A. Schmidt, Sviatoslav Batsyts, Felix Lederle
2019
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HETEROCYCLES
Abstract
4-[(Quinolin-4-yl)ethynyl]benzoic acid underwent a cycloreaction with tetraphenylcyclopentadienone to give the hexa(hetero)arylbenzene1-(quinolin-4-yl)-2-(4-benzoic acid)-3,4,5,6-tetraphenylbenzene which was N-methylated and finally deprotonated to give the title compound. The molecule adopts a propeller-shaped configuration in which the benzoate ring as well as the phenyl ring adjacent to the quinolinium substituent display non-isochronous resonance frequencies, respectively, due to a hindered rotation. Although the molecule formally belongs to the class of pseudo-cross-conjugated heterocyclic mesomeric betaines, the propeller-shape causes a charge distribution of cross-conjugated systems. DFT-calculations (frontier orbital profiles, geometries) have been carried out. Mesomeric betaines are conjugated molecules which can exclusively be represented by dipolar resonance forms which delocalize an equal number of positive and negative charges within a common -conjugated system.1 Their physical as well as chemical properties are governed by their type of conjugation.1-5 According to a recent classification, conjugated, cross-conjugated, pseudo-cross-conjugated, semi-conjugated, and pseudo-semi-conjugated heterocyclic mesomeric betaines can be differentiated.2-5 Sydnones 1 are well-known representatives of conjugated mesomeric betaines (CMB),6,7 whereas 6-oxo-pyrimidinium-4-olates 2 are examples of cross-conjugated mesomeric betaines (CCMB). One of the main differences between these two classes of compounds is that CMBs possess common atoms for either charge in the resonance forms, whereas the charges of CCMB are strictly separated according to the rules of resonance (Scheme 1). Heteroarenium-carboxylates like imidazolium-2-carboxylates 3,8-10 pyrazolium-3-carboxylates 4,11 indazolium-3-carboxylates,12,13 and pyridinium-2-carboxylates14-17 are pseudo-cross-conjugated mesomeric betaines (PCCMB). Characteristically, common atoms for the delocalization of the negative as well as of the positive charges exist, if electron-sextet structures without external octet-stabilization are taken into account as shown.1 Limited information is currently available about semi-conjugated and pseudo-semi-conjugated mesomeric betaines.2-5