A synthetic approach of fused tetrathiafulvalene - tetracyanoquinodimethane systems via a Knoevenagel condensation

International audienceIn the search for transformations of quinone moieties into the corresponding tetracyano-p-quinodimethane (TCNQ) systems, Knoevenagel-type condensations of 2-thioxo-1,3-dithiole- p-benzoquinone systems, TTF-p-benzoquinone dyads and a p-benzoquinone-TTF-p-benzoquinone triad with malononitrile under different experimental conditions were investigated. In order to avoid the well-known Michael 1,4- addition onto the quinone moiety, cyclopentadiene or cyclohexadiene units were incorporated as protecting groups through [4+2] Diels-Alder cycloadditions. On the one hand, aromatization leading to fused 2-thioxo-1,3-dithiole-hydroquinones was observed in the presence of acetic acid and pyridine. With use, on the other hand, of the nucleophilic malononitrile anion, an unprecedented ring-opening process of the 1,3-dithiole moiety occurred, affording stable ketene imines. Finally, successful conversion of 1,3-dithiole-pbenzoquinones into the corresponding TCNQ systems was achived by treatment with malononitrile in the presence of titanium(IV) chloride and pyridine as Lehnert's reagents. All new TCNQ derivatives were unambiguously characterized. So far, the application of such experimental conditions to protected TTF-p-benzoquinone dyads and a p-benzoquinone- TTF-p-benzoquinone triad has been unsuccessful, maybe due to the high reactivity of the TTF moiet