Visible-light-mediated facile reductive aromatization of quinols

Article abstract of DOI:10.1055/s-0035-1561404

A range of phenols bearing multiple useful functionalities at their meta positions were prepared from the corresponding quinols under the cooperative effects of visible light irradiation, Ru(bpy)₃Cl₂ photosensitizer, Hünig's base, Li

Full text of DOI:10.1055/s-0035-1561404

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Georg Thieme Verlag Stuttgart · New York
2016, 27, A–E
A
letter
Synlett
L. Wang, Z. Yan
Letter
Visible-Light-Mediated Facile Reductive Aromatization of Quinols
Leifeng Wang*
Zhiming Yan
external
reductant
O
OH
Key Laboratory of Chemical Genomics, School of Chemical
Biology and Biotechnology, Shenzhen Graduate School of
Peking University, Shenzhen University Town, Shenzhen
R1
R2
R1
5
7
18055, P. R. of China
06930267@qq.com
photo-
sensitizer
R3
6 examples
O
Lewis acid
1
yield: 71–97%
Received: 30.12.2015
Accepted after revision: 08.02.2016
Published online: 09.03.2016
OH
O
a)
1
987 Swenton: Zn(Cu)
1992 Coburn: Zn/HOAc/H2O
013 Fan: Zn/HOAc
active metal
DOI: 10.1055/s-0035-1561404; Art ID: st-2015-w1006-l
R
R
2
Abstract A range of phenols bearing multiple useful functionalities at
their meta positions were prepared from the corresponding quinols un-
der the cooperative effects of visible light irradiation, Ru(bpy) Cl pho-
HO
Ar
Ar
3
2
tosensitizer, Hünig’s base, LiBF , and MeCN solvent. The process in-
volves visible-light-enabled photocatalytic cleavage of C–O bond as the
strategic event.
4
b) OR2
R1
reducting
agent
1991 Coburn: BH3•SMe2
1998 Mo: SmI2
2006 Schwind: Et3SiH
R1
Key words visible light, photocatalysis, amine, C–O bond cleavage,
2
2
010 Ohe: SnCl2/HCl
015 Glöcklhofer: PCl3
aromatization
OR3
As a highly versatile class of substances, quinols are
c)
This work
known to serve as useful precursors to various compounds
in organic synthesis. Great efforts have been made to
achieve vital transformations of quinols, such as cycloaddi-
O
OH
R1
photocatalysis
Lewis acid
R2
1
6 examples
tion, Michael addition and reductive aromatization.3 As
1
2
R1
up to 97% yield
R3
we know, reductive aromatization of quinols was first ac-
complished with zinc and its alloys. Various reducing
agents were next well investigated involving LiAlH , Et SiH,
O
Scheme 1 Reductive aromatization of quinols and equivalents
4
3
BH ·SMe , SmI and SnCl (Scheme 1). Despite these signifi-
3
2
2
2
cant advances, there remains a strong need for discovering
mild and convenient methods for reductive aromatization
of quinols.
To the best of our knowledge, C–O bonds cleavage of
special substrates could be achieved in the presence of com-
mercially available photosensitizer as the catalyst and
amine as the real reducing agent. Therefore, meta-substi-
tuted phenols would be synthesized by photocatalytic re-
ductive aromatization of quinols.
In recent years, remarkable progress in photocatalytic
cleavages of C–C and C–X bonds has been achieved in un-
covering both new catalysis concepts and robust synthetic
4–6
applications. In turn, such advances serve as the key mo-
tivation for us to tackle a visible-light-mediated protocol for
the direct and mild reductive aromatization. Within this
context, we have reported the preparation of a range of
phenols bearing multiple useful functionalities at their
meta positions.
