Rh(III)-catalyzed synthesis of isoquinolines using the N-Cl bond of N-chloroimines as an internal oxidant

Article abstract of DOI:10.1016/j.tetlet.2020.151771

The Rh(III)-catalyzed coupling of N-chloroimines with alkynes for the efficient synthesis of isoquinolines is reported. This represents the first use of the N-Cl bond of N-chloroimines as an internal oxidant for construction of the isoquinoline skeleton. The synthesis features atom and step economy, a green solvent (EtOH), mild reaction conditions, and a broad substrate scope.

Full text of DOI:10.1016/j.tetlet.2020.151771

Tetrahedron Letters xxx (xxxx) xxx
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Rh(III)-catalyzed synthesis of isoquinolines using the N-Cl bond
of N-chloroimines as an internal oxidant
⇑
Bing Qi, Lili Fang, Qi Wang, Shan Guo, Pengfei Shi, Benfa Chu, Jin Zhu
State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
The Rh(III)-catalyzed coupling of N-chloroimines with alkynes for the efficient synthesis of isoquinolines
is reported. This represents the first use of the N-Cl bond of N-chloroimines as an internal oxidant for con-
struction of the isoquinoline skeleton. The synthesis features atom and step economy, a green solvent
(EtOH), mild reaction conditions, and a broad substrate scope.
Received 5 January 2020
Revised 15 February 2020
Accepted 20 February 2020
Available online xxxx
Ó 2020 Elsevier Ltd. All rights reserved.
Keywords:
CAH functionalization
Isoquinoline
N–Cl bond
Internal oxidant
Introduction
Me
R2
R3
Me
NCl
The isoquinoline skeleton is an important structural
Scheme 1motif commonly found in pharmaceuticals, biologically
active alkaloids, natural products, and chiral ligands [1]. Applica-
tions in organic light-emitting diodes have also been demonstrated
[2]. Traditionally isoquinolines have been synthesized by Bischler-
Napieralski cyclization [3], the Pictet-Gams reaction [4], and the
Pomeranz-Fritsch reaction [5]. However, these methods often
require laborious multistep efforts and harsh reaction conditions.
Transition metal-catalyzed CAH functionalization has emerged as
an extremely effective tool in organic synthesis, especially for the
construction of heterocycles [6]. The use of new directing groups
can contribute significantly to reaction development. Indeed, vari-
ous heterocycles have been accessed in an efficient manner due to
the expansion of the pool of directing groups [7–10]. Isoquinolines
have also attracted a great deal of attention from the transition
metal-catalyzed CAH functionalization community [2]. Initial
reaction development focused on the use of external oxidants for
effecting the catalytic turnover [11a–c]. The use of stoichiometric
transition metal salts can represent a serious concern from an envi-
ronmental sustainability perspective. Recent advances in this area
have demonstrated the feasibility of using part of the directing
group as an internal oxidant for turnover of the catalytic cycle
[2]. Thus far, only NAO bonds and NAN bonds have been reported
as internal oxidants.
Rh(III) (4 mol%)
EtOH, 60 oC,12 h
N
R1
+
R1
R2
R3
35 examples
25-91% yield
Scheme 1. Synthetic Access to Isoquinolines via the Rh(III)-Catalyzed Coupling of
N-Chloroimines with Alkynes.
Our group has pioneered the use of the N-Cl bond as an internal
oxidant for transition metal-catalyzed CAH functionalization reac-
tions [12]. We have recently demonstrated N-chloroimines as an
effective directing synthon, by coupling with a-diazo-a-phospho-
noacetates, for 2H-isoindole synthesis.
[13] Herein, N-chloroimines have been used in the coupling
with alkynes to provide isoquinolines. The N-Cl bond has been uti-
lised for the first time for the construction of this important skele-
ton, allowing the achievement of a broad substrate scope in a green
solvent and under mild conditions.
Results and discussion
At the outset of our studies, we probed the Rh(III)-catalyzed
coupling reaction using [RhCp*(CH3CN)3](SbF6)2 (4 mol%) as the
catalyst precursor, with N-chloro-1-phenylethan-1-imine (1a)
and diphenylacetylene (2a) as the model substrates (Table 1). First,
⇑
Corresponding author.
E-mail address: jinz@nju.edu.cn (J. Zhu).
https://doi.org/10.1016/j.tetlet.2020.151771
0040-4039/Ó 2020 Elsevier Ltd. All rights reserved.
Please cite this article as: B. Qi, L. Fang, Q. Wang et al., Rh(III)-catalyzed synthesis of isoquinolines using the N-Cl bond of N-chloroimines as an internal
oxidant, Tetrahedron Letters, https://doi.org/10.1016/j.tetlet.2020.151771

