Rhodium(III)-catalyzed regioselective C–H nitrosation/annulation of unsymmetrical azobenzenes to synthesize benzotriazole N-oxides via a RhIII/RhIII redox-neutral pathway

Article abstract of DOI:10.1016/j.tetlet.2021.153049

A Rh(III)-catalyzed regioselective C–H nitrosation/annulation reaction of unsymmetrical azobenzenes with [NO][BF₄] has been developed to achieve high-yielding syntheses of benzotriazole N-oxides with excellent functional group tolerance. Computational studies have revealed that this oxidative C–H functionalization reaction involves an interesting redox-neutral Rh(III)/Rh(III) pathway without the change of Rh oxidation state.

Full text of DOI:10.1016/j.tetlet.2021.153049

Tetrahedron Letters 72 (2021) 153049
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Tetrahedron Letters
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Rhodium(III)-catalyzed regioselective CAH nitrosation/annulation of
unsymmetrical azobenzenes to synthesize benzotriazole N-oxides via a
RhIII/RhIII redox-neutral pathway
Yuanfei Zhang a,b,1, Zhe-Ning Chen a,1, Weiping Su a,
⇑
a State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China
b Guangxi Key Laboratory of Natural Polymer Chemistry and Physics, Nanning Normal University, Nanning 530001, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A Rh(III)-catalyzed regioselective CAH nitrosation/annulation reaction of unsymmetrical azobenzenes
with [NO][BF4] has been developed to achieve high-yielding syntheses of benzotriazole N-oxides with
excellent functional group tolerance. Computational studies have revealed that this oxidative CAH func-
tionalization reaction involves an interesting redox-neutral Rh(III)/Rh(III) pathway without the change of
Rh oxidation state.
Received 21 January 2021
Revised 24 March 2021
Accepted 28 March 2021
Available online 8 April 2021
Ó 2021 Elsevier Ltd. All rights reserved.
Keywords:
Rh(III) catalyst
CAH nitrosation
Benzotriazole
Solar cell material
Rh(III)/Rh(III) pathway
Introduction
as the final products (Scheme 1, c) [7]. However, two problems
remained unsolved utilizing palladium catalytic system: i) the
Nitrogen-containing heterocyclic N-oxides feature very rich
chemistry and often serve as versatile building blocks in organic
synthesis [1], which provides a tremendous motivation for che-
mists to discover and develop synthetic methods for their facile
preparations [1a,b,2]. In this context, the transition-metal-
catalyzed CAH functionalization reactions enabling the formation
of two fragaments, namely the parent heterocycles and N-oxides,
reaction conditions were not versatile for unsymmetrical azoben-
zenes bearing substituents on ortho-positions, 3,5-positions or
2,5-positions; ii) the reaction yields were always low when two
substituents were located in the two ortho-positons of one aryl ring
of unsymmetrical azobenzenes. Hence, general methods allowing
the conversion of unsymmetrical azobenzenes to benzotriazole
N-oxides with structural diversities are still in demand. Herein,
we present a Rh(III)-catalyzed regioselective CAH nitrosation of
unsymmetrical azobenzenes with nitrosonium tetrafluoroborate
([NO][BF4]) (Scheme 1, d), affording diversely substituted benzotri-
azole N-oxides. This Rh(III)-catalyzed protocol tolerates a broad
range of functional groups and substitution types, and exhibits a
high level of regioselectivity. The computational studies of the
reaction mechanism revealed that the CAN bond formation step
involves a migration-insertion of NO cation into C-Rh bond, provid-
ing a new bond formation mode for the Rh(III)-catalyzed CAH
functionalization chemistry.
via
a tandem process provided effective accesses to those
compounds [3–5]. For instance, Glorius and coworkers reported a
Rh(III)-catalyzed synthesis of isoquinoline N-oxides through a car-
bene insertion/cyclization sequence (Scheme 1, a) [4a].
Subsequently, this strategy was extended to Ir(III) catalyst by
Ramana [4b]. In addition, Li’s group realized a sequential nitrene
insertion/condensative annulation reaction utilizing Rh(III)/Zn(II)
dual catalytic system to afford quinazoline N-oxides (Scheme 1,
b) [5]. Recently, we discovered that iodide could significantly
enhance the catalytic activity of Pd(II) catalyst via formation of
iodide-bridged binuclear palladium complex, delivering benzotria-
zole N-oxides, a class of common structural motifs in biologically
active compounds, pharmaceuticals, and functional materials [6],
Results and discussion
Cationic Rh(III) catalyst has been proved to be very active
towards chelation-assisted CAH functionalization [3c–g,8].
Accordingly, we hypothesized that cationic Rh(III) catalyst could
⇑
Corresponding author.
E-mail address: wpsu@fjirsm.ac.cn (W. Su).
These authors contributed equally to the work.
1
https://doi.org/10.1016/j.tetlet.2021.153049
0040-4039/Ó 2021 Elsevier Ltd. All rights reserved.

