A Cascade Cross-Coupling and in Situ Hydrogenation Reaction by Visible Light Catalysis

Article abstract of DOI:10.1002/adsc.201400588

An elegant cascade cross-coupling and in situ hydrogenation reaction has been established by visible light catalysis. Combining cheap and earth-abundant eosin Y as a photosensitizer and the cobalt chloride complex, Co(dmgH)₂Cl₂ (dmgH=dimethylglyoximate), as a catalyst, the desired oxidative cross-coupling products and reductive hydrogenation products could be achieved in good to excellent yields without use of any external oxidants and reductants under visible light irradiation.

Full text of DOI:10.1002/adsc.201400588

UPDATES
DOI: 10.1002/adsc.201400588
A Cascade Cross-Coupling and in Situ Hydrogenation Reaction
by Visible Light Catalysis
Jian-Ji Zhong,a Cheng-Juan Wu,a Qing-Yuan Meng,a Xue-Wang Gao,a Tao Lei,a
Chen-Ho Tung,a and Li-Zhu Wua,*
a
Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry
& University of Chinese Academy of Sciences, the Chinese Academy of Sciences, Beijing 100190, Peopleꢀs Republic of
China
Fax : (+86)-10-8254-3580; e-mail: lzwu@mail.ipc.ac.cn
Received: June 17, 2014; Revised: July 31, 2014; Published online: September 11, 2014
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201400588.
Abstract: An elegant cascade cross-coupling and in
situ hydrogenation reaction has been established by
visible light catalysis. Combining cheap and earth-
abundant eosin Y as a photosensitizer and the
visible light catalysis,[6–9] for example, molecular
oxygen is necessary to accept the electrons and pro-
tons eliminated from the substrates. In the CCHE re-
action, the whole reaction takes place under an inert
atmosphere. Combining a photosensitizer for visible
light absorption and subsequent electron transfer, and
a catalyst for H2 evolution as well, the CCHE reac-
tion proceeds smoothly to afford the desired cross-
coupling products and H2 in quantitative yields under
visible light irradiation.
cobalt chloride complex, CoACTHNUTRGNEUNG(dmgH)2Cl2 (dmgH=
dimethylglyoximate), as a catalyst, the desired oxi-
dative cross-coupling products and reductive hydro-
genation products could be achieved in good to ex-
cellent yields without use of any external oxidants
and reductants under visible light irradiation.
Inspired by the CCHE reaction, we wanted to com-
bine the unique cross-coupling reaction to hydrogena-
tion directly in a one-pot reaction. The most funda-
mental difficulty in such endeavors lies in the pres-
ence of a stoichiometric sacrificial oxidant for cross-
coupling and a reductant for hydrogenation in one re-
action vessel. In this contribution, we wish to report
the first example of an elegant cascade cross-coupling
Keywords: cross-coupling; cross-coupling hydrogen
evolution (CCHE) reaction; hydrogenation; redox
neutral reaction; visible light catalysis
The development of a step- and atom-economic ap- and in situ hydrogenation reaction under inert atmos-
proach to build complex molecular structures through phere. The redox-neutral transformation is capable of
À
À
selective functionalization of a rather unreactive C H activating C H bonds to afford a cross-coupling prod-
bond is a pressing demand in synthetic chemistry.[1] uct and an equivalent amount of hydrogenation prod-
Cross-dehydrogenation coupling (CDC) reactions,[2] uct in good to excellent yields without use of any sac-
À
one of the most powerful tools to make a C C bond rificial oxidant and reductant (Scheme 1). Herein, an
directly from two different C H bonds under oxida- organic dye, eosin Y, is employed as a photosensitizer
tive conditions, reduce the number of steps to the to initiate cross-coupling of amines with nucleophiles
target molecules. However, the use of stoichiometric by visible light catalysis, and at the same time, an
À
sacrificial oxidants may lead to low atom economy earth-abundant CoACTHNUTRGNEUNG(dmgH)2Cl2 (dmgH=dimethyl-
and possible generation of toxic wastes. Very recently, glyoximate) complex is used as a catalyst to capture
À
a new type of reaction, the cross-coupling hydrogen the electrons and protons eliminated from the C H
evolution (CCHE) reaction,[3] has appeared, which bonds of the substrates for in situ reduction of nitro-
avoids the use of any sacrificial oxidants and yields benzene.[10] In this regard, additional external reduc-
hydrogen gas (H2) as the sole by-product.[4] In the tra- tants might not be necessary if the electrons and pro-
ditional CDC reactions, H2 is not usually the by-prod- tons captured by the catalyst could be fully utilized in
À
uct because the thermodynamics of making a C C the following hydrogenation reaction. This expecta-
bond with loss of H2 are unfavorable, and the reaction tion was found to be indeed the case. With visible
thus requires an external driving force, stoichiometric light irradiation (lmax =525 nm) for 36 h, both the de-
sacrificial oxidants. In aerobic CDC reactions[5] by sired cross-coupling products and hydrogenation
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A Cascade Cross-Coupling and in Situ Hydrogenation Reaction
Scheme 1. Visible light-mediated cascade cross-coupling and in situ hydrogenation reaction in CH3CN/H2O.
