Metal-free, room-temperature, radical alkoxycarbonylation of aryldiazonium salts through visible-light photoredox catalysis

Article abstract of DOI:10.1002/anie.201408837

The first radical alkoxycarboxylation of aryldiazonium salts using CO gas through visible-light-induced photoredox catalysis (16 W blue LEDs) has been developed. This reaction is entirely metal-free, is carried out at room temperature with a low loading of an organic dye as a photocatalyst (0.5 mol%), and provides a wide range of arylcarboxylic acid esters in high yields. Importantly, this photocatalytic system can be successfully extended to other carboxylation reactions.

Full text of DOI:10.1002/anie.201408837

Angewandte
Chemie
DOI: 10.1002/anie.201408837
Synthetic Methods
Metal-Free, Room-Temperature, Radical Alkoxycarbonylation of
Aryldiazonium Salts through Visible-Light Photoredox Catalysis**
Wei Guo, Liang-Qiu Lu,* Yue Wang, Ya-Ni Wang, Jia-Rong Chen, and Wen-Jing Xiao*
Abstract: The first radical alkoxycarboxylation of aryldiazo-
nium salts using CO gas through visible-light-induced photo-
redox catalysis (16 W blue LEDs) has been developed. This
reaction is entirely metal-free, is carried out at room temper-
ature with a low loading of an organic dye as a photocatalyst
(0.5 mol%), and provides a wide range of arylcarboxylic acid
esters in high yields. Importantly, this photocatalytic system can
be successfully extended to other carboxylation reactions.
ology with mechanistic insights into the role of acyl radicals.
Not only have they significantly expanded the scope of
organic halides which can react with palladium and manga-
nese catalysts, but they have also identified a reasonable
reaction mechanism of these metal-catalyzed radical alkoxy-
carbonylations. Despite these advances, the vast majority of
room-temperature radical alkoxycarbonylations are generally
limited to highly reactive aliphatic iodides. In addition, high-
energy light sources such as 200 W high-pressure mercury
lamps or 500 W xenon lamps are necessary to efficiently
promote the generation of reactive radicals. Therefore,
radical alkoxycarbonylation has yet to reach its full poten-
tial[6] with respect to diversity of substrate and product, as well
as energy economy.
Interest in transition-metal-free cross-coupling reactions
of aryl halides[7,8] has increased since the first unexpected
finding from the group of Itami in 2008.[8a] Shortly thereafter,
Lei and co-workers[9] disclosed the first example of radical
alkoxycarbonylation of aryl iodides in the absence of
a transition-metal catalyst, thereby providing a range of
aromatic carboxylic tert-butyl esters under thermodynamic
conditions (Scheme 1a). In this transformation, potassium
Over the last century, the ester moiety has emerged as an
important structural unit in a great number of natural isolates
and synthetic materials. Not surprisingly, ester synthesis has
become one of the most important and fundamental tasks in
chemical science, and many elegant methods have been
established in both industrial and academic settings. One
approach towards efficient ester synthesis is direct alkoxy-
carbonylation utilizing carbon monoxide (CO). This
approach relies on the development of a wide range of
robust metal catalysts which elegantly combine many central
transition metals (i.e., palladium, ruthenium, rhodium, cobalt,
etc.) and diverse organic ligands (mainly organic phosphine,
nitrogen and carbene compounds).[1] In the last two decades,
radical alkoxycarbonylations[2] have attracted the attention of
the synthetic community because of their mild reaction
conditions (ambient temperature). In the 1990s, the groups
of Watanabe[3] and Miyaura[4] reported two isolated examples
of alkoxycarbonylation reactions of aliphatic iodides pro-
moted by photoirradiation in the presence of metal catalysts
such as platinum, rhenium, osmium, ruthenium, manganese,
and cobalt complexes. Recently, Ryu[5] and co-workers have
made substantial contributions to photocatalysis method-
[*] W. Guo, Dr. L.-Q. Lu, Y. Wang, Y.-N. Wang, J.-R. Chen,
Prof. Dr. W.-J. Xiao
Key Laboratory of Pesticide & Chemical Biology
Ministry of Education, College of Chemistry
Central China Normal University (CCNU)
152 Luoyu Road, Wuhan, Hubei 430079 (China)
E-mail: luliangqiu@mail.ccnu.edu.cn
wxiao@mail.ccnu.edu.cn
Scheme 1. Radical alkoxycarbonylation for aryl carboxylic acid esters.
