jo6b00561_si_002.pdf (3.95 MB)
Dipolar Bent and Linear Acetylenes Substituted by Cationic Quinolinium and Anionic Benzoates. Formation of Mesomeric Betaines
journal contribution
posted on 2016-04-18, 00:00 authored by Andreas Schmidt, Sviatoslav Batsyts, Alexey Smeyanov, Tyll Freese, Eike G. Hübner, Martin Nieger3-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.