Organic photoredox catalyzed C-H silylation of quinoxalinones or electron-deficient heteroarenes under ambient air conditions

Article abstract of DOI:10.1039/d0gc03697h

Direct C-H silylation of quinoxalinones was achieved by the combination of organic photoredox catalysis and hydrogen atom transfer (HAT) under ambient air conditions. Transition metal- and external oxidant-free conditions were the major features of this protocol. A series of silylated quinoxalinones with broad functional groups had been synthesized in moderate to high yields. This methodology was also applicable for the C-H silylation of some electron-deficient heteroarenes.

Full text of DOI:10.1039/d0gc03697h

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DOI: 10.1039/D0GC03697H
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Organic photoredox catalyzed C−H silylation of quinoxalinones or
electron-deficient heteroarenes under ambient air conditions
Received 00th November 2020,
Accepted 00th January 20xx
a
a
a
a
Changhui Dai, Yanling Zhan, Ping Liu* and Peipei Sun*
DOI: 10.1039/x0xx00000x
Direct C−H silylation of quinoxalinones was achieved by the bonds (Scheme 1d). Despite the significant advances such as the
combination of organic photoredox catalysis and hydrogen atom above examples have been made, several challenges, for instance,
transfer (HAT) under ambient air conditions. Transition metal- and reducing the amount of strong acids or bases and developing more
external oxidant-free conditions were the major features of this transition-metal-free conditions still remain for chemists.
protocol.
A series of silylated quinoxalinones with broad
functional groups had been synthesized in moderate to high
yields. This methodology was also applicable for the C−H silylation
of some electron-deficient heteroarenes.
Previous works:
Transition-metal-catalyzed cross-couplings of aryl halides or
aryl esters with hydrosilanes or disilanes.
a
X
[Si] resource
Pd] or [Pt]
There has been growing interest in biological activities evaluation of
organosilanes because of their high lipophilicity and other special
Het
Het
[Si]
[
X = Cl, Br, I or CO2R
1
properties. Organosilicon molecules also play important roles in
2
b
Friedel-Crafts C-H silylation of heteroarenes
the field of functional materials. It is well-known that the
3
H
heteroarene skeletons exist in a large number of marketed drugs,
Me3SiOTf, Et3N
Het
Het
Het SiMe3
Het SiR3
therefore the introduction of silicon motif to heteroarenes to access
heteroarylsilanes has continuously gained much attention in the
past decades. The conventional methodologies for the introduction
of silyl substituents to heteroarenes involved transition-metal-
H
3
R SiH
Catalyzed by Lewis acids
or BrØnsted acids
c
d
e
Strong base-catalyzed C-H silylation of heteroarenes
4
5
catalyzed cross couplings of aryl halides or aryl esters with
hydrosilanes or disilanes (Scheme 1a). The direct C−H silylation has
emerged as one of the most attractive alternatives without the
need of prefunctionalization of heteroarenes. A very early example
H
R SiH, KOtBu
3
Het
Het SiR3
or R SiBPin, KHMDS
3
Transition-metal-catalyzed C-H silylation of heteroarenes
involved the use of a silicon electrophile Me
Friedel-Crafts C−H silylation of heteroarenes (Scheme 1b). These
3
SiOTf to carry out
6
H
3
R SiH
Het
Het SiR3
7
transition-metal
reactions could be promoted effectively by Lewis acid or Brønsted
8
acid . In 2015, Grubbs and co-workers discovered that catalytic C–H
C-H silylation of heteroarenes involving radical processes
bond silylation of aromatic heterocycles could occur in the presence
H
3
R SiH
t
9a-b
of hydrosilanes and a strong base KO Bu (Scheme 1c). Moreover,
Het
Het SiR3
2
DTBP
a recent example about the site-selective sp C−H silylation of
or DTBP, [Cu]
(
poly)azines was demonstrated by the group of Martin using a
9
c
This works:
silicon reagent R
Transition-metal complexes based on Ir,
3
SiBPin and a strong base KHMDS (Scheme 1c).
