Efficient aerobic cross-dehydrogenative coupling of tertiary amines over NiGa layered double oxide

Article abstract of DOI:10.1016/j.tetlet.2020.152254

An efficient and convenient heterogeneous catalytic system has been developed for the aerobic cross-dehydrogenative coupling over NiGa layered double oxide (Ni₃Ga-LDO) without any additives. The results indicated that the catalytic system could

Full text of DOI:10.1016/j.tetlet.2020.152254

Journal Pre-proofs
Efficient aerobic cross-dehydrogenative coupling of tertiary amines over Ni‐
Ga layered double oxide
Zhonghua Sun, Zhenzhen Xia, Anwei Wang, Hui Wang, Zhong Zhang,
Weiyou Zhou, Junfeng Qian, Mingyang He
PII:
S0040-4039(20)30721-8
DOI:
Reference:
https://doi.org/10.1016/j.tetlet.2020.152254
TETL 152254
To appear in:
Tetrahedron Letters
Received Date:
Revised Date:
Accepted Date:
3 June 2020
5 July 2020
12 July 2020
Please cite this article as: Sun, Z., Xia, Z., Wang, A., Wang, H., Zhang, Z., Zhou, W., Qian, J., He, M., Efficient
aerobic cross-dehydrogenative coupling of tertiary amines over NiGa layered double oxide, Tetrahedron Letters
(2020), doi: https://doi.org/10.1016/j.tetlet.2020.152254
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Efficient aerobic cross-dehydrogenative coupling of tertiary
amines over NiGa layered double oxide

Zhonghua Sun, Zhenzhen Xia, Anwei Wang, Hui Wang, Zhong Zhang, Weiyou Zhou*, Junfeng Qian,
Mingyang He
Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, Changzhou University, Changzhou 213164, China
Abstract: An efficient and convenient heterogeneous catalytic system has been developed for the aerobic
cross-dehydrogenative coupling over NiGa layered double oxide (Ni3Ga-LDO) without any additives. The
results indicated that the catalytic system could tolerate various tetrahydroquinoline derivatives and nucleophiles
using molecular oxygen as a sustainable oxidant. The catalyst could be recycled four times without significantly
decrease of activity or change of XRD pattern, indicating that Ni3Ga-LDO was catalytically and structurally
stable in the cross-coupling reaction.
Keywords: cross-dehydrogenative coupling; NiGa layered double oxide; molecular oxygen; C–H activation;
N-phenyltetrahydroisoquinoline
Cross-dehydrogenative coupling (CDC) reaction has been an attractive strategy for constructing C–C bonds
under oxidative conditions, because it avoids prefunctionalization of the coupling partners [1]. During the past
decade, considerable progress has been achieved in oxidative CDC reactions and various homogeneous catalytic
systems based on Cu [2], Fe [3], Ru [4], Au [5], Rh [6], Co [7], and metal-free systems [8], have been developed
using molecular oxygen or peroxides as the oxidant. Even so, it is hard for homogeneous systems to achieve high
catalytic efficiency, sustainability and cost-effectiveness. Some heterogeneous catalysts originated from Au [9],
Ru [10] and some photocatalyzed systems [11] have been developed for CDC reactions. However, precious
metals applied for these catalysts will limit their wide application to some extent.
Some noble-metal-free materials have been introduced into CDC reactions. Hudson et al. [12] reported that
CuFe2O4 nanoparticles could effectively catalyze the coupling reaction of two sp3 carbons using DDQ
(2,3-dichloro-5,6-dicyanobenzoquinone) as the oxidant. Liu et al. developed a SBA-15-support iron terpyridine
[13] catalyst, which exhibited excellent catalytic activity for the C‒C cross-coupling reactions in the presence of
TBHP (tert-butyl hydroperoxide). Marset et al. [14] developed a magnetite supported copper(II) oxide catalyst
for the synthesis of different tetrahydroisoquinolines via CDC reaction using choline chloride/ethylene glycol as
*
Corresponding author. Tel./Fax: +86 51986330251.
E-mail address: zhouwy426@126.com.
1

