Copper-promoted direct amidation of isoindolinone scaffolds by sodium persulfate

Article abstract of DOI:10.1039/d1ob01054a

Isoindolinones are ubiquitous structural motifs in natural products and pharmaceuticals. Establishing an efficient method for structural modification of isoindolinones could significantly facilitate new drug development. Herein, we describe copper-promoted direct amidation of isoindolinone scaffolds mediated by sodium persulfate. The method exhibits mild reaction conditions and high site-selectivity, and enables the structural modification of the drug indobufen ester with various amides with yields of 49 to 98%. It is also gram-scalable. Additionally, the reaction mechanism appears to involve a radical and a carbocationic pathway.

Full text of DOI:10.1039/d1ob01054a

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Copper-promoted direct amidation of
isoindolinone scaffolds by sodium persulfate†
Cite this: DOI: 10.1039/d1ob01054a
a,b
Huifang Lai, a Jiexin Xu,a Jin Lina and Daijun Zha
*
Isoindolinones are ubiquitous structural motifs in natural products and pharmaceuticals. Establishing an
efficient method for structural modification of isoindolinones could significantly facilitate new drug devel-
opment. Herein, we describe copper-promoted direct amidation of isoindolinone scaffolds mediated by
sodium persulfate. The method exhibits mild reaction conditions and high site-selectivity, and enables the
structural modification of the drug indobufen ester with various amides with yields of 49 to 98%. It is also
gram-scalable. Additionally, the reaction mechanism appears to involve a radical and a carbocationic
pathway.
Received 31st May 2021,
Accepted 9th July 2021
DOI: 10.1039/d1ob01054a
rsc.li/obc
The amide is one of the most versatile functionalities in a great demand for methods that are more practical, con-
natural products and pharmaceutical chemistry.1 According to venient, sustainable, and environmentally compatible.10
the Comprehensive Medicinal Chemistry database, more than
Persulfates, such as K2S2O8, Na2S2O8, and (NH4)2S2O8, are
25% of pharmaceuticals are amide-containing compounds.2 strong inorganic oxidants.11 In addition, these agents are
Thus, chemists have developed many synthetic strategies to ‘green’ alternatives to other oxidants because they are low-cost,
create amide bonds. Traditional strategies depend on coupling readily available, essentially nontoxic, and easy to handle.12
reagents,3 such as 1-ethyl-3-(3′-dimethylaminopropyl)carbodi- Although persulfates have been used in many applications to
imide (EDC), 1,1′-carbonyldiimidazole (CDI) and (benzotriazol- transform C–H/C–C to C–X (X = N, O, S, P, B, Si, F, Br, I),11
1-yloxy)tris(pyrrolidine)phosphonium
hexafluorophosphate there have been relatively few studies on their use in mediating
(PyBOP), to condense the carboxylic acids and amine groups. direct amidation. Yuan13 et al. reported the use of K2S2O8 as
However, without relying on pre-installed functional groups, the oxidant in the direct oxidative amidation of quinoxalin-2
cross-dehydrogenative coupling (CDC) reaction is an alterna- (1H)-ones with a variety of aromatic and aliphatic amides.
tive method to construct amides and is a promising method Truscello14 et al. realized the α-amidoalkylation of
for late-stage modification of drug molecules.4 Cross-dehydro- N-heteroaromatic bases with amides in the presence of
genative reaction for the construction of amides is showing Na2S2O8. Nevertheless, persulfates have great potential for
significant advancement.5 For example, Chang et al.6 devel- broadening the number of direct amidation applications.