2
B. Qi et al. / Tetrahedron Letters xxx (xxxx) xxx
Table 1
Optimization of the Reaction Conditions.a,b
Me
[RhCp*(CH3CN)3](SbF6)2 (4 mol%)
Ph
Me
NCl
additive
N
+
solvent, temperature, 12 h
Ph
Ph
Ph
1a
2a
3aa
Entry
Additive (equiv)
Solvent
Temperature (oC)
Yield (%)
1
2
3
NaOAc (2)
HOAc (2)
DCE
DCE
DCE
DCE
DCE
TFE
MeCN
HFIP
1,4-dioxane
THF
80
80
80
80
80
80
80
80
80
80
80
80
80
60
100
88
0
0
trace
0
51
42
64
66
73
29
81
67
91
68
AgOAc (2)
NaOAc (0)
NaOAc (2)
NaOAc (2)
NaOAc (2)
NaOAc (2)
NaOAc (2)
NaOAc (2)
NaOAc (2)
NaOAc (2)
NaOAc (1)
NaOAc (2)
NaOAc (2)
4
5c
6
7
8
9
10
11
12
13
14
15
H2O
EtOH
EtOH
EtOH
EtOH
a
Reaction condition: 1a (0.2 mmol, 1.0 equiv), 2a (0.3 mmol, 1.5 equiv), [RhCp*(CH3CN)3](SbF6)2 (4 mol%), solvent (2 mL). bIsolated yield. cWithout [RhCp*(CH3CN)3]
(SbF6)2.
Me
[RhCp*(CH3CN)3](SbF6)2 (4 mol%)
NaOAc (2 equiv)
Ph
Me
NCl
N
R1
+
EtOH, 60 oC, 12 h
R1
Ph
Ph
Ph
1a-1r
Me
2a
3aa 3ra
-
Me
Me
R1
Me
N
N
N
R1
Ph
Ph
Ph
Ph
3aa:
Ph
Ph
1
1
R = H,
91%
86%
98%
79%
3ka:
3na:
3oa:
3pa:
86%
R = Cl,
35%
31%
25%
1
1
3ba:
R = Me,
R = Br,
1
1
3ca:
R = OMe,
R = CF3,
1
3da:
R = Ph,
Me
Me
Me
Me
1
3ea:
R = F,
73%
MeO
F
1
3fa:
3ga:
N
N
N
N
R = Cl,
80%
68%
+
+
Ph
1
R = Br,
Ph
Ph
Ph
Ph
1
3ha:
R = I,
59%
Ph
OMe Ph
72% (1:0.76)
Me
Ph
F
Ph
1
3ia:
R = CF3,
58%
47%
1
3ja:
R = CN,
3laI 3laII:
3maI 3maII:
+
+
73% (1:0.12)
R2
F
Me
N
N
N
N
Ph
Ph
Ph
Ph
3sa:
3ta:
Ph
Ph
36%
Ph
3ua:
89%
2
3qa:
3ra:
53%
R = Et,
81%
83%
2
R = Ph,
a
1a 1r
-
2a
(0.3 mmol, 1.5 equiv), EtOH (2 mL).
Reaction conditions:
bIsolated yields.
(0.2 mmol, 1.0 equiv),
Scheme 2. Substrate Scope for the N-Chloroimines.a,b
Please cite this article as: B. Qi, L. Fang, Q. Wang et al., Rh(III)-catalyzed synthesis of isoquinolines using the N-Cl bond of N-chloroimines as an internal
oxidant, Tetrahedron Letters, https://doi.org/10.1016/j.tetlet.2020.151771

