Understanding Mechanistic Differences between 3-Diazoindolin-2-Imines and N-Sulfonyl-1,2,3-Triazoles in the Rh2(II)-Catalyzed Reactions with Nitrosoarenes

Article abstract of DOI:10.1002/cjoc.202100033

The employment of α-iminometallocarbenes to construct valuable N-containing compounds has attracted significant research interest. Herein, the nucleophilic addition of nitrosoarenes with the α-imino rhodium carbene species (I), which is derived from Rh₂(II)-catalyzed denitrogenation of 3-diazoindolin-2-imines, to produce synthetically useful 2-iminoindolin-nitrones is described. Mechanistically, the N-attack of nitrosoarenes with the carbene site of I is proposed. For the analogous Rh₂(II)-catalyzed reaction of nitrosoarenes with N-sulfonyl-1,2,3-triazoles reported by Li and co-workers (Org. Lett. 2014, 16, 6394), however, the O-attack of nitrosoarenes with the carbene site of α-imino rhodium carbene species (II) is more favorable to occur than the N-attack. The subsequent transformation to yield the product of N-acylamidines is rationalized based on computational studies. The mechanistic differences for the reactions of nitrosoarenes with α-imino rhodium carbene species I and II are discussed.

Full text of DOI:10.1002/cjoc.202100033

Chinese Journal
of Chemistry
CJC
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Accepted Article
Title: Understanding Mechanistic Differences between 3-Diazoindolin-2-Imines and
N-Sulfonyl-1,2,3-Triazoles in the Rh2(II)-Catalyzed Reactions with Nitrosoarenes
Authors: Rui Fu, Luyao Kou, Ke Gao, Shaofang Zhou, and Xiaoguang Bao*
This manuscript has been accepted and appears as an Accepted Article online.
This work may now be cited as: Chin. J. Chem. 2021, 39, 10.1002/cjoc.202100033.
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published online in Early View: http://dx.doi.org/10.1002/cjoc.202100033.
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Cite this paper: Chin. J. Chem. 2021, 39, XXX—XXX. DOI: 10.1002/cjoc.202100XXX
Understanding Mechanistic Differences between
3-Diazoindolin-2-Imines and N-Sulfonyl-1,2,3-Triazoles in the
Rh2(II)-Catalyzed Reactions with Nitrosoarenes
Rui Fu,† Luyao Kou,† Ke Gao, Shaofang Zhou, and Xiaoguang Bao*
College of Chemistry, Chemical Engineering and Materials Science, Soochow University, 199 Ren-Ai Road, Suzhou Industrial Park, Suzhou 215123, China.
Keywords
α-imino rhodium carbene | nitrosoarene | reaction mechanism | chemo-selectivity | DFT calculation
Main observation and conclusion
The employment of α-imino metallocarbenes to construct valuable N-containing compounds has attracted significant research inter-
est. Herein, the nucleophilic addition of nitrosoarenes with the α-imino rhodium carbene species (I), which is derived from
Rh2(II)-catalyzed denitrogenation of 3-diazoindolin-2-imines, to produce synthetically useful 2-iminoindolin-nitrones is described.
Mechanistically, the N-attack of nitrosoarenes with the carbene site of I is proposed. For the analogous Rh2(II)-catalyzed reaction of
nitrosoarenes with N-sulfonyl-1,2,3-triazoles reported by Li and co-workers (Org. Lett. 2014, 16, 6394), however, the O-attack of ni-
trosoarenes with the carbene site of α-imino rhodium carbene species (II) is more favorable to occur than the N-attack. The subse-
quent transformation to yield the product of N-acylamidines is rationalized based on computational studies. The mechanistic differ-
ences for the reactions of nitrosoarenes with α-imino rhodium carbene species I and II are discussed.
Comprehensive Graphic Content
O
N
Ph
Ar
N
O
N2
Ar
Rh2 II)
(
NTs
NTs
N-attack
N
N
N2
(I)
are
• The Rh
reactions
3-diazoindolin-2-imines with nitrosoarenes to
2-iminoindolin-nitrones
described
to 94%
of
examples up
(23
produce
yield).
2(II)-catalyzed
Et
N
O
O
Ar
Ar
Ph
N
N
NTs
Et
Rh2 II)
(
N
N
Ts
O-attack
Ph
N2
(II)
α
and are revealed
II
• The mechanistic
the reactions
nitrosoarenes with -imino rhodium carbene
I
DFT calculations.
by
differences for
of
species
*E-mail: xgbao@suda.edu.cn
View HTML Article
Supporting Information
†These two authors contributed equally to this work.