R2
N
R2
1
0a-d
10e
10f-h
O
Rh, Ru
have also
R3
Si R1
R2
N
O
R1
R1
H
R3
R1
+
been proven to be effective in C−H/Si−H cross-coupling to form C−Si
4CzIPN (2.5 mol%)
N
Si
R2
N
quinuclidine (40 mol%)
pyridine, DMSO/CH3CN
Open air, rt
a. School of Chemistry and Materials Science, Jiangsu Provincial Key Laboratory of
Material Cycle Processes and Pollution Control, Jiangsu Collaborative Innovation
Center of Biomedical Functional Materials, Nanjing Normal University, Nanjing
H
blue LED, 24 h
Het
+
Si
Het Si
2
10023, People’s Republic of China. E-mail: sunpeipei@njnu.edu.cn,
pingliu@njnu.edu.cn
Scheme 1 Strategies for the direct silylation of heteroarenes
Electronic
Supplementary
Information
(ESI)
available.
See
DOI: 10.1039/x0xx00000x
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Silyl radicals, which could be generated from hydrosilanes in the
View Article Online
DOI: 10.1039/D0GC03697H
1
1
presence of peroxides, were used in the difunctionalization of
alkenes or radical addition cascade cyclization13 to synthesize
silylated products. However, the direct C−H silylation of
heteroarenes through a radical pathway was rarely reported.
Recently, DTBP or DTBP/copper salts was utilized to mediate the
C−H silylation of heteroaromatic compounds including furans,
thiophenes, coumarins, indole, pyridine, and quinoline under
heating conditions by the groups of Cai, Liu and Maruoka,
12
photocatalyst (2.5 mol%)
HAT catalyst (40 mol%)
N
N
O
N
N
O
+
H Si
base, solvent, air
blue LEDs, 24 h
Si
1a
2a
3a
N
NC
CN
N
N
1
4
respectively (Scheme 1e). In our recent works, a selective C-5
oxidative radical silylation of imidazopyridines was developed by
HAT catalyst A
N
iPr3SiSH
N
1
5
using DTBP as the oxidant. Visible-light-induced photoredox
catalysis of radical reactions has aroused much attention in the past
decade since these reactions can occur under very mild reaction
HAT catalyst B
4CzIPN
1
6
conditions. In 2019, Wang, Zhang and co-workers developed a
photocatalytic C–H silylation of heteroarenes by using
trialkylhydrosilanes as the silyl source in the presence of
Yieldb
(%)
Entry
1
Photocatalyst
Base (equiv.)
Solvent
.17 Both electron-deficient and -
Ir[dF(CF
3
)ppy]
none
(dtbbpy)PF
6
2
(dtbbpy)PF
6
K
2
CO
3
(1)
(1)
(1)
(1)
(1)
(1)
CH
CH
CH
CH
CH
CH
CH
CH
3
3
3
3
3
3
3
3
CN
CN
CN
CN
CN
CN
CN
CN
31
[
Ir(ppy) (dtbbpy)]PF and Na S O
2 6 2 2 8
rich heteroarenes were tolerated in this transformation. Organic
dyes are typically less expensive and less toxic, therefore have been
used as the attractive alternatives to transition-metal complexes in
2
3
K
2
K
2
K
2
K
2
CO
CO
CO
CO
3
3
3
3
0
0
Ir[ppy]
2
4
4CzIPN
4CzIPN
4CzIPN
4CzIPN
4CzIPN
4CzIPN
4CzIPN
4CzIPN
4CzIPN
47
0
1
8
photoredox catalysis. Organic photoredox catalysis in combination
with aerobic oxidation provides environmentally benign and
5c
6
1
9
efficient synthetic procedures using O
2
as the sole oxidant. As our
Cs
2
CO
3
32
41
65
73
0
continuing efforts on the direct C−H functionalization of
7
DBU (1)
1
5,20
heteroarenes,
herein, we disclose the C−H silylation of
8
pyridine (1)
pyridine (2)
pyridine (2)
pyridine (2)
pyridine (2)
quinoxalinones and some other electron-deficient heteroarenes by
the combination of organic photoredox catalysis and hydrogen
atom transfer under ambient air conditions.
