The right way to self-fuse bi- and terpyrenyls to afford graphenic cutouts

Article abstract of DOI:10.1039/c3cc45235b

In this work, we subject bi- and terpyrenyls to selective fusion for formation of extended polycyclic aromatic hydrocarbons (PAHs). Connecting the pyrene units at 4-4′- or 1-4′-positions led to smooth formation of extended PAHs, achieved via cyclodehydrog

Full text of DOI:10.1039/c3cc45235b

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The right way to self-fuse bi- and terpyrenyls to afford
graphenic cutouts†
Cite this: Chem. Commun., 2013,
49, 10578
Dominik Lorbach, Manfred Wagner, Martin Baumgarten and Klaus Mullen*
¨
Received 11th July 2013,
Accepted 16th September 2013
DOI: 10.1039/c3cc45235b
www.rsc.org/chemcomm
In this work, we subject bi- and terpyrenyls to selective fusion for
formation of extended polycyclic aromatic hydrocarbons (PAHs).
Connecting the pyrene units at 4-40- or 1-40-positions led to smooth
formation of extended PAHs, achieved via cyclodehydrogenation.
This is far more difficult if pyrene is coupled in the 1,10-position.
Precision synthesis of new polycyclic aromatic hydrocarbons
(PAHs) is a challenging task. PAHs are of great interest in view of
their key role as chromophores, mesogens and semiconductors.1–3
Also, large PAHs are attracting increasing attention as nanosized
graphene subunits.4 Our synthetic protocol leading to giant
PAHs uses cyclodehydrogenation of non-planar oligophenyl
and oligoaryl precursors. Illustrative cases are the transition from
hexaphenylbenzene (HPB) to the benzene-homologous hexa-
benzocoronene (HBC) or from biperylenyl molecules to soluble
quaterrylenes and quaterrylenebis(dicarboximide)s.5,6 We have
Scheme 1 Oxidative cyclodehydrogenation of the precursor molecules 7,70-
di-tert-butyl-1,10-bipyrenyl (1) and 4,40-bipyrenyl (2) in DCM at room temperature
for 1 h using FeCl3 as an oxidant.
recently used pyrene as a building block for the synthesis of light (2 and 4) and their higher oligomers 3 and 5. We demonstrate
emitting oligomers and polymers. This concept appeared to be that a connection in the 4-position is required to form the
appealing in particular regarding the photophysical properties of homologous PAHs 2a, 3a, 4a, and 5a in oxidative cyclodehydro-
the pyrene chromophore.7–9 This strongly suggests the use genation (Scheme 1).
of bi- and oligopyrenyl compounds en route to large PAHs.
Starting from 7-(tert-butyl)-1-bromopyrene or 4-bromopyrene,
Indeed, 1,10-bipyrenyl 1 has been subjected to oxidative cyclo- respectively, the bipyrenyls 1 and 2 were synthesized via Yamamoto
dehydrogenation in formation of PAH 1a, however, using rather cross coupling reaction using Ni(COD)2 as a stoichiometric
harsh conditions.10 The coupling of pyrenes to give oligomers is reducing agent, COD (1,5-cyclooctadiene) and 2,20-bipyridine
feasible at three different ring positions and later on it will be as ligands, as well as toluene and DMF (dimethylformamide) as
depicted that the mode of coupling is indeed decisive for solvents. The bipyrenyl 4 was synthesized by the reaction of
successful oxidative fusion resulting in PAHs. Our synthesis 4-bromopyrene and pyren-1-ylboronic acid under standard
of large PAHs thus faces two key questions: how can the various Suzuki conditions using tetrakis-(triphenylphosphino)-palladium(0)
bi- and oligopyrenyl precursors be made and can the cyclo- as catalyst and K2CO3 as base in a toluene–water mixture with
dehydrogenation with oxidants proceed smoothly without the Aliquat 336 as a phase transfer catalyst. The key step toward
occurrence of side reactions such as chlorination, oligomeriza- synthesis of trimer 3 is the functionalization of pyrene in the
tion or dealkylation of solubilizing alkyl groups. Herein, we 4,10-positions. Since no direct and selective access to these
report the synthesis of 4,40- and 1,40-connected bipyrenyls positions has so far been available, we pursue a multi-step route
starting from benzene derivatives (Scheme 2). Towards the
functionalized 7-(tert-butyl)-4,10-diiodopyrene (11), 2,6-dichloro-
1-iodobenzene (6) was reacted with 2-(4-(tert-butyl)phenyl)-
4,4,5,5-tetramethyl-1,3,2-dioxaborolane using Pd(dppf)Cl2 as
Max-Planck-Institut for Polymer Research, Ackermannweg 10, 55128 Mainz,
Germany. E-mail: muellen@mpip-mainz.mpg.de; Fax: +49 06131 379 350
† Electronic supplementary information (ESI) available: Experimental details,
catalyst and K2CO3 as base in DMF in order to form the
biphenyl compound 7. Sonogashira reaction of the obtained
synthesis, UV/Vis, quantum chemical calculation and NMR spectra. See DOI:
10.1039/c3cc45235b
c
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Scheme 2 Synthesis of 7-(tert-butyl)-4,10-diiodopyrene (11) (a) 2-(4-(tert-butyl)phenyl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane, Pd(dppf)Cl2, K2CO3, DMF, 60 1C
(dppf = 1,10-bis(diphenylphosphino)ferrocene) 35% yield; (b) Pd/C, XPhos, ethynyltriisopropylsilane, K2CO3, DMF, 110 1C (Xphos = 2-dicyclohexylphosphino-20,40,60-
triisopropylbiphenyl) 45% yield; (c) TBAF, THF, 25 1C, 1 h (TBAF = tetra-n-butylammonium fluoride) 99% yield; (d) I2, DMAP, DCM, 40 1C, 5 h (DMAP =
4-dimethylaminopyridine) 75% yield; (e) AuCl, toluene, 60 1C, 45% yield.
2,6-dichlorobiphenyl 7 and ethynyltriisopropylsilane applying