products are achieved smoothly under ambient condi- Table 1. Optimization of the reaction conditions.[a]
tions.
Our initial attempt focused on the cross-coupling of
N-phenyltetrahydroisoquinoline (1a) and indole (2a),
and hydrogenation of nitroarenes (3a) for their impor-
tance as central building blocks in organic synthesis.
First, catalytic amounts of eosin Y (3 mol%) and
Co
tion of 1a, 2a and 3a, and then the nitrogen-purged
solution was irradiated by green LEDs (lmax
ACHTUNGTRENNUNG(dmgH)2Cl2 (10 mol%) were added into the solu-
=
525 nm) at room temperature. To our delight, a 95%
yield of the desired cross-coupling product (4a) was
obtained without use of any sacrificial oxidant. More
importantly, aniline (5a) was produced without an ad-
ditional sacrificial reductant in 91% yield based on
the consumption of 1a (Table 1, entry 1). And the
cross-coupling product 4a and aniline 5a are the only
products detected. Increasing the amount of eosin Y
did not change the reaction performance very much
(Table 1, entries 1–4). However, decreasing the
Entry
Eosin Y
Co
G
Yield
4a [%][b]
5a [%][c]
1
2
3
4
5
6
3%
1%
5%
–
1%
1%
1%
1%
10%
10%
10%
10%
8%
5%
–
10%
95
94
97
–
76
60
–
91
89
90
–
79
66
20
–
amount of CoACHTUNGTRENNUNG(dmgH)2Cl2 resulted in a significant de-
crease in both conversion and yield of the reaction
(Table 1, entries 5–7). When 1 mol% of eosin Y and
7
8[d]
–
[a]
Reaction conditions: 0.35 mmol 1a, 1 mmol 2a, 0.1 mmol
3a, corresponding PS (eosin Y) and Cat. [Co(dmgH)2Cl2]
10 mol% of CoACHTUNGTRENNUNG(dmgH)2Cl2 were used, the cross-cou-
AHCTUNGTRENNUNG
pling product 4a and the hydrogenation product ani-
line 5a were achieved in yields of 94% and 89%, re-
spectively. When the reaction underwent in H2O, only
50% of the desired cross-coupling product 4a and
55% of aniline 5a could be detected. Importantly, in-
troduction of organic solvent CH3CN into the system
improved the performance of the reaction, and the
best ratio of H2O to CH3CN was found to be 2:1
were added in a mixture of H2O/CH3CN, the solution
was strictly deaerated, and irradiated for 36 h by 525 nm
green LEDs.
Yields determined by H NMR based on 3a.
Yields detected by GC-FID.
[b]
[c]
[d]
No light.
(Supporting Information, Table S1). When the reac- the cascade cross-coupling and in situ hydrogenation
tion was conducted in darkness, no conversion of sub- reaction is able to proceed efficiently under the opti-
strate 1a could be detected (Table 1, entry 8). Control mized conditions, i.e., 1 mol% eosin Y, 10 mol% Co-
experiments showed that neither cross-coupling prod-
uct nor hydrogenation product could be obtained LED irradiation for 36 h at room temperature.
when eosin Y was absent from the system (Table 1, With the optimized conditions in hand, we explored
entry 4), whereas the absence of Co(dmgH)2Cl2 led to the generality of the cascade cross-coupling and in
ACHTNUGTERN(NUNG dmgH)2Cl2, in H2O/CH3CN 2:1, under 525 nm green
AHCTUNGTRENNUNG
no cross-coupling product 4a formation but 20 mol% situ hydrogenation reaction on the scope of N-phenyl-
aniline 5a generation (Table 1, entry 7). As a result, tetrahydroisoquinolines 1. As shown in Table 2, N-
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Jian-Ji Zhong et al.
