Catalyst-Free Oxidative [3+2] Cycloaddition of Phenols and Styrenes in the Presence of a 2,3-Dichloro-5,6-dicyano-1,4-benzoquinone/1,1,1,3,3,3-hexafluoropropan-2-ol System

Article abstract of DOI:10.1055/s-0036-1589082

A catalyst-free oxidative [3+2] cycloaddition of phenols and styrenes was developed with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone as the oxidant and 1,1,1,3,3,3-hexafluoropropan-2-ol as the solvent at room temperature. With this method, a broad range of

Full text of DOI:10.1055/s-0036-1589082

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© Georg Thieme Verlag Stuttgart · New York
2017, 28, A–E
letter
A
en
Y. Wang et al.
Letter
Synlett
Catalyst-Free Oxidative [3+2] Cycloaddition of Phenols and Sty-
renes in the Presence of a 2,3-Dichloro-5,6-dicyano-1,4-benzoqui-
none/1,1,1,3,3,3-hexafluoropropan-2-ol System
R4
Yunxia Wang*
Na Cui
R2
R2
R3
R4
DDQ/HFIP
r.t., 15 min
R1
R1
+
Yu Zhao
O
R3
OH
R
R
1 = OMe, OEt, NHCOMe, CN;
College of Chemistry & Material Science, Key Laboratory of Synthetic
and Natural Functional Molecule Chemistry of Ministry of Education
of China, Northwest University, Xi’an 710127, P. R. of China
wyx27210@nwu.edu.cn
2 = Me, t-Bu, F, Cl, Br;
16 examples
up to 90% yield
R3 = Me, Ph, alkyl, H; R4 = alkyl, H
HFIP = 1,1,1,3,3,3-hexafluoropropan-2-ol
• catalyst-free • operationally simple • room temperature
Received: 06.05.2017
O
Accepted after revision: 03.07.2017
Published online: 03.08.2017
OH
HO
OH
DOI: 10.1055/s-0036-1589082; Art ID: st-2017-u0336-l
O
O
HO
HO
OH
Abstract A catalyst-free oxidative [3+2] cycloaddition of phenols and
styrenes was developed with 2,3-dichloro-5,6-dicyano-1,4-benzoqui-
none as the oxidant and 1,1,1,3,3,3-hexafluoropropan-2-ol as the sol-
vent at room temperature. With this method, a broad range of dihydro-
benzofurans were efficiently and quickly obtained from readily available
phenols and styrenes.
COOH
OH
OH
OH
O
O
HO
OH
(+)-lithospermic acid
ε-viniferin
Key words phenols, styrenes, cyclization, oxidation, dihydrobenzofu-
rans
HO
Me
O
O
HO
O
OH
OMe
O
The dihydrobenzofuran moiety is a key structural unit
of various bioactive natural products with promising anti-
oxidative,1 anti-HIV,2 anti-COX-2,5-LOX,3 or antihypoten-
sive4 activities, including ε-viniferin, (+)-lithospermic acid,
liliflol A, and cedrusin (Figure 1). Consequently, the devel-
opment of synthetic approaches to dihydrobenzofurans has
drawn considerable attention from organic chemists.
HO
OH
cedrusin
Liliflol A
Figure 1 Selected bioactive natural products containing a dihydroben-
zofuran unit
The oxidative [3+2] cycloaddition of phenols with sty-
renes has proven to be a concise and practical strategy for
the construction of dihydrobenzofuran units,5 because phe-
nols and styrenes are commercially available chemicals. Be-
cause of the significant electron donation by the hydroxy
group, phenols are readily oxidized in the presence of a va-
riety of oxidative systems. Reported oxidative [3+2] cyc-
loaddition strategies involve the use of such oxidants as
high-valence iodine reagents,6 DDQ,7 single-electron metal
oxidants,4,8 electrochemical oxidation,9 or Na2S2O8.10 Mean-
while, catalytic oxidations have been achieved by using
such conditions as FeCl3·(H2O)6/DTBP,1 FeCl3/DDQ,11–13 HOTf
/benzoquinone,14 PhI(OAc)2/MeOH, BF3·OEt2/CH2Cl2,15 visi-
ble light/Ru(bpz)3(PF6)2/(NH4)2S2O8,3 or visible light/meso-
porous graphitic carbon nitride/air.16 Despite these valuable
contributions, the exploration of simpler, milder, and more-
efficient oxidative [3+2] processes for constructing dihyd-
robenzofurans is still an attractive topic.
