A general and mild Cu-catalytic: N -arylation of iminodibenzyls and iminostilbenes using unactivated aryl halides

Article abstract of DOI:10.1039/c7ra07437a

A ligand-free, highly efficient Cu-catalytic N-arylation of iminodibenzyl and iminostilbene derivatives with a broad scope of unactivated aryl halides under mild conditions has been developed for the first time. Moreover, the first Ni-based catalytic syst

Full text of DOI:10.1039/c7ra07437a

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A general and mild Cu-catalytic N-arylation of
iminodibenzyls and iminostilbenes using
unactivated aryl halides†‡
Cite this: RSC Adv., 2017, 7, 49600
*
*
Bin Zhang, Rongrong Li, Huajiang Jiang, Xiaoying Chen and Fang Li
Wubing Yao,
A ligand-free, highly efficient Cu-catalytic N-arylation of iminodibenzyl and iminostilbene derivatives with
a broad scope of unactivated aryl halides under mild conditions has been developed for the first time.
Moreover, the first Ni-based catalytic system was also applied to the N-arylation of pre-existing
derivatives. These novel protocols provide facile and convenient access to the construction of
dibenzazepines and offer promising alternatives to the widely used palladium catalysts.
Received 6th July 2017
Accepted 25th September 2017
DOI: 10.1039/c7ra07437a
rsc.li/rsc-advances
Iminodibenzyl and iminostilbene analogues are value-adding syntheses.5 Recently, classic copper-catalysed Friedel–Cras
building blocks for the construction of drug molecules1 and cycliacylations have also been applied to the synthesis of these
notably the synthesis of organic functional materials, including functionalized rings.6 However, the above approaches still have
organic light emitting diodes (OLEDs) (Scheme 1).2 The appli- obvious limitations: (1) the cyclization reactions are oen
cations have generated tremendous attention and demands to limited by the availability of manufacturable substrates; (2) the
develop efficient, practical and general processes for the prep- multi-step processes always suffer from drawbacks such as
aration of these functionalized ring derivatives.
narrow functional group compatibility and low catalytic effi-
In recent years, a series of synthetic methods, mainly cata- ciency; and (3) the Friedel–Cras cycliacylations typically
lyzed by noble-metal catalytic systems, for the construction of require harsh reaction conditions (excessive reagents and
iminodibenzyls and iminostilbenes have been reported, such as super-lewis acid catalysts).6a
the cyclization of amines3 and anilines,4 or multi-step
An alternative, but less developed synthetic strategy for
iminodibenzyl and iminostilbene analogues is to use aryl
halides as the arylating agents. Industrially, aryl halides are the
preferred reagents because they are widely available and really
low-cost. However, the N-arylation of these dibenzazepines with
aryl halides is still rare, and mainly occupied by the precious
metal Pd.2a,e,i,7–10 Very recently, Buchwald reported that
a Ruphos-based palladacycle complex catalyses the N-arylation
of iminodibenzyls and iminostilbenes with aryl halides.11 The
reaction occurred at 80–100 ꢀC in 1,4-dioxane with 0.1–1 mol%
Pd in the presence of 1.1–2.2 equivalents of Li(NSiMe3)2. This
work represents a breakthrough in the catalytic N-arylation of
iminodibenzyls and iminostilbenes. However, to the best of our
knowledge, the earth-abundant, environmentally benign, and
low-cost base-metal catalyzed N-arylation of these functional-
ized rings has remained undisclosed, although a stoichiometric
arylating process with copper under high temperature (180–200
ꢀC) has been reported.2a So, for dibenzazepine derivatives, it is
still necessary to develop more efficient and practical methods.
Herein, we rstly demonstrate that copper oxide is remark-
ably active for the N-arylation of iminodibenzyls and iminos-
tilbenes with unactivated aryl iodides, bromides and even aryl
chlorides under mild conditions, providing the desired prod-
ucts in moderate to excellent yields. Furthermore, the rst Ni-
catalyzed arylation of dibenzazepines was also developed.
Scheme 1 The represented dibenzazepines.
Taizhou Univ, Dept Chem, Jiaojiang 318000, Zhejiang, PR China. E-mail: yaowb@tzc.
edu.cn; jhj@tzc.edu.cn
† Dedicated to Professor Qizhong Zhou on the occasion of his 45th birthday.
