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The development of efficient chiral catalysts plays a crucial role in asymmetric catalysis. Although many chiral ligands/catalysts have been developed in the past.
Table of contents
- Chiral ligand
- Reading | C2-Symmetric ligands
- Privileged chiral ligands and catalysts
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The side-product was possibly crosslinked phosphine formed by reaction of t -BuPCl 2 with two aryllithiums of the resin. In a second approach, the lithiated polystyrene was reacted with tert -butylchloro- N,N -diethylphosphinous amide [ 38 ] and transformation of the obtained supported phosphinous amide to the supported chlorophosphine was then carried out.
Unfortunately, subsequent reduction to the desired supported secondary phosphine led to the same side products in this route as well. Synthesis of synthon 6 via chloro-phosphinite 1. Solid-phase synthesis of immobilised phosphine 6 monitored by gel-phase 31 P NMR. Unfortunately, due to the borane-group, the compound became unreactive in the reaction with lithiated polystyrene 3. Therefore, the crude reaction mixture of 1 and 2 was used directly in excess without further purification in the reaction with lithiated polystyrene 3 Fig. After addition of the reaction mixture to 3 , the excess of reagent and all the other solution-phase side-products present Fig.
The immobilised phosphinite 4 was then protected with borane to prevent the Arbusov reaction as described by Crofts et al [ 40 ] in their studies during the synthesis of diethyl tert -butylphosphonite 2 from tert -butyldichlorophosphine and ethanol. The reactivity of the resulting phosphinite-borane in the following reduction step towards phosphine 6 was found to be very low. With this new methodology we were able to synthesise the ligands under mild conditions and in high purity. After each reaction step the work-up consisted of only a simple purification via filtration and washing of the resin, making it possible to use an excess of reagents.
Synthesis of immobilised diphosphines 9a — g. Solid-phase synthesis of immobilised diphosphine 9d monitored by gel-phase 31 P NMR. The reaction times varied depending on the sulfate and the secondary lithium phosphide used. Once again, the excess of secondary lithium phosphide and other side products formed were easily washed away once full substitution was confirmed by 31 P NMR Fig. The integral ratio of the two phosphine moieties of 9d and 9d - BH 3 is , confirming the formation of the desired supported diphosphine ligand in high purity.
The exact phosphorus loading of the supported ligands could be determined using elemental analysis see ESM for detailed characterisation data. Results of Rh-catalyzed asymmetric hydrogenation Open image in new window. This demonstrates why there is still a need for methods which allow the facile synthesis and screening of ligand libraries as small changes in ligand structure have a large effect on catalyst performance.
More interestingly, 9g is the only ligand that induced different absolute configurations of the hydrogenated products for the different substrates. Surprisingly for substrate I and III the R product was observed whereas 9d with the same absolute configuration provides the S product. However, for substrate II the expected opposite enantiomer S was obtained again. For ligands 9d and e changing the configuration of the ligand backbone led as expected to the opposite configuration of the products in catalysis for all three of the substrates. While the employed supported ligands are used as an epimeric mixture in a ratio, it is expected that this has only a minor influence on the catalytic selectivity.
Previous work by Deerenberg et al. When compared to the ligand BDPP [ S , S -2,4-bis diphenylphosphino pentane], a homogeneous counterpart, it can be seen that for substrate II the non-supported ligand performs better and shows higher activity and selectivity. Often immobilisation of catalysts has a detrimental effect on the selectivity and activity and this nicely showcases that with our facile and modular approach we are able to develop supported ligands which perform on the same level or better than their non-supported counterparts.
In summary we have demonstrated a novel modular solid-phase synthetic procedure for libraries of supported diphosphine ligands on polystyrene resin. Using this facile and efficient method, incorporating only a simple work-up after each step, the supported diphosphines were obtained in high purity. A new synthetic protocol has been developed for the synthesis of supported secondary phosphine 6 incorporating a bulky t -butyl group into the ligand structure.
Subsequently, immobilised bidentate phosphine ligands 9c — g were successfully screened in the Rh-catalyzed asymmetric hydrogenation of several benchmark substrates. The ligands displayed low to high activity and moderate selectivities, demonstrating that small changes in ligand structure can have a profound effect on the actual catalysis. The importance of trial-and-error in ligand discovery is demonstrated by these results and therefore the necessity of the development of facile combinatorial methods towards large ligand libraries.
The extension to larger structural diversity will enable to combine a wider screening of the substrate scope with the anticipated good recycling performance. We are currently extending both the polystyrene supported diphosphine library and the substrate scope. All reactions and manipulations were carried out under inert conditions using standard Schlenk techniques or in an MBraun glovebox unless stated otherwise. N-heterocyclic carbene catalyzed domino reactions.
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Enantioselective ruthenium-catalyzed ring-closing metathesis. Funk, T. Highly active chiral ruthenium catalysts for asymmetric ring-closing olefin metathesis. Powell, M. Chiral imidazolylidine ligands for asymmetric hydrogenation of aryl alkenes. Optically active iridium imidazolylidene-oxazoline complexes: preparation and use in asymmetric hydrogenation of arylalkenes.
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Reading | C2-Symmetric ligands
Chiral diene ligands for asymmetric catalysis. Acta 42 , 31—38 Breslow, R. On the mechanism of thiamine action. Kluger, R. Thiamin diphosphate catalysis: enzymic and nonenzymic covalent intermediates.
Kallfass, U. An efficient nucleophilic carbene catalyst for the asymmetric benzoin condensation. Liu, Q. Catalytic asymmetric intermolecular Stetter reaction of glyoxamides with alkylidenemalonates. Han, J. Asymmetric intermolecular Stetter reactions catalyzed by a novel triazolium derived N-heterocyclic carbene. Ryan, S.
Privileged chiral ligands and catalysts
Fu, Z. Fischer, C.
Umpolung of Michael acceptors catalyzed by N-heterocyclic carbenes. Biju, A. N-heterocyclic carbene catalyzed umpolung of Michael acceptors for intermolecular reactions. Zhang, Y. Chiral N-heterocyclic carbene catalyzed staudinger reaction of ketenes with imines: highly enantioselective synthesis of N-Boc beta-lactams.
Acyl anion free N-heterocyclic carbene organocatalysis. Schreiner, P. Metal-free organocatalysis through explicit hydrogen bonding interactions.
Seayad, J. Asymmetric organocatalysis. Taylor, M.
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Asymmetric catalysis by chiral hydrogen-bond donors. Doyle, A. Small-molecule H-bond donors in asymmetric catalysis.