The First Immobilization of Pyridine-bis(oxazoline) Chiral Ligands

Cornejo, Alfonso; Fraile, José M.; Garcı́a, José I.; García‐Verdugo, Eduardo; Gil, María José; Legarreta, Gorka; Luis, Santiago V.; Martínez‐Merino, Víctor; Mayoral, José A.

doi:10.1021/ol026787a

Cited by 75 publications

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References 46 publications

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“…Alternative C 1 -symmetric ligands, both sterically and electronically, derived from pyridine and quinoline were also tested ( Figure 11). 12,13 A similar behavior was observed, irrespective of the chelate size, 5 with pyridine ligands and 6 45 with quinoline ones. In particular, the clay-supported complexes displayed a high cis/trans selectivity (up to 85:15), but enantioselectivities were only moderate, and no reversal in the absolute configuration of the major cis-cyclopropane was observed, in contrast with bis(oxazoline) ligands and in 50 agreement with the initially proposed mechanism (Figure 10).…”

Section: Examples Of Clay Surface Effects On Enantioselectivitysupporting

confidence: 67%

Enantioselective catalysis with chiral complexes immobilized on nanostructured supports

et al. 2009

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5In spite of the inherent advantages of reuse and stability of immobilized chiral catalysts, they are frequently not enough to justify the effort necessary to immobilize the chiral complexes, unless additional advantages are found. When chiral catalysts are immobilized on solids with surface and pores of well controlled size, shape and dimensionality, the solid may act as a nano-reactor reducing the free movement of reagents, intermediates and, mainly, transition states. In this way 10 relative energies of the transition states leading to the different stereoisomers may be modified and, as a consequence, the stereochemical result of the reaction is modified when compared with the obtained in solution. The use of support effects to improve and/or change the stereochemical result of enantioselective reactions is currently emerging as an area of interest, but still with a limited number of examples. A better comprehension of the support influence will be needed in 15 order to allow, in the near future, the design of chiral ligands better adapted to this strategy.

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Section: Examples Of Clay Surface Effects On Enantioselectivitysupporting

confidence: 67%

Enantioselective catalysis with chiral complexes immobilized on nanostructured supports

et al. 2009

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“…In terms of recycling, starch immobilized catalyst 5a allowed more runs without decreasing of performance than reported copolymers. 8 In conclusion, we have shown the possibility of immobilizing chiral pybox system on modified starch. This fact, make us consider that other polysaccharides can be developed as new types of supports for organometallic species.…”

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confidence: 71%

“…5 Pybox ligands have been supported onto silica 6 and polymers 7 via grafting or copolymerized with styrene and divinylbenzene. 8 These studies showed that the nature of the support played a significant role on the activity of the supported catalyst as well as on the enantioselectivity of the reaction. 9 To our knowledge, carbohydrate type solids have not been developed as supports for chiral catalysts, and this could be of interest because the additional chirality of the support.…”

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“…As previously reported, it is quite difficult to obtain a local excess of chiral ligand on polymer. 8 So, the Pybox starch-Ru-ethylene 1:1:1 complex 5a was tested. This complex 5a exhibited lower activity and selectivity toward the cyclopropanation reaction than homogeneous complex 5b pointing out the dramatic influence of the support.…”

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confidence: 99%

“…However, ruthenium catalyst supported on starch 5a showed better performance on ACP than silica immobilized catalysts, 9 although with worse asymmetric induction than copolymers (specially on the trans/cis ratio and cis enantioselectivity). 8 After removing of the liquid solution and washing of the solid with degassed ethanol, the performance of the catalyst was established again. Over 3 runs, the trans/cis selectivity as well as the enantioselectivity keep constant.…”

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Immobilization of Pybox Ligand on Modified Starch

Cornejo¹,

Martínez‐Merino²,

Gil³

et al. 2005

Chemistry Letters

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Starch, one of the most abundant natural polymer was used as support for chiral pyridine-bis(oxazoline) ligand. The corresponding ruthenium complex was used as catalyst in cyclopropanation reaction of styrene with ethyl diazoacetate. Yield, diastereoisomeric excess and enantiomeric excess kept constant after 3 runs.The preparation of tethered organometallic complexes to perform enantioselective synthesis has been reported over several types of supports. Most commonly chiral catalysts were immobilized over solid support through covalent bondings. 1Ligands have been covalently grafted to inorganic materials such as amorphous silica or mesoporous structured inorganic solid. They can also be introduced in organic polymers either by modification of Merrifield type resin or by copolymerisation. On the other hand, natural polymers such as polysaccharides are largely available on great scale and can be applied as support of molecular catalysts. In this sense, the starch is a particularly attractive molecule: its swelling is strongly affected by the nature of the solvent. Moreover, it is very cheap and not toxic. Few reports are dealing with the use of polysaccharides as support for organometallic species. Allylic substitution was performed with Pd/TPPTS complex entrapped in different polysaccharides. Silica-supported starch-polysulfosiloxane-Pt complexes were used for asymmetric hydrogenation of 4-methyl-2-pentanone to 4-methyl-2-pentanol with 94% ee. 3 In that approach, the intrinsic chirality of the starch was responsible of the enantioselectivity. Recently, organometallic complexes were grafted to functionalized cellulose for potential application in nonlinear optic. 4Chiral pyridine-bis(oxazoline) (Pybox) ligands complexed to ruthenium are very efficient in cyclopropanation reaction of alkenes with diazoesters. 5 Pybox ligands have been supported onto silica 6 and polymers 7 via grafting or copolymerized with styrene and divinylbenzene. 8 These studies showed that the nature of the support played a significant role on the activity of the supported catalyst as well as on the enantioselectivity of the reaction. 9 To our knowledge, carbohydrate type solids have not been developed as supports for chiral catalysts, and this could be of interest because the additional chirality of the support. Thus, the aim of this work was the covalent grafting of Pybox ligands to starch and their test in the asymmetric cyclopropanation benchmark reaction (ACP). In order to get high flexibility of the catalyst over the support, it is necessary to insert a long chain as a linker between the starch and the ligand. We previously reported palladium-catalyzed telomerization of starch with butadiene.10 This transformation allowed the introduction of a 8-carbons chain in starch. Owing to the presence of unsaturated double bonds in the lateral chain, subsequent functionalization is conceivable. In this paper, we reported the grafting of a Pybox ligand to a telomerized starch and the preliminary results in asymmetric cyclopropanation reaction.Tre...

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