Palladium-catalyzed pentannulation of polycyclic aromatic hydrocarbons

Article abstract of DOI:10.1002/chem.201101126

We present a new and versatile one-step synthesis of a series of small molecular chromophores based on cyclopentannulated polycyclic aromatic hydrocarbons (PAH). Easily available pyrene, anthracene, and perylene bromides serve as starting materials for th

Full text of DOI:10.1002/chem.201101126

DOI: 10.1002/chem.201101126
Palladium-Catalyzed Pentannulation of Polycyclic Aromatic Hydrocarbons
Christian Lꢀtke Eversloh, Yuri Avlasevich, Chen Li,* and Klaus Mꢀllen*[a]
Abstract: We present a new and versa-
tile one-step synthesis of a series of
small molecular chromophores based
on cyclopentannulated polycyclic aro-
matic hydrocarbons (PAH). Easily
available pyrene, anthracene, and pery-
lene bromides serve as starting materi-
als for the reactions. The formation of
the five-membered ring is achieved by
the straightforward palladium(0)-cata-
lyzed carbannulation with various sub-
stituted acetylenes. This approach is
applicable either to single or multiple
annulation procedures leading to hith-
erto inaccessible PAH topologies. Ac-
cording to the resulting products of the
diverse reactions, a mechanistic explan-
ation is proposed. UV/Vis absorption
as well as cyclovoltammetric measure-
ments were performed for characteri-
zation demonstrating the value of this
annulation technique. Optical absorp-
tions of up to 780 nm and absorption
coefficients ranging from 8000 to
34000mÀ1 cmÀ1 were detected.
Keywords: absorption · anthracene ·
cyclovoltammetry · perylene · pyr-
ene
Introduction
PAHs since a long time[18] and further research on this is
still in progress.[19] Additionally the use of alkyne derivatives
as substrates for palladium-catalyzed annulations became an
essential part of the latter, as it enables a facile enlargement
of existing aromatic systems.[20]
Polycyclic aromatic hydrocarbons (PAHs) bearing cyclopen-
tadienide moieties can be regarded as subunits of fullerenes
or other nanospheres.[1] There is a great interest in new and
efficient syntheses of PAHs[2] containing functional groups
and structural motifs which control chemical and physical
properties like stability, charge-carrier mobilities and spec-
troscopic behavior.[3] Due to the remarkable optical[4] and
electronic[5] properties of aromatic cyclopentadienides, they
show characteristics of alternate PAHs, such as pentacene or
hexa-peri-hexabenzocoronenes, that are already being used
in organic field-effect transistors (OFETs),[6] organic light-
emitting diodes (OLEDs),[7] and organic photovoltaics
(OPVs).[8] In contrast to PAH structures which obtain their
properties from their sheer size and thus often need addi-
tional solubilizing groups,[9] small pentannulated PAHs can
be used without auxiliary substituents and easily purified.[10]
Surprisingly, only a few examples of cyclopentannulated
PAHs can be found in the literature.[11] Most of them con-
tain derivatives of the well known indenopyrene (1)[12] or
acenaphthylene (2) (see Scheme 1),[10] like the formation of
diphenylacenaphtylene using 1-iodonaphthalene and diphe-
nylacetylene.[13] It is noticeable at this point that palladium-
catalyzed reactions, such as Heck,[14] Negishi,[15] Suzuki–
Miyaura[16] and Sonogashira[17] reactions play an important
role, as they are known to be favorable for the formation of
Scheme 1. Examples for literature known cyclopentannulated PAH.
Herein, we present a straightforward pentannulation of
easily available pyrene, anthracene, and perylene bromides
with aryl or alkyl acetylenes under mild reaction conditions.
It is shown that this procedure is applicable to either single
or multiple annulation of PAHs. Additionally, it provides
the possibility to use different acetylenes, which makes this
approach a facile one-step synthesis of various PAHs includ-
ing five-membered rings. As a further advantage it is shown
that readily available palladium catalysts in combination
with well understood phosphine ligands[21] can be used.