Homepage: http://chem-xiao.ccnu.edu.cn/
Prof. Dr. W.-J. Xiao
Collaborative Innovation Center of Chemical Science and
Engineering, Tianjin (China)
tert-butoxide was critical, thus acting as a reducing agent and
the ester alkoxy component. The organocatalyst 1,10-phe-
nanthroline (40 mol%) was also necessary to facilitate the
generation of aryl radicals from the organic halides. Although
this work represents an impressive advance, the development
of a general alkoxycarbonylation process which involves aryl
radicals and diverse alcohol components at room temperature
remains an important goal.
[**] We are grateful to the National Science Foundation of China (NO.
21232003, 21202053, and 21272087) and the National Basic
Research Program of China (2011CB808603) for support of this
research. We greatly thank Dr. Li Rao and Prof. Jian Wan for the
theory calculation.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201408837.
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Table 1: Optimization of reaction conditions.
Since 2008, visible-light-induced photocatalysis has been
a powerful platform for the development of novel radical
synthetic methods because of the natural abundance of visible
light, thus inspiring potential applications and sustainabil-
ity.[10] By using these methods, readily available, reactive
arenediazonium salts[11] have been widely utilized as a con-
venient aryl radical source.[12] As a result, many unusual
transformations of these reagents have been carried out by
photocatalysis[13] or the combination of photocatalysis with
palladium[14] or gold catalysis.[15] Importantly, within the realm
of photoredox catalysis, some organic dyes have been
identified as efficient visible-light catalysts capable of pro-
moting numerous radical reactions under metal-free condi-
tions.[16] Continuing our work on visible-light-induced photo-
catalysis,[13d,17] we herein report a catalytic photoredox radical
alkoxycarbonylation of aryldiazonium salts using a low load-
ing of an organic photosensitizer and low-energy visible light
(Scheme 1b). By using this method, aryl carboxylic acids
esters having a wide variety of aliphatic alcohol components
can be easily obtained in high yields and with high selectivities
under metal-free and room-temperature conditions.
Initially, we examined this radical alkoxycarbonylation
reaction using phenyl diazonium tetrafluoroborate (1a, 0.2m
in methanol) as a model substrate in the presence of 3 mol%
of fluorescein as a photocatalyst under irradiation with 16 W
white LED and a CO pressure of 80 atm. To our delight, the
anticipated reaction did indeed proceed smoothly, thus
delivering the desired product, benzoyl methyl ester 3aa, in
high yield (Table 1, entry 1). The results of control experi-
ments showed that the reaction efficiency decreased notice-
ably in the absence of either light irradiation or a photo-
sensitizer (entries 2 and 3). In addition to fluorescein, other
organic dyes such as eosin Y, its sodium salt, rose Bengal, and
rhodamine B were also successful in accelerating this alkox-
ycarbonylation process, albeit with somewhat lower efficien-
cies (entries 4–7). Other reaction parameters such as the
substrate concentration, the CO pressure, and the visible-light
source were investigated.[18] As a result, the yield improved to
79% when the reaction was performed in dilute methanol
under irradiation by 16 W blue LEDs instead of white light
(entry 10). Notably, the loading of the photocatalyst can be
reduced to 0.5 mol% and gave the product in a better yield by
extending the reaction time (entry 11).
Entry
Variation from the initial conditions[a]
Yield [%][b]
1
2
none
without light
76
3
3
without Fluorescein
22
66
67
46
63
62
78
79
81
4
5
6
7
Eosin Y, instead of Fluorescein
Eosin Y disodium, instead of Fluorescein
Rose bengal, instead of Fluorescein
Rhodamine B, instead of Fluorescein
60 atm of CO
8
9
0.1m in CH3OH
10
11
16 W blue LEDs, 0.1m in CH3OH
0.5 mol% Fluorescein, 16 W blue LEDs
0.1m in CH3OH, 21 h
[a] Initial conditions: 1a (0.2 mmol, 0.1m) and Fluorescein (0.006 mmol,
3 mol%) were added into CH3OH. After flushing the autoclave three
times with CO, a pressure of 80 atm CO was set and the reaction was
performed under the irradiation of 16 W white LEDs for 10 h at RT.
