Carbonylation reductive

5 1 Reductive Carbonylation - Quinoline is reduced regiospecifically in the N-containing ring by [Fe(C0)5] 150- 

300 C in the presence of CO, H2O, base and a phase transfer catalyst (particularly 18-crown-6, 87 turnovers). Aromatic nitro compounds are reduced under mild conditions in a genuine bimetallic phase transfer catalysed reaction (reaction 7) , or by a (PtClj (PPhs )2 ]/SnCli,/Et3N system under more forcing conditions.  

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, 

AlCla and TiCli,) and EtaN (reaction 8) . Similarly, performing the reaction in carboxylic acids gives the corresponding N-substituted amides in moderate to good yield l  

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Organoaluminum compounds such as triphenylaluminum (527) are used for ketone synthesis[387]. On the other hand, the reaction of /-BuiAl affords the corresponding alcohol 528 by reductive carbonylation[388]. 

Reductive carbonylation of nitro compounds is catalyzed by various Pd catalysts. Phenyl isocyanate (93) is produced by the PdCl2-catalyzed reductive carbonylation (deoxygenation) of nitrobenzene with CO, probably via nitrene formation. Extensive studies have been carried out to develop the phosgene-free commercial process for phenyl isocyanate production from nitroben-zene[76]. Effects of various additives such as phenanthroline have been stu-died[77-79]. The co-catalysts of montmorillonite-bipyridylpalladium acetate and Ru3(CO) 2 are used for the reductive carbonylation oLnitroarenes[80,81]. Extensive studies on the reaction in alcohol to form the A -phenylurethane 94 have also been carried out[82-87]. Reaction of nitrobenzene with CO in the presence of aniline affords diphenylurea (95)[88]. 

The Pd-catalyzed reductive carbonylation of methyl acetate with CO and H2 affords acetaldehyde. The net reaction is the formation of acetaldehyde from MeOH, CO, and H2P4]. Methyl formate (109) is converted into AcOH under CO pressure in the presence of Lil and Pd(OAc)2[95],... 

This process comprises passing synthesis gas over 5% rhodium on Si02 at 300°C and 2.0 MPa (20 atm). Principal coproducts are acetaldehyde, 24% acetic acid, 20% and ethanol, 16%. Although interest in new routes to acetaldehyde has fallen as a result of the reduced demand for this chemical, one possible new route to both acetaldehyde and ethanol is the reductive carbonylation of methanol (85). 

Attempts have been made to develop methods for the production of aromatic isocyanates without the use of phosgene. None of these processes is currently in commercial use. Processes based on the reaction of carbon monoxide with aromatic nitro compounds have been examined extensively (23,27,76). The reductive carbonylation of 2,4-dinitrotoluene [121 -14-2] to toluene 2,4-diaLkylcarbamates is reported to occur in high yield at reaction temperatures of 140—180°C under 6900 kPa (1000 psi) of carbon monoxide. The resultant carbamate product distribution is noted to be a strong function of the alcohol used. Mitsui-Toatsu and Arco have disclosed a two-step reductive carbonylation process based on a cost effective selenium catalyst (22,23). 

Because transition metals even in a finely-divided state do not readily combine with CO, various metal salts have been used to synthesize metal carbonyls. Metal salts almost always contain the metal in a higher oxidation state than the resulting carbonyl complex. Therefore, most metal carbonyls result from the reduction of the metal in the starting material. Such a process has been referred to as reductive carbonylation. Although detailed mechanistic studies ate lacking, the process probably proceeds through stepwise reduction of the metal with simultaneous coordination of CO (90). 

Scheme 33 Iron-catalyzed reductive carbonylation reported by Reppe [108]...

Alkene-C=X (X=0, NR) Coupling (Reductive Carbonyl-Ene and Reductive Hydroacylation)... 

The first carbonyl complex of gold, [AuCl(CO)], was prepared in 192 52080,2081 and since then only a few more derivatives have been obtained. The [AuBr(CO)] derivative was prepared later and is unstable in the solid state.2082,2083 The reductive carbonylation of Au(S03F)3 in fluorosulfonic acid leads via [Au(CO)2]+ (solvent) to solid [Au(S03F)(C0)] (Scheme 30).2084 [Au(CO)2]+ salts are produced in strongly ionizing protic acids or in Lewis acids such as SbF5. 

Pd/Al203-FeCl3, and Ce-Pd/Al203-FeCl3 catalysts exhibit activity for the synthesis of ethylphenylcarbamate from the reductive carbonylation of nitrobenzene with ethanol at 453 K and 2.07 - 2.93 MPa. The advantage of the use of Al203-supported Pd catalyst is the easy of catalyst recovery form the reactants/product mixture. 

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 ... 

The reductive carbonylation has an advantage of low feedstock cost. A wide range of homogenous metal complexes have been tested for both reactions (1-16). The major drawback of the use of metal complex catalysts is the difficulty of catalyst recovery and purification of the reaction products (12). In addition, the gaseous reactants have to be dissolved in the alcohol/amine mixture in order to have an access to the catalyst. The reaction is limited by the solubility of the gaseous CO and 02 reactants in the liquid alcohol reactant (17). 

We have demonstrated that supported Pd and Cu catalysts are effective in catalyzing the oxidative carbonylation at low pressure reaction condition and the supported metal catalysts can be easily separated from the product mixture in both fixed bed and slurry phase reactors (12,17). The objective of this study is to investigate the feasibility of using Al203-supported Pd catalysts for catalyzing the reductive carbonylation of nitrobenzene with ethanol. 

Figure 1 Transmission IR spectra of reductive carbonylation over PdCl2(PPh3)2 at 2.93 MPa and 453 K.

The results of TOF and carbamate yields are summarized in Table 1. Although PdCl2(PPh3)2 exhibited the highest activity, supported Pd exhibits good activity for the carbamate synthesis for the reductive carbonylation. 

Preparation/Formation of Cp2Ti(CO)2 via the Reductive Carbonylation of Cp2TiCl2/ (Cp2TiCl)2... 

In 1975 Demerseman and co-workers reported two new preparations for Cp2Ti(CO)2 via the reductive carbonylation of Cp2TiCl2. The first of these involved the reaction of either Cp2TiCl2 or (Cp2TiCl)2 with AlEt3 in a CO atmosphere. After these heptane suspensions or benzene solutions were stirred for 20 hours at 20°C, Cp2Ti(CO)2 (1) could be isolated in 30% yield (26). No speculation as to the mechanism of this reduction was discussed however, alkylation and CO insertion steps are probably involved. 

Like zirconium, the first fully characterized carbonyl complex of hafnium was reported in 1976 by Thomas and Brown (6). This complex, bis(i7-cyclopentadienyl)dicarbonylhafnium (3) was prepared via the reductive carbonylation of Cp2HfCl2 using sodium amalgam. While the reaction proceeded to give a moderate yield of 3 (30%), this corresponded to only 60 mg of isolated product. 

In a similar manner, Demerseman et al. attempted the high pressure reductive carbonylation of Cp2HfCl2 using lithium metal. However, after treatment of a THF solution with 200 atm of CO for 10 hours, no Cp2Hf(CO)2 (3) could be isolated (7). 

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