Copper(II)-catalyzed oxidative [3+2] cycloaddition reactions of secondary amines with α-diazo compounds: A facile and efficient synthesis of 1,2,3-triazoles

Article abstract of DOI:10.1039/c5cc02092a

A novel copper-catalyzed [3+2] cycloaddition reaction of secondary amines with α-diazo compounds has been developed via a cross-dehydrogenative coupling process. The reaction involves a sequential aerobic oxidation/[3+2] cycloaddition/oxidative aromatizat

Full text of DOI:10.1039/c5cc02092a

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DOI: 10.1039/C5CC02092A
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Copper(II)-catalyzed oxidative [3 + 2] cycloaddition reactions of secondary amines with α-diazo
compounds: facile and efficient synthesis of 1,2,3-triazoles
Yi-Jin Li,a Xue Li,a Shao-Xiao Zhang,b Yu-Long Zhao*a and Qun Liua
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
broad range of α-amino acid derivatives, particularly N-alkylglycine
derivatives (Scheme 2, c). Moreover, the reaction takes place under
very mild conditions with molecular oxygen as a co-oxidant and
provides a new and efficient method for the construction of 1,2,3-
55 triazoles in a single step in an atom-economic manner.
A novel copper-catalyzed [3 + 2] cycloaddition reaction of
secondary amines with α-diazo compounds has been
developed via a cross-dehydrogenative coupling process. The
reaction involves
10 cycloaddition/oxidative
a
sequential aerobic oxidation/[3+2]
aromatization procedure and
provides an efficient method for the construction of 1,2,3-
triazoles in a single step in an atom-economic manner from
readily available starting materials under very mild
conditions.
R1 Nu
Nu-H
R
(a)
PGHN
O
R2
R1
R1
[O]
R2
PG = Ar
R
R
(b)
15 The transition-metal-catalyzed cross-dehydrogenative coupling
(CDC) reaction has been recognized as a powerful strategy to form
new C–C and C–heteroatom bonds because it allows the use of
simple nonfunctionalized substrates, thus making the synthetic routes
shorter and more efficient.1-4 Among these reactions, the CDC
20 reaction of α-C(sp3)–H centers of α-amino acid derivatives with
various nucleophiles has been used as an efficient method to prepare
α-substituted α-amino acid derivatives (Scheme 1, a).2 On the basis
of these reactions, taking the situ-generated α-aldimine A as an active
intermediate, the transition-metal-catalyzed cross-dehydrogenative
25 coupling-cyclization aromatization reaction of N-arylglycine
derivatives with alkynes or olefins was also carried out in the
presence of oxidants (Scheme 1, b).3 Very recently, Xiao and co-
workers reported a novel iridium-catalyzed [3 + 2] cycloaddition
reaction of N-arylglycine derivatives with isocyanides under light
30 irradiation condition (Scheme 1, c).4
PGHN
PGN
Mn+
R
O
O
N
O
A
O
Ts
R
Ar
R2
NC
R2
(c)
PG = Ar
light
N
N
Scheme 1 Transition-metal-catalyzed cross-dehydrogenative coupling of
α-C(sp3)–H centers of α-amino acid derivatives. PG = protecting group.
Previous work:
O
R2
N
R2
N
cat. [M]
R4
H
H
R4
R4
(a)
(b)
(c)
R1
+
R1
R3
R3
O
N2
N2
R2
N
R2 N2
N
O
O
R4
Photocatalyst
H
H
R1
R1
+
5 W green LED
R3
O
O
R1
R3
This work
R1HN
In addition, diazo compounds have attracted considerable interest
because of their utility in various useful transformation.5 These
compounds are ambiphilic reagents and electrophiles usually attack at
the carbon atom of the diazo group, while the terminal nitrogen of the
35 diazo group is attacked by nucleophiles.6 However, the harsh reaction
Cu, DBU, O2
DMF, rt
R
N
R
R2
+
N
O
N2
N
R2
60
Scheme 2 Coupling reactions of α-diazo compounds with amines.
