Synthetic Routes to the Cyclo[n]carbons

Cyclo[n]carbons (cyclo‐Cn) are n‐membered monocyclic rings of sp‐hybridized C‐atoms with unique electronic structures resulting from two perpendicular systems of conjugated π‐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 [Co2(CO)8] 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 groups, 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.