a deep eutectic solvent, but the efficiency was quite low. Sharma et al. [15] found that CeO2 could efficiently
catalyze the C‒H functionalization of N-aryltetrahydroisoquinolines via CDC reactions. Recently, Dutta et al.
[16] disclosed a heterogeneous catalytic approach for selective coupling of C1 of N-aryltetrahydroisoquinolines
with C3 of indoles over mesoporous manganese oxides using molecular oxygen as the oxidant. Still and all,
some of the above systems required complicated preparation methods for catalysts, or were only investigated in
limited substrate scope. In addition, peroxides were always introduced as the oxidant, which are corrosive and
unstable at room temperature, making them less suitable for large scale industrial reactions. In view of the
principles of green and practical chemistry, developing efficient heterogeneous catalytic system based on readily
available materials using molecular oxygen without any additives for the CDC reaction is still highly desirable.
In our study on the heterogeneous catalytic materials for the aerobic CDC reactions, we found that NiGa
layered double oxide (Ni3Ga-LDO) could efficiently accelerate the transformation using molecular oxygen as the
sole oxidant without any additives. Although nickel has emerged as a good choice to replace precious transition
metals for the C−H bond activation, including various CDC reactions [17], heterogeneous catalyst based on
nickel has rarely been reported. Henceforth, we report a facile and operationally convenient catalytic system for
the aerobic oxidative coupling of tertiary amines with various pro-nucleophiles based on Ni-containing material
under mild conditions.
Initially, the cross-coupling between N-phenyltetrahydroisoquinoline 1a and nitromethane 2a was chosen as a
model reaction to optimize the reaction conditions (Table 1). Various solvents were firstly screened in the model
reaction. It is observed that all the selected solvents could give good selectivity to the desired product except
acetonitrile and toluene (entries 1-8), suggesting that polar environment might benefit to the coupling reaction.
The main byproduct was 2-phenyl-3,4-dihydroisoquinolin-1(2H)-one (4), which should be produced from
over-oxidation, in consistent with reported results [7,17a]. The low selectivity might also imply that the rate of
over-oxidative amide formation was higher than that of enamine nucleophilic attack under acetonitrile or
toluene. However, the conversions were generally unsatisfactory under the selected solvents (entry 2), and the
highest conversion of 1a was observed using dioxane as the solvent with a satisfied selectivity. Concerning the
reaction temperature, decreasing the temperature to 60 ℃ resulted in significantly reduced 52% conversion of 1a
and 32% selectivity of 3aa (entry 9). When the reaction performed under an increased temperature of 100 ℃, the
selectivity of 3aa was 68% (entry 10), which should be due to the over-oxidation of the substrate. Decreasing the
ratio of 2a/1a to 2 or 4, reduced conversion and selectivity were observed, whilst further increasing the value to
8 only gave slightly improved yield of 3aa (etnries 11-13). Prolonging the reaction time to 16 h led to full
2

conversion and an increased 92% selectivity of 3aa. Further increasing the time could not improve the result
significantly. According to the proposed reaction path for the CDC reaction of N-aryltetrahydroisoquinolines
[2g,11c,11d], it is generally believed that iminium cation is firstly formed via oxidative dehydrogenation, and a
nucleophile attacks the intermediate to afford the coupled products. In the present catalytic aerobic CDC
reactions, the fact that the selectivity increased with the reaction time should be related to the formation of
iminium intermediate. These observations also suggested that the nucleophilic attack reaction was slower than
the oxidative dehydrogenation. Summary, an efficient heterogeneous catalytic system based on Ni3Ga-LDO has
been developed for the CDC reaction between N-phenyltetrahydroisoquinoline and nitromethane using
molecular oxygen as the sole oxidant.
Table 1 Optimization of the reaction conditions.a
Temp.
/℃
Conv. (Sel.)b
/%
Entry
Solvent
2a/1a
1
2
acetonitrile
1,4-dioxane
80
80
6
6
12 (67)
93 (85)
3
4
5
trifluorotoluene
mesitylene
DCE
80
80
80
6
6
6
21 (98)
18 (93)
13 (92)
6
7
toluene
H2O
80
80
80
60
100
80
80
80
80
80
6
6
6
6
6
2
4
8
6
6
32 (71)
43 (98)
18 (95)
52 (32)
>99 (68)
89 (72)
91 (75)
94 (86)
>99 (92)
>99 (93)
8
DMSO
9
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
10
11
12
13
14c
15d
a Reaction conditions: 1a 0.5 mmol, Ni3Ga-LDO 0.2 g, solvent 2 mL, 12 h, O2 atmosphere (1 atm).
b Based on the 1H-NMR analysis.
c 16 h.
d 20 h.
With the optimized reaction conditions have been obtained, various substrates have been tested in the CDC
reaction under analogous reaction conditions (Table 2). A variety of N-phenyltetrahydroquinoline derivatives
with different substituents were firstly introduced into the Ni3Ga-LDO-catalyzed aerobic cross-coupling
reaction. It can be observed that substrates with electron-donating substituents on the para-site of the aromatic
rings could be well tolerated by the catalytic system, and good to excellent yields of the coupled product were
3

