Reductive carbonylations

Beller and X.-F. Wu, Transition Metal Catalyzed Carbonylation Reactions, DOI 10.1007/978-3-642-39016-6 3, Springer-Verfag Berfin Heidelberg 2013 

Like alkoxycarbonylation, aminocarbonylation, and hydroxycarbonylation, the palladium-catalyzed reductive carbonylation reaction was originally discovered by Schoenberg and Heck in 1974 [15]. In the presence of a relatively large amount of [PdX2(PPh3)2l as a catalyst under 80-100 bar of synthesis gas and at 80-150 °C, aryl and vinyl bromides or iodides were converted into the corresponding aldehydes in good yields (Table 3.1). 

Halobi(triphenylphosphine)phenylnickel(II) complexes as catalysts were also tested for the reductive carbonylation of bromobenzene and iodobenzene by the same authors. The yield of benzaldehyde based on iodobenzene was only 20 % at 100 °C using triethylamine (NEta) as its base. Bromobenzene did not react under the usual conditions, and a stoichiometric reaction with chlorobi(triphenylphos-phine)phenylnickel gave only benzene and biphenyl at 100 °C. One decade later, 

Because of the toxicity and waste generation of tin hydrides, it should no longer be used despite the general apphcation of tin hydrides in the past. Organosilanes [18-20] are certainly a better choice to be based in conjunction with carbon monoxide. Ashfield and Barnard recently took up this concept by testing the practicabihty of various R3SiH systems for assorted known palladium catalysts [21]. The authors demonstrated that in many optimization experiments, finding the appropriate parameters (catalyst, base, solvent, temperature, pressure. 

The use of readily available and cheap formate salts is an economically attractive variant for performing palladium-catalyzed reductive carbonylations [22, 23]. For example, Cai and his associates developed a silica-supported phosphine palladium complex ( Si -P-Pd) for the formylation of aryl bromides and iodides with sodium formate (1 bar CO, 90-110 °C) [24]. The polymeric catalyst could be recovered afterwards and shown in simple model reactions comparable catalytic activity than homogeneous PdCl2(PPh3)2 (Table 3.3). 

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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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