Rhodium(iii)-catalyzed annulation of enamides with sulfoxonium ylides toward isoquinolines

Article abstract of DOI:10.1039/d1ra01063h

An efficient rhodium(iii)-catalyzed C-H activation followed by intermolecular annulation between enamides and sulfoxonium ylides has been developed. The transformation proceeds smoothly with a broad range of substrates, affording a series of isoquinoline derivatives in moderate to good yields under additive-free conditions.

Full text of DOI:10.1039/d1ra01063h

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Rhodium(III)-catalyzed annulation of enamides with
sulfoxonium ylides toward isoquinolines†
Cite this: RSC Adv., 2021, 11, 11490
a
a
a
ab
*
*
Chao Hong, Shuling Yu, Zhanxiang Liu and Yuhong Zhang
Received 8th February 2021
Accepted 12th March 2021
An efficient rhodium(III)-catalyzed C–H activation followed by intermolecular annulation between
enamides and sulfoxonium ylides has been developed. The transformation proceeds smoothly with
a broad range of substrates, affording a series of isoquinoline derivatives in moderate to good yields
under additive-free conditions.
DOI: 10.1039/d1ra01063h
rsc.li/rsc-advances
Enamides are valuable building blocks in organic synthesis
having tunable reactivity and potential usage in various trans-
formations.19 The potential of enamide chemistry has been
Introduction
Isoquinolines represent important structural motifs frequently
found in natural products, functional materials and pharma-
ceuticals.1 Their unusual bioactive properties, such as anti-
witnessed by transition-metal-catalyzed coupling reactions.20 As
part of our continuing interest in metal-catalyzed enamide-
directed C–H functionalization reaction and the synthesis of
heterocyclic compounds,21 herein, we present a novel Rh(III)-
catalyzed cascade transformation from enamides and sulfoxo-
nium ylides for the preparation of isoquinolines. This method
allows the approach of a range of diversied 1,3-disubstituted
isoquinolines with moderate to good yields under mild condi-
tions without the use of additives.
spasmodic,2
antiinammatory,3
antihypertensive4
and
antitumour5 activities, have attracted much attention (Fig. 1).
Consequently, the alternative efficient synthetic methodology
for isoquinolines is of great importance.6 Traditional methods,
including the famous Bischler–Napieralski,7 Pictet–Gams8 and
Pictet–Spengler9 reactions, are well-established. Most of these
protocols suffer from harsh reaction conditions and environ-
mental problems. In this regard, transition metal-catalyzed C–H
activation/annulation would be valuable and complementary to
the known classic methodology, enabling the direct access to
a series of isoquinoline derivatives with minimum environ-
mental impact and fewer synthetic steps.10–13 On the other hand,
sulfoxonium ylides are attractive starting materials that can be
easily accessed and widely be utilized as transition metal-
carbene precursors in coupling reactions.14,15 The use of sul-
foxonium ylide in the synthesis of isoquinoline through Rh(III)-
catalyzed C–H activation was reported by Li and co-workers. In
their transformation, the ortho C–H bond of amidines is acti-
vated by rhodium catalyst to give isoquinolines with sulfoxo-
Results and discussion
We initiated our investigation on the model reaction of enam-
ide (1a) and sulfoxonium ylide (2a) to optimize various reaction
parameters. The results have been summarized in Table 1. At
the outset, our study was treated by enamide (1a) with sul-
foxonium ylide (2a) in the presence of [RhCp*Cl2]2/AgSbF6 and
ꢀ
NaOAc in THF at 100 C under N2 for 20 h (entry 1, Table 1).
However, the target product 3aa was not detected in the reac-
tion. Subsequently, when different solvents such as DMF, DCE
nium ylides in the presence of 30% Zn(OTf)2 (Scheme 1a).16
A
ruthenium-catalyzed mono ortho-C–H annulation of benzimi-
dates with sulfoxonium ylides was developed for the synthesis
of substituted isoquinolines by Wang group in the presence of
mesitylenic acid (Scheme 1b).17 Very recently, preparation of
isoquinoline derivatives by Rh(III)-catalyzed coupling reaction of
benzylamine and sulfoxonium ylides using water as solvent is
achieved by Wu and coworkers (Scheme 1c).18
aDepartment of Chemistry, Zhejiang University, Hangzhou 310027, People's Republic
of China. E-mail: liuzhanx@zju.edu.cn; yhzhang@zju.edu.cn
bState Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou
730000, People's Republic of China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d1ra01063h
Fig. 1 Representative bioactive isoquinolines.
