Hydroxides rhodium-phosphine complexes

Ruthenium(III), d, is ruthenium s most stable oxidation state and resembles rhodium(III) and iridium(III) more than osmium(III). The salts inelude the halides, hydroxides, and oxides RuCls SHaO is most important because it is a good starting material for other compounds and reacts readily with olefins and phosphines. Complexes of this oxidation state are known with water, eyanide, oxygenated organies, sueh as diketones and earboxylates, pyridines, earbonyls, ey-elopentadienyls, phosphine, and arsine ligands. A notable differenee between ruthenium(II) and ruthenium(III) is the absenee of ruthenium(III) nitrosyl complexes. 

A few years ago, Patin and co-workers reported the synthesis under mild conditions of a series of ligands obtained from the mixture of Ph2PH and an activated alkene in the presence of a small amount of tetraethylammonium hydroxide (Scheme 1) [8], and observed, when the olefin is very hygroscopic, that some phosphine oxide is produced. During complexation with rhodium, oxidation is avoided using an Rh(I) dimer as a precursor. The synthetic reaction is shown in Scheme 1 and the various complexes obtained are listed in Table 1. 

If higher concentrations of aqueous sodium hydroxide (15%) are used, all of the re-immobilized ligands and the rhodium complex can be extracted into the aqueous phase. This can thus be submitted to the oxidative treatment for Rh recovery according to [34-36]. In this way, 92-95% of the Rh content may be recovered. The amine content of the organic phase was used again by treatment with fresh sulfonated phosphine and sulfuric acid. In the subsequent hydroformylation the same results were actually observed. 

The diphosphine (R,S)-BPPFA [(R,pS)-9] reacts analogously with acetic anhydride to give the corresponding acetate which can be derivatized. Replacement of the acetate by hydroxide leads to a useful ligand BPPFOH 1534, which has been used for the rhodium-catalyzed enantioselective reduction of a-oxo acids to a-hydroxy acids (Section D.2.3.1.). Recently, the chemistry of gold(I) complexes of such chiral phosphines has been developed they catalyze aldol-type cycloadditions of isocyanides to carbonyl compounds to give chiral dihydrooxazoles. which can be hydrolyzed to synthetically important chiral amino alcohols and amino acids 30,39,40. 

T he expectation that, by analogy to phosphines, thioethers should function as tt acceptor ligands and thereby stabilize low oxidation state compounds, led several investigators to try to synthesize thioether complexes of rhodium (I). Walton (I) treated [Rh(DTH)Cl2]Cl (DTH = CH3SCH2CH2SCH3) with ethanolic potassium hydroxide, a reducing system developed by Chatt and Shaw (2), but he failed to obtain a complex of the expected type. Attempts to obtain rhodium(I) derivatives by reducing [Rh(DTH)2Cl.]Cl with sodium borohydride or by electrochemical methods were equally unsuccessful. 

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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