Results and Discussion
[a] C. Lꢀtke Eversloh, Dr. Y. Avlasevich, Dr. C. Li, Prof. Dr. K. Mꢀllen
Max Planck Institute for Polymer Research
Ackermannweg 10, 55128 Mainz (Germany)
Fax : (+49)6131-379350
Syntheses: The initial coupling reaction was carried out be-
tween 1-bromopyrene (3) and diphenylacetylene (a). When
optimizing the reaction conditions, a mixture of toluene,
acetonitrile, and triethylamine at reflux appears as most
suitable in terms of yield and reaction time.[22] Thus, the de-
sired 3,4-diphenylcyclopenta[cd]pyrene (3a) could be ob-
E-mail: lichen@mpip-mainz.mpg.de
muellen@mpip-mainz.mpg.de
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201101126.
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FULL PAPER
tained in 82% yield using 1.2
equivalents of diphenylacety-
lene (a; throughout this article
the acetylene derivatives are
indicated as bold letters) in the
presence of 10 mol% [Pd-
A
PACHTUNGTRENNUNG
[23]
tol)3
a
core structure together with di-
phenylacetylene (a), the reac-
tion proceeded smoothly to
produce the respective 1,2-di-
phenylaceanthrylene (4a)[24] in
good yields, whereas the reac-
tion based on 3-bromoperylene
(5) gave lower yields probably
due to the poor solubility of
compound 5 (Scheme 2).
Scheme 3. Two-times pentannulation reactions of 9,10-dibromoanthracene, 1,6-dibromopyrene, and 1,8-dibro-
mopyrene.
direct bromination of pyrene according to the procedure of
Yip et al.[26] This reaction results in 1,6-dibromopyrene (7)
and 1,8-dibromopyrene (8) in equal amounts and the separa-
tion of 7 and 8 is known to be difficult.[27] When using the
mixture for the pentannulation, the desired products 7a and
8a were also formed in a 1:1 ratio as expected, but in this
case the purification of the isomers was considerably easier.
While the dicyclopenta
away from the cooled reaction mixture, the dicyclopenta-
[cd,fg]pyrene (8a) could be obtained via column chromatog-
raphy. The products were isolated in 54% yield. Finally the
dicyclopenta[cd,lm]perylene 9a was synthesized from 3,9-di-
ACHTUNGTRENN[UNG cd,jk]pyrene (7a) precipitated right
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
bromoperylene (9)[28] in 17% yield after recrystallization
(Scheme 4). Due to its low solubility, 9a could also be re-
ceived by direct precipitation from the cold reaction mix-
ture. To determine a potential correlation between the prod-
uct yield and the low solubility of compound 9a as well as
its intermediates, an analogous annulation using 1,2-bis(4-
butylphenyl)acetylene (e) was performed. However, the
solubilizing alkyl groups did not seem to have a positive
effect (Table 1). Moreover the annulation of perylene in par-
ticular is weakened by the occurrence of side products de-
rived from incomplete cyclization (11a) and addition of a
third acetylene derivative (12e) (see Scheme 4). Whereas
the first problem can be overcome by an increase of triethyl-
amine to enhance the removal of HBr (see also Scheme 6),
the second side reaction is caused by a higher concentration
of the reactants.[29]
Scheme 2. A) Overview about the palladium-catalyzed pentannulation re-
action of pyrene-, anthracene-, and perylenebromides with symmetric
acetylene derivatives. B) Pentannulation reaction of 1-bromopyrene with
diphenylacetylene.
Various aryl as well as alkyl substituted acetylenes
(a,b,c,d, Scheme 2A) were used as annulation partners.
Within this series the reaction with 4-octyne (b) was per-
formed in order to study the influence of solubilizing groups
attached to the respective cores. This time 3,4-dipropylcyclo-
penta[cd]pyrene (3b), 1,2-dipropylcyclopenta[cd]perylene
(5b), and 1,2,6,7-tetrapropylcyclopenta[hi]aceanthrylene
(6b) were obtained in slightly lower yields compared to
their phenyl congeners, which is caused by the lowered abili-
ty to precipitate within the last purification step. The synthe-
sis of thiophenyl- and pyridinyl-substituted pentannulated
We further investigated the twofold annulations of 9,10-
dibromoanthracene (6), 1,6-dibromopyrene (7), and 1,8-di-
bromopyrene (8). Applying 9,10-dibromoanthracene (6) in
this annulation chemistry, one would expect to obtain two
isomers as shown in Scheme 3. Remarkably, the formation
of isomer 10 was not observed, which is in agreement with
similar reports.[25] Only the cyclopenta[hi]aceanthrylenes 6a
and 6b were isolated in moderate yields.
