Seven 2-(N-cycloalkylamino-1,3-dithiocarbamoyl)-carboxylic acids and seven mesoionic 2-N-cycloalkylamino-5-alkyl-1,3-dithiolium-4-thiolates have been conveniently synthesized. They were characterized by elemental analysis, LR., U.V., mass and 'H NMR spectrometry, plus I3C N M R spectrometry in some cases. Important questions concerning the structure of the mesoionic compounds, relevant to mesoionic compounds in general, are addressed. In particular, we refer to the degree of separation of regions of positive and negative charge, bond orders, electron and charge delocalization and aromaticity. In this discussion we cite some of our X-ray diffraction and theoretical studies. We conclude that there are regions of positive and negative charge in which there is delocalization of electrons and charge with bond orders between 1 and 2. However, the shared regions are separated by what are essentially single bonds. Thus, they, and mesoionic compounds in general, should not be considered as formally aromatic. This is supported by estimates of Bird Aromaticity indices-experimentally based for two of the title mesoionic compounds-of the order of 50: substantially less than values for thiophene and pyrrole and much less than values for benzene and pyridine. Finally, based on this discussion, we introduce a new notation for the structures of mesoionic compounds which we believe more accurately represents both the bonding situation and the chemical patterns of reactivity of such compounds.
Uma série de Bases de Schiff foi sintetizada através da reação de condensação do piperonal (3,4-metilenodioxibenzaldeido) com as correspondentes aminas primárias aromáticas. Seus espectros de RMN de 1 H e 13 C foram obtidos e estudou-se as correlações de Hammett envolvendo os deslocamentos químicos e as constantes dos substituíntes (σp, σR e σI). Observou-se correlações lineares e bilineares significativas para o carbono imínico (C-α) e C-1', indicando maior contribuição do efeito de ressonância do substituinte sobre os deslocamentos químicos. Os assinalamentos dos C-4' apresentaram-se bem alterados pelos efeitos dos substituíntes, especialmente pelos halogênios, na direção esperada. A série de Bases de Schiff foi ensaiada contra microorganismos, sendo que apresentou atividade significativa frente a fungos filamentosos, especialmente o Epidermophyton floccosum. Não foram observadas correlações significativas entre a atividade biológica e os efeitos eletrônicos.A series of eleven Schiff Bases have been synthesized. They were obtained by condensation of piperonal (3,4-methylenedioxybenzaldehyde) with the corresponding aromatic primary amines. Their 1 H and 13 C-NMR spectra have been obtained and the Hammett correlations including chemical shifts and the subsitutent constants (σp, σR e σI) were studied. Linear and bilinear significant correlations were observed for iminic carbon (C-α) and C-1', showing a more significant ressonance effect on chemical shifts. The chemical shifts for C-4' were highly affected by substituent effects, especially for halogens in the expected direction. Their biological activity against microorganisms has also been measured and significant activity was showed against Epidermophyton floccosum.The biological activity did not give a reasonable relationship with electronic effects.
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