oped a CpRu(II) catalytic platform to achieve highly site-selec-
Isoindolinone skeletons are privileged structures in bio-
tive amidation at the benzylic position instead of at the tertiary active natural products and drug molecules (Fig. 1a).15 For
C–H bonds at the same distance. Nozawa-Kumada and Kondo7 instance, lenalidomide is used to treat multiple myeloma.16
et al. used tBuOOtBu as an oxidant to accomplish Cu-catalyzed Hericenone B and indobufen are antiplatelet aggregation
intramolecular oxidative C(sp3)–H amidation for the synthesis drugs.17 Many investigators have studied structural modifi-
of β-lactams. Landais8 et al. reported an efficient strategy to cations at the N-benzylic (C3) position of isoindolinone skel-
harness NFSI((PhSO2)2NF) or F-TEDA-PF6 as oxidants to access etons because of the significant influence of the modifications
benzylic carbamates. König9 et al. reported a photoinduced on their biological activity.18 However, to the best of our knowl-
copper(II)-di-tert-butyl peroxide catalytic system to realize the edge, direct amidation at the C3 position had not been rea-
amidation of alkanes. Yet, despite these achievements, there is lized. Hence, we report a direct benzylic amidation of isoindo-
linone scaffolds using Na2S2O8 as an oxidant. The method fea-
tures mild conditions, site-selectivity, good to excellent yields,
aDepartment of Medicinal Chemistry, School of Pharmacy, Fujian Medical University, and gram-scalability. In addition, we have determined the
Fuzhou 350004, Fujian Province, China. E-mail: zhadj@fjmu.edu.cn
details of the reaction mechanism.
For structural simplification, we began this investigation
with methylisoindolin-1-one as
Commercially available pyrrolidin-2-one, which is widely used
in drug discovery, was chosen as the amide source to optimize
bFujian Key Laboratory of Drug Target Discovery and Structural and Functional
Research, Fujian Medical University, China
a
model substrate.
†Electronic supplementary information (ESI) available: 1H, 13C and 19F NMR
spectra. CCDC 2065589 and 2065590. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/d1ob01054a
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result, albeit still affording a relatively low yield of 59%
(entries 2–5, Table 1).
Tetrabutylammonium bromide (TBAB) and tetrabutyl-
ammonium iodide (TBAI) are employed frequently as additives
to promote persulfate-mediated oxidation.20 Thus, we tested
the effects of the addition of TBAB and TBAI (20% mmol). In
the case of TBAB, the yield of 2a increased to a nearly quanti-
tative amount (97%), and TBAI afforded 91% yield (entries 6
and 7, Table 1). We also examined the effects of metal salts,
such as Pd, Mn, Ag, and Fe, in the presence of TBAB. However,
none of the yields were as high as that obtained with CuBr2
(entries 8–11, Table 1; for more details, see ESI Table S1,†
entries 10–16). We also evaluated other Cu(I)/Cu(II) species,
Fig. 1 (a) Typical products of the isoindolinone skeleton, (b) direct ami- such as CuBr, CuCl2, CuI, and CuCl (entries 12–15, Table 1).
dation of isoindolinone scaffolds mediated by Na2S2O8.
Again, CuBr2 was the most effective; other copper salts gave
56–86% yields (ESI Table S1,† entries 1–9). When the reaction
was performed in the absence of CuBr2, only a 15% yield of 2a
was obtained, indicating the significant role of the metal cata-
the reaction conditions. We mixed 0.2 mmol methylisoindolin-
lyst (entry 16, Table 1). The loadings of Na2S2O8, TBAB, and
1-one, 0.2 mmol pyrrolidin-2-one, and 0.4 mmol Na2S2O8 in
CuBr2 were also screened (ESI Table S2,† entries 15–20).
