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Synthesis of Novel Fluorinated Hexa-Peri-Hexabenzocoronenes Synthesisolivier of Fluorinated Hexa-Peri-Hexabenzocoronenes F

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Synthesis of Novel Fluorinated Hexa-Peri-Hexabenzocoronenes Synthesisolivier of Fluorinated Hexa-Peri-Hexabenzocoronenes F PAPER 2891 Synthesis of Novel Fluorinated Hexa-peri-hexabenzocoronenes SynthesisOlivier of Fluorinated Hexa-peri-hexabenzocoronenes F. Aebischer,a Patrick Tondo,a Bassam Alameddine,b Titus A. Jenny*a a Chemistry Department, University of Fribourg, 9 chemin du Musée, 1700 Fribourg, Switzerland Fax +41(26)3009739; E-mail: [email protected] b Chemistry Department, University of Balamand, P.O. Box 100, Tripoli, Lebanon Received 5 May 2006; revised 24 May 2006 volves the cobalt-catalysed7 cyclotrimerisation of the Abstract: The synthesis of several perfluoroalkylated hexabenzo- coronene derivatives is described. The substituents range from lin- corresponding disubstituted tolane derivatives 3, yielding ear semiperfluoroalkylated chains to branched perfluoroalkylated hexaphenylbenzene moieties 2, which are subsequently 8 chains. Novel strategies were applied to overcome the low solubili- cyclodehydrogenated, under the Kovacic conditions, in ty and the low reactivity of such chains. which the iron(III) chloride reagent acts as both Lewis Key words: coronenes, tolanes, perfluoroalkyl substituents, cross- acid and oxidant (Scheme 1). The latter conditions, how- coupling, cyclodehydrogenation ever, prevent the preparation of HBC derivatives in which perfluorinated side chains are directly attached to the core. An alkyl spacer of at least two carbons, or an ether or thio- Polycondensed aromatic hydrocarbons (PAHs) represent ether linkage is required to reduce the electron-withdraw- a widely investigated class of aromatic compounds and ing effect of the perfluorinated side chain to an acceptable have been extensively studied over the past decades.1 level for the mild iron(III) chloride reagent to be success- 5 Hexa-peri-hexabenzocoronene (HBC), an all-benzoid ful. The main task remains, therefore, in the preparation PAH containing thirteen fused benzene rings, possesses of the symmetrically substituted tolane derivatives 3 high thermal and chemical stability. It is also known to (Scheme 1). Nonetheless, the drastic change in both reac- self-assemble into highly ordered architectures,2 render- tivity and solubility of highly fluorinated derivatives did ing the HBC moiety a candidate of high potential for ap- not allow the application of the conditions already report- 9 plications in plastic electronics. Many functionalised ed in the literature to the preparation of alkylated tolane HBCs, either with extended conjugated structures, or with derivatives. The Kumada cross-coupling of a bis-bromi- alkyl groups on their periphery, have been prepared,3 but nated tolane with perfluoroalkyl Grignard intermediates, only few bearing perfluoroalkylated side chains have been for example, requires up to one week of reaction time and reported.4,5 Those highly fluorinated side chains are ex- yields the desired tolane only in unreliable and at best very 5 pected to clad the HBC molecule with a Teflon-like man- moderate yields, whereas for regular alkyl Grignard spe- tle that reduces the tendency for any lateral aggregation.5 cies the reaction works without difficulties in a short time and in acceptable yields.9 We report herein the synthesis of three new HBC deriva- tives bearing linear and branched fluorinated side chains A better way to prepare perfluoroalkylated tolane deriva- (Figure 1). tives consists therefore of synthesising first the halogenat- ed precursors 5a–c (Schemes 2 and 3), as these The best strategy for the synthesis of symmetrically sub- compounds are highly soluble in common organic sol- stituted hexa-peri-hexabenzocoronene6 derivatives 1 in- R R R R R R R R R R R R R R Scheme 1 Retrosynthesis3 showing strategic bond disconnection for the preparation of hexa-peri-hexabenzocoronenes SYNTHESIS 2006, No. 17, pp 2891–2896xx.xx.2006 Advanced online publication: 15.08.2006 DOI: 10.1055/s-2006-942550; Art ID: Z09406SS © Georg Thieme Verlag Stuttgart · New York 2892 O. F. Aebischer et al. PAPER R tions, only the iodine of the side chain of 6 reacts, without debromination of the aromatic core occurring. Finally, bromobenzene 7 was converted into the more reactive io- R R dobenzene 5b by a halogen-exchange13 reaction. Com- : R = (CH 1a ) (CF ) F 2 2 6 2 pound 5b was isolated in 40% overall yield for the three ) (CF ) F : R = (CH 1b 4 2 6 2 steps and in high purity after silica gel column chromatog- ) CH[CH (CF ) F] : R = (CH 1c 3 2 2 6 2 2 raphy. R R The preparation of precursor 5c started with the Kumada cross-coupling of the known fluorinated iodoalkene 814 with allylmagnesium bromide. The coupling only suc- R ceeded when the Grignard derivative was added very Figure 1 Hexa-peri-hexabenzocoronene target molecules slowly (over several hours) at 0 °C; otherwise, many