AgOTf-catalyzed electrophilic cyclization of triynols with NXS: Rapid synthesis of densely trisubstituted naphthalenes and quinolines

Article abstract of DOI:10.1002/asia.201301186

A silver triflate-catalyzed electrophilic cyclization reaction of acyclic triynols with NXS (X=I, Br) under mild conditions is reported. Three reactive functional groups, such as a carbonyl group, an alkyne group, and a halogen, could be selectively installed at the C1, C2, and C3 positions to obtain the naphthalene and quinoline products, respectively. The obtained densely trisubstituted products could be further transformed into more complex aromatic products by manipulating the alkynyl moiety and the other two functional groups as synthons. Copyright

Full text of DOI:10.1002/asia.201301186

COMMUNICATION
DOI: 10.1002/asia.201301186
AgOTf-Catalyzed Electrophilic Cyclization of Triynols with NXS: Rapid
Synthesis of Densely Trisubstituted Naphthalenes and Quinolines**
[
a]
[a]
[a]
[b]
[b]
Zhiyuan Chen,* Xuegong Jia, Changqing Ye, Guanyinsheng Qiu, and Jie Wu*
Abstract: A silver triflate-catalyzed electrophilic cyclization
reaction of acyclic triynols with NXS (X=I, Br) under mild
conditions is reported. Three reactive functional groups,
such as a carbonyl group, an alkyne group, and a halogen,
could be selectively installed at the C1, C2, and C3 positions
to obtain the naphthalene and quinoline products, respec-
tively. The obtained densely trisubstituted products could be
further transformed into more complex aromatic products
by manipulating the alkynyl moiety and the other two func-
tional groups as synthons.
envisioned that by adding a suitable metal catalyst to an
electrophile-mediated cyclization reaction, both good che-
moselectivity and regioselectivity would be easily achieved
under mild reaction conditions, thus producing complex car-
bocyclic or heterocyclic compounds in an economical and
ecological manner.
In line with these considerations, we have recently suc-
cessfully developed a silver-catalyzed electrophilic cascade
cyclization between 1,6-diyne-4-en-3-ol 1 and N-halosuccini-
mide (NXS) to give halogen-substituted benzo[a]fluorenols
[17]
2
(Scheme 1a).
The reaction was proposed to proceed
The synthesis of polysubstituted naphthalene derivatives
has been of increasing interest during the past few de-
[1]
cades, owing to their widespread applications in structural
and synthetic chemistry, material sciences, and medicinal
[2]
chemistry. Consequently, continuous efforts have been de-
voted to the development of methods for the synthesis of
[3]
substituted naphthalene derivatives. Methods for the con-
struction of naphthalene frameworks using transition metal
[
4]
[5]
[6]
[7]
[8]
[9]
[8f,10]
catalysts, such as Pd, Rh, Ru, Ir, Au, Ag, Cu,
and others,
[11]
have been extensively studied. Electrophilic
cyclizations of unsaturated alkenes or alkynes have also
proven to be powerful methods for the rapid generation of
[12,13]
functionalized naphthalenes under mild conditions.
De-
spite the advancements made in this area, the chemoselec-
tive and regioselective control of such aromatic compounds
Scheme 1. Strategies for the synthesis of densely trisubstituted naphtha-
lenes and quinolines.
[
14]
remains challenging, and the one-pot synthesis of polysub-
stituted naphthalenes, with different substituents at specific
locations, is far from being well-established.