obtained (entries 2-4), comparable with the reported Fe-based catalytic systems [3a]. When groups on the meta-
or/and ortho-sites of the aromatic rings, prolonged reaction time were required for the full conversion of the
substrates
(entries
5-8),
indicating
the
existence
of
steric
effect.
Delightedly,
N-(3-methylphenyl)-1,2,3,4-tetrahydroisoquinoline gave an excellent 88% isolated yield of the corresponding
CDC product (3ea) with a longer reaction time. On the other hand, for the substrates with electron-withdrawing
groups locating on the para-sites of aromatic rings, significantly longer reaction times were required to finish the
reaction and relatively lower yields of the corresponding coupled products were obtained (entries 9-11). These
observations indicated that electron-withdrawing groups suppressed the CDC reactivity, and the electronic effect
existed in the catalytic system. For the compound 1l with two methoxyl groups on 6- and 7-sites, it could also
give a good yield for the target product (entry 12), and methoxyl group activated the substrate. On the contrary,
electron-withdrawing substituent, such as Br on the 7-site reduced the reactivity, and longer reaction time was
required (entry 13). Even though, a good 75% yield of the coupling product was obtained. On the whole,
moderate to good yields of the products could be obtained under the selected conditions for these substrates.
N-phenylpiperidine was also tested in the present catalytic system. Only a 23% yield of the corresponding
coupled product could be obtained (entry 14), which might be due to its low reactivity in the formation of
iminium intermediate, because it is not stable in the case [18].
In addition, some other nucleophiles with Csp3‒H have also been tested. Nitroalkanes with different carbon
atoms could be well tolerated in the catalytic system (3ab and 3ac), and good yields were obtained, behaving
comparably performance with the homogeneous Cu systems [2a]. The reactivity discernibly decreased as the
carbon atom increase, which was probably due to the steric hindrance effect (entries 15-16). For the carbonyl
compounds with α-C‒H, cyclohexanone and acetophenone could not gave satisfied yield of the coupled product
(entry 17), which was possibly ascribed to their steric hindrance and the lower nucleophilicity. Trimethylsilyl
cyanide, as a typical cyanide source, was also investigated in the catalytic system, and a good 73% yield was
obtained (entry 19). It can be concluded from the above results that N-phenyl-1,2,3,4-tetrahydroisoquinoline and
its derivatives and some nucleophiles were compatible in the Ni3Ga-LDO -catalyzed aerobic CDC reaction. They
could uniformly provide the corresponding products in good to excellent isolated yields in appropriate reaction
times.
4

Table 2 Reaction scope for the Ni3Ga-LDO-catalyzed aerobic CDC reaction.a
Entry
1
1
2
3
Time /h
16
Yieldb /%
84
CH3NO2
2a
1a
1b
3aa
3ba
2
2a
16
78
3
4
2a
2a
16
16
72
69
1c
3ca
1d
3da
3ea
5
6
7
2a
2a
2a
24
24
36
88
72
76
11
e
1f
3fa
1g
3ga
8
2a
24
62
3ha
1h
9
2a
2a
24
24
59
62
1i
1j
3ia
3ja
10
5

11
12
2a
2a
24
20
68
69
1k
1l
3ka
3la
13
14
2a
2a
36
48
75
23
1m
3ma
3na
1n
1a
15
16
24
24
78
73
2b
2c
3ab
3ac
1a
1a
17
36
46
2d
3ad
18
19
1a
1a
36
24
40
73
2e
2f
3ae
3af
a Reaction conditions: 1 0.5 mmol, 2 3 mmol, Ni3Ga-LDO 0.2 g, 1,4-dioxane 2 mL, 80 ℃, O2 atmosphere (1 atm).
b Isolated yield.
To confirm the heterogeneity of Ni3Ga-LDO-catalyzed system, a hot filtration experiment has been conducted
under the standard conditions. After 4 h (～38% conversion of 1a), the filtrate afforded only slight additional
conversion for another 8 h under the same conditions (Fig. 1). These observations implied that Ni3Ga-LDO was
a true heterogeneous catalyst in the reaction, and leaching of active metals could be ignored. The catalytic
6

