Palladium-catalyzed synthesis of 7,9-diaryl-8 H-acenaphtho[1,2-c]pyrroles and their application in explosives detection

Article abstract of DOI:10.1002/chem.201101483

Making sensors of pyrroles: 7,9-Diarenyl-8H-acenaphtho[1,2-c]pyrroles were efficiently constructed by a palladium-catalyzed bicyclization of 1,8-diarenynyl naphthalenes and primary amines under air conditions (see scheme). The synthesized 8H-acenaphtho[1,

Full text of DOI:10.1002/chem.201101483

DOI: 10.1002/chem.201101483
Palladium-Catalyzed Synthesis of 7,9-Diaryl-8H-acenaphthoACTHNUTRGNE[UNG 1,2-c]pyrroles
and Their Application in Explosives Detection
Xiaopeng Chen, Jisong Jin, Yanguang Wang,* and Ping Lu*[a]
Pyrrole is one of the most important heterocyclic com-
alyzed by palladium(II) chloride, only dimethylsulfoxide
(DMSO) gave the desired product (Table 1, entries 1–5).
Other polar aprotic solvents, such as, dimethylforamide, ace-
tonitrile, nitromethane, and nitrobenzene did not facilitate
the reaction and no desired product could be detected. By
altering DMSO to sulfolane (Table 1, entry 6), 3a was also
obtained, but in relatively lower yield. When the base addi-
tive was added, the yields of the isolated product were in-
creased (Table 1, entries 1 and 7–10). Triethylamine provid-
ed the best yield among other bases, such as sodium carbon-
ate, potassium carbonate, diisopropylethylamine, and trie-
thylamine. Similarly, the use of sulfolane as a solvent also af-
forded 3a upon addition of triethylamine (Table 1,
entry 11). Decreasing the reaction temperature to 608C, the
yield of the isolated product was determined to be 23%, al-
though 1a completely disappeared based on tracking by
TLC (Table 1, entry 12). Altering the palladium to other
pounds in nature.[1] Its derivatives are broadly found and de-
veloped as drugs in medicinal industry.[2] Meanwhile, they
are closely related to optoelectronic material fields, such as
pyrrole-based oligomer or organic field-effect transistors
(OFET).[3] To satisfy the booming request of pharmaceutical
and material applications of various pyrroles, synthetic ap-
proaches of either pyrrole construction or pyrrole modifica-
tion have attracted considerable attention for achieving an
ideal state that fulfils reactions with multiple advantages:
readily available starting materials, simple processing, and
high economical and ecological values.[4]
On the other hand, explosives detection is of importance
in military operation, public safety, and environmental
cleaning. Several attempts have thereafter been made and
tested to be practical.[5] For example, Swager and co-workers
reported the detection of nitroaromatic explosives in the
vapor phase using fluorescent quenching of conjugated poly-
mers.[6] It offered a simple, exquisitely sensitive, and rapid
detection of explosives in the vapor phase.[7] The mechanism
of fluorescence quenching for explosives detection was at-
tributed to the photoinduced electron-transfer from the ex-
cited polymer donor to the explosive acceptors.
PdII sources, such as PdBr2 and Pd
ACTHNUTRGNE(UNG OAc)2, comparative
Table 1. Optimization of the reaction conditions for formation of 3a.[a]
With rigid parallel triple bonds in a molecule, the reactivi-
ty of diynes was greatly enhanced due to the existence of
the orbital repulsion between two triple bonds and the re-
sulting bend of the triple bonds.[8] By this consideration, a
series of products might be constructed by sequential reac-
tions from 1,8-diarenynyl naphthalenes in which the triple
bonds were fixed and influenced by each other. In this com-
munication, we tested the reaction of 1,8-diphenylacetylenyl
naphthalene (1a)[9] with aniline (2a) in the presence of
Entry
Sol.
Cat.
T
Base[b]
Yield
[%][c]
[oC]
1
2
3
4
5
6
7
8
DMSO
CH3CN
DMF
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PdBr2
100
80
100
80
None
None
None
None
None
None
Na2CO3
K2CO3
iPr2EtN
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
42
N.D[d]
N.D
N.D
N.D
38
PhNO2
CH3NO2
sulfolane
DMSO
DMSO
DMSO
DMSO
sulfolane
DMSO
DMSO
DMSO
DMSO
DMSO
PdCl2. To our delight, 8H-acenaphthoACTHNUTRGNEUNG
[1,2-c]pyrrole (3a)[10]
80
was easily constructed and its structure was established by
100
100
100
100
100
100
60
100
100
100
100
using X-ray crystal analysis.[11]
54
46
66
82
68
23
80
76
To efficiently perform this reaction, we optimized the re-
action conditions for this transformation. The reactivity
largely depended on the solvent; when the reaction was cat-
9
10
11
12
13
14
15
16[e]
[a] X. Chen, J. Jin, Prof. Y. Wang, Prof. P. Lu
Department of Chemistry, Zhejiang University
Hangzhou 310027 (P.R. China)
Fax : (+86)571-87952543
Pd
Pd
U
N.D
20
PdCl2
E-mail: orgwyg@zju.edu.cn
[a] Reaction was carried out under air with 1a (0.5 mmol), 2a
(0.75 mmol), Pd catalyst (0.05 mmol) in solvent (10 mL) for 8 h.
[b] Amount of base is 5 equiv to 1a. [c] Yield of isolated product.
[d] N.D means not detected. [e] Reaction was conducted under N2.
pinglu@zju.edu.cn
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201101483.
9920
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Chem. Eur. J. 2011, 17, 9920 – 9923