[b] Determined by GC analysis using dimethyl terephthalate as an
internal standard.
yields, respectively. Significantly, the sensitive functional
groups in traditional transition-metal-catalyzed alkoxycarbo-
nylation, such as bromo and iodo, were compatible with this
process, thus allowing subsequent transformation by cross-
coupling technologies.[19] This compatibility demonstrated the
complementarity of this radical route with numerous other
palladium-catalyzed alkoxycarboxylation protocols. In addi-
tion, this photocatalytic radical alkoxycarbonylation tolerated
heteroaryl diazonium salts. For example, the 2-(ethoxycarbo-
nyl)benzofuran derivative 1m was converted into the corre-
sponding ester product 3am in good yield (entry 13).
Next, we turned our attention to the generality of the
alcohol coupling partner of this transformation. As high-
lighted in Figure 1, a broad range of aliphatic alcohols readily
participate in this radical alkoxycarboxylation reaction,
regardless of the steric bulk of the substituents around the
reactive hydroxy group (3bb–bh). For example, when steri-
cally hindered isopropyl, cyclohexyl, and tert-butyl alcohols
were employed, the corresponding esters were obtained in
moderate to good yields (3be–bg). Notably, a wide range of
functional groups was compatible with these photoredox
catalysis conditions, including ether, free hydroxy, chloride,
alkyne, and alkene moieties. Through this approach, a series
of functionalized arylcarboxyl acid esters was synthesized
with good efficiency and selectivity (3bi–bm). Notably,
terminal alkenes and alkynes, which are known to be good
With the optimal reaction conditions in hand, we probed
the scope of this photocatalytic radical alkoxycarbonylation
reaction. As summarized in Table 2, a series of aryldiazonium
salts was successfully applied to this reaction. Variation of the
electronic properties of substituents on the benzene ring had
little influence on the catalytic efficiency. Both electron-
donating groups (MeO, PhO, Me, and OH) and electron-
withdrawing groups (Br, SO3À, CN, NO2, and CO2Et) were
successfully introduced at the para position of the benzene
ring, thus delivering the corresponding substituted benzoyl
methyl esters in good yields (entries 2–10). Additionally,
variation of the postion of the substituent on the benzene ring
of the diazonium salt was also explored. For example, when 3-
nitro- and 2-iodophenyl diazonium salts were used as
substrates, the corresponding 3-nitro- and 2-iodobenzoyl
methyl esters were conveniently formed with 60 and 50%
2
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Table 2: Radical alkoxycarbonylation of different aryl diazonium salts.[a]
Significantly, the success of this photocatalytic radical car-
boxylation protocol extended to chiral alcohols. For example,
when the natural isolate (À)-menthol (2n), methyl d-(À)-
mandelate (2o), and N-benzoyl l-(+)-prolinol (2p) were
subjected to standard photocatalytic reaction conditions
(Method C), the resultant ester products (3bn–bp) were
obtained in satisfactory yields. Notably, a gram-scale reaction
with 6 mmol of the 4-methoxybenzenediazonium tetrafluor-
oborate (1b) and 2 equivalents of 2n was implemented under
the visible-light photoredox catalysis conditions and it
proceeded in a similar efficiency, thus providing the corre-
sponding ester product 3bn in 73% yield.
Moreover, this visible-light-induced photocatalysis strat-
egy can be further applied to other radical carboxylation
reactions. For example, when ortho-allyl-substituted benze-
nediazonium 4 and ortho-propargyl-substituted benzenedia-
zonium 6 were subjected to the standard reaction conditions,
methyl 2-(2,3-dihydrobenzofuran-3-yl)acetate (5) and methyl
2-(benzofuran-3-yl)acetate (7), respectively, were obtained in
good yields through a radical addition/alkoxycarboxylation
sequence [Eq. (1) and (2)]. Beyond radical alkoxycarboxyla-
tion, the use of the (1,1’-biphenyl)-2-diazonium salt 8 resulted
in 9H-fluoren-9-one (9) in moderate yield with minimal
formation of the competitive byproduct 10 [Eq. (3)]. We
believe that our findings will open a new avenue for novel
reaction design utilizing visible-light-induced photocatalyzed
radical carboxylation protocols.
