Synthesis, structure, and photophysical properties of dibenzo-[de, mn]naphthacenes

Article abstract of DOI:10.1002/anie.201001929

Zethrenes were synthesized by Pd-catalyzed cyclodimerization of 1-ethynyl-8-iodonaphthalenes. The structure of these cycloadducts was confirmed by X-ray crystal analysis. The bond lengths and bond alternation in the crystal structures reveal that the central two six-membered rings of the title compounds lack aromaticity.

Full text of DOI:10.1002/anie.201001929

Angewandte
Chemie
DOI: 10.1002/anie.201001929
Zethrenes
Synthesis, Structure, and Photophysical Properties of Dibenzo-
[de,mn]naphthacenes**
Tsun-Cheng Wu, Chia-Hua Chen, Daijiro Hibi, Akihiro Shimizu, Yoshito Tobe, and
Yao-Ting Wu*
Dibenzo[de,mn]naphthacene (zethrene, 1a, R = H) is
a
derivatives exhibit interesting physical properties, which may
member of the benzenoid hydrocarbon family,[1] and has an
interesting structure with respect to the formal definitions of
aromaticity. The central two six-membered rings in zethrene
have potential applications as organic materials, including
electroluminescent devices[7] and organic transistors.[8] Theo-
retical investigations demonstrate that zethrene (1a) has
singlet biradical character[9b] and is also a suitable building
block for nonlinear optical materials[9] and near-infrared
absorbing pigments.[10] These versatile physical properties
have yet to be extensively explored because of the difficulty
involved in synthesizing these molecules.
Zethrene (1a) was first prepared from chrysene through
an inefficient route by Clar et al. in 1955.[11] Other synthetic
approaches involving several steps have been also elucidated.
Their key steps mainly involve either the transannular
reaction of cyclodeca[1,2,3-de:6,7,8-d’e’]dinaphthalene (2)[12]
and
naphthalene (3),[13] or the cyclization of 7H,9H,16H,18H-
dinaphtho[1,8-cd:1’,8’-ij][1,7]dithiacyclododecane
(4).[14]
However, preparation of functionalized zethrenes using
these protocols is inconvenient. Hence, the development of
an efficient synthetic method is necessary. Numerous poly-
aromatic hydrocarbons can be accessed by metal-catalyzed
annulation of haloarenes.[15] Accordingly, we observed that
zethrenes 1 can be obtained from 1-ethynyl-8-iodonaphtha-
lenes 5[16] in the presence of Pd catalysts.
The synthesis of zethrene 1b (R = Ph) from 1-iodo-8-
(phenylethynyl)naphthalene (5b) was explored under several
reaction conditions. The catalytic systems for the generation
of dibenzo[a,e]pentalenes by the cyclodimerization of 1-
ethynyl-2-halobenzenes[15f,g] are not efficient for producing
1b. Therefore, reaction conditions for metal-catalyzed anne-
lation[15a–e] or dimerization[17] of iodoarenes were utilized and
it was determined that condition F is more effective than A–E
(Table 1). Under condition E, 5b gave an acetonitrile-
mediated cycloadduct 6 as the major product (entry 5 in
7,8,15,16-tetradehydrocyclodeca[1,2,3-de:6,7,8-d’e’]di-
(1a) cannot present aromaticity associated with the Kekulꢀ
structure. Based on Clarꢁs aromatic sextet theory,[2] the p-
electron sextets of the two periphery naphthalenes and the
central butadiene moiety (i.e. C7, C7a, C14, and C14a) can be
identified as “essential”[3] (benzene-like) and “fixed”[2] (local-
ized) carbon–carbon double bonds, respectively.[4] Therefore,
the p-electrons in the central two six-membered rings are
localized and their index of local aromaticity[5] and induced p-
electron currents[6] are much lower than those of a normal
aromatic ring. However, no direct experimental evidence,
such as an X-ray structure, is currently available to confirm
the computational results. Although it can be viewed as
weakly coupled double naphthalene units,[5c] zethrene and its
[*] T.-C. Wu, C.-H. Chen, Prof. Y.-T. Wu
Department of Chemistry, National Cheng Kung University
No. 1 Ta-Hsueh Rd., 70101 Tainan (Taiwan)
Fax: (+886)6-274-0552
E-mail: ytwuchem@mail.ncku.edu.tw
D. Hibi, Dr. A. Shimizu, Prof. Y. Tobe
Division of Frontier Materials Science
Graduate School of Engineering Science, Osaka University
1–3 Machikaneyama, Toyonaka, Osaka, 560-8531 (Japan)
Fax: (+81)6-6850-6229
[**] Metal-Catalyzed Reactions of Alkynes. Part VI. Part V, see ref. [21].
