Palladium-Catalyzed Dual Coupling Reaction of 2-Iodobiphenyls with o-Bromoanilines through C-H Activation: An Approach for the Synthesis of Tribenzo[ b, d, f]azepines

Article abstract of DOI:10.1021/acs.orglett.0c04192

A novel and straightforward approach for the synthesis of tribenzo[b,d,f]azepines starting from 2-iodobiphenyls and 2-bromoanilines has been developed. A wide range of tribenzo[b,d,f]azepines were obtained in good to excellent yields via a cascade intermolecular palladium-catalyzed C-H activation/dual coupling reaction. C,C-palladacycles, which are generated by C-H activation of 2-iodobiphenyls, should be the reaction intermediates.

Full text of DOI:10.1021/acs.orglett.0c04192

pubs.acs.org/OrgLett
Letter
Palladium-Catalyzed Dual Coupling Reaction of 2‑Iodobiphenyls
with o‑Bromoanilines through C−H Activation: An Approach for the
Synthesis of Tribenzo[b,d,f]azepines
Cang Cheng, Dongdong Tu, Xiang Zuo, Zechen Wu, Bin Wan, and Yanghui Zhang*
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ABSTRACT: A novel and straightforward approach for the
synthesis of tribenzo[b,d,f ]azepines starting from 2-iodobiphenyls
and 2-bromoanilines has been developed. A wide range of
tribenzo[b,d,f ]azepines were obtained in good to excellent yields
via a cascade intermolecular palladium-catalyzed C−H activation/
dual coupling reaction. C,C-palladacycles, which are generated by
C−H activation of 2-iodobiphenyls, should be the reaction
intermediates.
enzazepines are seven-membered aromatic-ring-fused aza-
However, synthetic methods for tribenzo[b,d,f]azepines are rare
and should be developed.7
B
heterocycles.1 They are the molecular scaffold of many
pharmaceutical compounds and biologically active natural
Transition-metal-catalyzed C−H activation has made striking
advances over recent decades.8 Compared with classical organic
reactions via the transformation and conversion of reactive
functionalities, C−H functionalization reactions are usually
more step- and atom-economical. More importantly, C−H
functionalization provides innovative strategies for retrosyn-
thetic analysis. However, to achieve high site selectivity,
directing groups have to be used in C−H functionalization
reactions.9 The directing groups need to be preinstalled and
often have to be removed after the reactions. To address this
issue, traceless directing groups have gained considerable
interest.10 In this context, halogens represent ideal traceless
directing groups. Halogens are omnipresent in organic
compounds and can be installed easily. For palladium-catalyzed
halogen-directed reactions, the reactions proceed via a Pd(0)−
Pd(0) mechanism, and therefore, the use of external oxidants is
avoided. Notably, the reactions generate essential synthetic
intermediate C,C-palladacycles, which are particularly useful for
the synthesis of ring compounds. Most of the current reactions
utilizing halogens as the directing groups involve intramolecular
cyclization,11 and intermolecular reactions are comparatively
underdeveloped, except for Catellani reactions.12 Among the
current intermolecular reactions, almost all of them involve the
insertion of one functional group into C,C-palladacycles.
Notably, it has been reported that arylnorbornylpalladacycles
products (Figure 1).2 Developing facile and efficient synthetic
Figure 1. Selected examples of benzazepine drugs and materials.
methods for benzazepines is of paramount significance in drug
discovery and organic synthesis. Although quite a few synthetic
methods for benzazepines have been developed, they are still
relatively underexplored.3 Tribenzo[b,d,f ]azepines, special
benzazepines with three fused aromatic rings, are particularly
intriguing. Tribenzo[b,d,f ]azepines are essential organic electro-
luminescent compounds and have found wide applications in
OLED materials.4 Some tribenzo[b,d,f ]azepine derivatives
exhibit novel pharmacological properties.5 Furthermore, 9H-
tribenzo[b,d,f ]azepines are also essential intermediates in the
synthesis of the functional materials and bioactive molecules.5,6
Received: December 18, 2020
Published: February 8, 2021
https://dx.doi.org/10.1021/acs.orglett.0c04192
© 2021 American Chemical Society
Org. Lett. 2021, 23, 1239−1242
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Letter
can react with aryl bromide/chlorides bearing an ortho chelating
group.13 These reactions are very intriguing, because aryl iodides
and aryl bromide/chlorides react with different Pd species in a
well-defined sequence. Inspired by these novel reactivities, we
envisaged that C,C-palladacycles derived from 2-iodobiphenyl
could be difunctionalized with two different functionalities. By
reacting with 2-bromoanilines, the reaction would form C−C
and C−N bonds and provide tribenzo[b,d,f ]azepines as the
products. However, the amino group of anilines could be
diarylated by C,C-palladacycles to form carbazoles.14 This
undesired reaction should be overcome to achieve the azepine-
forming reaction. Herein we report the Pd-catalyzed cross-
coupling reaction of 2-iodobiphenyls with 2-bromoanilines
through C−H activation, which provides a facile and efficient
synthetic method for tribenzo[b,d,f ]azepines.