Table 2. Scope of tetrahydroisoquinolines.[a]
Table 3. Scope of indoles.[a]
R1
Yield
Entry
R2
R3
R4
Yield
Entry
4 [%][b]
5a [%][c]
4 [%][b]
5a [%][c]
1
2
3
4
5
6
7
8
CH3
H
H
H
H
H
H
H
H
CH3
H
H
H
H
H
H
H
H
4h, 90 (84)
4i, 91 (81)
4j, 89 (84)
4k, 92 (82)
4l, 88 (76)
4m, 86 (78)
4n, 85 (80)
4o, 95 (88)
88
87
92
91
89
84
91
94
1
2
3
4
5
6
p-F
p-Cl
p-Br
p-CH3
p-OCH3
o-OCH3
4b, 92 (83)
4c, 88 (80)
4d, 85 (76)
4e, 87 (78)
4f, 84 (77)
4g, 81 (73)
90
87
89
85
86
83
5-CH3
5-OCH3
5-Br
6-Cl
6-OCH3
7-CH3
[a]
Reaction conditions: 0.35 mmol 1, 1 mmol 2a, 0.1 mmol
3a, 1 mol% PS (eosin Y) and 10 mol% of the catalyst
[CoACHTUNGTRENNUNG(dmgH)2Cl2] were added in a 2:1 mixture of H2O/
[a]
Reaction conditions: 0.35 mmol 1a, 1 mmol 2, 0.1 mmol
3a, 1 mol% PS (eosin Y) and 10 mol% of the catalyst
[CoACTHUNTGRENUNG(dmgH)2Cl2] were added in a 2:1 mixture of H2O/
CH3CN, the solution was strictly deaerated, and irradiat-
ed for 36 h by 525 nm green LEDs.
Yields determined by H NMR based on 3a.
CH3CN, the solution was strictly deaerated, and irradiat-
ed for 36 h by 525 nm green LEDs.
[b]
[c]
Yields detected by 1H NMR based on 3a.
Yields detected by GC-FID.
[b]
[c]
1
Yields detected by GC-FID.
phenyltetrahydroisoquinolines with electron-donating
groups or electron-withdrawing groups at N-aryl were
Furthermore, various substituted nitrobenzenes
converted to the desired cross-coupling products ac- could be reduced to their corresponding anilines effi-
companied with the formation of aniline smoothly in ciently in the absence of external reductants
good to excellent yields. Steric hindrance seemed to (Table 4). When the system containing nitrobenzenes
have little influence on the efficiency of the reaction with either electron-donating or electron-withdrawing
(Table 2, entry 6). Significantly, when the substrates groups on the ring was irradiated, the cross-coupling
containing other reducible groups like halogen were products 4 and the corresponding hydrogenation
presented in such an elegant reaction, only nitroben- products aniline derivatives 5 were achieved under an
zene 3 was chemoselectively reduced to the corre- inert amosphere. Notably, only the nitro group was
sponding anilines leaving these halogen substituents reduced to the corresponding aniline, and other re-
unaffected. The products containing halogens could ducible groups including bromo, chloro, iodo or car-
serve as potential intermediates for further organic bonyl substituents in the nitrobenzene derivatives re-
transformation.
mained unchanged.
Good to excellent yields were also obtained by
In the cascade cross-coupling and in situ hydroge-
using a variety of substituted nucleophilic indoles. nation reaction, no external oxidant and reductant are
Table 3 shows the representative results. Evidently, N- needed to realize the oxidative cross-coupling prod-
protected indole performed well with 90% of cross- ucts and the reductive hydrogenation. The perfect
coupling yield and 88% of aniline yield (Table 3, performance prompted us to shed light on the mecha-
entry 1). For free indoles with electron-donating or nism of the redox-neutral transformation. For consis-
electron-withdrawing group, the desired cross-cou- tence with the real reaction condition, the subsequent
pling product 4 and the hydrogenation product aniline mechanistic studies were performed under the opti-
5 were also obtained in excellent yields. No steric hin- mal conditions. In the whole reaction system, only
drance was found to affect the reaction yield (Table 3, eosin Y shows the visible light absorption in the
entry 2). The products with bromo- or chloro-func- range of green LED light (lmax =525 nm). Upon exci-
tionalities at the indole ring also provided a potential tation of eosin Y by light, two reaction pathways are
pathway for further functionalization.
