Reactions Involving Rhodium, Iron, and Cobalt

The metals rhodium, iron, and cobalt each participate in several reactions that are of value in organic synthesis. Rhodium and cobalt are active catalysts for the reaction of alkenes with hydrogen and carbon monoxide to give aldehydes. This reaction is called hydroformylation.  

SECTION 8.4. REACTIONS INVOLVING RHODIUM, IRON, AND COBALT 

The key steps in the reaction are addition of hydridometal to the double bond of the alkene and migration of the alkyl group to the complexed carbon monoxide. 

The steps in the hydroformulation reaction are closely related to those that occur in the Fischer-Tropsch process. The Fischer-Tropsch process is the reductive conversion of carbon monoxide to alkanes. It occurs by a series of carbonylation, migration, and reduction steps. 

The Fischer-Tropsch process is of interest because of its potential for conversion of carbon monoxide to synthetic hydrocarbons fuels. 

A key step in this reaction is the migration of the alkyl group from the metal atom to give complexed carbonyl group. 

A related version of great economic interest is the Fischer-Tropsch process for reductive conversion of carbon monoxide to hydrocarbons. This reaction is catalyzed by a number of metals but cobalt and iron have been most closely studied. The key reaction steps are reduction of metal-complexed carbon monoxide and carbonyl insertion reactions. The hydrocarbon chain is built up by a series of successive carbonyl insertion and reduction steps. 

The key carbonyl insertion reaction which is involved in both hydroformylation and the Fischer.-Tropsch reaction can be reversible. This reaction is of occasional 

SECTION 6.4. SYNTHETIC APPLICATIONS OF OTHER TRANSITION METALS 

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Structural types for organometallic rhodium and iridium porphyrins mostly comprise five- or six-coordinate complexes (Por)M(R) or (Por)M(R)(L), where R is a (T-bonded alkyl, aryl, or other organic fragment, and Lisa neutral donor. Most examples contain rhodium, and the chemistry of the corresponding iridium porphyrins is much more scarce. The classical methods of preparation of these complexes involves either reaction of Rh(III) halides Rh(Por)X with organolithium or Grignard reagents, or reaction of Rh(I) anions [Rh(Por)] with alkyl or aryl halides. In this sense the chemistry parallels that of iron and cobalt porphyrins. 

In the above examples, the nucleophilic role of the metal complex only comes after the formation of a suitable complex as a consequence of the electron-withdrawing effect of the metal. Perhaps the most impressive series of examples of nucleophilic behaviour of complexes is demonstrated by the p-diketone metal complexes. Such complexes undergo many reactions typical of the electrophilic substitution reactions of aromatic compounds. As a result of the lability of these complexes towards acids, care is required when selecting reaction conditions. Despite this restriction, a wide variety of reactions has been shown to occur with numerous p-diketone complexes, especially of chromium(III), cobalt(III) and rhodium(III), but also in certain cases with complexes of beryllium(II), copper(II), iron(III), aluminum(III) and europium(III). Most work has been carried out by Collman and his coworkers and the results have been reviewed.4-29 A brief summary of results is relevant here and the essential reaction is shown in equation (13). It has been clearly demonstrated that reaction does not involve any dissociation, by bromination of the chromium(III) complex in the presence of radioactive acetylacetone. Furthermore, reactions of optically active... 

Olefin isomerization has been widely studied, mainly because it is a convenient tool for unravelling basic mechanisms involved in the interaction of olefins with metal atoms (10). The reaction is catalyzed by cobalt hydrocarbonyl, iron pentacarbonyl, rhodium chloride, palladium chloride, the platinum-tin complex, and by several phosphine complexes a review of this field has recently been published (12). Two types of mechanism have been visualized for this reaction. The first involves the preformation of a metal-hydrogen bond into which the olefin (probably already coordinated) inserts itself with the formation of a (j-bonded alkyl radical. On abstraction of a hydrogen atom from a diflFerent carbon atom, an isomerized olefin results. 

Some insight into the mechanisms of the iodine-promoted carbonylation has been obtained by radioactive tracer techniques [17] and low-temperature NMR spectroscopy [18]. The mechanism involves the formation of HI, which in a series of reactions forms with rhodium a hydrido iodo complex which reacts with ethylene to give an ethyl complex. Carbonylation and reductive elimination yield propionic acid iodide. The acid itself is then obtained after hydrolysis. The rate of carboxylation was reported to be accelerated by the addition of minor amounts of iron, cobalt, or manganese iodide [19]. The rhodium catalyst can be stabilized by triphenyl phosphite [20]. However, it is doubtful whether the ligand itself would meet the requirements of an industrial-scale process. 

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