product. The impact of inorganic bases on the reactions was also
probed. Replacing Cs2CO3 with K2CO3 resulted in a yield of
86% (entry 8). However, the reactions gave very low yields when
Na2CO3, KOAc, or K3PO4 was used (entries 9−11). In those
reactions, most of 1a and 2a were recovered. The reactions were
considerably or even completely suppressed in other solvents
(entries 12−14). The reaction almost failed to occur when it was
carried out at 100 °C (entry 15). Notably, a yield of 96% was still
obtained when the catalyst loading of Pd(OAc)2 was reduced to
5 mol % by running the reaction for 36 h (entry 16), and a 2 mol
% loading of Pd(OAc)2 gave a yield of 70% (entry 17).
Furthermore, the yield decreased slightly when 1.2 equiv of 2a
was used (entry 16). 2-Iodoaniline was also suitable, and 3aa was
formed in 86% yield (entry 19). However, 2-chloroaniline was
almost inert toward the coupling reaction, and only a trace
amount of 3aa was observed (entry 20). 2-Bromo-1,1′-biphenyl
showed lower reactivity, and a large amount of 2-bromo-1,1′-
biphenyl and 2a were recovered in the reaction (entry 21).
Finally, the reaction could be scaled up to 1 mmol to afford 3aa
in 88% yield (entry 22).
We first investigated the reaction of 2-iodobiphenyl (1a) and
2-bromoaniline (2a) (Table 1). Gratefully, the desired 9H-
Table 1. Survey of the Reaction Conditions
Next, we set out to explore the substrate scope of the protocol
for the synthesis of tribenzo[b,d,f ]azepines. We first examined
the reactions of various 2-bromoanilines. As shown in Scheme 1,
a range of 5-substituted 2-bromoanilines underwent the cross-
coupling reaction efficiently to yield the desired tribenzo[b,d,f ]-
azepine products (3ab−ag). A variety of substituents were
compatible, including electron-donating methoxy and acetami-
do groups, an electron-withdrawing trifluoromethyl group, and
even halo groups. Notably, for the reactions with unsatisfactory
a
entry
ligand (x mol %)
base
solvent
yield (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
−
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
K2CO3
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMSO
MeCN
toluene
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
68
88
73
94
96
40
−
86
11
7
PPh3 (20%)
P(4-FC6H4)3 (20%)
P(p-tol)3 (20%)
P(o-tol)3 (20%)
BANIP (12%)
Scheme 1. 2-Bromoaniline Scope
Xantphos (12%)
P(o-tol)3 (20%)
P(o-tol)3 (20%)
P(o-tol)3 (20%)
P(o-tol)3 (20%)
P(o-tol)3 (20%)
P(o-tol)3 (20%)
P(o-tol)3 (20%)
P(o-tol)3 (20%)
P(o-tol)3 (10%)
P(o-tol)3 (4%)
P(o-tol)3 (10%)
P(o-tol)3 (10%)
P(o-tol)3 (10%)
P(o-tol)3 (10%)
P(o-tol)3 (10%)
Na2CO3
CsOAc
K3PO4
8
12
−
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
−
trace
b
c
h
96 (94 )
d
70
e c
,
91
f c
86 ,
gc
,
trace
ic
12 ,
c hj
,
,
88
a
The yields were determined by 1H NMR analysis of the crude
b
c
d
reaction mixtures. 100 °C. Pd(OAc)2 (5 mol %), 36 h. Pd(OAc)2
(2 mol %), 36 h. 0.24 mmol of 2a. 2-Iodoaniline instead of 2a. 2-
Chloroaniline instead of 2a. Isolated yield. 2-Bromo-1,1′-biphenyl
instead of 1a, 36 h. 1 mmol of 1a and 1.5 mmol of 2a, 48 h.
e
f
g
h
i
j
tribenzo[b,d,f ]azepine product 3aa was obtained in 68% yield
using 10 mol % Pd(OAc)2 and 2 equiv of Cs2CO3 (entry 1).
Next, we screened different ligands to improve the yield (entries
2−5). The yield increased to 88% in the presence of 20 mol %
PPh3 (entry 2) and was further improved to 96% with P(o-tol)3
(entry 5). On the contrary, bidentate phosphine ligands
suppressed the formation of 3aa (entries 6 and 7). In those
reactions, 1-phenyl-9H-carbazole was obtained as the main
a
Pd(OAc)2 (10 mol %), P(o-tol)3 (20 mol %), TBAI (1 equiv).
Pd(OAc)2 (10 mol %), P(o-tol)3 (20 mol %), TBAI (1 equiv), 140
b
°C.