Motivated by this possibility, we initially explored a set
of reaction conditions using Ru(bpy) Cl as the photosensi-
3
2
tizer. Eventual success was achieved after an elaborate
screening (Table 1), proving that the formation of phenols
in synthetically useful efficiency depends on the synergy of
several factors critically. With quinol 1 as the model sub-
strate and a 45 W household bulb as the light source, we
employed the Ru(bpy) Cl (0.05 equiv) as the photosensitiz-
3
2
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Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–E

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L. Wang, Z. Yan
Letter
er, amine i-Pr NEt (Hünig’s base) (5.00 equiv) as the reduc-
was all furnished in lower yields when MeCN was replaced
by THF, 1,4-dioxane, MeOH, Et O, CH Cl or DMF (entries 8–
2
ing agent and MeCN (0.05 M) as the solvent. Unfortunately,
only trace of the desired product was detected (entry 1, Ta-
ble 1). A significant observation was noted when mild Lew-
2
2
2
13). Furthermore, reaction parameters from the experi-
ments shown in entries 14–18 revealed the critical impor-
tance of amine, as the use of Et N, Et NH, pyrrolidine, ani-
is acid (LiClO , 1.00 equiv) was added into the reaction sys-
4
3
2
tem (42%, entry 2). In contrast, strong Lewis acid, such as
line or monomethylaniline led to significant reductions or
even complete inhibitions in reactivities. The different reac-
tivities would, thus, suggest that single electron transfer
from amine to the photoexcited catalyst should be crucial
ZnCl (1.00 equiv), made substrate 1 decompose (entry 4).
2
Interestingly, a simple switch of the Lewis Acid La(OTf) to
3
LiBF was found to dramatically improve the reactivity (en-
4
in this transformation.4 Ir(ppy) (5% mol) and eosin Y (5%
d,7
tries 5 and 6). To our delight, a doubled loading of LiBF de-
4
3
livered the product in nearly quantitative yield (97% isolat-
ed yield). Compared with Lewis acid, the solvent effect was
shown to be fairly significant during this transformation.
Under otherwise identical reaction conditions, the product
mol) were examined while the Ru catalyst was absent, both
leading to rather low yields (entries 19 and 20). Finally,
complete inhibition of the reactivity could be identified
from control experiments conducted under the absence of
Table 1 Optimization of Reaction Conditions
OH
O
Ru(bpy)3Cl2•6H2O (0.05 equiv)
amine, additive, solvent, r.t., Ar
visible light (45 W bulb)
O
1
2
OH
Entry
Catalyst (0.05 equiv)
Amine (5.00 equiv)
Solvent
Additive (equiv)
–
Yield (%)a
1
2
3
4
5
6
7
8
9
Ru(bpy)
Ru(bpy)
Ru(bpy)
Ru(bpy)
Ru(bpy)
Ru(bpy)
Ru(bpy)
Ru(bpy)
Ru(bpy)
Ru(bpy)
Ru(bpy)
Ru(bpy)
Ru(bpy)
Ru(bpy)
Ru(bpy)
Ru(bpy)
Ru(bpy)
Ru(bpy)
Eosin-Y
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
·6H
·6H
·6H
·6H
·6H
·6H
·6H
·6H
·6H
·6H
·6H
·6H
·6H
·6H
·6H
·6H
·6H
·6H
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
i-Pr
i-Pr
i-Pr
i-Pr
i-Pr
i-Pr
i-Pr
i-Pr
i-Pr
i-Pr
i-Pr
i-Pr
i-Pr
2
2
2
2
2
2
2
2
2
2
2
2
2
Net
Net
NEt
NEt
NEt
NEt
NEt
NEt
NEt
NEt
NEt
NEt
NEt
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
THF
trace
42
LiClO
Mg(ClO
ZnCl (1.00)
La(OTf) (1.00)
4
(1.00)
4
)
2
(1.00)
31
2
dec.