This work was supported by the Landmark Project of National
Cheng Kung University and the National Science Council of Taiwan
(NSC 98-2113M-006-002-MY3) and a Grant-in-Aid for Scientific
Research from the Ministry of Education, Culture, Sports, Science
and Technology (Japan). We also thank Prof. S.-L. Wang and P.-L.
Chen (National Tsing Hua University, Taiwan) for the X-ray structure
analyses. The authors are indebted to Prof. Jay Siegel (University of
Zurich, Switzerland) for his useful suggestions.
Table 1).[18] Fine-tuning condition F by varying the Pd
catalyst, phosphine ligand, and solvent did not increase the
yield of 1b (entries 6–11 in Table 1). Under the best
condition, the desired product 1b was obtained in 73%
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201001929.
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Table 1: Optimization of reaction conditions for preparation of 1b.[a]
Table 2: Preparation of compounds 1 from alkynes 5.[a]
Entry
Condition
Ligand ([mol%])
Solvent
Yield [%]
Entry
Alkyne
R
Product
Yield [%]
1
2
3
4
5
6
7
8
A
B
C
D
E
F
F
F
F
F
F
–
–
–
–
–
–
p-xylene
DMF
34, traces[b]
38
1
2
3
4
5
6
7
8
5b
5c
5d
5e
5 f
5g
5h
5i
Ph
4-FC6H4
4-ClC6H4
4-BrC6H4
4-CH3C6H4
4-CF3C6H4
4-OCH3C6H4
4-CO2CH3C6H4
2,6-Cl2C6H3
3,5-(CH3)2C6H3
2,4,6-(CH3)3C6H2
3,4,5-F3C6H2
1b
1c
1d
1e
1 f
1g
1h
1i
73
56[b]
CH3CN
2-pentanone
CH3CN
o-xylene
o-xylene
CH3CN
o-xylene
o-xylene
o-xylene
32[c]
36[b]
<37[d,e]
3[f]
24[b]
46,[c] 35[b]
59[b]
14
P(2-furyl)3 (15)
P(2-furyl)3 (15)
PPh3 (15)
PCy3 (15)
P(2-furyl)3 (15)
73
22,[b] 61[d]
40
26[c]
9
10
11
29
9
5j
5k
5l
1j
1k
1l
26[d]
45
10
11
12
34[b]
<37[e,g]
24,[b] 26[d]
51[b]
5m
1m
[a] Amounts of catalysts and additives relative to alkyne 5b (0.5 mmol):
Condition A: Pd(OAc)2 (5 mol%), AgOAc (1 equiv), 1108C, 36 h.[15a] B:
Pd(OAc)2 (5 mol%), K2CO3 (4 equiv), nBu4NBr (2 equiv), 1308C,
20 h.[15b] C: Pd(OAc)2 (5 mol%), K2CO3 (2.4 equiv), 1208C, 36 h.[18] D:
Pd(OAc)2 (5 mol%), NaOAc·H2O (2 equiv), LiCl (0.5 equiv), 1308C,
40 h.[15d] E: [NiBr2(dppe)] (5 mol%), Zn (3 equiv), 1108C, 12 h.[15e] F:
Pd(OAc)2 (5 mol%), Ag2CO3 (1 equiv), 1308C, 36 h.[15c] [b] AgOAc
(2 equiv) was used. [c] A mixture of 1b and 7 was obtained. [d] 5b
(63%) remained. [e] Yield according to NMR spectroscopy. [f] 6 (84%)
was obtained. [g] [Pd2(dba)3] (2.5 mol%; dba=trans,trans-dibenzylide-
neacetone) was used. 5b (63%) remained.
13
14
5n
5o
1n
1o
44[d]
14[d]
9-anthracenyl
15
5p
1p
16
16
17
18
5q
5r
5s, 5t
Si(CH3)3
nC4H9
tC4H9, CꢀCPh
1a
1r
1s, 1t
20
20
0
[a] Reaction was conducted with alkyne 5 (0.5 mmol) in o-xylene
(condition F). [b] Ag2CO3 (1.5 equiv) was used. [c] 5 f (33%) was
recovered. [d] Pd(OAc)2 (10 mol%), Ag2CO3 (1.5 equiv), and P(2-furyl)3
(30 mol%) were used.
yield (entry 7 in Table 1). Notably, reactions in acetonitrile
under either condition C or F gave a mixture of 1b and 11-
phenylbenzo[a]naphtha[2,1,8-cde]perylene (7),[16,19] which
was observed as the minor product and whose structure was
verified by the X-ray crystal analysis. Compound 7 should be
generated from 1b by the cyclodehydrogenation.[20]
examples of crystal structures of this compound class.
