Palladium catalyzed synthesis of highly substituted naphthalenes via direct ring construction from amides with alkynes

Article abstract of DOI:10.1039/c0cc01448f

The direct ring construction of amides with alkynes catalyzed by palladium acetate with cheap oxidant under an air atmosphere has been realized. A variety of novel highly substituted naphthalenes 3a-3l have been prepared chemo- and regioselectively in 55-97% yields under mild conditions. Product 3j emits intense blue luminescence peaked at 435 nm with a good blue purity.

Full text of DOI:10.1039/c0cc01448f

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Palladium catalyzed synthesis of highly substituted naphthalenes via
direct ring construction from amides with alkynesw
Junliang Wu, Xiuling Cui,* Xia Mi, Ying Li and Yangjie Wu*
Received 17th May 2010, Accepted 14th July 2010
DOI: 10.1039/c0cc01448f
The direct ring construction of amides with alkynes catalyzed by
palladium acetate with cheap oxidant under an air atmosphere
has been realized. A variety of novel highly substituted naphthalenes
3a–3l have been prepared chemo- and regioselectively in
55–97% yields under mild conditions. Product 3j emits intense
blue luminescence peaked at 435 nm with a good blue purity.
Initially, the condensation of acetanilide 1a with diphenyl-
acetylene 2a was chosen as a model reaction in the presence of
palladium acetate (5 mol%) to examine the impact of various
parameters on the reaction (Table 1). The results revealed that
5,6,7,8-tetraphenyl-N-acetyl-1-aminonaphthalene 3a was
obtained as a main product in 83% yield in toluene at 80 1C
when potassium persulfate and TsOH (p-toluenesulfonic acid)
(0.5 equiv.) were used as oxidant and additive, respectively
(Table 1, entry 11). Polar solvents, such as HOAc, DMSO and
a mixture of HOAc and toluene (1 : 2 v/v), did not favour this
reaction (Table 1, entries 1–6). Cu(OAc)2 (Table 1, entry 9)
and Ag2O (Table 1, entry 10) afforded only 13% and trace
amount of the desired product, respectively. The use of
potassium persulfate as oxidant showed a similar performance
as benzoquinone (BQ) (Table 1, entry 11 vs. 7), and was
cheaper and easier to be isolated from the product. TsOH
was superior to trifluoroacetic acid (TFA) and trifluoromethane-
sulfonic acid (TfOH) as additives (Table 1, entry 11 vs. entries
12 and 13). Sulfuric acid was not suitable as additive since
acetanilide was partly decomposed although the yield was
improved slightly (see ESI).w
The synthesis of substituted polycyclic aromatic compounds
has attracted considerable attention because of their increasing
applications as p-conjugated functional materials, such as
organic semiconductors and luminescent materials.1 Of particular
interest is the synthesis of polyarylated aromatic cores owing
to their enhanced ability to transport charge and fluorescent
properties brought by the aryl groups. The synthetic
methodologies for such structures have been developed
recently, among which, transition metal-catalyzed ring
construction of an aromatic substrate with two alkyne
molecules has emerged particularly as an effective tool.2
However, the di- or mono- functionalized aromatic substrates,
such as aryl halide and aryl boronates, were required.
Polyarylated heteroaromatic compounds, for instance, indoles,
isoquinolines, benzothiazoles and pyridines, could be obtained
by catalytic activation of C–H bonds with subsequent C–C
bond formation,3 which is one of most valuable endeavours
from atom- and step-economic points of view.4 Miura et al.
has obtained naphthalene derivatives via the dual C–H bond
Table 1 Screening of various reaction conditions for ring construction of
1a with 2a via dual C–H bond cleavagea
activation of 1-phenylpyrazole catalyzed by
a rhodium
complex in the presence of excessive Cu(OAc)2 as the oxidant.5a
However, the requirement of Cu(OAc)2 as the oxidant, an
inert gas atmosphere and ortho-azole as the directing group,
which requires several steps for installation and detachment,
may limit its structural modification and applications. Thus,
cycloaromatization of alkynes with arenes through activation
of C–H bonds still remains challenge.5 Based on the ability of
N-acetyl anilines to undergo ortho-metalation,6 the ease to be
de-protected and modified, and stability of Pd(OAc)2 under an
air atmosphere, here we studied the synthesis of 5,6,7,8-tetra-
aryl-N-acetyl-1-aminonaphthalenes from N-acetyl anilines
with two alkyne molecules through dual C–H bond activation
catalyzed by Pd(OAc)2 using a cheap oxidant under mild
conditions. The products with fluorescent properties were
obtained.
Entry Solvent
Additives Oxidants
T/1C Yield (%)b
1
2
3
HOAc TsOH
DMSO TsOH
HOAc–toluene TsOH
BQ
BQ
BQ
BQ
BQ
BQ
BQ
BQ
80
80
80
80
80
80
80
80
58
3
66
15
58
36
82(78)
7
13
4
5
6
Acetonitrile
Ethanol
DMF
TsOH
TsOH
TsOH
TsOH
—
7
8
9
10
11
12
13
14
15
16
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
TsOH
TsOH
TsOH
Cu(OAc)2ÁH2O 80
Ag2O
K2S2O8
80
80
80
80
60
70
Trace
83(80)
10
71
65
CF3COOH K2S2O8
CF3SO3H K2S2O8
TsOH
TsOH
TsOH
K2S2O8
K2S2O8
K2S2O8
Henan Key Laboratory of Chemical Biology and Organic Chemistry,
Key Laboratory of Applied Chemistry of Henan Universities,
Department of Chemistry, Zhengzhou University, 75 Da Xue Road,
Zhengzhou 450052, China. E-mail: cuixl@zzu.edu.cn,
79
100 83
a
Reaction conditions: 1a (0.3 mmol), 2a (0.63 mmol), Pd(OAc)2
(5 mol%), additives (0.15 mmol), oxidant (0.6 mmol), solvent (1.5 mL),
wyj@zzu.edu.cn; Fax: (+86)-371-67767753
b
16 h. Yields determined by HPLC analysis and based on 1a. Isolated
yield in parentheses.
w Electronic supplementary information (ESI) available: Preparation
and characterization. CCDC 778161. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c0cc01448f
c
This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 6771–6773 6771