18
3
LiBF
LiBF
LiBF
LiBF
LiBF
LiBF
LiBF
LiBF
LiBF
LiBF
LiBF
LiBF
LiBF
LiBF
LiBF
LiBF
LiBF
LiBF
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
(1.00)
(2.00)
(2.00)
(2.00)
(2.00)
(2.00)
(2.00)
(2.00)
(2.00)
(2.00)
(2.00)
(2.00)
(2.00)
(2.00)
(2.00)
(2.00)
(2.00)
(2.00)
70
97b
58
1,4-dioxane
MeOH
61
10
11
12
13
14
15
16
17
18
19
20
21
22
23
59
Et
CH
DMF
2
O
trace
35
2
Cl
2
51
Et
3
N
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
31
Et
2
NH
15
pyrrolidine
trace
trace
trace
35
aniline
monomethylaniline
i-Pr
i-Pr
i-Pr
i-Pr
i-Pr
2
2
2
2
2
NEt
NEt
NEt
NEt
NEt
Ir(ppy)
/
3
26
NR
Ru(bpy)
3
Cl
2
2
·6H
2
O
O
NR
Ru(bpy)
3
Cl
·6H
2
NR
a
1
Determined by H NMR analysis using 1,3,5-trimethoxybenzene as internal standard.
Isolated yield.
b
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Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–E

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L. Wang, Z. Yan
Letter
either a photocatalyst, visible light, or amine (entries 21–
3). Collectively, these screening results established the op-
timal reaction conditions: 5% mol of Ru(bpy) Cl photocata-
fluence on the reaction efficiencies; the presence of a bulky
substituent retarded the reduction to some extent. Aromat-
ic (4d), naphthenic (4e) and oxygen-containing substitu-
tions (4f) were also well tolerated. 3,4,5-Trisubstituted and
2,4,5-trisubstituted phenols (4g and 4h) were also success-
fully prepared by this facile reductive process with high ef-
ficiencies.
2
3
2
lyst, 5.00 equiv of Hünig’s base, 2.00 equiv of LiBF additive
4
and MeCN as the solvent under a balloon-argon atmo-
sphere at room temperature.
With the above optimal conditions in hand, we next
probed the scope of the facile reductive aromatization of
quinols. A range of quinols were prepared from simple phe-
nols bearing electron-donating functional groups rather
To further examine the substrate scope of the method-
4
ology, the effect of the aromatic residues (R ) was next in-
vestigated. As summarized in Scheme 2 (4i–p), the corre-
sponding phenols were generally delivered in moderate-to-
high isolated yields. These results revealed the fact that the
substitution patterns (meta, para) on the aryl rings seemed
to pose no obvious influence on the reaction efficiency ex-
cept in 4k. Although methoxy-substituted aryl ring inhibit-
ed the reaction activity dramatically compared with other
8
than electron-withdrawing functional groups (see the
Supporting Information). As compiled in Scheme 2, in each
case the reaction proceeded smoothly to furnish the de-
sired phenols in moderate-to-high isolated yields (71–97%).
In the cases 4a–c, steric effect of the aliphatic substitution
3
patterns (R ) on the quinol rings seemed to pose a slight in-
OH
Ru(bpy)3Cl2•6H2O (0.05 equiv)
i-Pr2NEt (5.00 equiv)
O
O
R3
LiBF4 (2.00 equiv)
R4
R3
R4
MeCN, r.t., Ar, visible light, 24 h
(45 W household bulb)
3
4
OH
OH
OH
Et
OH
OH
i-Pr
Ph
4a: 97%
4b: 93%
4c: 91%
4d: 90%
OH
OH
OH
OH
OH
OH
OH
OH
Cy
O
4e: 88%
4f: 90%
4g: 87%
4h: 89%
OH
OH
OH
OH
OH
OH
OH
OH
O
Ph
4i: 80%
4j: 93%
4k: 71%
4l: 81%
OH
OH
OH
OH
OH
OH
OH
OH
4m: 84%
4n: 83%
4o: 86%
4p: 79%
OH
Scheme 2 Survey of reaction scope
OH
OH
OH
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L. Wang, Z. Yan
Letter
LA
N
N
*Ru2+
LA
visible light
LA
–
e
photoredox
catalysis
N
N
C
H
A
Ru2+
Ru+
hydrogen
transfer
+
e
O
R2
LA
R2
O
OH
O
R3
LA
B
D
E
R1
R1
R2
R1
proton
R3
R1
R3
R3
O
O
R2
OH
OH
LA
Scheme 3 Proposed mechanism
cases, the isolated yield was acceptable. An aryl ring with
more conjugation (4l) was also a comparably competent
substrate. Bulky substitution patterns (4m–n) and disubsti-
tuted (4o–p) aryl rings were well tolerated.
