Visible-Light-Driven Reductive Carboarylation of Styrenes with CO2 and Aryl Halides

Article abstract of DOI:10.1021/jacs.0c03144

The first example of visible-light-driven reductive carboarylation of styrenes with CO₂ and aryl halides in a regioselective manner has been achieved. A broad range of aryl iodides and bromides were compatible with this reaction. Moreover, pyridyl halides, alkyl halides, and even aryl chlorides were also viable with this method. These findings may stimulate the exploration of novel visible-light-driven Meerwein arylation-addition reactions with user-friendly aryl halides as the radical sources and the photocatalytic utilization of CO₂

Full text of DOI:10.1021/jacs.0c03144

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Communication
Visible-Light-Driven Reductive Carboarylation
2
of Styrenes with CO and Aryl Halides
Hao Wang, Yuzhen Gao, Chunlin Zhou, and Gang Li
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.0c03144 • Publication Date (Web): 20 Apr 2020
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Visible-Light-Driven Reductive Carboarylation of Styrenes with CO2
and Aryl Halides
Hao Wang,†,‡ Yuzhen Gao,† Chunlin Zhou,†,‡ and Gang Li*,†,‡
10
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†Key Laboratory of Coal to Ethylene Glycol and Its Related Technology, State Key Laboratory of Structural Chemistry, Center for Excel-
lence in Molecular Synthesis, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian,
350002, China
‡Fujian College, University of Chinese Academy of Sciences, Beijing, 100049, China
Supporting Information Placeholder
Scheme 1. Visible-light-driven Difunctionalization of Al-
kenes
ABSTRACT: The first example of visible-light-driven reductive
carboarylation of styrenes with CO2 and aryl halides in a
regioselective manner has been achieved. A broad range of
aryl iodides and bromides were compatible with this reaction.
Moreover, pyridyl halides, alkyl halides and even aryl chlorides
were also viable with this method. These findings may stimulate
the exploration of novel visible-light-driven Meerwein arylation-
addition reactions with user-friendly aryl halides as the radical
sources and the photocatalytic utilization of CO2.
(a) visible-light-driven Meerwein-arylation type alkene difunctionalization
previous works
X
R1
Meerwein-arylation type
difunctionalization
X = N2BF4, IArBF4
IArPF6, SO2Cl, SAr2OTf
,
visible-light-driven
radical generation
Ph
R
R
this work
X
X = I, Br or Cl
(b) visible-light-driven alkene carboxylative difunctionalization with CO2 (previous works)
photocatalyst, blue LEDs
radical precursors
R2
CO2
H
R1
Visible-light-driven photoredox catalysis (PRC) has proven to
be a powerful method to access complex organic molecules
under mild reaction conditions.1 Difunctionalization of alkenes is a
fruitful research filed that is valuable for building molecular com-
plexity in a highly efficient manner, which could be achieved
through PRC processes besides transition-metal-catalyzed or other
radical-mediated pathways.2-6 Particularly, the visible-light-driven
Meerwein-arylation-type difunctionalization of alkenes, which
generally utilizes aryl radical sources from aryl diazonium salts, di-
aryliodonium salts, arylsulfonyl chlorides and so on, has recently
received much attention (Scheme 1a) and was pioneered by Kö-
nig.1a,2,3 However, aryl halides, which are bench-stable, inexpen-
sive and widely available, have not been successfully employed in
this type of reaction to date.1a,2,3 One of the challenges could be it
is difficult to generate highly reactive aryl radicals from aryl hal-
ides with visible-light since much higher negative reduction poten-
tial is required for aryl halides than aryl diazonium salts, diarylio-
donium salts or arylsulfonyl chlorides.1a,2,7-10
In recent years, the photocatalytic utilization of carbon dioxide
(CO2), which is an ideal one-carbon (C1) building block and sus-
tainable, abundant, low-cost and nontoxic as well, has been a re-
search topic of great interest in fine chemical synthesis.11-14 Nota-
bly, a limited number of redox-neutral photocatalytic difunctional-
izations of alkenes with CO2 were achieved to provide rapid access
to carboxylic acids that are important motifs in a number of biolog-
ically active molecules (Scheme 1b).11 Consequently, visible-light-
induced regioselective carbocarboxylation, silacarboxyaltion,
phosphonocarboxylation as well as thiocarboxylation of alkenes
have been developed with CO2 and various radical precursors by
the groups of Martin,11a Yu11b,d and Wu11c. However, these redox-
neutral protocols should not be applicable for developing three-
component carboarylation of alkenes with CO2 and aryl halides,
since a novel highly-reducing protocol is probably necessary for
utilizing aryl halides in such type of visible-light-driven reaction.
