Synthesis of 2,9-dialkylated phenanthro[1,2-b:8,7-b 0 ]dithiophenesvia cross-coupling reactions and sequential Lewis acid-catalyzedregioselective cycloaromatization of epoxideKeita Hyodoa , Hikaru Nonobe a , Shuhei Nishinaga a , Yasushi Nishihara a,b, ⇑a Division of Earth, Life, and Molecular Sciences, Graduate School of Natural Science and Technology, Okayama University, 3-1-1 Tsushimanaka, Kita-ku, Okayama 700-8530, Japanb Japan Science and Technology Agency, ACT-C, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japana r t i c l e i n f oArticle history:Received 26 March 2014Revised 26 April 2014Accepted 9 May 2014Available online 22 May 2014Keywords:PhenacenesSuzuki–Miyaura couplingNegishi couplingBoronZinca b s t r a c tPhenanthro[1,2-b:8,7-b 0 ]dithiophene (PDT) was prepared via Suzuki–Miyaura or Negishi cross-couplingof a 2-thienylboron or -zinc compound with 1,4-dibromobenzene, followed by Lewis acid-catalyzed reg-ioselective cycloaromatization of the epoxide. A series of 2,9-dialkylated phenanthro[1,2-b:8,7-b 0 ]dithi-ophene (PDT) derivatives could also be synthesized in good yields by Suzuki–Miyaura coupling of thebrominated PDT with alkylboranes by introducing linear alkyl substituents.? 2014 Elsevier Ltd. All rights reserved.Organic f i eld-effect transistors (OFETs) have attracted consider-able interest as key components in future ubiquitous electronicsdue to advantages such as f l exibility, light weight, and ease ofdesign. 1 One particular acene-type molecule, pentacene, 2 hasserved as the active semiconducting layer in OFETs owing to thehigh f i eld-effect mobility ( l ) of 5.5 cm 2 V ?1 s ?1 that it has shownin a thin-f i lm transistor, 3 and its state-of-the-art value of 40 cm 2 -V ?1 s ?1 in single crystals. 4 However, pentacene is unstable underatmospheric conditions, and readily photodegrades owing to itsrelatively high HOMO energy (?5.0 eV), which arises from itsextended p -conjugation. 5 Recently, a phenacene-type molecule,picene, incorporating the same number of benzene rings as penta-cene, has become the focus of considerable interest because itbecomes superconductive 6 with alkali-metal doping, and alsoshows high f i eld-effect mobility in a transistor (1.1 cm 2 V ?1 s ?1 ina thin-f i lm OFET). 7 Moreover, a picene-based FET is stable in airbecause picene has a larger energy bandgap (E g = 3.3 eV) and alower HOMO energy (?5.5 eV) than pentacene. 8 The potential util-ity of phenacene-type molecules makes the development of moreeff i cient synthetic methods 9 and further improvement in theirOFET properties 10–12 matters of some importance.We have recently reported the synthesis of picenes 13 and ful-minene 14 by the palladium-catalyzed Suzuki–Miyaura couplingof (Z)-alkenylboronates with polyhalobenzene and sequentialintramolecular double cyclization via CAH activation. This protocolis also applicable to the synthesis of phenanthro[1,2-b:8,7-b 0 ]dithi-ophene (PDT) by replacing the terminal phenyl rings in picene withthiophene rings, aiming at increased intermolecular p – p interac-tions due to the large atomic radius of sulfur, which may enhanceits performance in OFETs. 15 Results showed that OFET devicesfabricated with thin f i lms of PDT formed by thermal depositionexhibited carrier mobility as large as 1.1 ? 10 –1 cm 2 V ?1 s ?1 , andsuggested that fabrication using a solution process might be possi-ble owing to PDT’s high solubility in common organic solvents.However, we found that this synthetic strategy is not suitablefor the large-scale synthesis of PDT because in some cases a mix-ture of stereoisomers of the coupled products is formed through(E)/(Z) isomerization upon Suzuki–Miyaura coupling, leading to alower yield of the desired products. To produce derivatives ofPDT for use as organic semiconductors, a more eff i cient syntheticmethod is highly desirable. Here we report a new synthetic routeto the PDT core structure using cross-coupling reactions. Further-more, the solubility of PDT might be improved by introducing longalkyl chains, as this may induce a self-assembly process by the ‘fas-tener effect’, leading to high crystallinity in thin f i lms. Some exam-ples of this have already been reported in alkyl-substitutedpicene 16 and alkylated thienoacene. 17,18To explore a new synthetic route to PDT, we investigated thepalladium-catalyzed Suzuki–Miyaura coupling of commerciallyhttp://dx.doi.org/10.1016/j.tetlet.2014.05.0350040-4039/? 2014 Elsevier Ltd. All rights reserved.⇑ Corresponding author. Tel./fax: +81 86 251 7855.E-mail address: ynishiha@okayama-u.ac.jp (Y. Nishihara).Tetrahedron Letters 55 (2014) 4002–4005Contents lists available at ScienceDirectTetrahedron Lettersjournal homepage: www.elsevier.com/locate/tetlet
