Radical Addition/Cyclization Reaction of 2-Vinylanilines with Alkynes: Synthesis of Naphthalenes via Electron Catalysis

Article abstract of DOI:10.1002/asia.201801496

A cascade radical addition/cyclization reaction of 2-vinylanilines with alkynes for the synthesis of biologically important naphthalene derivatives is reported. In this transformation, the in-situ-formed diazonium salts from 2-vinylanilines served as efficient aryl radical precursors and the reaction was run under metal-free conditions.

Full text of DOI:10.1002/asia.201801496

CHEMISTRY
AN ASIAN JOURNAL
www.chemasianj.org
Accepted Article
Title: Radical Addition/Cyclization Reaction of 2-Vinylanilines with
Alkynes: Synthesis of Naphthalenes via Electron Catalysis
Authors: Xia Cao, Bao-Gui Cai, Guo-Yong Xu, and Jun Xuan
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10.1002/asia.201801496
Chemistry - An Asian Journal
COMMUNICATION
Radical Addition/Cyclization Reaction of 2‑Vinylanilines with
Alkynes: Synthesis of Naphthalenes via Electron Catalysis
Xia Cao,[a] Bao-Gui Cai,[a] Guo-Yong Xu,[b] Jun Xuan,*[a][b]
Abstract: Cascade radical addition/cyclization reaction of
2‑vinylanilines with alkynes for the synthesis of biologically important
naphthalene derivatives is reported. In this transformation, the in situ
formed diazonium salts from 2‑vinylanilines served as efficient aryl
radical precursors and the reaction was run under metal-free
conditions.
heterocyclic compounds. In 2016, the group of Jiao reported an
elegant method to synthesize highly valuable tryptophol
derivatives via the reaction of 2‑vinylanilines with alkynes under
metal-free reaction conditions (Scheme 1, a).[9] Shortly after,
Zeng et al. disclosed a palladium catalyzed domino oxidative
annulation of 2-vinylanilines with internal alkynes, providing the
corresponding cyclopentaquinoline derivatives in good yields
(Scheme 1, b).[10] Very recently, Yan and co-workers developed
a convenient method to the synthesis quinolines from the
reaction of 2-vinylanilines with alkynoates through palladium
catalyzed cleavage of C≡C bond (Scheme 1, c).[11] Despite
these advances, to the best of our knowledge, the react of
2‑vinylanilines with alkynes towards the construction of valuable
carbocyclic ring structures has not yet been explored.
Naphthalenes are important carbocycles which can be found
in
a wide range of biologically active natural isolates,
pharmaceuticals, and drug candidates, such as parvinaphthol B,
Duloxetine, and Cinacalcet showing in Figure 1.[1] Moreover,
they also play important roles in nanotechnology, electronic
materials, supramolecular chemistry.[2] Driven by their rich
activities, the development of efficient synthetic approaches
towards the construction of naphthalene derivatives have
attracted considerable attentions in the past decades.
Representative synthetic strategies included the transition-metal
catalyzed annulation reactions,[3] Diels−Alder reactions,[4] aryne
insertion reactions,[5] rearrangements of strained rings[6] and
some others.[7] Despite the above elegant achievements, it is still
highly desirable to develop robust and novel methodologies in
the synthesis of naphthalene derivatives so that the substitution
patterns of these important carbocycles can be further expanded,
especially under the transition-metal free conditions.
R2
R5
R4
HO
)
l
R5
%
D
R3
o
M
)
a
m
b
)
S
r
0
1
(
O
)
.
N
,
.
v
i
R5
R4
O
u
R3
R4
)
2
c
A
q
2
,
8
0
e
3
R1
H
o
C
O
(
d
(
2
e
N
H
o C
f
N
R1 = Ts;
R4 = R5 = aryl
)
P
c
9
0
.
A
9
O
,
(
0
1
F
M
e
u
R2
C
R1 = H;
R5 = CO2R'
f
D
r
R3
:
k
r
)
o
P
d
%
l
w
o
m
r
(
O
s
i
e
t
i
c
)
+
A
(
h
0
y
c
t
i
T
1
(
)
n
R4
R5
I
2
(
1
T
f
t
o
l
u
l
R4
0
A
R2
O
e
m
o C
B
H
m
T
a
R3
1
0
o
0
7
o
R5
s
i
l
%
)
,
F
m
n
T
e
o
l
%
B
,
R3
1
r
e
2
)
f
0
o
.
1
C
1
N
CO2Me
metal free reaction conditions
broad sustrate scopes
R2
R1 = H;
R4 = R5 = CO2Me
R1 = H; R4 = H;
R5 = aryl, alkyl
high-valued naphthalene derivatives
S
H
OH
O
N
Scheme 1. Reactions of 2‑vinylanilines with alkynes for the construction of
important carbo- and heterocycles.
