Overcrowded enes of the tricycloindane-1,3-dione series: interplay of twisting, folding and pyramidalization

Abstract

MNDO-PM3 calculations have revealed three low energy conformations for the overcrowded ene 2-(thioxanthen-9-ylidene)indane-1,3-dione 2, two of which, a syn-pyramidalized Cs conformer (yellow solvated crystals) and a C1 folded-twisted conformer (orange unsolvated crystals), were characterized by single crystal X-ray diffraction techniques, and a C2 twisted, non-pyramidalized conformer, which may be present in the thermochromic deep-red crystals obtainable by sublimation at T > 140°C.