Rhodium(III)-Catalyzed Oxidative Cyclization of Oxazolines with Cyclopropanols: Synthesis of Isoindolinones

Article abstract of DOI:10.1021/acs.orglett.1c02031

The synthesis of C3-substituted isoindolin-1-ones from oxazolines and cyclopropanols has been achieved with oxazoline as a bifunctional nucleophilic directing group. The reaction proceeds by the cleavage of three chemical bonds and allows the formation of three new chemical bonds, a C-N bond, a C-C bond, and a C-O bond, in a single step.

Full text of DOI:10.1021/acs.orglett.1c02031

pubs.acs.org/OrgLett
Letter
Rhodium(III)-Catalyzed Oxidative Cyclization of Oxazolines with
Cyclopropanols: Synthesis of Isoindolinones
Jidan Liu,* Zhenke Yang, Jinyuan Jiang, Qiaohai Zeng, Liyao Zheng, and Zhao-Qing Liu*
Cite This: Org. Lett. 2021, 23, 5927−5931
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ABSTRACT: The synthesis of C3-substituted isoindolin-1-ones
from oxazolines and cyclopropanols has been achieved with
oxazoline as a bifunctional nucleophilic directing group. The
reaction proceeds by the cleavage of three chemical bonds and
allows the formation of three new chemical bonds, a C−N bond, a
C−C bond, and a C−O bond, in a single step.
Scheme 1. Construction of C3-Substituted Isoindolin-1-
ones via C−H Activation/Annulation
soindolin-1-ones are important N-heterocycles found in
many natural products, pharmaceuticals, and bioactive
I
molecules, exhibiting a broad spectrum of biological activities
and therapeutic potential, such as anesthetic, antipsychotic,
antiulcer, antihypertensive, and anxiolytic properties.1 In
particular, the C3-substituted isoindolin-1-ones are present as
core structural subunits in a number of alkaloids and designed
pharmaceutical molecules.2 Considering the enormous poten-
tial of C3-substituted isoindolin-1-ones, a variety of synthetic
methods for the assembly of these useful scaffolds have been
developed over the past few decades. Prominent approaches to
this class of heterocycle include the addition reaction of
organometallic reagents with isoindoline-1,3-diones,3 lithiation
of isoindolinones at the C3 position using organo-lithium
reagents followed by treatment with an electrophile,4 and
cyclization of ortho-substituted aryllithium species with differ-
ent imines.5 Alternatively, the C3-substituted isoindolin-1-ones
can also be constructed by palladium-catalyzed coupling of o-
halobenzylamine derivatives with carbon monoxide (CO)6 or
cascade carbonylation/annulation of aryl halides with amines
and carbon monoxide.7 However, most of these reported
methods still have some limitations, such as the harsh reaction
conditions and/or tedious multiple reaction steps, and the
requirement of preactivated raw materials. Thus, the develop-
ment of more straightforward synthetic processes to access
diversely functionalized isoindolin-1-ones should be appealing.
Recently, direct C−H bond functionalization under transition-
metal catalysis has emerged as a powerful strategy for the
preparation of C3-substituted isoindolin-1-ones in a highly
atom-economical and environmentally friendly manner. For
instance, transition-metal-catalyzed oxidative couplings be-
tween benzamides and numerous terminal olefins followed
by aza-Michael cyclization constituted a straightforward route
to isoindolinone derivatives (Scheme 1a).8 Moreover, the C3-
substituted isoindolin-1-ones can be obtained via direct C−H
carbonylation of protected or free benzylamines with various
carbonyl sources by using Pd(II), Ru(II), or Co(II) catalysis
(Scheme 1b).9 Despite considerable progress in this area,
further exploration of new general strategies to afford
structurally diverse C3-substituted isoindolin-1-one skeletons
from readily available starting materials is still needed.