Y. Zhang, Zhe-Ning Chen and W. Su
Tetrahedron Letters 72 (2021) 153049
and [NO][BF4] (2a). Our investigations disclosed that the expected
orthoACAH nitrosation/annulation indeed occurred to give benzo-
triazole N-oxide (3a) in various solvents and DCM was the best
choice (Table 1, entries 1–4, see the Supporting Information for
details). 71% yield was obtained in the absence of AgSbF6, indicat-
ing cationic Rh(III) is more reactive than its neutron counterpart
(Table 1, entry 5). The reaction of 1a with 2 equiv. of 2a carried
out in 2 mL of DCM at 80 °C in the presence of 1 mol% of
[Cp*RhCl2]2 as catalyst precursor and 4 mol% of AgSbF6 as an addi-
tive gave the optimal result, delivering the annulated product in
92% isolated yield (Table 1, entry 6). Lowering the catalyst loading,
reaction temperature and the addition of acids or bases could lead
to decreased yields (Table 1, entries 7–12).
As shown in Scheme 2, the Rh(III) catalytic system worked well
when symmetrical azobenzene bearing fluoro substituent was
employed, yielding the product 3b with satisfaction. After the
identification and justifying of the optimized reaction conditions
regarding to symmetrical azobenzenes, we subsequently investi-
gated their generality and selectivity using unsymmetrical sub-
strates. Unfortunately, we found that the optimal reaction
conditions for symmetrical azobenzenes were not suitable for
unsymmetrical azobenzenes. However, with the increase of the
loading of [Cp*RhCl2]2 and AgSbF6, and the adjustment of the sol-
vent, the reactions of unsymmetrical azoarenes, which are of more
synthetic interest, exhibited a high level of regioselectivity, and
satisfying yields were generally obtained. In these reactions, elec-
tronic and steric effects of substituents are the key factors to con-
trol reaction regioselectivity. When sterically hindered
substituents were located on ortho-positions of azobenzenes, reac-
tions selectively occurred on benzene ring bearing this ortho-sub-
stituent regardless of the electronic property of these
substituents (3c-f). This regioselectivity governed by ortho-sub-
stituent was also observed in the reaction of azobenzene bearing
less sterically hindered fluoro on ortho-position of the other phenyl
Scheme 1. Methods for the synthesis of heterocyclic N-oxides.
effect the rapid formation of cyclometalation intermediate via
directing group-assisted, steric or electronic effect controlled
CAH activation and that electrophilic attack of [NO][BF4] would
occur preferentially at the carbon atom of C-Rh bond in the newly
formed rhodacycle intermediate, due to the electron-rich property
of this carbon atom, to selectively afford CAH nitrosation/annula-
tion product, and therefore provides a complementary to our pre-
viously reported work.
To verify the aforementioned working hypothesis, we firstly
checked the reactivity of cationic Rh(III) complex towards the
CAH nitrosation/annulation reaction between azobenzene (1a)
Table 1
Selected results from optimization of the reaction conditions.a
Entry
Additive
Solvent
Temp (oC)
Yield (%)b
1
2
3
4
–
–
–
–
–
–
–
–
Dioxane
DMF
Toluene
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
80
80
80
80
80
80
80
60
50
80
80
80
16
0
0
95
71
95 (92)
82
90
75
44
79
0
5c
6d
7e
8
9
–
10
11
12f
LiOAc
PivOH
–
DCM
a
b
c
d
e
f
1a (0.2 mmol), 2a (2 equiv), [Cp*RhCl2]2 (2 mol%) , AgSbF6 (8 mol%), Additive (1 equiv), 2 mL of solvent, 80 °C, 24 h.
Yield determined by HPLC analysis; isolated yield given in parentheses.
AgSbF6 was not added.
[Cp*RhCl2]2 (1 mol%) and AgSbF6 (4 mol%) were used.
[Cp*RhCl2]2 (0.5 mol%) and AgSbF6 (2 mol%) were used.
[Cp*RhCl2]2 was not added.
2

