Hetero Diels-Alder Reactions with a Dicationic Urea Azine Derived Azo Dienophile and Their Use for the Synthesis of an Electron-Rich Pentacene.

Herein, we report the first hetero Diels-Alder (DA) reactions with a stable, dicationic urea azine derived azo dienophile, synthesised by two-electron oxidation of the neutral urea azine. Several charged DA products were synthesised in good yield and fully characterised. The DA adduct of anthracene is in thermal equilibrium with the reactants at room temperature, and the reaction enthalpy and entropy were determined from the temperature-dependent equilibrium constant. Furthermore, base addition to solutions of the pentacene DA product leads to deprotonation, cleavage of the N-N bond and formation of an electron-rich 6,13-bisguanidinyl-substituted pentacene. The redox and optical properties of this new pentacene derivative were studied. Furthermore, the dication resulting from its two-electron oxidation was synthesised and fully characterised. The results disclose a novel elegant route to electron-rich pentacene derivatives.