Recently, the combination of C−H bond activation and C−
C bond cleavage has been realized as one of the most efficient
methods for the generation of molecular complexity.10 Among
the developed methodologies, the use of strained rings as
coupling partners has attracted considerable attention owing to
the high activity of the coupling associated with their
Received: June 17, 2021
Published: July 8, 2021
https://doi.org/10.1021/acs.orglett.1c02031
© 2021 American Chemical Society
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inherently strained nature.11 In particular, cyclopropanols and
their derivatives have been widely employed as readily available
starting materials for various transformations due to their
relatively easy preparation as well as rich reactivity.12 In 2012,
Orellana and co-workers developed a new strategy for
constructing 1-indanones via palladium-catalyzed tandem
silyloxycyclopropane rearrangement with intramolecular C−
H arylation.13 Very recently, the Li group reported the
synthesis of various β-functionalized ketones or quinolines via
Rh(III)-catalyzed oxidative coupling of arenes with cyclo-
propanols.14 Inspired by these achievements and as a
continuation of our interest in cyclopropanols and C−H
activation,15 herein we disclose a Rh(III)-catalyzed oxazolinyl-
assisted oxidative cyclization of oxazolines with cyclopropanols
(Scheme 1c). Notably, the ring opening of oxazolines via
nucleophilic attack of acetate provided facile access to C3-
substituted isoindolin-1-ones.
Scheme 2. Substrate Scope with Cyclopropanols
Our initial investigations showed that the coupling of 2-
phenyloxazoline (1a) with readily available 1-benzylcyclopro-
panol (2a) in methanol at 130 °C for 2 h afforded the desired
C3-substituted isoindolin-1-one 3a in 61% isolated yield using
[Cp*RhCl2]2 as a catalyst and Cu(OAc)2 as an oxidant (Table
S1, entry 1). The yield of 3a was further increased to 72%
when the reaction was carried out with the cationic rhodium
catalyst [Cp*Rh(CH3CN)3](SbF6)2 (Table S1, entry 3).
Other common oxidants, including Cu(OAc)2·H2O, Mn-
(OAc)3·2H2O, PhI(OAc)2, and AgOAc, were less effective
than Cu(OAc)2 (Table S1, entries 7−10, respectively). Further
screening of solvents revealed that t-AmOH, dioxane, CH3CN,
toluene, and DCE were inferior to MeOH (Table S1, entries
11−15, respectively). An attempt to decrease or increase the
reaction temperature resulted in only inferior results (Table S1,
entries 16 and 17). Finally, the control experiment also
confirmed that the Rh(III) catalyst was necessary for the
transformation (Table S1, entry 18).
Scheme 3. Substrate Scope with Oxazolines
With the standard conditions identified (Table S1, entry 3),
we then tested the generality and scope of this tandem
annulation protocol with respect to cyclopropanols and
oxazolines. First, a variety of cyclopropanols 2a−2l that can
be readily prepared from the precursor esters in one step were
investigated (Scheme 2). 2-Phenyloxazoline 1a reacted well
with 1-benzylcyclopropanols 2b−2e bearing different sub-
stituents, such as Me, MeO, Cl, or Br, at the para or meta
position of the benzene ring to afford the C3-substituted
isoindolin-1-ones 3b−3e in 64−75% yields. Moreover, various
arylcyclopropanols participated well in this coupling, providing
products 3f−3h in 58%, 60%, and 52% yields, respectively.
Thiophene-containing cyclopropanol was also converted to the
expected product 3i in 55% yield. In addition, the alkyl-
substituted cyclopropanols gave the corresponding products
3j−3l in 74−78% yields. Unfortunately, the relatively bulky
1,2-disubstituted cyclopropanols as well as the cyclobutanol
and cyclopentanol with a less strained ring were unreactive
under the reaction conditions.
We next examined the scope of the oxazoline substrate
(Scheme 3). It was found that aryloxazolines with various
substituents at the para positions of the phenyl ring, such as
Me, MeO, t-Bu, and Ph, reacted well with 2a to generate
products 3m−3p, respectively, in 58−74% yields. Different
electron-withdrawing groups, such as F, Cl, Br, CF3, and
CO2Me, all coupled smoothly to afford the target isoindolin-1-
ones 3q−3u, respectively, in 48−70% yields. The halogen (F,
Cl, and Br) groups in isoindolin-1-ones 3q−3s remained
intact, providing the possibility for further chemical trans-
formations. The reaction of m-methyl- or chloro-substituted
aryloxazolines with 2a afforded a mixture of regioisomeric
products. The m-methyl aryloxazoline was found to react
preferentially at the less hindered position while the
regioselectivity for m-chloro aryloxazoline was contrary,
probably due to the coordination between the chloro group
and the rhodium(III) catalyst.16 Moreover, the ortho-
substituted aryloxazoline can be well tolerated to give
isoindolin-1-one 3x in 52% isolated yield. Finally, 2-
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naphthyloxazoline was also a suitable substrate for producing
the corresponding product 3y as a single regioisomer.