possible. One is that the photoexcited eosin Y might
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UPDATES
A Cascade Cross-Coupling and in Situ Hydrogenation Reaction
Table 4. Scope of nitrobenzenes.[a]
demonstrate that the reaction is dominated by the
electron transfer from 1a to the photoexcited eosin Y
to generate [eosin Y]CÀ radical anion. Further addition
of CoACHTNURTGNEUNG(dmgH)2Cl2 into the mixture of eosin Y, 1a and
3a resulted in a distinct quenching of [eosin Y]CÀ upon
532 nm laser excitation. The lifetime of [eosin Y]CÀ
changed from 24.6 ms to 0.6 ms indicating the electron
transfer from [eosin Y]CÀ to Co
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
action. In the present work, H2 could not be detected
throughout the reaction. Instead, nitrobenzene 3a was
converted to aniline efficiently. This result implied
Entry
R5
Yield
that the reduced CoACTHNUTRGENN(UG dmgH)2Cl2 and its CoACHTUNGTRENNUNG(III)-H
4a [%][b]
5 [%][c]
hydrid might be consumed by nitrobenzene. To con-
firm the assumption, the real catalytic system was
monitored by absorption changes. Before irradiation,
the absorption spectrum was simply the sum of the in-
dividual components and eosin Y showed absorption
in the visible region. Upon irradiation the system con-
1
2
3
4
5
6
7
8
H
m-Cl
p-Cl
p-Br
p-I
p-COCH3
p-OCH3
p-CH3
95 (87)
87 (78)
92 (86)
91 (85)
87 (78)
92 (87)
90 (83)
92 (85)
5a, 91
5b, 90
5c, 89
5d, 94
5e, 85
5f, 90
5g, 87
5i, 96
taining eosin Y, 1a, and CoACHTNUTRGNEUNG(dmgH)2Cl2 by green
LEDs for 2 min, the characteristic absorption ap-
peared at 450 nm and 550–650 nm, respectively, sug-
gesting the formation of the reduced form of
[a]
Reaction conditions: 0.35 mmol 1a, 1 mmol 2a, 0.1 mmol
3, 1 mol% PS (eosin Y) and 10 mol% of the catalyst
CoACTHNUTRGNEUNG
(dmgH)2Cl2, Co(II) and Co(I) species[3b,13]
[CoACHTUNGTRENNUNG(dmgH)2Cl2] were added in a 2:1 mixture of H2O/
(Figure 2, a). Importantly, the absorptions were quick-
ly decreased with the subsequent addition of nitro-
benzene to the solution (Figure 2, b). In this situation,
the cascade electron transfer (i) from 1a to the excited
CH3CN, the solution was strictly deaerated, and irradiat-
ed for 48 h by 525 nm green LEDs.
[b]
[c]
1
Yields determined by H NMR based on 3a.
Yields detected by GC-FID.
3[eosin Y]* to produce [eosin Y]CÀ, (ii) from the [eosin
ACHTUNGTRENNUNG
Y]CÀ to Co
be oxidatively quenched to generate [eosin Y]C+. The Co
(dmgH)2Cl2 and its Co
(dmgH)2Cl2 and its CoACHTUNGTRENNUNG
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
other is that the photoexcited eosin Y could be reduc- reduced Co
C
tively quenched to produce [eosin Y]CÀ. To determine trobenzene, is believed to be responsible for the in
the dominant reaction pathway, a flash photolysis in- situ hydrogenation.
vestigation was undertaken in terms of the rich spec-
On the basis of the above observation, we proposed
troscopic property of eosin Y.[11] With a laser pulse by a plausible mechanism for the cascade cross-coupling
525 nm light, the characteristic absorption assigned to and in situ hydrogenation reaction (Scheme 2). Upon
the triplet excited state (3
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
3[eosin Y]*. Subsequent electron transfer from
absorption bands appeared at 410 nm and 370 nm, re- state ACHTUNGRTENNUNG
+
spectively (Figure 1, b). Following the spectroscopic 1a to ACHTUNGTRENNUNG
+
and electrochemical studies in our previous work,[3b] generated amine radical cation 1aC releases an elec-
the new absorption band at 410 nm was attributed to tron and a proton to form the reactive intermediate
the characteristic absorption of the eosin Y radical iminium, which can effectively react with nucleophiles
anion ([eosin Y]CÀ), and the absorption band at to yield the desired cross-coupling product. The
370 nm was assigned to N-phenyltetrahydroisoquino- formed radical anion [eosin Y]CÀ delivers an electron
line radical cation 1aC+. On the other hand, nitroben- to Co
ACHTUNGTRENNUNG
+ [12]
eosin Y to generate [eosin Y]C . To figure out the Co
ACHTUNGTRENNUNG
main pathway under the reaction conditions, we Co(I) and the CoAHCTUNTGRENNUNG
added 1a to the solution containing eosin Y and 3a in situ hydrogenation of nitrobenzenes to complete
(Figure 1, c). The characteristic absorption of [eosin the catalytic cycle.
