Visible-light-induced C=C bond cleavage of enaminones for the synthesis of 1,2-diketones and quinoxalines in sustainable medium

Article abstract of DOI:10.1002/cctc.201500139

The C=C double bond cleavage of enaminones has been realized under ambient conditions through visible-light catalysis in the presence of Rose Bengal, which leads to the synthesis of a class of 1,2-diketones without using any metal catalyst. In addition, the one-pot synthesis of quinoxalines has also been achieved under identical photocatalytic conditions by making use of the in situ generated 1,2-diketones as intermediates.

Full text of DOI:10.1002/cctc.201500139

DOI: 10.1002/cctc.201500139
Full Papers
Visible-Light-Induced C=C Bond Cleavage of Enaminones
for the Synthesis of 1,2-Diketones and Quinoxalines in
Sustainable Medium
Shuo Cao,[a] Shanshan Zhong,[a] Luoting Xin,[a] Jie-Ping Wan,*[a] and Chengping Wen*[b]
The C=C double bond cleavage of enaminones has been real-
ized under ambient conditions through visible-light catalysis in
the presence of Rose Bengal, which leads to the synthesis of
a class of 1,2-diketones without using any metal catalyst. In ad-
dition, the one-pot synthesis of quinoxalines has also been
achieved under identical photocatalytic conditions by making
use of the in situ generated 1,2-diketones as intermediates.
Introduction
1,2-Diketones are valuable organic compounds possessing
highly enriched reactivity owing to the presence of the charac-
teristic adjacent electron-deficient double carbonyls. Therefore,
as the main building blocks, 1,2-diketones have been broadly
used in the synthesis of many important organic products
such as a-hydroxyl ketone,[1] N-containing heterocycles,[2] and
carbohydrates.[3] More notably, the 1,2-diketone fragment has
been found to be present in a number of biologically relevant
organic molecules.[4] During the past decades, the efforts of
chemists have led to the establishment of several different
types of strategies toward the synthesis of 1,2-diketones. Typi-
cally, the main synthetic toolbox consists of the oxidation of
the a-hydroxyl ketone,[5] stilbenes,[6] vicinal dibromoalkanes,[7]
vicinal diols,[8] enones,[9] methylene ketones, etc.[10] And more
generally, the oxidative dicarbonylation of internal alkynes has
been widely employed to access 1,2-diketones.[11] Additionally,
oxidative cleavage of 1,3-diketones using different oxidants is
also known as independently reported by Zhang,[12]
Smonou[13a] and Yuan.[13b] On the other hand, new synthetic
methods involving unconventional transformation have also
been achieved. Jiao and co-workers developed the copper-cat-
alyzed synthesis of 1,2-diketones by the cascade coupling and
oxygenation reaction using olefins and hydrazines [Eq. (1) in
Scheme 1],[14] and Yang and Tang et al. reported the 1,2-dike-
tone synthesis using b-ketoaldehydes in the presence of NaClO
[Eq. (2) in Scheme 1].[15] Upon the sustaining efforts, the palla-
Scheme 1. Different routes for the synthesis of 1,2-diketone.
dium-catalyzed bifunctionalization of olefins using nitroal-
kanes[16] and the palladium- or copper-catalyzed decarboxyla-
tive coupling reactions of aryl propiolic acids and aryl io-
dides[17] have also been found as attractive methods for 1,2-di-
ketone synthesis. Generally, most of these known methods en-
counter one or more limits such as the dependence on
transition-metal catalyst, high-temperature heating or employ-
ment of additional oxidants. In this context, searching for alter-
natively sustainable and mild approaches is still highly desir-
able work in the area of 1,2-diketone synthesis.
[a] S. Cao, S. Zhong, L. Xin, Dr. J.-P. Wan
Key Laboratory of Functional Small Organic Molecules
Ministry of Education, College of Chemistry and Chemical Engineering
Jiangxi Normal University
Nanchang 330022 (P.R. China)
Enaminones are a class of broadly investigated organic com-
pounds in organic synthesis. Owing to their easy availability
and highly versatile reactivity, enaminones have been exten-
sively employed as main building blocks for the synthesis of
a great variety of organic products.[18] Predominantly, most en-
aminone- and similar enamine-based organic syntheses utilize
one or more of enaminones’ reactive sites such as the amino
E-mail: wanjieping@jxnu.edu.cn
[b] Prof. Dr. C. Wen
College of Basic Medical Sciences
Zhejiang Chinese Medical University
Hangzhou 310053(P.R. China)
E-mail: cpwen.zcmu@yahoo.com
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201500139.
