Rhodium complexes organometallic reactions

Abstract The purpose of this chapter is to present a survey of the organometallic chemistry and catalysis of rhodium and iridium related to the oxidation of organic substrates that has been developed over the last 5 years, placing special emphasis on reactions or processes involving environmentally friendly oxidants. Iridium-based catalysts appear to be promising candidates for the oxidation of alcohols to aldehydes/ketones as products or as intermediates for heterocyclic compounds or domino reactions. Rhodium complexes seem to be more appropriate for the oxygenation of alkenes. In addition to catalytic allylic and benzylic oxidation of alkenes, recent advances in vinylic oxygenations have been focused on stoichiometric reactions. This review offers an overview of these reactions... 

Another chemical approach to the chemical conversion of methane involves organometallic reactions.85-89 Interesting work with iridium complexes and other transition metal insertion reactions (rhodium, osmium, rhenium, etc.) were carried out. Even iron organometallics were studied. These reactions take place in the coordination spheres of the metal complexes, but so far the reactions are stoichiometric and noncatalytic.77 In terms of synthetic hydrocarbon chemistry, these conversions are thus not yet practical, but eventually it is expected that catalytic reactions will be achieved. 

The reaction of bis-phenylpropargyl ether (321) with tris(triphenylphosphine)rhodium chloride in benzene or toluene led to the formation of the unusual organometallic compound (322), which can be viewed as a derivative of an oxygen-rhodium pentalene system. Reaction of the rhodium complex (322) with sulfur leads to the corresponding 4,6-diphenyl-l,3-dihydro[3,4-c]furan (323). The selenium and tellurium analogs (324) and (325) were made in a similar manner (Scheme 111) (76LA1448). 

Subsequent major events, up until the early 1980s, have been reviewed [2], with one of the major reactions involved being that of asymmetric hydrogenation, which is especially useful and efficient. This was first developed using rhodium complexes equipped with chiral mono- or diphosphines [3-6], though many other types of reaction (e.g., hydroformylation, Diels-Alder reaction) are now well controlled in the presence of chiral organometallic catalysts. Over the past few years there has been a clear renewal of interest for organocatalysis [7], and consequently this chapter will review the specific and unusual case of the catalytic enantioselective reduction of C=C, C=0, and C=N double bonds. 

Jimenez-Catano R, Niu S, Hall MB. Theoretical studies of inorganic and organometallic reaction mechanisms. 10. Reversal in stability of rhodium and iridium j2-ethene and hydridovinyl complexes. Organometallics 1997 16(9) 1962-1968. 

Catalyst decomposition is, overall, receiving little attention in academic work on homogeneous catalysis, and only in recent years has research on decomposition and stabilization of organometallic catalysts started to expand (116), with emphasis on reactions of significant commercial interest such as hydroformylation (117), metathesis 118), crosscoupling, and polymerization 119). Ligand decomposition seems to be a key issue for industrial application, because it affects the total number of turnovers, TON. Phosphine decomposition is an unavoidable side reaction in metal-phosphine complex-catalyzed reactions and the main barrier for commercial application of homogeneous catalysts. There are a few exceptions to this statement for example, the rhodium tppts-catalyzed hydroformylation of propene, a process developed by Ruhrchemie-Rhone Poulenc (now Celanese). 

Organometallic compounds of rhodium have the metal center in oxidation states ranging from +4 to -3. but the most common oxidation states are +1 and +3. The Rh(I) species have a d electron configuration and both four coordinated square planar and five coordinated trigonal bipyramidal species exist. Oxidative addition reactions to Rh(I) form Rh(III) species with octahedral geometry. The oxidative addition is reversible in many cases, and this makes catalytic transformations of organic compounds possible. Presented here are important reactions of rhodium complexes in catalytic and stoichiometric transformations of organic compounds. 

Rhodium catalysts were prepared by hydrogen reduction at atmospheric pressure of a cationic organometallic rhodium complex and anchored onto lamellar and zeolitic products. The effect of the structure and characteristics of the support on metal load and dispersion was studied in the heterogeneous catalysts prepared. The new rhodium catalysts were applied in the hydrogenation of acetone. The reaction was carried out under milder conditions. 

Other reactions. Blum et a/.15 report that the organometallic complex causes decarbonylation of carbonyl compounds and desulfonation of aromatic sulfonyl chlorides. They noted also that 20-g. samples of ethylbenzene, fluorene, and ace-naphthene on being heated with 100 mg. of rhodium complex for 5 hrs. at 130° are oxidized to the extent of 10-60% to acetophenone, fluorenone, and acenaphthenone. 

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