Due to this challenge, the Meerwein-arylation-type carboarylation
R2
R
: (fluorinated) alkyl, silyl, or phosphonyl
R
CO2
R1
R2
[Fe], blue LEDs
ArSH, KOtBu
SAr
R1
CO2H
(c) visible-light-driven alkene carboarylation using aryl halides with CO2 (this work)
[Ir], blue LEDs
R
R
HAT cat., reductant, rt
CO2H
Ar1
+
CO2
Ar1
ArX
(x = I, Br, Cl)
Ar
(1 atm)
user-friendly aryl halides
photocatalytic CO2 utilization
broad scope & mild conditions
heteroaryl halide & alkyl halides compatible
of alkenes with CO2 via visible-light PRC has not been realized so
far.15,16 Inspired by previous studies by König,2a,10b Stephenson,7a
Jui,7h,i and other groups,7-10 herein, we report the development of
the first visible-light-driven reductive carboarylation of styrenes
with CO2 and aryl halides in a highly regioselective manner using
the readily available and low-cost HCO2K as the terminal reductant,
leading to the rapid access to valuable hydrocinnamic acid deriva-
tives.17 Notably, the scope of aryl halides is very broad, and pyridyl
halides, alkyl halides and even aryl chlorides are also compatible
with this reaction (Scheme 1c).
To start our investigation, 1,1-diphenylethylene was employed
as the model substrate, which reacted with iodobenzene under 30
W blue LEDs irradiation in the presence of commercially available
[Ir(ppy)2(dtbbpy)]PF6 photocatalyst (PC) and an atmospheric pres-
sure of CO2 at ambient temperature (Table 1, see Supporting Infor-
mation (SI) for photocatalyst optimization).7b After extensive in-
vestigation of the reaction conditions, the desired carboarylation
product 1 was produced regioselectively in a 78% isolated yield in
the presence of the hydrogen atom transfer (HAT) catalyst DABCO
with HCO2K as the terminal reductant and K2CO3 as the base in
DMSO (entry 1). It should be mentioned that for the ease of analy-
sis and product isolation, the original carboxylic acid product was
converted to a methyl ester. Importantly, no product was detected
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Table 1. Optimization of Reaction Conditionsa
in the absence of either a photocatalyst or light, demonstrating the
reaction was induced by light (entries 2 and 3). Slight decrease in
the yield was observed when the reaction was carried out under ni-
trogen atmosphere (entry 4), suggesting that CO2 could be also
formed from oxidation of HCO2K or possibly the acidification of
K2CO3.16j,18a Notably, not any product can be detected without add-
ing HCO2K, which was the vital terminal reductant (entry 5, see SI
for other reductants tested).7a,i,18b-d And only a trace of product was
formed with 2.5 equivalents of DABCO but without HCO2K (entry
6). However, DABCO was also crucial for the reaction (entry 7),
and it acted better than other amine HAT catalysts (entries 8-10).
These results might suggest the combination of HCO2K/DABCO
was an effective electron donor for this reductive carboarylation.
Moreover, it was found that K2CO3 was only slightly beneficial for
the reaction (entry 11), and other bases such as Na2CO3 and Cs2CO3
were equally effective (entries 12 and 13). Finally, solvents were
also evaluated, and DMSO was found to be the best out of a variety
of solvents such as DMF and DMA (entries 14 and 15).
With the optimized reaction conditions in hand, the generality of
this carboarylation was investigated with a variety of aryl iodides
and bromides as well as some representative alkyl halides with 1,1-
diphenylethylene (Table 2). First, cheaper bromobenzene was only
slightly less effective than iodobenzene (1). Notably, a large num-
ber of substituents that are electron-donating such as methyl, meth-
oxy and methylthio groups, or electron-withdrawing such as fluoro,
choloro, boronate, trifluoromethyl, carbonyl and cyano groups
were tolerated in this reaction without much difference in reactivity
(2-33), delivering the target products in generally good yields.
However, aryl iodides worked better than their bromide counter-
parts for some substrates with electron-donating groups such as
4
5
6
7
8
9
Entry
Deviation from standard conditions
Yield [%]b
1
none
82 (78)c
N.D.
N.D.
