Chiral host-guest catalysis : enantioselective epoxidation of unfunctionalized alkenes

Li, Zengmin (1999) Chiral host-guest catalysis : enantioselective epoxidation of unfunctionalized alkenes. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Catalytic asymmetric synthesis is the most effective and challenging approach to obtain enantiomerically enriched organic compounds. This research focused on development of new chiral salen-based macrocyclic catalysts for catalytic asymmetric synthesis, particularly asymmetric epoxidation of unfunctionalized alkenes. -- Template-directed reactions of dialdehydes and (1R.2R)- or (1S.2S)-cyclohexane-1.2- diamine gave three series of macrocyclic salen dimers. A single-crystal X-ray study of the first series. 26-membered macrocycles having -CH₂- links, showed a novel calixarene-like structure with a 1.3-alternate conformation in the solid state. NMR studies revealed that a single conformer was maintained in solution for this series of macrocycles. However, the related 32-membered macrocyclic salen dimer containing longer -(OCH₂)₂- links was flexible and showed four conformers in CDCl₃ at room temperature. -- Sequentially, controlled complexation of the macrocyclic salen dimers afforded mononuclear (C₁ symmetric) and binuclear (C₂ symmetric) complexes. Host/guest interactions of the mononuclear complexes were demonstrated through ¹H NMR titration experiments. For the binuclear complexes, two cofacial salen units adopted a syn- or anti- conformation with respect to the cyclohexyl rings relative to a cavity defined by four benzene units. An intramolecular inclusion compound between a binuclear Ni(II) macrocyclic complex and a non-aromatic guest, acetonitrile. was structurally determined by X-ray diffraction. -- Electrocatalytic studies using the rotating ring-disk technique showed that the activity and selectivity of a binuclear calixsalen cobalt complex compared favourably with that of a face-to-face porphyrin catalyst reported by Collman for catalytic reduction of dioxygen at biological pH. -- Catalytic epoxidation studies showed that binuclear Mn(III) complexes had moderate enantioselectivity. An ee of 72% was achieved in the epoxidation of styrene at ambient temperature. The new binuclear catalysts were size-selective and both X-ray and MMX modelling studies of a model complex supported the experimental observations. This research provided the first strong experimental evidence for host/guest catalysis in asymmetric epoxidation.

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