Polymer‐Bound Pyridine‐Bis(oxazoline). Preparation through Click Chemistry and Evaluation in Asymmetric Catalysis

Abstract

A pyridine-bis(oxazoline) ligand was efficiently immobilized by copper(I)-catalyzed azidealkyne cycloaddition onto a polystyrene resin. The so obtained click-pybox resin 1a was associated with various metal salts (YbCl 3 , LuCl 3 , CuOTf) and the resulting resin-bound catalysts were explored in ring-opening of cyclohexene oxide, silylcyanation of benzaldehyde and alkynylation of imines. These new polymer-supported catalysts exhibit good to excellent performances in terms of catalytic activity, enantioselectivity and recyclability.Keywords: asymmetric catalysis; click chemistry; copper(I); immobilization; lanthanides; pybox In recent years, pyridine-2,6-bisoxazoline (pybox) derivatives have emerged as a popular class of tridentate ligands, highly efficient in a large range of asymmetric catalytic processes.[1] The demands for economical, eco-friendly and recyclable polymer-supported catalysts led some groups to investigate various strategies to attach pybox ligands on solid supports.[2] The first immobilization of pybox was reported by Mayoral et al. by means of copolymerization of 4-vinylpybox derivatives with styrene and divinylbenzene. [2a,b] Moberg et al. subsequently reported the grafting of pybox ligands onto a TentaGel resin via the formation of ester bonds.[2c] Finally, Portnoy at al. reported a five-step solid-phase synthesis of various pybox ligands from a Wang trichloroacetimidate resin.[2d] Although these polymer-supported pybox ligands demonstrated promising results in a number of transformations, all these approaches suffer from serious drawbacks and limitations, regarding either the number of steps or the stability of the linker connecting pybox to the polymeric matrix. We report herein a rapid and straightforward method for the preparation of a new polystyrene-supported pybox by means of "click chemistry".[3] The performance of the resulting click-pybox resin 1a was investigated in asymmetric ring opening of cyclohexene oxide, [4] silylcyanation of benzaldehyde, [5] and alkynylation of imines.[6]We were especially interested in investigating the Cu(I)-catalyzed "click" azide-alkyne cycloaddition, [7] since the resulting triazole ring connecting the ligand to the polymer is formed in high yield and under mild conditions, [8] providing a stable linker compatible with various reaction conditions. The robustness of this triazole linker and the simplicity of the method make this approach very attractive for accessing new polymer-supported catalysts. This strategy has recently been exploited with success for the immobilization of hydroxyproline,[9a] TEMPO, [9b] P,N-ligands, [9c] aza(bisoxazoline) [9d] and Cinchona alkaloid derivatives.[9e]The click-pybox resin 1a was uneventfully prepared in a three-step sequence from the readily available 4-bromo-substituted phenyl-pybox 2[2c] and polystyrenesupported azide 5, [10] using the Gmeiner CuI protocol.[8] The required "alkyne anchor" was installed through a Sonogashira cross-coupling between 2 and (trimethylsilyl)acetylene in refluxing CH 2 Cl...

Recyclable Stereoselective Catalysts

“…1.17 ) was tested in the cyanosilylation of aromatic aldehydes promoted by ytterbium and lutetium. High conversions (68% − 87%) and high enantioselectivities (67% − 78% ee) were obtained for this reaction in four consecutive cycles (10 mol% of catalyst, acetonitrile/dichloromethane 3:2) [77] .…”

Rare earth metal oxazoline complexes in asymmetric catalysis

“…The use of a polymer-supported (via the pybox ligand) [Yb(pybox)Cl 3 ] gave 65% ee, but with a reduction to 21% ee in the second run and only 7% in the third. 52 The addition of aldehydes to allylsilanes and allenylsilanes, resulting in homopropargylic alcohols and dihydrofurans (formed from addition and annulation reactions, respectively) (Scheme 18) was catalysed by the scandium complex [Sc(Ph-pybox)(OTf) 3 ], allowing the reactions to proceed with up to 98% ee and 94% ee, respectively, with the dihydrofuran being produced when View Online the steric bulk of the silane moiety was increased from SiMe 3 to Si i Pr 3 . 53 In 2003 Evans reported that the scandium triflate complex [Sc(Ind-pybox)(OTf) 3 ] catalyses the addition of N-methylindole to a,b-unsaturated acylphosphonates with good stereocontrol (Scheme 19 and Table 2).…”

Polymer “Clicking” by CuAAC Reactions

“…Tilliet et al [183] modified chiral pyridine-bis(4-R-phenyloxazoline) (pybox) with TMS-acetylene at the 4 position of the pyridine ring and immobilized the catalyst on polystyrene beads. The polymer catalyst was complexed with transition metals and used for chiral silylcyanation of aldehydes and addition of alkyne to imines.…”