OMe
O
H
N
HO
CF3
OH OMe
Within the frame of our ongoing research interest devoted to
the development of valuable radical cascade reactions,[12] we
describe herein a new route to construct naphthalene derivatives
through radical addition/cyclization reaction of 2‑vinylanilines
with alkynes (Scheme 1). Note that, the in situ formed diazonium
salts from 2‑vinylanilines served as efficient aryl radical
precursors.[13] Transition metals were not required to run these
cascades and the transformations proceeded by electron
catalysis.[14]
parvinaphthol B
Duloxetine
Cinacalcet
Figure 1. Examples of some biologically important molecules containing
naphthalene motifs.
2‑Vinylanilines have been identified as highly versatile
substrates for the synthesis of nitrogen-containing heterocycles
in oxidative annulation reactions.[8] Among the reactions
developed, the reaction of 2‑vinylanilines with alkynes
represented one of the most efficient methods to those
Initially, we used 4-methyl-2-(1-phenylvinyl)aniline 1a and
phenylacetylene 2a as model substrates to commence our
investigations (Table 1). The desired naphthalene product 3aa
was obtained by using isoamyl nitrite for in situ diazonium salt
generation and LiI as a radical initiator in CH3CN at 70 °C for 24
h, albeit with low yield (Table 1, entry 1). Encouraged by this
preliminary result, some other reaction parameters were then
systematically varied in order to further enhance the reaction
yield. Switching the radical initiator from LiI to other iodide salts,
such as NaI, KI, could slightly increase the yield of 3aa to 12%
and 13%, respectively (Table 1, entries 2-3). To our delight, the
yield could be further improved to 42% when TBAI was
employed as the radical initiator (Table 1, entry 4). It should be
[a]
X. Cao, B.-G. Cai, Prof. Dr. J. Xuan
Department of Chemistry and Anhui Province Key Laboratory of
Chemistry for Inorganic/Organic Hybrid Functionalized Materials,
Anhui University, Hefei, Anhui 230601, China.
E-mail: xuanjun@ahu.edu.cn
Homepage: www.chem-xuan.com
Dr. G.-Y. Xu, Prof. Dr. J. Xuan
[b]
Institute of Physical Science and Information Technology, Anhui
University, Hefei, Anhui 230601, China.
Supporting information for this article is given via a link at the end of the
document.
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10.1002/asia.201801496
Chemistry - An Asian Journal
COMMUNICATION
pointed out that transition-metal salts were not suitable initiators
for this radical cascade process (Table 1, entries 5-6). Solvent
screen revealed benzotrifluoride (BTF) to be best suited,
improving the yield to 61% (Table 1, entry 7). Other reaction
medias such as DCE, EtOAc, DMF, 1,4-dioxane and toluene
provided lower yields (Table 1, entries 8-12). No further
improvements were achieved upon increasing the amount of 2a
to 10 equivalents or the reaction temperature to 80 oC (Table 1,
entries 13-14). Notably, without the addition of any initiator, 3aa
was isolated in 12% yield. Likely, isoamyl nitrite acted as the
radical initiator in this case (Table 1, entry 15).
isolated in 47% and 18% yield, respectively. It is worth noting
that, the desired product 3na was obtained in 17% yield when R2
is a hydrogen atom.
Scheme 2. Substrate scope of 2‑vinylanilines.[a],[b]
NH2
Ph
R1
R1
TBAI (10 mol%)
+
Ph
isoamyl nitrite
BTF, 70 oC, 24 h
R2
R2
1
2a
3
Ph
Ph
Table 1. Reaction optimization between 1a and 2a [a]
Ph
Me
MeO
NH2
initiator (10 mol%)
Ph
, 61%
Ph
3ba
, 51%
+
Ph
3aa
Me
Me
isoamyl nitrite
solvent, 70 oC, 24 h
Br
Ph
Ph
Ph
Ph
Ph
1a
2a
3aa
F3CO
Cl
Entry
Initiator
Solvent
Yield[%][b]
Ph
Ph
Ph
3ea, 24%
1
LiI
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
BTF
7
3ca, 41%
3da, 50%
Ph
2
NaI
12
13
42
NR
NR
61
19
28
22
8
Me
3
KI
Ph
Ph
Me
4
TBAI
CuBr
FeCl2
TBAI
TBAI
TBAI
TBAI
TBAI
TBAI
TBAI
H
Me
Me
5
Ph
Ph
3fa, 15%
6
3ga, 31%
3ha, 46%
Me
7
Ph
Ph
Ph
8
DCE
Me
9
EtOAc
DMF
Me
10,
11
12
1,4-dioxnae
toluene
BTF
3ia, 47%
3ja, 43%
3ka, 40%
Me
F
Cl
15
37
Ph
Ph
Ph
[c]
13
[d]
TBAI
--
BTF
BTF
34
12
14
Me
3la, 47%
H
Me
3ma, 18%
Me
15
3na, 17%
[a] Reaction conditions: 1a (0.2 mmol), 2a (1.0 mmol), initiator (0.02 mmol) and
isoamyl nitrite (0.4 mmol) in solvent (2.0 mL) were stirred at 70 oC for 24 h
under argon atmosphere. [b] Isolated yield. [c] 10 equiv. 2a was used. [d] stirred
at 80 oC. TBAI = tetrabutylammonium iodide, DCE = 1,2-dichloroethane, BTF
= benzotrifluoride.