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group, the nucleophilic a-carbon, the electrophilic b-carbon,
and the ketone carbonyl, etc.[19] Interestingly, rather rare re-
ports on enaminone-based synthesis involving the functionali-
zation of the C=C double bond have been known. During our
research efforts on the enaminone and analogous substrates
participated organic synthesis,[20] we have recently disclosed
that the enaminones are able to undergo the C=C bond cleav-
age to take part in the construction of pyridines by acting as
C4 building block.[21] Inspired by this transformation, we have
conducted investigation on the catalytic production of 1,2-di-
ketones by making use of the C=C bond cleavage-based trans-
formation of proper enaminone substrates. Primary attempts
by Wasserman and Ives have revealed that the 1,2-diketones
are accessible from enaminones under catalytic conditions of
650 W lamp heating and À788C to room temperature opera-
tion.[22a,b] This method has been seriously restricted from practi-
cal synthetic application by the inconvenient and harsh condi-
tions. Therefore, an operationally practical catalytic protocol of
improved applicability is urgently required to complement this
type of 1,2-diketone synthesis. Herein, we report the synthesis
of 1,2-diketones based on the C=C bond cleavage of enami-
nones under sustainable catalytic conditions consist of Rose
Bengal assistance (metal-free), visible-light radiation and room-
temperature stirring in bio-based ethyl lactate (EL) medium
[Eq. (3) in Scheme 1].[23] Notably, this catalytic transformation
also allows the direct one-pot synthesis of useful quinoxa-
lines[24] by employing o-phenylenediamines as reaction part-
ners without any modification on the reaction conditions
[Eq. (4) in Scheme 1].
Table 1. Optimization on reaction conditions.[a]
Entry
Solvent
t
[h]
Yield
[%][b]
1[c]
2[d]
3[c,d]
4
5
6
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
H2O
12
12
12
12
24
48
72
48
48
48
48
48
48
48
48
trace
trace
trace
54
66
88
87
trace
47
40
92
7
8
9
LA
10
11
12
13
14[e]
15[f]
EtOH
EL
DMF
AcOH
EL
88
62
78
90
EL
[a] General conditions: enaminone 1a (0.3 mmol), Rose Bengal sodium
salt (5 mol%) in 2 mL solvent, radiation with 15 W LED bulb at room tem-
perature (EL=ethyl lactate, LA=lactic acid). [b] Yield of isolated product.
[c] Reaction in black. [d] Reaction without Rose Bengal. [e] 2.5 mol% Rose
Bengal. [f] 7.5 mol%Rose Bengal.
Table 2. Scope of metal-free photocatalytic 1,2-diketone synthesis.
Results and Discussion
Initially, enaminone 1a was selected as the model substrate to
optimize the reaction parameters. At first, control experiments
in the absence of visible light and/or photocatalyst (Rose
Bengal) were conducted. The results demonstrated that in the
absence of either visible light or Rose Bengal no production of
expected 1,2-diketone product 2a occurred (entries 1–3,
Table 1). On the other hand, the conditions with both visible-
light radiation and Rose Bengal presence allowed successful
C=C bond cleavage to produce diketone 2a with moderate
yield (entry 4, Table 1). On the basis of the result, prolonging
the reaction time to 48 h gave the highest yield (entries 5–7,
Table 1). Notably, further attempts in improving the reaction ef-
ficiency by varying the reaction medium using different candi-
dates such as water, lactic acid (LA), ethanol, EL, DMF, and
AcOH proved that EL, which was well known as a nontoxic,
biomass-available, cheap, and easily degradable green organic
solvent,[25] was found as the best medium for the reaction by
providing a yield of 92% (entries 8–13, Table 1). Finally, the fa-
vorable loading of Rose Bengal was found as 5 mol% accord-
ing to the data provided by the entries using different
amounts of Rose Bengal (entries 14–15, Table 1).
