Chiral Propargylic Cations as Intermediates in S1-Type Reactions: Substitution Pattern, Nuclear Magnetic Resonance Studies, and Origin of the Diastereoselectivity.
Nine propargylic acetates, bearing a stereogenic center (-CHXR) adjacent to the electrophilic carbon atom, were prepared and subjected to S1-type substitution reactions with various silyl nucleophiles employing bismuth trifluoromethanesulfonate [Bi(OTf)] as the Lewis acid. The diastereoselectivity o...
| Publicado en: | Journal of the American Chemical Society Vol. 136; no. 7; pp. 2851 - 2858 |
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| Autores principales: | , , , , , , |
| Formato: | Artículo |
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American Chemical Society
2/19/2014
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| Materias: | |
| Acceso en línea: | Ver este registro en EBSCOhost |
| fields | @attributes: recordID: 1 pdfLink: plink: https://search.ebscohost.com/login.aspx?direct=true&db=hlh&AN=94872797&site=ehost-live header: @attributes: shortDbName: hlh uiTerm: 94872797 longDbName: Humanities International Complete uiTag: AN controlInfo: bkinfo: jinfo: jid: 00027863 ACS jtl: Journal of the American Chemical Society issn: 00027863 maglogo: N pubinfo: dt: 2/19/2014 vid: 136 iid: 7 pid: 997 pub: American Chemical Society artinfo: ui: 94872797 10.1021/ja411772n ppf: 2851 ppct: 7 formats: tig: atl: Chiral Propargylic Cations as Intermediates in S1-Type Reactions: Substitution Pattern, Nuclear Magnetic Resonance Studies, and Origin of the Diastereoselectivity. aug: au: Nitsch, Dominik Huber, Stefan M. Pöthig, Alexander Narayanan, Arjun Olah, George A. Surya Prakash, G. K. Bach, Thorsten affil: Department Chemie and Catalysis Research Center (CRC), Technische Universität München, D-85747 Garching, Germany Loker Hydrocarbon Research Institute and Department of Chemistry, University of Southern California, Los Angeles, California 90089-1661, United States su: Density functionals Scission (Chemistry) Chemical reactions Native element minerals Acid-base chemistry Magnetic resonance Cations sug: subj: Density functionals Scission (Chemistry) Chemical reactions Native element minerals Acid-base chemistry Magnetic resonance Cations ab: Nine propargylic acetates, bearing a stereogenic center (-CHXR) adjacent to the electrophilic carbon atom, were prepared and subjected to S1-type substitution reactions with various silyl nucleophiles employing bismuth trifluoromethanesulfonate [Bi(OTf)] as the Lewis acid. The diastereoselectivity of the reactions was high when the alkyl group R was tertiary (tert-butyl), irrespective of the substituent X. Products were formed consistently with a diastereomeric ratio larger than 95:5 in favor of the anti-diastereoisomer. If the alkyl substitutent R was secondary, the diastereoselectivity decreased to 80:20. The reaction was shown to proceed stereoconvergently, and the relative product configuration was elucidated. The reaction outcome is explained by invoking a chiral propargylic cation as an intermediate, which is preferentially attacked by the nucleophile from one of its two diastereotopic faces. Density functional theory (DFT) calculations suggest a preferred conformation in which the group R is almost perpendicular to the plane defined by the three substituents at the cationic center, with the nucleophile approaching the electrophilic center opposite to R. Transition states calculated for the reaction of allyltrimethylsilane with two representative cations support this hypothesis. Tertiary propargylic cations with a stereogenic center (-CHXR) in the α position were generated by ionization of the respective alcohol precursors with FSOH in SOClF at -80 °C. Nuclear magnetic resonance (NMR) spectra were obtained for five cations, and the chemical shifts could be unambiguously assigned. The preferred conformation of the cations as extracted from nuclear Overhauser experiments is in line with the preferred conformation responsible for the reaction of the secondary propargylic cations. pubtype: Academic Journal doctype: Article src: R language: English refInfo: copyright: @attributes: flag: Y dt: @attributes: year: 2014 holdings: @attributes: islocal: N |
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