In case of the twofold annulation of 1,6- and 1,8-dibromo-
pyrene (7, 8), both starting materials were synthesized via
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K. Mꢀllen, C. Li et al.
Table 1. Synthesis of pentannulated PAH.[a]
PAHs was carried out analogously. For this purpose, the di-
thiophenylacetylene (c) and the dipyridinylacetylene (d)
were prepared in a two-step synthesis derived from the bro-
moaryl precursors and 2-methylbut-3-yn-2-ol according to
the literature.[30] The desired products 3c, 4c, and 5c could
be obtained in moderate to good yields by reaction of 3–5
with dithiophenylacetylene (c). This is comparable to the
formation of the diphenyl derivatives 3a, 4a, and 5a. Annu-
lation with dipyridinylacetylene (d) furnished the respective
products 3d, 4d, and 5d in yields ranging from 8 to 40%,
most probably because of the electron-withdrawing charac-
ter of the pyridinyl groups. An analogous attempt using the
more electron-demanding dicarboxyacetylene dimethylester
was not successful. A summary of the annulation reactions
leading to cyclopentannulated PAHs is given in Table 1.
Similar to a single pentannulation of 9-bromoanthracene
(4), 9-bromophenanthrene (13) was tested. Surprisingly, the
expected acephenanthrylene 13a was observed only in trace
amounts, instead triphenylene 14[31] was obtained as main
product derived from hexannulation of 13 (Scheme 5). Dou-
bling the amount of acetylene applied, the yield of 14 could
be improved to 20%. The fact that an attachment of a
second alkyne can occur in a single annulation reaction
leads to the following mechanistic explanation.
Entry Bromoaryl
Acetylene
Product (yield [%])[b]
1
2
3
3
3
3
a: R=phenyl
b: R=nPr
c: R=3-thio-
phenyl
3a (82)
3b (60)
3c (74)
4
3
d: R=3-pyri-
dinyl
3d (30)
5
6
4
4
a: R=phenyl
c: R=3-thio-
phenyl
4a (76)
4c (68)
7
4
d: R=3-pyri-
dinyl
4d (40)
Based upon a mechanism for the hexannulation of bi-
phenyl compounds already described by Larock,[32] it is pro-
posed that the reaction starts with an insertion of the alkyne
into the aryl–palladium(II) species, resulting in an open vi-
nylic palladium intermediate (B). Subsequently two process-
es can take place: i) a ring closure by electrophilic attack
(C), forming the desired cyclopentadienide derivative after
reductive elimination of Pd0 (D); or ii) a second alkyne in-
serts (E) followed by ring closure to yield the respective
hexannulated product (F+G). (Scheme 6)
8
9
10
5
5
5
a: R=phenyl
b: R=nPr
c: R=3-thio-
phenyl
d: R=3-pyri-
dinyl
5a (30)
5b (16)
5c (27)
11
5
5d (8)
Photophysical characterization: The described compounds
are soluble in common organic solvents such as dichlorome-
thane or chloroform. Whereas the annulations of the differ-
ent core structures significantly affect the shape of the ab-
sorption spectra, only little effects were observed upon vary-
ing the substituents (Table 2). A discussion can therefore be
restricted to the di- and tetraphenyl derivatives.
