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, 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.

1,3-thiazolium-5-thiolates mesoionic compounds: semiempirical evaluation of their first static hyperpolarizabilities and synthesis of new examples

“…Figure 1 shows that the mesoionic ring possesses two charged regions separated by what are essentially single bonds. 8 It is this charge separation (asymmetry) that is responsible for the large values of b(0) obtained for this class of compounds. Recently, it was proposed by Kuzyk and co-workers 9,10 that conjugated bridges with sites of reduced conjugation may be excellent prototypes for increasing the values of b(0).…”

Síntese e caracterização do 4,5-difenil-1,3,4-tiadiazólio-2- fenilamina e seus intermediários

“…Segundo Oliveira (1996) [2] os elétrons dos compostos mesoiônicos estão deslocalizados sobre duas regiões do anel separadas apenas por ligações simples. Percebe-se então que os compostos mesoiônicos não obedecem aos critérios de aromaticidade para sistemas cíclicos planares conjugados.…”

Theoretical nonlinear optics equivalence between mesoionic and polyenic bridges in push-pull compounds

“…8 Important questions concerning the structure of mesoionic compounds, in general, can be addressed. de Oliveira et al 9 referred to the degree of separation of regions of positive and negative charges, bond orders, electron and charge delocalization, and aromaticity. In that discussion, we have cited some of our X-ray diffration and theoretical studies on mesoionic compounds, from which we concluded that there are regions of positive and negative charges, each presenting both electron and charge delocalizations.…”

“…Based on this discussion, we introduced a new notation for the structures of mesoionic compounds, Figure 2a, which we believe more accurately represents both the bonding situation and the chemical patterns of reactivity of such compounds. 9,10 In summary, we have postulated the following definition: mesoionic rings are cyclic dipolar structures in which the positive and negative charges are both separated and delocalized within the π eletron system. 9 These compounds, whose basic structure is shown in Figure 2a, are planar five-membered heterocyclic betaines with at least one side-chain, whose c-atom is also in the ring plane, and with dipole moments of the order of 5D.…”