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pubs.acs.org/OrgLett
Letter
yields, the yields could be considerably improved by using
tetrabutylammonium iodide (TBAI) (3ac, 3ad, 3af, and 3ag).15
Intriguingly, for substrate 2g bearing two bromo groups, 2-
iodobiphenyl selectively coupled with the bromo group at the
position ortho to the amino group. The compatibility of 2-
bromoanilines bearing a 4-substituent was also investigated. An
array of 4-substituted 2-bromoanilines 2h−k reacted with 1a,
providing the cross-coupling products in good or excellent yields
in the presence of TBAI. For substrate 2j, which bears a carbonyl
group, the reaction temperature was enhanced to 140 °C to
obtain a satisfactory yield. Furthermore, a range of disubstituted
2-bromoanilines also underwent the dual coupling reaction
smoothly (3al−aq). It is noteworthy that the pyridine substrate
was suitable, affording product 3ar in 55% yield. Finally,
secondary amines were also reactive, and the reations provided
N-protected tribenzo[b,d,f ]azepine products 3as and 3at.
Next, the substrate scope with respect to 2-iodobiphenyls was
also probed (Scheme 2). Various symmetrically substituted 2-
Scheme 3. Mechanistic Studies
Scheme 4. Proposed Mechanism
Scheme 2. 2-Iodobiphenyl Scope
which is produced in situ. Intramolecular C(sp2)−H activation
of the resulting intermediate A gives key palladacycle B. Next,
oxidative addition of 2-bromoaniline to palladacycle B generates
chelated Pd(IV) species C. Subsequent reductive elimination
gives intermediate D. Finally, intramolecular C−N reductive
elimination of D affords the tribenzo[b,d,f]azepine and releases
the Pd(0) species.
In conclusion, we have developed a highly efficient palladium-
catalyzed dual coupling reaction of 2-iodobiphenyls and 2-
bromoanilines for the synthesis of tribenzo[b,d,f ]azepines. The
cross-coupling reactions form C−C and C−N bonds via C−H
activation. This protocol represents the first synthesis of
tribenzo[b,d,f]azepines via C−H activation.
a
Pd(OAc)2 (10 mol %), P(o-tol)3 (20 mol %), TBAI (1 equiv).
iodobiphenyls reacted with 2-bromoaniline efficiently. Electron-
donating methyl and methoxy groups and an electron-
withdrawing trifluoromethyl group were compatible (3ba−
da), and fluoro and chloro groups were well-tolerated in the dual
coupling reaction (3ea and 3fa).
The mechanism of the dual coupling reaction was then
studied. We prepared C,C-palladacycle 4 via the reaction of 2-
iodobiphenyl and Pd(PPh3)4 (Scheme 3).16 Palladacycle 4
could react with 2a under the analogous conditions, and
tribenzo[b,d,f ]azepine 3aa was formed in 30% yield. The low
yield could be caused by slightly different conditions. For
example, when Pd(PPh3)4 was used as the catalyst, the reaction
of 1a and 2a was also low-yielding. In addition, in the case of
unsymmetrically substituted 2-iodobiphenyl 1g, the reaction
produced two isomeric products (3ga-1 and 3ga-2) in a 1:1 ratio
in 91% overall yield. Similarly, other unsymmetrically
substituted 2-iodobiphenyls (1h and 1i) reacted with 2-
bromoaniline to yield a mixture of regioisomers (3ha and 3ia)
in a 3:2 ratio. These experimental results are consistent with a
mechanism involving a C,C-palladacycle as the intermediate.
Therefore, the mechanism shown in Scheme 4 is proposed for
the Pd-catalyzed dual coupling reaction.13b The reaction is
started by the oxidative addition of 2-iodobiphenyl to Pd(0),
ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c04192.
Experimental procedures, characterization data, and
NMR spectra of the products (PDF)
AUTHOR INFORMATION
Corresponding Author
■
Yanghui Zhang − School of Chemical Science and Engineering,
Shanghai Key Laboratory of Chemical Assessment and
Sustainability, Tongji University, Shanghai 200092, China;
orcid.org/0000-0003-2189-3496;
Email: zhangyanghui@tongji.edu.cn
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Organic Letters
Authors
pubs.acs.org/OrgLett
Letter
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Cang Cheng − School of Chemical Science and Engineering,
Shanghai Key Laboratory of Chemical Assessment and
Sustainability, Tongji University, Shanghai 200092, China
Dongdong Tu − School of Chemical Science and Engineering,
Shanghai Key Laboratory of Chemical Assessment and
Sustainability, Tongji University, Shanghai 200092, China
Xiang Zuo − School of Chemical Science and Engineering,
Shanghai Key Laboratory of Chemical Assessment and
Sustainability, Tongji University, Shanghai 200092, China
Zechen Wu − School of Chemical Science and Engineering,
Shanghai Key Laboratory of Chemical Assessment and
Sustainability, Tongji University, Shanghai 200092, China
Bin Wan − School of Chemical Science and Engineering,
Shanghai Key Laboratory of Chemical Assessment and
Sustainability, Tongji University, Shanghai 200092, China
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c04192
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Natural Science
Foundation of China (21971196).
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