Initially, we selected 1-methylquinoxalin-2(1H)-one (1a) and
tert-butyldimethylsilane (2a) as the model substrates to evaluate
the photochemical reaction (Table 1. For details, see ESI). Using
9
3
CH CN
1
0e
1f
12
CH
CH
3
CN
CN
1
3
0
DMSO/
CH CN
3:1, v/v)
77
3
Ir[dF(CF
the hydrogen atom transfer (HAT) catalyst and K
the model reaction was performed in CH CN under blue light
irradiation and open air. We were delighted to obtain the silylated
product 3-(tert-butyldimethylsilyl)-1-methylquinoxalin-2(1H)-one
3a) in 31% yield after 24 h (entry 1). The reaction could not be
3 2
)ppy] (dtbbpy)PF
6
as the photocatalyst, quinuclidine (A) as
(
2
CO as the base,
3
aReaction conditions: 1a (0.2 mmol, 1.0 equiv.), 2a (1.0 mmol, 5.0 equiv.),
photocatalyst (2.5 mol%), HAT catalyst (40 mol%), base and solvent (4 mL)
were stirred under irradiation (12 W blue LEDs) at room temperature under
open air, 24 h. bIsolated yield based on 1a. Triisoproplysilylthiol (B) was
3
c
(
d
e
f
used as the HAT catalyst. HAT catalyst A (20 mol%). Without light. Under
carried out in the absence of a photocatalyst (entry 2). In the
several photocatalysts we examined, the organic photoredox
catalyst 4CzIPN was the most appropriate one, providing 3a in 47%
yield (entries 1, 3 and 4). The presence of HAT catalyst is necessary
Ar (1 atm).
With the optimal reaction conditions in hand, we then
investigated the scope of 1-methylquinoxalin-2(1H)-ones 1, and the
results are summarized in Scheme 2. This reaction proceeded
smoothly with a wide range of substituted quinoxalin-2(1H)-ones by
employing tert-butyldimethylsilane as the reaction partner. The 1-
methylquinoxalin-2(1H)-ones substituted with electron-donating
2
1a
for the formation of silyl radicals in photochemical reaction.
When triisoproplysilylthiol (B) was used as the HAT catalyst instead
of quinuclidine, no desired product 3a was observed (entry 5). To
improve the reaction yield, other inorganic base Cs CO as well as
2 3
organic bases (1,8-diazabicyclo[5,4,0]undec-7-ene (DBU) and
pyridine were examined (entries 6−8). To our delight, the reaction
efficiency was obviously improved when pyridine was employed,
and the yield of 3a was further increased to 73% with 2 equiv.
pyridine to be added (entry 9). The results revealed that both light
(
Me) or electron-withdrawing group (F, Cl, Br, NO
and the corresponding products 3b−3h were obtained. However,
the 1-methylquinoxalin-2(1H)-ones bearing Br or NO (1c, 1d and
h) gave the products 3c, 3d and 3h in relatively low yields under
2
) were tolerated
2
1
the same conditions, which might be attributed to the poor
solubility and low conversion of the starting materials. In addition,
-(tert-butyldimethylsilyl)-1-methylbenzo[g]quinoxalin-2(1H)-one
3i) could also be obtained in 66% yield. A variety of quinoxalin-
(1H)-ones (1j−1u) with N-protected groups such as n-butyl,
irradiation and O
0 and 11). In addition, the mixed solvent of DMSO and CH
2
were necessary to promote the reaction (entries
CN with
1
3
3
(
2
the 3:1 volume ratio could increase the solubility of substrates and
give 3a in 77% yield (entry 12).
Table 1 Optimization of the reaction conditionsa
cyclopropylmethyl, ester group, allyl, propargyl, aryl, benzyl, etc.
2
| J. Name., 2012, 00, 1-3
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4
CzIPN (2.5 mol%)
were well compatible in the reaction, and gave the corresponding
products 3j−3u in 51–81% yields. It was worth mentioning that
unsaturated allylic and propargylic substrates were tolerated under
aerobic conditions, affording the desired products 3m and 3n in
H
quinuclidine (40 mol%)
View Article Online
Het
+
Si
Het Si
DOI: 10.1039/D0GC03697H
pyridine, DMSO/CH3CN
Open air, rt
blue LED, 24 h
4
2a
5
Cl
5
1% and 55% yields, respectively.
