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Suzuki reaction of 9-halophenanthrenes and boronic acids,10 arylation of phenanthrene ..... (p) M. Catellani, E. Motti, and S. Baratta, Org. Lett. 2001, 3,. 3611; (q) ...

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This article can be cited before page numbers have been issued, to do this please use: F. Jafarpour, M. Ayoubi-Chianeh, M. Abbasnia and M. Azizzade, Catal. Sci. Technol., 2017, DOI: 10.1039/C7CY01040K. Volume 6 Number 1 7 January 2016 Pages 1–308

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A new route to π-extended polycyclic aromatic hydrocarbons via Cross-Dehydrogenative Coupling Received 00th January 20xx, Accepted 00th January 20xx

Farnaz Jafarpour,* Mojgan Ayoubi-Chianeh, Masoumeh Abbasnia and Meysam Azizzade

DOI: 10.1039/x0xx00000x www.rsc.org/

Palladium-catalyzed cross-dehydrogenative coupling of phenanthrenes with simple arenes is innovated. This protocol provides an opportunity for a waste–minimized construction of πextended PAHs with high atomic efficiency under mild and ligandfree conditions. Double C-H activation due to its simplicity and atom-economy has been a landmark in the history of organic synthesis. 1 In this respect, coupling of two simple arenes has aroused a great deal of attention as they merely contain C and H atoms in their scaffolds and not any directing group nor heteroatom to assist the regioselectivity and coupling. Given the versatile application of phenanthrene in naturally occurring substances,2 pharmaceuticals,3 material science4 and polymers,5 many efforts have been directed onto their construction. Some typical methods for construction of these privileged motifs comprise Haworth synthesis,6 cycloaddition of biphenyls with alkynes,7 intramolecular arylation via direct C-H activation8 and Pschorr cyclization.9 However, direct instalment of arenes at K-region of a phenanthrene is by far the most favourable route rather than foregoing procedures, from economical and environmental standpoint. Some limited reports disclosed on direct arylation of phenanthrenes include Suzuki reaction of 9-halophenanthrenes and boronic acids,10 arylation of phenanthrene with aryltin reagents 11, arylboronic acids12 and arylboroxins13 (Scheme 1). Shi et. al. have demonstrated a method for direct construction of biaryl C-C bonds by a Pd-catalyzed cross coupling of (hetero)arenes and aryl boronic acids using O2 as the final oxidant in acidic media.12 Very recently Glorius et. al. have developed an interesting arylation of PAHs (polycyclic aromatic hydrocarbons) and disclosed one example of direct arylation of phenanthrene with aryliodonium salts and Pd/C.14 Furthermore, Itami et. al. have

School of Chemistry, College of Science, University of Tehran, P.O. Box 141556455, Tehran, Iran. E-mail: [email protected] Electronic Supplementary Information (ESI) available: Experimental procedures and characterization data. See DOI: 10.1039/x0xx00000x

Scheme 1. Solutions for functionalization of phenanthrenes at K-region

pioneered one-shot annulative π-extension reaction of phenanthrenes with silicon-bridged aromatics which finds significant use in the nanographene synthesis and in the latestage fine-tuning of nanographene properties.15 Despite the importance of these reports, the liabilities of prefunctionalization of at least one of the coupling partners however, have limited the applicability of these procedures. Nevertheless, the most elegant and environmentally attractive method for arylation of phenanthrenes via a two-fold dehydrogenative functionalization of phenanthrenes with

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Pd-Catalyst Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 PdCl2 Pd(acac)2 Pd(dba)2

Oxidant K2S2O8 K2S2O8 K2S2O8 K2S2O8 K2S2O8 K2S2O8 K2S2O8 (NH4)2S2O8 DTBP TBHP AgOAc Na2S2O8 Na2S2O8 Na2S2O8 Na2S2O8

Table 2 Scope of C-H functionalization of phenanthrene with simple arenes a

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Solvent 1-propanol 2-propanol t-Butanol TFE AcOH PivOH TFA TFA TFA TFA TFA TFA TFA TFA TFA

Yield 0% 0% 0% 25% 10% 20% 62% 60% 64% 50% 40% 75% 21% 32% 15%

DOI: 10.1039/C7CY01040K

(%)b

aReaction

conditions: Phenanthrene 1a (0.1 mmol), anisole 2a (5 equiv), [Pd] (10 mol %), Na2S2O8 (1 equiv), solvent (10 equiv), 80 °C, 12 h; DTBP= di-t-butyl peroxide, TBHP= t-butyl hydrogen peroxide, TFE= trifluoroethanol, PivOH= pivalic acid, TFA= trifluoroacetic acid.