B. Qi et al. / Tetrahedron Letters xxx (xxxx) xxx
3
Me
R2
[RhCp*(CH3CN)3](SbF6)2 (4 mol%)
NaOAc (2 equiv)
Me
NCl
N
+
EtOH, 60 oC, 12 h
R2
R3
R3
3ab-3ao
1a
2b-2o
Me
Me
Me
N
N
N
OMe
R5
MeO
R4
R5
R4
4
5
3ab:
R = OMe,
3ag:
3ak:
66%
R = Et,
74%
3ac:
56%
98%
69%
71%
R = Me,
R = OMe,
R = F,
R = Cl,
87%
3ah:
86%
4
5
4
5
3ad:
3ae:
3af:
3ai:
R = F,
53%
4
5
3aj:
R = Cl,
65%
4
R = Br,
Me
Me
N
N
Ph
CO2Me
Ph
R6
6
3ao:
3al:
3am:
3an:
20%
R = Me,
86%
35%
31%
6
R = Et,
6
n
R = Pr,
a
1a
2b 2o
- (0.3 mmol, 1.5 equiv.), EtOH (2 mL).
Reaction condition:
bIsolated yield.
(0.2 mmol, 1.0 equiv.),
Scheme 3. Substrate Scope for the Alkynes.a,b
the reaction in dichloroethane (DCE), with 2 equiv. of NaOAc,
HOAc, or AgOAc as the additive (Entries 1–3, Table 1), showed that
NaOAc gave the highest yield for the cyclized target isoquinoline
derivative 3aa, with comparable reaction efficiency under nitrogen
and in the air (88% yield after 12 h reaction at 80 °C; for reaction
setup convenience, all reactions are performed with the corre-
sponding yields reported without the strict exclusion of air). No
conversion was observed using AgOAc and HOAc. The control
experiment in the absence of NaOAc resulted in virtually no reac-
tion (Entry 4, Table 1). Moreover, [RhCp*(CH3CN)3](SbF6)2 proved
to be indispensable as no conversion occurred without this catalyst
precursor (Entry 5, Table 1). With these initial findings, the effect of
solvent was then examined. The screening of various solvents
(Entries 6–12, Table 1) demonstrated EtOH as an alternative, green
reaction medium (81% yield). Upon reducing the amount of NaOAc
to 1 equiv., a lower yield was observed (Entry 13, Table 1). The
investigation of reaction temperature in the range of 60 °C to
100 °C (Entries 14–15, Table 1) revealed 60 °C as the best temper-
ature (91% yield). Preliminary experiments with [CoCp*I2]2 as the
catalyst precursor showed that no reaction occurs, likely due to
the inability of Co(III) to activate the CAH bond.
With the optimum reaction conditions established, we next
explored the substrate scope of the N-chloroimines using 2a as
the coupling partner (Scheme 2). For para-substituted
N-chloroimines, both electron-donating (Me, 1b; OMe, 1c; Ph,
1d) and electron-withdrawing (F, 1e; Cl, 1f; Br, 1g; I, 1h; CF3, 1i;
CN, 1j) groups were well tolerated, with electron-rich N-chloroimi-
nes exhibiting a higher reactivity. The reaction also proceeded with
meta-substituted N-chloroimines (Me, 1k; OMe, 1l; F, 1m; Cl, 1n;
Br, 1o; CF3, 1p). N-Chloroimines 1k, 1n, 1o, and 1p showed exclu-
sive regioselectivity at the sterically less hindered site (para to the
meta-substituent), whereas 1l and 1m generated a mixture of
regioisomers (both ortho and para to the meta-substituent).
ortho-Substitution (F, 1q) was also compatible with the methodol-
ogy. In addition, N-chloroimine bearing a fused ring system (1r)
also reacted. Changing the imino C-Me group to either a C-Et (1s)
Please cite this article as: B. Qi, L. Fang, Q. Wang et al., Rh(III)-catalyzed synthesis of isoquinolines using the N-Cl bond of N-chloroimines as an internal
oxidant, Tetrahedron Letters, https://doi.org/10.1016/j.tetlet.2020.151771

