Showing papers on "Transfer hydrogenation published in 1990"

Optically active phenanthrolines in asymmetric catalysis. III. Highly efficient enantioselective transfer hydrogenation of acetophenone by chiral rhodium/3-alkyl phenanthroline catalysts.

Abstract: The in situ catalysts prepared from [Rh(Diol)Cl] 2 (Diol = 1,5-Hexadiene or 1,5-Cyclooctadiene) and 3-alkylphenanthrolines display an extremely high catalytic activity in the transfer hydrogenation of acetophenone. Turn over rates up to 10,000 cycles-per-hour (c/h) have been recorded in 2-propanol solution at 83°C in the presence of KOH as a promoter. Asymmetric inductions up to 655 e.e. are obtained with the ligand 3c bearing a chiral trimethylpropyl substituent. This is an extraordinarily high value in view of the long distance existing between the chiral carbon atom and the reactive site of the catalyst. Experimental evidences suggest that in the asymmetric process the most active and stereoselective catalytic species might be a rhodium hydride complex containing two phenanthroline ligands in a chiral C 2 array.

Studies on [Ru3(CO)12]-catalysed homogeneous transfer hydrogenation reactions; X-ray structure of [Ru4(CO)10Cl12(OPh)2]

Abstract: Using [Ru3(CO)12](1) as the homogeneous precatalyst, transfer hydrogenations of cyclohex-2-en-1-one, benzylideneaniline, and carbon tetrahalides by donor alcohols, in particular propan-2-ol, have been studied. Conversion of cyclohex-2-en-1-one into cyclohexanol has been found to proceed via the intermediate formation of cyclohexanone. From the temperature dependence of the overall reaction rates, the precatalysts [Ru3(CO)12] and [Ru4H4(CO)12] are involved in processes with comparable activation energies. Two catalytically active cluster complexes, [Ru4(CO)12(C6H6O)] and [Ru3(CO)10(C6H8O)], were isolated from the reaction of (1) with cyclohex-2-en-1-one. Hydrogenation of the tetranuclear cluster led to the formation of [Ru4H4(CO)12] and [Ru3H2(CO)9(C6H8O)]. The reaction of complex (1) with benzylideneaniline gave a catalytically active cluster [Ru3H(CO)9(PhNCPh)]. With (1) as the precatalyst, analogues of benzylideneaniline of general formula RC6H4CHNPh can all be transfer hydrogenated, with the exception of the o-methoxy derivative. The complex [Ru3H(CO)9(PhNCPh)] was found to undergo reversible carbonylation. Rational syntheses for [Ru3(CO)7X2(OR)2] and [Ru4(CO)10X2(OR)2] were designed by treating (1) with the appropriate alcohol and carbon tetrahalide. The X-ray structure of [Ru4(CO)10Cl2(OPh)2] has been determined. The halogenoalkoxo clusters are considered to be active intermediates in the overall catalytic cycle for the transfer hydrogenations of carbon tetrahalides.

Right or left - this is the question (enantioselective catalysis with transition metal compounds)

Abstract: New methods to prepare optically active phosphines for enantioselective transition metal catalysts are presented. The transfer hydrogenation of itaconic acid to give methylsuccinic acid using HCOOH is optimized to optical purity. The enantioselective monophenylation of meso-diols with Cu(0Ac) lpyridineoxazoline catalysts is introduced as a new reaction. The stereodemi stry of the sto i ch i ometr i c cyc 1 opropanat i on of styrene with the carbene ligand of a Fe complex is determined by the metal configuration, the optically active phosphine ligand making only a small contribution. in the Khand-Pauson reaction of phenylacetylene with norbornene to give optically pure 3a,4,5,6,7,7a-hexahydro-2-phenyl-4,7-methano-lH-inden~-l-one, the stereochemistry is determined by the cluster configuration and not by the optically active phosphine ligand. The conclusion is drawn that a control of the metal configuration during enantioselective catalysis would be of major importance for the optical induction. Similarly,

Carbonyl complexes as homogeneous and supported catalysts

Abstract: The use of carbonyl complexes in homogeneous and heterogeneous catalysis is very briefly presented. The demonstrated and expected advantages of such catalysts and the knowledge-gap in these areas of research are highlighted. The results obtained in the author’s laboratory, for two specific reactions — the carbonylation of nitrobenzene and transfer hydrogenation of organic carbonyl compounds with ruthenium carbonyl clusters as catalysts — are discussed. Where empirical and rigorous kinetic analyses are not possible, the usefulness of building hypothetical catalytic cycles with fully characterised clusters to rationalise observed activity and/or selectivity is emphasised. A summary of recent work on organic polymer supported mono- and polynuclear metal carbonyls is also presented.

Heterogeneous Catalytic Transfer Hydrogenation Reactions of 4-Nitrodiphenylamine.

Abstract: A new method for the preparation of p-phenylenediamine analogues from 4-nitrodiphenylamine by transfer hydrogenation is described here. Two catalysts used here are Raney nickel and palladium supported on carbon. A class of amines could be prepared by using different alcohols or ketones with this technique. The steric effect of alcohols plays an important part in the product yield, with the final product yield decreasing with the bulkiness of the alcohol. Atomic absorption and XPS (X-ray photoelectron spectroscopic) studies on the Raney nickel catalyst indicate that increasing aluminium content in the final catalyst by modifying the catalyst preparation conditions increases the final product yield. Palladium is found to be more active than Raney nickel for this transfer hydrogenation reaction. Both catalysts are compared by kinetics and modelling studies. The rate constants and activation energies for the reaction are determined by fitting the simulation to the experimental data.