Interception of Radicals by Molecular Oxygen and Diazo Compounds: Direct Synthesis of Oxalate Esters Using Visible-Light Catalysis

Article abstract of DOI:10.1021/acs.orglett.8b02487

The synthesis of oxalate esters through a radical process, rather than the traditional ionic reaction, has been well developed in which the radicals induced by visible light are trapped by molecular oxygen and diazo compounds under room temperature. This reaction is operationally simple, mild, and shows broad substrate scopes in α-bromo ketones and diazo compounds.

Full text of DOI:10.1021/acs.orglett.8b02487

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Interception of Radicals by Molecular Oxygen and Diazo
Compounds: Direct Synthesis of Oxalate Esters Using Visible-Light
Catalysis
Meihua Ma, Weiwei Hao, Liang Ma, Yonggao Zheng, Pengcheng Lian, and Xiaobing Wan*
Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry, Chemical Engineering and Materials Science,
Soochow University, Suzhou 215123, P. R. China
S
* Supporting Information
ABSTRACT: The synthesis of oxalate esters through a
radical process, rather than the traditional ionic reaction, has
been well developed in which the radicals induced by visible
light are trapped by molecular oxygen and diazo compounds
under room temperature. This reaction is operationally
simple, mild, and shows broad substrate scopes in α-bromo ketones and diazo compounds.
xalate esters are a prevalent class of compounds that are
widely found in natural products1 and pharmaceutical
Since 2008, visible light-initiated radical reactions con-
tributed by MacMillan and co-workers have grown remarkably
in modern organic synthesis,8 and many excellent works have
been reported.9,10 Given the abundance and accessibility of
oxygen, it is considered to be an ideal oxidant and perfect
source of oxygen for the construction of organic molecules.11
In this context, development of a new reaction strategy
employing visible light as energy and molecular oxygen as the
oxidant or oxygen source under mild conditions seems highly
desirable for modern organic synthesis. Recently, significant
developments have been made for visible-light-induced aerobic
oxidative transformation.12 For instance, in 2011, Jiao’s
group13 developed an aerobic oxidation of benzyl halides in
which molecular oxygen participated in the photocatalyst cycle
and was introduced into the products under visible-light
conditions. Very recently, Nicewicz and co-workers14 reported
a photoredox-catalyzed direct C−H cyanation of arenes using
molecular oxygen as an oxidant to regenerate the photo-
catalyst. Herein, we envisioned that organic bromide may serve
as radical precursor under photoredox catalysis, which could
then be trapped by dioxygen and diazo compounds15 to give
the oxalate esters (Scheme 1c).
O
agents2 and can serve as widespread building blocks in organic
synthesis.3 The most classical methods for the synthesis of
oxalate esters are the coupling reactions of oxalic acid or oxalic
acid derivatives with alcohol (Scheme 1a).4 Transition-metal-
Scheme 1. Synthetic Strategies for the Synthesis of Oxalate
Esters
Initially a reaction between benzyl bromide and ethyl
diazoacetate as the model substrates was attempted using 1.0
mol % eosin Y as photoredox catalyst and CH3CN as solvent
under O2 atmosphere under irradiation from 12 W green LED,
but the reaction was unsuccessful. Alternatively, however,
when the reaction was attempted with α-bromoacetophenone
1a,16 we were delighted to observe the formation of the desired
product 3a in 56% yield (Table 1, entry 1). Screening different
solvents for the reaction revealed that the reaction in DMF
afforded 3a in improved yield of 85% (entries 2−12). Eosin Y
proved to be the optimal catalyst for the transformation during
investigations of the reaction under different photocatalysts
catalyzed oxidative coupling of CO with CH3OH also
consititutes a good strategy to deliver dimethyl oxalate
(Scheme 1b).5 In addition, ozonolysis of alkenes or alkynes
in the presence of alcohol has emerged as a powerful method
for the synthesis of oxalate esters.6 Other alternative pathways
to oxalate esters include dimerization of formates, Oxone-
mediated oxidative cleavage of β-keto esters, alcoholysis of the
trichloroacetyl compounds, and oxidation of ascorbic acid and
ascorbic acid derivatives.7 In general, the present approaches
for the systhesis of oxalate esters mainly rely on the ionic
process. To the best of our knowledge, the radical-promoted
formation of unsymmetrical oxalate esters has not been
discovered until now.
Received: August 3, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b02487
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
a
Table 1. Optimization of the Reaction Conditions
Scheme 2. Scope of α-Bromo Ketones
b
entry
photocatalyst
eosin Y
solvent
CH3CN
yield (%)
1
56
<5
<5
85
24
<5
17
<5
<5
24
<5
55
30
78
84
67
<5
7
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
eosin Y
eosin Y
eosin Y
eosin Y
eosin Y
eosin Y
eosin Y
eosin Y
eosin Y
eosin Y
eosin Y
RhB
Ru(bpy)3Cl2.6H2O
RhB-6G
rose bengal
alizarin red
Acr+-MeClO4
eosin Y
DME
1,1,2-trichloroethane