4
B. Qi et al. / Tetrahedron Letters xxx (xxxx) xxx
H-D scrambling:
H
Me
D/H Me
[RhCp*(CH3CN)3](SbF6)2 (4 mol%)
NaOAc (2 equiv)
Cl
NCl
N
(a)
(b)
CD3CO2D (3 equiv)
EtOH, 60 oC, 12 h
H
D/H
~9%D
1a
Competition:
Me
NCl
Me
[RhCp*(CH CN) ](SbF ) (4 mol%)
Ph
3
3
6 2
MeO
F
NaOAc (2 equiv)
EtOH, 60 oC, 12 h
N
1c
1e
Me
NCl
MeO/F
Ph
Ph
2a
Ph
OMe/F = 1.3
KIE determination:
H
Me
H
H
NCl
H/D Me
N
H
Ph
[RhCp*(CH3CN)3](SbF6)2 (4 mol%)
NaOAc (2 equiv)
H/D
H/D
H
1a
(c)
EtOH, 60 oC, 12 h
Ph
Ph
D
Me
H/D Ph
2a
D
D
NCl
kH/kD = 1.7
D
D
1a-d5
Scheme 4. H-D Scrambling, Competition, and KIE Experiments.
or C-Ph (1t) group maintained the reactivity. The method was also
compatible with ring-fused substrate (1u).
experiment involving the reaction of 2a with 1c and 1e, shows that
the reaction slightly prefers electron-rich 1c (product distribution:
3ca/3ea = 1.3) (Scheme 4b), which is consistent with an elec-
trophilic aromatic substitution pathway. Third, we performed the
kinetic isotope effect (KIE) test using the intermolecular competi-
tion between 1a and 1a-d5 with 2a (Scheme 4c). The KIE was deter-
mined as 1.7 by calculating the product ratio of 3aa and 3aa-d4
using 1H NMR analysis. This moderate KIE value illustrates that
the CAH activation step might not be rate-determining for the
reaction. In addition, we have previously isolated two types of
five-membered rhodacycles 1e-Rh and 1f’-Rh [13]. When com-
bined with the use of chloride-abstracting AgSbF6, they can react
both as a stoichiometric reagent with 2a and as a catalyst for the
reaction, respectively, for 1e, 2a and 1f, 2a (Scheme 5).
We next proceeded to examine the substrate scope for alkynes
using 1a as the coupling partner (Scheme 3). Both electron-rich (Et,
2b; OMe, 2c), and electron-poor (F, 2d; Cl, 2e; Br, 2f) para-substi-
tuted diarylalkynes afforded the respective products. Further, the
optimized reaction condition enables the accommodation of
meta-substituted (Me, 2g; OMe, 2h; F, 2i; Cl, 2j) and ortho-substi-
tuted (OMe, 2k) diarylalkynes, all of which display good reactivity.
Unsymmetrically substituted phenyl/alkyl alkynes (Me, 2l; Et, 2m;
nPr, 2n) and phenyl/ester alkyne CO2Me, 2o) were also tolerated.
The regioselectivity is excellent and likely caused by the coordina-
tion capability of phenyl ring for the Rh(III) center.
Several mechanistic experiments were conducted to probe the
reaction pathway. We first conducted the H-D scrambling experi-
ment which evaluated the feasibility of CAH activation (Scheme 4).
Indeed, reacting 1a under deuterating conditions (with the addi-
tion of CD3CO2D) resulted in an approximately 9% deuterium sub-
stitution at the ortho position (Scheme 4a). Second, a competition
The reactivity supports the ability to maintain the Rh catalytic
center at the III valence state for each catalytic cycle by using the
N-Cl bond as an internal oxidant.
Based on the above experimental results, a plausible mecha-
nism is depicted in Scheme 6 using 1a and 2a as the illustrative
Please cite this article as: B. Qi, L. Fang, Q. Wang et al., Rh(III)-catalyzed synthesis of isoquinolines using the N-Cl bond of N-chloroimines as an internal
oxidant, Tetrahedron Letters, https://doi.org/10.1016/j.tetlet.2020.151771

