Abstract
Immobilization of chiral bis(oxazoline)-copper complexes by electrostatic interactions with anionic supports leads to recyclable catalysts for aziridination between styrene and (p- toluensulfonylimino)phenyliodinane as a nitrene precursor. Although the yields are good in all the cases, enantioselectivity depends on the nature of the chiral ligand. With a bis(oxazoline) bearing phenyl substituents, the enantioselectivity is always around 25% ee, as in solution. When the chiral ligand has tert-butyl groups, enantioselectivity is noticeably lower than that observed in homogeneous phase as a consequence of the presence of free copper on the solid.
Highlights
The importance of aziridines comes from their presence in many natural products and from their use as building blocks in the synthesis of nitrogen-containing compounds, and as chiral auxiliaries or ligands in asymmetric synthesis.[1,2] In spite of this importance, the studies on the enantioselective preparation of aziridines are rather scarce, when compared with the development of epoxidation chiral catalysts.[3]Bis(oxazoline)-copper complexes were tested by Evans and coworkers as chiral catalysts in the aziridination of alkenes with nitrene precursors,[4] and modifications on the tether between the oxazoline rings were later described.[5]
Our group has shown that bis(oxazoline)-copper complexes, immobilized by electrostatic interactions on anionic supports, are efficient and recoverable catalysts for enantioselective cyclopropanation reactions.[7,8,9]
Based on previous cyclopropanation results, bis(oxazoline)-copper(II) complexes were immobilized on three different anionic supports: laponite, a synthetic clay; SAC-13, a nafionsilica nanocomposite with 13% nafion content;[10] and SiO2-CF2SO3H, prepared by grafting a partially fluorinated chain with a sulfonic acid group [≡Si–(CH2)3–(CF2)2–O–(CF2)2SO3H] on silica gel.[11]
Summary
The importance of aziridines comes from their presence in many natural products and from their use as building blocks in the synthesis of nitrogen-containing compounds, and as chiral auxiliaries or ligands in asymmetric synthesis.[1,2] In spite of this importance, the studies on the enantioselective preparation of aziridines are rather scarce, when compared with the development of epoxidation chiral catalysts.[3]Bis(oxazoline)-copper complexes were tested by Evans and coworkers as chiral catalysts in the aziridination of alkenes with nitrene precursors,[4] and modifications on the tether between the oxazoline rings were later described.[5]. Our group has shown that bis(oxazoline)-copper complexes, immobilized by electrostatic interactions on anionic supports, are efficient and recoverable catalysts for enantioselective cyclopropanation reactions.[7,8,9] In this paper we explore the use of these solids to promote
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