Rhodium reactions

An aspect of the hydroformylation reaction which is of particular importance in continuous commercial operation is the separation of the catalyst from product aldehyde and/or alcohol, together with its recovery and recycle into the reactant stream. This feature is of considerable economic and process importance for cobalt reactions and of extreme economic importance for rhodium reactions. 

Extensive investigations in our laboratories on the deactivation of rhodium and iridium catalysts has shown there to be a number of different mechanisms involved. Both, rhodium and iridium catalysts are generally less stable at higher temperatures, and have more labile ligands than their ruthenium counterparts. All of the catalysts are affected by pH, but the ruthenium catalysts seem to be more readily deactivated by acid. Indeed, these reactions are often quenched with acetic acid, whilst stronger acids are used to quench the rhodium reactions. Each of the catalysts can be deactivated by product inhibition, the ruthenium catalyst with aromatic substrates such as phenylethanol, and the rhodium and iridium ones by bidentate chelating products. 

A very closely related process is the Tennessee Eastman (Kodak) carbonylation of methyl acetate to produce acetic anhydride. The rhodium-catalyzed portion of the mechanism is the same as shown in Scheme 19. Differences occur in the iodide-promoted pre- and post-rhodium reactions shown in Scheme 20. 

The reversibility of the rhodium reaction indicates that the reaction is not as energetically favorable as for the iridium analogues. The rate of addition to Rh(I) is more affected by the nature of the phosphine than for iridium. For these rhodium complexes the addition reaction was autocatalytic, the formation of the acetyl product speeded the reaction. Oxidative addition of C2H5I or C4H9I did not occur. An alternate proposal for the preparation of the acyl product has been offered . 

Both palladium and rhodium are effective catalysts, but the rhodium reactions seem to be more subject to steric effects. The reaction proceeds via a catalytic cycle similar to cross coupling, except that the formation of the diorgano(palladium) is brought about by electrophilic attack by the arylpalladium halide, rather than a transmetallation reaction, hence the need for electron-rich systems. The equivalent alkynylation can also be carried out using haloalkynes. ... 

Consideration could also be given to possible involvement of an (yhde carbene)/Pd complex, perhaps assisted by coordination with the oxygen. (Carbene intermediates are well known in azoles, cf. the rhodium reactions shown later.)... 

After copper and palladium, rhodium is the third most important transition metal for the synthesis of the indole ring. For a 2007 review on this reaction, see Patil and Paiil [1], Some early examples (Scheme 1) are Alper s rhodium reaction of 2-aryl-2/7-azirines to give 2-styiylindoles (equation 1) [2], Watanabe s Rh-catalyzed Fischer indole synthesis (equation 2) [3], Ucciani s 3-methylindole synthesis via the hydroformylation of o-nitrostyrene (equation 3) [4], and Burst s preparation of 3-acetyl-2-hydrox-yindoles from the Rh-catalyzed decomposition and carbenoid aromatic C-H bond insertion (equation 4) [5]. Narasaka extended Alper s 2-aryl-2//-azirine reaction to a Rh(II)-catalyzed synthesis of 2,3-disubstituted indoles [6], and both Cenini [7] and Alper [8] stretched the deoxygenation of o-nitrostyrenes to give indoles. Burst s Rh-catalyzed decomposition of a-diazo carbonyl compounds was used by Bauban [9] and Jha [10] in the synthesis of substituted oxindoles. 

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