Visible-light photoredox-mediated oxidation of N-methyl tertiaryamines under catalyst free conditions: Direct synthesis of methylene-bridged bis-1,3-dicarbonyl compounds

Article abstract of DOI:10.1016/j.cclet.2014.01.021

Mediated by visible light-induced photoredox catalysis and free of other catalysts, a new and efficient synthesis of methylene-bridged bis-1,3-dicarbonyl derivatives has been developed. A variety of N-methyl tertiaryamines and 1,3-dicarbonyl compounds were investigated in this reaction.

Full text of DOI:10.1016/j.cclet.2014.01.021

Chinese Chemical Letters 25 (2014) 545–548
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Chinese Chemical Letters
journal homepage: www.elsevier.com/locate/cclet
Original article
Visible-light photoredox-mediated oxidation of N-methyl
tertiaryamines under catalyst free conditions: Direct synthesis
of methylene-bridged bis-1,3-dicarbonyl compounds
b
a,
Xiao-Jun Dai a, Xiao-Liang Xu a, , Dong-Ping Cheng , Xiao-Nian Li
*
*
a Institute of Industrial Catalysis, College of Chemical Engineering and Materials Science, Zhejiang University of Technology, Hangzhou 310014, China
b College of Pharmaceutical Science, Zhejiang University of Technology, Hangzhou 310014, China
A R T I C L E I N F O
A B S T R A C T
Article history:
Mediated by visible light-induced photoredox catalysis and free of other catalysts, a new and efficient
synthesis of methylene-bridged bis-1,3-dicarbonyl derivatives has been developed. A variety of
N-methyl tertiaryamines and 1,3-dicarbonyl compounds were investigated in this reaction.
ß 2014 Xiao-Liang Xu and Xiao-Nian Li. Published by Elsevier B.V. on behalf of Chinese Chemical
Society. All rights reserved.
Received 6 November 2013
Received in revised form 25 December 2013
Accepted 27 December 2013
Available online 15 January 2014
Keywords:
Photochemistry
Oxidation
Aliphatic tertiaryamine
Catalyst free
1. Introduction
low yield. Although the exact mechanism was unclear, the
observation may be ascribed to the differences in reaction
The catalytic oxidation of sp3 C–H bond adjacent to a nitrogen
atom to form a new carbon–carbon or carbon–hetero bond has
been widely applied in organic synthesis [1]. Using the iminium
ions generated from the direct oxidation of tertiaryamines in situ,
Strecter reaction, Mannich reaction, aza-Henry reactions and so on
have been successfully realized [2]. Recently, visible light
photoredox catalysis aroused great interest due to its green
chemistry characteristics [3]. Many metal complexes and organic
dyes were frequently used as photocatalysts in photoredox
chemistry [4,5]. In particular, several highly efficient chemical
transformations were established using Ir or Ru photocatalysts [5].
The photocatalytic oxidation of tertiaryamines is a very important
research topic.
Kobayashi reported that a Ru complex, in the presence of visible
light, catalyzed the cross oxidative coupling of 1,3-diphenylpro-
pane-1,3-dione and tertiary aliphatic amines to form methylene-
bridged bis-1,3-dicarbonyl compounds [6,7]. However, in the
course of our study, it was found that without metal photocatalyst,
1,3-diphenylpropane-1,3-dione and N,N-dimethylcyclohexyla-
mine reacted smoothly to give the methylene-bridged bis-1,3-
dicarbonyl compounds even using only visible light, albeit in very
conditions between Kobayashi’s and ours.
2. Experimental
All reactions were carried out without the exclusion of air or
moisture. Toluene, cyclohexane, DCE, DCM, DMSO, DMF, EtOAc,
1,4-dioxane and MeCN were analytical reagents and used directly
as received. N,N-dimethylcyclohexylamine, N,N-dimethylbutyla-
mine, N,N-dimethylbenzylamine, N,N-dimethylaniline, N,N,N0,N0-
tetramethylethane-1,2-diamine, N-methyldiisopropylamine, N-
methyldicyclohexylamine were purchased from the Sigma–
Aldrich company and used directly as received. 1,3-Dicarbonyl
compounds were prepared according to literature procedures [8].
1H NMR and 13C NMR spectra were recorded at ambient
temperature in CDCl3 on a Bruker AMX-500 MHz instrument with
TMS as an internal standard. Low-resolution MS was obtained
using EI or ESI ionization. HRMS was obtained using ESI ionization.
General procedure for the preparation of compounds 2: to a
mixture of 1,3-diphenylpropane-1,3-dione 1a (0.5 mmol) and 9/1
(v/v) DMSO/H2O (2 mL) in a 10 mL two-necked round-bottom flask
was added N-methyl-dicyclohexanamine (0.25 mmol). The result-
ing mixture was stirred under 1 atm of O2 (balloon) for 24 h at
ambient temperature under the irradiation by three 45 W white
fluorescent bulbs (Arrow BHSL 45) at a distance of 5 cm. Then the
reaction mixture was washed three times with dilute hydrochloric
*
Corresponding authors.
E-mail addresses: xuxiaoliang@zjut.edu.cn (X.-L. Xu), xnli@zjut.edu.cn (X.-N. Li).
1001-8417/$ – see front matter ß 2014 Xiao-Liang Xu and Xiao-Nian Li. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
http://dx.doi.org/10.1016/j.cclet.2014.01.021