As a widely used and highly active oxidant, DDQ has
been previously utilized in oxidative [3+2] cycloadditions of
phenols and styrenes (Scheme 1). In 2008, Janin and co-
workers reported that the DDQ-mediated oxidation of 2,5-
dihydroxybenzaldehyde in ethyl acetate and subsequent
[3+2] cycloaddition with three enol ethers gave the corre-
sponding dihydrobenzofurans in 18–56% yields [Scheme
1(a)].7 However, this method shows low yields and a limited
scope, so a more-effective and general approach for DDQ-
mediated oxidative [3+2] cycloadditions of phenols and sty-
renes was required. In 2013, Lei and co-workers demon-
strated that a catalytic amount of FeCl3significantly en-
hanced the DDQ-mediated oxidative radical coupling of
phenols and styrenes in toluene to give various dihydro-
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–E

B
Y. Wang et al.
Letter
Synlett
Previous work:
R3
HO
O
HO
OR1
R2
R1O
R2
R3
DDQ
+
(a)
(b)
(c)
O
AcOEt, over night
OH
O
3 examples, 18–56%
R2
R3
R5
FeCl3, DDQ
toluene, 4 h
R5
R1
R2
R1
+
R4
R4
O
OH
R3
21 examples, 38–93%
R1
R1
Ts
Ts
N
R2
N
FeCl3, DDQ
CH2Cl2:toluene = 7:1, 4 h
+
O
OH
R2
9 examples, 40–96%
O
R2
O
Ts
Ts
N
N
FeCl3, DDQ
R2
(d)
R1
R1
+
HFIP, 2 h
O
OH
11 examples, 29–67%
This work:
R4
R3
R2
R2
R4
DDQ/HFIP
r.t., 15 min
R1
(e)
R1
+
R3
O
OH
16 examples, 35–90%
Scheme 1 DDQ-mediated oxidative [3+2] cycloaddition between phenols and styrenes for the construction of dihydrobenzofurans
benzofurans in good yields [Scheme 1(b)].11 In 2016, Xia
and co-workers used a FeCl3/DDQ system in an oxidative
cross-coupling between 5-hydroxyindoles and styrene de-
rivatives to construct the corresponding indolofurans
[Scheme 1(c)].12 However, Xia’s study on 5-hydroxyindoles
with cinnamic acid derivatives failed to provide the oxida-
tive cross-coupling product under the same conditions. A
detailed examination found that 1,1,1,3,3,3-hexafluoropro-
pan-2-ol (HFIP) promotes the FeCl3/DDQ-mediated cross-
coupling between 5-hydroxyindoles and cinnamic acid de-
rivatives [Scheme 1(d)].13 Here, we report an alternative ox-
idative [3+2] cycloaddition of phenols and styrenes in the
presence of the DDQ/HFIP system at room temperature as
an efficient approach to dihydrobenzofurans [Scheme 1(e)].
Compared with previous work,7,11–13 our reaction displays a
broad substrate scope and proceeds smoothly without the
need for a catalyst.
Our investigation began with a screen of oxidants in
acetonitrile, with 4-methoxyphenol (1a) and styrene (2a)
as model substrates (Table 1 entries 1–6). Disappointingly,
no reaction was observed when air, H2O2, tert-butyl hy-
droperoxide (TBHP), K2S2O8, or m-chloroperoxybenzoic acid
(mCPBA) was employed as the oxidant (Table 1, entry 1–5).
However, the desired oxidative [3+2] cycloaddition product
dihydrobenzofuran 3a was obtained in 12% yield in the
presence of DDQ at room temperature (Table 1, entry 6).