11490 | RSC Adv., 2021, 11, 11490–11494
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NaOAc (compare entry 14 with entry 9, Table 1). The other
transition metal catalysts such as [Rh(cod)Cl]2 and RhCl3 were
probed as well. The reaction results indicated that they were less
effective than [RhCp*Cl2]2 (entry 15–16, Table 1). Control
experiment showed that an absence of the Rh catalyst led to no
formation of 3aa (entry 17, Table 1). The temperature reduction
(80 ꢀC) or elevation (120 ꢀC) had an adverse effect (entry 18–19,
Table 1). Therefore, the best result (71%ꢀ) was achieved by using
[RhCp*Cl2]2 (2.5 mol%) in HFIP at 100 C under N2 for 20 h.
With the optimized reaction conditions in hand, we then
investigated the generality and scope of enamide. Diversied
enamides bearing various aryl moieties substituted by electron-
donating groups and electron-withdrawing groups reacted
smoothly, affording the desired products in moderate to good
yields. Among them, the enamide substrates with the para-
position substituted by various electron-donating groups, such
as –Me, –Et and –OMe could be smoothly converted into the
desired products (Scheme 2, 3ba–3da). When the electron-
withdrawing groups including –F, –Cl, –Br and –CF3 were
introduced to the para-position of benzene ring of enamide, it
was also tolerated to the standard reaction conditions, and gave
the good yield (Scheme 2, 3ea–3ha). We further introduced –F
and –CF3 groups into the 3-position of benzene ring of enamide
1a and good results were also obtained (Scheme 2, 3ia–3ja).
However, the yield decreased markedly when ortho position of
the benzene ring was substituted by –OMe or –F (Scheme 2, 3ka–
3la). This may be due to steric effects. Interestingly, fused iso-
quinoline 3oa could also be obtained by this method in the yield
of 60%. It was noteworthy that the methyl or phenyl substituted
enamide on olenic bond could also be converted into
Scheme 1 Synthetic strategies toward isoquinolines.
and HFIP were studied on this reaction (entry 2–4, Table 1), we
were pleased to nd that the reaction occurred in HFIP,
affording the desired annulation product 3aa in a yield of 25%.
The experimental results show that the protic solvent-HFIP has
a better promotion effect on the reaction. Different inorganic
bases (LiOAc, KOAc, CsOAc and Cu(OAc)2) were then screened
(entry 5–8, Table 1), but the results were no better than that
obtained with NaOAc (entry 4, Table 1). The yield was improved
to 46% when AgSbF6 was removed (compare entry 9 with entry
4, Table 1). Next, the yield increased to 71% in the absence of
Table 1 Optimization of the reaction conditionsa
Entry
Catalyst
Additive/base
Solvent
Yieldb (%)
1
2
3
4
5
6
7
8
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
[Rh(cod)Cl]2
RhCl3
AgSbF6/NaOAc
AgSbF6/NaOAc
AgSbF6/NaOAc
AgSbF6/NaOAc
AgSbF6/LiOAc
AgSbF6/KOAc
AgSbF6/CsOAc
AgSbF6/Cu(OAc)2
—/NaOAc
—/LiOAc
—/KOAc
—/CsOAc
—/Cu(OAc)2
—
—
—
THF
DMF
DCE
N.R.
N.R.
N.R.
25
19
18
20
5
46
30
28
29
12
71
N.R.
N.R.
N.R.
53
HFIP
HFIP
HFIP
HFIP
HFIP
HFIP
HFIP
HFIP
HFIP
HFIP
HFIP
HFIP
HFIP
HFIP
HFIP
HFIP
9
10
11
12
13
14
15
16
17
18c
19d
—
—
—
—
[RhCp*Cl2]2
[RhCp*Cl2]2
65
a
Reaction conditions: 1a (0.3 mmol), 2a (0.2 mmol), [RhCp*Cl2]2
a
Scheme 2 Scope of the enamides.
