Rh(III)-Catalyzed C-H Activation of Boronic Acid with Aryl Azide

Article abstract of DOI:10.1021/acs.orglett.8b02247

A Rh(III)-catalyzed C-H activation of boronic acid with aryl azide to obtain unsymmetric carbazoles, 1H-indoles, or indolines has been developed. The reaction constructs dual distinct C-N bonds via sp²/sp³ C-H activation and rhodium nitrene insertion. Synthetically, this approach represents an access to widely used carbazole derivatives. The practical application to CBP and unsymmetric TCTA derivatives has also been performed. Mechanistic experiments and DFT calculations demonstrate that a five-membered rhodacycle species is the key intermediate.

Full text of DOI:10.1021/acs.orglett.8b02247

Letter
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Rh(III)-Catalyzed C−H Activation of Boronic Acid with Aryl Azide
†
†
Shiyang Xu, Baoliang Huang, Guanyu Qiao, Ziyue Huang, Zhen Zhang, Zongyang Li, Peng Wang,
and Zhenhua Zhang*
Beijing Advanced Innovation Center for Food Nutrition and Human Health, and Department of Applied Chemistry, China
Agricultural University, Beijing 100193, China
*
S Supporting Information
ABSTRACT: A Rh(III)-catalyzed C−H activation of boronic acid with aryl azide to obtain unsymmetric carbazoles, 1H-
2
3
indoles, or indolines has been developed. The reaction constructs dual distinct C−N bonds via sp /sp C−H activation and
rhodium nitrene insertion. Synthetically, this approach represents an access to widely used carbazole derivatives. The practical
application to CBP and unsymmetric TCTA derivatives has also been performed. Mechanistic experiments and DFT
calculations demonstrate that a five-membered rhodacycle species is the key intermediate.
itrogen-containing compounds are extensively distrib-
Scheme 1. C−H Aminations with Azides via TM-Nitrene
N
uted in natural products, materials, pharmaceuticals, and
1
pesticides. The effective and selective construction of C−N
bonds has continuously been an important research topic. In
recent years, transition-metal (TM)-catalyzed C−H amina-
2
tions have been widely explored. Among them, reactions with
azides via TM-nitrene transformation have been established as
3
,4
a succinct and efficient method.3 Typically, there are two
4
general strategies, C−H insertion and C−H activation. The
former usually presents in an intramolecular manner due to the
high activity of metal nitrene intermediate and the difficult
control of regioselectivity (Scheme 1a). The latter mainly
involves TM-mediated C−H activation at first, then TM-
nitrene intermediate formation, and subsequent migratory
insertion. The reports about this pathway usually required the
assistance of ortho-directing groups which restrict the scope of
accessible products (Scheme 1b). Notably, these two strategies
are merely limited in constructing only one C−N bond. Dual
selective C−N bond formations in a single catalytic cycle has
rarely been reported. Herein, we reported a Rh(III)-catalyzed
intermolecular C−H activation of biphenyl boronic acid with
aryl azide to form two distinct C−N bonds in a single catalytic
cycle, which utilized the boronic acid as a traceless directing
group. To gain insight into mechanistic details, controlled
experiments, kinetic isotope effects, and DFT calculations were
carried out, which indicated that the reaction constructed dual
distinct C−N bonds via C−H bond activation, rhodium
nitrene formation, and migratory insertion steps. A five-
membered rhodacycle was the key intermediate (Scheme 1c).
In addition, carbazoles have shown promising biological
templated formation and TM-catalyzed intramolecular cycliza-
5
,7,8
tion.
According to the importance of carbazoles, further
development of a facile synthetic method is still highly desired.
This report provides an efficient and practical route to obtain
various unsymmetric carbazoles. The application to synthesize9
photoelectric material N,N′-dicarbazolyl-4,4′-biphenyl (CBP)
5
6
activities and photoelectricity properties. Previous prepara-
tion of carbazole skeletons mainly concentrated on indole-
Received: July 18, 2018
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b02247
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
and unsymmetric polysubstituted 4,4′,4′′-tris(carbazol-9-yl)-
electron-withdrawing groups gave better results, in which
cyano- and acetyl-substituents presented 91% and 86%
excellent yields (3ag, 3ah). 1-Azido-4-halobenzene derivatives
also smoothly proceeded with good yields (3ai−3al). More-
over, disubstituted substrate 2m also gave a 73% yield (3am).