Ar1
Ar2
Product
Yield
[%][a]
Ph
4-BrC6H4
4-ClC6H4
Ph
Ph
Ph
Ph
Ph
Ph
2a
2b
2c
2d
2e
2 f
2g
2h
2i
2j
2k
2l
2m
2n
92
94
95
80
75
85
77
61
83
76
75
68
27
63
4-MeC6H4
2,5-(Me)2C6H3
4-MeOC6H4
4-ClC6H4
4-CH3OC6H4
4-CH3C6H4
4-BrC6H4
2,5-(Me)2C6H3
4-iPrC6H4
4-CH3OC6H4
naphthyl
4-FC6H4
4-FC6H4
4-FC6H4
4-FC6H4
4-FC6H4
4-FC6H4
4-CH3OC6H4
Ph
[a] Yield of isolated product.
sults showing in Table 2, a-aryl enaminones were generally ap-
plicable for the synthesis of enaminones 2 through photocatal-
ysis. A variety of different functional groups, including alkyl, al-
koxyl, and halogen in both Ar1 and Ar2 tolerated the synthesis
of corresponding 1,2-diketones well. The effect of substituent
in aryls was not observed, and most products were provided
With the results from the systematic optimization in hand,
the application scope of this photocatalytic protocol on the
synthesis of different 1,2-diketones was then explored by sub-
jecting various enaminone substrates 1. According to the re-
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in good to excellent yields. The attempts on running identical
synthesis using alkyl functionalized enaminones were not suc-
cessful. As extended investigation, an entry directly using 1,2-
diphenylethanone as starting material was also run under the
standard reaction conditions, however, no reaction was ob-
served, demonstrating the special reactivity of enaminone
1 for the phtotocatalytic synthesis of 1,2-diketone.
observed if o-phenylenediamines containing electron-with-
drawing groups such as nitro and halogen were used (4k, 4l,
4r) because of their weaker amino nucleophilicity. Good to ex-
cellent yields of quinoxalines were afforded by all other entries.
According to known literature on both the Rose Bengal-
based photocatalysis and the enaminone C=C double-bond
cleavage, we proposed the reaction mechanism as shown in
Scheme 2. Firstly, the inducement of the photon to Rose
Bengal (RB) provides the excited state of RB as RB* which re-
turns to the stage of RB by promoting the production of sin-
glet oxygen.[26]
Considering the versatile reactivity of 1,2-ketones, we en-
visioned that this photocatalytic generation of 1,2-diketones
could be employed as a key step for running cascade synthesis
of more complex molecules. Under the inspiration, we attempt-
ed to subject o-phenylenediamine 3 together with the enami-
none under the standard reaction conditions in hope of achiev-
ing the direct synthesis of quinoxaline. However, the direct op-
eration gave no expect transformation probably because of
the interaction between enaminone 1 and diamine 3 before
the formation of diketone. Therefore, we modified the opera-
tion by running the first step of 1,2-diketone synthesis before
employing diamine 3. To our delight, synthesis of quinoxalines
was realized by using the modified operation without employ-
ing any other catalyst or additive. The results on the quinoxa-
line synthesis were given in Table 3. It could be seen from the
products that the scope of the synthesis was broad and relia-
ble synthesis of quinoxalines containing different substructures
was allowed. Functional groups of different properties such as
electron-donating group and electron-withdrawing groups
were compatible both to this one-pot synthetic protocol. Gen-
erally, relatively lower yields of corresponding products were
The singlet oxygen then incorporates to the enaminone to
afford peroxide intermediates 5 as described by Wasserman
and Ives and others.[22] The automatic decomposition of 5 pro-
vides the 1,2-diketones 2, which could be captured in situ by 3
Scheme 2. Proposed reaction mechanism.
to give quinoxalines 4. The good
Table 3. Scope of the one-pot photocatalytic synthesis of quinoxalines (isolated yield based on enaminone 1 is
effect of EL, as observed in previ-
ous studies, may be attributed
to its special property in solving
oxygen.[25c,d]
reported).
