Iridium-catalyzed carbonylations

Scheme 2. Catalytic cycles for the iridium-catalyzed carbonylation of methanol.

Murakami and Ito reported a novel iridium-catalyzed carbonylative ring expansion of allenylcyclopropanes [29]. When a mixture of substituted allenylcyclo-propane 49a (R = R = Et, R = R = H) and 5 mol% of IrCl(CO)(PPh3)2 in xylene is heated at 130°C under 5 atm pressure of CO for 35 h, cyclohexenone 53 a is obtained in 81% yield. The reaction is proposed to proceed through in-... 

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... 

Scheme 3. Proposed mechanism for the iridium-catalyzed carbonylations of methanol.

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. 

The iridium-catalyzed carbonylation of methanol known as the Cotiva process was aimounced by BP Chemicals in 1996 it now operates on a number of plants worldwide [43—46]. The advantages of iridium catalysts are better stability because of stronger metal—ligand bonding, broad reaction conditions tolerability, and others. 

Han SB, Kim IS, Krische Ml (2009) Enantioselective iridium-catalyzed carbonyl allylation from the alcohol oxidation level via transfer hydrogenation minimizing pre-activation for synthetic efficiency. Chem Commun 7278-7287... 

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. 

Iridium-Catalyzed Carbonylation of Methanol BP s Cativa Process... 

Iridium-catalyzed carbonylation ofMeOHdS, 14) Ruthenium or osmium Removal of halide to speed rate determining step in the catal5rtic cycle... 

Allenylcyclopropane 51 generated in situ from 3-cyclopropylprop-2-yn-l-yl ester 52 via rhodium-catalyzed 1,3-acyloxy migration underwent carbonylative annulation to give the six-membered ketone 53 (Scheme 2.42) [61]. The [5+1] annulation reaction proceeds via a mechanism analogous to that described for iridium-catalyzed carbonylation of allenylcyclopropanes [62]. 

The conditions employed for iridium-catalyzed carbonylation (ca. 180-190 °C, 20-40 bar) are comparable to those of the rhodium-based process. A variety of iridium compounds (e.g., I1CI3, IrU, H2I1CI6, Ir4(CO)i2) can be used as catalyst precursors, as conversion into the active iodocarbonyl species occurs rapidly under process conditions. In a working catalytic system, the principal solvent component is acetic acid, so the methanol feedstock is substantially converted into its acetate ester (Equation (2)). Methyl acetate is then activated by reaction with the iodide co-catalyst (Equation (3)). Catalytic carbonylation of methyl iodide formally gives acetyl iodide (Equation (4)) prior to rapid hydrolysis to the product acetic acid (Equation (5)). However, it is difficult to establish the true intermediacy of acetyl... 

The rate of iridium-catalyzed carbonylation displays a rather complicated dependence on a range of process variables such as pCO, [Mel], [MeOAc], and [H20]. " The catalytic rate displays a strong positive dependence on [MeOAc], but is zero order in [Mel] above a limiting threshold, and independent of CO partial pressure above ca. 10 bar. A particularly notable aspect is the dependence of rate on [H2O], which attains a maximum at ca. 5wt.% H2O as illustrated in Figure 1. This behavior contrasts with the rhodium catalyst, for which catalytic activity attains a plateau above ca. lOwt.% H2O. Indeed, under conventional high water conditions the performance of an iridium catalyst is inferior to rhodium. 

The reactivity of [Ir(GO)2l3Me] with other species has also been investigated, in particular, reactions leading to methane, a known byproduct of iridium-catalyzed carbonylation. Methane formation occurs on reaction of [Ir(GO)2l3Me] either with carboxylic acids or with H2 at elevated temperature. In both cases, the reaction is inhibited by the presence of GO, suggesting that GO dissociation from the reactant complex is required. For the protonolysis reaction with carboxylic acids, a mechanism was proposed (Scheme 8(a)) in which the acid coordinates to a vacant site created by GO loss, and methane is then liberated via a cyclic transition state. The hydrogenolysis reaction, which leads cleanly to [Ir(GO)2l3H] , could proceed via oxidative addition of H2 or an rf-Hz complex as shown in Scheme 8(b). 

Figure 4.3 Iridium-catalyzed carbonylation of methanol. Hydrolysis of 4.14 and 4.15 is not shown and the organic cycle is on the left.

Extensive spectroscopic and other evidences are available for all the three catalytic cycles. For the Co-based catalytic cycle, good kinetic, spectroscopic, and structural data on model complexes exist. For rhodium-catalyzed carbonylation, oxidative addition is found to be the rate-determining step. In contrast, for iridium-catalyzed carbonylation, insertion of CO is the rate-determining step. Thus kinetic measurements show that for 4.13 the insertion reaction is about 700 times faster than that for 4.11. Computational studies, as mentioned earlier (see 3.5), are also in agreement with the kinetic data. 

Evidences for the rhodium- and iridium-catalyzed carbonylation cycles also come from isolated and model complexes. From the reaction of CHjI with 4.5, under noncatalytic conditions, a solid has been isolated. 

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