title: Dipolar Bent and Linear Acetylenes Substituted by Cationic Quinolinium and Anionic Benzoates. Formation of Mesomeric Betaines.
abstract: 3-Ethynylquinoline was subjected to a Sonogashira-Hagihara reaction with methyl 2-, 3-, and 4-bromobenzoates, respectively, and then N-methylated to give 3-[((methoxycarbonyl)phenyl)ethynyl]-1-methylquinolinium salts (two X-ray analyses). On saponification of the 3- and 4-substituted benzoates, the mesomeric betaines 3- and 4-[(1-methylquinolinium-3-yl)ethynyl]benzoates were formed. By contrast, the 2-benzoate derivative gave either the corresponding (1-oxo-1H-isochromen-3-yl)quinolinium derivative or the mesomeric betaine 2-(1-methylquinolinium-3-yl)-1,3-dioxo-2,3-dihydro-1H-inden-2-ide depending on the reaction conditions. A DFT calculation predicts a transoid conformation of the acetylene bond in the intermediate 2-[(1-methylquinolinium-3-yl)ethynyl]benzoate which is due to a strong hydrogen bond between the carboxylate group and 2H of the quinolinium ring, in addition to a 1,5-interaction between the carboxylate group and the CC triple bond. The bond angles of the transoid CC triple bond were calculated to be 211.6° and -175.1° in vacuo. The corresponding linear triple bond is 50.4 kJ/mol less stable in vacuo according to the calculation, and the N-heterocyclic carbene quinoline-2-ylidene is not formed as a tautomer.
authors.affiliation: Clausthal University of Technology , Institute of Organic Chemistry, Leibnizstrasse 6, D-38678 Clausthal-Zellerfeld, Germany.
authors.forename: Andreas
authors.initials: A
authors.lastname: Schmidt
authors.affiliation: Clausthal University of Technology , Institute of Organic Chemistry, Leibnizstrasse 6, D-38678 Clausthal-Zellerfeld, Germany.
authors.forename: Sviatoslav
authors.initials: S
authors.lastname: Batsyts
authors.affiliation: Clausthal University of Technology , Institute of Organic Chemistry, Leibnizstrasse 6, D-38678 Clausthal-Zellerfeld, Germany.
authors.forename: Alexey
authors.initials: A
authors.lastname: Smeyanov
authors.affiliation: Clausthal University of Technology , Institute of Organic Chemistry, Leibnizstrasse 6, D-38678 Clausthal-Zellerfeld, Germany.
authors.forename: Tyll
authors.initials: T
authors.lastname: Freese
authors.affiliation: Clausthal University of Technology , Institute of Organic Chemistry, Leibnizstrasse 6, D-38678 Clausthal-Zellerfeld, Germany.
authors.forename: Eike G
authors.initials: EG
authors.lastname: Hübner
authors.affiliation: University of Helsinki , Department of Chemistry, Laboratory of Inorganic Chemistry, P.O. Box 55, FIN-00014 University of Helsinki, Finland.
authors.forename: Martin
authors.initials: M
authors.lastname: Nieger