Conclusions
A photocatalytic synthesis of 1,2-
diketones through the cleavage
of
enaminones
has
been
achieved under mild and metal-
free conditions. Besides offering
a sustainable synthetic option in
the field of vicinal diketone syn-
thesis, the present catalytic pro-
tocol is also applicable for the
sustainable one-pot synthesis of
quinoxalines by simply employ-
ing o-phenylenediamine as the
reaction partner. The simplicity
in operation and easy availability
of all reagents ensure the pres-
ent work as useful complement
to known methods for both the
synthesis of 1,2-diketones and
quinoxaline heterocycles.
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General procedure for the photocatalytic synthesis of 1,2-di-
ketones
Enaminone 1 (0.3 mmol) and Rose Bengal (0.015 mmol) were
charged in a 25 mL round-bottom flask equipped with a stirring
bar. EL (2 mL) was added and the mixture was stirred at RT and
open-air atmosphere for 48 h under visible-light irradiation (15 W
white LED bulb). After completion of the reaction, water (5 mL)
was added, and the resulting mixture was extracted with ethyl ace-
tate (3ꢁ8 mL). The organic layers were combined and washed
with water for 3 times (3ꢁ20 mL). The acquired solution was dried
overnight with anhydrous MgSO4. After filtration and removing the
solvent under reduced pressure, the residue was subjected to silica
gel column chromatography to provide pure products.
Benzil (2a).[13b] Yield: 58 mg, 92%; yellow solid; m.p. 93–968C;
1H NMR (400 MHz, CDCl3): d=7.97 (d, J=7.6 Hz, 4H), 7.66 (t, J=
7.6 Hz, 2H), 7.51 ppm (t, J=7.6 Hz, 4H); 13C NMR (100 MHz, CDCl3):
d=194.6, 134.9, 133.0, 129.9, 129.0 ppm.
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General procedure for the one-pot synthesis of quinoxalines
Enaminone 1 (0.3 mmol) and Rose Bengal (0.015 mmol) were
charged in a 25 mL round-bottom flask equipped with a stirring
bar. EL (2 mL) was added. The resulting mixture was stirred at RT
and open-air atmosphere for 48 h under irradiation of light (15 W
white LED bulb). Subsequently, o-phenylenediamine 3 (0.3 mmol)
was employed and further stirring of 24 h under identical light irra-
diation was performed. Upon reaction completion, water (5 mL)
was added, and the resulting mixture was extracted with ethyl ace-
tate (3ꢁ8 mL). The collected organic layer was then washed with
water for 3 times (3ꢁ20 mL). The organic solution was dried over-
night with anhydrous MgSO4. After filtration and removing the sol-
vent under reduced pressure, the residue was subjected to silica
gel column chromatography to provide pure products.
2,3-Diphenylquinoxaline (4a).[24a] Yield: 68 mg, 80%; white solid;
m.p. 120–1228C; 1H NMR (400 MHz, CDCl3):d=8.17 (q, J=3.2 Hz,
2H), 7.74 (q, J=3.2 Hz, 2H), 7.51 (d, J=7.6 Hz, 4H), 7.32 ppm (d,
J=7.2 Hz, 6H); 13C NMR (100 MHz, CDCl3):d=153.5, 141.3, 139.1,
130.0, 129.9, 129.2, 128.9, 128.3 ppm.
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Acknowledgements
The work is financially supported by the National Natural and
Science Foundation of China (no. 21102059) and a research pro-
ject from The Department of Education of Jiangxi Province
(GJJ13245). C. Wen thanks the support from the National Basic
Research Program “973” of China (No. 2014CB5430).
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Keywords: amines · ketones · photochemistry · reaction
mechanism · sustainable chemistry
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Received: February 11, 2015
Revised: March 9, 2015
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Published online on && &&, 0000
ChemCatChem 0000, 00, 0 – 0
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&
These are not the final page numbers! ÞÞ

FULL PAPERS
S. Cao, S. Zhong, L. Xin, J.-P. Wan,*
C. Wen*
&& – &&
Visible-Light-Induced C=C Bond
Cleavage of Enaminones for the
Synthesis of 1,2-Diketones and
Quinoxalines in Sustainable Medium
White LED power to diketones: A sus-
tainable synthetic protocol towards the
synthesis of 1,2-diketones is achieved
by visible-light-induced cleavage of the
C=C double bond in enaminones.
&
ChemCatChem 0000, 00, 0 – 0
www.chemcatchem.org
6
ꢀ 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!