Chin. J. Chem. 2021, 39, XXX－XXX
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This article has been accepted for publication and undergone full peer review but has not been
through the copyediting, typesetting, pagination and proofreading process which may lead to
differences between this version and the Version of Record. Please cite this article as doi:
10.1002/cjoc.202100033
This article is protected by copyright. All rights reserved.

First author et al.
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Scheme 1 Reactions of carbene precursors with nitrosoarenes
Background and Originality Content
reaction nitrosobenzene with
of
carbenoid
a) Cycloaddition
O
alkenylgold
The intermediacy of α-imino metallocarbenes, which could be
derived from appropriate transition-metal-catalyzed denitrogena-
tion of 1,2,3-triazoles, has attracted substantial research interest
due to the versatile transformations to afford synthetically useful
N-containing compounds.1 The main reaction types of α-imino
metallocarbenes could be categorized as cycloaddition,2 X–H
insertion3 (X = C, N, O etc.), cyclopropanation,4 ring expansion,5
ylide formation,6 and others.7 For instance, Gevorgyan and
co-workers2a,2b reported that the Rh2(II)-catalyzed denitrogenative
transannulation of 1,2,3-triazoles with alkynes and nitriles to con-
struct pyrroles and imidazoles, respectively, which proceeds via
the intermediacy of α-imino rhodium carbenes.
N
O
O
OEt
Ph
O
LAuCl/AgNTf2
Ph
Ph
OEt
OEt
N2
[Au]+
[Au]+
N
O
N2
Divergent Rh
reactions of enoldiazo
with nitrosoarenes
compounds
b)
Ar
2(II)-catalyzed cyclization
OSi
O
SiO
N
NR2
N
O
Rh
Rh
2(oct)4
2(cap)4
N
+
Ar
O
NR2
O
NR2
+
+
2]
1]
[3
[5
Ar
O
O
N2
OSi
Rh
reaction of nitrosoarenes with N
c)
2(II)-catalyzed
-sulfonyl-1,2,3-triazoles
Et
N
O
O
[Rh]
Ph
N
N
N
NTs
Et
Rh2 II)
(
NTs
N
Ts
Ph
Ph
N2
reaction of nitrosoarenes with 3-diazoindolin-2-imines
2(II)-catalyzed
[Rh]
Apart from 1,2,3-triazoles, 3-diazoindolin-2-imines can also
undergo analogous conversion with suitable rhodium catalysts to
yield α-imino rhodium carbenes, which could be efficiently con-
verted to functionalized indoline derivatives.8 For instance, Wang
work:
This
Rh
d)
N
O
Ph
N
O
N2
[Rh]
Rh2 II)
(
NTs
NTs
NTs
N
N
N
and
co-workers9
reported
the
annulation
of
N2
[Rh]
3-diazoindolin-2-imines with alkynes, indoles, furans, and others,
to construct the molecular complexity for cyclic compounds. Lee
et al.10 described the employment of 3-diazoindolin-2-imines to
generate α-imino rhodium carbenes and subsequent reactions
with conjugated dienes, azides, and sulfimides. A concise ap-
proach for the synthesis of 2-(3-arylallylidene)-3-oxindoles was
Results and Discussion
1.
Rh2(II)-catalyzed
3-diazoindolin-2-imines to synthesize 2-iminoindolin-nitrones.
reaction
of
nitrosoarenes
with
achieved
by
the
Rh2(II)-catalyzed
reaction
of
Initially, substrates 3-diazoindolin-2-imine (1a) and nitro-
sobenzene (2a) were used to obtain optimal reaction conditions
(Table 1). A variety of Rh2(II) catalysts were screened, such as
Rh2(OAc)4, Rh2(oct)4, and Rh2(TFA)4, in toluene at 85 °C for 13
hours. Among these catalysts, Rh2(OAc)4 demonstrates better
efficiency in catalyzing this reaction to afford the product 3a in 85%
yield (entries 1~4). Other solvent, such as DCE, CH3CN, THF, DMSO,
and DMF, does not show good performance compared with tolu-
ene (entries 5~9). No improvement of the yield of 3a is found
when the amount of 2a increases to 1.8 and 2.5 equiv., respec-
tively (entries 10~11). The reaction can be completed in 6 hours
(entries 12~13). When the reaction is carried out at 65 or 105 °C,
the yield of 3a is not good (entries 14~15). No desired product is
detected in the absence of the Rh2(II) catalyst (entry 16).
