Nickel-Catalyzed Regio- And Stereospecific C-H Coupling of Benzamides with Aziridines

Article abstract of DOI:10.1021/acs.orglett.1c01821

A nickel-catalyzed C-H coupling of 8-aminoquinoline-derived benzamides with aryl- and alkyl-substituted aziridines has been disclosed. The current strategy provides direct access to benzolactams by the C-H alkylation-intramolecular amidation cascade event with the concomitant removal of the aminoquinoline auxiliary. The regioselectivity of ring opening of aziridines can be controlled by the substituents. The reaction with chiral aziridines proceeds with inversion of configuration, thus suggesting an SN2-type nucleophilic ring-opening pathway.

Full text of DOI:10.1021/acs.orglett.1c01821

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Letter
Nickel-Catalyzed Regio- and Stereospecific C−H Coupling of
Benzamides with Aziridines
Shibo Xu, Koji Hirano,* and Masahiro Miura*
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ABSTRACT: A nickel-catalyzed C−H coupling of 8-aminoquinoline-derived benzamides with aryl- and alkyl-substituted aziridines
has been disclosed. The current strategy provides direct access to benzolactams by the C−H alkylation−intramolecular amidation
cascade event with the concomitant removal of the aminoquinoline auxiliary. The regioselectivity of ring opening of aziridines can be
controlled by the substituents. The reaction with chiral aziridines proceeds with inversion of configuration, thus suggesting an SN2-
type nucleophilic ring-opening pathway.
Scheme 1. Metal-Catalyzed Directed C−H Transformations
ue to the innate ring strain, aziridines have been regarded
Das important building blocks for the construction of
structurally complex N-containing compounds via ring-open-
ing reactions.1 Among them, the Lewis-acid-promoted
Friedel−Crafts alkylation2 of electron-rich aromatic rings and
the metal-catalyzed cross-coupling reaction with aryl halides3
or organometallic reagents4 have witnessed a significant
progress in C−C bond formation. In particular, as an efficient
and step- and atom-economical strategy, metal-catalyzed C−H
coupling of arenes5 with aziridines to furnish the β-arylethyl-
amine skeleton has attracted growing attention. As a seminal
work, the research group of Li reported the Cp*Rh(III)-
catalyzed pyridine-directed ortho-C−H alkylation of 2-
arylpyridines with aziridines.6 The same transformation was
subsequently disclosed with the Co-NHC catalytic system by
the Yoshikai group (Scheme 1a, top).7 However, these
protocols are limited to the relatively activated aryl-substituted
aziridines that favor the benzylic C−N cleavage, and the C−H
coupling reaction with alkyl-substituted aziridines is less
explored. As only one successful example, Zhao’s group
recently developed the Pd-catalyzed carboxylic-acid-assisted
ortho-C−C coupling of benzoic acids with alkyl-substituted
aziridines (Scheme 1a, bottom), in which the more sterically
accessible C−N bond is selectively cleaved.8
Involving Ring Opening of Aziridines
removal of the aminoquinoline directing group. Particularly
notable is the stereochemistry observed in the reaction with
internal epoxides: the C−C bond formation occurred with
Meanwhile, our research group has recently reported the
nickel-catalyzed C−H coupling of benzamides9 with small-
sized O-heterocycles, including epoxides and oxetanes.10 The
reactions proceeded with the assistance of the N,N-bidentate
aminoquinoline auxiliary, which was originally introduced by
Daugulis,11 and the corresponding six- and seven-membered
benzolactones were directly formed with the spontaneous
Received: May 31, 2021
Published: July 1, 2021
https://doi.org/10.1021/acs.orglett.1c01821
© 2021 American Chemical Society
Org. Lett. 2021, 23, 5471−5475
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Letter
retention of configuration.10a During our continuing interest in
this chemistry, we herein describe a nickel-catalyzed C−H
coupling of benzamides with aziridines (Scheme 1b). The
N,N-bidentate chelation-promoted C−H alkylation was
followed by the intramolecular amidation to form the
corresponding six-membered lactam derivatives with concom-
itant removal of the directing group. Both the 2-aryl- and alkyl-
substituted aziridines were successfully accommodated to
afford the functionalized 3,4-dihydroisoquinolinones. Notably,
the nickel catalysis was stereospecific,12 and the chiral
aziridines were converted into the corresponding products
with inversion of configuration, suggesting a redox-neutral SN2-
type ring-opening pathway, which is in contrast to the reaction
with epoxides.10a
Scheme 3. Products of Nickel-Catalyzed Regioselective C−
H Coupling of Various Benzamides 1 with N-Benzyl
Aziridine 2a
a
We selected benzamide 1a and N-benzyl aziridine (2a; 3.0
equiv) as model substrates and started optimization studies
(Scheme 2). After extensive screening of various reaction
Scheme 2. Effects of Directing Groups and N-Substituents
