Carbonylation rhodium-catalysed

Rhodium-catalysed addition (10) of hydridosilanes (Chapter 17) to a/3-unsaturated carbonyl compounds can be performed regioselectively, to afford either the product of 1,2-addition, or, perhaps more usefully, that of 1,4-addition, i.e. the corresponding silyl enol ether this latter process is an excellent method for the regiospecific generation of silyl enol ethers. Of all catalyst systems investigated, tris(triphenylphosphine)rhodium(l) chloride proved to be the best. 

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. 

One approach which enables lower water concentrations to be used for rhodium-catalysed methanol carbonylation is the addition of iodide salts, especially lithium iodide, as exemplified by the Hoechst-Celanese Acid Optimisation (AO) technology [30]. Iodide salt promoters allow carbonylation rates to be achieved at low (< 4 M) [H2O] that are comparable with those in the conventional Monsanto process (where [H20] > 10 M) while maintaining catalyst stability. In the absence of an iodide salt promoter, lowering the water concentration would result in a decrease in the proportion of Rh existing as [Rh(CO)2l2] . However, in the iodide-promoted process, a higher concentration of methyl acetate is also employed, which reacts with the other components as shown in Eqs. 3, 7 and 8 ... 

Diphosphine ligands (e.g. dppe, dppp) were originally found, by Wegman and coworkers [43], to be effective ligands in the rhodium-catalysed reductive carbonylation of methanol to give acetaldehyde (Eq. 9) or ethanol (Eq. 10) ... 

A mechanistic study by Haynes et al. demonstrated that the same basic reaction cycle operates for rhodium-catalysed methanol carbonylation in both homogeneous and supported systems [59]. The catalytically active complex [Rh(CO)2l2] was supported on an ion exchange resin based on poly(4-vinylpyridine-co-styrene-co-divinylbenzene) in which the pendant pyridyl groups had been quaternised by reaction with Mel. Heterogenisation of the Rh(I) complex was achieved by reaction of the quaternised polymer with the dimer, [Rh(CO)2l]2 (Scheme 11). Infrared spectroscopy revealed i (CO) bands for the supported [Rh(CO)2l2] anions at frequencies very similar to those observed in solution spectra. The structure of the supported complex was confirmed by EXAFS measurements, which revealed a square planar geometry comparable to that found in solution and the solid state. The first X-ray crystal structures of salts of [Rh(CO)2l2]" were also reported in this study. 

Monsanto developed the rhodium-catalysed process for the carbonylation of methanol to produce acetic acid in the late sixties. It is a large-scale operation employing a rhodium/iodide catalyst converting methanol and carbon monoxide into acetic acid. At standard conditions the reaction is thermodynamically allowed,... 

The proposed mechanism for this carbonylation reaction, which occurs in the absence of water, involves basic catalytic steps similiar to the rhodium-catalysed methanol carbonylation process (see Section 2.1.2.1.1). The mechanism leads to the formation of acetyl iodide, which reacts with methyl formate to produce the mixed anhydride [133]. 

Asymmetric hydrosilylation of ketones. Chiral alkoxysilanes have also been obtained in the rhodium-catalysed hydrosilylation of carbonyl compounds50-52 (equation 6). 

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