3-diazoindolin-2-imines with 1-aryl-substituted allylic alcohols.11
Nitrosoarenes involving cycloaddition reactions have been
extensively explored.12 In particular, nitrosoarenes demonstrate
versatile reaction styles with various transition-metal carbene
intermediates. For instance, Liu and co-workers13 reported a for-
mal [3 + 3] cycloaddition reaction of nitrosoarenes with alkenyl-
gold carbenoids (Scheme 1a). In addition, Doyle and co-workers14
described divergent Rh2(II)-catalyzed [3 + 2] and [5 + 1] cyclization
reactions of enoldiazo compounds with nitrosoarenes (Scheme
1b). Interestingly, Li and co-workers15 reported a Rh2(II)-catalyzed
reaction of nitrosoarenes with N-sulfonyl-1,2,3-triazoles to afford
N-acylamidine (Scheme 1c). An unprecedented C−C bond breaking
of α-imino rhodium carbene and subsequent rearrangement was
suggested. Nevertheless, the detailed mechanistic pathway re-
mains unclear. In this work, we disclose a convenient approach to
synthesize 2-iminoindolin-nitrones via the Rh2(II)-catalyzed reac-
tion of 3-diazoindolin-2-imines with nitrosoarenes (Scheme 1d).
Remarkably, the two α-imino rhodium carbene species derived
Table 1 Optimization of reaction conditions.a
O
N2
Ph
N
N
catalyst, solvent
O
+
NTs
NTs
time
temperature,
N
from
two
precursors,
3-diazoindolin-2-imine
and
N
N-sulfonyl-1,2,3-triazole, demonstrate different reactivity with
nitrosoarenes. To continue our research interest in understanding
the chemo-selectivity of α-imino metallocarbenes involving reac-
tions,16 the mechanistic differences between Rh2(II)-catalyzed
reactions of nitrosoarenes with 3-diazoindolin-2-imine and with
N-sulfonyl-1,2,3-triazole are discussed by DFT calculations.
2a
3a
1a
Entry
Catalyst
Solvent
toluene
toluene
toluene
toluene
DCE
T (oC)
85
Time (h)
13
Yield (%)b
71c
1
2
3
4
5
6
7
8
Rh2(OAc)4
Rh2(oct)4
Rh2(TFA)4
Rh2(OAc)4
Rh2(OAc)4
Rh2(OAc)4
Rh2(OAc)4
Rh2(OAc)4
85
13
67c
30c
85
13
85
13
85
85
13
20
MeCN
THF
85
13
trace
N.D.
N.D.
85
13
DMSO
85
13
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Chin. J. Chem. 2021, 39, XXX－XXX

Running title
Chin. J. Chem.
ra-methyl (3q, 49%), para-ethyl (3r, 68%), para-isopropyl (3s, 78%),
and para-tbutyl (3t, 46%), the yields of the desired product are
moderate to good. For substrates with electron-withdrawing sub-
stituents at the para-position, good yields of the corresponding
products 3u, 3v and 3w are isolated in 80%, 65% and 60% yields,
respectively.
9
Rh2(OAc)4
DMF
85
85
85
85
85
65
105
85
13
13
13
24
6
N.D.
78d
77e
83
10
11
12
13
14
15
16
Rh2(OAc)4
Rh2(OAc)4
Rh2(OAc)4
Rh2(OAc)4
Rh2(OAc)4
Rh2(OAc)4
-
toluene
toluene
toluene
toluene
toluene
toluene
toluene
86
6
66
Table 3 Substrate scope of nitrosoarenes.a,b
R4
6
67
O
N2
N
mol
4
N
Rh2 OAc)
,
%
(
4
+
O
R4
6
N.D.
NTs
NTs
toluene, 85°C, 6 h
N
a Reaction condition: 1a (0.2 mmol), 2a (0.22 mmol), Rh2(OAc)4 (4 mol%),
N
solvent (1 mL), under air atmosphere. b Isolated yield. c Rh2(II) (2 mol%).
d
1a
2
3
2a (0.36 mmol). e 2a (0.5 mmol). N.D. = Not Detected.
O
N
O
O
N
N
O
With the optimal reaction conditions in hand, we next ex-
plored the substrate scope of 3-diazoindolin-2-imines (Table 2).
