Acetic acid synthesis, carbonylation

Acetic Acid and Anhydride. Synthesis of acetic acid by carbonylation of methanol is another important homogeneous catalytic reaction. The Monsanto acetic acid process developed in the late 1960s is the best known variant of the process. 

Acetic Acid Synthesis by Catalytic Carbonylation of Methanol... 

Haynes, A. (2005) Acetic acid synthesis by catalytic carbonylation of methanol, in Topics in Organometallic Chemistry, Vol. 18 (ed. M. Beller), Springer-Verlag, Berlin, pp. 179-205. 

The carbonylation of methyl acetate to acetic anhydride is likely to become an industrial process in the near future 424,427 RhCl3-3H20 is typically used as catalyst precursor and an iodide promoter is used. A mechanistic study indicated that methyl iodide formed from the ester and HI is carbonylated as in acetic acid synthesis (Scheme 26). The resulting acyl, perhaps via reductive elimination of acetyl iodide, converts the acetic acid formed in the ester cleavage to acetic anhydride.428 430 [RhI(CO)(PPh3)2] also catalyzes the reaction though apparently more slowly than complex (95).430,431 The mechanism of this reaction is given in Scheme 27. 

This direct, oxidative condensation of methane to acetic acid in one-pot could be competitive with the current three-step, capital intensive process for the production of acetic acid based on methane reforming to CO, methanol synthesis from CO, and generation of acetic acid by carbonylation of methanol. Key improvements required with the PdS04/H2S04 system, however, will be to develop more stable, faster, and more selective catalysts. Although it is possible sulfuric acid could be utilized industrially as a solvent and oxidant for this reaction, it would be desirable to replace sulfuric acid with a less corrosive material. This chemistry has recently been revisited, verified, and extended by Bell et al., who used Cu(II)/02 as the oxidizing system [22],... 

Haynes A (2006) Acetic Acid Synthesis by Catalytic Carbonylation of Methanol. 18 He Y, see Nicolaou KC (1998) 1 73-104... 

Monsanto acetic acid synthesis 4), and the hydroformylation or 0X0 reaction (5). A key mechanistic step in catalytic carbonylation reactions is the migration of an alkyl group onto an adjacent carbonyl ligand. This reaction involves the formation of a new carbon-carbon bond and has been termed a carbonyl insertion reaction since a CO ligand has been formally inserted into the transition metal-carbon (r-bond. Because of the industrial and commercial importance of these catalytic reactions, the search for stoichiometric systems in which this step can be observed directly has been, and still is, one of great endeavor. 

Another important and commercially essential example proving the variability of homogeneous catalysis is the synthesis of acetic acid via carbonylation of methanol. Here, too, a breakthrough was achieved by employing milder reaction... 

Various unsaturated compounds can be inserted into the metal alkyl, aryl, and alkenyl complexes to give new organometallic complexes having various functional groups. The insertions of carbon monoxide (CO) and isocyanide (CNR) into transition metal-carbon a-bond are particularly important processes, since a carbon unit can be increased in the process and the acyl type complexes formed by the insertion processes can be subjected to further transformations to synthesize useful organic compounds. For example, the CO inserhon constitutes a fundamental step in industrially important processes such as hydroformylation of olefins, acetic acid synthesis from methanol and CO, Fischer-Tropsch process, amidocarbonylation, olefin and CO copolymerizahon processes as well as in a variety of laboratory syntheses of carbonyl containing compounds. 

In the presence of a small amount of (CF3C0)20, even the economical CaCl2 catalyzes the acetic acid synthesis by K2S2O8 (eq. (34)) (58). The yield based on the methane exceeds 94%. Isotope ( C) labeled experiments clearly showed that the methyl group and the carbonyl group of acetic acid come from methane and CO, respectively. The C—H bond is presumably cleaved by radical mechanism. 

Synthesis from Carbon Dioxide. Acetic acid synthesis from methane and carbon dioxide was reported under the conditions similar to those for methane carbonylation (eqs. (38-40)) (55,60,63). The reaction shown in equation (39) is fascinating because it uses H2O2 as the oxidant and water as the solvent. 

In addition, acetic acid synthesis from methane alone was also achieved in the presence of catalytic PdS04 using sulfuric acid as the reaction medium. The stoichiometry of the reaction is summarized in equation (43). TON (CH3C02H/Pd) of 4.1 was achieved under the conditions of equation (44). i C-labeling experiments demonstrated that both the methyl group and the carbonyl groups of acetic acid come from methane (Scheme 3). Methanol is rapidly converted to CO, which further reacts with methyl palladium species to give acetic acid (66,67). The mechanism is closely related to the results of C-labeld experiments in methane carbonylation (eq. (34)). 

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