546
X.-J. Dai et al. / Chinese Chemical Letters 25 (2014) 545–548
Table 1
Table 2
Solvent screening.a
Evaluation of tertiaryamines as methylene donors.a
O
O
O
O
O
O
O
Ph
Ph
Ph
Ph
Me
N
Ph
Ph
Ph
O
O
visible light
solvent, 24 h
O
O
visible light
toluene, 24 h
+
amine
+
Cy
Cy
Ph
Ph
Ph
Ph
Ph
1a
1a
3
O
2a
2a
Entry
Solvent
Toluene
Yield (%)b
Entry
1
Amine
Yield (%)b
10
1
2
3
4
5
6
7
8
9
12
26
17
18
70
20
22
30
32
N
Cyclohexane
DCM
2
3
9
4
DCE
N
DMSO
DMF
EtOAc
N
Ph
1,4-Dioxane
MeCN
4
5
<5
N
a
Reaction conditions: 1,3-diketone 1a (0.5 mmol), amine 3 (0.6 mmol), in solvent
(2 mL) at r.t. for 24 h under irradiation by a 45 W fluorescent lamp at a distance of
5 cm.
5
b
N
Isolated yield based on 1a.
6
15
7
Cy
N
Cy
Table 3
Solvent and light source screening.a
7
N
O
O
O
O
N
Me
N
Ph
Ph
Ph
Ph
a
O
O
visible light
solvent
Cy
Reaction conditions: 1,3-diketone 1a (0.5 mmol), amine (0.6 mmol), in toluene
(2 mL) at r.t. for 24 h under irradiation by a 45 W fluorescent lamp at a distance of
5 cm.
+
Cy
Ph
Ph
b
Isolated yield based on 1a.
1a
3
2a
acid. Purification was done by column chromatography on silica
gel (200–300 mesh) using petroleum ether and ethyl acetate (5:1–
10:1) as the eluent to give the pure product 2a.
Synthesis and characterization of all products are described in
Supporting information.
Entry Solvent
Light source
Time (h) Yield (%)b
1
2
DMSO
H2O/DMSO (1:1) Three 45 W fluorescent lights 20
Three 45 W fluorescent lights 12
74
86
89
78
90
82
80
n.r.
n.r.
3
H2O
Three 45 W fluorescent lights 23
4
DMSO
Two 45 W fluorescent lights
24
35
20
30
24
24
5
DMSO/H2O (9/1) Two 45 W fluorescent lights
6c
7d
8
DMSO
DMSO
DMSO
H2O
Two 45 W fluorescent lights
3. Results and discussion
Two 45 W fluorescent lights
Dark
Dark
Our initial studies focused on the visible light-mediated
reaction of 1,3-diphenylpropane-1,3-dione 1a and N,N-dimethyl-
cyclohexylamine. Unfortunately, the reaction proved to be very
sluggish and the starting material could not be fully consumed
within 30 h (Table 1, entry 1). Various N-methyl tertiaryamines as
potential methylene donors were examined. To our disappoint-
ment, all aliphatic N-methyl tertiaryamines tried led to the desired
product with very low yields. When N,N-dimethylaniline was
introduced, the reaction resulted in a complex mixture (Table 1,
entry 4).
However, it was interesting to find that the photocatalytic
reaction of N-methyldicyclohexylamine was significantly faster,
compared with other N-methyl tertiaryamines. This reaction
could occur in all solvents tried. It was gratifying that, in contrast
to other solvents (Table 2), the desired product was obtained in
70% isolated yield after 24 h using DMSO as a solvent (Table 2,
entry 5).
9
a
Reaction conditions: 1,3-diketone 1a (0.5 mmol), amine 3 (0.6 mmol), in solvent
(2 mL) at r.t. for 24 h under irradiation by visible light at a distance of 5 cm under air
until otherwise noted.
b
Isolated yield based on 1a.
c
Under O2.
d
Solvent was dried, under N2.
when the reaction was conducted in the dark (Table 3, entries 8
and 9). Under O2, the reaction was more rapid than under air, while
a very slow background reaction was observed under N2. These
observations indicated that oxygen was essential in this reaction.
Under the optimized conditions, the scope of this photon-
mediated coupling reaction was investigated. In general a variety
of different dicarbonyl nucleophiles (Table 4), including aryl and
alkyl 1,3-diketones,
b-ketoesters and 1,3-diester, could react
Based on these results, various conditions were evaluated. It
was found that strong light emission could accelerate the reaction.
Interestingly, use of H2O as a solvent also resulted in high yields
(Table 3, entry 3). The mixed solvent of DMSO and H2O gave
reasonable result as well. Importantly, no conversion was observed
with N-methyl-dicyclohexylamine. Substrates containing sub-
stituents like Me and OMe on the aryl rings underwent the
reaction smoothly (Table 4, entries 2 and 3). For substrates with
halogenated phenyl rings, a fluoro appeared to be more favorable
than a bromo and some unidentified compounds formed in certain