‡ Electronic supplementary information (ESI) available: Experimental procedures,
characterization data, and NMR and HRMS (ESI) spectra. See DOI:
10.1039/c7ra07437a
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Our initial studies focused on the direct N-arylation of imi- even aryl chlorides containing electron-decient groups (2b and
nodibenzyl (1a) with bromobenzene (2a) as a coupling partner 2c) or electron-rich groups (2d–2h) were all well-tolerated in this
by testing the catalytic activities of commercially available transformation, offering the products in moderate to excellent
copper catalysts (Table 1, entries 1–5). Gratifyingly, the reaction yields. Furthermore, the amine- or phenyl-substituted
with copper oxide showed the topmost efficiency and gave the
desired product 3a in 71% yield (entry 1). More importantly,
aer evaluating a variety of bases, the catalytic efficiency was
Table 2 The Cu-catalyzed N-arylation of dibenzazepinesa
improved by the substitution of the base from KOH to KOtBu
(95%, entry 9), while the processes using KOMe, KOAc and
K2CO3 showed unsatisfactory yields (entries 6–8). Further opti-
mization revealed that the solvents also have enormous impact
on this arylation reaction. DMSO proved to be the best medium
among the solvents studied, giving 95% of 3a in the presence of
5 mol% CuO at 100 ꢀC (entry 9). However, when using other
solvents, including toluene (PhMe), THF, DMF, 1,4-dioxane,
CH3CN or CH3OH, the reactions were not suitable for arylation
(entries 10–15). To our delight, the high activity of CuO allowed
ꢀ
this transformation to proceed at 80 C, furnishing 3a in 95%
yield (entry 16). On the other hand, it should be noted that this
ꢀ
process will not proceed at 25 C or in the absence of copper
catalysts (entries 17–18).
Using the highly active copper catalyst and optimal reaction
conditions (Table 1, entry 16), the substrate scope of the reac-
tion was explored by altering the dibenzazepines and aryl
halides (Table 2). Importantly, the aryl bromides, iodides, or
Table 1 The optimization of the reaction conditionsa
Entry
[Cu]
Solvent
Base
T (ꢀC)
Yieldb (%)
1
2
3
4
5
6
7
8
CuO
CuCl
CuCN
Cu(OAc)2
CuI
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
PhMe
KOH
KOH
KOH
KOH
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
80
71
42
35
47
42
40
0
0
95
7
KOH
CuO
CuO
CuO
CuO
CuO
CuO
CuO
CuO
CuO
CuO
CuO
CuO
—
KOMe
KOAc
K2CO3
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
9
10
11
12
13
14
15
16
17
18
THF
DMF
16
77
19
0
1,4-Dioxane
CH3CN
CH3OH
DMSO
DMSO
DMSO
0
95(90)
0
0
25
25
a
a
Reaction conditions: iminodibenzyl (1.0 mmol), bromobenzene (2.0
Reaction conditions: 1 (1.0 mmol), 2 (2.0 mmol), CuO (50 mmol,
mmol), [Cu] (50 mmol), base (2.0 mmol) and solvent (2.0 mL) under Ar 5 mol%), KOtBu (2.0 mmol) and THF (2.0 mL) under Ar atmosphere
b
atmosphere at 80–100 ꢀC for 18 h. The GC yield based on at 80 ꢀC for 18 h; the yields of the isolated products are shown; the
iminodibenzyl (1a) with mesitylene as an internal standard, and the approximate ArH yields in the parentheses were estimated by GC
isolated yield is in parentheses.
analysis. b CuO (0.1 mmol, 10 mol%), at 100 ꢀC.
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substrates (2h and 2i) also proceeded smoothly with iminodi- an aryl bromide as the arylating partner. However, the intro-
benzyl and produced chloroiminodibenzyl, which also under- duction of a stronger electron-withdrawing group (–CF3) at the
went arylation smoothly using the standard conditions. para-position of the aryl halides gave reduced yields of the
Moreover, the yields of the arylating products in most cases corresponding products (3c). In addition, iminostilbenes are
indicate that the reaction occurred more smoothly when using suitable substrates in the same arylation reaction, as demon-
strated by the coupling with bromo-, iodo- or even chloro-
benzene to form 3k and 3l in moderate to useful yields.
Similarly, nickel-based catalysts are also recognized in C–N
bond-formation reactions, constituting an essential part of the
Table 3 The Ni-catalyzed N-arylation of dibenzazepinesa
amination methodologies.12 Moreover, the catalytic N-arylation
of iminodibenzyls and iminostilbenes has not been disclosed so
far. Therefore, we focused our attention on developing the N-
arylation of dibenzazepines catalyzed by the low-cost and
abundant metal Ni.
Aer the optimization of the reaction conditions,13 we eval-
uated dibenzazepines and various aryl halides (Table 3). The
unactivated aryl bromides or iodides were also smoothly con-
verted into the corresponding products in moderate to excellent
yields at 100 ꢀC aer 24 h with 5 mol% NiO and 10 mol% PPh3.