heterogeneous copper-free conditions with Pd/C as catalyst, XPhos
(2-dicyclohexylphosphino-20,40,60-triisopropylbiphenyl) as a ligand
and K2CO3 as base in DMF at 110 1C afforded the protected
diacetylene 8.11 Stirring the TIPS (triisopropylsilane) protected
diacetylene biphenyl 8 over tetra-n-butylammonium fluoride
(TBAF) in a tetrahydrofuran (THF) solution quantitatively yielded
deprotected diacetylene 9. Without further purification 9 was
halogenated by adding iodine and 4-dimethylaminopyridine
(DMAP) in dichloromethane at 40 1C giving diiodo-biphenyl 10.
The key step was an endo-selective 6p-electrocyclization forming
a metal vinylidene as the reactive intermediate in simultaneous
1,2-migration of the halogens in the 4,10 positions.12 The reac-
tion took place by adding Au(I)Cl to a solution of 10 in toluene
at 25 1C for 16 h and afforded the resulting 7-(tert-butyl)-4,10-
diiodopyrene (11).
All solution-based oxidative cyclodehydrogenation reactions
were carried out using FeCl3 as an oxidant in dry DCM at room
temperature for appropriate times as given in the ESI.† Three
equivalents of FeCl3 per reacting hydrogen atom were added and
the reaction mixtures were subsequently purged by a continuous
and gentle argon flow. Under these conditions, cyclodehydro-
genation of 1 could not be achieved, instead, only chlorinated
Fig. 1 (A) Spin densities for the radical ions of pyrene. The calculation was
and oligomerized products were observed by MALDI-TOF MS. In
performed using density functional theory (DFT) with the BLYP functional and
contrast to the precursor 1 with a 1,10-connection, the oxidative
6311G(d) basis set. (B) UV-Vis absorption spectra of 2, 3, 4, and 5 (open form =
cyclodehydrogenation took place smoothly for the 4,40-precursor
solid line) in THF, of 2a, 4a (closed form = dashed line) in toluene and 3a in
1,3,5-trichlorobenzene at 180 1C.
2 to give the pentaphene derivative 2a. Self-fusing of 2 led to a
new bond in the 1,10-position in elimination of hydrogen. High-
resolution MALDI-TOF MS analysis of 2a was performed and a
peak at 400.1247 g molꢀ1 was observed, being consistent with the also subjected to the cyclodehydrogenation. By analogy to the
loss of two hydrogen atoms (calculated mass of 400.1252 g molꢀ1
)
previous cases, the C–C bond formation between the 1- and the
and providing evidence for a successful cyclodehydrogenation. 4-positions occurred as proven by the loss of 2 and 4 hydrogen
1
The formation of a six-membered ring was clarified by H-NMR atoms, respectively, using the same reaction conditions and a
spectroscopy of the symmetric compounds 2a and was addi- longer reaction time of 20 h (Scheme 3).
tionally validated by investigation of 14a. The UV-Vis absorp-
As a consequence, the potential for forming a new C–C bond
tion spectra in THF of 2a show an expected strong bathochromic is significantly higher in the 1,10- compared to the 4,40-positions,
shift of 174 nm in comparison to precursor 2 with an absorption as clearly indicated by the formation of 2a. The radical ion of the
maximum at 516 nm (Fig. 1). Extending the 4,40-connected pyrene unit possesses three different ring positions all strongly
precursor to the terpyrenyl 3 also allowed successful cyclode- differing in their spin density (Fig. 1A) and therefore in their
hydrogenation resulting in the heptacene structure 3a. The loss ability to undergo a Scholl reaction. Simplified, the Scholl
of four hydrogens is consistent with the formation of compound 3a. reaction can be understood as an oxidative intramolecular
Connecting the pyrene units in 4,40- and 4,40-100,400- positions to C–C bond formation reaction, generally accomplished by the
precursor molecules 2 and 3, respectively, is the key step towards use of a variety of oxidants.13–22 The new C–C bond is being
successful C–C bond formation. In order to study the effect of formed by the connection of two aromatic C–H groups and
chemically non-identical connecting positions on the fusion subsequent elimination of hydrogen. Thus, the reaction can be
reaction the 1,40-bipyrenyl (4) and the extended terpyrenyl 5 were distinguished as intramolecular C–H activation, although a
c
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use of an oxidative cyclodehydrogenation with FeCl3. Under
these conditions the reactions of 1 and 2 clearly showed that
formation of a new 4,40-bond is very difficult using a 1,10-linked
precursor, whereas the 1,10-bond was successfully closed when
using a 4,40-linked bipyrenyl. Higher spin density in the 1-position
of the radical ion of pyrene, compared to the one in the 4-position,
was figured out to be key toward oxidative intramolecular C–C
bond formation. Pyrene-based molecules as building blocks for
PAHs benefit from their size in comparison to benzene-based
systems. The successful cyclodehydrogenation and the synthesis
of higher oligomers open up a new route to achieve extended
PAHs or toward defined graphene nanoribbons.
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4 demonstrated that the combination of a 1- and a 4-position
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In conclusion, it was for the first time demonstrated that
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c
10580 Chem. Commun., 2013, 49, 10578--10580
This journal is The Royal Society of Chemistry 2013