2 mL 1,2-dichloroethane and reacted the mixture by refluxing
at 80 °C. The desired product, 2-methyl-3-(2-oxopyrrolidin-1-yl)
isoindolin-1-one (2a), was obtained in only 5% yield (entry 1,
Table 1). Because transition metals can facilitate the formation
We also screened several common solvents. Highly polar
solvents such as dimethyl sulfoxide (DMSO) and dimethyl-
formamide (DMF) caused a considerable decrease in yields to
13% and 12%, respectively, whereas acetonitrile and tetra-
of the SO4•− radical and electron transfer,19 we added catalytic
hydrofuran (THF) gave the products in 56% and 76% yields
amounts of transition metals, such as CuBr2, Pd(OAc)2,
(ESI Table S2,† entries 4–10). K2S2O8, even under microwave-
AgNO3, and FeCl2. Among them, CuBr2 produced the best
assisted conditions,13 and (NH4)2S2O8 produced lower yields
compared with Na2S2O8 (ESI Table S2,† entries 1–3), which
might be ascribed to their different oxidation power and solu-
bility in DCE.11 Finally, we hypothesized that aerobic oxidation
Table 1 Optimization of the reaction conditionsa
could inhibit the metal catalyst. Indeed, running the reaction
in open air resulted in lower yield (entry 17, Table 1).
In summary, the foregoing experiments established the
optimal reaction conditions for the transformation: 2 equiv.
Na2S2O8 as an oxidant, 10% mmol CuBr2, 20% mmol TBAB,
1,2-dichloroethane as the solvent, and a reaction temperature
of 80 °C (entry 6, Table 1).
With the optimized conditions in hand, we next assessed
the versatility of the protocol with a variety of isoindolinone
derivatives. N-Alkylisoindolinones (Scheme 1, 2a–2g), which
have different length or branched carbon chains in their
N-termini, produced excellent yields (81–96%). Similarly, good
to excellent yields (80–97%) were obtained with the
N-phenylisoindolinone derivatives (Scheme 1, 2h–2s). Notably,
high site selectivity was achieved, and the amide was installed
primarily at the C3 position, which was exemplified by pro-
ducts 2b, 2c, 2g, 2j, and 2k. These results indicated that the
C(sp3)–H at the C3 position of the isoindolinone skeleton was
more prone to be activated than other secondary, tertiary, or
benzylic centers. Furthermore, the reaction of 2-phenylisoindo-
Entry
Additive
PTC
Yieldb (2a)
1
2
3
4
5
6
7
8
—
—
—
—
—
5
CuBr2
Pd(OAc)2
AgNO3
FeCl2
CuBr2
CuBr2
Pd(OAc)2
AgNO3
FeCl2
Mn(OAc)3
CuBr
CuCl2
CuI
CuCl
—
CuBr2
59
22
37
4
97
91
31
52
4
19
85
86
64
56
15
54
—
TBAB
TBAI
TBAB
TBAB
TBAB
TBAB
TBAB
TBAB
TBAB
TBAB
TBAB
TBAB
9
10
11
12
13
14
15
16
17c
lin-1-one with N-pyrrolidone occurred successfully on
a
5 mmol scale and produced 1.27 g of 2h without apparent
erosion (87% yield).
In addition, the substituents, either electron-donating or
electron-withdrawing groups, located in the phenyl ring of the
isoindolinone skeleton or N-terminus, affected the product
a Reaction conditions: Methylisoindolin-1-one (0.2 mmol), pyrrolidin-
2-one (0.2 mmol), Na2S2O8 (0.4 mmol), additives (10% mmol), PTCs
(20% mmol), and 1,2-dichloroethane (2 mL) at 80 °C for 24 h under a
N2 atmosphere. b Yield detected by HPLC based on three runs of each
reaction. c Air conditions.
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91% yield of the desired product (3b). We obtained similar
results with 2-piperidone and caprolactam (3c–3d). However,
with obvious steric hindrance, the substituted 2-pyrrolidone
and 2-piperidone, which show important biological activities,
were introduced successfully (3e–3g). Other coupling partners,
such as 2-oxazolinone and 2-imidazolidone, resulted in 98%
and 85% yields (3h–3i).
In addition to the lactam mentioned above, we employed
primary amides as coupling partners (3j–3s) and obtained
similar yields with the aliphatic ring-substituted amides. In
addition, the cyclopropyl group-substituted amide was well tol-
erated (3j) without producing any by-product, which further
indicated the mildness of our protocol. In the presence of
another benzylic position, 2-phenylacetamide reacted well and
afforded a 77% yield of 3l with high site-selectivity. Good to
excellent yields were also obtained when we replaced aliphatic
amides with aryl amides, and even when the bromo moiety was
substituted at the ortho, meta, and para positions (3o–3q).