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Table 2 Palladium catalyzed direct ring construction of amides 1 with alkynesa
Entry
1
2
Product yield (%)b
Entry
1
2
Product yield (%)b
1
7
1c
2b
2
2a
8
2b
3
2a
9
1a
4
2a
10
1b
2c
5
2a
11
1c
2c
6
1a
12
2a
a
b
Reaction conditions: 1 (0.3 mmol), 2 (0.63 mmol), Pd(OAc)2 (5 mol%), TsOH(0.15 mmol), K2S2O8 (0.6 mmol), toluene (1.5 mL), 16 h. Isolated
yield in parentheses.
c
6772 Chem. Commun., 2010, 46, 6771–6773
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Under the optimized reaction conditions {5% Pd(OAc)2,
1.0 equiv. anilide, 2.1 equiv. diarylacetylene, 0.5 equiv. TsOH,
2.0 equiv. K2S2O8, at 80 1C in toluene}, the scope of the
substrates was examined (Table 2). Diphenylethylene 2a
reacted smoothly with acetanilide and its derivatives 1a–1e
to give 5,6,7,8-tetraphenyl-N-acetyl-1-aminonaphthalenes
3a–3e in good to excellent yields (entries 1–5). 5,6,7,8-Tetra-
(4-methyl)phenyl-N-acetyl-1-aminonaphthalenes 3f–3h were
obtained as main products from the reaction of di-(4-methyl)-
phenylacetylene 2b with acetanilides 1a, 1c and 1f (entries
6–8). Di-(4-trifluoromethyl)phenylacetylene was also
a
suitable substrate, providing 5,6,7,8-tetra-(4-trifluoromethyl)-
phenyl-N-acetyl-1-aminonaphthalenes 3i–3k in 81–97% yields
(entries 9–11). N-Propionylaniline can also be applied, affording
the desired product 3l in moderate yield (entries 12). It is worth
noting that the reactions of 2,3-dimethylacetanilide 1d and
2-methylacetanilide 1f with diarylacetylene provided the
corresponding desired products in 77% (entry 4) and 61%
(entry 8) yields, respectively, although it was reported that the
direct ortho-alkenylation of ortho-substituted acetanilides
catalyzed by Pd(OAc)2 via C–H bond activation failed.7 No
products were afforded when mono aryl-, alkyl- substituted
and dialkyl- substituted alkyens were used as substrates.
The results of Table 2 revealed that this procedure exhibited
electronic dependence. Electron-deficient diarylacetylenes
exhibited higher reactivity than those of electron-rich diaryl-
acetylenes. The substituents on the moiety of the acetanilides
also influenced the efficiency of the cycloaromatization
significantly. Acetanilides substituted with electron-donating
group, such as methyl, converted into the desired products
smoothly, whereas, no desired products were observed for
acetanilides with electron-withdrawing groups, e.g. trifluoro-
methyl and nitro, indicating that the lower electron density on
ortho-carbon of the N-acetyl group has the cyclometallation
obstructed. These results suggested a reaction pathway via
electrophilic attack of cationic [PdOAc]+ species on the
p-system of the arenas.8
Fig. 1 Fluorescence spectra of products 3 in the solid state.
also thank Prof. Chenxia Du from Zhengzhou University for
fluorescence spectra and valuable discussion.
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In summary, we have developed an efficient method for the
direct ring construction of amides with alkynes catalyzed by
Pd(OAc)2 under mild reaction conditions. A series of novel,
highly substituted naphthalenes have been prepared in good to
excellent yields. The products obtained can be applied as a
promising blue-emitting material. Further investigations on
the scope of the reaction and employing the products as blue-
emitting materials to fabricate OLED device are in progress.
This work was supported by NSF of China (20772114,
20972139), NSF of Henan (082300423201) and Outstanding
Doctoral Dissertation Fund of Zhengzhou University. We
c
This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 6771–6773 6773