[a] Reaction conditions: Reaction conditions: 1 (0.2 mmol), 2a (1.0 mmol), TBAI
(0.02 mmol) and isoamyl nitrite (0.4 mmol) in BTF (2.0 mL) were stirred at 70
oC for 24 h under argon atmosphere. [b] Isolated yield.
On the other hand, a variety of alkynes were also subjected to
this metal-free protocol (Scheme 3). It was found that both
electron-donating (-Me, -OMe, -tBu) and electron-withdrawing (-
F, -Cl) substituents are well tolerated at the para-position of the
R-aryl group and the corresponding products 3ab-3af could be
obtained in 34-53% isolated yields. Furthermore, heteroaryl
alkynes are also tolerated as aryl radical acceptors. For example,
the reaction of 2-ethynylpyridine and 2-ethynylthiophene with 1a
provided the corresponding heteroaryl substituted naphthalenes
3ag and 3ah in 60% and 50% yield, respectively. It should be
pointed out that unactivated alkyl substituted alkynes and
electron-deficient alkynes do not engage in this cascade, which
might be due to the less efficient addition reaction.
With the optimal reaction conditions established, the substrate
scope with regard to the 2-vinylanilines was evaluated next. As
shown in Scheme 2, a wide range of electron-rich (-Me, -OMe, -
OCF3) and electron-deficient (-Cl, -Br) substituents can be
successfully introduced at C4-positions or C2-positions of aniline
phenyl ring, affording the corresponding naphthalenes 3aa-3ga
in moderate to good yields. The slightly lower yield obtained for
3ea and 3fa can be attributed to steric effects. The structure of
product 3ba was unambiguously confirmed by X-ray diffraction
[15]
analysis (Scheme 2).
We then turned our attention to
investigate the effects of the R2 group adjacent to the alkene
moiety to this cascade radical addition/cyclization sequence. No
significant electronic effects on the reaction yield was observed
by incorporation of different substituents (-Me, -F, -Cl) on the R2
aromatic ring and the corresponding naphthalene derivatives
3ha-3ka could be isolated in 40%-47% yields. More importantly,
The R2-aryl substituent can be replaced by an alkyl substituent
as documented for the methyl and isopropyl congener. The
corresponding carbocyclic compound 3la and 3ma could be
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10.1002/asia.201801496
Chemistry - An Asian Journal
COMMUNICATION
Scheme 3. Substrate scope of alkynes.[a],[b]
C, which subsequently underwent an intramolecular radical
cyclization to give cyclohexadienyl radical D. Deprotonation of D
with the assistance of the formed alcoholate delivered radical
NH2
R
TBAI (10 mol%)
+
R
Me
Me
isoamyl nitrite
BTF, 70 oC, 24 h
anion species E.[16] Finally, the oxidation of
E provided
Ph
Ph
1a
2
3
naphthalene product 3aa, formally liberating an electron to
complete the catalytic cycle.
tBu
Me
OMe
Ph
standard conditions
O
Me
Me
Me
1a + 2a
3aa
+
(trace)
N
2.0 equiv. TEMPO
Ph
Ph
Ph
Me
3ac
, 53%
3ad
, 34%
3ab, 37%
8
dectected by LCMS
F
Cl
Ph
N2OR
TBAI
N
isoamyl
1a
nitrite
Me
Me
Me
Me
Me
Ph
A
Ph
Ph
Ph
Ph
e
3aa
3ae
, 41%
3af
3ag
, 60%
, 42%
Ph
Bu
CO2Me
S
Me
Me
Me
Me
Me
E
Ph
Ph
Ph
Ph
Ph
3ah, 50%
B
3ai, failed
3aj, failed
electron
catalysis
HOR
Ph
[a]
Reaction conditions: 1a (0.2 mmol), 2 (1.0 mmol), TBAI (0.02 mmol) and
isoamyl nitrite (0.4 mmol) in BTF (2.0 mL) were stirred at 70 oC for 24 h under
argon atmosphere. [b] Isolated yield.
