Biphenalenylidene:  The Forgotten Bistricyclic Aromatic Ene. A Theoretical Study

Abstract

The Lawesson reagent and P(2)S(5) mediated reductive coupling of phenalenone (6) gives LPAH peropyrene (5) in 47% and 54% yields. The mechanism of the reaction involves the formation of phenalenethione (10), Z- and E-1,1'-biphenalenylidene (3), and 9 as intermediates. The electrocyclization reaction of Z-3 to 9, followed by aromatization, gives 5. The results of an ab initio and DFT study of 3 and 2,2'-biphenalenylidene (12) are reported. E-3 and Z-3 have a diradical character with twist angles of 44.8 degrees and 57.8 degrees (at UB3LYP/6-311G**). Delta E(++)(Tot) = 10.2 kJ/mol and Delta G(++)(298) = 10.6 kJ/mol for E-3 <==> Z-3 diastereomerization. These unusually low energy barriers are due to the ground-state diradical destabilization and the aromatic stabilization of the transition state TS-3. Triplet Z-3 is higher in energy than singlets E-3 and Z-3 by 10.4 and 3.1 kJ/mol. In the concealed non-Kekulé 12, singlet 12 is more stable than the triplet by 1.3 kJ/mol. Singlet 12 is more stable than singlet E-3 by 2.0 kJ/mol, and orthogonal singlet TS-12 is lower in energy than singlet TS-3 by 6.0 kJ/mol. The energy barriers for the hexatriene-cyclohexadiene electrocyclization Z-3 --> 9 are Delta E(++)(Tot) = 94.8 and Delta G(++)(298) = 98.3 kJ/mol (at (U)B3LYP/6-31G). The reaction occurs thermally in a conrotatory mode.

Dimethylcethrene: A Chiroptical Diradicaloid Photoswitch

“…Nonetheless, this process could be validated in an analogous system biphenalenylidene 15 (o- 3 ; Scheme 1, bottom) by Kubo and co-workers, who demonstrated 15a that the closed form c- 3 underwent an electrocyclic ring opening to give o- 3 upon irradiation by UV light. In analogy to o- 1a , the open form o- 3 also underwent a thermal ring-closing reaction, which proceeded with a barrier (∼16 kcal mol –1 ) close to that of o- 1a .…”

Recent Advances in the Chemistry of Phenalenyl

“…4 Later, Agranat proposed reactive intermediates, E and Z biphenalenylidene (E, Z 10) and dihydroperopyrene (11), for the decomposition pathway from 2 to 3 (Scheme 4). 39 However, these intermediates have not been experimentally determined because of the rapid decomposition into 3.…”

Cethrene: A Helically Chiral Biradicaloid Isomer of Heptazethrene

“…Its intensity increased and the band gradually disappeared within 5h at 298 Ki na ccord with the NMR observations.T he calculations (Table S2) for 3a predicted aS 0 -S 1 transition at l calc = 708 nm. Theo bserved low-energy absorption band can therefore be attributed to 3a,i na greement with the absorption [17] of biphenalenylidene.T he absorption spectrum of 1 reported [1] by Clar showed an absorption band at l max = 586 nm (l calc = 587 nm). Theb athochromic shift observed for 3a indicates that its HOMO-LUMO gap is smaller than that of the planar 1 (DE HOMO-LUMO = 1.79 and 1.68 eV for 1 and 3,r espectively;T able S2).…”