For ortho-substituted 2n, corresponding carbazole 3an was
obtained with a slightly diminished yield, probably due to the
steric effect. The reaction of heterocyclic 3-azidoquinoline gave
the product in 32% yield (3ap).
10
triphenylamine (TCTA) has been performed.
Initially, we attempted the reaction by using [1,1′-biphenyl]-
2
-ylboronic acid (1a) and 1-azido-4-methylbenzene (2a) as
substrates. After investigating different reaction parameters, we
obtained the desired product in 80% yield using [RhCp*Cl ] ,
2
2
AgOAc, and Na CO (Table 1). A variety of solvents were
2
3
Table 1. Selected Results of the Optimization of the
Reaction Conditions
Different substituted boronic acids were also studied
(
Scheme 3). For the boronic acids with different substituents
a
Scheme 3. Reaction Scope of Boronic Acid Derivatives
a
entry
variations from the standard conditions
yield (%)
1
2
3
4
5
6
7
8

80
47
n.r.
28
67
n.r.
68
toluene instead of dioxane
DMF instead of dioxane
Na S O instead of AgOAc
2
2
8
Cs CO instead of Na CO
3
2
3
2
RhCp*PPh Cl instead of [RhCp*Cl ]
2 2
[RhCp*OAc ] instead of [RhCp*Cl ]
2 2
[RhcodCl ] instead of [RhCp*Cl ]
2 2
3
2
2
2
n.r.
2
2
a
Isolated yields refers to 2a.
examined, in which dioxane gave the best result (entries 2−3).
Different oxidants have also been examined, and AgOAc gave
the best yield (entries 4). After screening different bases,
Na CO showed the best effect (entries 5) (for more details,
2
3
see Supporting Information (SI)).
Then, we investigated the substrate scope of different aryl
azides with [1,1′-biphenyl]-2-ylboronic acid (1a) under the
optimized conditions (Scheme 2). The aryl azide substrates
bearing electron-donating and -withdrawing groups were all
tolerated well (3aa−3ah). In general, the aryl azide containing
a
b
Isolated yields refers to 2. AgOAc (2.5 equiv), toluene instead of
a
Scheme 2. Reaction Scope of Aryl Azide Derivatives
dioxane, 120 °C instead of 80 °C, yield were based on recovered
starting materials.
on the same ring, such as Me- (3ba), F- (3bb), and Cl-groups
(
3bc) substituted at the 3-position, all gave the corresponding
product in good yields. Other functionalized carbazoles with
Me- (3bd), MeO- (3be), Ph- (3bf), F- (3bg), and CF -groups
3
(
3bh) substituted at the 7-position could also be synthesized in
moderate to high yields. In particular, CF -substituted at the 6-
3
position (3bi) exhibited a higher yield of 80%. Mono- and
multisubstituted arylboronic acids were also subjected to this
reaction with different azides; all afforded the corresponding
products (3bj−3bl) in good yields. (2-Vinylphenyl)-boronic
acid could react with azide but only give a 10% yield (3ca
b.r.s.m. = 62%). Moreover, (2-(tert-butyl)phenyl)boronic acid
could give 3,3-dimethyl-indoline derivatives in around 40%
yield (3cb−3cd b.r.s.m.= 90%, 65%, 91%). Based on our
calculations and mechanistic experiments, the decomposition
of boric acid may be the cause of the low yield of 3ca−3cd (for
more details, see SI).
Considering their practical use, we synthesized CBP and
a
9,10
Isolated yields refers to 2.
TCTA derivatives,
which serve as important scaffolds in
B
DOI: 10.1021/acs.orglett.8b02247
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
OLED material research. When 1a and 4,4′-diazido-1,1′-
biphenyl (2r) were used for the one-step synthesis of CBP
under the standard conditions, it gave CBP product in 70%
yield (Scheme 4a). Unsymmetric TCTA derivatives are hard to
93% of 4b, and 97% of 4c were recovered after 12 h (Scheme
5a), which ruled out the possibility that the carbazole product
was formed from the intramolecular cyclization of a secondary
amine intermediate which was generated from Rh-catalyzed
8
,13
direct amination of boronic acid with azide.