conditions,
employment
of
transition
metals
and
of
Initially, the reaction of glycine-derived amine 1a with ethyl
diazoacetate (EDA) 2a was examined carefully to optimize the
reaction conditions (Table 1). It was found that the [3+2]
cycloaddition reaction of 1a (0.2 mmol) with 2a (1.0 mmol) could
oxidants in CDC reactions dramatically limited the application
diazo compounds as nucleophiles due to their facile decomposition
under these conditions. In fact, the typical reaction between tertiary
40 amines and diazo compounds in the presence of transition metals is
the direct C−H insertion involving metal carbenoid intermediates
(Scheme 2, a).7 Recently, the first visible-light induced cross-
dehydrogenative coupling reaction between tertiary amines and diazo
compounds was developed by Zhu and co-workers (Scheme 2, b).8 In
45 this context, the [3 + 2] cycloaddition reaction of secondary amines
with α-diazo compounds via a CDC process is highly desired. As part
of our continuing research on the carbon–carbon and carbon-
heteroatom bonds-formation reactions,9,12a herein we report the first
copper-catalyzed [3 + 2] cycloaddition reaction of secondary amines
50 with α-diazo compounds via a CDC process, which works with a
65 afford the desired 1,2,3-triazole 3a in 84% yield in the presence of
CuBr2 (30 mol%) and DBU (0.6 mmol; DBU 1,8-
diazabicyclo[5.4.0]undec-7-ene) in DMF (1.5 mL) at room
temperature for 10
under oxygen atmosphere (entry 3).10
=
h
Decreasing the amount of CuBr2 and the ratio of 1a/2a led to lower
70 yields (entries 1, 2, 5 and 6). In addition, the desired 1,2,3-triazole 3a
was also achieved in 73% yield using air instead of pure oxygen
(entry 4). However, no reaction was observed under otherwise
identical conditions but in the absence of CuBr2 (entry 7). Other
catalysts, such as Cu(OTf)2, CuBr, Cu(OAc)2, CuI, FeCl3 and AgOAc,
75 were less (Table 1, entries 8-11) or not effective (entries 12 and 13).
This journal is © The Royal Society of Chemistry [year]
[journal], [year], [vol], 00–00 | 1

ChemComm
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9
2-Naphthyl
1-Naphthyl
C6H5(CH2)2
4-MeC6H4
4-MeC6H4
OEt
OEt
OEt
NHMe
NMe2
10
16
10
8
75d
60d
43
1i
1j
1k
1l
1m
3i
3j
3k
3l
3m
Among the solvents tested, DMF seemed to be the best choice. Other
solvents, such as CH3CN, dichloroethane (DCE) and toluene, gave
lower product yields (entries 14-16).
10
11
12
13
View Article Online
80
DOI: 10.1039/C5CC02092A
Table 1 Optimization of reaction conditionsa
5
96
a Reaction conditions: 1 (0.2 mmol), 2a (1.0 mmol), CuBr2 (0.06 mmol),
O
O
O
EtO
[M], [O], DBU
(3.0 eq.)
DBU (0.6 mmol), DMF (1.5 mL) at room temperature under oxygen
H
N
b
c
O
atmosphere for 5-17 h. Isolated yield. The reaction was performed at
80 0C. d The reaction was performed at 60 0C.
OEt
OEt
+
N
N
solvent, rt
N2
EtO
N
1a
2a
3a
5
The tandem process mentioned above not only carries out a copper-
30 catalyzed [3 + 2] cycloaddition reaction of secondary amines with
ethyl diazoacetate with molecular oxygen as a green co-oxidant for
the first time, but also provides a simple and efficient route to 1,2,3-
triazoles from readily available starting materials in one-pot in an
atom-economic manner. To test the generality of the new approach,
35 the [3 + 2] cycloaddition reaction of selected secondary amines 1
with a series of α-diazocarbonyl compounds, such as 2-diazo-1-
phenylethanone 2b, 2-diazo-1-(4-methoxyphenyl)ethanone 2c, 2-
diazo-1-p-tolylethanone 2d, 1-(4-chlorophenyl)-2-diazoethanone 2e
and 2-diazo-1-(thiophen-2-yl)ethanone 2f, was investigated. As a
40 result, under otherwise identical conditions as above, polysubstituted
1,2,3-triazoles 3n-v can also be prepared in moderate to excellent
Entry
[M]
[O]
Ratio
(1a/2a)
1/5
1/5
1/5
1/5
1/4
1/3
1/5
1/5
1/5
1/5
1/5
1/5
1/5
1/5
1/5
Solvent T/h
Yieldb
(%)
55
74
84c
73
70
59
0
60
43
67
63
0
(mol%) (1 atm)
1
2
3
4
5
6
7
8
9
CuBr2 (10)
O2
O2
O2
air
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
10
10
10
12
12
12
12
10
12
10
12
12
12
CuBr2 (20)
CuBr2 (30)
CuBr2 (30)
CuBr2 (30)
CuBr2 (30)
----
Cu(OTf)2 (30)
CuBr (30)
10 Cu(OAc)2 (30)
11 CuI (30)
12 FeCl3 (30)
13 AgOAc (30)
14 CuBr2 (30)
15 CuBr2 (30)
0
yields (Scheme 3).