Next, various solvents were examined to improve the DDQ-
mediated oxidative [3+2] cycloaddition of 1a and 2a. As
shown in Table 1, three common solvents failed to provide
the desired products (entries 7–9). Inspired by the signifi-
cant enhancement effect of fluoro alcohols in the cross-
coupling of phenols and 5-hydroxyindoles,13,17 we then per-
formed this reaction in HFIP and 2,2,2-trifluoroethanol
(TFE) (entries 10 and 11). To our delight, the reaction was
complete in one minute in HFIP at room temperature, af-
fording dihydrobenzofuran 3a in 88% yield. Obviously, HFIP
is effective in this catalyst-free oxidative [3+2] cycloaddi-
tion of 1a and 2a mediated by DDQ. According to previous
reports,17 HFIP can stabilize oxidation intermediates and re-
duce the oxidation potential of phenols, thereby facilitating
this reaction under mild conditions.
The substrate scope of this reaction under the opti-
mized conditions was explored. As illustrated in Scheme 2,
phenols containing strongly electron-donating group (OMe
or OEt) in the para-position reacted well with various sty-
renes to give the desired dihydrobenzofurans 3b and 3e–3q
in moderate to good yields. This reaction also proceeded
smoothly with the less-electron-rich N-(4-hydroxyphe-
nyl)acetamide and 6-hydroxy-2-naphthonitrile to give the
corresponding products 3c and 3d in yields of 35 and 37%,
respectively. However, other phenols, such as 4-tert-butyl-
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–E

C
Y. Wang et al.
Letter
Synlett
phenol, 4-bromophenol, biphenyl-4-ol, and 3-methoxy-
phenol failed to deliver the expected products. A variety of
styrenes were also tested in this work. Electron-donating
substituents on styrenes, such as 4-methyl, 4-tert-butyl, or
2,4,6-trimethyl, were tolerated and the desired dihydroben-
zofurans 3e–g were obtained in 48–89% yield. Moreover,
styrenes bearing weakly electron-withdrawing halogen
groups (4-fluoro; 2-, 3-, or 4-chloro; or 4-bromo) were also
effective substrates for the [3+2] process, providing prod-
ucts 3h-–l in 40–58% yield. Notably, hindered styrenes
were confirmed to function in the process, generating the
polysubstituted dihydrobenzofurans 3m–q in 37–90% yield.
Based on previous explorations,11–13 a plausible mecha-
nism for this [3+2] transformation was proposed (Scheme
3). Initially, phenol 1 is oxidized by DDQ to produce a phe-
noxyl radical 4 and an HDDQ radical. Next, the oxygen-
based radical 4 is delocalized to give a carbon-based radical
5, which then undergoes [3+2] radical addition with sty-
rene 2 to generate the cyclic radical intermediate 6. Finally,
the corresponding dihydrobenzofuran 3 is delivered by ab-
straction of a hydrogen radical from 6 by the HDDQ radical.
In summary, a DDQ/HFIP-mediated oxidative [3+2] cy-
cloaddition between phenols and styrenes has been suc-
cessfully achieved, establishing an efficient approach to di-
hydrobenzofuran units at room temperature.18 This novel
Table 1 Optimization of the Oxidative [3+2] Cycloaddition of 4-Me-
thoxyphenol (1a) and Styrene (2a)
MeO
MeO
conditions
+
O
OH
1a
2a
3a
Entrya
Oxidant
Solvent
Yieldb (%)
1
2
Air
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
CH2Cl2
toluene
EtOAc
HFIP
0
0
H2O2
TBHP
K2S2O8
mCPBA
DDQ
DDQ
DDQ
DDQ
DDQ
DDQ
3
0
4
0
5
0
6
12
0
7
8
0
9
0
10
11
88
27
TFE
a Reaction conditions: 1a (0.15 mmol), 2a (0.3 mmol), oxidant (0.18
mmol), solvent (1.5 mL), r.t.
b Yield of isolated product based on 1a.