B. Qi et al. / Tetrahedron Letters xxx (xxxx) xxx
5
Me
Me
Ph
AgSbF6 (1.1 equiv)
NaOAc (2 equiv)
Cl
N
N
(a)
EtOH, 60 oC,12 h
Rh Cp*
Cl
F
F
Ph
Ph
Ph
: 83%
1e-Rh
2a
3ea
3ea
Me
Me
Ph
1e-Rh
(4 mol%)
AgSbF6 (4 mol%)
Cl
N
N
(b)
(c)
(d)
NaOAc (2 equiv)
F
Ph
Ph
Ph
EtOH, 60 oC,12 h
F
Ph
Ph
: 80%
1e
2a
Me
NH
Rh Cp*
Cl
Me
Ph
AgSbF6 (1.1 equiv)
NaOAc (2 equiv)
N
EtOH, 60 oC,12 h
Cl
Cl
Ph
Ph
: 91%
1f'-Rh
2a
3fa
Me
Me
Ph
1f'-Rh
(4 mol%)
AgSbF6 (4 mol%)
Cl
N
N
NaOAc (2 equiv)
Cl
EtOH, 60 oC,12 h
Cl
Ph
Ph
: 68%
1f
2a
3fa
Scheme 5. Mechanistic Studies Involving Rhodacycles.
reactants. The first step is the transformation of Rh catalyst precur-
sor [RhCp*(CH3CN)3](SbF6)2 to a catalytically active species I via
ligand exchange with NaOAc. Intermediate I then coordinates with
the N-chloroimine directing group of 1a and activates the ortho
CAH bond to form II. Subsequently, 2a coordinates with II by dis-
placing the weakly coordinated HOAc to afford III. Intermediate III
further experiences migratory insertion to give IV. Finally, ring
closure occurs with formation of the CAN bond and cleavage of
the N-Cl bond; proto-demetalation generates 3aa and releases
the Rh catalytic center as I.
Declaration of Competing Interest
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
Acknowledgments
We gratefully acknowledge support from the National Natural
Science Foundation of China (21774056,21425415), and Science
and Technology Department of Jiangsu Province (BRA2017360,
BK20181255).
Conclusion
We have developed a Rh(III)-catalyzed method for the synthesis
of isoquinolines using the N-Cl bond of N-chloroimines as an inter-
nal oxidant. The coupling of N-chloroimines with alkynes provides
access to a diverse range of isoquinoline derivatives in a green
solvent under mild conditions. We expect that this valuable
N-chloroimine synthon will find wide applications in transition
metal-catalyzed CAH functionalization due to its high coordinating
and oxidizing reactivity.
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.tetlet.2020.151771.
Please cite this article as: B. Qi, L. Fang, Q. Wang et al., Rh(III)-catalyzed synthesis of isoquinolines using the N-Cl bond of N-chloroimines as an internal
oxidant, Tetrahedron Letters, https://doi.org/10.1016/j.tetlet.2020.151771

6
B. Qi et al. / Tetrahedron Letters xxx (xxxx) xxx
[RhCp*(CH3CN)3](SbF6)2 + NaOAc
Rh
1a
AcO
Cp*
3aa
+
HOAc + NaCl
I
HOAc + NaOAc
Me
Me
Cl
Cl
N
N
Rh
Cp*
Rh Cp*
Ph
HOAc
II
Ph
IV
Me
2a
Cl
N
Rh Cp*
HOAc
Ph
Ph
III
Scheme 6. Proposed Reaction Pathway.
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