side products formed. The Heck reaction between fluorinated alkene 9 and benzenediazonium 10 proved to be a chal- vents, allowing for their preparation in high purity. The lenge. Coupling was attempted in the presence of palladi- tolane derivatives 3a–c are then obtained in a one-pot um(II) acetate and methanol, but no product formed at all. Sonogashira reaction,10 as white, sparingly soluble com- Fortunately, the coupling was finally achieved when pal- pounds (Scheme 4). ladium(II) acetate, calcium carbonate, and a methanol– Precursor 5a is already described in the literature,11 tetrahydrofuran mixture (3:1) was used at 50 °C. Purifica- whereas precursors 5b and 5c are, to our knowledge, new tion of the crude product by column chromatography over derivatives and represent very interesting fluorinated silica gel afforded styrene derivative 11 as a colourless oil building blocks. in moderate yield. Hydrogenation was achieved under mild conditions with the use of a rhodium–carbon cata- For the synthesis of compound 5b (Scheme 2), 1-bromo- lyst,11 to prevent the dehalogenation of the aromatic core. 4-but-3-enylbenzene (4) was first coupled with perfluoro- Finally, compound 5c was isolated as a colourless liquid hexyl iodide in a radical reaction. Sodium metabisulfite in 34% isolated overall yield. was added to temper the reaction and to reduce the amount of side products.12 Of all tested dehalogenation methods, The most efficient way to convert an aromatic halide into the use of lithium aluminum hydride was best for removal a tolane derivative involves a Sonogashira coupling. of the iodine of the side chain of 6. Under these condi- Many different conditions are proposed in literature, but Rf I, Na S O 30% 6 2 2 5 Br Br I AIBN, 80 °C 98% Rf 6 4 6 LiAlH , THF, Ni, KI, I , DMF, 4 2 r.t. 150 °C I Br 77% 83% Rf Rf 6 6 5b 7 Rf = C F 6 6 13 Scheme 2 Three-step synthesis of fluoroalkyl-substituted iodobenzene precursor 5b MgBr Rf Rf 6 6 Rf Rf 6 6 CH Cl , Pd(dppf)Cl ⋅ 2 2 2 THF, 0 °C to r.t. I 9 85% 8 Rf Rf 6 6 Rf Rf 6 6 N BF 2 4 , 9, Pd(OAc) 2 MeOH–THF (3:1), (60 bar), Rh/C, H 2 , 50 °C CaCO pentane, r.t. 3 51% 98% Br 11 10 5c Br Br Scheme 3 Synthesis of fluoroalkyl-substituted bromobenzene precursor 5c by Kumada and Heck coupling Synthesis 2006, No. 17, 2891–2896 © Thieme Stuttgart · New York PAPER Synthesis of Fluorinated Hexa-peri-hexabenzocoronenes 2893 PdCl (PPh ), CuI, DBU 2 2 O, TMSC CH, r.t. benzene, H ≡ 2 R R X R R = (CH ) (CF ) F, X = I ( ) R = (CH ) (CF ) F (33%) ( ) 5a 3a 2 2 2 6 2 2 2 6 ) (CF ) F, X = I ( ) ) (CF ) F (44%) ( ) R = (CH R = (CH 5b 3b 2 4 2 6 2 4 2 6 ) CH[CH (CF ) F] , X = Br ( ) ) CH[CH (CF ) F] (47%) ( ) R = (CH R = (CH 5c 3c 2 3 2 2 6 2 2 3 2 2 6 2 Scheme 4 Sonogashira one-pot coupling of fluoroalkylated halobenzenes with (trimethylsilyl)acetylene to yield tolane derivatives 3a–3c most of them proceed stepwise, starting with the coupling drogen chloride, to yield the HBC derivatives 1a–c in fair of the appropriate halobenzene precursor with (trimethyl- to good yields (45–70%) (Scheme 5). Final purification silyl)acetylene, followed by deprotection of the trimethyl- was by recrystallisation from benzotrifluoride, hexafluo- silyl group and, finally, by coupling this product with the robenzene, and diethyl ether. second halobenzene. In our case, however, this approach In conclusion, three new symmetrically substituted hexa- failed at the deprotection stage for solubility reasons. For- peri-hexabenzocoronenes bearing linear or branched par- tunately, two methods were found to perform the coupling 15 tially fluorinated alkyl chains in their periphery were pre- in a one-pot reaction. The first uses a constant stream of pared and characterised. These HBC derivatives 1a–c are acetylene passing through the reaction mixture at 80 °C. potential candidates in plastic electronics, while their pre- As the obtained yields were generally low, a second meth- 10 cursors are new, interesting partially fluorinated building od was applied, in which (trimethylsilyl)acetylene was blocks. used instead. Despite the problems encountered in the stepwise procedure, the highly fluorinated precursors 5a– c yielded the tolane derivatives 3a–c, although in moder- All reagents and solvents were purchased from commercial sources ate yields owing to their difficult purification: linear per- and used without further purification. THF and CH2Cl2 were dried and deoxygenated by the Grubbs method.16 All reactions were per- fluoroalkylated tolane derivatives 3a and 3b are only 1 13 formed under inert atmosphere (N2 or Ar). H NMR and C NMR slightly soluble in common organic solvents, and were pu- spectra were recorded on a Bruker DPX 360 or a Bruker DRX 500 rified by recrystallisation and filtration over basic alumi- spectrometer, using CDCl3 or hexafluorobenzene as solvent. Chem- num oxide. Nevertheless, the method avoids the ical shifts (d) are reported in ppm, and coupling constants (J) are formation of homocoupled products, normally occurring given in Hz.
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