Stepwise cope rearrangement of cyclo-biphenalenyl via an unusual multicenter covalent π-bonded intermediate

Multicenter covalent π-bonding between π-conjugated radicals has been recently recognized as a novel and important bonding interaction. The Cope rearrangement of cyclo-biphenalenyl 9 is studied by exploring its potential energy surface with density functional theory (DFT), and it is found that π-bonding plays a critical role in the rearrangement process. Affected by this, the rearrangement of 9 takes place by a stepwise mechanism through an unusual π-intermediate 10, of C_2h symmetry, which can be characterized as a 27times;13π+2×2π system. The π-intermediate has a long inter-phenalenyl distance of R ≈ a 2.8 Å, which is shorter than the sum of the van der Waals radii displaying multicenter covalent π-bonding between the two phenalenyl units. The energy of the π-intermediate 10 is higher than that of the σ-bonded reactant 9 by ≃2 kcal/mol according to the employed spin-restricted DFT. NMR chemical shift calculations support the σ-bonded 9 as the global minimum. The calculated activation barrier of ∼6 kcal/mol for the Cope rearrangement is consistent with the stepwise mechanism. A covalent π-bonding effect in the π-intermediate 10 is demonstrated indirectly by the shortening of inter-naphthalene distance of the dianion and dication of the cyclophane 14 compared to that of its neutral counterpart. The unusual π-bonded structure with a long inter-phenalenyl distance becomes the most stable structure for the ethano-bridged derivative 13, which should have observable paramagnetism according to the calculated paramagnetic susceptibility.