COMMUNICATION
yields were achieved, respectively (Table 1, entries 13 and
14). However, 1a was almost quantitatively recovered in the
presence of [PdACHTUNGTRENNUNG(PPh3)2Cl2] (Table 1, entry 15), which implied
smoothly. n-Butylamine afforded the corresponding product
3p in a moderate yield and others with relatively poor
yields (Table 2, entries 15–18). By using aniline as the
amine, we investigated the tolerance of 1,8-diarenynyl naph-
thalenes under the optimized reaction conditions (Table 2,
entries 19–24). The substituent effect of the para position of
the aryl part of 3 would not affect the yield significantly re-
gardless of its electron-withdrawing or electron-donating
nature. Flexible 1,6-diynes, such as hepta-1,6-diyne and 1,7-
diphenylhepta-1,6-diyne, did not produce the desired bicy-
cled products.
that this palladium-catalyzed reaction was a phosphine-free
reaction.[12] When the reaction was conducted under nitro-
gen, 3a was obtained in yield of 20%, whereas 7-phenylben-
zo[k]fluoranthene was isolated as the major product
(55%).[9a,13] Finally, we selected PdCl2 as the catalyst, trie-
thylamine as the base additive, and performed the reaction
under air in DMSO at 1008C for 8 h.
With the optimized reaction conditions in hand, we next
tested the substrate diversity of this transformation. A
number of aryl amines (such as 2a–2n) were compatible
with the reaction conditions and afforded the corresponding
products in moderate to high yields (46–92%; Table 2, en-
tries 1–14). The substituent effect of aryl amines, including
the inductive effect and conjugate effect, was negligible
when the substituent occupied either the para position or
meta position of aniline (Table 2, entries 1–12). However,
the steric hindrance increased when the substituent was at-
tached on the ortho position of aniline (Table 2, entries 13
and 14). Not only the aromatic amines worked for this trans-
formation, the aliphatic amines also processed the reaction
Based on the survey of the reaction condition and the
substrate diversity, a possible mechanism was postulated and
shown in Scheme 1. Upon coordination of 1a with palladi-
Table 2. Various substrates for constructing 8H-acenaphtho
G
s.[a]
Entry
1
Ar
Ph
2
R
Product
Yield
[%][b]
Scheme 1. Proposed mechanism for the formation of 3a.
1
2
3
4
5
6
7
8
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1b
1c
1d
1e
1 f
1g
2a
2b
2c
2d
2e
2 f
2g
2h
2i
Ph
3a
3b
3c
3d
3e
3 f
3g
3h
3i
3j
3k
3l
3m
3n
3o
3p
3q
3r
82
86
80
82
78
86
92
82
83
85
88
75
46
68
32
56
28
25
82
88
66
62
83
68
p-FC6H4
p-ClC6H4
p-BrC6H4
p-NO2C6H4
pMeOC6H4
pMeC6H4
p-HOC6H4
m-ClC6H4
m-BrC6H4
mMeC6H4
m-NO2C6H4
o-ClC6H4
oMeC6H4
n-C8H17
um, an alkyne–palladium complex (A) was formed and the
triple bond became electron deficient through the electron
flow from triple bond to palladium.[14] The nucleophilic
attack of aniline (2a) on this triple bond would lead to the
formation of B by aminopalladation.[15] The syn addition
was predominantly controlled by the weak interaction of ni-
trogen and palladium. Insertion of the adjacent triple bond
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
2j
2k
2l
À
to the C Pd bond of B generated C. Similarly, the weak in-
teraction between nitrogen and palladium resulted in the in-
2m
2n
2o
2p
2q
2r
2a
2a
2a
2a
2a
2a
sertion with high regioselectivity. Finally, 3a was constructed
À
by C N coupling and the oxidation state of palladium was
n-C4H9
PhCH2
tBu
returned to its origin by DMSO[16] or sulfolane under air.
To demonstrate the utility of 8H-acenaphthoACHTNUTRGNE[NUG 1,2-c]pyr-
roles, we investigated the absorption and emission proper-
ties as well as the energy gap by theoretical calculations
(Gaussian 03) of 3a, 3b, 3e, 3 f, 3p, 3s, and 3x, which were
selected as representatives because the photophysical prop-
erties were largely dependent on the key fluorophore with a
slight influence of the substituted group (see the Supporting
Information, Table S1). It was noticed that most of these
compounds absorbed light around 405 nm and emitted light
p-FC6H4
3s
3t
3u
3v
3w
3x
p-BrC6H4
p-CNC6H4
pAcC6H4
pMeC6H4
pMeOC6H4
[a] The reaction was carried out in
(0.75 mmol), and PdCl2 (0.05 mmol) in DMSO (10 mL) for 8 h. [b] Yield
of isolated product.
a
flask with
1
(0.5 mmol),
2
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www.chemeurj.org
9921