additional controlled experiments were carried out to clarify
the role of the Rh(III) catalyst and Cu(OAc)2. When
phenylcyclopropanol 2f was treated with the [Cp*Rh-
(CH3CN)3](SbF6)2 catalyst without Cu(OAc)2, ethyl phenyl
ketone 7 was produced in a high isolated yield while only a
trace amount of enone 6 and methoxy ketone 8 were
obtained.17 Additionally, when phenylcyclopropanol 2f was
treated with Cu(OAc)2 without a catalyst, methoxy ketone 8
was isolated in 81% yield along with ethyl phenyl ketone 7
(5%) and enone 6 (4%). This result indicates that Cu(OAc)2
alone is effective enough to facilitate the formation of enone
intermediate 6 via direct β-H elimination of the in situ-
generated Cu(II) ketone homoenolate.18 The newly formed
enone 6 can be easily converted into methoxy ketone 8 via
Cu(II)-promoted Michael addition in methanol.18a Finally,
methoxy ketone 8 was afforded in 94% yield when enone 6 was
heated in methanol with Cu(OAc)2, which further confirmed
that enone 6 was involved as an intermediate in this oxidative
coupling (Scheme 4e).
To further prove the synthetic utility of this methodology, a
scale-up reaction of 2-phenyloxazoline 1a and 1-benzylcyclo-
propanol 2a was performed to afford 3a in 65% yield (see the
Supporting Information). Furthermore, the chemical trans-
formations of isoindolin-1-one 3a were also conducted.
Deacetylation of 3a with K2CO3 in methanol provided alcohol
4 in 90% yield, which can be further transformed into the
corresponding azide 5 (see the Supporting Information).
To understand the mechanistic pathway of this oxidative
annulation, a series of experiments were performed (Scheme
4). First, when the substrate phenyloxazoline 1a was subjected
Scheme 4. Mechanistic Studies
We continued to explore the substrate scope of activated
alkenes under the Rh(III)-catalyzed oxidative cyclization
conditions presented here (Scheme 5). Initially, various
Scheme 5. Substrate Scope with Alkenes
to the reaction in CD3OD without cyclopropanol, a significant
H/D exchange was detected at both ortho positions of the
phenyl ring in phenyloxazoline, confirming the C−H bond
cleavage of 1a occurs in the presence of the Rh(III) catalyst
(Scheme 4a). A moderate kinetic isotope effect (kH/kD = 2.1)
was also obtained from an intermolecular competition
experiment, suggesting the ortho C−H bond cleavage of 1a
is likely involved in the turnover-determining step (Scheme
4b). A small or trace amount of enone byproducts was
observed in some reactions, indicating that this oxidative
coupling may take place via olefin species. Therefore, a set of
control experiments were conducted to further explore the
reaction mechanism. When enone 6 was applied in the
reaction system instead of cyclopropanol 2f, the corresponding
isoindolin-1-one 3f was afforded in 62% yield (Scheme 4c). To
probe the role of the cyclopropanol substrate in this
transformation, phenylcyclopropanol 2f was run under the
standard conditions without 2-phenyloxazoline 1a, and ethyl
phenyl ketone 7 could be obtained in 67% yield, together with
methoxy ketone 8 (15%) and enone 6 (3%). Then, two
substituted enones were examined. To our delight, benzyl
vinyl ketone 9, aryl vinyl ketones (10 and 11), and alkyl vinyl
ketones (12 and 13) all coupled smoothly with 2-phenyl-
oxazoline 1a to furnish the C3-substituted isoindolin-1-ones in
45−78% yields. Moreover, treatment of 1a with 2 equiv of
methyl acrylate 14 under otherwise identical conditions
afforded alkenylated isoindolinone 3z in 51% yield. Unfortu-
nately, when β-substituted or α-substituted enone was used,
only a trace of the desired product was observed, which is
likely affected by steric hindrance. Other activated olefins such
as N,N-dimethyl acrylamide and acrylonitrile were also not
suitable for this oxidative coupling.
