 6.10 Palladium-catalyzed oxidative carbonylation of aniline derivatives...

The synthesis of MDI (methylenediphenyidiisocyanate, one of the most important monomers for the production of polyurethanes together with TDI, 2,4-toluendiisocyanate) without phosgene has been carried out via the catalytic oxidative carbonylation of aniline (Scheme 1) [11, 12] ... 

Use of FeOCl in place of FeCls has also been reported [65]. The results seem comparable to those obtainable by the use of FeCb. The same catalytic system was also used to catalyse the oxidative carbonylation of diphenylurea to ethyl phenylcarbamate using nitrobenzene as an oxidant (oxygen could also be used) [66]. A 2 1 ratio urea/PhN02 was employed, with quantitative conversion of the urea (Under the reaction conditions, diphenylurea is surely alcoholysed to ethyl phenylcarbamate and aniline, so the reaction is in effect an oxidative carbonylation reaction of aniline). 

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