Synthesis and properties of a novel Cu(ii)–pyridineoxazoline containing polymeric catalyst for asymmetric Diels–Alder reaction

Abstract

A novel pyridineoxazoline (PyOx) containing optically active polymer, poly[(−)-(S)-4-tert-butyl-2-(3-vinylpyridin-2-yl)-oxazoline], was prepared via the radical polymerization of (−)-(S)-4-tert-butyl-2-(3-vinylpyridin-2-yl)-oxazoline. Its complex with Cu(OTf)₂ was employed to catalyse the homogeneous Diels–Alder (D–A) reaction of 2-alkenoyl pyridine N-oxide and cyclopentadiene in tetrahydrofuran. The polymeric catalyst showed a 5 fold faster reaction rate and a 2.5 fold higher enantio-selectivity than the Cu(II) complex of (−)-(S)-4-tert-butyl-2-(3-methylpyridin-2-yl)-oxazoline, a low molecular mass model compound. This was rationalized by the constrained environment around catalytic site provided by the polymer backbone in terms of CD spectrometry results. The polymer complex was easily recycled by the precipitation method, and only a slight decrease of reactivity and enantio-selectivity was observed after 5 cycles.