When methyl group attached to N1 is replaced by H, the desired
product 3b17 can be obtained in excellent yield (94%). However,
moderate yield of 3c (60%) is isolated when the methyl group is
changed to Et group. The derivatives of 3-diazoindolin-2-imines
with substituents attached at the C5/C6 sites are investigated and
both electron-withdrawing and electron-donating groups are tol-
erant for this reaction, affording the desired products (3d-3k) in
moderate to good yields.
N
Cl
NTs
N
NTs
NTs
N
N
NTs
N
3o, 63
3n, 45
%
3l. 43
%
%
3m. 47
%
O
O
N
O
N
O
N
N
Br
NTs
NTs
NTs
NTs
N
N
N
N
3s, 78
%
3r, 68
%
49
%
82
%
3q,
3p,
I
CN
O
N
O
Cl
O
N
O
N
N
NTs
NTs
NTs
NTs
Table 2 Substrate scope of 3-diazoindolin-2-imines.a,b
N
N
N
N
O
N2
3w, 60 %
Ph
3u, 80
%
3v, 65 %
3t, 46
%
N
R2
N
R2
R3
mol
4
Rh2 OAc)
,
%
(
4
O
+
NTs
a Reaction conditions: 1a (0.2 mmol), 2 (0.22 mmol), toluene (1 mL), under
NTs
R3
N
air atmosphere. b Isolated yield.
toluene, 85 °C, 6 h
N
R1
R1
2a
1
3
2. Computational studies for the Rh2(II)-catalyzed reaction of
nitrosobenzene with 3-diazoindolin-2-imine.
O
O
O
Ph
N
Ph
Ph
N
N
NTs
NTs
Computational studies were carried out to provide a mecha-
nistic insight into the aforementioned reaction. An initial complex
INT1a could be formed after the coordination of substrate 1a with
the catalyst Rh2(OAc)4. Next, the Rh2(II)-catalyzed denitrogenation
of 1a could occur to afford the key α-imino rhodium carbene spe-
cies INT2a (I). The corresponding transition state (TS) is located as
TS1a, in which the C…N distance is lengthened to 1.81 Å. The pre-
dicted activation barrier (ΔG≠) is 14.5 kcal/mol and the formation
of α-imino rhodium carbene intermediate (I) is exergonic by 10.9
kcal/mol relative to INT1a (Figure 1).
Subsequently, the intermolecular nucleophilic attacks of both
N and O atoms of nitrosobenzene (2a) to the carbene moiety of
INT2a are considered computationally. For the N-attack of 2a, two
orientations of TS are optimized (TS2a and TS3a), which could lead
to trans and cis structure of the nitrone product 3a, respectively.
Computational results suggest that TS2a is 4.3 kcal/mol lower in
energy than TS3a. Therefore, the formation of trans conformer of
3a via TS2a is more favorable, which is consistent with experi-
mental observation. A distortion/interaction analysis for TS2a and
TS3a is performed to rationalize the origin of stereo-selectivity
(Figure S1). The greater distortion energy penalty is found in TS3a,
which is mainly due to the steric hindrance between the Ph group
of 2a and aryl group of INT2a (in an almost perpendicular position,
Figure 2). For TS2a, however, such steric hindrance is avoided.
Thus, the N-attack of 2a via TS2a is more favorable. In addition,
the competitive O-attack of 2a is also considered and the corre-
NTs
N
N
N
H
3c, 60%
3a, 86%
3b, 94%
O
O
O
O
N
Ph
Ph
Ph
N
Ph
NTs
N
N
F
F
F
Cl
NTs
NTs
NTs
N
N
N
N
H
H
88%
3e, 75%
3d, 80%
3g,
3f, 71%
O
O
N
O
N
O
Ph
Ph
NTs
Ph
NTs
Ph
N
N
O
NTs
NTs
N
N
N
H
N
Cl
H
3k, 86%
3h, 58%
3i, 85%
62%
3j,
a Reaction conditions: 1 (0.2 mmol), 2a (0.22 mmol), toluene (1 mL), under
air atmosphere. b Isolated yield.
Further exploration of the substrate scope was focused on ni-
trosoarenes (Table 3). Various substituents with different elec-
tronic properties are tolerable, giving the corresponding products
in moderate to good yields. For substrates with electron-donating
substituents at the ortho-position, such as methyl (3l, 43%), ethyl
(3m, 47%), and 2,5-dimethyl (3n, 45%), moderate yields of the
desired product were obtained. When the electron-withdrawing
groups are present at the meta-position of nitrosobenzene, the
corresponding products (3o, 63%) and (3p, 82%) are obtained in
moderate to good yields. When the electron-donating groups are
attached to the para-position of nitrosobenzene, such as pa-
Chin. J. Chem. 2021, 39, XXX－XXX
© 2021 SIOC, CAS, Shanghai, & WILEY-VCH GmbH
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First author et al.
