Enhancement of Push–Pull Properties of Pentafulvene and Pentafulvalene Derivatives by Protonation at Carbon

5 years ago

Enhancement of Push–Pull Properties of Pentafulvene and Pentafulvalene Derivatives by Protonation at Carbon

François Diederich, Nicolas Kerisit, Karl Kaupmees, Przemyslaw Gawel, Corinne Boudon, Laurent Ruhlmann, Märt Lõkov, Yann Trolez, Daniel Himmel, Nils Trapp, Claudine Katan, Sophie Haberland, Ivo Leito, Aaron D. Finke, Robert Järviste

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. Acid-induced protonation of novel fulvenes and fulvalenes under ambient conditions leads to the formation of stable carbocations with unusual absorption spectra having bathochromically shifted longest-wavelength absorptions as compared to their neutral forms.

Publisher URL: http://onlinelibrary.wiley.com/resolve/doi

DOI: 10.1002/ejoc.201800039