Immobilization of Pybox Ligand on Modified Starch

Abstract

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...

The catalytic oxidation of biomass to new materials focusing on starch, cellulose and lignin

“…The supported catalyst exhibited lower activity and selectivity than the unsupported complex but higher than silica immobilised catalysts. It was demonstrated that the yield, diastereoisomeric excess and enantiomeric excess remained constant over three runs [142].…”

Enantioselective Heterogeneous Heck–Matsuda Reaction with Polymer-Supported PyOx Ligands

“…The selection of C5-carboxymethyl-functionalized PyOx for immobilization onto polymeric resins was grounded in the well-documented catalytic efficiency of chiral PyOx in enantioselective homogeneous 14,20,21 and heterogeneous [22][23][24][25] catalysis. Additionally, the C5 carboxyl group enhances the catalytic performance of the ligand and, at the same time, serves as a versatile anchoring group, facilitating seamless attachment to the solid matrix (Scheme 2A).…”