Report
sponding TS is shown as TS4a. The calculated ΔG≠ is 7.2 kcal/mol
higher than the favored N-attack via TS2a, suggesting the O-attack
pathway is not reasonable to take place. A distortion/interaction
analysis between TS2a and TS4a shows that interaction energy in
TS4a is much lower than that in TS2a. The greater nucleophilic
character of N- than O-atom in 2a is mainly responsible for the
chemo-selectivity. Consequently, the N-attack to the carbene site
of INT2a is proposed for the Rh2(II)-catalyzed reaction of
3-diazoindolin-2-imines with nitrosoarenes.
O
Et
N
oct)
Rh2
mol%)
Ph
N
N
(
4 (2
NTs
Et
N
N
Ts
O
DCE, reflux, N2
Ph
1a'
2a'
3a'
Li and co-workers15 reported the Rh2(II)-catalyzed reaction of
nitrosoarene (2aʹ) with N-sulfonyl-1,2,3-triazole (1aʹ) to afford
N-acylamidine (3aʹ), which is quite different from the previous
reaction with 3-diazoindolin-2-imines although similar α-imino
rhodium carbene species are proposed to be the key intermedi-
ates. An unprecedented C–C bond breaking of α-imino carbene
moiety, derived from 1aʹ, is disclosed in Li’s reaction. Herein,
computational studies were carried out to provide mechanistic
insights for this reaction and reveal the mechanistic differences
between these two reactions. First, the tautomerization of 1aʹ to
diazoimine and subsequent denitrogenation was explored com-
putationally to yield the α-imino rhodium carbene species INT3aʹ
(II).2i The calculated activation barrier for this step is 24.6 kcal/mol
relative to separated 1aʹ and the catalyst (Figure 3). It should be
noted that the orientation of imino group in INT3aʹ is almost per-
pendicular to the adjacent phenyl group (the dihedral angle
N-C1-C2-C3 is 90.6° in INT3aʹ) (Figure 4). In contrast, the imino
group in INT2a is coplanar with indolin skeleton.
ω
ω
∆
Gsol
B97XD/6-311++G(d,p)-SDD/SMD(toluene)// B97XD/6-31G(d)-LANL2DZ/SMD(toluene)
[Rh]=Rh2(OAc)4
-attack trans
N
N
O
(
)
-attack cis
(
)
Ph
N2
1.81
-attack
N
9.4
TS1a
[Rh]
2.08
NTs
O
2.07
2.03
[Rh]
N
NTs
0.0
N
1a
TS4a
N2
-5.1
-5.9
O
Ph
INT1a
N2
[Rh]
TS4a
-8.8
TS3a
-13.1
TS2a
N
Ph
O
N
[Rh]
2.26
2.04
2.39
[Rh]
2.01
[Rh]
NTs
NTs
NTs
N
N
N
-16.0
TS3a
TS2a
INT2a
2a
[Rh]
[Rh]
Ph
NTs
-22.9
N
N
O
INT3a
NTs
N
After the formation of INT3aʹ, the two possible nucleophilic N-
and O-attacks for 2aʹ to the carbenoid carbon (C2), were explored
respectively. For the N-attack at C2 of INT3aʹ, two orientations
leading to trans and cis structures are also considered and the
corresponding transition states are located as TS4aʹ and TS5aʹ,
respectively. Computational results suggest that TS4aʹ is 3.0
kcal/mol lower in energy than TS5aʹ. For TS4aʹ,18 the phenyl group
of 2aʹ adopts a face-to-face attacking mode with the carbenoid
moiety of INT3aʹ (Figure 4), which is different from the attacking
mode of 2a with INT2a. Due to the specific orientation of imino
group in INT3aʹ, the steric hindrance between C–H moiety of
imino group with the Et group of 2aʹ is responsible for the unfa-
vorable mode via TS5aʹ. For the O-attack, interestingly, the pres-
ence of C–H moiety of imino group in INT3aʹ could form an
H-bonding interaction with the N-atom of 2aʹ. The distance (C1–H1
⋯Na) is 2.12 Å in TS3aʹ (Figure 4). The favorable H-bonding interac-
tion could play an important role in stabilizing the transition state
TS3aʹ.19 In addition, no substantial steric hindrance is found be-
tween 2aʹ and INT3aʹ for the O-attack pathway. Consequently, the
calculated ΔG≠ is 1.2 kcal/mol lower for O-attack via TS3aʹ than
N-attack via TS4aʹ, suggesting that the O-attack pathway is kinet-
ically more favorable.20 A distortion/interaction analysis for TS3aʹ,
TS4aʹ, and TS5aʹ was performed (shown in Figure S2). Although
the N atom is more nucleophilic than the O atom in nitrosoarenes,
the N-attack pathway of 2aʹ is not favorable with INT3aʹ because
of pronounced steric congestion caused by either the C–H group
of imino moiety or the Et group adjacent to the nitroso group.