76
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
2
without [Ir(ppy)2(dtbbpy)]PF6
in the dark
3
4
under a nitrogen atmosphere
without HCO2K
5
N.D.
2
6
DABCO (2.5 equiv), without HCO2K
without DABCO
7
3
8
quinuclidine instead of DABCO
quinuclidin-3-yl acetate instead of DABCO
trisobutylamine instead of DABCO
without K2CO3
33
9
6
10
11
12
13
14
15
64
76
Na2CO3 instead of K2CO3
Cs2CO3 instead of K2CO3
DMF instead of DMSO
DMA instead of DMSO
82
82
21
13
aReaction conditions: 1,1-diphenylethylene (0.2 mmol), iodobenzene (0.4
mmol), [Ir(ppy)2(dtbbpy)]PF6 (2 mol %), DABCO (0.1 mmol), K2CO3 (0.5
mmol), HCO2K (0.4 mmol), DMSO (2 mL), 1 atm CO2, 30 W blue LEDs,
rt, 24 h; then MeI (2.0 mmol), K2CO3 (1.0 mmol), acetone (10 mL), 70 oC,
2 h. bYield was determined by 1H NMR with CH2Br2 as internal standard.
cYield of isolated products in parentheses. DABCO = triethylenediamine;
N.D. = not detected.
Table 2. Scope of of Aryl Halides and Alkyl Halidesa
[Ir(ppy)2(dtbbpy)]PF6 (2 mol %)
DABCO (50 mol %)
K2CO3 (2.5 equiv)
Ph
Ph
MeI, K2CO3
CO2Me
R
R-X
+
+
CO2
Ph
Ph
(1 equiv)
HCO2K (2 equiv)
DMSO (0.1 M)
blue LEDs, rt, 24 h
R = Aryl, Alkyl
X = I , Br
(2 equiv)
(1 atm)
1-39
Ph
Ph
Ph
Ph
CO2Me
Ph
CO2Me
Ph
Ph
Ph
Ph
CO2Me
CO2Me
CO2Me
CO2Me
CO2Me
Me
CO2Me
Ph
Ph
Ph
Ph
Ph
Ph
Ph
OMe
Me
OMe
SMe
Me
OMe
1, X = I, 78%
X = Br, 68%
2, X = I, 72%
X = Br, 64%b
3, X = I, 71%
X = Br, 73%b
4, X = I, 76%
X = Br, 67%b
5, X = I, 56%
6, X = I, 71%
7, X = I, 67%
8, X= Br, 66%
X = Br, 68%
Ph
Ph
Ph
Ph
CO2Me
CO2Me
Ph
CO2Me
Ph
Ph
Ph
CO2Me
CO2Me
CO2Me
CO2Me
CO2Me
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
F
Cl
OCF3
10, X = I, 67%
F
Cl
OCF3
SMe
F
15, X = I, 56%c
9, X = I, 66%
11, X = I, 65%
12, X = I, 58%
13, X = I, 67%
X = Br, 77%
14, X = I, 70%
16, X = I, 65%
X = Br, 73%
X = Br,73%
X = Br, 67%
X = Br,60%
X = Br, 64%
Ph
Ph
Ph
Ph
Ph
CO2Me
Ph
CO2Me
Ph
CO2Me
CO2Me
CO2Me
Ph
CO2Me
CO2Me
CO2Me
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
CF3
CF3
CO2Me
Bpin
CF3
CO2Me
SO2Me
Cl
20, X = I, 61%c
21, X = I, 68%c
24, X = Br, 64%c
17, X = I, 67%
X = Br, 62%
18, X = Br, 55%
19 X = Br, 69%
22, X = I, 64%
23, X = I, 66%
X = Br, 70%
X = Br, 77%
X = Br, 70%
X = Br, 70%
Ph
Ph
Ph
Ph
Ph
CO2Me
CO2Me
Ph
CO2Me
Ph
Ph
CO2Me
CO2Me
CO2Me
CO2Me
CO2Me
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
CN
F3
C
CF3
COMe
CN
Me
Me
COMe
COPh
CN
26, X = I, 60%c
32, X = Br, 50%c
25, X = Br, 68%
27, X = Br, 53%
28, X = Br, 60%
29, X = I, 58%
X = Br, 67%
30, X = I, 65%
31, X = I, 72%
X = Br, 68%
X = Br, 66%
Ph
Ph
Ph
CO2Me
CO2Me
Ph
Ph
Ph
Ph
CO2Me
CO2Me
CO2Me
CO2Me
CO2Me
Ph
Ph
Ph
Ph
Ph
Ph
Ph
N
O
Boc
Ph
33, X = I, 67%
34, X = I, 63%
35, X = Br, 50%
36, X = I, 59%
37, X = Br, 64%
38, X = Br, 72%
39, X = Br, 65%
X = Br, 71%
aReaction conditions: standard conditions (Table 1, entry 1). b48h. cTrisobutylamine (50 mol %) was used instead of DABCO.