through an iodonium-mediated triple bond activation, an in-
tramolecular 6-endo-dig electrophilic cyclization, followed
by a Friedel–Crafts-type cyclization, and then an intramolec-
ular rearrangement. Based on these studies, it was natural
for us to assume that by using a triynol substrate as a starting
material, such as compound 3, the intramolecular Friedel–
Crafts cyclization could be suppressed in the presence of
a suitable transition-metal catalyst, thus giving rise to dense-
[
15]
Given our sustained interest in metal-catalyzed
electrophile-mediated
and
[16]
tandem cyclization reactions, we
[
a] Dr. Z. Chen, X. Jia, C. Ye
Key Laboratory of Functional Small Organic Molecules
Ministry of Education
College of Chemistry & Chemical Engineering
Jiangxi Normal University
ly trisubstituted naphthalene in
a
one-pot reaction
Ziyang Road 99, Nanchang, Jiangxi 330022 (P.R. China)
E-mail: zchenjx@hotmail.com
(Scheme 1b). The resulting polysubstituted naphthalenes
and quinolines bearing three reactive functional groups are
synthetically useful building blocks, which are difficult to
access by conventional stepwise methods. These building
blocks could be used in further transformations to afford
other types of aromatic derivatives.
[
b] G. Qiu, Prof. Dr. J. Wu
Department of Chemistry
Fudan University
2
20 Handan Road, Shanghai 200433 (P.R. China)
[
**] NXS=N-halosuccinimides.
Our studies began with acyclic triynol 3a (0.2 mmol), NIS
(1.2 equiv), and AgBF (10 mol%) in anhydrous CH Cl at
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201301186.
4
2
2
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Zhiyuan Chen et al.
[
a]
Table 1. Screening of the reaction conditions.
+
[b]
Entry Catalyst
[x mol%]
I
(equiv)
Solvent Yield [%]
ACHTUNGTRENNUNG
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
AgBF
–
AgSbF
4
(10)
NIS (1.2 equiv) CH
NIS (1.2 equiv) CH
NIS (1.2 equiv) CH
NIS (1.2 equiv) CH
NIS (1.2 equiv) CH
NIS (1.2 equiv) CH
2
2
2
2
2
2
2
2
2
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
2
2
2
2
2
2
2
2
2
40
NR
21
50
33
17
37
33
39
[
c]
6
(10)
AgOTf (10)
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
[(PPh
AuBr
[(Ph
3
)AuCl] (10)
(10)
3
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
3
P)Au
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(OTf)](10) NIS (1.2 equiv) CH
AgOTf (10)
–
–
I
I
I
2
2
2
(1.2 equiv)
(1.5 equiv)
(1.5 equiv)
CH
CH
DCE
[
d]
0
1
2
3
4
5
6
7
8
9
31
21
61
57
47
65
39
38
45
AgOTf (10)
AgOTf (10)
AgOTf (10)
AgOTf (5)
AgOTf (10)
AgOTf (10)
AgOTf (10)
AgOTf (10)
AgOTf (10)
ICl (1.2 equiv)
CH
2
Cl
2
Cl
2
Cl
2
Cl
2
2
2
2
NIS (1.5 equiv) CH
NIS (2.0 equiv) CH
NIS (1.5 equiv) CH
NIS (1.5 equiv) DCE
NIS (1.5 equiv) CHCl
NIS (1.5 equiv) CCl
NIS (1.5 equiv) DCE
NIS (1.5 equiv) DCE
3
4
[
e]
[
d]
complex
[
a] Reaction conditions: 3a (0.2 mmol), NIS, catalyst, solvent (2.0 mL)
and stirring for 12 h under N . [b] Yield of isolated product. [c] NR=no
2
reaction. [d] Reaction was carried out at 508C. [e] Reaction was carried
out at 08C. DCE=1,2-dichloroethane; NIS=N-iodosuccinimide; Tf=
Trifluoromethylsulfonate.
room temperature. To our delight, the desired (3-iodo-2-
(
phenylethynyl)naphthalen-1-yl)
A
H
U
G
R
N
U
(phenyl)methanone
(4a)
Scheme 2. Scope of the AgOTf-catalyzed electrophilic cyclization of acy-
clic triynol into 1,2,3-trisubstituted naphthalene. Reaction conditions: 3
was formed and isolated in 40% yield (Table 1, entry 1).
