Regiospecifically Fluorinated Polycyclic Aromatic Hydrocarbons via Julia–Kocienski Olefination and Oxidative Photocyclization. Effect of Fluorine Atom Substitution on Molecular Shape

Abstract

A modular synthesis of regiospecifically fluorinated polycyclic aromatic hydrocarbons (PAHs) is described. 1,2-Diarylfluoroethenes, synthesized via Julia-Kocienski olefination (70–99% yields), were converted to isomeric 5- and 6-fluorobenzo[c]phenanthrene, 5-and 6-fluorochrysene, and 9- and 10-benzo[g]chrysene (66–83% yields) by oxidative photocyclization. Photocyclization to 6-fluorochrysene proceeded more slowly than conversion of 1-styrylnaphthalene to chrysene. Higher fluoroalkene dilution led to a more rapid cyclization. Therefore, photocyclizations were performed at higher dilutions. To evaluate the effect of fluorine atom on molecular shapes, X-ray data for 5- and 6-fluorobenzo[c]phenanthrene, 6-fluorochrysene, 9- and 10-fluorobenzo[g]chrysene, and unfluorinated chrysene as well as benzo[g]chrysene were obtained and compared. The fluorine atom caused a small deviation from planarity in the chrysene series, decreased nonplanarity in the benzo[c]phenanthrene derivatives, but its influence was most pronounced in the benzo[g]chrysene series. A remarkable flattening of the molecule was observed in 9-fluorobenzo[g]chrysene, where the short 2.055 Å interatomic distance between bay-region F-9 and H-8, downfield shift of H-8, and a 26.1 Hz coupling between F-9 and C-8 indicate a possible F-9··· H-8 hydrogen bond. Also, in 9-fluorobenzo[g]chrysene, stacking distance is short at 3.365 Å and there is an additional interaction between the C-11–H and C-10a of a nearby molecule that is almost perpendicular.

A General Synthetic Approach and Photophysical Properties of Regioselectively Fluorinated [5]‐ and [6]‐Helical Bispiroindenofluorenes

“…Most of the respective dispirodihydroindenofluorenes were synthesized in 5 steps (the octafluorinated DSF-IF 1 h was prepared in 9 steps) from commercially available starting compounds with reasonable efficiency and constitutes the first application of catalytic [2 + 2 + 2] cyclotrimerization for synthesis of fluorinated helical compounds. The mono-to tetrafluorinated [5]-helical DSF-IFs 1 a-1 g exhibited similar optical properties like their nonfluorinated counterpart, but their emission maxima were slightly red shifted by 7-15 nm (λ em = 386-395 nm). Whereas the octafluorinated [5]-helical DSF-IF 1 h and the tetrafluorinated [6]-helical DSF-BIF 2 showed much more red shifted emission maxima at 413 nm and 412 nm, respectively.…”

“…[21,22] Herein we were interested to analyze the effect of fluorine atoms directly attached to the helical π-system of the DSF-IFs. As a result, compounds 1 a-g (mono-to tetrafluorinated DSF-IFs, Figure 3) presented very similar absorption spectra and molar attenuation coefficients with the characteristic strong absorption band at 308-312 nm and the moderate intensity band at 319-329 nm typical for all of known [5]-helical DSF-IFs. [13,21,22] Although some differences can be noted in the absorption spectra, such as an additional band of lowest energy absorption (343 nm) for the tetrafluorinated DSF-IF (1 g) and a slightly red-shifted lowest energy absorption of 1 b and 1 e (both with at least one fluorine in position 2), we can conclude that fluorination of the one terminal benzene ring of the SBIFs helical core (neither the number nor position of fluorine atoms) does not have a significant effect on the absorption properties.…”

“…Recently we have reported a synthetic pathway to regioselectively substituted compounds possessing the fluorene and dihydroindenofluorene scaffolds based on inter or intramolecular cyclotrimerization reaction of triple bonds. This method allows synthesis of specifically and regioselectively substituted fluorenes [10][11][12] and [5]-and [7]-helical dihydroindenofluorenes (DSF-IFs). [13] Since the precursors for the cyclotrimerization step were formed by a rather simple transformation of halobenzaldehydes, we envisioned that the use of fluorinated halobenzaldehydes could serve our purpose and be used to synthesize the desired target compounds.…”

“…[3,4] It should be mentioned that attaching of fluorine atoms to specific position of the polyaromatic scaffold (pinpoint-fluorinated polycyclic aromatic hydrocarbons -F-PAHs) is often a synthetic challenge and requires special approaches. [5][6][7] Interestingly, a very little attention has been payed to synthesis and property evaluation of other types of helical aromatic compounds with potential application in organic electronics such as fluorinated compounds possessing the dihydroindenofluorene scaffold. A rare exception is a synthesis and evaluation of properties of donor/acceptor dihydroindeno[1,2-a]-and dihydroindeno [2,1-b]fluorenes bearing trifluoromethyl substituents.…”

“…A rare exception is a synthesis and evaluation of properties of donor/acceptor dihydroindeno[1,2-a]-and dihydroindeno [2,1-b]fluorenes bearing trifluoromethyl substituents. [8] Herein, we would like to present a general synthetic approach to regioselectively fluorinated [5]-helical DSF-IFs [9] 1 (6,7- (Figure 1). Their photophysical characterization to understand the impact of fluorine substitution on the properties of the synthesized compounds was evaluated as well.…”