Diphosphane, hydrogenation catalyst

There arc two main classes of catalysts neutral chlororhodium(I) diphosphane complexes and cationic rhodium(I) complexes having the general structure [Rh(diphosphane)(olefin)2]+. The cationic complexes are often more active and more selective than the corresponding chloro complexes60. In contrast to heterogeneous hydrogenation catalysts rhodium phosphane complexes are not pyrophoric. However, they are sensitive to oxygen and should be stored and handled under an inert atmosphere. 

Iridium(I)-diphosphane complexes are known as effective hydrogenation catalysts for a long time. Lately, BINAP, BCPM, and dihydrooxazol derivatives were used as ligands for enantioselective hydrogenation of imines (244). Iridium-(P-Phos) catalysts have been used for the asymmetric hydrogenation of the C=N bond of quinolines (245). 

Hydrogenation of a series of /Z-isomeric mixtures of a-arylenamides with a MOM-protected /3-hydroxyl group catalyzed by a BICP-Rh complex or an Me-DuPhos complex leads to the formation of chiral /3-amino alcohol derivatives with excellent enantioselectivities.70b A 1,4-diphosphane 26 with a rigid 1,4-dioxane backbone is also very effective for this transformation (Equation (28)).76 DIOP -Rh72a and Me-DuPhos-Rh219 catalysts are also effective for this transformation. 

The asymmetric hydrogenation of unfunctionalized ketones is a much more challenging task than that of functionalized ketones [3 j, 115]. Many chiral catalysts which are effective for functionalized ketones do not provide useful levels of enantioselectivity for unfunctio-nalized ketones, due to a lack of secondary coordination to the metal center. Zhang demonstrated the enantioselective hydrogenation of simple aromatic and aliphatic ketones using the electron-donating diphosphane PennPhos, which has a bulky, rigid and well-defined chiral backbone, in the presence of 2,6-lutidine and potassium bromide [36]. 

The surface-active diphosphane 12 was applied in the hydrogenation of methyl a-acetamidocinnamate [Eq. (10)] with [RhCl(COD)]2 as the catalyst precursor in homogeneous methanolic solution and, alternatively, in ethyl acetate-water biphasic systems (96). 

High rates and selectivities are attainable only by the coordination of structurally well-designed catalysts and suitable reaction conditions. The base system employs a [RuCl2(phosphane)2(l,2-diamine)] complex as precatalyst, isopropanol serving both as solvent and hydrogen donor and in the presence of base, typically t-BuOK. Use of chiral diphosphanes, particularly BINAP compounds, and/or chiral diamines... 

A successful codimerization between methylenecyclopropanes and carbondioxide is only possible in the presence of triphenylphosphane or tetraphenylethylene-diphosphane modified Pd(0) catalysts. With methylenecyclopropane itself, a high yield preparation of the butenolid 71 is somewhat tricky, because 71 contains at least three acidic hydrogens, which can react further with methylenecyclopropane. Therefore 71 is obtained in 80 % yield only under special reaction conditions, e.g. by pumping a solution of the catalyst and the methylenecyclopropane in DMF slowly into a 165 °C hot autoclav which contains the same solvent and is under a pressure of 40 bar C02... 

Using the same in situ catalyst, l-(2-methylphenyl)ethanone and l-(l-naphthyl)ethanone were hydrogenated in 77% and 84% ee, respectively When 1-phcnylcthanonc was reduced with an alternative in situ catalyst, [Rh(nbd)CI]2/BDPP, (,S )-l-phenylcthanol in 82% ee resulted9. The diphosphane BDPP7, which forms six-membered chelate rings with metal atoms, is also commercially available. 

In the hydrogenation of optically active azomethines (prepared from chiral a-phenylcthyl-amine and ketones), the effect of reaction parameters, i.e., catalyst dispersion, mass and solvent polarity, on the diastereoselectivity has been studied20. The diastereoselective hydrogenation of several related chiral imines using nonchiral or chiral diphosphane ligands/rhodium has been reported to yield the corresponding amines with diastereomeric ratios up to 99.7 0.3 141. 

Dicarboxylato complexes such as 4, 8, and 10 and cationic arene complexes, such as 6 and 9, all serve as efficient catalysts for enantioselective hydrogenation. (Diphosphane)bis(2-methylal-lyl)ruthenium(II) complexes, prepared from (cod)bis(2-methylallyl)ruthenium(II) and the corresponding diphosphane, have also been used7. Other versatile catalyst precursors are (diphosphane)(allyl)(l,l.l,5,5,5-hexafluoroacetylacetonato)ruthenium(II) complexes which are obtained from (cod)(allyl)(hexafluoroacetylacetonato)ruthenium by displacement of cy-clooctadiene with diphosphanes,B. Ruthenium diphosphane catalysts may also be generated in situ from suitable precursors, e.g., 7 and the corresponding phosphane ligand 76,77,1 27. 

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