However, for O-attack, a favorable H-bonding interaction exists in
TS3aʹ, thus promoting the chemoselective addition of the O atom
of 2aʹ to INT3aʹ.
Ph
O
N
O
-54.1
Ph
N
NTs
3a-cis
-59.3
3a
N
NTs
N
Figure 1 Energy profiles (in kcal/mol) for the Rh2(II)-catalyzed reaction of
nitrosobenzene (2a) with 3-diazoindolin-2-imine (1a). Bond distances are
shown in Å.
Ar
2.39
Ar
TS2a
INT2a
Ar
Ar
2.26
2.07
TS4a
TS3a
Figure 2 The 3D structures of key intermediate and transition states. Bond
distances are shown in Å. For clarity, the 4-methylphenyl group is denoted
as Ar.
After the formation of INT4aʹ, an intramolecular cyclization to
form a three-membered ring intermediate INT6aʹ via TS6aʹ was
investigated. The predicted ΔG≠ for this step is only 8.9 kcal/mol
relative to INT4aʹ and the yielded INT6aʹ is thermodynamically
3. Mechanistic insights into the Rh2(II)-catalyzed reaction of ni-
trosoarenes with N-sulfonyl-1,2,3-triazoles.
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Chin. J. Chem. 2021, 39, XXX－XXX

Running title
Chin. J. Chem.
stable. Alternatively, one may propose that the formed INT4aʹ
might undergo another intramolecular cyclization with the imino
moiety to afford a five-membered ring intermediate INT9aʹ. The
located TS for this possible route is shown as TS9aʹ. However, the
predicted ΔG≠ for this step is 1.3 kcal/mol higher than the afore-
mentioned path via TS6aʹ. In addition, the formed intermediate
INT9aʹ is less favorable than the formation of INT6aʹ from ther-
modynamical point of view. Thus, the intramolecular cyclization to
form a three-membered ring intermediate INT6aʹ via TS6aʹ is
more ready to occur. Moreover, the potential intramolecular cy-
INT6aʹ with Rh via the O...Rh interaction can stabilize the corre-
sponding TS. The optimized TS with the presence of Rh catalyst is
shown as TS8aʹ. The calculated activation barrier is lowered to
23.0 kcal/mol with the assistance of Rh2(II) catalyst. It should be
noted that the formation of INT6aʹ is rather exergonic and the
reverse ring-opening of the three-membered ring for INT6aʹ via
either C-N or C-O bond cleavage needs to overcome a much high-
er energy barrier than the desired migration of the imino group.
Thus, it is feasible for INT6aʹ to undergo the migration of the
imino group assisted by the Rh2(II) catalyst and hence to afford
clization of INT5aʹ/INT8aʹ to form INT6aʹ was considered. However, the final product 3aʹ. Overall, based on the computational analy-
the predicted ΔG≠ for this possibility is significantly high (~40
kcal/mol), suggesting that the conversion of the intermediates
generated via N-attack pathway to INT6aʹ is not feasible to occur.
Next, the migration of the imino group for the generated
INT6aʹ could follow, accompanying the N–O bond breaking, to
afford the final product. The predicated activation barrier for this
isomerization of INT6aʹ via TS7aʹ is 31.7 kcal/mol, indicating this
step is not facile to occur. Nevertheless, the aforementioned pro-
cess could be facilitated by the Rh2(II) catalyst. The coordination of
sis, steric hindrance is mainly responsible for the unfavored
N-attack of 2aʹ with the α-imino rhodium carbene intermediate
(II), which is derived from the Rh2(II)-catalyzed denitrogenation of
1aʹ. In addition, a favored C–H...N interaction is revealed to facili-
tate the O-attack of 2aʹ with II. Afterwards, the yielded adduct via
O-attack of 2aʹ could undergo an intramolecular cyclization to
form the intermediate with a N,O-containing three-membered
ring. Finally, a Rh2(II)-assisted migration of imino group could fol-
low to produce the desired product 3aʹ.