Zethrene (1a) is planar, whereas 7,14-disubstituted zethrenes
1b, 1l, and 1r deviate significantly from planarity (approx-
imately 458)[16] because they contain two substructures of 4-
substituted phenanthrene (Figure 1).[23] The central two six-
membered rings in both planar and twisted zethrenes 1
exhibit remarkable bond alternation with a range of 0.070–
0.116 ꢂ,[16,24] and, accordingly, they lack aromaticity.
The reactivity of several alkynes 5 for the preparation of
zethrenes 1 was investigated under the optimized conditions
described above (condition F). Most of them are less reactive
than 5b (Table 2). It was necessary to increase the amount of
silver carbonate and/or Pd catalysts (10 mol%) to ensure
complete consumption of the starting material. Aryl-substi-
tuted reactants are more appropriate than alkyl and phenyl-
ethynyl analogues in this reaction. The steric congestion and
the electronic properties of the substituents strongly affect the
yield. The electron-deficient aryl substituent increased the
reaction efficiency relative to the electron-rich moiety
(entries 2–8, Table 2). As expected, bulky groups, such as
mesityl, 9-anthracenyl, and 2,6-dichlorophenyl, gave unsat-
isfactory results, and tert-butyl-substituted alkyne 5s did not
undergo the cyclodimerization (entries 9, 11, 14, and 18,
Table 2). 1-Iodo-8-(trimethylsilylethynyl)naphthalene (5q)
formed zethrene (1a) in low yield through the in situ
desilylation of 7,14-bis(trimethylsilyl)zethrene (entry 16,
Table 2).[21] Cycloadducts 1 cannot be obtained in good
yields for two possible reasons: 1) The structure is signifi-
cantly out-of-plane and 2) zethrenes are unstable and they
significantly decompose in solution after a few days.[16,22]
Additionally, 5-bromo-6-(phenylethynyl)acenaphthene did
not give the corresponding cycloadduct. The crossed cyclo-
addition between 5b and 1,2-diphenylacetylene was ineffi-
cient and only 1b was obtained.
Figure 1. Molecular structure of 1l as a space-filling model.
The character of two fixed double bonds in 1 was
examined by Pd-catalyzed hydrogenation under ambient
pressure (condition I, Scheme 1), and tetrahydrozethrenes 8
are the expected products. However, compound 1a generated
a complex mixture, and hexahydrozethrene was identified to
be the major product based on GC–MS analysis. After careful
purification of the product, the structure was determined to
X-ray-quality crystals of 1a, 1b, 1l, and 1r were obtained
by slow evaporation of the CH2Cl2/MeOH solvent mixture at
48C.[16,19] To the best of our knowledge, these are the first
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In conclusion, this investigation developed a simple
method for synthesizing zethrenes. The central two six-
membered rings of zethrenes are confirmed to lack aroma-
ticity. Further studies of their physical properties and their
applications as organic materials are in progress.
Experimental Section
Preparation of 1b: A mixture of alkyne 5b (177 mg, 0.50 mmol), P(2-
furyl)3 (18.0 mg, 77.6 mmol), Ag2CO3 (138 mg, 0.50 mmol), Pd(OAc)2
(5.60 mg, 25.0 mmol), and o-xylene (5 mL) in a thick-walled pyrex
tube was purged with nitrogen for 5 min. The sealed tube was kept in
an oil bath at 1308C for 36 h. The mixture was cooled to room
temperature and filtered over celite, and the solvent of the filtrate was
removed under reduced pressure. The residue was subjected to
chromatography on silica gel; eluting with hexane/CH2Cl2 (4:1)
afforded 1b (83.0 mg, 73%) as red solids. A suitable crystal of 1b
[m.p. 331–3328C (dec.)] for the X-ray diffraction analysis was grown
from degassed CH2Cl2/MeOH at 48C. 1H NMR (300 mhz, CDCl3):
d = 6.98–7.04 (brs, 4H), 7.27–7.32 (m, 8H), 7.40 ppm (brs, 10H).