Y. Wang, P. Lu et al.
around 450 nm with quantum yields about 0.30. The energy
gaps of these compounds were calculated to be about
3.50 eV. However, 3e was exceptional in that no emission
was observed, although it presented similar absorption to
others, whereas the energy gap of 3e was only 2.54 eV. Thus,
we performed fluorescent quenching experiments of 3a with
nitroarenes in solutions because of the importance of the
trace explosive detection.[5–7] Upon gradual addition of 100
equivalents of arene to a solution of 3a (1ꢁ10À5 m in
CH2Cl2), no significant changes of fluorescence intensity
were observed when the arenes were benzene, toluene, ani-
line, bromobenzene, or chlorobenzene, respectively (see the
Supporting Information, Figure S63). However, the fluores-
cence was efficiently quenched when either nitrobenzene
(NB) (see the Supporting Information, Figure S64) or 2,4,6-
trinitrotoluene (TNT) (Figure 1) was gradually added.
Based on the Stern–Volumer plot, the quenching constant
(Ksv) was determined to be 1500mÀ1 for NB (see the Sup-
porting Information, Figure S66) and 4100mÀ1 for TNT (see
the Supporting Information, Figure S67), respectively. These
values were comparable to previous reports.[7a,17] Fluorescent
quenching experiments indicated that 3a presented both
quenching selectivity and sensitivity towards nitroaromatic
analytes. Furthermore, it was obvious that no shift in emis-
sion spectra was seen, which indicated that exciplex was not
formed.[7c] The formation of a charge-transfer complex be-
tween the electron-deficient nitroaromatics and the elec-
Figure 2. Fluorescent quenching of 3a upon exposure to saturated vapor
of TNT at different times (lex =410 nm).
sessed the high stability against photobleaching[18] and ex-
hibited its potential application for real-time vapor detec-
tion of TNT.
In conclusion, 8H-acenaphthoACTHNUTRGNEUGN[1,2-c]pyrroles were easily
constructed by using a palladium-catalyzed bicyclization of
1,8-diarenynyl naphthalenes and primary amines under air
in dimethylsulfoxide. This palladium-catalyzed reaction is
phosphine-free and is carried out with high atom efficiency.
More significantly, the synthesized 8H-acenaphthoACTHNUTRGENUG[N 1,2-c]pyr-
roles might be used as an alternative sensory materials for
the trace detection of nitroaromatics by fluorescent quench-
ing, based on the formation of the charge-transfer complex
tron-rich 8H-acenaphtho
G
between 8H-acenaphthoACHTNUGETRN[UNG 1,2-c]pyrrole and nitroaromatics.
for this fluorescent quenching process.
Further study on this application is in progress.
Experimental Section
General procedure: In a 25 mL round-bottom flask (25 mL), 1a (164 mg,
0.5 mmol), 2a (70 mg, 0.75 mmol), Et3N (252 mg, 2.5 mmol) was dis-
solved in dimethyl sulfoxide (DMSO, 10 mL). PdCl2 (10 mol%, 9 mg,
0.05 mmol) was added to the reaction solution after the reaction temper-
ature rose to 1008C. After complete consumption of starting material 1a
(as tracked by TLC after 8 h), the solution was dissolved with CH2Cl2,
washed with water (3 x 25 mL) and dried over anhydrous Na2SO4. After
filtration, the filtrate was distilled under reduced pressure and the residue
was purified by using flash chromatography on silica gel (using hexane as
the eluent solvent). 3a (172 mg, 82%) was obtained as yellow solid. For
additional details, see the Supporting Information.
Figure 1. Fluorescent quenching of 3a (10À5 m) with TNT in CH2Cl2 solu-
tion (lex =410 nm).
Acknowledgements
Fluorescent quenching could also be observed when 3a
was exposed to the saturated vapour of NB (see the Sup-
porting Information, Figure S68) or TNT (Figure 2). No
emission changes were detected for 3a exposed to vapors of
other arenes, such as benzene, toluene, aniline, bromoben-
zene, or chlorobenzene (see the Supporting Information,
Figure S70). The emission intensity of 3a was quenched to
about 50% after 3a was exposed to TNT vapor for 2 min.
Moreover, the fluorescent intensity could be recovered after
3a was removed from the vapor of nitroaromatics for five
minutes. This reversible phenomenon implied that 3a pos-
This work has been supported by the National Nature Science Founda-
tion of China (Nos. 21032005, 20972137 and J0830413).
Keywords: cyclization
· diynes · explosives detection ·
fluorescence · palladium · sensors
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Synthesis of 7,9-Diaryl-8H-acenaphthoACTHNUGRTENUNG[1,2-c]pyrroles
COMMUNICATION
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Received: May 14, 2011
Published online: July 27, 2011
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