R4
R2
R2
R3
R4
DDQ/HFIP
R1
R1
+
r.t., 15 min
R3
O
OH
1
3
2
Ph
H
O
MeO
EtO
N
O
O
O
O
NC
3e
48%
Cl
3b
3c
3d
3h
63%
35%
37%
MeO
MeO
MeO
MeO
MeO
F
O
O
O
O
O
89%
40%
69%
45%
3f
3j
3g
50%
47%
58%
3i
MeO
MeO
MeO
MeO
Cl
Br
O
O
O
Cl
3k
3l
3m
37%
H
H
EtO
EtO
EtO
Ph
Ph
Ph
O
O
O
O
H
3p
68%
50%
3n
3q
40%
3o
90%
Scheme 2 Scope of DDQ/HFIP-mediated oxidative [3+2] cycloaddition of phenols and styrenes. Reaction conditions: 1 (0.15 mmol), 2 (0.3 mmol), DDQ
(0.18 mmol), HFIP (1.5 mL), r.t., yields of isolated products based on 1.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–E

D
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Letter
Synlett
R.-B.; Yang, S.-D. Tetrahedron 2012, 68, 5216. (f) Althagafy, H. S.;
Meza-Aviña, M. E.; Oberlies, N. H.; Croatt, M. P. J. Org. Chem.
2013, 78, 7594.
R1
R1
R1
HFIPH2
DDQ
OH
O
O
HDDQ
(9) (a) El-Seedi, H. R.; Yamamura, S.; Nishiyama, S. Tetrahedron
2002, 58, 7485. (b) Kirste, A.; Schnakenburg, G.; Stecker, F.;
Fischer, A.; Waldvogel, S. R. Angew. Chem. Int. Ed. 2010, 49, 971.
(c) El-Seedi, H. R.; Yamamura, S.; Nishiyama, S. Tetrahedron Lett.
2002, 43, 3301. (d) Okada, Y.; Yoshioka, T.; Koike, M.; Chiba, K.
Tetrahedron Lett. 2011, 52, 4690. (e) Chiba, K.; Fukuda, M.; Kim,
S.; Kitano, Y.; Tada, M. J. Org. Chem. 1999, 64, 7654. (f) Kim, S.;
Noda, S.; Hayashi, K.; Chiba, K. Org. Lett. 2008, 10, 1827.
(10) Zhao, Y.; Huang, B.; Yang, C.; Li, B.; Xia, W. Synthesis 2015, 47,
2731.
1
5
4
R2
R4
[3+2] radical cycloaddition
R3
2
R4
R4
R2
R2
R1
R1
HDDQH HDDQ
R3
R3
O
O
3
6
(11) Huang, Z.; Jin, L.; Feng, Y.; Peng, P.; Yi, H.; Lei, A. Angew. Chem.
Int. Ed. 2013, 52, 7151.
Scheme 3 Plausible reaction mechanism
(12) Liang, K.; Yang, J.; Tong, X.; Shang, W.; Pan, Z.; Xia, C. Org. Lett.
2016, 18, 1474.
(13) Liang, K.; Wu, T.; Xia, C. Org. Biomol. Chem. 2016, 14, 4690.
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2013, 52, 10195.
(15) Shama, S.; Parumala, S. K. R.; Peddinti, R. K. Synlett 2017, 28,
239.
approach features catalyst-free conditions, a broad sub-
strate scope, and short reaction times, which suggests it has
good potential for practical application.
(16) Song, T.; Zhou, B.; Peng, G.-W.; Zhang, Q.-B.; Wu, L.-Z.; Liu, Q.;
Wang, Y. Chem. Eur. J. 2014, 20, 678.
Funding Information
(17) (a) Gaster, E.; Vainer, Y.; Regev, A.; Narute, S.; Sudheendran, K.;
Werbeloff, A.; Shalit, H.; Pappo, D. Angew. Chem. Int. Ed. 2015,
ó54, 4198. (b) Elsler, B.; Wiebe, A.; Schollmeyer, D.; Dyballa, K.
M.; Franke, R.; Waldvogel, S. R. Chem. Eur. J. 2015, 21, 12321.
(c) Dohi, T.; Yamaoka, N.; Kita, Y. Tetrahedron 2010, 66, 5775.
(d) Libman, A.; Shalit, H.; Vainer, Y.; Narute, S.; Kozuch, S.;
Pappo, D. J. Am. Chem. Soc. 2015, 137, 11453.