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Table 4. Scope of Heterocyclic Substratesa
halides for products 2-5. Similarly, aryl bromides worked much
better than their iodide counterparts for some substrates with elec-
tron-withdrawing groups such as bromides for products 25, 28 and
32. Moreover, ortho-substituted aryl halides generally gave lower
yields than their meta- and para-substituted counterparts such as 5-
7, 12-14 and 28-30. In addition, iodo- and bromonaphthalenes were
also viable in this reaction (34 and 35). It is noteworthy that the aryl
iodides/bromides bearing a chloro or boronate group were feasible
for our carboarylation (15-18), leaving the opportunity for further
derivatization. Stimulated by the success in using aryl halides, we
examined the feasibility of alkyl halides. Pleasingly, representative
secondary alkyl halides (36-38) proceeded smoothly under the op-
timized conditions. Finally, tertiary bromide (39) could also be em-
ployed in this method. However, primary halides did not work well
at present.
The scope of styrenes was studied subsequently (Table 3). The
α-substituted styrenes including α-methyl styrenes (62-65) were
excellent substrates (40-65), regardless of the electronic nature and
the position of substituents on the aromatic ring. Notably, hindered
α,-disubstituted styrene (66) also delivered the expected products
in a moderate yield. However, α,-nonsubstituted styrene deriva-
tives were less effective, leading to generally moderate yields of
products with electron-deficient substrates (67-74). Notably, syn-
thetically useful boronate group (70) was also tolerated. However,
aliphatic alkenes and cyclic styrenes such as 1-cyclopentenylben-
zene were not compatible with this method at this stage.
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
aReaction conditions: standard conditions (Table 1, entry 1); methylation
conditions (75 used standard conditions): SOCl2 (0.4 mL), MeOH (4 mL),
o
b
100 C, 6 h, see SI for details. The methyl ester of 78 was transferred to
acid 78 while isolating with silical gel.
Table 5. Scope of Aryl Chloridesa
Table 3. Scope of Styrenesa
CO2 (1 atm)
[Ir(ppy)2(dtbbpy)]PF6 (2 mol %)
DABCO (50 mol %)
CO2Me
R1
R2
R1/H
K2CO3 (2.5 equiv)
MeI, K2CO3
R3
Ph
40-74
R3/H
Ph-I
+
R2
(1 equiv)
HCO2K (2 equiv)
DMSO (0.1 M)
blue LEDs, rt, 24 h
(2 equiv)
(a) -substituted &
-disubstituted styrenes
R
Ph
Ph
Ph
CO2Me
F
Ph
CO2Me
CO2Me
CO2Me
R
Ph
Ph
Ph
Ph
R
F
40, R = OMe, 68%
41, R = Me, 66%
42, R = F, 71%
43, R = Cl, 67%
44, R = OMe, 73%
45, R = Me, 69%
46, R = F, 66%
51, R = OMe, 76%
52, R = SMe, 70%
53, R = Me, 71%
54, R = F, 72%
55, R = Cl, 72%
56, R = Ph, 71%
57, 71%
47, R = Cl, 70%
48, R = CF3, 70%
49, R = CO2Me, 67%
50, R = CN, 66%
aReaction conditions: standard conditions (Table 1, entry 1) in 48 h. b4 equiv
R
c
aryl chloride were employed. methylation conditions: SOCl2 (0.4 mL),
Ph
Me
CO2Me
Me
Ph
Ph
CO2Me
CO2Me
MeOH (4 mL), 100 oC, 6 h, see SI for details.
CO2Me
Ph
R
Ph
Ph
Ph
viability of aryl chlorides that are generally much less reactive in
visible-light driven photocatalytic reactions (Table 5).7i,9,10 Re-
markably, the carboarylation could be successfully extended to sev-
eral electron-deficient (hetero)aryl chlorides that are often much
cheaper than corresponding bromides and iodides (Table 5).
