Dipolar Bent and Linear Acetylenes Substituted by Cationic Quinolinium and Anionic Benzoates. Formation of Mesomeric Betaines

Andreas Schmidt; Sviatoslav Batsyts; Alexey Smeyanov; Tyll Freese; Eike G Hübner; Martin Nieger

doi:10.1021/acs.joc.6b00561

Dipolar Bent and Linear Acetylenes Substituted by Cationic Quinolinium and Anionic Benzoates. Formation of Mesomeric Betaines

J Org Chem. 2016 May 20;81(10):4202-9. doi: 10.1021/acs.joc.6b00561. Epub 2016 Apr 28.

Authors

Andreas Schmidt¹, Sviatoslav Batsyts¹, Alexey Smeyanov¹, Tyll Freese¹, Eike G Hübner¹, Martin Nieger²

Affiliations

¹ Clausthal University of Technology , Institute of Organic Chemistry, Leibnizstrasse 6, D-38678 Clausthal-Zellerfeld, Germany.
² University of Helsinki , Department of Chemistry, Laboratory of Inorganic Chemistry, P.O. Box 55, FIN-00014 University of Helsinki, Finland.

PMID: 27088664
DOI: 10.1021/acs.joc.6b00561

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.