Notably, the electron-withdrawing nitro-group on aromatic
amides (3r) led to a dramatic drop in their reactivity, and more
reaction time was required for the completion of the reaction.
Moreover, the yield was relatively lower than that with the elec-
tron-donating methoxyl-group (3s). Unlike lactam and cyclic
amides, acyclic amides, even secondary acyclic amides, showed
greatly reduced yields (49–54%), because their relatively higher
reactivity led to the occurrence of side reactions (3u–3x).
Scheme 1 Amidation of substituted isoindolinones with pyrrolidin-2-
one. Reaction conditions: Isoindolinones (1 mmol), N-pyrrolidone
(1 mmol), Na2S2O8 (2 mmol), CuBr2 (10% mmol), TBAB (20% mmol), and
DCE (4 mL) at 80 °C for 24 h under a N2 atmosphere, isolated yield.
a Isoindolinone as the only substrate.
After amide installation, diastereomers appeared because of
1
the creation of new chiral centers. We used crude H NMR to
yields (2h–2x). By comparing the yields of 2m and 2o, 2t and measure the diastereomeric ratio (dr) of chromatographically
2x, we found that the electron-donating groups improved the inseparable products (3t and 3u), whereas the diastereomeric
yield, which could be attributed to the stabilization of the ratios of other molecules were determined after isolation.
intermediates at the C3 position by electronic effects. Notably, the diastereomeric ratios varied from 1.5 : 1 to 10 : 1,
Furthermore, the susceptible functionalities, such as –OCH3, which implied that the reaction was not stereoselective.
–CN and –COOCH3, were well tolerated under the optimized
We further tested the practicality of this protocol by increas-
reaction conditions (2m–2o and 2w). Although the phenolic ing the scale of the reaction to 1.29 g ethyl ester indobufen
hydroxyl group was not tolerated (2w–1), the corresponding (4 mmol) with 0.34 g pyrrolidin-2-one (4 mmol). The reaction
product could be obtained by the removal of the protective proceeded successfully and produced the corresponding
group, such as by demethylation with BBr3, in a two-step pro- product in 90% yield without any loss of efficiency. Moreover,
cedure with 82% yield (ESI 3.2†). We obtained yields of 3b was readily hydrolyzed with 98% yield. Therefore, by a two-
88–96% for halogen-substituted isoindolinone derivatives (2q– step process, we accomplished the benzylic structural modifi-
2s, 2u and 2v). The survival of a halogen substituent offered an cation of indobufen on a gram-scale (Fig. 2).
opportunity for further functionalization. Interestingly, when
To obtain information about the reaction mechanism, we
the substituent was a hydrogen atom in the N-terminus, both performed the reaction in the presence of the radical scaven-
cross-coupling and self-coupling reactions of isoindolinone gers 2,2,6,6-tetramethylpiperidinooxy (TEMPO) and butylated
occurred, and gave the corresponding products 2y and 2z in hydroxytoluene (BHT) (Fig. 3a and b). In these cases, the yield
40% and 35% yield, respectively. The dimer (2z) was obtained of 2a was significantly reduced, and the TEMPO and BHT
in 80% yield by using isoindolinone as the only substrate. adducts were observed by high-resolution mass spectrometry
Moreover, we confirmed the structures of 2y and 2z by X-ray (HRMS). When 5.0 equiv. of TEMPO were added to the reaction
crystallographic analysis (Scheme 2).
