MESOIONIC 2-N-CYCLOALKYLAMINO-5-ALKYL-1,3-DITHIOLIUM-4-THIOLATES

Abstract

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, I.R., U.V., mass and ¹H NMR spectrometry, plus ¹³C NMR 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.

Key Words:

• Mesoionic
• bond orders
• charge separation
• aromaticity