Carbonylation iridium based

The formation of C-C bonds is of key importance in organic synthesis. An important catalytic methodology for generating C-C bonds is provided by carbonylation. In the bulk chemicals arena this is used for the production of acetic acid by methanol carbonylation (Eqn. (9)) in the presence of rhodium- or, more recently, iridium-based catalysts (Maitlis et al, 1998). 

The carbonylation of methanol was developed by Monsanto in the late 1960s. It is a large-scale operation employing a rhodium/iodide catalyst converting methanol and carbon monoxide into acetic acid. An older method involves the same carbonylation reaction carried out with a cobalt catalyst (see Section 9.3.2.4). For many years the Monsanto process has been the most attractive route for the preparation of acetic acid, but in recent years the iridium-based CATIVA process, developed by BP, has come on stream (see Section 9.3.2) ... 

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. 

The commercialisation of an iridium-based process is the most significant new development in methanol carbonylation catalysis in recent years. Originally discovered by Monsanto, iridium catalysts were considered uncompetitive relative to rhodium on the basis of lower activity, as often found for third row transition metals. The key breakthrough for achieving high catalytic rates for an iridium catalyst was the identification of effective promoters. Recent mechanistic studies have provided detailed insight into how the promoters influence the subtle balance between neutral and anionic iridium complexes in the catalytic cycle, thereby enhancing catalytic turnover. 

In 1970, the first rhodium-based acetic acid production unit went on stream in Texas City, with an annual capacity of 150 000 tons. Since that time, the Monsanto process has formed the basis for most new capacities such that, in 1991, it was responsible for about 55% of the total acetic acid capacity worldwide. In 1986, B.P. Chemicals acquired the exclusive licensing rights to the Monsanto process, and 10 years later announced its own carbonylation iridium/ruthenium/iodide system [7, 8] (Cativa ). Details of this process, from the viewpoint of its reactivity and mechanism, are provided later in this chapter. A comparison will also be made between the iridium- and rhodium-based processes. Notably, as the iridium system is more stable than its rhodium counterpart, a lower water content can be adopted which, in turn, leads to higher reaction rates, a reduced formation of byproducts, and a better yield on CO. 

Although the carbonylation of methanol using an iodide-promoted iridium complex was first reported by Monsanto researchers Roth and Pauhk in 1968, and its mechanism studied by Forster and others, it was the rhodium system that was initially developed for commercialization. A more complex mechanism for iridium, involving both anionic and neutral intermediates was discovered, but it would take over twenty years to coimnercialize an iridium-based system for methanol carbonylation (Scheme 21). In the Cativa process, the iridium complex is promoted by two distinct... 

Since 1979, numerous reviews have appeared on the kinetics, mechanisms, and process chemistry of the metal-catalyzed methanol carbonylation reaction [11, 14-20], especially the Monsanto rhodium-catalyzed process. In this section, the traditional process chemistry as patented by Monsanto is discussed, with emphasis on some of the significant improvements that Monsanto s licensee, Celanese Chemicals (CC) has contributed to the technology. The iridium-based methanol carbonylation process recently commercialized by BP Chemicals Ltd. (BP) will be discussed also. 

In addition to rhodium-based catalysts, iridium-based catalysts have also been developed for carbonylation of methanol. The iridium system, known as the Cativa process, follows a cycle similar to the rhodium system in Figure 14.20, beginning with oxidative addition of... 

Recently, the Cativa process for liquid-phase carbonylation to produce acetic acid has been com-mercialized.2 This process uses an iridium-based catalyst instead of rhodium, produces less propionic acid and acetaldehyde, and uses a much lower water concentration in the reaction mixture. The last aspect results in a reaction product stream that contains less water, so much less energy is needed for distillation to separate water from acetic acid. 

Detailed mechanistic and theoretical analysis of the key mechanistic steps of the Cativa process, for the Cativa process See Ref 257,257a that is, the iridium-based catalytic carbonylation of methanol to acetic acid, have allowed several groups, " particularly Haynes and co-workers, to unravel the mechanism of the catalytic process. Ir(l) complexes [Ir(CO)(L-L)I] (LL = dppms, dppe, dppmo) provided important mechanistic information about the influence of stereoelectronic ligand effects on the organometallic reactivity of modified metal centers with Mel. The carbonylation of methanol promoted by iridium and rhodium complexes which is at the basis of both Cativa and Monsanto processes for the synthesis of acetic acid will be described in detail in a different chapter of this volume. 

Meanwhile, Wacker Chemie developed the palladium-copper-catalyzed oxidative hydration of ethylene to acetaldehyde. In 1965 BASF described a high-pressure process for the carbonylation of methanol to acetic acid using an iodide-promoted cobalt catalyst (/, 2), and then in 1968, Paulik and Roth of Monsanto Company announced the discovery of a low-pressure carbonylation of methanol using an iodide-promoted rhodium or iridium catalyst (J). In 1970 Monsanto started up a large plant based on the rhodium catalyst. 

Acyl nitroso compounds (3, Scheme 7.2) contain a nitroso group (-N=0) directly attached to a carbonyl carbon. Oxidation of an N-acyl hydroxylamine derivative provides the most direct method for the preparation of acyl C-nitroso compounds [10]. Treatment of hydroxamic acids, N-hydroxy carbamates or N-hydroxyureas with sodium periodate or tetra-alkyl ammonium periodate salts results in the formation of the corresponding acyl nitroso species (Scheme 7.2) [11-14]. Other oxidants including the Dess-Martin periodinane and both ruthenium (II) and iridium (I) based species efficiently convert N-acyl hydroxylamines to the corresponding acyl nitroso compounds [15-18]. The Swern oxidation also provides a useful alternative procedure for the oxidative preparation of acyl nitroso species [19]. Horseradish peroxidase (HRP) catalyzed oxidation of N-hydroxyurea with hydrogen peroxide forms an acyl nitroso species, which can be trapped with 1, 3-cyclohexanone, giving evidence of the formation of these species with enzymatic oxidants [20]. 

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 an earlier report, Maitlis et al. showed that 1 could be easily converted into a hydrido complex [Cp lrHCl]2 (2) under ambient conditions by treatment with alcohol and a weak base (Scheme 5.1) [19], probably accompanied by the formation of carbonyl compounds. This fact means that the hydrogen atom in an alcohol can be rapidly transferred to the iridium center in the form of a hydride but then, if the hydride on the iridium could be re-transferred to another hydrogen acceptor, a new catalytic system using alcohols as substrates might be realized. In fact, a wide variety of Cp Ir complex-catalyzed hydrogen transfer systems using alcohols as substrates, and based on the above hypothesis, have been reported to date [20]. 

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