Palladium-catalyzed highly efficient synthesis of tetracenes and pentacenes

Article abstract of DOI:10.1039/c2cc36700a

Pd(0)-catalyzed cascade reactions of 1,7-diyn-3,6-bis(propargyl carbonate) with organoboronic acids have been developed, which provide one-pot access to functionalized tetracenes or pentacenes. The Royal Society of Chemistry 2012..

Full text of DOI:10.1039/c2cc36700a

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COMMUNICATION
Palladium-catalyzed highly efficient synthesis of tetracenes and
pentacenesw
Ming Chen, Yifeng Chen and Yuanhong Liu*
Received 14th September 2012, Accepted 31st October 2012
DOI: 10.1039/c2cc36700a
Pd(0)-catalyzed cascade reactions of 1,7-diyn-3,6-bis(propargyl
carbonate) with organoboronic acids have been developed, which
provide one-pot access to functionalized tetracenes or pentacenes.
1,6-diynyl carbonates to benzo[b]fluorenes,10 which is initialized
through the formation of an allenyl carbonate via 3,3-rearrange-
ment reaction. Inspired by these successes and our continuous
interest in the synthesis of polyacenes,11 we postulated that the
efficient generation of the allenic intermediates might also be
achieved through transition metal-catalyzed reactions of
bis(propargyl carbonate). Along this line, we discovered an
entirely new route to linear acenes via palladium-catalyzed
allenylation/cyclization/cross-coupling reaction of 1,7-diyn-3,6-
bis(propargyl carbonate) with organoboronic acids (Scheme 1).
It is noted that the metal-catalyzed one-pot synthesis of higher
acenes from acyclic precursors is quite rare.12
Polyacenes, particularly tetracenes and pentacenes, have attracted
considerable attention owing to their fascinating structural
features and their unique electronic properties. They are active
semiconductor materials that have been utilized extensively in
organic field-effect transistors (OFETs) and organic light-emitting
diode applications.1 The most common route to synthesise poly-
acenes2 is through the nucleophilic addition of an organometallic
reagent to an acene quinone followed by reduction.3 Others
include iterative strategies as reported by Takahashi et al. via
zirconium-mediated cyclizations,4 by Bowles and Anthony via
Bergman cyclization,5 by Lin et al. via homo-elongation,6 and
Diels–Alder or aldol condensations,7 etc. However, these
syntheses usually suffer from multiple, tedious steps, and often
need further reduction or oxidation reactions to furnish the
acene structure. Thus, more straightforward methods for the
preparation of polyacenes are highly desired. On the other
hand, it is documented that ene-diallenes or benzene-bridged
diallenes are highly efficient for the construction of various
polycyclic organic compounds.8 The reactions generally proceed
via first formation of the transient o-quinodimethane intermediates,
which then undergo [1,5]-hydrogen migration or inter- or intra-
molecular cyclizations to furnish the final product. The diallenes
are usually generated by treatment of bis(propargyl alcohol) with
SOCl2 or HX,8b,c or through the [2,3]-sigmatropic rearrangement
of the in situ formed propargyl sulfenates or phosphites.8d–f
For example, Lin et al. reported that tetracene sulfoxides and
tetracene sulfones could be synthesized by the reactions of
bis(propargyl alcohol) with sulfenyl or sulfinyl chlorides via
sequential cyclizations of the diallene intermediates followed
by aromatization through the elimination of a leaving group.9
Recently, we have developed a gold-catalyzed cyclization of
It is well known that Pd(0)-catalyzed cross-coupling reac-
tions of propargylic carbonates with organoborons afford
allene derivatives.13 Here we found that by simple treatment
of phenyl-substituted dicarbonate 1a with 4.0 equiv. of phenyl-
boronic acid in the presence of 5 mol% Pd(PPh3)4 in THF at
50 1C for 2 h, 5,12-diphenyltetracene 2a was formed in a high
yield of 90% (Table 1, entry 1). Meanwhile, the homocoupling
product, biphenyl, could be detected during the reaction (44%
based on boronic acid). Decreasing the amount of PhB(OH)2
to 2.0 equiv. resulted in a slight decrease in product yield
(86%). However, the use of 1.0 equiv. of PhB(OH)2 afforded
2a in a lower yield of 42%. The results indicated that only one
molecule of phenylboronic acid coupled with substrate 1a, in
addition, a phenyl group, either from 1a, or from phenylboronic
acid, provided a benzo unit to furnish the tetracene core.