N
N
N
N
Cl
Si
S
N
N
Si
R2
N
Si
N
R2
Si
4
CzIPN (2.5 mol%)
N
O
H
quinuclidine (40 mol%)
O
Cl
R1
Si
R1
+
pyridine, DMSO/CH3CN
Open air, rt
N
Si
5a
, 51%a
5b, 55%
5c, 65%b
5d, 77%
N
1
2a
blue LED, 24 h
3
Si
N
N
O
N
O
b, R1 = Cl, 72%
c, R1 = Br, 47%
R1
3e, R1 = Me, 77%
Si
Si
N
O
Si
O
3
3
3
N
O
N
N
N
3f, R1 = F, 65%
3g, R1 = Cl, 64%
R1
N
Si
N
O
O
Si
d, R1 = NO2, 40% R1
N
Si
Cl
N
Cl
N
3
h, R1 = Br, 31%
5f, 44%
5g, 21%
a, 77%a
5ea, 54%
5eb, 5%
3
CN
CN
COOEt
Si
EtO
O
CN
CN
N
N
O
N
N
O
N
N
O
N
N
O
Si
N
Si
N
N
Si
Si
Si
Si
Si
Si
N
Si
CN
3i, 66%
3j, 71%
3k, 63%
3l, 73%
5h, 33%
5i, 65%
5j, 68%
5ka, 56%
5kb, 19%
O
O
N
O
N
N
O
N
N
N
Si
Si
Si
Scheme 3 Substrates scope of heteroarenes 4 and electron-
deficient arene 4i. Reaction conditions: 4 (0.2 mmol, 1.0 equiv.), 2a
3m, 51%
3n, 55%
3o, 67%
(
1.0 mmol, 5.0 equiv.), 4CzIPN (2.5 mol %), quinuclidine (40 mol%),
R2
R2
pyridine (0.4 mmol, 2.0 equiv.), DMSO/CH CN (3:1, (v/v), 4 mL)
were stirred under irradiation (12 W blue LEDs) at room
temperature under open air, 24 h. Isolated yield based on 4. 2a
(2.0 mmol, 10.0 equiv.), quinuclidine (80 mol%).
3
3
r, R2 = H, 77%
p, R2 = H, 71%
3q, R2 = Cl, 73%
3s, R2 = OMe, 71%
3
N
N
O
N
N
O
3t, R2 = CN, 68%
a
b
3
u, R2 = CF3, 81%
Si
Si
Finally, the procedure was extended successfully to other
Scheme 2 Substrates scope of 1-methylquinoxalin-2(1H)-ones 1. trialkylsilanes (Scheme 4). The results showed that aliphatic silanes,
Reaction conditions: 1 (0.2 mmol, 1.0 equiv.), 2a (1.0 mmol, 5.0 such as triethylsilane (2b), triisopropylsilane (2c) were compatible
equiv.), 4CzIPN (2.5 mol%), quinuclidine (40 mol%), pyridine (0.4 under ambient air conditions to yield the silylated products 6a and
mmol, 2.0 equiv.), DMSO/CH
3
CN (3:1, (v/v), 4 mL) were stirred 6b in 72% and 66% yields, respectively. However, a limitation was
under irradiation (12 W blue LEDs) at room temperature under that arylsilanes, such as triphenylsilane, diphenylsilane,
a
open air, 24 h. Isolated yield based on 1.
phenylsilane and methyldiphenylsilane were incompatible under
our reaction conditions and the substrate 1a could be recovered
To further extend the scope of this reaction, we texted the completely. From di-tert-butylsilane we also failed to obtain the
applicability of several other heteroaromatic substrates (Scheme 3). desired product.
The results showed that a variety of heterocycles, including
4
CzIPN (2.5 mol%)
quinoxalines (4a, 4b), benzothiazole (4c), pyridazine (4d),
imidazo[1,2-b]pyridazine (4e), coumarin (4f), benzooxazinone (4g),
ethyl isonicotinate (4h), quinoline-4-carbonitrile (4j) and
isoquinoline-4-carbonitrile (4k) could provide the corresponding
silylated products under the same reaction conditions. Moreover,
the electron-deficient 1,4-dicyanobenzene (4i) could also be
employed as a substrate and generated the product 5i in
satisfactory yield (65%).
