Rhodium catalysed homogeneous hydrogenation

Interest in the chemistry of the 6ji-phosphinine (phosphabenzene) system has continued, although considerably fewer papers have appeared compared to recent years. The synthesis, coordination chemistry and catalytic applications of phosphi-nines have been reviewed." New chiral bidentate phosphinine ligands (220) have been prepared and their coordination chemistry and applications in rhodium-catalysed asymmetric hydrogenations assessed." The diphosphinine (221) continues to find new applications as a ligand in homogeneous catalysis" and a new mode of coordination to a metal has been identified for the phosphinine (222), two such ligands simultaneously bridging a Mn-Mn bond." ... 

In contrast to the generally accepted mechanism for the homogeneous hydrogenation of C=C double bonds catalysed by rhodium or iridium complexes, which is assumed to occur through M(III)/M(I) species, the postulated cycle for the hydrogenation of imines involves only Ir(III) species. Many aspects remain unclear for example the simple cycle neither explains the origin of the enantioselectivity nor the effect of acids and iodide used as promoters. 

Chiral ionic liquids have also been used to aid enantioselective metal catalysed reactions. For example, homogeneous rhodium catalysed hydrogenations using tropoisomeric biphenylphosphine ligands have been reported using chiral ionic liquids derived from L-proline and L-valine. Enantioselectivities of up to 69% could be achieved and the catalytic system could be reused after extraction with SCCO2. 

The discovery of the chiral atropisomeric ligand, BINAP, greatly expanded the number of asymmetric homogeneous hydrogenation catalyses. Rhodium and ruthenium complexes that contain BINAP and similar ligand systems have demonstrated an amazing versatility in the reduction of a wide variety of substrate classes in excellent stereoselectivities and reactivities. (-)-Menthol, a variety of... 

Brown, J. M. Chaloner, P. A. Structural characterisation of a transient intermediate in rhodium-catalysed asymmetric homogeneous hydrogenation. /. Chem. Soc. Chem. Commun. 1980,344-346. 

An important modern example of homogeneous catalysis is provided by the Monsanto process in which the rhodium compound 1.4 catalyses a reaction, resulting in the addition of carbon monoxide to methanol to form ethanoic acid (acetic acid). Another well-known process is hydro-formylation, in which the reaction of carbon monoxide and hydrogen with an alkene, RCH=CH2, forms an aldehyde, RCH2CH2CHO. Certain cobalt or rhodium compounds are effective catalysts for this reaction. In addition to catalytic applications, non-catalytic stoichiometric reactions of transition elements now play a major role in the production of fine organic chemicals and pharmaceuticals. 

Hydrosilylation.—This reaction is catalysed by the usual homogeneous catalysts. In some cases the mechanism involves insertion of the alkene into a metal-hydrogen bond, as in hydrosilylation of butadiene in the presence of PdL(PPh3)2, with L = p-benzoquinone or maleic anhydride. In other cases concerted addition of the silicon hydride to the carbon-carbon double bond is indicated, as in hydrosilylations catalysed by rhodium(i) catalysts such as RhCl(PPh3)3. In the reaction of silanes with hex-l-ene in the presence of this catalyst, rates depend on the stability of the intermediate adduct RhClH(SiR3)(PPh3)2 such an adduct was isolated in one case. Hydrosilylation of ethylene by trimethylsilicon hydride... 

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