Catalytic methanol carbonylation rhodium-complex-catalyzed

SCHEME 1 Catalytic cycle for the rhodium-complex-catalyzed methanol carbonylation. 

The Monsanto carbonylation of methanol to acetic acid catalyzed by Rh/H is a well-understood example of an organometallic catalytic cycle and can act as a good model with well defined steps (shown schematically in Chapter 4, Section 4.2.4). The starting material is the square planar Rh(I) complex, [Rh(CO)2l2] which is easily accessible by reaction of rhodium trichloride in solution with CO in the presence of iodide. This undergoes oxidative addition with Mel very readily to give the methyl-Rh(III) complex [Rh(Me)(CO)2l3] as an unstable... 

The reaction of divalent metals, such as copper, nickel, and so on, with dioxetanes in methanol leads to clean catalytic decomposition into carbonyl fragments/ The reaction rates increase with increasing Lewis acidity of the divalent metal and indicate, therefore, typical electrophilic cleavage of the dioxetane. On the other hand, univalent rhodium and iridium complexes catalyze the decomposition of dioxetanes into carbonyl fragments via oxidative addition. 

Ditertiary phosphines such as (86), (92), and (98) (100) (Scheme 6) have found important uses as ligands for metal-catalyzed transformations, including e.g., palladium-catalyzed Grignard cross couplings,194,205 rhodium-catalyzed Michael additions,2 hydrocyanations,206 copolymerizations,20 and palladium-catalyzed animations.208 Rhodium complexes of (86) are catalysts for the carbonylation of methanol.188 More recently the ligand bite angle of ditertiary phosphines such as (100) has been shown to influence catalytic activity/selectivity in several important catalytic processes.209-213... 

The most common oxidation states and corresponding electronic configurations of rhodium are +1 (tf8), which is usually square planar although some five coordinate complexes are known, and +3 (T) which is usually octahedral. Dimeric rhodium carboxylates are +2 (oxidation states —1 (industrial applications include rhodium-catalyzed carbonylation of methanol to acetic acid and acetic anhydride, and hydroformylation of propene to tf-butyraldehyde. Enantioselective catalytic reduction has also been demonstrated. 

As mentioned in the previous section, the carbonylation of methanol to acetic acid is an important industrial process. Whereas the [Co2(CO)s]-catalyzed, iodide-promoted reaction developed by BASF requires pressures of the order of 50 MPa, the Monsanto rhodium-catalyzed synthesis, which is also iodide promoted and which was discovered by Roth and co-workers, can be operated even at normal pressure, though somewhat higher pressures are used in the production units.4,1-413 The rhodium-catalyzed process gives a methanol conversion to acetic acid of 99%, against 90% for the cobalt reaction. The mechanism of the Monsanto process has been studied by Forster.414 The anionic complex m-[RhI2(CO)2]- (95) initiates the catalytic cycle, which is shown in Scheme 26. 

Reaction (78) regenerates Mel from methanol and HI. Using a high-pressure IR cell at 0.6 MPa, complex (95) was found to be the main species present under catalytic conditions, and the oxidative addition of Mel was therefore assumed to be the rate determining step. The water-gas shift reaction (equation 70) also occurs during the process, causing a limited loss of carbon monoxide. A review of the cobalt-, rhodium- and iridium-catalyzed carbonylation of methanol to acetic acid is available.415... 

Rhodium-catalyzed carbonylation of methanol is known as the Monsanto process, which has been studied extensively. From the reaction mechanism aspect, the study of kinetics has proved that the oxidative addition of methyl iodide to the [Rh(CO)2l2] is the rate-determining step of the catalytic cycle. It was also observed that acetyl iodide readily adds to [Rh(CO)2l2], indicating that the acetyl iodide must be scavenged by hydrolysis in order to drive the overall catalytic reaction forward. An alternative to sequential reductive elimination and the hydrolysis of acetyl iodide is the nucleophilic attack of water on the Rh acetyl complex and the production of acetic acid. The relative importance of these two alternative pathways has not yet been fully determined, although the catalytic mechanism is normally depicted as proceeding via the reductive elimination of acetyl iodide from the rhodium center. The addition of iodide salts, especially lithium iodide, can realize the reaction run at lower water concentrations thus, byproduct formation via the water gas shift reaction is reduced, subsequently improving raw materials consumption and reducing downstream separation. In addition to the experimental studies of the catalytic mechanism, theoretical studies have also been carried out to understand the reaction mechanism [17-20]. 

The influences of diphosphines Ph2P(CH2) PPh2 on the rhodium-catalyzed carbonylation and hydroformylation of methanol to acetic add and acetaldehyde has been examined. [(P-P)Rhl(CO)] reacts with Mel to give the unstable but detectable Rh(III) complex [(P-P)RhMe(I)2CO], which is readily converted into [(P-P)Rh(COMe)l2]. The latter can be isolated after catalytic runs and liberates CH3CHO on treatment with hydrogen. The reaction is first order in [Rh—COMe] and zero order in [Mel]. The... 

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