The structure of compound 4a was identified by X-ray dif-
fraction analysis. A control experiment revealed that the
reaction failed to proceed in the absence of a metal catalyst
(
0.2 mmol), NIS (0.3 mmol), AgOTf (0.02 mmol), and DCE (2.0 mL)
[18]
with stirring at room temperature.
(
Table 1, entry 2). Further screening of the silver salts dem-
onstrated that AgOTf was the catalyst of choice, and afford-
temperatures, but no improvement in the reaction yield was
observed (Table 1, entries 17 and 18).
ed 4a in 50% yield (Table 1, entries 3–7). Interestingly,
I
III
when Au or Au catalysts were employed, the reaction
With the optimized reaction conditions in hand, we next
surveyed the substrate scope of this catalytic electrophilic
cyclization reaction (Scheme 2). A range of electron-rich
and electron-poor substrates could participate smoothly in
the reaction. For instance, electron-withdrawing halogen
functionalities (F, Cl) and electron-donating alkyl groups at
became rather sluggish (Table 1, entries 5 and 6). [(PPh )Au-
3
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(OTf)], which was prepared in situ by treating [(PPh )AuCl]
3
with AgOTf, was also shown to be less reactive (Table 1,
entry 7). While other electrophiles such as I and ICl failed
2
to improve the reaction, increasing the loading of NIS to 1.5
equivalents gave an enhanced yield of 61% (Table 1, en-
tries 8–12). It should be noted that the yields of the desired
1
the R position were well tolerated, and delivered the de-
sired halo-substituted naphthalenes in good yields
(Scheme 2, compounds 4–8). Note that NBS also proved to
be a viable electrophile for this reaction; the corresponding
bromonaphthalenes could be obtained, albeit in a slightly
product 4a were not improved when I was used as an elec-
2
trophile in the absence of metal catalyst, these results high-
lighted the crucial role of the silver catalyst in the cycliza-
tion process (Table 1, entries 9 and 10). Attempts to reduce
the catalyst loading to 5 mol% failed to increase the yield
[19]
2
lower yield. A range of functional groups at R were tol-
erated such as fluorine, chlorine, ester, and cyclopropyl
(Scheme 2, compounds 10–13). The desired 1,2,3-trisubsti-
tuted naphthalenes were obtained in yields ranging from
37% to 78%. It is noteworthy that the reaction efficiency
improved slightly in the presence of an electron-withdrawing
(
Table 1, entry 14). Among the solvents examined, 1,2-di-
chloroethane (DCE) was found to be the best choice, and
furnished the desired product 4a in 65% yield (Table 1, en-
tries 14–16). The reaction was also conducted at different
Chem. Asian J. 2014, 9, 126 – 130
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Zhiyuan Chen et al.
2
group at R . A similar electronic effect was observed in the
AgBF -catalyzed sequential electrophilic cyclization reaction
4
of 1,6-diyne-4-en-3-ol 1 to halide-containing benzo[a]fluore-
[17]
nols 2. The diminished product yield of the electron-rich
substrates might be attributed to the enhanced reactivity of
the triple bond owing to the presence of an electron-donat-
2
ing group at R , and this subsequently resulted in lower se-
lectivity of the electrophilic cyclization step. Likewise, simi-
3
lar functional group tolerance was displayed at R , reactions
with alkylated, methylated, or fluorinated substrates gave
rise to the corresponding products 14–16 in acceptable
yields, while the reaction of chlorinated triynol proceeded
smoothly to afford product 17a and 17b in good yields.
The scope of this reaction could be further extended to
the synthesis of densely trisubstituted quinoline derivatives
(
Table 2). Pyridine-based acyclic triynols 18 reacted with
Table 2. AgOTf-catalyzed electrophilic cyclization of pyridine-based acy-
clic triynols 18 to polysubstituted quinolines 19 and 1,3-diiodinated qui-
[
a]
nolines 19’.
