Iridium complexes carbonylation

Mechanistic Pathways in the Catalytic Carbonylation of Methanol by Rhodium and Iridium Complexes... 

Nagata, T., Pohl, M., Weiner, H., and Finke, R.G., Polyoxoanion-supported organometallic complexes carbonyls of Rhenium(I), Iridium(I), and Rhodium(I) that are soluble analogs of solid-oxide-supported M(CO) + and that exhibit novel M(CO)n+ mobility, Inorg. Chem., 36, 1366, 1997. 

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. 

There have been many reports of the use of iridium-catalyzed transfer hydrogenation of carbonyl compounds, and this section focuses on more recent examples where the control of enantioselectivity is not considered. In particular, recent interest has been in the use of iridium A -heterocyclic carbene complexes as active catalysts for transfer hydrogenation. However, alternative iridium complexes are effective catalysts [1, 2] and the air-stable complex 1 has been shown to be exceptionally active for the transfer hydrogenation of ketones [3]. For example, acetophenone 2 was converted into the corresponding alcohol 3 using only 0.001 mol% of this... 

Recently, Shibita et al. reported catalysis of alkyne insertion into an arylamide sp C-H bond to give allylamides (42) by a cationic iridium complex [118]. An interesting aspect of this work is the unusually selective cleavage of an sp C-H bond over sp aromatic C-H bonds so that the alkenyl arylamide (43) is only a very minor product (30). The carbonyl group is required for the reaction as no coupling... 

The behavior shown in Schemes 2.22 and 2.23 suggests that these hydrido arene-iridium complexes might act as hydrogenation catalysts. Thus, the [(T -C6H6)IrH2(P Pr3)]BF4 complex has been shown to be an efficient hydrogenation catalysts for alkenes and alkynes, as well as some carbonyl groups [22]. 

Recent mechanistic studies using HP infrared equipment, as well as HP-NMR measurements involving the use of CO and CH3I, have allowed the iridium intermediates which are present in solution as methyl acetate and water, and are consumed to produce acetic acid [.12, 34, 41-43], to be followed. All of these observations can be rationalized by a single catalytic cycle (see Figure 8.5), in which equilibria exist between the neutral and anionic complexes for all species. The main species involved in the carbonylation, which are detected in batch mode under carbonylation conditions [34], and correspond to the slower steps of catalysis, are the methyl—iridium and acetyl-iridium complexes [Ir(CH3)l3(CO)2] and [Ir(COCH3)l3(CO)2] respectively. 

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. 

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