Preliminary studies were conducted to provide some insight into
the mechanism of the reaction. Stern-Volmer luminescence studies
(see SI) showed the light-activated Ir catalyst (PC*) was quenched
by DABCO (퐸ꢀꢂꢁ = + 0.69 V vs. SCE in MeCN) rather than HCO2K
and 1,1-diphenylethylene (see SI Figures S1-S4).19 The control ex-
periments without using aryl halides produced carboxylation prod-
ucts 83 and 84, suggesting that a CO2 radical anion might be gen-
erated under the reaction conditions (Scheme 2a, 2b), though fur-
ther studies are needed.13c,20 When a “radical clock” substrate was
used, ring opening product 85 was afforded, indicating a benzyl
radical might be involved (Scheme 2c). The isotope-labeling study
and the investigation of using aldehyde as the electrophile (Scheme
2d, 2e) suggested indirectly the possible presence of a benzylic an-
ion intermediate.11a,c,21 To determine the carboxyl source of the
product, 13CO2 (99% 13C) gas was employed and 74% 13C incorpo-
ration was found in the carboxyl of the product
61, 66%
62, R = 3-Cl, 68%
63, R = 4-Cl, 59%
64, R = 4-CF3, 61%
65, R = 4-Ph, 71%
66, 56%
R
58, R = OMe, 66%
59, R = Me, 68%
60, R = F, 75%
(b)
-nonsubstituted styrenes
F
F
CO2Me
CO2Me
R
CO2Me
NC
CO2Me
Ph
F
Ph
Ph
F
F
Ph
R
67, R = Cl, 52%
68, R = CN, 51%
69, 47%
70, R = Bpin, 56%
71, R = CF3, 54%
72, R = CN, 55%
73, R = Ph, 55%
74, 50%
aReaction conditions: standard conditions (Table 1, entry 1).
In light of these results, we turned our attention to test heterocy-
clic substrates, especially the challenging electron-deficient pyridyl
halides. As shown in Table 4, alkenes with a thiofuryl or pyridyl
group could afford the desire products (75 and 76) in modest yields.
Remarkably, this reaction also proceeded efficiently with several
pyridyl halides to give corresponding products (77-81) in generally
good yields. In addition, unambiguous proof of the structure of 78
was achieved by single-crystal X-ray analysis.
To demonstrate the potential of this reaction, we tested the
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regioselective addition to alkene to afford a benzyl radical (F). Sin-
Scheme 2. Mechanistic Studies and Scaled-up Reaction.
gle-electron transfer (SET) from the reduced photocatalyst B (Ered
[IrIII/IrII] = –1.51 V vs. SCE in MeCN) to benzyl radical F (for ben-
(a) Control experiment without using aryl halide:
[Ir(ppy)2(dtbbpy)]PF6 (2 mol %)
zyl radical of 1,1-diphenyl ethane, Ered = –1.34 V vs. SCE in MeCN)
produces benzyl anion intermediate G.11a,c,23, Then G undergoes
nucleophilic addition to CO2 to produce the carboxylate H which
is then converted to methyl ester after methylation. It should be
noted that it cannot be ruled out at present that the benzyl radical
intermediate may react reversibly with CO2 followed by SET from
the reduced PC B to the carboxyl radical intermediate, affording
the carboxylate H. Further mechanistic studies to provide more in-
sight into the mechanism are underway in our laboratory.
In conclusion, we have developed the first effective reductive
protocol of regioselective visible-light-driven carboarylation of
styrenes with CO2 and aryl halides, leading to an efficient method
for producing valuable hydrocinnamic acid derivatives. Notably,
this highly-reducing protocol allows the use of a wide range of aryl
halides, including electron-deficient and low-cost aryl chlorides.
Moreover, pyridyl halides as well as alkyl halides were also viable
with this method. These findings may open up a new opportunity
for exploring novel visible-light-driven Meerwein type arylation-
addition reactions employing user-friendly aryl halides as the radi-
cal sources, as well as developing new photocatalytic utilization re-
actions of CO2. Further exploration of this discovery is under way
in our laboratory and will be reported in due course.