Y]CÀ at 410 nm emerged instead of the corresponding
In summary, an elegant cascade cross-coupling and
absorption band of [eosin Y]C+ at 460 nm. The results in situ hydrogenation reaction has been established by
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UPDATES
Jian-Ji Zhong et al.
Figure 1. Transient absorption spectra observed after laser excitation (lex =525 nm) of system containing: a) eosin Y (1.0ꢂ
10À5 M); b) eosin Y (1.0ꢂ10À5 M), 1a (3.0ꢂ10À3 M); c) eosin Y (1.0ꢂ10À5 M), 1a (3.0ꢂ10À3 M), 3a (1.0ꢂ10À3 M); d) eosin Y
ACTHNUTRGNEUNG
(1.0ꢂ10À5 M), 1a (3.0ꢂ10À3 M), 3a (1.0ꢂ10À3 M), Co(dmgH)2Cl2 (1.0ꢂ10À4 M).
Figure 2À.4UV-vis absorption spectra of a) the solution containing eosin Y (1.0ꢂ10À5 M), 1a (3.0ꢂ10À3 M), Co
(1.0ꢂ10À5 M), 1a (3.0ꢂ10À3 M), Co(dmgH)2Cl2 (1.0ꢂ10À4 M) with gradually addition of nitrobenzene 3a.
ACTHNUTRGNEUNG
ACHTUNGRTEN(NUNG dmgH)2Cl2
(1.0ꢂ10 M) before and after irradiation without addition of nitrobenzene; b) the irradiated solution containing eosin Y
visible light catalysis. With no use of any external oxi- reaction is accomplished under an inert atmosphere
À
dant and reductant, the desired oxidative cross-cou- by activation of C H bonds via oxidative cross-cou-
pling products and the reductive hydrogenation prod- pling and reduction via in situ hydrogenation. Spec-
ucts are achieved in good to excellent yields using troscopic studies and product analysis demonstrate
cheap and noble metal free eosin Y as photosensitizer the photoinduced electron transfer from tertiary
and molecule CoACHTUNGTRENNUNG
(dmgH)2Cl2 as catalyst. The cascade amine to eosin Y to generate [eosin Y]CÀ radical
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UPDATES
A Cascade Cross-Coupling and in Situ Hydrogenation Reaction
Scheme 2. A plausible mechanism.
anion, which delivers an electron to CoACTHNUGTRNEUNG(dmgH)2Cl2
Science Foundation of China (21390404, 21090343 and
91027041), the Chinese Academy of Sciences is gratefully ac-
knowledged.
for in situ reduction of nitrobenzenes to anilines. The
whole process is redox-neutral and represents a cheap-
er, safer, more step- and atom-economic transforma-
tion.
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General Procedure for the Visible Light-Induced
Cascade Cross-Coupling and in situ Hydrogenation
Reaction
The
tetrahydroisoquinoline
derivative
(0.35 mmol,
3.5 equiv.), indole derivative (1 mmol, 10 equiv.), nitroben-
zene derevative (0.1 mmol, 1 equiv.), eosin Y (0.001 mmol,
1 mol%), and CoACHTUNGTRENNUNG(dmgH)2Cl2 (0.01 mmol, 10 mol%) were
dissolved in a mixture solution of 4 mL H2O and 2 mL
CH3CN in a 15-mL reaction tube equipped with magnetic
stirring bar, the reaction tube was equipped with a rubber
septum and sealed, the resulting mixture solution was strict-
ly deaerated with N2 for 20 min, then the reaction tube was
irradiated by green LEDs for 36–48 h. After reaction, the
organic layer was extracted with ethyl acetate (3ꢂ5 mL).
The combined organic phases were washed with brine and
dried over sodium sulfate. The solvent was removed by
rotary evaporation. Then the yield of the aniline derivatives
was determined by GC-FID, and the yield of the desired
cross-coupling product was obtained after purification by
column chromatography on silica gel using hexane/ethyl
acetate (50:1) as eluent.
Acknowledgements
Financial support for this research from the Ministry of Sci-
ence
and
Technology
of
China
(2013CB834804,
2014CB239402 and 2013CB834505), the National Natural
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UPDATES
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Adv. Synth. Catal. 2014, 356, 2846 – 2852