Then,
Scheme 4. Efficient Synthesis of CBP and TCTA Derivative
biphenylene (5a), a typical substrate that usually reacts with
a transition metal to form dibenzofused metallacyclopenta-
14
diene species, was employed with [Rh(COD)Cl ] as the
2
2
catalyst. Carbazole products were obtained when p-Me, p-H,
and p-CN substituted aryl azides were used (Scheme 5b). This
result indicates that the reaction consisted of the generation of
a five-member rhodacycle intermediate. Moreover, when single
phenyl boronic acids were performed with aryl azides 2a−2c
under the standard conditions, there were no secondary
amines, and 94% of 2a, 80% of 2b (2b is easy to volatize under
rotary evaporation), and 96% of 2c could be recovered
(
Scheme 5c). These results indicate that the secondary amine
could not be formed from azide and boronic acid directly, and
the five-membered rhodacycle intermediate was the key
intermediate to decompose azide. Intermolecular parallel
experiments were also carried out using 1a and 1a-d under
5
the standard conditions by two separate reactions to measure
the kinetic isotopic effect (KIE) (Scheme 5d). The reactions
were terminated after 10 min to keep the yield below 10%.
GC-MS analysis showed k /k = 1.33, indicating that the C−
H
D
15
obtain through traditional methods. Then, we applied this
method to synthesize a densely functionalized TCTA
derivative (Scheme 4b). First, 3bk was obtained in 75%
yield. Then, the following reduction by hydrazine hydrate gave
H activation was not involved in the rate-determining step.
When 10 equiv of D O were added to the reaction of 1a, 3aa
2
and 3aa-d were obtained with the ratio 1:2.3 (Scheme 5e).
1
This result further disclosed that the C−H activation of this
reaction was reversible. Furthermore, we studied the
decomposition of (2-(tert-butyl)phenyl)boronic acid (Scheme
5f). With oxidant Ag CO or base Na CO , the ortho-alkanyl
1
1
3
da in 96% yield. After a two-step sequential Buchwald−
12
Hartwig cross-coupling with 3bl and 3am, which were also
synthesized by this method, unsymmetric TCTA 3dc was
achieved in four steps with a 60% yield.
2
3
2
3
substituted boronic acid alone was very easy to decompose.
And catalyst [Rh(COD)Cl ] would further accelerate the
To gain insight into the mechanistic details of this reaction,
several experimental studies were carried out. First, 4a−4c
were subjected to the reaction under standard conditions. The
result showed that no product was detected and 95% of 4a,
2
2
decomposition.
Theoretically, to further explore the dual C−N bonds
16
formation process, DFT calculations were carried out by
Scheme 5. Controlled Experiments
C
DOI: 10.1021/acs.orglett.8b02247
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
2
Figure 1. Energy profile of Rh(III)-catalyzed sp C−H activation of boronic acid with aryl azide and 3D structure of selected computed transition
states.
1
7
using Gaussian 09 programs. Azidobenzene (2b) and [1,1′-
biphenyl]-2-ylboronic acid (1a) were used as the model
substrates for this calculation study (Figure 1). On the basis of
the above-mentioned experiments, a Rh(III) species [Cp*Rh-
Cp*Rh(OAc) INT-6 to finish the catalytic cycle. Another
2
pathway where the first step was N release was also calculated.
2
The kinetic barrier from INT-1 to TS-5 via INT-7 was up to
40.20 kcal/mol, which further indicated the route forming the
secondary amine was not favorable and the five-member
rhodacyclic intermediate was a key in this reaction. In addition,
the kinetic barrier of a direct 1,2 shift process with C−H
(
III)(biphenyl)](OAc) (INT-1) was postulated to be formed
in this theoretical study through transmetalation of 1a with a
Cp*RhOAc (INT-6) species, which was generated by a
counteranion exchange process between AgOAc and precata-
lyst [Cp*RhCl ] . The energy of the endergonic process of an
sp C−H activation from INT-1 to TS-1 was calculated to be
3.64 kcal/mol. Subsequently, TS-1 further generates a five-
membered Rh(III) species INT-2 (ΔG = 5.80 kcal/mol). This
result also suggested that the C−H activation step was a
reversible process. The activation energy for ligand exchange
process (INT-2 to INT-3) was calculated to be 1.45 kcal/mol,
resulting in a facile coordination of an azide to the rhodium
center. The calculated bond elongation between N and N of
the azide moiety in INT-3 (bond length, 1.25 Å) suggested
that π-back-donation from the Rh atom to an antibonding
orbital of azide exists slightly. In contrast, the same bond
length of unbound azidobenzene was calculated to be 1.24 Å.