Encouraged by this initial result, we next investigated the reac-
tions of 1a with various arylboronic acids. Since both aryl groups
in carbonate 1a and arylboronic acid might undergo ring-closure
reaction, two tetracene regioisomers may be formed. To our
delight, when arylboronic acids bearing electron-withdrawing
groups such as p-Cl were employed, the corresponding tetra-
cene 2b was produced as a single regioisomer in 94% yield
(Table 1, entry 2), along with only 7% of 4,40-dichlorobiphenyl
State Key Laboratory of Organometallic Chemistry, Shanghai
Institute of Organic Chemistry, Chinese Academy of Sciences,
345 Ling ling Lu, Shanghai 200032, P. R. China.
E-mail: yhliu@mail.sioc.ac.cn
w Electronic supplementary information (ESI) available: Experimental
details, NMR spectra of all new products, X-ray crystallography of
compounds 2b and 2u. CCDC 892574 (2b) and 892575 (2u). For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c2cc36700a
Scheme 1 A Pd(0)-catalyzed strategy to synthesize polyacenes.
Chem. Commun., 2012, 48, 12189–12191 12189
c
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Table 1 Formation of tetracenes via reactions with arylboronic acids
Yielda
(%)
Product
EntrySubstrate ArB(OH)2
R1 =H
(1a)
90
Ar=Ph
1
PhB(OH)2
2a
2
3
1a
1a
p-ClC6H4B(OH)2 Ar=p-ClC6H4 2b 94
p-CF3C6H4B(OH)2 Ar=p-CF3C6H4 2c 77
Scheme 2 Formation of tetracene regioisomers.
electron-donating (p-MeO, 3,4,5-triMeO) or electron-withdrawing
groups (p-Cl, p-CF3) on the aryl ring all worked well under the
present reaction conditions, indicating that this method is highly
useful for the preparation of a wide variety of tetracenes.
R1 =
p-OMe
(1b)
90
Ar=Ph
4
PhB(OH)2
2d
5
6
7
8
1b
1b
1b
1b
p-MeOC6H4B(OH)2 Ar=p-OMeC6H4 2e 77
p-ClC6H4B(OH)2 Ar=p-ClC6H4 2f 88
p-CF3C6H4B(OH)2 Ar=p-CF3C6H4 2g 88
It should be noted that when electron-rich arylboronic acids
were employed, two regioisomers were usually obtained. For
example, the reaction of 1e with 4-methoxyphenylboronic acid
provided 2q and 2q0 in 40% and 50% NMR yields, respectively
(Scheme 2, eqn (1)), implying that the aryl ring of arylboronic
acids could also be involved in the tetracene core. 2q and 2q0
could be separated by column chromatography. Additionally,
substrate 1f bearing ortho-methoxy-substituted aryl groups under-
went cyclization to give, not only the desired product 2r, but also
a mono-MeO-substituted tetracene 2r0 in 7% yield (Scheme 2,
eqn (2)). The latter might be formed via b-MeO elimination of
the palladium intermediate during the ring-closure process
(vide infra).14 When substrates 1 bearing two alkyl groups, or
one alkyl, one aryl group on the alkyne terminus were employed,
the desired tetracenes could not be obtained.
o-FC6H4B(OH)2
Ar=o-FC6H4
2h 90
9
1b
Ar=2-thienyl
2i 78
R1 =3,4,5-
triMeO
(1c)
10
PhB(OH)2
PhB(OH)2
2j 89
R1 =p-Cl
(1d)
87
Ar=Ph
11
2k
12
13
1d
1d
p-ClC6H4B(OH)2 Ar=p-ClC6H4 2l 87
p-CF3C6H4B(OH)2 Ar=p-CF3C6H4 2m86
14
15
1d
Ar=2-thienyl
2n 45
71
2o Ar =
R1 =p-CF
(1e)
The reactions of dicarbonates 1 with alkylboronic acids
were also investigated. To our delight, the reaction of 1a with
n-butylboronic acid at 50 1C afforded the desired 5-butyl-12-
phenyltetracene 2s in 62% yield (Table 2, entry 1). Further
optimizations revealed that Boc-protected propargyl alcohols
led to better results at 90 1C in a sealed tube, furnishing 2s in
74% yield (entry 3). Methyl, i-butyl and phenethylboronic
acids were also suitable in this cyclization reaction to give
3 p-ClC6H4B(OH)2
p-ClC6H4
16
1e
p-CF3C6H4B(OH)2 Ar=p-CF3C6H4 2p 82
a
Isolated yields.
