Dications of Benzylidenefluorene and Diphenylmethylidene Fluorene: The Relationship between Magnetic and Energetic Measures of Antiaromaticity

Abstract

Oxidation of m-and p-substituted benzylidene fluorenes to antiaromatic dications was attempted by electrochemical and chemical means. Electrochemical oxidation to dications was successful for benzylidene fluorenes with p-methoxy, p-methyl, p-fluoro, and unsubstituted phenyl rings in the 3-position; attempts to oxidize the m-substituted derivatives via electrochemistry were unsuccessful. Chemical oxidation with SbF 5 /SO 2 ClF gave the dication of 9-[(4-methoxyphenyl)methylene]-9H-fluorene cleanly; oxidation of all other substituted benzylidene fluorenes resulted in mixtures of products. The excellent linear relationship between the chemical shifts calculated by the GIAO method and the experimental shifts for the p-methoxy-substituted benzylidene fluorene dication suggests that the calculations satisfactorily reflect the magnetic properties of this dication and potentially those of the other dications studied. The redox potentials from electrochemical oxidation, a measure of the stability of the dications, showed a good linear relationship with another measure of stability, the calculated difference in energy between each dication and its neutral precursor. The dications of benzylidene fluorenes were less stable than the dications of diphenylmethylidene fluorenes; within each type of compound, dications with p-substituted phenyl rings were more stable than dications with m-substituted phenyl rings and dications with phenyl rings substituted with electron-donating groups were more stable than dications with phenyl rings substituted with electron-withdrawing groups. The antiaromaticity of the fluorenyl system was assessed through the nucleus-independent chemical shift (NICS) that was also calculated by the GIAO method. The plot of the NICS values per square area versus the calculated energy difference for the dications showed a moderate degree of linearity; the plot of NICS values per square area versus the oxidation potentials was less linear. Thus, a suggestive, but not conclusive, relationship between magnetic and energetic measures of antiaromaticity was observed.

Using Antiaromaticity To Illuminate Aromaticity during a Research Career with Undergraduates

“…This work was extended to the formation of dications from substituted benzylidenefluorenes 19/20. 53 The relationship between calculated and experimental chemical shifts for the protons of 19, Z = OCH 3 , was good, as shown in Figure 1, which also supports the magnitude of NICS values calculated in the same way. Some researchers have concluded that the calculations of NICS for antiaromatic species give values that are abnormally large, but it is clear that the relationship of calculated and experimental chemical shift for the protons on the benzyl substituent are the same as for the fluorenyl systems, which cannot then be abnormally large.…”

“Deconvoluted Fullerene” Derivatives: Synthesis and Characterization

“…Interestingly, as a result of species that can reversibly hold and donate electrons, compounds 8 a-c demonstrated an amphoteric electrochemical behavior, relative to other simpler overcrowded polycycles recently reported. [16] Particularly, the formation of the cation radical produces a stable antiaromatic species as in the case of benzylidenefluorene systems described by Mills et al, [17] whereas the formation of the reduced species is due to two main driving forces: 1) A gain in aromaticity resulting from the formation of 14-and 22-pelectron systems, and 2) strain relief from the interaction of the protons of the fulvalene-like fragments. [18] In fact, the two fluorene moieties in the case of 8 a, b and the benzeneannulated moieties in 8 c (linked to the core by a double bond) are forced to be coplanar.…”

Reduction of Benzylidene Dibenzo[a,d]cycloheptenes: Over-Reduction of Antiaromatic Dianions to Aromatic Tetraanions

“…We anticipated that the electron-donating (destabilizing) group on the benzyl substituent would enhance the antiaromaticity of the dibenzotropylium anion just as electron-withdrawing groups on the benzyl substituent of the analogous benzylidene fluorene dications had enhanced the antiaromaticity of the fluorenyl system. 7 We observed instead the over-reduction of the desired dianion (1 2À ) to an unexpected tetraanion (1 4À ), which contains an aromatic dibenzotropylium trianion (Scheme 1). Of particular interest was our inability to observe the dianion in the 1 H NMR spectrum of the reaction mixture.…”