Rhodium complexes with arylphosphine ligands

Activation of a C-H bond requires a metallocarbenoid of suitable reactivity and electrophilicity.105-115 Most of the early literature on metal-catalyzed carbenoid reactions used copper complexes as the catalysts.46,116 Several chiral complexes with Ce-symmetric ligands have been explored for selective C-H insertion in the last decade.117-127 However, only a few isolated cases have been reported of impressive asymmetric induction in copper-catalyzed C-H insertion reactions.118,124 The scope of carbenoid-induced C-H insertion expanded greatly with the introduction of dirhodium complexes as catalysts. Building on initial findings from achiral catalysts, four types of chiral rhodium(n) complexes have been developed for enantioselective catalysis in C-H activation reactions. They are rhodium(n) carboxylates, rhodium(n) carboxamidates, rhodium(n) phosphates, and < // < -metallated arylphosphine rhodium(n) complexes. 

Phosphorus donor ligands have also been used to activate Ru3(CO)i2 in the catalytic reduction of nitrobenzene by CO/H2O in the presence of sodium hydroxide [32], Reaction conditions are mild (room temperature and one atmosphere of CO). In a three hours reaction, a turnover of 95 was observed by adding l,2-bis(diphenylphosphino)ethane (DPPE) to Ru3(CO)i2 in a 0.5 molar ratio. By adding PPh3 or in the absence of any phosphorus ligand, the observed turnover was 43 and 51 respectively. However, better results have been obtained with rhodium catalysts (see later) [32]. Preformed phosphine-substituted clusters of the type Ru3(CO)9L3 (L = arylphosphine) have also been used as catalysts for the same reaction [33] and the same complex was also supported on a polystyrene-divinylbenzene copolymer. 

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