Supporting Information
Supporting information for this article is available online at
http://dx.doi.org/10.1055/s-0035-1561404.
S
u
p
p
o
nrtIo
i
g
f
rm oaitn
S
u
p
p
ortioIgnfmr oaitn
Enabled by these observations, a plausible mechanistic
network is briefly depicted in Scheme 3. We believe that
three intimately coupled pathways could play critical roles
in this unusual photocatalytic reduction: Ru(I)/Ru(II)-pho-
toredox cycle (in blue), amine dehydrogenative oxidation
References and Notes
(
(
(
1) For selected examples of Michael addition reactions of quinols,
see: (a) Dipakranjan, M.; Sutapa, R. Eur. J. Org. Chem. 2008,
014. (b) Fringuelli, F.; Minuti, L.; Pizzo, F.; Taticchi, A.; Halls, T.
3
(in green), and quinol substrate reduction (in red). Under
D. J.; Wenkert, E. J. Org. Chem. 1983, 48, 1811. (c) Nicolaou, K. C.;
Stepan, A. F.; Lister, T.; Li, A.; Montero, A.; Tria, G. S.; Turner, C.
I.; Tang, Y.; Wang, J.; Denton, R. M.; Edmonds, D. J. J. Am. Chem.
Soc. 2008, 130, 13110. (d) Liotta, D.; Saindane, M.; Barnum, C.
J. Am. Chem. Soc. 1981, 103, 3224.
visible light irradiation, the excited state species *Ru(II) ini-
tially captures an electron from amine–Lewis acid com-
6
a,7b,c
plex,
giving rise to Ru(I) and the corresponding radical
cation A. Activated by free Lewis acid, the electron deficien-
cy of quinol could thus grant its oxidation capacity that ab-
stracts an electron from the Ru(I) intermediate, closing the
photoredox catalytic cycle. The concomitantly generated
oxygen radical B could be immediately quenched by the
radical cation A through a hydrogen transfer process, fur-
nishing oxygen anion D and amine cation C. Finally, proton-
ation of oxygen anion D leads to the phenol product F.
In summary, visible-light-mediated facile reductive aro-
2) For selected examples of Michael addition reactions of quinols,
see: (a) Brocksom, T. J.; Coelho, F.; Deprés, J. P.; Greene, A. E.;
Freire de Lima, M. E.; Hamelin, O.; Hartmann, B.; Kanazawa, A.
M.; Wang, Y. J. Am. Chem. Soc. 2002, 124, 15313. (b) Giomi, D.;
Piacenti, M.; Brandi, A. Eur. J. Org. Chem. 2005, 4649. (c) Imbos,
R.; Brilman, M. H. G.; Pineschi, M.; Feringa, B. L. Org. Lett. 1999,
1
2
, 623. (d) Meister, A. C.; Sauter, P. F.; Bräse, S. Eur. J. Org. Chem.
013, 7110.
3) For selected examples of reductive aromatization of quinols,
see: (a) Zhang, L.; Wang, W.; Fan, R. Org. Lett. 2013, 15, 2018.
9
matization of quinols was well developed. A series of phe-
(b) Capparelli, M. P.; DeSchepper, R. E.; Swenton, J. S. J. Org.
nols bearing multiple useful functionalities at their meta
positions were prepared based on the optimized conditions
which may serve as new intermediates for natural products
syntheses and pharmaceutical research. Furthermore,
many useful synthetic utilities could be well exploited on a
wider platform of photoredox catalysis.