2
3
[b]
[b]
Entry
R , R
Yield 19 [%]
Yield 19ꢀ [%]
1
2
3
4
5
Ph, Ph
45 (19a)
49 (19b)
52 (19c)
38 (19d)
41 (19e)
38 (19aꢀ)
20 (19aꢀ)
15 (19aꢀ)
40 (19dꢀ)
35 (19eꢀ)
4-FPh, Ph
4-ClPh, Ph
Ph, 4-FPh
Ph, 4-ClPh
[
(
[
a] Reaction conditions: 18 (0.2 mmol), NIS (0.3 mmol), AgOTf
0.02 mmol), and CH Cl (2.0 mL) and stirring at room temperature.
b] Yield of isolated product.
2
2
Scheme 3. a) Proposed reaction mechanism for the synthesis of densely
trisubstituted naphthalene 4a. b) Proposed alternative mechanism.
NIS in the presence of AgOTf in CH Cl to give trisubstitut-
2
2
ed quinolines 19 in moderate yields. The basic pyridine triy-
nol 18a generated (6-iodo-7-(phenylethynyl)quinolin-8-yl)-
the reaction was initiated by OH-chelation-assisted activa-
I
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(phenyl)methanone 19a in 45% yield. Slightly higher yields
tion of the alkyne group to form an active Ag –p–alkyne
2
3
[8d]
were observed when R or R is a halogen-substituted aro-
matic ring (Table 2, entries 2–5). A highly substituted 1,3-
diiodinated quinoline by-product 19’, which was probably
formed through a sequential cascade iodocyclization process,
was consistently isolated in 15–40% yield in these cases.
Quinolines are important core structures ubiquitously found
species A, which could undergo an intramolecular 6-endo-
dig cyclization of the triple bond to produce a vinylic carbo-
[12c,22]
cation B.
This cationic intermediate was quickly cap-
tured by water to form enol species C. The keto–enol tauto-
merization of C, and subsequent electrophilic substitution of
the active iodonium species generates the iodinated dihydro-
naphthalenol D. This species quickly eliminates the hydroxy
[20]
[21]
in natural products.
Their derivatives are developed as
+
drug leads in the pharmaceutical industry or as building
blocks for fine chemicals. The rigid densely polysubstituted
N-heterobicyclic aromatics arising from this transformation
can rapidly be accessed, thus suggesting that the method
could potentially serve as a convenient entry to some bio-
logically active scaffolds.
group upon activation with Ag to produce a secondary
cation intermediate F. Final intramolecular aromatization of
F affords product 4a (Scheme 3a). According to this mecha-
nism, the silver catalyst should offer two advantages: 1) re-
gioselectivity control, and 2) activation of the alkyne group.
No reaction occurred and compound D was not detected in
the absence of the Lewis acid catalyst (Table 1, entry 2). A
trace amount of water in the reaction system would be suffi-
cient for the cyclization reaction, as one molecule of water
would be released in the latter aromatization process.
Based on the above observations and our recent stud-
[
17]
ies, we have proposed a plausible mechanism, as outlined
+
in Scheme 3a. Upon activation with Ag , NIS could act as
an active iodonium source, and therefore we believe that
Chem. Asian J. 2014, 9, 126 – 130
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Zhiyuan Chen et al.
However, considering the
strong coordination nature of
the p-electrophilic
coinage
metal (Au, Ag) Lewis acids to
[9,13c,d,23]
alkynes,
an alternative
mechanism can be considered,
as shown in Scheme 3b. In this
mechanism, the reaction was
possibly initiated by the coordi-
+
nation of the Ag to one of the
triple bonds, followed by nucle-
ophilic attack of the hydroxy
I
group in species G to the Ag –
p–alkyne group to give oxacy-
clic species H, which could then
undergo CÀO cleavage to
[9]
afford allenol species I. The
allene moiety in I could be acti-
vated by the silver catalyst to
give silver complex J, and its
subsequent aldol-type conden-
sation followed by the release
of one molecule of water would
Scheme 4. Transformations of the densely 1,2,3-trifunctionalized naphthalene 4a. See the Supporting Informa-
tion for more details on the reaction conditions.
