Rhodium RhCl diphos

The occurrence of multinuclear catalysts in hydrogenations catalyzed by rhodium-DIOP systems seems unlikely, although the trans-RhCl(CO)(DIOP) complex 43 is dimeric (276), and in basic methanolic solution the 1 1 diphos complex exists as Rh3(diphos)3(OMe)2 + (138a, Section II, B, 1). 

The hydride route involves the initial reaction with hydrogen followed by coordination of the substrate the well-known Wilkinson catalyst [RhCl(PPh3)3] is a representative example. A second possible route is the alkene (or unsaturated) route which involves an initial coordination of the substrate followed by reaction with hydrogen. The cationic catalyst derived from [Rh(NBD)(DIPHOS)]+ (NBD = 2,5-norbornadiene DIPHOS = l,2-bis(diphenyl)phosphinoethane) is a well-known example. The above-mentioned rhodium catalysts will be discussed, in the detail, in the following sections. 

The most accessible complex appears to be [Rh(diphos)2]Cl which can be obtained from a variety of starting materials (Scheme 10). The ligand will reduce rhodium(JII) chloride or displace other ligands from many rhodium(I) complexes. Unlike tertiary monophosphines it will totally displace carbon monoxide from [RhCl(CO)2]2 (equation 80). It does, however, require excess ligand to bring about the reaction.214,233 Less diphos forms an unstable carbonyl complex,233 which slowly dispropor-tionates (equation 81).244... 

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