Ph
DABCO (50 mol %)
K2CO3 (2.5 equiv)
CO2Me
CO2Me
Ph
MeI, K2CO3
Ph
+
CO2
Ph
HCO2K (2 equiv), DMSO (0.1 M)
blue LEDs, rt, 24 h
(1 atm)
83, 10%
(b) Control experiment without using aryl halide in the presence of water:
[Ir(ppy)2(dtbbpy)]PF6 (1 mol %)
DABCO (50 mol %)
Ph
H
Ph
MeI, K2CO3
K2CO3 (2.5 equiv)
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Ph
H2
O
+
CO2 +
CO2Me
Ph
HCO2K (2 equiv),DMSO (0.1 M)
blue LEDs, rt, 24 h
(1 atm) (10 equiv)
84, 24%
Ph
(c) Reaction with radical clock:
[Ir(ppy)2(dtbbpy)]PF6 (2 mol %)
MeO2C
DABCO (50 mol %)
K2CO3 (2.5 equiv)
Ph
MeI, K2CO3
PhI
CO2
+
+
Ph
Ph
HCO2K (2 equiv)
DMSO (0.1 M)
(2 equiv)
(1 atm)
Ph
blue LEDs, rt, 24 h
85, 49% (Z/E = 5.9/1)
(d) Isotope-labelling study:
[Ir(ppy)2(dtbbpy)]PF6 (2 mol %)
DABCO (50 mol %), K2CO3 (2.5 equiv)
Ph
D
Ph
Ph
PhI
+
+
D2
O
Ph
Ph
(2 equiv)
(20 equiv)
HCO2K (2 equiv), DMSO (0.1 M)
blue LEDs, rt, N2, 24 h
86, 82% (75% D)
(e) Investigation of using aldehyde as the electrophile:
[Ir(ppy)2(dtbbpy)]PF6 (2 mol %)
O
DABCO (50 mol %)
K2CO3 (2.5 equiv)
OH
Ph
H
Ph
PhI
+
+
Ph
HCO2K (2 equiv), DMSO (0.1 M)
blue LEDs, rt, N2, 24 h
Ph Ph
F
F
(2 equiv)
(8 equiv)
87, 73%
(f) Reaction with 13CO2
:
[Ir(ppy)2(dtbbpy)]PF6 (2 mol %)
DABCO (50 mol %)
Ph
13CO2Me
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the ACS
Publications website.
Ph
K2CO3 (2.5 equiv)
MeI, K2CO3
Ph
PhI
+
+
13CO2
Ph
Ph
HCO2K (2 equiv), DMSO (0.1 M)
blue LEDs, rt, 48 h
(2 equiv) (1 atm)
1-13C, 80% (74%, 13C)
(g) Gram-scale synthesis:
[Ir(ppy)2(dtbbpy)]PF6 (2 mol %)
DABCO (50 mol %)
Ph
CO2Me
Ph
K2CO3 (2.5 equiv)
MeI, K2CO3
Ph
CO2
+
PhI
Detailed experimental procedures and characterization data for new
compounds (PDF)
+
Ph
(5 mmol) (2 equiv) (1 atm)
Ph
HCO2K (2 equiv), DMSO (0.1 M)
blue LEDs, rt, 24 h
1, 69%, 1.09 g
AUTHOR INFORMATION
Scheme 3. Proposed Catalytic Cycle.
Corresponding Author
*gangli@fjirsm.ac.cn
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
We gratefully thank the financial supports from NSFC (Grant Nos.
21871257, 21801240), the Natural Science Foundation of Fujian
Province (2017J06007), the Strategic Priority Research Program of
the Chinese Academy of Sciences (Grant No. XDB20000000), and
the 100 Talents Program of Fujian Province. We also thank profes-
sor Armido Studer at Westfälische Wilhelms-Universität Münster
for valuable suggestions.
(Scheme 2f). Moreover, only slight decarboxylation was observed
by subjecting potassium salt of 1 under the standard reaction con-
ditions in the presence or absence of CO2 (see SI). Finally, the re-
action could be easily scaled up to 5 mmol without significant de-
crease in the yield of 1 (Scheme 2g).
A tentative mechanism was proposed based on the above mech-
anistic studies (Scheme 3). Upon blue light irradiation, the excited
PC* (A) is produced and subsequently quenched reductively by
DABCO to give reduced PC (B) and radical cation of DABCO (C).
The radical cation C promotes a hydrogen atom transfer from
HCO2K to give a CO2 radical anion, together with cation D. The
CO2 radical anion possesses high reduction potential (E0 = –2.21 V
vs. SCE in DMF), which is typically supplemented by an overpo-
tential of 0.1–0.6 V.22 Thus, aryl halides can be reduced by the CO2
radical anion to afford aryl radical E, which undergoes
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