simple arenes which excludes installment of any disposal functionalities, is not precedented. Herein we set out to explore a rare example of two-fold direct arylaton of phenanthrenes with simple arenes which could have significant implications for construction of π–extended phenanthrenes with applications in construction of bioactive phenanthrenes or organic light emitting diodes (OLEDs).16 This protocol provides a straightforward and environmentally benign route to functionalized phenanthrenes in which employment of prefunctionalized starting materials are fully avoided. The first step to establish the optimized conditions for direct arylation of phenanthrenes was to find an appropriate oxidant since in CDC reactions the utility of an oxidant to convert Pd(0) generated in the course of the reaction to Pd(II) seems to be a pivotal point to retrieve the catalyst to the cycle. To this end, the reaction of phenanthrene 1a and anisole 2a as model substrates in presence of various solvents and oxidants were investigated where, among all oxidants, Na 2S2O8 acted by far the best (Table 1). As was shown, using alcohols as solvent suppressed the reaction drastically. This phenomenon can be rationalized by the fact that on heating with alcohols palladium(II) acetate decomposes to palladium(0) which retards the reaction.17 Only TFE (trifluoroethanol) gave rise to a low yield of product (entry 4). This inspired us to exploit TFA (trifluoroacetic acid) as solvent which built up the yield fundamentally (entry 7). Then, different palladium sources were examined among which Pd(OAc)2 was the catalyst of the choice (entry 12). Additionally, use of any base regardless of their kinds had a detrimental effect on the reaction. Having established the optimized condition, we surveyed the scope of the reaction. To this end, arenes possessing electrondonating and withdrawing substituents were employed. The results showed that various groups including alkyl, methoxy, halo and hydroxyl groups were tolerated. Chloro- and fluorosubstituted arenes were tolerated in this procedure, leaving

aAll

reactions were run under the optimized reaction conditions. bIsolated yields. (10 equiv.) was used.

carene

behind π-extended phenanthrenes to be functionalized even more through cross-coupling reactions (3c and 3k). Employing o-, m- and p-xylenes, synthetically useful quantities of desired products were obtained (3d-3f). Gratifyingly, when hydroxylsubstituted arenes were exposed to this reaction, satisfactory results were attained (3g, 3h). Also, more sterically encumbered naphthalene successfully participated in this reaction with a high selectivity for β-arylation (3l). It is noteworthy that this two-fold direct arylation of phenanthrene could also be scaled up to a gram scale without any significant loss of the product yield. Direct arylation of 1 gram phenanthrene (5.6 mmol) with benzene proceeded smoothly to generate the desired product in 50% yield. To expand the scope of the reaction, phenanthrene was exposed to pyridine-N-oxide. Pyridine moiety is a key component of many naturally occurring bioactive compounds and pharmacophores and 2-arylpyridine-N-oxides are proved to inhibit gastric secretion.18 To our delight, when pyridine-Noxide was exposed to phenanthrene under some altered reaction conditions, the desired product was produced in a satisfactory 55% isolated yield (Scheme 2). Furthermore, the rapid assembly of highly π–extended symmetrical biaryls due to their wide applications in pharmaceuticals, dyes, semiconductors and organic materials have attracted considerable interest in recent years.19 Construction of much larger aromatic biaryls via homo-coupling reactions however, have been problematic with an increase in the size of the substrates.20 Very Recently Tobisu and Chatani developed a homo-coupling of methoxyarenes through sequential nickel-catalyzed C−O borylation and C−O/C−B crosscoupling (Scheme 3).20a Later, Konar et. al. disclosed a copper(I)based metal–organic framework as a heterogeneous catalyst for the homo-coupling of arylboronic acids (Scheme 3).20b

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Table 1 Optimization of reaction conditions a

Entry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

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Scheme 2. CDC reaction of phenanthrene and pyridine-N-oxide

Scheme 5. Proposed reaction mechanism.

Scheme 3. Highly π–extended systems via Homo-coupling of phenanthrenes

In spite of the importance of these contributions, the approaches relied on prefunctionalized starting materials with low yields of dimerized products not exceeding 60%. Under the presented catalytic system, however, the two-fold homocoupling of unactivated phenanthrene proceeded smoothly to afford the desired biaryl compound, 9,9'-biphenanthrene 3n in promising 85% yield (Scheme 3). This approach may find some applications in fabricating graphene, a two-dimensional sheet of sp2-hydridized carbon, which due to its electronic and optoelectronic properties has grabbed appreciable attention. 21 In addition to cross-dehydrogenative coupling, we found that with the present catalytic conditions direct arylation of phenanthrene is also possible employing iodoarenes. In this regard iodoarenes possessing various electronic and steric properties were subjected into this reaction and a wide range

of substituents were tolerated (Scheme 4). Furthermore, direct arylation of phenanthrene with p-iodotoluene was scaled up to gram scale and to our delight, the desired product was obtained without any decrease in the yield. A putative mechanism is outlined in Scheme 5. As has been represented to date, TFA seems to be crucial for C-H activation of carbocyclic rings. It renders Pd(OAc)2 more electrophilic hence facilitating cross-coupling reactions.22 Electrophilic palladation of excess arene generates intermediate I. The second electrophilic palladation generates diaryl palladium intermediate II which on reductive elimination, affords the desired π–extended PAH. In summary, without recourse of any proximate chelating groups or functionalities, we flourished to arylate phenanthrene with simple arenes via two-fold C-H activation. This protocol provides an unprecedented approach to atomand step-economical assemble of π-extended PAHs with several applications. Moreover, based on this protocol a satisfactory direct arylation of phenanthrene with various iodoarenes was also achieved.

Acknowledgements We acknowledge the financial support from the University of Tehran.

Notes and references 1

Scheme 4. Scope of the C-H functionalization of phenanthrene with iodoarenes.

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Abstract  A new route to π‐extended polycyclic aromatic hydrocarbons via Cross‐ Dehydrogenative Coupling  Farnaz Jafarpour,* Mojgan Ayoubi‐Chianeh, Masoumeh Abbasnia and Meysam Azizzade    A rare example of fruitful construction of highly π‐extended PAHs expressly via palladium‐catalyzed CDC  reaction is demonstrated.   

Catalysis Science & Technology Accepted Manuscript

DOI: 10.1039/C7CY01040K