DMF
DMSO
cyclohexane
PE
H2O
i-PrOH
THF
Tol
NMP
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
a
Reaction conditions: 1 (0.5 mmol, 1.0 equiv), 2a (2.0 mmol, 4.0
equiv), eosin Y (0.005 mmol, 1.0 mol %), LED (12 W, green) and
DMF (2.0 mL) for 36 h under O2 atmosphere.
−
shown in Scheme 3. The reaction of alkyl 2-diazoacetates with
α-bromo ketone 1a provided the corresponding products in
c
<5
<5
<5
65
a
d
Scheme 3. Scope of Diazo Substrates
eosin Y
eosin Y
DMF
DMF
e
a
Reaction conditions: 1a (0.5 mmol, 1.0 equiv), 2a (2.0 mmol, 4.0
equiv), catalyst (0.005 mmol, 1.0 mol %), LED (12 W, green), and
b
c
solvent (2.0 mL) for 36 h under O2. Isolated yields. In the dark.
N2. Air.
d
e
(entries 13−18). Moreover, a control experiment suggested
that both photoredox catalyst and light were essential for this
transformation (entries 19 and 20). No oxalate ester 3a was
observed under N2 atmosphere, and a yield of 65% was
achieved when the reaction was performed under air
atmosphere, which demonstrated that O2 was the crucial
participant in this reaction (entries 21 and 22).
With the optimal reaction conditions having been
determined, we began to explore the scope of α-bromo
ketones using ethyl diazoacetate 2a as the reaction partner
(Scheme 2). The reactions of 2a with α-bromo ketones
bearing ortho-substitution groups of the benzene rings gave the
corresponding products in good yields (3b−d). Meta-
substituted α-bromo ketones containing either electron-
withdrawing or electron-donating groups also underwent
reaction to give the anticipated products in moderate to
good yields (3e−h). A variety of functional groups, such as F,
Cl, Br, OCH3, and CF3 at the phenyl rings, were well tolerated
and afforded the desired products in good to excellent yields.
Notably, the hydroxy group was also compatible under the
standard conditions (product 3m). The employment of
disubstituted α-bromoacetophenone could generate the
corresponding product 3o in 84% yield. When 2-bromo-1-
(naphthalen-2-yl)ethanone was used to couple with ethyl
diazoacetate 2a, the reaction proceeded well to deliver product
3p in high 88% yield. It is worth noting that when 2-bromo-
1,2-diphenylethanone was employed as the reaction partner,
the desired product 3q was isolated in 71% yield.
a
Reaction conditions: 1a (0.5 mmol, 1.0 equiv), 2 (2.0 mmol, 4.0
equiv), eosin Y (0.005 mmol, 1.0 mol %), LED (12 W, green) and
DMF (2.0 mL) for 36 h under O2 atmosphere.
excellent yields (4a, 4b, and 4g). The α-diazo esters with
double and triple bonds also afforded the desired products in
moderate to high yields (4c−e). It is noteworthy that diazo
compound bearing phenyl was proven to be a good substrate
in this transformation, leading to the target product in
moderate yield (4f).
To verify the mechanism of this oxalate ester formation
reaction, fluorescence quenching experiments were performed.
The Stern−Volmer plot (Figure 1) illustrated that excited-state
eosin Y* was quenched by α-bromoacetophenone 1a, rather
than ethyl diazoacetate 2a. To determine the source of oxygen
in the oxalate ester, we added 18O-labeled water to the model
reaction; however, no 18O atom was detected in the target
product 3a (Scheme 4a). The combination of this result with
To further explore the generality of this transformation, a
variety of diazo esters were evaluated, and the results are
B
DOI: 10.1021/acs.orglett.8b02487
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
In summary, we have successfully developed a radical
process to build oxalate esters skeleton using α-bromo ketones,
diazo compounds and O2 as the starting materials under
visible-light conditions. This green and environmentally
friendly methodology features broad functional group compat-
ibility, wide substrate scopes, and mild reaction conditions.
Further investigations on the mechanism determine that two
molecular oxygens participate in this oxalate ester formation
reaction.
Figure 1. Stern−Volmer analyses for substrates 1a and 2a.
Scheme 4. Probe for the Possible Mechanism
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b02487.
1
Experimental procedure, characterization data, and H
and 13C NMR spectra (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: wanxb@suda.edu.cn.
ORCID
the control reactions (as shown in entries 21 and 22 in Table
1) suggests that the oxygen atom in the oxalate ester was
derived from molecular oxygen rather than water. Using 2-
hydroxy-1-phenylethanone 5 instead of 2-bromoacetophenone
1a did not give product 3a under the standard conditions,
which indicated that 2-hydroxy-1-phenylethanone was not an
intermediate in this transformation (Scheme 4b).
On the basis of the above experimental results and the
relevant literature, we propose a plausible catalytic cycle as
shown in Scheme 5. First, eosin Y is excited by visible-light
Xiaobing Wan: 0000-0001-9007-2128
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This project has been funded by the Priority Academic
Program Development of Jiangsu Higher Education Institu-
tions (PAPD) and the NSFC (21572148, 21272165).
Scheme 5. Proposed Mechanism
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