Phosphine, iridium complex rhodium complexes

The mechanistic basis of iridium-complex-catalyzed enantioselective hydrogenation is less secure than in the rhodium case. It is well known that square-planar iridium complexes exhibit a stronger affinity for dihydrogen than their rhodium counterparts. In earlier studies, Crabtree et al. investigated the addition of H2 to their complex and observed two stereoisomeric intermediate dihydrides in the hydrogenation of the coordinated cycloocta-1,5-diene. The observations were in contrast to the course of H2 addition to Ms-phosphine iridium complexes [69]. 

The products of oxidative addition of acyl chlorides and alkyl halides to various tertiary phosphine complexes of rhodium(I) and iridium(I) are discussed. Features of interest include (1) an equilibrium between a five-coordinate acetylrhodium(III) cation and its six-coordinate methyl(carbonyl) isomer which is established at an intermediate rate on the NMR time scale at room temperature, and (2) a solvent-dependent secondary- to normal-alkyl-group isomerization in octahedral al-kyliridium(III) complexes. The chemistry of monomeric, tertiary phosphine-stabilized hydroxoplatinum(II) complexes is reviewed, with emphasis on their conversion into hydrido -alkyl or -aryl complexes. Evidence for an electronic cis-PtP bond-weakening influence is presented. 

In the event, only the rhodium(I) complex features sulfur coordination, whereas the iridium(I) complex prefers a second carbene Ugand over the sulfur mediated chelate effect. The two complexes were tested for their activity in the hydrogenation of dimethyl itaco-nate. The iridium complex was inactive and the rhodium complex showed 44% conversion with a disappointingly low chiral resolution of 18% ee (R). The corresponding phosphine functionalised NHC rhodium(I) complex reacted under milder conditions, but without improvement of chiral resolution, 13% ee (S). 

Among the complexes which may function in this way are pentacyano-cobaltate ion, iron pentacarbonyl, the platinum-tin complex, and iridium and rhodium carbonyl phosphines. It has been suggested that with tristriphenylphosphine Rh(I) chloride, a dihydride is formed and that concerted addition of the two hydrogen atoms to the coordinated olefin occurs (16). There are few examples of the homogeneous reduction of other functional groups besides C=C, C=C, and C=C—C=C penta-cyanocobaltate incidentally is specific in reducing diolefins to monoolefins. 

QH P, Phosphine, dimethylphenyl-, iron complex, 26 61 molybdenum complex, 27 11 osmium complex, 27 27 osmium-rhodium complex, 27 29 osmium-zirconium complex, 27 27 ruthenium complex, 26 273 CkH,2, 1,5-Cyclooctadiene, iridium complex, 26 122, 27 23 osmium-rhodium complex, 27 29 ruthenium complexes, 26 69-72, 253-256 CbH 20,PS, 2-Butenedioic acid, 2-(dimeth-ylphosphinothioyl)-, dimethyl ester, manganese complex, 26 163 ChH,4, Cyclooctene, platinum complex,... 

In early patents by Halcon, molybdenum carbonyls are claimed to be active catalysts in the presence of nickel and iodide [23]. Iridium complexes are also reported to be active in the carbonylation of olefins, in the presence of other halogen [24] or other promoting co-catalysts such as phosphines, arsines, and stibines [25]. The formation of diethyl ketone and polyketones is frequently observed. Iridium catalysts are in general less active than comparable rhodium systems. Since the water-gas shift reaction becomes dominant at higher temperatures, attempts to compensate for the lack of activity by increasing the reaction temperature have been unsuccessful. 

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