Iridium-complex catalyzed carbonylation

Iridium complexes catalyze carbonylation of CH3OH to acetic acid, also with an iodide promoter. The reaction rate relative to Rh is much slower. The steps in the reaction sequence are similar to those for Rh, but the kinetics are more complex . Complex interactions involve HjO, the form of the iodide promoter and CO pressure . For example, at high concentration of 1 ion, the rate increases with increasing pressure. At low 1 levels and low HjO concentration, the reaction rate is inversely dependent on CO pressure. Catalyst species under these different reaction conditions include IrfCOljI, IrH(CO)2l2(OH2), [Ir(CH3)(CO)2l3] and [IrH(CO)2l3] . In acetophenone solvent at 175°C and 3 MPa, the reaction is first-order in CH3OH and independent of CH3I at concentrations in which the I Ir ratio is >20 . Under some conditions, the water gas shift reaction becomes important careful control is necessary for high efficiencies to acetic acid. 

CPDN 394 is an important intermediate for the synthesis of corannulene 23 it can be prepared by [2-F2-F1] reaction of 1,8-diethynylnaphthalene 395 using Fe(CO)5 upon demetallation procedure (Scheme 6.97) [235]. Later, Shibata reported the iridium complex-catalyzed carbonylative alkyne-alkyne coupling, which provides 394 in high isolated yields without demetallation procedure. The iridium phosphine complexes, [IrCl(CO)2(PPh3)2l enables the catalytic coupling under carbon monoxide at atmospheric pressure or less (Scheme 6.97) [236]. 

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. 

The stereospecific polymerization of alkenes is catalyzed by coordination compounds such as Ziegler-Natta catalysts, which are heterogeneous TiCl —AI alkyl complexes. Cobalt carbonyl is a catalyst for the polymerization of monoepoxides several rhodium and iridium coordination compounds... 

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... 

A wide range of carbon, nitrogen, and oxygen nucleophiles react with allylic esters in the presence of iridium catalysts to form branched allylic substitution products. The bulk of the recent literature on iridium-catalyzed allylic substitution has focused on catalysts derived from [Ir(COD)Cl]2 and phosphoramidite ligands. These complexes catalyze the formation of enantiomerically enriched allylic amines, allylic ethers, and (3-branched y-8 unsaturated carbonyl compounds. The latest generation and most commonly used of these catalysts (Scheme 1) consists of a cyclometalated iridium-phosphoramidite core chelated by 1,5-cyclooctadiene. A fifth coordination site is occupied in catalyst precursors by an additional -phosphoramidite or ethylene. The phosphoramidite that is used to generate the metalacyclic core typically contains one BlNOLate and one bis-arylethylamino group on phosphorus. 

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]. 

Many transition metal complexes catalyze homogeneous activation of molecular hydrogen in solution, forming hydrido complexes. Such complexes include pentacyanocobaltate(II) anion, [Co(CN)5], many metal carbonyls, and several complexes of rhodium, iridium, and palladium. 

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... 

Iridium complexes in the presence of iodide also catalyze the carbonylation of methyl acetate to acetic anhydride (equation 69). The reaction mechanism is similar to that of Scheme 33. The ester reacts with HI to give methyl iodide which is carbonylated as in Scheme 33 to acetyl iodide. This reacts with acetic acid to give the anhydride.429 430... 

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

The major drawback in the development of efficient catalytic PK protocols is the use of carbon monoxide. Many groups probably refuse to use this reaction in their synthetic plans in order to avoid the manipulation of such a highly toxic gas. Carbonylation reactions without the use of carbon monoxide would make them more desirable and would lead to further advances in those areas. Once the use of rhodium complexes was introduced in catalytic PKR, two independent groups realized these species were known for effecting decarbonylation reactions in aldehydes, which is a way to synthesize metal carbonyls. Thus, aldehydes could be used as a source of CO for the PKR. This elegant approach begins with decarbonylation of an aldehyde and transfer of the CO to the enyne catalyzed by rhodium, ruthenium or iridium complexes under argon atmosphere (Scheme 36). 

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. 

Other reactions of [Ir(CO)2I3Me] have also been investigated, in particular those leading to methane, a known by-product of iridium-catalyzed carbonylation [153]. Methane formation occurs on reaction of [Ir(CO)2I3Me] with either carboxylic acids or with H2 at elevated temperatures. In both cases, the reaction is inhibited by CO, suggesting that CO dissociation from the reactant complex is required. 

Iridium complexes containing triphenylphosphine, e.g., HIr(CO)2(PPh3)2, in propionic acid catalyze ethylene carbonylation to propionic anhydride . Reaction occurs at a reasonable rate at 195°C and 5 Pa of CO/ethylene pressure. The corresponding Rh complexes are ineffective. The reaction is inoperative with higher olefins, even propylene. 

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