2a
OR
Ph
H
Ph
To further demonstrate the versatility of this methodology, we
also applied this metal-free radical addition/cyclization cascade
to the construction of other biologically important polycyclic ring
systems (Scheme 4). To our delight, we found that our approach
can be extended to the synthesis of anthracene derivative 5 and
phenanthrene derivative 7 that were isolated in 45% and 36%
yield, respectively.
Me
Me
Ph
C
Ph
D
Scheme 5. Radical trapping experiment and plausible reaction mechanism.
NH2
Ph
TBAI (10 mol%)
isoamyl nitrite
In
conclusion,
we
have
developed
a
radical
+
addition/cyclization cascade of 2‑vinylanilines with alkynes,
which provided an efficient route to various important
polysubstituted naphthalene derivatives in moderate to good
yields. To the best of our knowledge, this is the first reported
example of the reaction of 2‑vinylanilines with alkynes towards
the construction of carbocyclic ring structures. The in situ formed
diazonium salt from 2‑vinylanilines served as aryl radical
precursors. Transition metals were not required to run these
radical cascades and these processes proceeded by electron
catalysis. The further discovery of new radical cascade reactions
towards the efficient synthesis of other carbo- or heterocyclic
compounds is currently underway in our laboratory.
BTF, 70 oC, 24 h
45%
Ph
Ph
4
2a
5
NH2
Ph
TBAI (10 mol%)
isoamyl nitrite
+
BTF, 70 oC, 24 h
36%
Ph
Ph
6
2a
7
Scheme 4. Synthesis of anthracene derivative 5 and phenanthrene derivative
7
Some control experiments were conducted in order to gain
some mechanistic insight into this radical addition/cyclization
process. The cascade was completely suppressed by adding 2.0
equivalents of TEMPO to the reaction system (Scheme 5).
Moreover, the radical trapping adduct 4 was detected by LC-MS
analysis, proving the existence of a radical mechanism. Based
on above results and some previous reports, a plausible reaction
mechanism was proposed in Scheme 5. Initially, 2-vinylaniline
1a reacted with isoamyl nitrite to give the corresponding
diazonium salt A.[13] Then, a single electron transfer from TBAI
to diazonium salt A generated aryl radical B.[14] Chemoselective
radical addition of B to C≡C bond of 2a provided alkenyl radical
Experimental Section
General procedure: A flame-dried Schlenk-tube equipped with
a magnetic stir bar was charged with 1a (1.0 equiv., 0.2 mmol),
2a (5.0 equiv., 1.0 mmol), TBAI (0.1 equiv., 0.02 mmol), isoamyl
nitrite (2.0 equiv., 0.4 mmol), sealed with a septum, and
degassed by alternating vacuum evacuation and argon
backfilling (three times). Then, BTF (2.0 mL) was added under a
flow of argon. The reaction mixture was then stirred at 70 C for
24 h. Then solvent was then removed under reduced pressure
with the aid of a rotary evaporator. The crude residue was
o
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COMMUNICATION
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purified by silica gel column chromatography to afford pure
product 3aa as a yellow solid in 61% yield.
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Acknowledgements
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We are grateful to the National Natural Science Foundation of
China (No. 21702001), Natural Science Foundation of Anhui
Province (No. 1808085MB47), Open fund for Discipline
Construction, Institute of Physical Science and Information
Technology, Anhui University and the Start-up Grant from Anhui
University for financial support of this work. We thank Prof.
Armido Studer, Westfälische Wilhelms-Universität, for many
helpful discussions and language polishing
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Keywords: radical cyclization • metal free • electron catalysis •
naphthalene compounds • diazonium salts
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10.1002/asia.201801496
Chemistry - An Asian Journal
COMMUNICATION
COMMUNICATION
Xia Cao, Bao-Gui Cai, Guo-Yong Xu,
Jun Xuan*
Page No. – Page No.
Radical Addition/Cyclization Reaction
of 2‑Vinylanilines with Alkynes:
Synthesis of Naphthalenes via
Electron Catalysis
Cascade radical addition/cyclization reaction of 2-vinylanilines with alkynes for the
synthesis of biologically important naphthalene derivatives is reported. In this
transformation, the in situ formed diazonium salts from 2-vinylanilines served as
efficient aryl radical precursors and the reaction was run under metal-free conditions
This article is protected by copyright. All rights reserved.