Gas-Phase Formation of 1,3,5,7-Cyclooctatetraene (C₈H₈) through Ring Expansion via the Aromatic 1,3,5-Cyclooctatrien-7-yl Radical (C₈H₉) Transient

Gas-phase 1,3,5,7-cyclooctatetraene (C₈H₈) and triplet aromatic 1,3,5,7-cyclooctatetraene (C₈H₈) were formed for the first time through bimolecular methylidyne radical (CH)-1,3,5-cycloheptatriene (C₇H₈) reactions under single-collision conditions on a doublet surface. The reaction involves methylidyne radical addition to the olefinic πelectron system of 1,3,5-cycloheptatriene followed by isomerization and ring expansion to an aromatic 1,3,5-cyclooctatrien-7-yl radical (C₈H₉). The chemically activated doublet radical intermediate undergoes unimolecular decomposition to 1,3,5,7-cyclooctatetraene. Substituted 1,3,5,7-cyclooctatetraene molecules can be prepared in the gas phase with hydrogen atom(s) in the 1,3,5-cycloheptatriene reactant being replaced by organic side groups. These findings are also of potential interest to organometallic chemists by expanding the synthesis of exotic transition-metal complexes incorporating substituted 1,3,5,7-cyclooctatetraene dianion (C₈H₈^2-) ligands and to untangle the unimolecular decomposition of chemically activated and substituted 1,3,5-cyclooctatrien-7-yl radical, eventually gaining a fundamental insight of their bonding chemistry, electronic structures, and stabilities.