Y. Zhang, Zhe-Ning Chen and W. Su
Tetrahedron Letters 72 (2021) 153049
monomer of the polymer containing multi-fluorinated benzotria-
zole core structural unit (Scheme 3). Our synthetic route makes
it possible to study the physical properties of polymers of this kind
and develop efficient solar cell polymers from this kind of materi-
als, further highlighting the value of the Rh-catalyzed CAH nitrosa-
tion/annulation reaction in organic synthesis.
DFT calculations have been carried out to gain mechanistic
insights into the Rh(III)-catalyzed CAH nitrosation/annulation
reaction of azobenzene with [NO][BF4] (Fig. 1), which indicates
that this process includes three consecutive stages: i) chelation-
assisted Rh(III)-promoted CAH activation to generate a rhodacycle
intermediate; ii) adduction of NO+ as 2e cationic ligand to rhodacy-
cle intermediate and subsequent migration-insertion of NO+ frag-
ment into Rh-C bond to form six-membered chelation complex;
and iii) product formation via a concerted decomplexation/NAN
bond formation.
Initially, cationic Rh(III) species (A) is generated via abstracting
a chloride anion by a silver salt from its neutral precursor to trigger
catalytic cycle. Species A is bound to an azobenzene molecule to
form intermediate B, which leads to a slight increase in free energy
likely due to the weak bond between Rh(III) and nitrogen atom in
azobenzene, and steric repulsion between the ligands around Rh
center (See Figs. S1 and S3 in Supporting Information for details).
Then, intermediate B is converted to 16e rhodacycle intermediate
C via a concerted metalation-deprotonation (CMD) pathway with
an activation free energy of 20.9 kcal/mol (TS1) with respect to
species A, in which the reactant azobenzene also plays as base to
assist the CAH activation process [11]. Subsequently, NO+ cationic
ligand is transferred to intermediate C from [NO][BF4] to generate
18e nitrosyl-Rh complex (D) with exothermicity of 21.7 kcal/mol.
The resultant nitrosyl-Rh complex D proceeds to six-membered
chelation complex E via the migration-insertion of cationic NO+
into Rh-C bond, of which the activation free energy is 14.0 kcal/mol
(TS2) with respect to complex D. Finally, benzotriazole N-oxide is
produced from complex E via a concerted decomplexation/NAN
bond formation pathway with a free energy barrier of 23.9 kcal/mol
(TS3). This product formation step involves the highest free energy
barrier, and is the rate-determined step of the whole catalytic cycle
Scheme 2. Variation of the unsymmetrical azoarenes. Reaction conditions:
1
(0.2 mmol), 2a (3 equiv), [Cp*RhCl2]2 (2.5 mol%) , AgSbF6 (10 mol%), PivOH (1
equiv), Solvent (2 mL), 100 °C, 24 h. Yields are of the isolated products. See the
Supporting Information for details.
ring (3c). The effect of ortho-substituents on the regioselectivity
reflects that bulky substituents hinder coordination of proximal
nitrogen atom to Rh center. When two substituents were located
on 3,5-positions or 2,5-positions of one phenyl ring of azoben-
zenes, the reactions occurred exclusively on the less sterically hin-
dered phenyl rings (3g-q). Two substituents on 3,5-positions of one
phenyl ring in azobenzenes of this kind include electron-with-
drawing two trifluoromethyl groups (3g-i), electron-donating
two methyl groups (3j-k), two ester groups (3l), and combination
of trifluoromethyl group and ester group (3m). In the cases of
2,5-disubstitution, less bulky chloro, bromo and fluoro groups are
present on 2-positions, the other substituents on 5-positions are
ester (3n), nitro (3o), trifluoromethyl (3p) and sulfonyl (3q)
groups. Interestingly, when both ortho-positions of one phenyl ring
of azobenzene were blocked by two substituents that may influ-
ence coordination of proximal nitrogen atom to Rh catalyst, the
reactions still worked well with good yields obtained (3r-3z). Thus,
our protocol is compatible with a broad range of functional groups
and complementary to the previous methods for syntheses of ben-
zotriazole N-oxides since many of benzotriazole N-oxides contain-
ing electron-withdrawing groups that are reported here are not
accessible by the previously reported methods [7,9].
The prevalence of benzotriazole structural motif in organic solar
cell polymers, and the drastic improvement of power conversion
efficiencies of such materials by the fluoro substitutes [6c–e,10]
motivated us to explore the potential application of our protocol
to syntheses of such materials. A six-step synthetic route based
on the Rh-catalyzed CAH nitrosation/annulation reaction as the
key step has been established to achieve gram-scale synthesis of
Scheme 3. Gram-scale synthesis of monomer of the polymer based on multi-
fluorinated benzotriazole.
3