[8b]
afford intermediate L.
Finally, the electrophilic substitu-
method to industrially valuable transformations are current-
ly underway.
+
tion of I with L would afford the polysubstituted naphtha-
lene 4a, along with the regeneration of the active silver cat-
alyst (Scheme 3b).
The present densely polysubstituted products could be
further transformed into more complex compounds by ma-
nipulation of the alkynyl moiety and the other two function-
al groups as synthons (Scheme 4). By using product 4a, as
a starting material, this idea was realized by the following
transformations: 1) synthesis of 1H-benzo[f]indole (20) by
treating 4a with aniline through a palladium-catalyzed
Buchwald–Hartwig coupling reaction, and subsequent intra-
Experimental Section
Standard procedure for the synthesis of (3-iodo-2-
(
phenylethynyl)naphthalen-1-yl)
ACHTUNGERTNNUNG( phenyl)methanone (4a)
A solution of triynol 3a (0.20 mmol, 66.4 mg) in 1,2-dichloroethane
1.0 mL) was slowly added to an ice-cold solution of N-iodosuccinimide
0.30 mmol, 51.9 mg) and AgOTf (0.02 mmol, 5.12 mg) in 1,2-dichloro-
(
(
ethane (1.0 mL) under N
temperature for 12 h. The reaction mixture was quenched by adding satu-
rated NH Cl solution (3 mL), stirred for an additional 10 min, and ex-
tracted with CH Cl (10 mLꢁ3). The combined organic layers were dried
over Na SO , filtered, and concentrated in vacuum. The crude residue
2
. The resulting suspension was stirred at room
[24]
4
molecular nucleophilic cyclization (Scheme 4a),
dophenanthrene (21), which is synthesized by treating 4a
with 4-methyl phenylacetylene and Cu(OTf) as the catalyst
2) 10-io-
2
2
2
4
A
H
U
G
R
N
U
G
was purified by flash chromatography in silicon (eluent: petroleum ether/
EtOAc=50:1) to afford 59.5 mg (65%) of 4a as an orange solid.
2
[8f,25]
(
Scheme 4b),
and, 3) phenanthrene (23), which is syn-
thesized by the palladium catalyzed Suzuki–Miyaura cou-
pling followed by a copper-catalyzed benzannulation (Sche-
me 4c,d).
Acknowledgements
In summary, we have developed an efficient silver triflate-
catalyzed electrophilic cyclization of acyclic triynols with
NXS (X=I, Br). The cyclization was selective under mild
conditions, and this led to a series of densely trisubstituted
naphthalenes in moderate to good yields with excellent
functional group tolerance. Three distinct reactive functional
groups could be directly installed at the C1, C2, and C3 po-
sitions to obtain naphthalene skeletons in a regiocontrolled
manner. The reaction could be further extended to synthe-
size trisubstituted quinolines, simply by using the pyridine-
based triynols as starting materials. This indicates that this
transformation is an efficient method for the synthesis of
highly complex heterocyclic aromatics from inexpensive acy-
clic starting materials. The mechanism of the reaction war-
rants further studies, and investigations on extending this
We thank the National Natural Science Foundation of China (21202065),
the Specialized Research Fund for the Doctoral Program of Higher Edu-
cation (20123604120003) and
a Sponsored Program for Cultivating
Youths of Outstanding Ability from Jiangxi Normal University for finan-
cial support.
Keywords: electrophilic cyclization
·
heterocycles
·
homogeneous catalysis · silver triflate · triynol
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Received: September 1, 2013
Published online: October 24, 2013
Chem. Asian J. 2014, 9, 126 – 130
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