(18) Dihydrobenzofurans 3a–q; Typical Procedure
A 5 mL cylindrical glass bottle equipped with magnetic stirrer
bar was charged with the appropriate phenol 1 (0.15 mmol),
styrene 2 (0.3 mmol), and HFIP (1.5 mL). When the phenol and
the styrene were completely dissolved in the HFIP, DDQ (1.2
equiv) was gradually added. The mixture was stirred at r.t. for
15 min, and then concentrated under vacuum. The crude
product was purified by flash chromatography [silica gel, PE–
EtOAc (50:1)].
We are grateful for financial support from the Open Foundation of
Key Laboratory of Synthetic and Natural Functional Molecule Chemis-
try of the Ministry of Education of China, Science and Technology Pro-
gram of Xi'an City and Shaanxi Province (2017KW-066).2(
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Supporting Information
Supporting information for this article is available online at
https://doi.org/10.1055/s-0036-1589082.
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References and Notes
(1) Kshirsagar, U. A.; Regev, C.; Parnes, R.; Pappo, D. Org. Lett. 2013,
15, 3174.
(2) Varadaraju, T. G.; Hwu, J. R. Org. Biomol. Chem. 2012, 10, 5456.
(3) Blum, T. R.; Zhu, Y.; Nordeen, S. A.; Yoon, T. P. Angew. Chem. Int.
Ed. 2014, 53, 11056.
(4) Magoulas, G. E.; Papaioannou, D. Molecules 2014, 19, 19769.
(5) Cui, N.; Zhao, Y.; Wang, Y. Chin. J. Org. Chem. 2017, 37, 20.
(6) (a) Wang, S.; Gates, B. D.; Swenton, J. S. J. Org. Chem. 1991, 56,
1979. (b) Dohi, T.; Nakae, T.; Toyoda, Y.; Koseki, D.; Kubo, H.;
Kamitanaka, T.; Kita, Y. Heterocycles 2015, 90, 631. (c) Mohr, A.
L.; Lombardo, V. M.; Arisco, T. M.; Morrow, G. W. Synth.
Commun. 2009, 39, 3845.
5-Methoxy-2-phenyl-2,3-dihydro-1-benzofuran (3a)
Yellow oil; yield: 30 mg (88%). 1H NMR (400 MHz, CDCl3):
δ = 7.46–7.30 (m, 5 H), 6.83–6.76 (m, 2 H), 6.72 (dd, J = 8.7, 2.6
Hz, 1 H), 5.75 (t, J = 8.8 Hz, 1 H), 3.78 (s, 3 H), 3.61 (dd, J = 15.7,
9.3 Hz, 1 H), 3.21 (dd, J = 15.7, 8.2 Hz, 1 H). 13C NMR (100 MHz,
CDCl3): δ = 154.3, 153.78, 142.0, 128.7, 128.0, 127.5, 125.8,
113.0, 111.2, 109.2, 84.2, 56.1, 38.9. HRMS (ESI+): m/z [M + Na]+
calcd for C15H14NaO2: 249.0886; found: 249.0888.
2-(4-tert-Butylphenyl)-5-methoxy-2,3-dihydro-1-benzofu-
ran (3f)
Pale-yellow solid; yield: 38 mg (89%). 1H NMR (400 MHz,
CDCl3): δ = 7.39 (d, J = 8.3 Hz, 2 H), 7.34 (d, J = 8.3 Hz, 2 H), 6.83–
6.72 (m, 2 H), 6.69 (dd, J = 8.6, 2.2 Hz, 1 H), 5.71 (t, J = 8.7 Hz, 1
H), 3.76 (s, 3 H), 3.57 (dd, J = 15.7, 9.3 Hz, 1 H), 3.22 (dd, J = 15.7,
8.2 Hz, 1 H), 1.31 (s, 9 H). 13C NMR (100 MHz, CDCl3): δ = 154.2,
153.8, 151.0, 138.8, 127.7, 125.7, 125.5, 112.9, 111.2, 109.2,
84.2, 56.0, 38.6, 34.6, 31.3. HRMS (ESI+): m/z [M + Na]+ calcd for
(7) Alvey, L.; Prado, S.; Huteau, V.; Saint-Joanis, B.; Michel, S.; Koch,
M.; Cole, S. T.; Tillequin, F.; Janin, Y. L. Bioorg. Med. Chem. 2008,
16, 8264.
