Biphenalenylidene: The Forgotten Bistricyclic Aromatic Ene. A Theoretical Study

Sergey Pogodin, Israel Agranat*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

38 Scopus citations

Abstract

The Lawesson reagent and P2S5 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° and 57.8° (at UB3LYP/6-311G**). ΔE‡ Tot = 10.2 kJ/mol and ΔG‡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 ΔE‡Tot = 94.8 and ΔG‡298 = 98.3 kJ/mol (at (U)B3LYP/6-31G*). The reaction occurs thermally in a conrotatory mode.

Original languageEnglish
Pages (from-to)12829-12835
Number of pages7
JournalJournal of the American Chemical Society
Volume125
Issue number42
DOIs
StatePublished - 22 Oct 2003

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