The five-membered rhodacyclic intermediate INT-3 subse-
2
activation and N release at the same time was also calculated,
2
which was 42.65 kcal/mol from INT-2 to TS-1,2shift via INT-
2
2
2
3
. It was also much higher than TS-2, which suggested that the
3
1,2-shift was not favorable. DFT calculations of sp C−H
1
activation of (2-(tert-butyl)phenyl)boronic acid (1b) was also
carried out. For more details, see SI, Part 12.
In summary, we have developed a Rh-catalyzed direct
amination of boronic acid using aryl azides as the amine source
2
3
via an sp /sp C−H activation and Rh-nitrene migratory
insertion process, constructing two distinct C−N bonds in a
cascade manner. Experimental studies and theoretical calcu-
lations indicated that C−H bond activation, nitrene formation,
and the migratory insertion of a five-membered rhodacycle
intermediate were the key steps. From a synthetic point of
view, a series of functionalized carbazoles, especially CBP,
TCTA derivatives as important materials in OLEDs, can be
obtained through this reaction. Thus, it can be expected that
such a strategy for azide transformations via C−H activation to
construct two distinct C−N bonds will be further explored in
the synthesis of N-containing polycyclic compounds.
1
2
quently released N passing through the transition state TS-2
2
to give a cycle Rh(V)-nitrenoid intermediate INT-4 (a kinetic
barrier of −22.47 kcal/mol from INT-2 to TS-2). The
Rh(V)−nitrogen bond length in the stable structure of INT-4
was calculated to be 1.86 Å, suggesting that the distance was
consistent with the double bond feature. Subsequently, INT-4
underwent a migratory insertion process via a three-centered
transition state TS-3 with a kinetic barrier (15.55 kcal/mol)
affording a six-membered Rh(III)-amido species INT-5 (ΔG =
ASSOCIATED CONTENT
Supporting Information
■
*
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b02247.
−
54.92 kcal/mol). Finally, Rh(III) complex INT-5 passed
through reductive elimination TS-4 (ΔG = −27.98 kcal/mol)
and the simultaneous oxidation by AgOAc to regenerate
Procedures and NMR spectra (PDF)
D
DOI: 10.1021/acs.orglett.8b02247
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Org. Chem. 2009, 74, 4720. (c) Ozaki, K.; Zhang, H.; Ito, H.; Lei, A.;
Itami, K. Chem. Sci. 2013, 4, 3416. (d) Laha, J. K.; Dayal, N. Org. Lett.
AUTHOR INFORMATION
Corresponding Author
■
*
2
015, 17, 4742. (e) Chen, S.; Li, Y.; Ni, P.; Huang, H.; Deng, G. Org.
Lett. 2016, 18, 5384.
8) (a) Yoshikai, N.; Wei, Y. Asian J. Org. Chem. 2013, 2, 466.
b) Jiang, Q.; Duan-Mu, D.; Zhong, W.; Chen, H.; Yan, H. Chem. -
E-mail: zhangzhh@cau.edu.cn.
ORCID
(
(
Zhenhua Zhang: 0000-0002-6579-1670
Eur. J. 2013, 19, 1903.
Author Contributions
(9) Kozlov, V. G.; Parthasarathy, G.; Burrows, P. E.; Forrest, S. R.;
You, Y.; Thompson, M. E. Appl. Phys. Lett. 1998, 72, 144.
†S.X. and B.H. contributed equally.
(10) Kang, J.-W.; Lee, S.-H.; Park, H.-D.; Jeong, W.-I.; Yoo, K.-M.;
Notes
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11) Bavin, P. M. G. Can. J. Chem. 1958, 36, 238.
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The authors declare no competing financial interest.
Alcazar-Roman, L. M. J. Org. Chem. 1999, 64, 5575.
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ACKNOWLEDGMENTS
■
(
This project is supported by the National Natural Science
Foundation of China (No. 21672256). Prof. Haixiang Gao of
China Agricultural University are acknowledged for providing
computational resources.
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DOI: 10.1021/acs.orglett.8b02247
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