R3
Si R1
R2
N
N
O
N
N
O
quinuclidine (40 mol%)
+
H
R3
R1
pyridine, DMSO/CH3CN
Open air, rt
Si
R2
blue LED, 24 h
6
1a
2
N
N
O
N
O
i
SiEt3
N
Si Pr3
6
a, 72%a
6b, 66%b
Scheme 4 Substrates scope of silanes 2. Reaction conditions: 1a (0.2
mmol, 1.0 equiv.), 2 (1.0 mmol, 5.0 equiv.), 4CzIPN (2.5 mol%),
quinuclidine (40 mol%), pyridine (0.4 mmol, 2.0 equiv.),
DMSO/CH
3
CN (3:1, (v/v), 4 mL) were stirred under irradiation (12 W
a
blue LEDs) at room temperature under open air, 24 h. Isolated
yield based on 1a. 2 (2.0 mmol, 10.0 equiv.), quinuclidine (80
b
mol%).
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Next, gram scale was performed to prove the applicability of
the reaction, and the product 3a was obtained in 63% yield (1.04 g,
N
N
O
N
O
View Article Online
TEMPO (3 equiv.)
SiH
DOI: 10.1039/D0GC03697H
N
+
+
(a)
standard conditions
N
Si
O
3
.8 mmol) under the irradiation of blue LEDs (Scheme 5a). In
addition, the product 3a could be used for a further transformation.
When 3a was treated with PhI(OAc) (1.5 equiv.) and Pd(OAc) (5
Si
8
1a
2a
3
a, trace
detected by LC-MS
2
2
mol %) in AcOH (1 mL) at 100 °C for 24 h, followed by the addition
of water (1 mL) and heating at 100 °C for another 24 h, 1-methyl-
N
N
O
O
N
N
O
1
,1-diphenylethylene (3 equiv.)
SiH
+
(b)
standard conditions
Si
1
,4-dihydroquinoxaline-2,3-dione (7) was obtained in 53% yield
(Scheme 5b).
1
a
a
2a
3a, trace
4CzIPN (2.5 mol%)
N
N
DABCO (1 equiv.)
N
N
O
quinuclidine (40 mol%)
pyridine (12 mmol)
N
N
O
SiH
N
N
O
+
(c)
SiH
standard conditions
+
(a)
Si
CH3CN (30 mL)
Open Air, rt
Si
1
2a
3a, N.D.
2
30W blue LEDs, 60 h
1a (6 mmol)
2a (30 mmol)
3a, 63%, 1.04 g
(2) Kinetic isotope effect experiment
1
) PhI(OAc)2 (1.5 equiv.)
Pd(OAc)2 (5 mol%)
AcOH, 100 oC, 24 h
N
N
O
N
N
O
N
N
O
N
O
O
i
(b)
+
H Si Pr3
i
Si
Si Pr
3
) H2O (1 mL), 100 oC, 24 h
N
H
2
4CzIPN (2.5 mol%)
quinuclidine (80 mol%)
1
a, 0.2 mmol
2.0 mmol
6b, 25.3%
(d)
3
a
7, 53%
pyridine, DMSO/CH3CN
Open air, rt
N
N
O
N
N
O
Scheme 5 Scaling-up synthesis and further transformation of the
product 1a.
i
+
D
Si Pr3
blue LED, 4 h
i
Si Pr3
kH/kD = 1.27
1
a, 0.2 mmol
2.0 mmol
6b, 19.9%
To understand the detail of this C–H silylation reaction, several
control experiments were conducted. The reaction was obviously
suppressed in the presence of TEMPO (2,2,6,6-tetramethyl-1-
piperidinyloxy, 3 equiv.) under the standard conditions, and
meanwhile, the adduct 8 was detected by LC-MS (Scheme 6-a).
