Transition Metal-Free vs. Metal-Catalyzed Cyclotrimerization of Didehydro[8]annulenes (COTynes): A Complex Pathway to Non-Planar PAHs – Dewar Benzenes vs. Benzotri[8]annulenes

Abstract

The formation of non-planar PAHs from didehydro[8]annulene (COTyne) cycloadditions was investigated under both transition metal-catalyzed (Pd, Ru) and metal-free conditions. The observed reactivity depended on the planarity of the COTyne and the reaction conditions. Parent COTyne 1a dimerized into naphthocyclooctatetraene under TM-free conditions, whereas Pd(0) catalysis promoted its cyclotrimerization into benzotri[8]annulene 7. X-ray characterization and its dynamic behavior in solution was investigated. Planar dibenzoCOTyne 1b exhibited different reactivity depending on its formation method (in situ or preformed), the metal catalyst (Pd, Ru), and absence of catalysts. Under Pd(0) catalysis, cyclotrimerization yielded benzo-fused tri(dibenzo[8]annulene) 3 with moderate efficiency, regardless of how 1b was generated. The presence of K⁺ had no significant effect compared to tribenzoCOTyne 1c. Without metal catalysts, 1b predominantly formed the corresponding Dewar benzene 2. With Ru(II) catalysts, reactivity was influenced by both the generation method of 1b and the Cp ligand. When generated in situ, 1b was an inefficient ligand for CpRu, leading to Dewar benzene formation, whereas preformed 1b produced 3 in moderate yields. The competition between Dewar benzene and benzo-fused tri(dibenzo[8]annulene) formation increased with greater steric hindrance at the Ru center (CpRu vs. Cp*Ru catalysts). Dewar benzene formation likely proceeds via a cyclobutadiene intermediate followed by cycloaddition.