12[c]
13[c]
6
6
a: R=phenyl
b: R=nPr
6a (44)
6b (34)
All three cyclopentapyrene compounds (3a, 7a, and 8a)
show sharp bands in the region of shorter wavelengths up to
450 nm with high absorption coefficients ranging from
22000mÀ1 cmÀ1 (3a, l=380 nm, and 7a, l=419 nm) to
34300mÀ1 cmÀ1 (8a, l=430 nm). In the long wavelength
region of the spectrum of the single annulated pyrene 3a a
weak broad band appears between 420 and 600 nm being re-
sponsible for the red color (Figure 1). The spectrum of 7a
shows a similar but much stronger band located at 450 to
560 nm, whereas the mirror-symmetric 8a exhibits only min-
imal absorption beyond 450 nm. Upon going from unsubsti-
tuted over alkyl- to aryl-substituted analogues of 3a, 7a,
and 8a the additional phenyl groups appear to have a cru-
cial effect on the absorption spectrum, as compounds 3a,
7a, and 8a show a larger bathochromic shift and a higher in-
14[c]
15[c]
7
8
a: R=phenyl
a: R=phenyl
7a (54)
8a (54)
16[c]
17[c]
9
9
a: R=phenyl
e: R=4-butyl- 9e (18)
phenyl
9a (17)
[a] Reaction conditions: Bromoaryl (1.0 mmol), acetylene (1.2 mmol),
[Pd(dba)2] (10 mol%), P(o-tol)3 (15 mol%), toluene (90 mL), CH3CN
G
ACHTUNGTRENNUNG
(10 mL), Et3N (5 mL), 1118C, 16 h. [b] Yield of the isolated product.
[c] 2.4 mmol of the acetylene were used instead of 1.2 mmol, the amount
of catalyst was doubled.
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Polycyclic Aromatic Hydrocarbons
FULL PAPER
780 nm for 6a, leading to a
bright green appearance of the
tetraphenylcyclopenta[hi]a-
ceanthrylene (Figure 2). The
observed absorption coeffi-
cients are 7800mÀ1 cmÀ1 (4a,
l=375 nm) and 20400mÀ1 cmÀ1
(6a, l=393 nm) (Table 2). Sur-
prisingly the unsubstituted cy-
clopenta[hi]aceanthrylene is re-
Scheme 4. Two-times pentannulation reaction of 3,9-dibromoperylene.
Table 2. Absorption maxima (lmax) and absorption coefficients (e) of all
compounds derived from palladium-catalyzed pentannulation.
Compound
lmax [nm]
e (lmax) [mÀ1 cmÀ1
]
3a
3b
3c
3d
4a
4c
4d
5a
5b
5c
5d
6a
6b
7a
8a
9a
380/400
340/356/376
384/405
21000/17100
14100/16900/12400
22700/19900
382/401
22000/19000
375/417/439
376/417/440
374/420/443
480/507
456/484
480/507
7800/6400/5100
7900/7300/5800
9400/7400/6300
30000/28500
34100/24900
29000/27700
Scheme 5. Hexannulation reaction of 9-bromophenanthrene with diphe-
nylacetylene.
488/513
15900/15600
tensity of the broad bands compared both to the alkyl sub-
stituted 3b and to the unsubstituted parent compounds.[4c]
Pyrene 8a plays a special role, since two of its phenyl
groups are twisted out of the plane by about 808 due to the
steric hindrance and thus do not conjugate with the pyrene
core.[33]
393/441/468
382/424/450
398/419
406/430
462/493/530
20400/11800/10800
15800/5900/5000
20500/22000
31500/34300
10800/20800/26700
The spectra of the anthracene derivatives 4a and 6a also
possess sharp peaks from 300 to 450 nm, and 490 nm respec-
tively, followed by a broad band located between 450 and
650 nm for the red colored 4a and even between 500 and
ported to be dark-brown and does not show absorption
beyond 500 nm.[34] The alkyl-substituted analogue 6b reveals
a decrease of the absorption between 500 and 780 nm of ap-
proximately 50%. This again
proves that the phenyl groups
enlarge the conjugation of the
core, compared to alkyl- or
non-substituted analogues and
thus affect the spectral proper-
ties of the compounds.
The analogous annulation of
perylene leads to a bathochro-
mic shift of about 70 nm (5a)
and 93 nm (9a) compared with
the
parent
compound
(Figure 3). This results in a red
color with a lmax of 507 nm
(5a) and 530 nm (9a) and ab-
sorption coefficients of 30000
and 26700mÀ1 cmÀ1. As already
stated in the case of the
pyrene derivative 8a, both
mirror-symmetric perylenes do
not show any broad bands in
the region of longer wave-
lengths.