(based on arylboronic acid). Similarly, the use of 4-(trifluoro-
methyl)-phenylboronic acid resulted in the formation of 2c in
77% yield (entry 3). The results suggest that ring-closure
selectively occurs with the phenyl ring of the dicarbonate 1a.
We proceeded to examine the scope and limitations of this
process. When electron-neutral or electron-poor arylboronic
acids were subjected to the reaction conditions, the corres-
ponding tetracenes were obtained as a single regioisomer in
good to high yields and within short reaction time, regardless
of the electronic properties of the substituents on carbonate
aryl rings (Table 1, entries 4, 6–8, 10, 11, 13 and 15). In entries
5, 12 and 16, since aryl groups of carbonates 1 and arylboronic
acids are same, only single products 2e, 2l and 2p were
observed, respectively, with isolated yields in the range of
77–87%. Heteroarylboronic acid such as 2-thienylboronic acid
is also compatible for this cyclization, and the corresponding
products 2i and 2n were obtained in 78% and 45% yields,
respectively (entries 9 and 14). Again in these cases, ring-closure
occurred exclusively with the aryl ring of the dicarbonate
substrates. As for propargylic carbonates, substrates bearing
Table 2 Formation of tetracenes via reactions with alkylboronic
acids
Temp Time
(1C)
Yield
Product of 2a (%)
Entry Dicarbonate R2
(h)
1
1a
1a
1g
1g
1g
1g
nBu
nBu
nBu
Me
iBu
50
90
90
90
90
2.5
2.5
2.5
3
3
2.5
2s
2s
2s
2t
2u
2v
62
56
74
59
74
50
2b
3b
4b
5b
6b
–CH2CH2Ph 90
a
b
Isolate yields. In a sealed tube.
c
12190 Chem. Commun., 2012, 48, 12189–12191
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easily available starting materials. High regioselectivity has
been achieved when coupled with electron-neutral or electron-
poor aryl boronic acids. Further studies to elucidate the
precise mechanism and to extend the scope of synthetic utility
are in progress in our laboratory.
We thank the National Natural Science Foundation of
China (Grant No. 21121062, 21125210), Chinese Academy
of Science, and the Major State Basic Research Development
Program (Grant No. 2011CB808700) for financial support.
Scheme 3 One-pot synthesis of pentacenes.
Notes and references
1 For reviews, see: (a) M. Bendikov, F. Wudl and D. F. Perepichka,
Chem. Rev., 2004, 104, 4891; (b) J. E. Anthony, Chem. Rev., 2006,
106, 5028.
2 For review, see: H. Qu and C. Chi, Curr. Org. Chem., 2010,
14, 2070.
3 For examples, see: (a) M. S. Taylor and T. M. Swager, Angew.
Chem., Int. Ed., 2007, 46, 8480; (b) J. E. Anthony, J. S. Brooks,
D. L. Eaton and S. R. Parkin, J. Am. Chem. Soc., 2001, 123, 9482.
4 (a) T. Takahashi, M. Kitamura, B. Shen and K. Nakajima, J. Am.
Chem. Soc., 2000, 122, 12876; (b) T. Takahashi, K. Kashima, S. Li,
K. Nakajima and K. Kanno, J. Am. Chem. Soc., 2007, 129, 15752.
5 D. M. Bowles and J. E. Anthony, Org. Lett., 2000, 2, 85.
6 C.-H. Lin, K.-H. Lin, B. Pal and L.-D. Tsou, Chem. Commun.,
2009, 803.
7 (a) Z. Chen, P. Muller and T. M. Swager, Org. Lett., 2006, 8, 273;
(b) Y. Kuninobu, T. Seiki, S. Kanamaru, Y. Nishina and K. Takai,
Org. Lett., 2010, 12, 5287; (c) M. M. Payne, S. A. Odom,
S. R. Parkin and J. E. Anthony, Org. Lett., 2004, 6, 3325.
8 (a) H. A. Staab and B. Draeger, Chem. Ber., 1972, 105, 2320;
(b) F. Toda, K. Tanaka, I. Sano and T. Isozaki, Angew. Chem., Int.