X.-J. Dai et al. / Chinese Chemical Letters 25 (2014) 545–548
547
Table 4
Substrate scope.a
4. Conclusion
O
O
O
O
In summary, without any other photocatalyst, we have
developed a useful protocol for the synthesis of methylene-
bridged bis-1,3-dicarbonyl compounds in aqueous media using
visible light photoredox catalysis. In contrast to the similar
reactions reported in recent literature, the mechanism of this
reaction needs more experimental studies. Further investigations
into the mechanism of this reaction under visible light are
currently underway.
1
2
2
1
R
R
Me
N
visible light
O
O
+
1
2
DMSO/H O (9/1)
2
Cy
Cy
R
R
R
R
1a-j
3
2a-j
Entry
Substrate
R1
R2
Product
Time (h)
Yield (%)b
Acknowledgment
1a
2
1a
1b
1c
1d
1e
1f
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
OEt
Me
Ph
2a
2b
2c
2d
2e
2f
24
24
24
17
24
24
38
40
48
39
91
80
75
66
26
67
48
78
54
15
4-MeO-C6H4
4-Me-C6H4
4-F-C6H4
4-Br-C6H4
3-thienyl
Me
The authors are grateful to Zhejiang Provincial Natural Science
Foundation of China (No. LY12B02017).
3
4
5
6
7
1g
1h
1i
2g
2h, 2h0
c
8
OEt
Appendix A. Supplementary data
9
OEt
2i
2j
10
1j
OEt
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.cclet.2014.01.021.
a
Reaction conditions: 1,3-diketone 1a (0.5 mmol), amine 3 (0.3 mmol), in DMSO/
H2O (9/1, 2 mL) at r.t. under irradiation by two 45 W white fluorescent lamps under
1 atm of O2 or air balloon; please see ESI for details.
b
Isolated yield based on 1.
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