Scheme 1. Palladium-catalyzed hydrogenation of zethrenes 1.
be 4,5,6,11,12,13-hexahydrozethrene (9a, R = H), which was
verified by X-ray crystal analysis.[19] In addition, 9a was
predicated to be the final hydrogenation product of zethrene
(1a) almost 60 years ago.[4b] Although the mechanism of the
formation of 9a is not clear, Coulson et al. suggested that 9a is
formed via 8a (R = H) through hydrogenation and hydrogen
shift.[4b] Alternatively, the singlet biradical property of
zethrene (1a), as shown by the structure 1a’,[9b] also provides
the possibility to generate 9a by hydrogenation. In contrast,
when compound 1b was conducted under conditions either I
or II, it remained unchanged. This is perhaps caused by the
crowdedness in central butadiene moiety and the twisted
structure, which could decrease the biradical property.
1H NMR (300 mhz, C6D6): d = 7.02 (t, J = 7.8 Hz, 2H), 7.14 (t, J =
7.7 Hz, 2H), 7.21 (brs, 4H), 7.25 (brs, 6H), 7.26 (d, 3J = 7.7 Hz, 2H),
7.42–7.49 ppm (m, 6H). 13C NMR (75.5 mhz, C6D6, plus DEPT): d =
124.8 (CH), 125.6 (CH), 126.4 (CH), 127.1 (CH), 127.3 (CH), 127.9
(CH), 129.5 (CH), 129.9 (CH), 130.0 (Cquat), 131.7 (Cquat), 132.0 (CH),
132.7 (Cquat), 133.3 (Cquat), 135.0 (Cquat), 137.4 (Cquat), 140.7 ppm
(Cquat). EI MS (70 eV), m/z (%): 454 (100) [M+], 422 (34), 328 (29), 57
(39). HRMS (EI) calcd for C36H22: 454.1722; found: 454.1728.
3
3
The photophysical properties of zethrenes are strongly
influenced by the conformation and substituents (Table 3).
The twisted backbone would cause the absorption and
Received: March 31, 2010
Revised: June 20, 2010
Published online: August 16, 2010
Table 3: Photophysical properties of zethrenes.[a]
Keywords: alkynes · aromaticity · cycloaddition · nickel ·
palladium
.
Entry
Cpd
lmax(abs) [nm]
(e [mÀ1 cmÀ1
lmax(em)
[nm]
FPL
]
1
2
3
4
5
6
7
8
1a
1b
1h
1i
544 (42900)[b]
523 (29000)
526 (25900)
521 (43600)
521 (35500)
514 (38000)
526 (26800)
526 (27700)
531 (27800)
499 (37400)
576 (31600)
571
569
578
577
541
565
580
552
593
525[c]
610
0.34
0.38
0.34
0.25
0.75
0.28
0.32
0.60
0.31
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Angew. Chem. Int. Ed. 2010, 49, 7059 –7062
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1
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[19] X-ray crystallographic data for 1a (CCDC 762793), 1b
(CCDC 762790), 1l (CCDC 762792), 1r (CCDC 762794), 7
(CCDC 762791), and 9a (CCDC 771823) contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
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zene undergoes the partial cyclodehydrogenation to generate
Crystallization of this compound was not successful because it
slowly decomposed.
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which
contains phenanthrene substructures, is twisted acene:
a
e) H. M. Duong, M. Bendikov, D. Steiger, Q. Zhang, G.
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106, 4809.
[24] Bond alternation (or localization) in aromatic systems is also a
method to distinguish between benzene and cyclohexatriene.
For example, the bond alternation in tris(bicyclo-
[2.1.1]hexeno)benzene is estimated to be 0.089 ꢂ, see: a) N. L.
Frank, K. K. Baldridge, J. S. Siegel, J. Am. Chem. Soc. 1995, 117,
2102; b) H.-B. Bꢄrgi, K. K. Baldridge, K. Hardcastle, N. L.
Frank, P. Gantzel, J. S. Siegel, J. Ziller, Angew. Chem. 1995, 107,
1575; Angew. Chem. Int. Ed. Engl. 1995, 34, 1454. Epoxidation
and cyclopropanation of tris(bicyclo[2.1.1]hexeno)benzene give
triexpoxide and tercyclopropane, respectively, see: c) A. Mat-
suura, K. Komatsu, J. Am. Chem. Soc. 2001, 123, 1768.
[25] F. L. Arbeloa, P. R. Ojeda, I. L. Arbeloa, J. Lumin. 1989, 44, 105.
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