stability of Ni3Ga-LDO was also tested in the research. After the reaction, the catalyst was filtrated, washed with
ethyl acetate and dried at 120 ℃ for 4 h to recycle the catalyst. The results showed that only slight reduction of
activity and selectivity were observed for the catalyst even after four runs (Fig. S1). XRD pattern for the
recycled catalyst indicates that the structure of the NiGa hydrotalcite double oxide was basically preserved after
several reuses (Fig. S2).
Fig. 1. The hot-filtration experiment. Reaction conditions: 1a 0.5 mmol, 2a 3 mmol, Ni3Ga-LDO 0.2 g,
1,4-dioxane 2 mL, 80 ℃, O2 atmosphere (1 atm).
To validate the possible reaction pathways, some controlled experiments have been conducted under the
standard reaction conditions (Table 3). In the absence of catalyst, Only a 45% selectivity could be obtained with
a significantly lower conversion, implying the catalytic activity of Ni3Ga-LDO for the CDC reaction (entry 2).
When oxygen was replaced by air, markedly decreased catalytic activity was observed, indicating that molecular
O2 was important for the Ni3Ga-LDO-catalyzed CDC reaction. In addition, performing the reaction under
nitrogen atmosphere still led to a 20% conversion of 1a, implying that oxygen was not necessary at the initial
stage of the reaction. In all cases with catalyst, the selectivity for the coupled product were significantly higher
than that without catalyst (entries 1-4), indicating that Ni3Ga-LDO was effective for the coupling reaction, which
might be related to the surface acidity of catalyst [19]. To further elucidate the catalytic performance of
Ni3Ga-LDO, Mg3Ga-LDO and Ni3Al-LDO were subjected into the reaction as comparisons. Under the standard
reaction conditions, lower conversion of 1a was observed in the case of Mg3Ga-LDO, while Ni3Al-LDO could
gave full conversion, suggesting that Ni might be the catalytic active site. However, quite lower selectivity to
3aa was obtained in both cases, which might be related to the varied surface property of these catalysts, which is
affected mainly by the compositions of layered double oxide [20].
7

When the reaction was conducted with 2,6-di-tert-butyl-4-methylphenol (BHT) or CCl3Br as a radical
scavenger, quite lower conversion of the substrate and selectivity to 3aa were obtained (entry 6), although only a
slight decrease of conversion was observed in the case of CCl3Br. These results implied that a radical pathway
was probably involved in the Ni3Ga-LDO-catalyzed aerobic CDC reaction of N-aryl tetrahydroisoquinolines.
Table 3 The results of some controlled experiments.a
Reaction
Conv.b
/%
Sel.b
/%
Entry
1
Catalyst
conditions
Ni3Ga-LDO
O2
>99
92
2
3
-
O2
air
79
67
45
84
Ni3Ga-LDO
4
5
6
7
Ni3Ga-LDO
Ni3Al-LDO
Mg3Ga-LDO
Ni3Ga-LDH
N2
O2
O2
O2
20
>99
92
80
51
62
65
97
8
9
Ni3Ga-LDO
Ni3Ga-LDO
O2/BHTc
56
97
76
48
O2/CCl3Brc
a Reaction conditions: 1a 0.5 mmol, 2a 3 mmol, Ni3Ga-LDO 0.2 g, 1,4-dioxane 2 mL, 80 ℃, 16 h, O2 atmosphere (1 atm).
b Based on the 1H-NMR analysis.
c 2 Equiv. of substrate.
The possibly formed iminium intermediate was also examined in the CDC reactions by analysing the reaction
mixture performed without a nucleophile [2,3]. The iminium could be obviously observed at 208.1 apart from
the substrate peak (210.1) (Fig. S3), confirming the formation of iminium ion. The controlled experiment has
shown that both of the conversion and selectivity decreased without catalyst (Table 3, entries 1-2), implying that
Ni3Ga-LDO might accelerate not only the transform of the substrate, but also the nucleophilic attack of the
formed iminium. On the basis of the obtained and reported results [2,3,7,17b,17c], we believe that N-aryl
tetrahydroisoquinoline (1a) is oxidized to iminium ion intermediate (II) in the catalytic system (Scheme 1).
Subsequently, the intermediate reacts nucleophilic species to furnish the desired coupled product 3aa with the
catalysis of Ni3Ga-LDO.
8

Scheme 1. A possible reaction path for the Ni3Ga-LDO-catalyzed aerobic CDC reaction of
N-phenyltetrahydroisoquinolines with nitromethane
In summary, a green and sustainable catalytic system based on NiGa layered double oxide for the aerobic
CDC reaction has been developed. Various substrates could be tolerated by the present protocol, and it provides
an alternative synthetic strategy to achieve CDC reaction for the a-functionalization of N-aryl
tetrahydroisoquinolines via heterogeneous catalysis. Further studies on the application of the protocol to other
substrates and the detailed catalytic mechanism are under way in our laboratory.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could
have appeared to influence the work reported in this paper.
Acknowledgment
This work was supported by Advanced Catalysis and Green Manufacturing Collaborative Innovation Center of
Changzhou University, Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology (BM2012110),
Postgraduate Research &Practice Innovation Program of Jiangsu Province (KYCX19_1746), Qinglan Project of
Jiangsu Province and Natural Science Foundation of Jiangsu Province of China (BK20181461).
Supplementary data
1
Supplementary data (general experimental information, and H and 13C NMR spectral data for all compounds)
associated with this article can be found, in the online version, at doi:0000000.
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12

1. NiGa layered double oxide can catalyze the aerobic oxidative cross-dehydrogenative coupling.
2. A convenient heterogeneous catalytic procedure is developed for the CDC reaction.
3. The catalyst can tolerate various substrates.
4. Good catalytical and structural stable in the coupling reaction have been observed.
13