The overcrowded thermochromic bistricyclic aromatic enes (BAEs) 10-(9ЈH-fluoren-9Ј-ylidene)-9(10H)-anthracenone (6), 10-(11ЈH-benzo [b]fluoren-11Ј-ylidene)-9(10H)-anthracenone (7), and 10-(1Ј,8Ј-diaza-9ЈH-fluoren-9Ј-ylidene)-9(10H)-anthracenone (8) were synthesized by applying Barton's twofold extrusion diazo-thione coupling method and their crystal and molecular structures were determined. BAEs 6-8 exhibit thermochromic behavior at room temperature due to the equilibrium between the yellow anti-folded conformations and the thermochromic purple, blue, or red twisted conformations. The NMR experiments demonstrate a fast interconversion of the twisted and the anti-folded conformers of 6-8 in solution. BAE 7 readily undergoes E,Z-topomerization at room temperature with the coalescence point at 297 K and
Intramolecular overcrowding in bistricyclic enes (1) requires out-of-plane deformations in order to accommodate the sterically demanding tricyclic moieties without prohibitively close contacts of nonbonded atoms. The nonplanarity of 1 introduces the notion of chirality to the arena of overcrowded aromatic enes. Deviation from coplanarity occurs by twisting around the double bond and out-of-plane bending (hence pyramidalizatwn). The bending is realized by folding of the tricyclic moieties. The conformations of homomerous (1, X = Y) and heteromerous (1, X f Y) bistricyclic enes and their mono-and disubstituted derivatives are analyzed along with their point group symmetries and chrality. The implications are illustrated using the structures of characteristic examples calculated with the semiempirical method PM3. o 1995 WiIey-Liss, inc.
Oxidation of 3,6-disubstituted tetrabenzo[5.5]fulvalenes by SbF(5) results in the formation of dications that behave like two antiaromatic fluorenyl cations connected by a single bond. Both fluorenyl systems exhibit the paratropic shifts and nucleus independent chemical shifts (NICS) characteristic of antiaromatic species. Comparison with analogous 2,7-disubstituted tetrabenzo[5.5]fulvalenes reveals that the antiaromaticity of the substituted ring system can be altered substantially by changes in the placement of the substituents, possibly due to changes in the delocalization of charge in the system. Substituents in the 3,6-position decrease the antiaromaticity because of the increase in the benzylic resonance compared to 2,7-substituents.
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