New chiral ligands in asymmetric catalysis. Application in stabilization of metal nanoparticles
Summary Thesis M. Rosa Axet
This thesis deals with the development and application of diphosphite ligands derived from carbohydrates to rhodium-catalysed asymmetric hydroformylation and hydrogenation reactions. The use of various carbohydrate derivative ligands as stabilisers of metal nanoparticles is also studied.
The synthesis and the characterisation of the series of diphosphite ligands are described in Chapter 2. The results of the asymmetric hydroformylation of styrene and related vinyl arenes are also described. High-pressure NMR studies of [RhH(CO)2(L)] (L:diphosphite) under catalytic conditions reveal detailed information about the coordination behaviour of these diphosphite ligands. The respective cationic rhodium complexes of diphosphite ligands are prepared and characterised to be used in the hydrogenation of methyl acetamidoacrylate.
New diphosphite ligands with C2-symmetry and a tetrahydrofuran backbone have been synthesised in moderate to good yields starting from D-glucosamine, D-glucitol and (2S,3S)-diethyl tartrate. The rhodium cationic complexes of diphosphite ligands of general formula [Rh(cod)(L)]BF4, (L= diphosphite ligand) were prepared.
This new series of diphosphite ligands has been applied to the rhodium-catalysed asymmetric hydroformylation of styrene and related substituted vinyl arenes. High regioselectivities to the branched aldehyde (up to 90%) and moderate enantioselectivities (up to 46%ee) were obtained in the asymmetric hydroformylation of styrene. In the hydroformylation of p-methoxystyrene was obtained 60% of enantioselectivity. The configuration and substitution of the remote stereocenters at positions 2 and 5 of the tetrahydrofuran ring were observed to have a considerable influence on the enantioselectivity. The most significant result is that the configuration of the major isomer obtained in the hydroformylation reaction can be controlled by changing the configuration of these stereocenters.
The intermediate species in hydroformylation with diphosphite ligands were studied by high pressure-NMR spectroscopy. These species were prepared in situ under hydroformylation conditions, observing that the formation of the hydridorhodiumcarbonyl specie was very slow. The ligands coordinate in an equatorial-equatorial fashion in the trigonal-bipyramidal hydridorhodiumcarbonyl species.
Rhodium complexes were tested in the asymmetric hydrogenation of methyl acetamidoacrylate. The conversions and the enantioselectivities were low and were mainly influenced by the substitution in the biphenyl moiety and by the configuration of the remote centres at positions 2 and 5 of the tetrahydrofuran ring.
Chapter 3 discusses the use of diphosphite chiral ligands in the rhodium-catalysed hydroformylation of allylbenzene and propenylbenzene. The hydroformylation of these substrates, easily available from biomass, enables the facile synthesis of aldehydes that are useful in the perfume and pharmaceutical industry. The rhodium-diphosphite system was used in the hydroformylation of trans-anethole and led to high selectivities (as high as 86%) under mild conditions (60ºC, 40 bar). We also studied the asymmetric induction of the two diphosphite chiral ligands in this reaction. We only observed low enantioselectivities in the case of diphosphite in trans-anhetole hydroformylation. In the hydroformylation of estragole, we used rhodium-diphosphite ligand. In this case, regioselectivities on the branched aldehyde were low (47% of and 53% of lineal aldehyde) when excess of ligand was added. We also investigated the enantioselectivity but we did not observe asymmetric induction in the conditions studied.
Chapter 4 describes the novel application of carbohydrate derivative ligands as stabilisers of metal nanoparticles. Rhodium, platinum and ruthenium nanoparticles are prepared by the organometallic approach in the presence of carbohydrate derivative ligands. The proprieties of the nanoparticles were strong influenced by the metal precursor and the stabiliser used. The palladium nanoparticles were applied as catalyst in the allylic alkylation reaction showing excellent results. The application of rhodium nanoparticles as catalyst in the styrene hydroformylation provided low activities. The results of the colloidal system, the respective molecular system, and the poisoning test were suggesting the formation of small amounts of homogeneous catalyst from colloidal system.
Advisor:Claver Cabrero, Carmen; Castillón Miranda, Sergio
School:Universitat Rovira i Virgili
Source Type:Master's Thesis
Keywords:departament de química física i inorgànica
Date of Publication:01/27/2006