Computational Studies on the Cyclization of Polycyclic Aromatic Hydrocarbons in the Synthesis of Curved Aromatic Derivatives

10.1002/cphc.200500345.absComputational studies on the cyclization reactions of some polycyclic aromatic hydrocarbons (PAHs) were performed at the DFT level. Compounds C26H14 and C24H14, which show the connectivity of C60 fullerene fragments, were chosen as suitable models to study the formation of curved derivatives by six‐ or five‐membered ring formation, upon oxidation to their radical cations. Four possible pathways for the cyclization process were considered: a) initial CC bond formation to afford a curved derivative, followed by dehydrogenation; b) homolytic CH cleavage prior to cyclization; c) initial concerted H2 elimination and subsequent cyclization; and d) deprotonation of the radical cations prior to cyclization. Computed reaction and activation energies for these reactions show that direct cyclization from radical cations (pathway a) is the lowest‐energy mechanism. The formation of five‐membered rings is somewhat more favourable than benzannulation. After new cycle formation, homolytic CH dissociation to afford the corresponding cations is the most favourable process. These cations react with H. without barrier to give H2. Intermediate deprotonations are strongly disfavoured. The relatively low activation energies compared with carbon cage rearrangements suggest that ionization of PAHs can be used for the tailored preparation of nonplanar derivatives from suitable precursors. Carbon cycles: The cyclization reactions of polycyclic aromatic hydrocarbons (PAHs) to form additional five‐ and six‐membered rings (see picture) are studied at the DFT level, and intermediates and transition states for the possible pathways are determined. The relatively low activation energies suggest that ionization of PAHs can be used for the tailored preparation of nonplanar derivatives and new fullerenes.