Carbonylation iridium-catalysed

It is now nearly 40 years since the introduction by Monsanto of a rhodium-catalysed process for the production of acetic acid by carbonylation of methanol [1]. The so-called Monsanto process became the dominant method for manufacture of acetic acid and is one of the most successful examples of the commercial application of homogeneous catalysis. The rhodium-catalysed process was preceded by a cobalt-based system developed by BASF [2,3], which suffered from significantly lower selectivity and the necessity for much harsher conditions of temperature and pressure. Although the rhodium-catalysed system has much better activity and selectivity, the search has continued in recent years for new catalysts which improve efficiency even further. The strategies employed have involved either modifications to the rhodium-based system or the replacement of rhodium by another metal, in particular iridium. This chapter will describe some of the important recent advances in both rhodium- and iridium-catalysed methanol carbonylation. Particular emphasis will be placed on the fundamental organometallic chemistry and mechanistic understanding of these processes. 

It was discovered by Monsanto that methanol carbonylation could be promoted by an iridium/iodide catalyst [1]. However, Monsanto chose to commercialise the rhodium-based process due to its higher activity under the conditions used. Nevertheless, considerable mechanistic studies were conducted into the iridium-catalysed process, revealing a catalytic mechanism with similar key features but some important differences to the rhodium system [60]. 

In 1996, BP Chemicals announced a new methanol carbonylation process, Cativa , based upon a promoted iridium/iodide catalyst which now operates on a number of plants worldwide [61-69]. Promoters, which enhance the catalytic activity, are key to the success of the iridium-based process. The mechanistic aspects of iridium-catalysed carbonylation and the role of promoters are discussed in the following sections. 

Scheme 12 Catalytic cycles for iridium-catalysed methanol carbonylation...

Forster also reported HP IR measurements on iridium catalysed reactions [59]. It was noted that the iridium speciation is dependent on reaction conditions, with three different regimes being distinguishable. At intermediate [H2O], the dominant Ir species are [MeIr(CO)2l3] and [Ir(CO)2l4] . The anionic methyl complex is regarded as the active form of the catalyst in a cycle analogous to the Rh system, with carbonylation of [MeIr(CO)2l3] being rate determining. The Ir(III) tetraiodide... 

There has been a recent resurgence of interest in iridium catalysed methanol carbonylation, arising from the commercialisation by BP Chemicals of the Cativa process. This uses a promoted iridium catalyst and has now superseded the rhodium catalyst on a number of plants. Its success relies on the discovery of promoters which increase catalytic activity, particularly at commercially desirable low water concentrations. HP IR spectroscopy has been used to investigate the behavior of... 

Scheme 3.1 Anionic and neutral cycles proposed by Forster for iridium catalysed methanol carbonylation and WGS reaction (adapted from Ref [59] by permission of The Royal Society of Chemistry).

Two classes of promoter have been identified for iridium catalysed carbonylation (i) transition metal carbonyls or halocarbonyls (ri) simple group 12 and 13 iodides. Increased rates of catalysis are achieved on addition of 1-10 mole equivalents (per Ir) of the promoter. An example from each class was chosen for spectroscopic study. An Inis promoter provides a relatively simple system since the main group metal does not tend to form carbonyl complexes which can interfere with the observation of iridium species by IR. In situ HP IR studies showed that an indium promoter (Inl3 Ir = 2 1) did not greatly affect the iridium speciation, with [MeIr(CO)2l3] being converted into [Ir(CO)2l4] as the batch reaction progressed, as in the absence of promoter. 

Scheme 3.2 Mechanism for promoted iridium catalysed methanol carbonylation. The red arrows indicate the dominant pathway for catalytic turnover (Ac = C(O)Me). (Adapted from Ref [39] by permission of the American Chemical Society).

The kinetics of hydrogenolysis of a metal-alkyl have been monitored by HP IR spectroscopy for [MeIr(CO)2l3] , the resting state in the cycle for iridium catalysed methanol carbonylation [113]. On treatment with H2 at elevated temperatures, the v(CO) bands of [MeIr(CO)2l3] decayed and were replaced by new r(CO) bands at slightly higher frequency and a v(Ir-H) absorption, corresponding to Eq. (10). 

This represents one pathway to the formation of methane, a knovm by-product in iridium catalysed methanol carbonylation. The hydrogenolysis reaction was severely retarded by the presence of excess CO, indicating a mechanism involving initial dissociation of CO from [MeIr(CO)2l3] , prior to activation of H2. The mechanism therefore resembles that for hydrogenolysis of Rh acetyl complexes in hydroformylation. 

L = P(CH3)3 or CO, oxidatively add arene and alkane carbon—hydrogen bonds (181,182). Catalytic dehydrogenation of alkanes (183) and carbonylation of bensene (184) has also been observed. Iridium compounds have also been shown to catalyse hydrogenation (185) and isomerisation of unsaturated alkanes (186), hydrogen-transfer reactions, and enantioselective hydrogenation of ketones (187) and imines (188). 

With a ruthenium promoter (added as [Ru(CO)4l2]), r(CO) bands due to Ru iodo-carbonyls dominated the spectrum, precluding the easy observation of iridium species. Before injection of the Ir catalyst, absorptions due to [Ru(CO)2l2(sol)2], [Ru(CO)3l2(sol)] and [Ru(CO)3l3] are present. After injection of the iridium catalyst (Ru Ir = 2 1), [Ru(CO)3l3] becomes the dominant Ru species (Figure 3.11(b)). The observations indicate that the Ru(II) promoter has a high affinity for iodide and scavenges Hl(aq) as H30 [Ru(CO)3l3] . An indium promoter is believed to behave in a similar manner to form H30 [Inl4] . These promoter species also catalyse the reaction of Hlj q) with methyl acetate (Eq. (3)), which is an important organic step in the overall process. 

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