CH3CN 10
36
41
40
O
O
R1
O
DCE
toluene
10
10
O
H
N
CuBr2 (30 mol%), O2
DBU (3.0 eq.), DMF, rt
R2
R2
16
CuBr2 (30)
1/5
+
R
O
R1
R
N
a Reaction conditions: 1a (0.2 mmol), 2a (0.6-1.0 mmol), [M] (0.02-
N2
2b-f
N
3
N
1
0.06 mmol), DBU (0.6 mmol), solvent (1.5 mL), at room temperature
Cl
EtO
EtO
EtO
for 10-12 h. Estimated by 1H NMR spectroscopy using dimethyl
b
O
O
O
O
O
phthalate as an internal standard. c Isolated yield.
S
N
N
N
N
N
N
N
N
N
3n (80%, 5 h)
3o (71%, 5 h)
3p (40%, 10 h)
Under the optimal reaction conditions (Table 1, entry 3), the scope
and generality of the copper-catalyzed [3 + 2] cycloaddition reaction
of secondary amines 1 with ethyl diazoacetate 2a was studied and the
Me2N
O
MeHN
EtO
O
O
O
O
O
N
N
N
N
N
N
N
N
MeO
N
10 results are summarized in Table 2. It is obvious that the cycloaddition
reaction showed broad tolerance for various R substituents of 1. All
selected substrates 1a-k, bearing phenyl (entry 6), electron-rich
(entries 1-5), electron-deficient (entry 7) aryl, heteroaryl (entry 8), 2-
naphthyl (entry 9), 1-naphthyl (entry 10) and alkyl R groups (entry
15 11) on the nitrogen atom, reacted with ethyl diazoacetate 2a to give
the corresponding 1,2,3-triazoles 3a-k in moderate to high yields. In
contrast, substrate 1g with electron-deficient aromatic R group and 1e
with sterically hindered ortho-substituted aromatic R group required
longer reaction times and gave comparatively lower product yields.
20 Similarly, the desired 1,2,3-triazoles 3l and 3m could also be
obtained in 80% and 96% yields from substrates 1l and 1m bearing a
methylamino (entry 12) and dimethylamino (entry 13) R1 group,
respectively.
3q
3r
3s
(78%, 5 h)
(90%, 5 h)
(82%, 5 h)
Me2N
O
Me2N
O
Me2N
O
O
O
O
N
N
N
S
Cl
N
N
N
N
N
N
3t (85%, 5 h)
3u (56%, 5 h)
3v (76%, 5 h)
Scheme 3 CuBr2-catalyzed [3 + 2] cycloaddition reaction of secondary
45 amines 1 with α-diazo compounds 2b-f.
To further probe the mechanisms for formation 3, some control
experiments were designed and investigated. As a result, it was found
that the yield of product 3a decreased greatly from 84 to 30% under
the standard condition when BHT (2.0 equiv.; BHT = 2,6-di-tert-
50 butyl-4-methylphenol) was used as a radical inhibitor (Scheme 4, a).
This result implies that a radical process was involved in the overall
CDC. In addition, according to the results of this transformation, we
predicted that the glycine imine 4a may be a key intermediate in the
reaction. Therefore, the reaction of glycine imine 4a with ethyl
55 diazoacetate 2a was further investigated. As expected, under
otherwise identical conditions as above, the [3 + 2] cycloaddition
reaction of glycine imine 4a with ethyl diazoacetate 2a could proceed,
affording the desired product 3a in 55% yield (Scheme 4, b). In
addition, the reaction of 4a with 2a under a nitrogen atmosphere
60 produced only a trace amount of 3a (Scheme 4, c), indicating that
molecular oxygen is crucial for the reaction. No desired product 3a
was observed under otherwise identical conditions but in the absence
of CuBr2 (Scheme 4, d), which demonstrates that copper salt is also
crucial for the above [3 + 2] cycloaddition reaction.
