Novel Electron Acceptors Bearing a Heteroquinonoid System III: 2,5-Bis(dicyanomethylene)-2,5-dihydrofuran and Its Conjugated Homologues as Novel Oxygen-Containing Electron Acceptors

Abstract 2,5-Bis(dicyanomethylene)-2,5-dihydrofuran with a 2,5-furanoquinonoid skeleton and its extensive conjugated homologues, 5,5′-bis(dicyanomethylene)-5,5′-dihydro-Δ2,2′-bifuran (8a), and 5,5″-bis(dicyanomethylene)-5,5″-dihydro-Δ2,2′:5′,2″-terfuran (12a), have been prepared as a new type of oxygen-containing electron acceptors. In the synthetic course of the tetrabromo derivative of 12a, 3′,4-dibromo-5-(3,5-dibromo-2-furyl)-3,5′-bis(dicyanomethylene)-3,5′-dihydro-Δ2,2′-bifuran was also obtained as an unsymmetrical isomer possessing an unusual 2,3-furanoquinonoid conjugation, whose structure was confirmed on the basis of X-ray crystallographic analysis. In addition, X-ray analysis of the TTF complex of 8a revealed that the bifuranoquinonoid conjugation has a trans form. These furanoquinonoid compounds behaved as weak electron acceptors, and accordingly failed to form conductive complexes. However, the 3,3′-dibromo derivative of 8a gave the TTT complex of a very high electrical conductivity.