(8) (a) Chen, P.-Y.; Wu, Y.-H.; Hsu, M.-H.; Wang, T.-P.; Wang, E.-C.
Tetrahedron 2013, 69, 653. (b) Sako, M.; Hosokawa, H.; Ito, T.;
Iinuma, M. J. Org. Chem. 2004, 69, 2598. (c) Takaya, Y.;
Terashima, K.; Ito, J.; He, Y. H.; Tateoka, M.; Yamaguchi, N.; Niwa,
M. Tetrahedron 2005, 61, 10285. (d) Bruschi, M.; Orlandi, M.;
Rindone, B.; Rummakko, P.; Zoia, L. J. Phys. Org. Chem. 2006, 19,
592. (e) Wang, G.-W.; Wang, H.-L.; Capretto, D. A.; Han, Q.; Hua,
C
19H22NaO2: 305.1512; found: 305.1512.
2-Mesityl-5-methoxy-2,3-dihydro-1-benzofuran (3g)
colorless oil; yield: 28 mg (69%). 1H NMR (400 MHz, CDCl3):
δ = 6.86 (s, 2 H), 6.80 (s, 1 H), 6.73 (d, J = 8.7 Hz, 2 H), 6.11 (t, J =
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–E

E
Y. Wang et al.
Letter
Synlett
10.4 Hz, 1 H), 3.78 (s, 3 H), 3.43 (dd, J = 15.9, 9.9 Hz, 1 H), 3.25
(dd, J = 15.9, 11.0 Hz, 1 H), 2.29 (d, J = 14.9 Hz, 9 H). 13C NMR
(100 MHz, CDCl3): δ = 154.1, 153.7, 137.4, 136.7, 133.4, 130.0,
128.0, 113.2, 110.9, 109.4, 81.6, 56.0, 36.4, 20.8, 20.4. HRMS
(ESI+): m/z [M + Na]+ calcd for C18H20NaO2: 291.1356; found:
291.1350.
126.7, 112.9, 110.8, 109.5, 82.1, 56.1, 41.7, 27.9, 27.7. HRMS
(ESI+): m/z [M + Na]+ calcd for C17H16NaO2: 275.1043; found:
275.1044.
5-Ethoxy-2-methyl-2-phenyl-2,3-dihydro-1-benzofuran (3o)
White solid; yield: 33 mg (90%); mp 58–59 °C. 1H NMR (400
MHz, CDCl3): δ = 7.50–7.43 (m, 2 H), 7.33 (t, J = 7.6 Hz, 2 H), 7.22
(dd, J = 4.9, 2.2 Hz, 1 H), 6.77 (d, J = 8.5 Hz, 1 H), 6.74–6.65 (m, 2
H), 3.93 (q, J = 7.0 Hz, 2 H), 3.36 (q, J = 15.5 Hz, 2 H), 1.75 (s, 3 H),
1.36 (t, J = 7.0 Hz, 3 H).
8-Methoxy-5,6,6a,11a-tetrahydrobenzo[b]naphtho[2,1-
d]furan (3n)
Yellow oil; yield: 26 mg (68%). 1H NMR (400 MHz, CDCl3):
δ = 7.61 (d,J = 7.1 Hz, 1 H), 7.40–7.29 (m, 2 H), 7.22 (d,J = 7.1 Hz,
1 H), 6.92 (d, J = 1.8 Hz, 1 H), 6.80–6.71 (m, 2 H), 5.72 (d, J = 8.4
Hz, 1 H), 3.85 (s, 3 H), 3.71 (q, J = 8.0 Hz, 1 H), 2.86–2.66 (m, 2
H), 2.18–2.07 (m, 1 H), 1.94–1.82 (m, 1 H). 13C NMR (100 MHz,
CDCl3): δ = 154.3, 153.5, 138.8, 133.6, 132.5, 130.2, 128.5, 128.3,
13C NMR (100 MHz, CDCl3): δ = 153.4, 153.0, 146.9, 128.3, 127.4,
127.0, 124.5, 113.9, 112.1, 109.4, 89.1, 64.3, 45.2, 29.2, 15.0.
HRMS (ESI+): m/z [M + Na]+ calcd for C17H18NaO2: 277.1199;
found: 277.1199.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–E