10.1021/acs.joc.6b00561.s002 Andreas Schmidt Andreas Schmidt Sviatoslav Batsyts Sviatoslav Batsyts Alexey Smeyanov Alexey Smeyanov Tyll Freese Tyll Freese Eike G. Hübner Eike G. Hübner Martin Nieger Martin Nieger Dipolar Bent and Linear Acetylenes Substituted by Cationic Quinolinium and Anionic Benzoates. Formation of Mesomeric Betaines American Chemical Society 2016 methylquinolinium bond Linear Acetylenes Substituted CC DFT carboxylate group 2016-04-18 00:00:00 Journal contribution https://acs.figshare.com/articles/journal_contribution/Dipolar_Bent_and_Linear_Acetylenes_Substituted_by_Cationic_Quinolinium_and_Anionic_Benzoates_Formation_of_Mesomeric_Betaines/3204916 3-Ethynylquinoline was subjected to a Sonogashira–Hagihara reaction with methyl 2-, 3-, and 4-bromobenzoates, respectively, and then <i>N</i>-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-1<i>H</i>-isochromen-3-yl)­quinolinium derivative or the mesomeric betaine 2-(1-methylquinolinium-3-yl)-1,3-dioxo-2,3-dihydro-1<i>H</i>-inden-2-ide depending on the reaction conditions. A DFT calculation predicts a <i>transoid</i> 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 2<i>H</i> 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 <i>transoid</i> 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.