Sequential Formation of Yellow, Red, and Orange 1‐Phenyl‐3,3‐Biphenylene‐Allene Dimers Prior to Blue Tetracene Formation: Helicity Reversal in trans‐3,4‐Diphenyl‐1,2‐bis(fluorenylidene)cyclobutane
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abstract
Abstract1‐Phenyl‐3,3‐biphenyleneallene (2), the base‐catalyzed rearrangement product of 9‐phenylethynylfluorene (1) yields a yellow, head‐to‐tail dimer 6 that, upon gentle warming, is converted to the red tail‐to‐tail isomer trans‐3,4‐diphenyl‐1,2‐bis(fluorenylidene)cyclobutane (7), in which the two fluorenylidene moieties severely overlap. The helical sense of the fluorenylidene moieties in 7 matches that of the phenyl substituents, and the interplanar angle between the fluorenylidene moieties is 41°. At 80 °C, 6 isomerizes to orange cis‐3,4‐diphenyl‐1,2‐bis(fluorenylidene)cyclobutane (8), which at 110 °C is converted to orange trans diastereomer 9, whereby the helicity of the overlapping fluorenylidene moieties is reversed from that in 7 such that they are aligned with the ring hydrogen atoms, and the interplanar angle between the fluorenylidene moieties is now 60°. At 180 °C, 6 rearranges to dispirodihydrotetracene 3 and blue, electroluminescent diindenotetracene 4, which is readily oxidized to peroxide 5. In the solid state, both 3 and 4 adopt structures with Ci symmetry (only an inversion center) such that the central polycyclic framework is nonplanar. Deprotonation of yellow head‐to‐tail allene dimer 6 with tBuOK in DMSO and reprotonation with HOAc yields the [1,3]‐hydrogen migration product 10, in which the proton originally on the cyclobutane ring is now sited at C9 on the exocyclic fluorenyl substituent. Analogously, deprotonation and reprotonation of orange dimer 9 furnishes [1,3]‐hydrogen migration product 11. Side product 17, formed during the synthesis of 1 from 9‐phenylethynylfluoren‐9‐ol, BF3 and Et3SiH, was shown to be a silyl‐indene spiro‐linked to C9 of fluorene. All products were characterized by NMR spectroscopy and X‐ray crystallography, and the mechanisms of these interconversions are discussed.