system, this conversion was completely inhibited. These
Having noticed the mildness, site selectivity, and satisfac- results revealed that the reaction proceeded by a radical
tory yields of the foregoing reactions, we applied the protocol mechanism, and confirmed the generation of isoindolinone
to late-stage functionalization of indobufen. However, when radicals. In addition, on omitting the copper salt, the yield of
we reacted indobufen with pyrrolidin-2-one under the stan- 2a decreased from 97% to 15%, suggesting that CuBr2 could
dard conditions, we obtained only 45% yield of the title com- greatly promote the transformation (Fig. 3c). Conversely, when
pound 3a. After the carboxylic group was converted to the we added 0.1 mmol H2O under the standard conditions and
corresponding ethyl ester (3), the ester reacted well and gave replaced dichloroethane with acetonitrile, except for the separ-
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Scheme 2 Amidation of indobufen ethyl ester. Reaction conditions: indobufen ethyl ester (1 mmol), amides (1 mmol), Na2S2O8 (2 mmol), CuBr2
(10% mmol), TBAB (20% mmol), and DCE (4 mL) at 80 °C for 24 h under a N2 atmosphere, isolated yield. a The diastereomeric ratio of 3t and 3u was
determined by crude 1H NMR, whereas the diastereomeric ratios of other molecules were determined by isolated yields. b Standard conditions for
36 h.
Considering the foregoing control experiments and earlier
methodologies,8,21 we propose a probable reaction mechanism
in Fig. 4. Initially, sodium persulfate (Na2S2O8) was reacted with
TBAB to generate bis(tetrabutylammonium)peroxydisulfate,
which is readily convertible to tetrabutylammonium sulfate
radical-anions (I) by thermal decomposition.11,15,22 The tetra-
butylammonium sulfate radical (I) abstracts a hydrogen atom
from isoindolinone to give the corresponding radical (a).23 The
planar resonance structure makes (a) more stable than other
secondary, tertiary, or benzylic radicals. Then the isoindolinone
radical (a) combines with CuBr2 to afford Cu(III) adducts (b),
Fig. 2 Gram-scale preparation of the indobufen derivative.
ation of 2a and 2a-1, we observed the Ritter-type product 2a-2 which could lead to intermediate (c) after ligand exchange with
by HRMS. This result pointed toward the formation of a carbo- pyrrolidin-2-one, providing the title product (2a) upon reductive
cation in this reaction (Fig. 3d).
elimination. However, we cannot exclude a carbocationic
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process.24 Benefitting from the stability, the isoindolinone
radical (a) readily transforms into carbocation (d) by single-elec-
•−
tron oxidation mediated by Cu or SO4 species, and the carbo-
cation reacts with nucleophiles, such as amides, to generate 2a.
The carbocation (d) also undergoes a nucleophilic addition with
H2O to produce (e), which is further oxidized to give 2a-1.
Under the same conditions, the carbocationic pathway can
explain the generation of the Ritter-type product 2a-2.
Conclusions
In conclusion, we have developed a copper-promoted method
mediated by Na2S2O8 to introduce amides into the benzylic
position of isoindolinone scaffolds. Our protocol exhibits mild-
ness, a broad substrate scope, and satisfactory yields to furnish
a series of amide products, by a practical and convenient
process. We also applied the method to the modification of
indobufen with various amides, a process that will be useful in
measuring structure–activity relationships and expanding practi-
cal applications of indobufen. We envision that this protocol
will have broad applications in late-stage diversification of isoin-
dolinone-containing natural products and drug molecules.
Fig. 3 Control experiments. Radical scavenging by TEMPO (a); Radical
scavenging by BHT (b); The absence of copper salt (c); Carbocation ver-
ification experiment (d).
Author contributions
HFL and JXX performed the experiments. HFL and JL were
responsible for data analysis. DJZ conceived and designed the
experiments. HFL and DJZ co-wrote the manuscript. All
authors read and approved the final version to be published.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
We acknowledge financial support from the Research
Foundation for Talented Scholars of Fujian Medical University
(Grant number: XRCZX2018014), the Joint Funds for the
Innovation of Science and Technology, Fujian province (Grant
number: 2018Y9072) and the Startup Fund for Scientific
Research, Fujian Medical University (Grant number:
2018QH2020). We thank AiMi Academic Services for English
language editing and review services.
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