A plausible catalytic cycle for this oxidative coupling was
proposed on the basis of the mechanistic investigations
mentioned above and previous DFT studies (Scheme 6).19
Initially, a highly reactive Cp*Rh(OAc)2 species was generated
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Guangzhou University, Guangzhou 510006, P. R. China;
orcid.org/0000-0002-0727-7809; Email: lzqgzu@
gzhu.edu.cn
Scheme 6. Proposed Mechanism
Authors
Zhenke Yang − School of Chemistry and Chemical
Engineering, Institute of Clean Energy and Materials,
Guangzhou Key Laboratory for Clean Energy and Materials,
Guangzhou University, Guangzhou 510006, P. R. China
Jinyuan Jiang − School of Chemistry and Chemical
Engineering, Institute of Clean Energy and Materials,
Guangzhou Key Laboratory for Clean Energy and Materials,
Guangzhou University, Guangzhou 510006, P. R. China
Qiaohai Zeng − School of Chemistry and Chemical
Engineering, Institute of Clean Energy and Materials,
Guangzhou Key Laboratory for Clean Energy and Materials,
Guangzhou University, Guangzhou 510006, P. R. China
Liyao Zheng − School of Chemistry and Chemical Engineering,
Institute of Clean Energy and Materials, Guangzhou Key
Laboratory for Clean Energy and Materials, Guangzhou
University, Guangzhou 510006, P. R. China; orcid.org/
0000-0002-1996-7727
via ligand exchange. Then, an oxazolinyl-assisted ortho C−H
bond activation of phenyloxazoline (1a) with the active
Rh(III) catalyst afforded a five-membered rhodacycle A. Next,
the enone that derived from ring opening/β-H elimination of
cyclopropanol 2 promoted by Cu(OAc)2 inserted into the C−
Rh bond of intermediate A to afford a rhodacycle species B.
The subsequent β-H elimination of rhodacycle B delivered
ortho-alkenylated phenyloxazoline C and the resulting Cp*Rh-
(I) species. The intramolecular aza-Michael addition of
intermediate C provided oxazolinium salt D, which was
attacked by acetate to generate isoindolin-1-one 3 via ring
opening of oxazoline. Finally, the Rh(III) active catalyst would
be regenerated under the oxidation of Cu(OAc)2.
In conclusion, we have realized an oxazolinyl-assisted
rhodium(III)-catalyzed C−H oxidative cyclization of oxazo-
lines with cyclopropanols. The reaction proceeds by the
cleavage of three chemical bonds and allows the formation of
three new chemical bonds, a C−N bond, a C−C bond, and a
C−O bond, in a single step. A number of functional group-
bearing oxazolines and cyclopropanols react smoothly to give
structurally diverse isoindolinones in a highly atom-economical
way. Because of the ready availability of raw materials, the
simplicity of the reaction conditions, and the wide use of
isoindolin-1-ones, this protocol may find significant applica-
tions in synthetic and medicinal chemistry.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.1c02031
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Natural Science
Foundation of China (21875048), Research Projects in
Guangzhou University (YG2020015), the Science and
Technology Research Project of Guangzhou (202102080146
and 202002010007), and the Outstanding Youth Project of
Guangdong Natural Science Foundation (2020B1515020028).
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ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.1c02031.
Experimental details, characterization data, and copies of
NMR spectra (PDF)
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AUTHOR INFORMATION
Corresponding Authors
■
Jidan Liu − School of Chemistry and Chemical Engineering,
Institute of Clean Energy and Materials, Guangzhou Key
Laboratory for Clean Energy and Materials, Guangzhou
University, Guangzhou 510006, P. R. China; orcid.org/
0000-0001-6469-8100; Email: jdliu@gzhu.edu.cn
Zhao-Qing Liu − School of Chemistry and Chemical
Engineering, Institute of Clean Energy and Materials,
Guangzhou Key Laboratory for Clean Energy and Materials,
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