Scheme 6. Proposed mechanism of the palladium-catalyzed pentannulation and hexannulation reaction.
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K. Mꢀllen, C. Li et al.
Figure 3. Absorption spectra of cyclopenta[cd]perylene 5a (solid line),
and dicyclopentaACTHNUGTRNEUNG[cd,lm]perylene 9a (dashed line) (solvent: chloroform).
Figure 1. Absorption spectra of cyclopenta[cd]pyrene 3a (solid line),
dicyclopenta[cd,jk]pyrene 7a (dotted line), and dicyclopenta[cd,fg]pyrene
8a (dashed line) (solvent: chloroform).
N
ACHTUNGTRENNUNG
HOMO and LUMO, which are both located on the core
and resemble the orbitals of the parent perylene.[36] Thus the
absorption spectrum of 9a consists of sharp peaks only. Sim-
ilar correlations between the shapes of the orbitals and the
optical absorption were obtained for the other pentannulat-
ed structures, which are listed in the Supporting Informa-
tion. It appears that all presented mirror-symmetric mole-
cules only show sharp peaks in their absorption spectra, due
to the similarity between their HOMOs and LUMOs (no
charge transfer), whereas non-symmetric annulation induces
a strong difference in the shape of the respective orbitals
and thus a broadening of the absorption bands.
Cyclic voltammetry: In order to determine the respective or-
bital energies, cyclovoltammetric measurements were per-
formed in dichloromethane.[37] The results are compiled in
Table 3. Almost all PAHs show at least one reversible reduc-
tion reaction. Some of them even exhibit partially reversible
oxidation behavior, even though they are reported to be
mostly irreversible for unsubstituted pentannulated PAH,
such as aceanthrylene, cyclopenta[cd]pyrene, or cyclopen-
ta[cd]perylene.[5] In the case of the anthracene derivatives
4a and 6a, it can be seen from the data, that the LUMO as
well as the HOMO energies are lowered by introducing a
second five-membered ring. In contrast to that, the pyrene
compounds 3a, 7a, and 8a do not show any noticeable
change in the HOMO–LUMO energies. The same is true
for the resulting HOMO–LUMO gaps. Whereas the DE
values of 3a, 7a and 8a do not vary much, the difference of
the orbital energies is much larger when going from anthra-
cene 4a to 6a, leading to a narrowing of the HOMO–
LUMO gap. This conforms to the bathochromic shift com-
paring the absorption spectra of 4a and 6a (see Table 3 and
Figure 2). Similar results are obtained for the perylene de-
rivatives 5a and 9a. In this case the second pentannulation
also leads to a strong decrease of the energy levels and at
the same time results in a convergence of the orbital ener-
Figure 2. Absorption spectra of aceanthrylene 4a (solid line), and cyclo-
penta[hi]aceanthrylene 6a (dashed line) (solvent: chloroform).
Simple semiempirical quantum mechanical calculations
(Gaussian 03W, AM1)[35] of the electronic structure of the
phenyl-substituted pentannulated compounds give an ex-
planation for the difference in the shape of the respective
spectra. As an example the frontier orbitals of cyclopenta-
[hi]aceanthrylene 6a and dicyclopenta [cd,lm]perylene 9a
are depicted (Figure 4). The anthracene derivative exhibits a
significant difference in the shape of the respective orbitals.