Chem. 1987, 52, 4953. (c) Parker, K. A.; Coburn, C. A. J. Org. Chem.
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Acknowledgment
(
k) Parker, K. A.; Georges, A. T. Org. Lett. 2000, 2, 497.
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(
We thank the Shenzhen Bureau of Science and Technology for finan-
cial support.
(
5
©
Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–E

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Synlett
L. Wang, Z. Yan
Letter
1
6
239176. (c) Xuan, J.; Xiao, W.-J. Angew. Chem. Int. Ed. 2012, 51,
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tive Aromatization of Quinols: A flame-dried round-bottom
flask (10 mL) was equipped with magnetic stirring bar and
charged with quinol compound (0.1 mmol, 1.0 equiv), tris(2,2′-
bipyridyl)ruthenium(II) chloride hexahydrate (0.005 mmol,
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4
2
011, 480, 224. (d) Qvortrup, K.; Rankic, D. A.; MacMillan, D. W.
the mixture was then added amine (0.5 mmol, 5.0 equiv), and it
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argon atmosphere at r.t. until the starting material disappeared
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pressure, and the residue was purified by silica gel column
chromatography to afford the pure product.
2
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1
(k) Nguyen, J. D.; D’Amato, E. M.; Narayanam, J. M. R.;
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1218. (m) Kalyani, D.; McMurtrey, K. B.; Neufeldt, S. R.; Sanford,
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3
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+
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2
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M.; Fuldner, S.; Harlander, C.; Kummel, S.; Dankesreiter, S.;
Pfitzner, A.; Zeitler, K.; König, B. Angew. Chem. Int. Ed. 2012, 51,
(Z)-2-(3-Hydroxy-1-phenylprop-1-en-1-yl)-[1,1′-biphenyl]-
4-ol (4d): white solid; yield: 27.2 mg (90%). H NMR (500 MHz,
1
MeOD): δ = 7.21 (d, J = 8.4 Hz, 1 H), 7.06–7.17 (m, 10 H), 6.89
(dd, J = 8.4, 2.6 Hz, 1 H), 6.69 (d, J = 2.6 Hz, 1 H), 6.06 (t, J = 6.4
Hz, 1 H), 4.00–4.14 (m, 1 H), 3.80 (dd, J = 13.2, 5.3 Hz, 1 H). 13
C
NMR (126 MHz, acetone): δ = 156.51 (s), 141.53 (s), 141.29 (s),
140.95 (s), 138.76 (s), 133.35 (s), 131.26 (s), 129.30 (s), 128.78
(s), 127.92 (s), 127.56 (s), 126.89 (s), 126.66 (s), 126.12 (s),
+
117.41 (s), 115.05 (s), 60.17 (s). HRMS: m/z [M + Na ] calcd for
C21H18NaO : 325.1204; found: 325.1200.
2
(Z)-3-{1-([1,1′-Biphenyl]-4-yl)-3-hydroxyprop-1-en-1-yl}-4-
4062. (w) Kisch, H. Angew. Chem. Int. Ed. 2013, 52, 812.
methylphenol (4l): white solid; yield: 25.6 mg (81%). 1H NMR
(
(
2
x) Mitoraj, D.; Kisch, H. Angew. Chem. Int. Ed. 2008, 47, 9975.
y) Parrino, F.; Ramakrishnan, A.; Kisch, H. Angew. Chem. Int. Ed.
008, 47, 7107. (z) Kisch, H.; Zang, L.; Lange, C.; Maier, W. F.;
(500 MHz, CDCl ): δ = 7.53 (d, J = 7.4 Hz, 2 H), 7.44 (d, J = 8.3 Hz,
3
2 H), 7.39 (t, J = 7.6 Hz, 2 H), 7.32 (t, J = 7.3 Hz, 1 H), 7.25 (d, J =
8.3 Hz, 2 H), 7.05 (d, J = 8.3 Hz, 1 H), 6.78 (dd, J = 8.2, 2.6 Hz, 1
H), 6.67 (d, J = 2.6 Hz, 1 H), 6.36 (t, J = 6.9 Hz, 1 H), 3.98–4.10 (m,
Antonius, C.; Meissner, D. Angew. Chem. Int. Ed. 1998, 37, 3034.
