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) u nits were synthesized either by an oxidative coupling with copper(I) u nder air or by a palladium-copper-catalyzed coupling. The first series of dehydroannulenes consisted of tetradehydro[l2]annulene (12), hexad ehydro[l8]annulene (13), octadehydro[24]annulene (14), and decadehydro [30]annulene (15), which all have the BCO units connected by the butad iyne linkage. The second consisted of tridehydro[12]annulene (16) and tetradehydro[16]annulene (17), which have the BCO units connected by t he acetylene bond. The molecular structures of the relatively stable a nnulenes 13 and 17 were determined by X-ray crystallography, which sho wed that 13 has a planar ct-system, whereas 17 has a tub structure lik e cyclooctatetraene. The H-1 NMR signal for the bridgehead proton of t he BCO units is a good measure for the ring current effect. This signa l clearly indicated the presence of diatropicity in [18]annulene 13 an d paratropicity in [12]annulenes 12 and 16. The redox properties of th ese annulenes were examined by cyclic voltammetry. The antiaromatic [1 2]- 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 a nnulenes as expected from the energy level of HOMOs calculated by the PM3 semiempirical MO method.