The HOMO of 6a shows a strong influence of two phenyl
substituents resulting in a linear extension of this orbital,
whereas the LUMO is mostly centered in the core of the
molecule. The difference between the orbitals leads to an in-
tramolecular charge transfer, yielding the broad band in the
absorption spectrum (Figure 2). In contrast, the calculations
of the perylene 9a indicate only little changes between the
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Polycyclic Aromatic Hydrocarbons
FULL PAPER
achieved in a feasible one-step reaction. The successful ap-
plication of mono- and dibromo derivatives of pyrene, an-
thracene, and perylene as well as the use of 4-octyne, di-
phenyl-, dithiophenyl-, and dipyridinylacetylene as coupling
partners demonstrates the versatility of this reaction. Fur-
ther, one can envision the use of this pentannulated PAHs
in organic photovoltaics, as the performed UV/Vis absorp-
tion measurements show that the synthesized compounds
exhibit colors going from yellow (8a) over red (3a, 4a, and
5a) up to green in the case of the cyclopenta[hi]aceanthry-
lene 6a. Thus they provide easily accessible dyes covering a
large spectral range together with high absorption coeffi-
cients. The cyclovoltammetric measurements are in good
agreement with the spectroscopic data. Furthermore they
reveal illustrative redox behavior similar to literature known
PAH.[5] The compounds derived from palladium-catalyzed
pentannulations possess good solubility in common organic
solvents, even without additional solubilizing groups, ena-
bling an easy solution processing for organic electronic devi-
ces.
Experimental Section
General methods: See Supporting Information.
Cyclic voltammetry: Cyclic voltammetry for spectroelectrochemistry ex-
periments was carried out with an EG&G Princeton Applied Research
potentiostat, model 273. The working electrode consisted of an inlaid
platinum disk (1.5 mm diameter) that was polished on a felt pad with
0.05 mm alumina and sonicated in milli-Q water for 5 min before each ex-
periment. A platinum wire was used as the counter electrode and an Ag
wire was used as the reference electrode internally calibrated with ferro-
cene/ferrocenium (Fc/Fc+) in the measurement. The CV measurements
were carried out in a solution of Bu4NPF6 (0.1m) in dry CH2Cl2 with a
scan rate of 50 mVsÀ1 at room temperature under argon. Redox poten-
tials were determined via the onset method.
Figure 4. Molecular orbitals of cyclopenta[hi]aceanthrylene 6a and
dicyclopentaACHTUNGTRENNUNG[cd,lm]perylene 9a. HOMO=highest occupied molecular or-
bital, LUMO=lowest unoccupied molecular orbital (Gaussian 03W,
AM1).
Representative synthetic procedure: A stirred solution of 3 (1.0 mmol)
and a (1.2 mmol) in toluene (90 mL),
acetonitrile (10 mL), and triethyla-
Table 3. HOMO–LUMO calculations according CV measurements.
mine (5 mL) was purged with argon
for 20 min. Subsequently, [Pd
ACHTUNGTRENNUNG(dba)2]
Compound
EHOMO [eV]
ELUMO [eV]
DE [eV]
lmax [nm]
e (lmax) [mÀ1 cmÀ1
]
(10 mol%) and P(o-tol)3 (15 mol%)
AHCTUNGTRENNUNG
3a
4a
5a
6a
7a
8a
9a
À5.57
À5.12
À5.48
À5.42
À5.57
À5.46
À5.57
À3.59
À3.10
À3.24
À3.55
À3.61
À3.56
À3.83
1.98
2.02
2.24
1.87
1.96
1.90
1.74
380/400
375/417/439
480/507
393/441/468
398/419
406/430
21000/17100
7800/6400/5100
30000/28500
20400/11800/10800
20700/22500
were added. The resulting solution
was evacuated and purged with argon
two times and then refluxed for 16 h.
After cooling to room temperature,
the solvents were evaporated in
vacuo and the crude product was pu-
rified by column chromatography
(silica gel, petroleum ether/dichloro-
methane 4:1) to give product 3a in
82% yield.
31500/34300
10800/20800/26700
462/493/530
gies, which is again accompanied by a bathochromic shift in
the absorption spectrum.
Acknowledgements
Conclusion
We gratefully acknowledge the financial support from the Bundesminis-
terium fꢀr Bildung und Forschung (BMBF) and BASF SE.
In summary, the palladium-catalyzed pentannulation of aryl-
bromides with aryl and alkyl acetylenes appears as a power-
ful method for the preparation of functionalized pentannu-
lated PAHs, since novel cyclopentadienide moieties can be
´
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Received: April 12, 2011
Revised: August 22, 2011
Published online: September 28, 2011
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