Y. Zhang, Zhe-Ning Chen and W. Su
Tetrahedron Letters 72 (2021) 153049
Fig. 1. Free energy profiles for Rh(III)-catalyzed CAH nitrosation/annulation reaction.
due to stability of six-membered chelation complex E, which is in
accordance with the experimentally observed KIE value (kH/
kD = 1.02) (Fig. 2) [12].
The Rh(III)-catalyzed oxidative CAH functionalization redox
processes were generally proposed to involve Rh(III)-Rh(I)-Rh(III)
or Rh(III)-Rh(V)-Rh(III) catalytic cycles [3e–h,8a,13,14]. In
contrast to these proposals, our calculations demonstrate that
the Rh-NO unit in complex D is nearly linear with an angle of
169.3° (Fig. 3). Therefore, the coordination NO should behave as
a 2e cationic ligand, namely NO+. The apparent valence of Rh(III)
center thus remains unchanged in nitrosyl-Rh complex D ([Rh
(III)]–NO+). Further, the natural bond orbital (NBO) analysis
indicates that the coordinating NO possesses a positive charge of
0.453 au. Moreover, the electron density of Rh center in complex
C is lower than that in nitrosyl-Rh complex D, which again
suggests that the coordinating NO should be considered as a 2e
Fig. 3. Optimized structures and NBO charges (in parentheses) for intermediate C
and nitrosyl-Rh complex D. Atoms of Rh, N, O, C are labeled in cyan, blue, red and
gray respectively.
donor NO+. As
a result, the Rh-catalyzed CAH nitrosation
described here is a redox-neutral Rh(III)/Rh(III) process without
change in the apparent valence of Rh center.
Based on the above mechanistic studies, a plausible catalytic
cycle is postulated (Scheme 4). Firstly, cationic Rh(III) complex A
reacts with 1 through a sequential coordination/concerted metala-
tion-deprotonation to form rhodacycle C. Adduction of NO+ catio-
nic ligand to C subsequently takes place, affording 18e nitrosyl-
Rh complex D. Next, the migration insertion of NO ligand into
Rh-C bond to generate chelation complex E from which product
3 is obtained via decomplexation/annulation.
Conclusion
In conclusion, we have developed a novel Rh(III)-catalyzed CAH
nitrosation/annulation reaction for the rapid construction of ben-
zotriazole N-oxides using [NO][BF4] as the nitroso source. This
operationally simple method enabled the highly regioselective
conversions of the unsymmetrical azobenzenes into diversely sub-
stituted benzotriazole N-oxides in good-to-excellent yields, and
Fig. 2. KIE experiments.
4

Y. Zhang, Zhe-Ning Chen and W. Su
Tetrahedron Letters 72 (2021) 153049
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culations revealed that this transformation proceeds through a
RhIII/RhIII redox-neutral pathway and the rate-limiting step is the
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Declaration of Competing Interest
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The authors declare that they have no known competing finan-
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to influence the work reported in this paper.
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Financial support from the National Science Foundation of
China (Grant No. 21602221 and 21973094), Guangxi Natural
Science Foundation (2020GXNSFBA159005), the Start-up Grant
from Nanning Normal University (Grant No. 602021239229) and
the ‘‘BAGUI Scholar” program of Guangxi province is greatly
appreciated.
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Appendix A. Supplementary data
Supplementary data to this article can be found online at
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