Entry
Aryl source
Product
Yield [%][b]
1
2
3
1a: C6H5
3aa
3ab
3ac
3ad
3ae
3af
3ag
3ah
3ai
81[c]
75
80
73
62
64
80
68
65
66
60
50
1b: 4-CH3OC6H5
1c: 4-PhOC6H5
1d: 4-OH-C6H5
1e: 4-CH3C6H5
1 f: 4-BrC6H5
4[d]
5
6
7[e]
8[f]
9[f]
10
11[f]
12[f]
1g: 4-(SO3À)-C6H5
1h: 4-CNC6H5
1i: 4-NO2C6H5
1j: 4-(CO2Et)-C6H5
1k: 3-NO2C6H5
1l: 2-IC6H3
3aj
3ak
3al
13[f]
58
1m
3am
[a] Unless otherwise noted, the reaction was carried out on a scale of
0.4 mmol under the standard reaction conditions as given in Table 1,
entry 11. [b] Yield of the isolated products. [c] Determined by GC analysis
using dimethyl terephthalate as an internal standard. [d] EtOH was used
as the solvent, and the product is the corresponding ethyl ester. [e] 4-
Diazophenylsulfonic acid was used as the substrate. [f] A mixture of
0.2 mL of MeOH and 3.8 mL of CH3CN was used as the solvent.
As shown in Scheme 2, a plausible mechanism is proposed
using radical methoxycarboxylation of benzenediazonium salt
(1a) as an example of this novel photocatalytic process. The
initial single-electron reduction of the substrate with the
excited state of the Dye photocatalyst to generate the phenyl
radical A and simultaneously deliver the oxidized Dye radical
+
cation (DyeC ) has been elegantly demonstrated in previous
Figure 1. Generality of alcohols. Method A: 1b (0.4 mmol) in 4.0 mL
corresponding alcohol. Method B: 1b (0.4 mmol) and 0.2 mL alcohol
in 3.8 mL CH3CN. Method C: 1b (0.4 mmol) and 2 equivalents alcohol
in 4.0 mL CH3CN. Yield is that of the isolated product. [a] Yield of
6 mmol scale reaction given within parentheses. [b] Alcohol 3o:
98% ee; the product 3bo: 98%ee.
reports. Trapping of a CO molecule by this reactive inter-
mediate forms the new benzoyl radical B. Further oxidation
of B by DyeC results in the benzylidyneoxonium C, thus
completing the visible-light-induced photoredox catalysis
cycle. Finally, electronic trapping of C by methanol gives
the desired 3aa with concomitant generation of fluoroboric
acid. The byproduct of the radical carbonylation is the
corresponding arene, which was generated through the
+
acceptors in radical reactions, remained intact, thereby
indicating the high selectivity for CO in this process.
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+
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(i.e., 3bo) shows that this process has no influence on the
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the hydroxy group of the alcohol functions as the nucleophile
which attacks C to afford the ester product.
In conclusion, by using visible-light-induced photoredox
catalysis, we have developed the first radical alkoxycarbox-
ylation of aryldiazonium salts using CO gas. This reaction was
entirely metal free and was carried out at room temperature
with low loading of the organic photocatalyst to deliver
a range of esters using various aryl carboxylic acid and alcohol
partners in moderate to good yields. Importantly, the
application of this protocol to chiral alcohols has also been
proven to be feasible, including natural isolates such as (À)-
menthol and their derivatives such as methyl d-(À)-mande-
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Received: September 5, 2014
Revised: October 31, 2014
Published online: && &&, &&&&
Keywords: diazo compounds · esters · photocatalysis · radicals ·
.
synthetic methods
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[21] For experimental details, please see the Supporting Information.
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Communications
Synthetic Methods
W. Guo, L.-Q. Lu,* Y. Wang, Y.-N. Wang,
J.-R. Chen, W.-J. Xiao*
&&&& — &&&&
Metal-Free, Room-Temperature, Radical
Alkoxycarbonylation of Aryldiazonium
Salts through Visible-Light Photoredox
Catalysis
All lit up: The radical alkoxycarboxylation
of aryldiazonium salts using CO gas was
accomplished through visible-light-
induced photoredox catalysis. The reac-
tion is metal-free, and proceeds at room
temperature with a low loading of organic
photocatalyst. A wide range of esters were
conveniently synthesized in moderate to
good yields. Importantly, this strategy can
be further applied to other carboxylation
reactions.
6
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Angew. Chem. Int. Ed. 2014, 53, 1 – 6
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