25 Table 2 CuBr2-Catalyzed [3 + 2] cycloaddition reaction of secondary
amines 1 with ethyl diazoacetate 2aa
O
R1
O
O
O
CuBr2 (30 mol%), O2 (1 atm)
DBU (3.0 eq.), DMF, rt
H
+
N
R
OEt
2a
R1
N
R
EtO
N2
N
N
1
3
Entry
R
R1
OEt
T/h
10
10
10
10
17
12
16
10
Yieldb (%)
1
1a
1b
1c
1d
1e
1f
3
1
2
3
4
5
6
7
8
4-MeC6H4
4-MeOC6H4
3-MeOC6H4
3-MeC6H4
2-MeC6H4
C6H5
84
88
88
75
48
75
50c
72
3a
OEt
OEt
OEt
OEt
OEt
OEt
OEt
3b
3c
3d
3e
3f
4-ClC6H4
3-Thienyl
1g
1h
3g
3h
2
|
Journal Name, [year], [vol], 00–00
This journal is © The Royal Society of Chemistry [year]

Page 3 of 3
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O
H
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+
OEt
OEt
OEt
OEt
3a (30%) (a)
standard conditions
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2a
CuBr2 (30 mol%), O2
DBU (3.0 eq.), DMF, rt
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(b)
3a
(55%)
O
CuBr2 (30 mol%), N2
X
N
(c)
(d)
3a
3a
+
DBU (3.0 eq.), DMF, rt
N2
2a
DBU (3.0 eq.), O2
4a
X
DMF, rt
Scheme 4. Control experiments for mechanistic studies.
On the basis of the above experimental results together with related
reports,11,12 a possible mechanism of [3 + 2] cycloaddition reaction is
proposed in Scheme 5. Initially, the glycine-derived amines 1 are
oxidized by CuBr2 through a single-electron transfer (SET) process to
generate the ammoniumyl radical cation intermediate I (Scheme 5).11
Subsequently, hydrogen transfer or a combination of electron and
proton transfer forms the iminium salt intermediate II (Scheme 5).11
5
10 Finally, the reactive intermediate II undergoes
a 1,3-dipolar
cycloaddition reaction with α-diazo compounds 2 to afford the
intermediate III, which then is oxidized by copper ion to produce the
corresponding 1,2,3-triazole product 3 (Scheme 5).12
65
1
CuBr2
O2
70
H
N
O
O
CuBr2
H
N
-e-, -H+or -H
R
R1
O
R
R1
CuBr
CuBr2
2 I
II
O
O
O
75
R1
N R
R1
O
R2
[O]
[Cu2+
N2
[3+2] cycloaddition
R2
R2
]
N R
N
N
N
N
III
15 Scheme 5. Proposed mechanism for formation of 3.
3
80
In conclusion, we have developed a novel copper-catalyzed [3 + 2]
cycloaddition reaction of secondary amines with α-diazo compounds
for the first time via a CDC process. The reaction involves a
85
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4232.
20 sequential
aerobic
oxidation/[3+2]
cycloaddition/oxidative
aromatization procedure and provides a new and efficient method for
the construction of 1,2,3-triazoles in a single step in an atom-
economic manner. The reaction features readily available starting
materials, simple operation, mild reaction conditions, good to
25 excellent yields, broad substrate scope and using molecular oxygen as
a green co-oxidant. Further studies are in progress.
90
95
9 For selected recent reports, see: (a) L. Li, Y.-L. Zhao, Q. Wang, T. Lin
and Q. Liu, Org. Lett., 2015, 17, 370; (b) H. Wang, Y.-L. Zhao, L. Li,
S.-S. Li and Q. Liu, Adv. Synth. Catal., 2014, 356, 3157; (c) H. Zhang,
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Chem. Int. Ed., 2013, 52, 2529; (d) X. Xu, L. Zhang, X. Liu, L. Pan
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10 CCDC 1026335 (3a) contains the supplementary crystallographic data
in the Cambridge Crystallographic Data Centre.
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Acknowledgements
Financial supports of this research by the National Natural
Sciences Foundation of China (21472017 and 21172032) are
30 greatly acknowledged.
100
105
110
Notes and references
a
Department of Chemistry, Northeast Normal University, Changchun,
130024, China. E-mail: zhaoyl351@nenu.edu.cn
b Changchun No. 104 Junior High School, Changchun, 130041, China.
35 † Electronic Supplementary Information (ESI) available: Experimental
procedures, characterization of data for all new compounds. CCDC
1026335 (3a). See DOI: 10.1039/b000000x/
12 (a) H. Wang, Y.-L. Zhao, L. Li, Z.-W. Zhang and Q. Liu, Adv. Synth.
Catal., 2013, 355, 1540; (b) T. Kano, T. Hashimoto and K. Maruoka, J.
Am. Chem. Soc., 2006, 128, 2174.
1 For selected reports, see: (a) S. A. Girard, T. Knauber and C.-J. Li,
40
Angew. Chem. Int. Ed., 2014, 53, 74; (b) C.-L. Sun and Z.-J. Shi, Chem.
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