6) For selected examples of visible-light-enabled photocatalytic
cleavages of C–C and C–X bonds, see: (a) Espelt, L. R.;
McPherson, I. S.; Wiensch, E. M.; Yoon, T. P. J. Am. Chem. Soc.
13
(
2 H), 1.92 (s, 3 H). C NMR (126 MHz, CDCl ): δ = 153.85 (s),
3
143.02 (s), 140.50 (s), 140.33 (s), 139.09 (s), 138.79 (s), 131.44
(s), 128.79 (s), 128.14 (s), 127.35 (s), 127.03 (s), 126.92 (s),
2
015, 137, 2452. (b) Silvi, M.; Arceo, E.; Jurberg, I. D.; Cassani, C.;
Melchiorre, P. J. Am. Chem. Soc. 2015, 137, 6120.
c) Cuthbertson, J. D.; MacMillan, D. W. C. Nature (London)
015, 519, 74. (d) Noble, A.; McCarver, S. J.; MacMillan, D. W. C.
126.87 (s), 126.27 (s), 116.58 (s), 114.98 (s), 60.78 (s), 18.68 (s).
+
HRMS: m/z [M + Na ] calcd for C22H20NaO : 339.1361; found:
2
(
2
339.1357.
(Z)-3-[1-(3,4-Dimethylphenyl)-3-hydroxyprop-1-en-1-yl]-4-
J. Am. Chem. Soc. 2015, 137, 624. (e) Huang, H.; Jia, K.; Chen, Y.
Angew. Chem. Int. Ed. 2015, 54, 1881. (f) Guo, W.; Lu, L.; Wang,
Y.; Chen, J.; Xiao, W.-J. Angew. Chem. Int. Ed. 2015, 54, 2265.
methylphenol (4o): colorless liquid; yield: 23.0 mg (86%). 1
H
NMR (400 MHz, CDCl ): δ = 6.99–7.13 (m, 3 H), 6.94 (d, J = 7.9
3
Hz, 1 H), 6.75 (dd, J = 8.2, 2.7 Hz, 1 H), 6.59 (d, J = 2.6 Hz, 1 H),
6.31 (t, J = 6.8 Hz, 1 H), 4.04 (d, J = 6.8 Hz, 2 H), 2.23 (s, 3 H), 2.21
(g) Majek, M.; von Wangelin, A. J. Angew. Chem. Int. Ed. 2015, 54,
13
2270. (h) Zhang, Y.; Qian, R.; Zheng, X.; Zeng, Y.; Sun, J.; Chen,
(s, 3 H), 1.97 (s, 3 H). C NMR (101 MHz, CDCl ): δ = 153.5 (s),
3
Y.; Dinga, A.; Guo, H. Chem. Commun. 2015, 51, 54. (i) Jin, J.;
MacMillan, D. W. C. Angew. Chem. Int. Ed. 2015, 54, 1565.
142.9 (s), 138.9–139.5 (m), 137.5 (s), 136.4 (s), 136.2 (s), 131.2
(s), 129.6 (s), 128.2 (s), 127.5 (s), 125.7 (s), 124.0 (s), 116.5 (s),
(
j) Primer, D. N.; Karakaya, I.; Tellis, J. C.; Molander, G. A. J. Am.
114.5 (s), 60.9 (s), 19.8 (s), 19.4 (s), 18.6 (s). HRMS: m/z [M +
+
Chem. Soc. 2015, 137, 2195. (k) He, Y.; Wuab, H.; Toste, F. D.
Chem. Sci. 2015, 6, 1194.
Na ] calcd for C18H20NaO : 291.1361; found: 291.1354.
2
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Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–E