Structures of the Reactive Intermediates in Organocatalysis with Diarylprolinol Ethers |
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| Authors: | Uroš Grošelj Dieter Seebach D. Michael Badine W. Bernd Schweizer Albert K. Beck Ingo Krossing Petra Klose Yujiro Hayashi Tadafumi Uchimaru |
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| Affiliation: | 1. Laboratorium für Organische Chemie, Departement für Chemie und Angewandte Biowissenschaften, ETH‐Zürich, H?nggerberg, Wolfgang‐Pauli‐Strasse 10, CH‐8093 Zürich, (phone: +41‐44‐632‐2990;2. fax: +41‐44‐632‐1144);3. On leave from Faculty of Chemistry and Chemical Technology, University of Ljubljana, A?ker?eva 5, P.O. Box 537, 1000 Ljubljana, Slovenia;4. financed by Slovene Human Resources Development and Scholarship Fund (Vilharjeva 27, 1000 Ljubljana, Slovenia) and by Novartis Pharma AG, Basel.;5. Present address: BASF SE, GCl/P‐M311, D‐67056 Ludwigshafen.;6. Postdoctoral Research Fellow at ETH‐Zürich (2006/2007), financed by Novartis Pharma AG, Basel.;7. Lehrstuhl für Molekül‐ und Koordinationschemie, Institut für Anorganische und Analytische Chemie, Albert‐Ludwigs Universit?t Freiburg, Albertstrasse 21, D‐79104 Freiburg i. Br. (phone: +49‐761‐203‐6122);8. Department of Industrial Chemistry, Faculty of Engineering, Tokyo University of Science, 1‐3 Kagurazaka, Shinjuku‐ku, Tokyo 162‐8601, Japan (phone: +81‐3‐5228‐8318);9. Research Institute for Computational Sciences, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305‐8568, Japan, (phone: +81‐29‐861‐4800) |
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| Abstract: | Structures of the reactive intermediates (enamines and iminium ions) of organocatalysis with diarylprolinol derivatives have been determined. To this end, diarylprolinol methyl and silyl ethers, 1 , and aldehydes, Ph? CH2? CHO, tBu? CH2? CHO, Ph? CH=CH? CHO, are condensed to the corresponding enamines, A and 3 (Scheme 2), and cinnamoylidene iminium salts, B and 4 (Scheme 3). These are isolated and fully characterized by melting/decomposition points, [α]D, elemental analysis, IR and NMR spectroscopy, and high‐resolution mass spectrometry (HR‐MS). Salts with BF4, PF6, SbF6, and the weakly coordinating Al[OC(CF3)3]4 anion were prepared. X‐Ray crystal structures of an enamine and of six iminium salts have been obtained and are described herein (Figs. 2 and 4–8, and Tables 2 and 7) and in a previous preliminary communication (Helv. Chim. Acta 2008 , 91, 1999). According to the NMR spectra (in CDCl3, (D6)DMSO, (D6)acetone, or CD3OD; Table 1), the major isomers 4 of the iminium salts have (E)‐configuration of the exocyclic N?C(1′) bond, but there are up to 11% of the (Z)‐isomer present in these solutions (Fig. 1). In all crystal structures, the iminium ions have (E)‐configuration, and the conformation around the exocyclic N‐C? C‐O bond is synclinal‐exo (cf. C and L ), with one of the phenyl groups over the pyrrolidine ring, and the RO group over the π‐system. One of the meta‐substituents (Me in 4b , CF3 in 4c and 4e ) on a 3,5‐disubstituted phenyl group is also located in the space above the π‐system. DFT Calculations at various levels of theory (Tables 3–6) confirm that the experimentally determined structures (cf. Fig. 10) are by far (up to 8.3 kcal/mol) the most stable ones. Implications of the results with respect to the mechanism of organocatalysis by diarylprolinol derivatives are discussed. |
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| Keywords: | Organocatalysis Catalysis Prolinol ethers Enamines Iminium ions Density‐functional theory (DFT) X‐Ray crystallography |
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