Reaction conditions: 1 (0.3
(2.5 mol%), additive (20 mol%), base (1.0 equiv.), solvent (2.0 mL),
b
100 ꢀC, under N2, for 20 h. N.R. ¼ no reaction. Isolated yields.
mmol), 2a (0.2 mmol), [RhCp*Cl2]2 (2.5 mol%), in HFIP (2.0 mL), at
80 ꢀC. d 120 ꢀC.
c
100 ꢀC, under N2, for 20 h.
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corresponding isoquinoline products 3pa or 3qa, which further
expanding the scope of this protocol.
Next, the scope of sulfoxonium ylides was examined and the
results are summarized in Scheme 3. Both the electron-
donating and electron-withdrawing groups of the phenyl rings
tolerated well, giving the desired isoquinolines in moderate to
good yields (Scheme 3, 3ab–3am). When the phenyl group in
sulfoxonium ylide 2a was switched to alkyl group, the reaction
still performed smoothly to give the corresponding products
(Scheme 3, 3ao–3aq). However, when the phenyl ring was
replaced by a furan ring, 3an was obtained in a lower yield.
With the established substrate scope of the products, we
conducted a series of experiments to investigate the possible
mechanism. Initially, when the reaction of 1a was performed
with D2O and sulfoxonium ylide 2a under standard conditions
for 0.5 h, 25% of 1a were recovered and no deuterium was found
at the ortho position of the benzene ring, showing no H/D
exchange (Scheme 4a). It should indicate that the C–H bond
activation of enamide might follow an irreversible process.
Then, two parallel independent reactions (Scheme 4b) and the
one-pot deuterium competition reaction (Scheme 4c) of
substrates 1a and d5-1a were carried out, giving kH/kD values of
1.8 and 3.5 respectively. Both results indicate that the ortho C–H
bond cleavage of enamide was likely involved in the turnover-
limiting step. Moreover, the intermolecular competition exper-
iment between electron-rich and electron-decient enamides
(1b vis 1e) shows a ratio of products 3ba and 3ea of 1 : 1.30
based on the yields, suggesting that the aryl Csp2–H bond
activation possibly proceeded through concerted metallation–
deprotonation (CMD) process instead of electrophilic rho-
diumization pathway (Scheme 4d). Finally, the competitive
Scheme 4 Mechanism study experiments.
coupling-cyclization of a-(4-methylbenzoyl)-sulfoxonium ylide
(2b) and a-(4-uorobenzoyl) sulfoxonium ylide (2d) with
enamide (1a) led to the yields of 3ab and 3ad with a ratio of
1.14 : 1 based on the yields (Scheme 4e), indicating that
electron-rich sulfoxonium ylide more easily forms a rhodium-
carbene than the electron-decient sulfoxonium ylide, which
facilitates the formation of isoquinoline.
On the basis of the above experimental results and precedent
literatures,14d,15d,18
a
plausible reaction mechanism was
proposed as shown in Scheme 5. The process begins with
coordination of nitrogen atom of enamide 1a to the rhodium
atom of A and subsequent Csp2–H activation via the concerted
metalation-deprotonation (CMD) afford the key ve-membered
rhodacycle C, which is trapped by the sulfoxonium ylide 2a to
form the rhodium-carbene D through the elimination of DMSO.
Scheme 3 Scope of the sulfoxonium ylides. a Reaction conditions: 1a
(0.3 mmol), 2 (0.2 mmol), [RhCp*Cl2]2 (2.5 mol%), in HFIP (2.0 mL), at
100 ꢀC, under N2, for 20 h.
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Conclusions
In summary, we have disclosed a novel strategy for the synthesis
of isoquinolines via rhodium(III)-catalyzed C–H activation and
annulation. The useful building blocks of enamides and sul-
foxonium ylides are applied, and a range of substituted iso-
quinolines are prepared under mild reaction conditions. This
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Conflicts of interest
There are no conicts to declare.
Acknowledgements
This research was supported by the Natural Science Foundation
of China (No. 21672186) and Important Project of Zhejiang
province (2020C01149).
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