Synthesis, structure, and redox behavior of the dehydroannulenes fused with bicyclo[2.2.2]octene frameworks

Two series of dehydroannulenes fused with bicyclo[2.2.2]octene (BCO) units were synthesized either by an oxidative coupling with copper(I) under air or by a palladium-copper-catalyzed coupling. The first series of dehydroannulenes consisted of tetradehydro[l2]annulene (12), hexadehydro[l8]annulene (13), octadehydro[24]annulene (14), and decadehydro[30]annulene (15), which all have the BCO units connected by the butadiyne linkage. The second consisted of tridehydro[12]annulene (16) and tetradehydro[16]annulene (17), which have the BCO units connected by the acetylene bond. The molecular structures of the relatively stable annulenes 13 and 17 were determined by X-ray crystallography, which showed that 13 has a planar ct-system, whereas 17 has a tub structure like cyclooctatetraene. The H-1 NMR signal for the bridgehead proton of the BCO units is a good measure for the ring current effect. This signal clearly indicated the presence of diatropicity in [18]annulene 13 and paratropicity in [12]annulenes 12 and 16. The redox properties of these annulenes were examined by cyclic voltammetry. The antiaromatic [12]- and [16]annulenes 18 and la having only one acetylene bond between the BCO units were found to be more readily oxidized than the other annulenes as expected from the energy level of HOMOs calculated by the PM3 semiempirical MO method.