Application of Different Spectral Methods to Examine Structure of the Complexes Found in the Cycloaddition Reactions of
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Appl Magn Reson (2013) 44:1359–1371 DOI 10.1007/s00723-013-0495-1
Magnetic Resonance
Application of Different Spectral Methods to Examine Structure of the Complexes Found in the Cycloaddition Reactions of 4-Trifluoromethylbenzonitrile Oxide to Olefins M. Gucma • W. M. Gołe˛biewski • E. Romanowska W. Skupin´ski
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Received: 4 June 2013 / Revised: 2 August 2013 / Published online: 15 November 2013 Ó Springer-Verlag Wien 2013
Abstract Structure of chiral complexes of ytterbium triflate with (R)-1,10 -bi-2naphthol and 1,2:3,4-di-O-isopropylidene-a-D-galactopyranose, highly effective catalysts in enantioselective cycloaddition reaction of 4-trifluoromethylbenzonitrile oxide to Z-2-pentenol and 2-methoxyphenylcrotonamide, was examined with the application of electron paramagnetic resonance (EPR), 1H, 13C, 19F nuclear magnetic resonance, infrared spectroscopy, mass spectroscopy, and spectrofluorimetry. EPR spectroscopy has demonstrated the presence of the octahedral and tetrahedral environments of the complexes. Observation of strong emission bands proved electron delocalization to the remote areas which enabled an interaction of the ligand with the rest of the catalytic complex. 1 Introduction Some isoxazoline derivatives show fungistatic activity against plant pathogens which depends on their optical purity [1, 2]. One of the methods of enantioselective synthesis is the use of the chiral catalytic complexes of Yb?3 [3–6]. Effectiveness of such complexes is influenced by their structure in different reaction stages and a type of metal–ligand interaction [7]. The character of such interactions is conveniently elucidated after a photoactivation with a specific light length and M. Gucma W. M. Gołe˛biewski (&) Institute of Industrial Organic Chemistry, Annopol 6, 03-236 Warsaw, Poland e-mail: [email protected] E. Romanowska Department of Molecular Plant Physiology, University of Warsaw, Miecznikowa 1, 02-096 Warsaw, Poland W. Skupin´ski Department of High-Energetic Materials, Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland
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registration of the emitted radiation [8]. One of the best methods for investigation of interactions in the complexes is measurement of light emission after excitation of complexes with specific light wavelengths. The structure of the complexes can be analyzed, on the other hand, with the help of electron paramagnetic resonance (EPR) (X band) (3.5–40.0 K) spectroscopy. The type of the complexed atoms can be established using nuclear magnetic resonance (NMR) and infrared (IR) spectroscopy [9–11]. IR spectroscopy can identify functional groups engaged in the ligand–ytterbium interactions and indicate a change in bond lengths. Similarly, instructive is the analysis of NMR spectra. Recording of EPR spectra at liquid helium temperature can contribute to ytterbium complex structure elucidation (tetrahedral versus octahedral environment). Herein we present results of our studies aimed at structure elucidation of the transient co
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