The electron-accepting ability of 6,6-dicyanopentafulvenes (DCFs) can be varied extensively through substitution on the five-membered ring. The reduction potentials for a set of 2,3,4,5-tetraphenyl-substituted DCFs, with varying substituents at the para-position of the phenyl rings, strongly correlate with their Hammett σp-parameters. By combining cyclic voltammetry with DFT calculations ((U)B3LYP/6-311+G(d)), using the conductor-like polarizable continuum model (CPCM) for implicit solvation, the absolute reduction potentials of a set of twenty DCFs were reproduced with a mean absolute deviation of 0.10 eV and a maximum deviation of 0.19 eV. Our experimentally investigated DCFs have reduction potentials within 3.67-4.41 eV, however, the computations reveal that DCFs with experimental reduction potentials as high as 5.3 eV could be achieved, higher than that of F4-TCNQ (5.02 eV). Thus, the DCF core is a template that allows variation in the reduction potentials by about 1.6 eV.

We report visible color changes and new intense, bathochromically shifted bands in electronic absorption spectra that reach into the near‐infrared region (up to 862 nm) upon protonation of nine pentafulvene and expanded pentafulvalene derivatives. This phenomenon can only be explained by the formation of carbocations with highly delocalized charges. Solution pKa values in organic solvents were determined, making use of the method of relative basicity measurements. All seven 6‐phenylpentafulvenes are weak bases, and pKa values of the protonated forms range from 0.92 to 10.29 in acetonitrile and from 1.3 to 5.9 in 1,2‐dichloroethane. For 6‐phenylfulvenes with varying para‐substituents on the phenyl ring, pKa values correlate well with the Hammett parameters σpara. Furthermore, for most compounds, electrochemical reduction is significantly facilitated by protonation. Extensive theoretical and NMR studies strongly support the postulated protonation at carbon.

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Walk this way: Mehr als 40 Jahre nach der Entdeckung, dass Fulven sich thermisch zu Benzol umlagern kann, wurde entdeckt, dass 6,6‐Dicyanpentafulvene sich in dipolaren aprotischen Lösungsmitteln unter milden Bedingungen quantitativ zu 1,3‐ und 1,4‐Dicyanbenzolderivaten umlagern können. Zur Erklärung dieser neuartigen Reaktivität wird ein „Ring‐walk“‐Mechanismus vorgeschlagen.

A Mild, Thermal Pentafulvene-to-Benzene Rearrangement. -(FINKE, A. D.; HABERLAND, S.; SCHWEIZER, W. B.; CHEN, P.; DIEDERICH*, F.; Angew. Chem., Int. Ed. 52 (2013) 37, 9827-9830, http://dx.