Iridium-complex catalyzed carbonylation methanol

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. 

SCHEME 13 Catalytic cycles for iridium-complex-catalyzed methanol carbonylation and WGS reaction. Adapted with permission from reference [115], copyright 1979, Royal Society of Chemistry. 

SCHEME 16 Mechanism for ruthenium-complex-promoted, iridium-complex-catalyzed methanol carbonylation. Alternative geometrical isomers of complexes and coordinated solvent molecules are omitted for clarity. Adapted with permission from Scheme 9 in reference [15], copyright 2006, Elsevier. 

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... 

Mel, in Rh-catalyzed methanol carbonylation, 7, 256 to monocarbonyl iridium complexes, 7, 284 in mononuclear ruthenium and osmium alkynyl formations,... 

A process for the coproduction of acetic anhydride and acetic acid, which has been operated by BP Chemicals since 1988, uses a quaternary ammonium iodide salt in a role similar to that of Lil [8]. Beneficial effects on rhodium-complex-catalyzed methanol carbonylation have also been found for other additives. For example, phosphine oxides such as Ph3PO enable high catalyst rates at low water concentrations without compromising catalyst stability [40—42]. Similarly, iodocarbonyl complexes of ruthenium and osmium (as used to promote iridium systems, Section 3) are found to enhance the activity of a rhodium catalyst at low water concentrations [43,44]. Other compounds reported to have beneficial effects include phosphate salts [45], transition metal halide salts [46], and oxoacids and heteropolyacids and their salts [47]. 

The original mechanistic investigations of iridium/iodide-catalyzed methanol carbonylation were conducted by Forster [6,7,19,115,132-135]. Some other studies were also reported in the late 1970s [136-138]. Since the 1990s, interest in the fundamental aspects of the reaction mechanism has been rekindled by the industrial significance of iridium-complex catalysts. 

The analogous anionic iridium complex reacts with methyl iodide 150 times faster than the rhodium complex. The iridium complex also reacts 140-200 times faster than the rhodium analog with higher alkyl iodides/ but competing radical mechanisms appear to occur during the addition of the higher alkyl iodides. More details on the mechanism of rhodium and iridium-catalyzed carbonylation of methanol are provided in Chapter 17. 

If ethylene is present during the carbonylation of methanol catalyzed by IrCl4, once again with Mel as promoter, methyl propionate is formed.416 The reaction depends on the presence of iridium hydride species in solution, and a rhodium analogue of the reaction exists. The full details of the mechanism are not known but the basic steps are shown in Scheme 34. The intermediates are all believed to be complexes of iridium(IIl). 

The diphosphine complexes [Ir(dppe)2]+ and [Ir(dppe)(cod)]+ reportedly catalyze the decar--bonylation of aldehydes.504,505 Methanol carbonylation is catalyzed by lr(Cl)4]-H20 to yield acetic acid. The active species is thought to be an acetyl iridium(III) species, wherein the rate-determining step involves electrophilic attack of this species on methanol. The effects of added iodide ion on reaction rates have also been investigated.506... 

The Ir-catalyzed methanol carbonylation reaction has been studied extensively by several groups 9f-h. The mechanism for the reaction is more complex than for the Rh reaction. The reaction involves a neutral and an anionic catalytic cycle. The extent of participation by each cycle depends on the reaction conditions. The anionic carbonylation pathway predominates in the Cativa process. The active Ir catalyst species is the iridium carbonyl iodide complex, [Ir(CO)2l2]. The carbonylation reaction proceeds through a series of reaction steps similar to the Rh catalyst process shown in Figure 1 however, the kinetics involve a different rate determining step. 

Among the most studied dicarbonyl iridium(l) complexes are the anionic [IrX2(GO)2] (X = G1 141a, Br, 141b, I 141c) due to their implications in catalytic processes such as methanol carbonylation See for example Refs 72, 72a-72g. vide infra), both from experimental and theoretical viewpoint, as for derivatives such as tfr-[Ir(GO)2(py)2]PF6 142 which was found to catalyze the WGS reaction. " From the synthetic point of view, it is worth mentioning here a few examples of systems derived from [IrX2(GO)2] ... 

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