Many synthetically important functional groups, including
uorine (5b), triuoromethyl (5c), alkyl (5d and 5e), methoxyl
(5f and 5g), N,N-dimethyl (5h) and phenyl (5i) groups, are all
well tolerated with the yields of iminodibenzyl ranging from
49% to 91%. On the other hand, chlorine-containing iminodi-
benzyl (4j) also gave the desired dibenzazepine in excellent yield
(90%). Furthermore, the coupling of iminostilbenes (4k and 4l)
with aryl halides also formed the desired products 6k–6l in
useful yields.
To investigate the possibility of a radical-mediated pathway,
we explored the Cu-catalyzed arylation of dibenzazepines (Table
4). Consequently, the reactions of 1a with bromobenzene in
standard conditions using commonly available radical scaven-
gers, including 9,10-dihydroanthracene (entry 2), BHT (butyl-
ated hydroxytoluene) (entry 3), or TEMPO (2,2,6,6-
Table 4 The effects of radical scavengers on the N-arylation of 1aa
Entry
Additive
Equivalent
Yieldb (%)
1
2
—
—
95(90)
77
56
74
49
9,10-Dihydroanthracene
0.5
1.0
0.5
1.0
0.5
1.0
3
4
BHT
a
Reaction conditions: 4 (1.0 mmol), 5 (2.0 mmol), NiO (50 mmol,
TEMPO
76
58
5 mol%), PPh3 (0.1 mmol, 10 mol%), KOtBu (2.0 mmol) and THF (2.0
mL) under Ar atmosphere at 100 ꢀC for 24 h; the yields of the isolated
a
products are shown; the approximate ArH yields in the parentheses
Reaction conditions: 1a (1.0 mmol), ArBr (2.0 mmol), CuO (50 mmol,
b
were estimated by GC analysis. NiO (0.1 mmol, 10 mol%) and PPh3 5 mol%), KOtBu (2.0 mmol), additive (0.5 or 1.0 equiv.), and THF (2.0
c
b
(0.2 mmol, 20 mol%). NiO (0.1 mmol, 10 mol%) and PPh3 mL), under Ar atmosphere at 80 ꢀC for 18 h. The yields were
(0.2 mmol, 20 mol%) at 120 ꢀC for 24 h.
determined by GC, and the isolated yield is in parentheses.
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tetramethylpiperidinooxy) (entry 4), were conducted. However, 100 equivalents of Hg or 1.0 equivalent of PMe3 (relative to Ni).
these reactions indicated slightly decreased yields of 3a Therefore, these results suggest that the Cu catalyst is likely to
compared with the reaction in the absence of radical be homogeneous under the standard conditions, whereas the
scavengers.
Meanwhile, in order to gain insight into the reaction
reactions catalyzed by nickel are possible with metal particles.
pathway, we also carried out a radical-clock experiment using
substrate 7, which is a radical well-known for 5-exo-trig cycli-
Conclusions
zation to form 1-methylindane aer H-atom abstraction.14 We have developed the rst and efficient approach for the N-
Under the current conditions, the reaction of 7 with iminodi- arylation of iminodibenzyl and iminostilbene derivatives with
benzyl 1a failed to produce the cyclization product 8 (Scheme 2). aryl bromides, iodides and even aryl chlorides, using the really
In summary, these results all indicated that this Cu-catalytic simple copper reagent CuO in the absence of any ligand.
reaction might not appear to go through a radical process.
Moreover, a Ni-based catalyst was also applied to the N-arylation
To distinguish homogeneous from heterogeneous catalysis, of these derivatives for the rst time. Featuring broad substrate
we conducted the reactions of iminodibenzyl with bromo- scopes, low-cost metal-based catalysts, and mild conditions,
benzene in the presence of the commonly used heterogeneous these novel methods are practical and general processes for the
catalyst poisons liquid Hg15 and PMe3 (ref. 15d) (Table 5). The N-arylation of dibenzazepines, which are useful for a variety of
addition of Hg or PMe3 resulted in a slight decrease of the yield value-adding applications.
of 3a in comparison with that without the heterogeneous cata-
lyst poison in the Cu-based catalytic system. However, the
reactions catalysed by the metal Ni showed that the yield of the
Conflicts of interest
N-arylation process decreased signicantly in the presence of There are no conicts to declare.
Acknowledgements
We gratefully acknowledge the nancial support from the
National Natural Science Foundation of China (21542010),
Science and Technology Plan Project of the Zhejiang Province
(LY14B020012, 2016C37040), and Taizhou Science and Tech-
nology Project (No. 131KY08).
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