Iridium-complex catalyzed carbonylation promoters

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. 

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. 

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 mechanism of the cobalt- (BASF), rhodium- (Monsanto), and iridium- (Cativa) catalyzed reaction is similar but the rate-determining steps differ and different intermediate catalyst complexes are involved. In all three processes two catalytic cycles occur. One cycle involves the metal carbonyl catalyst (II) and the other the iodide promoter (i). For a better overview only the catalytic cycle of the rhodium-catalyzed Monsanto process is presented in detail (Figure 6.15.4). Initially the rhodium iodide complex is activated with carbon monoxide by forming the catalytic active [Rhi2(CO)2] complex 4. Further the four-coordinated 16-electron complex 4 reacts in the rate-determining step with methyl iodide by oxidative addition to form the six-coordinated 18-electron transition methyl rhodium (I II)... 

In 2004, Frontier et al. found that iridium complex (154) is very reactive as a Lewis acidic promoter of Nazarov cyclization, especially relative to Cu(OTf)2 [48]. Recently, they have found that Ir (III) catalyst (154) is a strong Lewis acid capable of catalyzing Nazarov cyclization with great efficiency for various divinyl ketones (Scheme 16.41) [49]. The efficiency of the cyclization is attributed to the electrophilicity of the Ir(III) complex and substrate activation via 0,0 -chelation that employs two substrate carbonyl groups or one carbonyl and an ether function, and encourages the s-trans/s-trans conformation required for cyclization. 

If ethylene is present during the carbonylation of methanol catalyzed by IrCl4, once again with Mel as promoter, methyl propionate is formed.416 The reaction depends on the presence of iridium hydride species in solution, and a rhodium analogue of the reaction exists. The full details of the mechanism are not known but the basic steps are shown in Scheme 34. The intermediates are all believed to be complexes of iridium(IIl). 

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