Ed., 1994, 33, 1757; (c) K. Tanaka, N. Takamoto, Y. Tezuka,
M. Kato and F. Toda, Tetrahedron, 2001, 57, 3761; (d) S. Kitagaki,
K. Katoh, K. Ohdachi, Y. Takahashi, D. Shibata and C. Mukai,
J. Org. Chem., 2006, 71, 6908; (e) S. Kitagaki, K. Ohdachi,
K. Katoh and C. Mukai, Org. Lett., 2006, 8, 95; (f) S. Kitagaki,
Y. Okumura and C. Mukai, Tetrahedron, 2006, 62, 10311.
9 Y.-C. Lin and C.-H. Lin, Org. Lett., 2007, 9, 2075.
10 Y. Chen, M. Chen and Y. Liu, Angew. Chem., Int. Ed., 2012,
51, 6493.
11 G. Li, S. Zhou, G. Su, Y. Liu and P. G. Wang, J. Org. Chem.,
2007, 72, 9830.
12 C. Kitamura, A. Takenaka, T. Kawase, T. Kobayashi and
H. Naito, Chem. Commun., 2011, 47, 6653.
13 (a) T. Moriya, N. Miyaura and A. Suzuki, Synlett, 1994, 149;
(b) N. Miyaura and A. Suzuki, Chem. Rev., 1995, 95, 2457;
(c) W. Shu, G. Jia and S. Ma, Org. Lett., 2009, 11, 117; For the
trapping of the in situ formed CO2, see: (d) J. Ye, S. Li and S. Ma,
Org. Lett., 2012, 14, 2312.
14 (a) Z. Zhang, X. Lu, Z. Xu, Q. Zhang and X. Han, Organometallics,
2001, 20, 3724; (b) G. Zhu and X. Lu, Organometallics, 1995,
14, 4899; (c) L. Zhou, Y. Liu, Y. Zhang and J. Wang, Chem.
Commun., 2011, 47, 3622.
Scheme 4 Possible reaction mechanism.
2t–2v in 50–74% yields (entries 4–6). These tetracenes are
highly soluble in common organic solvents. The structure of
tetracene was unambiguously determined by X-ray crystallo-
graphic analysis of compounds 2b and 2u.
The present cascade cyclization methodology has been success-
fully extended to the synthesis of pentacenes (Scheme 3). Aryl and
alkylboronic acids were well accommodated, leading to pentacenes
4a–4c in moderate to good yields as purple solids. The method
presented here represents one of the most efficient routes for the
synthesis of pentacenes from acyclic precursors.
We tentatively propose the following plausible mechanism
for this transformation (Scheme 4). The reaction is initiated by
double SN20 attack of Pd(0) on the propargylic carbonate13 to form
bis[(s-allenyl)palladium(II)] intermediate 5.15 6p-electrocyclic ring-
closure of ene-diallene8 5 leads to 2,3-naphthoquinodimethane 6 as
the reactive alkene isomer, which can also be represented as the
resonance structure 7, a highly stabilized biradical.16 A disrotatory
6p-electrocyclization of 6 occurs to afford the intermediate 8, in
which the palladium and the hydrogen are in the cis configuration.
b-Hydride elimination furnishes arylpalladium intermediate 9.17
Suzuki-type coupling of intermediate 9 with organoboronic acid
gives the tetracene 2 and releases Pd(0). The formation of tetracene
regioisomers 2q and 2q0 might be due to the competitive coupling
reactions of intermediate 5 or 6/7 with arylboronic acid.
15 For Pd(0)/SmI2 mediated formation of diallenes from o-bis-
(a-acetoxypropargyl)benzene possibly via the bis[(s-allenyl)palla-
dium(II)] intermediate, see: Y. Sugimoto, T. Hanamoto and
J. Inanaga, Appl. Organomet. Chem., 1995, 9, 369.
16 J. L. Segura and N. Martın, Chem. Rev., 1999, 99, 3199.
´
17 The reaction of 1a in the absence of aryl boronic acids (5 mol%
Pd(PPh3)4, 100 1C, in THF, 3 h) afforded 5-phenyltetracene in 21%
yield. This result may support the formation of intermediate 9.
In summary, we have developed a novel Pd(0)-catalyzed
cascade reaction for the synthesis of linear polyacenes from
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 12189–12191 12191