Dipolar Bent and Linear Acetylenes Substituted by Cationic Quinolinium and Anionic Benzoates. Formation of Mesomeric Betaines
2016-04-18T00:00:00Z (GMT) by
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.