Synthetic Routes to the Cyclocarbons

Article abstract of DOI:10.1002/hlca.19940770522

Cyclocarbons (cyclo-Cn) are n-membered monocyclic rings of sp-hybridized C-atoms with unique electronic structures resulting from two perpendicular systems of conjugated ?-orbitals, one in-plane and one out-of-plane.Several synthetic approaches to generate cyclo-C18 from stable precursors were investigated.In a six-step sequence from anthracene, tris(anthraceno)hexadehydro<18>annulene 5 was prepared and shown by laser-desorption time-of-flight mass spectrometry to generate C18 in three successive retro-Diels-Alder reactions, together with anthracene as the by-product.The attempted preparation of C18 by flash-vacuum pyrolysis of 5 using solvent-assisted sublimation only afforded anthracene next to polymers.The reaction of 1,6-bis(triisopropylsilyl)hexa-1,3,5-triyne with followed by exchange of two CO groups for a bridging bis(diphenylphosphino)methane (dppm) ligand gave Co complex 22 which, after removal of the silyl goups, was oxidatively cyclized to afford the very stable trimeric and tetrameric macrocycles 6 and 7, complexes with cyclo-C18 and cyclo-C24, respectively.An X-ray crystal structure established the identity of 6 and showed that the butadiyne units within the C18 core are considerably bent.Attempts to free cyclo-C18 from the coordinating metal atoms in 6 did not succeed, presumably due to the steric shielding of the Co-atoms by the dppm ligands.Low-temperature matrix isolation studies using IR and UV/VIS spectroscopy showed that irradiation of carbon oxide 2 (C24O6) leads to ketene intermediates and, by subsequent loss of six CO molecules, presumably to cyclo-C18.