ω
ω
∆
Gsol
B97XD/6-311++G(d,p)-SDD/SMD(DCE)// B97XD/6-31G(d)-LANL2DZ/SMD(DCE)
=
[Rh]=Rh
2(OAc)4
Ar
Ar
-attack trans
N
N
O
(
)
Et
-attack cis
Ph
1.91
Ar
O
N2
Ar
Ph
(
)
Ar
N
O
-attack
N
2.12
N
1.81
N
N
O
2.22
N
O
Ts
N
2.29
2.36
Ts
Ts
Ar
N
H
Ph
N
Ts
2.03
N
28.3
TS1a'
2.17
Ph
[Rh]
1.91 O
N
Ts
Ph
[Rh]
Ph
[Rh]
Ts
N
[Rh]
N
Ar
N
Ts
24.6
TS11a'
TS10a'
TS5a'
TS3a'
TS4a'
TS2a'
Ph
[Rh]
13.7
13.7
INT2a'
Ph
Ph
N2
TS5a'
TS4a'
TS3a'
1.97
N
N
1.59
Ts
N
10.7
9.5
Ts
O
N
O
N2
Ts
O
2.00 N
Ar
N
2.19
2.32
N
Ts
Ph
2.6
INT1a'
Ph
TS9a'
Ar
[Rh]
1.9
TS6a'
1.2
0.0
1a'
N
TS7a'
Ts
INT3a'
-0.2
-1.5
-2.2
N
N
2a'
TS9a'
TS6a'
TS10a'
TS11a'
Ts
N
Ph
[Rh]
Ph
[Rh]
Ts
[Rh]
[Rh]
N
O
-2.6
-6.8
TS8a'
Ph
2.07
1.65
Ar
O
N
N
Ph
Ar
Ts
Ts
O
-10.4
N
N
2.28
N
INT4a'
Ar
INT4a'
-17.3
[Rh]
Ph
INT9a'
[Rh]
Ts
-29.5
-29.8
INT6a'
Ph
[Rh]
N
INT7a'
Ts
N
Ar
N
[Rh]
Ph
Ph
O
O
N
Ar
N
Ts
N
Ar
-41.8
-43.2
[Rh]
O
INT5a'
INT8a'
Ph
O
N
O
Ts
Ts
N
N
Ph
N
-103.1
3a'
Ar
Ar
Figure 3 Energy profiles (in kcal/mol) for the Rh2(II)-catalyzed reaction of nitrosoarene (2aʹ) with N-sulfonyl-1,2,3-triazole (1aʹ). Bond distances are shown
in Å.
Chin. J. Chem. 2021, 39, XXX－XXX
© 2021 SIOC, CAS, Shanghai, & WILEY-VCH GmbH
www.cjc.wiley-vch.de

First author et al.
Report
Ar
Ar
2
1
3
2.12
2.29
2.36
2.22
Ar
Ar
TS5a'
TS4a'
TS3a'
INT3a'
Ar
Ar
2.28
2.07
2.00
Ar
Ar
TS8a'
INT7a'
TS6a'
INT4a'
Figure 4 The 3D structures of key intermediates and transition states. Bond distances are shown in Å. For clarity, the 4-methylphenyl group is denoted as
Ar.
imaginary frequency) or transition states (one imaginary frequen-
cy). In addition, intrinsic reaction coordinate (IRC) calculations
were used to verify that the transition state connect with appro-
Conclusions
priate reactant and product.25 The Gibbs energies for all species
were obtained at 298.15 K and 1 atm at their respective optimized
structures. Single-point energy computations at larger basis sets
(SDD26 for Rh and 6-311++G(d,p) for other atoms) using the SMD
model were performed based on the optimized structures. The
solution–phase Gibbs energy was determined by adding the solva-
tion single-point energy at larger basis set and the thermal correc-
tion to the Gibbs energy obtained from the vibrational frequency
analyses. All calculations were carried out with the Gaussian 09
suite of programs.27 The CYLView software28 was employed to
show the 3D structures.