When 1,1-diphenylethylene (3 equiv.) was added, the reaction was
also inhibited (Scheme 6-b). These results revealed that the silyl
radical might be a pivotal intermediate in this transformation. The
reaction was completely inhibited by the addition of a singlet
oxygen quencher such as 1,4-diazabicyclo[2.2.2]octane (DABCO),
indicating that singlet oxygen probably played a role in this catalytic
(
3) Stern–Volmer experiment
e)
(
1
0000
000
9
HAT = 0 M
HAT = 0.001 M
8000
7000
6000
HAT = 0.002 M
HAT = 0.004 M
HAT = 0.006 M
HAT = 0.008 M
5
4
3
000
000
000
Quenching
2
1c
2000
H D
cycle (Scheme 6-c). The value of k /k (1.27) from two parallel
1
000
0
reactions of 1a with 2c and 2c-d respectively showed that the Si–H
bond cleavage might not be the kinetically rate-determining step in
this reaction (Scheme 6-d). The results of the Stern−Volmer
experiment indicated that the luminescence of 4CzIPN could be
gradually quenched with the increase of the concentration of
quinuclidine (Scheme 6-e and f, see ESI for details).
500
600
700
Wavelength (nm)
(
f)
3
y=187.3x+1.111
2
2
.5
2
R =0.9833
(1) Radical-trapping experiment
1
.5
1
0
0.002
0.004
0.006
0.008
Concentration of HAT [M]
Scheme 6 Control experiments. (a) Radical-trapping experiment
with TEMPO. (b) Radical-trapping experiment with 1,1-
diphenylethylene. (c) The addition of a singlet oxygen quencher
DABCO. (d) Control experiments about kinetic isotope effects. (e)
Emission spectra of 4CzIPN at different concentration of
4
| J. Name., 2012, 00, 1-3
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quinuclidine. (f) Stern-Volmer plot of 4CzIPN at different
concentrations of quinuclidine.
Acknowledgements
View Article Online
DOI: 10.1039/D0GC03697H
This work was supported by the National Natural Science
Foundation of China (Project 21672104, 21502097) and the Priority
Academic Program Development of Jiangsu Higher Education
Institutions.
Based on the above observations as well as some previous
literature reports, a possible mechanism of this reaction was
proposed (Scheme 7). Initially, the photocatalyst 4CzIPN was
excited into 4CzIPN* under the irradiation of blue LEDs. A single
electron was transferred from quinuclidine to 4CzIPN*, resulting in
Notes and references
•
−
the generation of a quinuclidinium radical cation and a 4CzIPN
2
1a-b
1
3
1
For selected reviews, please see: (a) P. K. Pooni and G. A.
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radical anion.
Meanwhile,
O
2
generated from
2
O with
2
1c
activated 4CzIPN* under light radiation. Subsequently, another
1
single-electron-transfer (SET) process occurred between O
2
and
•
−
•−
4
CzIPN , giving O
2
and regenerating 4CzIPN for the next
Then, the resulting quinuclidinium radical
3
779−3798.
2
1d-e
photoredox cycle.
2
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011, 40, 789–800; (b) T. A. Su, H. Li, R. S. Klausen, N. T. Kim,
cation served as the HAT catalyst to produce the t-
butyldimethylsilyl radical by abstracting the hydrogen atom from t-
butyldimethylsilane. After that, the addition of t-butyldimethylsilyl
radical to 1a formed the intermediate I. Finally, the intermediate I
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088−1095.
3
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R. D. Taylor, M. MacCoss and A. D. G. Lawson, J. Med. Chem.,
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•−
underwent direct hydrogen atom transfer to O
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to give the
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3
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*
4
CzIPN
4CzIPN
3O2
5
1O2
SET
4
CzIPN
6
7
U. Frick and G. Simchen, Synthesis, 1984, 929–930.
For selected examples and review, please see: (a) H. T. Klare,
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O2
4
CzIPN
*
CzIPN
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HO2-
3
663−3666; (c) Q. Yin, H. F. T. Klare and M. Oestreich,
SET
N
N
O
N
N
O
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Py
N
Si
Si
N
7
399−7407; (e) S. Bähr and M. Oestreich, Angew. Chem., Int.
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Si
N
PyH
8
9
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H
N
N
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SiH
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7
2
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the C−H silylation of quinoxalinones by using a combination of
organic photocatalyst 4CzIPN and hydrogen atom transfer reagent
quinuclidine under ambient conditions. In this transition-metal-free
process, the silyl radicals generated in situ regioselectively added to
quinoxalinones or a series of electron-deficient heteroarenes with
high functional group tolerance. This methodology reported herein
offered an environmentally friendly way for the synthesis of
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Conflicts of interest
There are no conflicts to declare.
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