In summary, we developed a Rh2(II)-catalyzed reaction of ni-
trosoarenes with 3-diazo-indolin-2-imines to produce
2-iminoindolin-nitrones via the key α-imino rhodium carbene in-
termediate (I). The N-attack of nitrosobenzene with the carbene
site of I is favorable because the N atom is more nucleophilic than
the O atom in nitrosobenzene and no substantial steric hindrance
is present. On the other hand, for the analogous Rh2(II)-catalyzed
reaction of nitrosoarene (2aʹ) with N-sulfonyl-1,2,3-triazole (1aʹ),
the O-attack of 2aʹ with the carbene site of α-imino rhodium car-
bene species (II), derived from 1aʹ, is more favorable to occur than
the N-attack. A favored C–H...N interaction and no substantial
steric hindrance is mainly responsible for the favorable O-attack of
2aʹ with II. In contrast, the unfavored N-attack of 2aʹ with II could
be attributed to substantial steric hindrance. After the favored
O-attack, an intramolecular cyclization of the adduct could occur
to form an intermediate with a N,O-containing three-membered
ring. Finally, the Rh2(II)-facilitated migration of imino group could
follow to produce the product of N-acylamidine.
General procedure for synthesis of 3.
The mixture of 1a (0.2 mmol), 2a (0.22 mmol), Rh2(OAc)4 (4
mol%) and toluene (1 mL) were stirred under air at 85 °C for 6
hours. After cooling to room temperature, the solvent was re-
moved under vacuum. Then crude residue was purified by column
chromatography (PE/EA = 10:1) to afford the desired product 3a in
86% yield (69.7mg) as a red solid.
Experimental
Supporting Information
Computational Methods: The ωB97XD21 density functional
method (DFT) was employed to carry out all the calculations. The
LANL2DZ basis set22 in conjunction with the LANL2DZ pseudopo-
tential was used for Rh atom. The 6-31G(d) basis set23 was used
for other atoms. To consider solvation effects, geometry optimiza-
tions were performed with the SMD solvation model24 (toluene
and dichloroethane was employed for 3-diazoindolin-2-imine and
N-sulfonyl-1,2,3-triazole involving reactions, respectively, accord-
ing to the optimal conditions). Vibrational frequency analyses at
the same level of the theory were performed on all the optimized
geometries to characterize stationary points as local minima (no
The supporting information for this article is available on the
WWW under https://doi.org/10.1002/cjoc.2021xxxxx.
Acknowledgement
We are grateful to the National Natural Science Foundation of
China (21973068), the project of scientific and technologic infra-
structure of Suzhou (SZS201708), and the Priority Academic Pro-
gram Development of Jiangsu Higher Education Institutions (PAPD)
for financial support.
www.cjc.wiley-vch.de
© 2021 SIOC, CAS, Shanghai, & WILEY-VCH GmbH
Chin. J. Chem. 2021, 39, XXX－XXX

Running title
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Chin. J. Chem. 2021, 39, XXX－XXX

Running title
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Manuscript received: XXXX, 2021
Manuscript revised: XXXX, 2021
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Manuscript accepted: XXXX, 2021
Accepted manuscript online: XXXX, 2021
Version of record online: XXXX, 2021
Chin. J. Chem. 2021, 39, XXX－XXX
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First author et al.
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Entry for the Table of Contents
Understanding Mechanistic Differences between 3-Diazoindolin-2-Imines and N-Sulfonyl-1,2,3-Triazoles in the Rh2(II)-Catalyzed Reactions with
Nitrosoarenes
Rui Fu, Luyao Kou, Ke Gao, Shaofang Zhou, and Xiaoguang Bao*
Chin. J. Chem. 2021, 39, XXX—XXX. DOI: 10.1002/cjoc.202100XXX
O
N
Ph
N
[Rh]
NTs
(I)
N2
O
Rh2 II)
(
NTs
NTs
N-attack
N
N
N
N2
Et
N
O
O
Ph
N
NTs
Et
[Rh]
Ph
Rh2 II)
(
N
N
NTs
(II)
N
Ts
O-attack
Ph
N2
•
2-iminoindolin-nitrones; 23
to 94%
yield
examples up
Synthesize
and
•
Rationalize different chemo-selectivities between I
II
The Rh2(II)-catalyzed reactions of 3-diazoindolin-2-imines with nitrosoarenes to produce 2-iminoindolin-nitrones are described. In addition, the
mechanistic differences for the reactions of nitrosoarenes with α-imino rhodium carbene species I and II are revealed by DFT calculations.
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Chin. J. Chem. 2021, 39, XXX－XXX