in Nickel-Catalyzed C−H Coupling of Benzamides with
a
Aziridines
a
Conditions: 1 (0.10 mmol), 2a (0.30 mmol), Ni(OAc)2·4H2O
(0.020 mmol), diglyme (0.5 mL), microwave irradiation (200 °C), 1
h, N2. Isolated yields are shown. The hydrodechlorinated product
3aa was also formed in ∼8% yield. The hydrodebrominated product
3aa was also formed in ∼9% yield.
b
c
tolerated, and the coupling products (3ea and 3fa) were
obtained along with the protodehalogenated product 3aa in
small amounts (∼8%). When meta-substituted benzamides
were employed in the reaction, the C−H coupling preferred to
occur at the less sterically hindered position with good to
excellent regioselectivity, regardless of the electronic nature of
substituents (3ga−ja). The ortho substitutions were also
tolerated albeit with somewhat lower efficiency (3ka and
3la). Naphthalene derivatives 1m and 1n could also be
coupled with 2a: the reaction with 2-naphthalenecarboxamide
occurred selectively at the more sterically accessible C3
position to form the corresponding 3ma in 83% yield, whereas
3na was obtained in a moderate yield from the congested 1-
naphthyl isomer. The structure of 3ma was unambiguously
confirmed by X-ray crystallographic analysis (CCDC
2049819). Moreover, several thiophene-derived carboxamides
were also applicable for this reaction, successfully leading to
thiophene-fused lactams (3oa−pa) in synthetically useful
yields. Notably, the aminoquinoline directing group could be
spontaneously removed and subsequently recovered.14 The
model reaction of 1a with 2a could be easily conducted on a
2.0 mmol scale to deliver the coupling product 3aa in 73%
yield along with 79% recovery of 8-aminoquinoline auxiliary,
which exhibits remarkable reproducibility and reliability.
The scope of aziridines 2 was also investigated with 1a. As
shown in Scheme 4a, the 2-alkyl-substituted aziridines, which
were challenging substrates under previous Cp*Rh(III)6 and
Co7 catalysis, were successfully accommodated in the nickel-
catalyzed C−C coupling protocol to deliver the 3-substituted
a
Conditions: 1 (0.10 mmol), 2 (0.30 mmol), Ni(OAc)2·4H2O (0.020
mmol), diglyme (0.5 mL), microwave irradiation (200 °C), 1 h, N2.
1H NMR yields are shown.
parameters,13 we pleasingly found that the reaction proceeded
smoothly in the presence of a Ni(OAc)2 catalyst with
microwave irradiation (200 °C) in diglyme for 1 h to form
1
the corresponding benzolactam 3aa in 83% H NMR yield.
Due to the dimerization side reaction of the aziridine, an excess
amount (3.0 equiv) of aziridine is necessary to maintain the
satisfactory yield. The evaluation of directing groups
demonstrated that the aminoquinoline auxiliary was indis-
pensable, and other monodentately and bidentately coordinat-
ing amide substrates resulted in no or much less formation of
product 3aa under the present conditions. On the other hand,
the N-benzyl protecting group of aziridines was also critical for
the success: N-Ts substitution resulted in just decomposition
of the aziridine, whereas the N-Ph-substituted aziridine showed
no reactivity.
With the optimal conditions in hand, the scope of
benzamides 1 was first explored with 2a as the reaction
partner. As shown in Scheme 3, benzamides bearing electron-
donating tert-butyl and methoxy substituents at the para
position smoothly afforded the corresponding 3,4-dihydroiso-
quinolinones 3ba and 3ca in high yields. The reaction was also
compatible with an electron-withdrawing trifluoromethyl
group to furnish the targeted product 3da in a moderate
yield. Of note, the chloro and bromo substitutions were also
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Scheme 4. Nickel-Catalyzed C−H Coupling of Benzamide
1a with Substituted N-Benzyl Aziridines 2
Scheme 5. Investigation of Regio- and Stereochemistry in
the C−H Alkylation Step
3,4-dihydroisoquinolinones. The reaction with 2-ethyl-sub-
stituted aziridine 2b preferably occurred at the less hindered
terminal position to furnish the desired 3ab in 70% yield. The
ether substituents were also compatible for the C−H
alkylation, and the corresponding products (3ac and 3ad)
were isolated in good yields with high regioselectivity.
Additionally, when the optically active aziridine (S)-2e was
used, its chirality was successfully transferred to the product
(S)-3ae without losing enantiopurity (Scheme 4b). We also
tested the 2,2-dimethyl-substituted aziridine for the C−H
coupling, but only decomposition of aziridine was observed,
and no desired product was formed (data not shown).
To probe the possible reaction pathway, some mechanistic
experiments were performed. Initially, when the enantiopure 2-
phenyl aziridine (S)-2f was subjected to the reaction
conditions, as observed in previous reports,6,7 the benzylic
C−N bond was primarily cleaved and coupled with 1a to
deliver the stereochemically inverted (S)-3af15 albeit with
some erosion of the enantiopurity (Scheme 5a). This result
may imply that nickel-promoted accumulation of positive
charge at the benzylic position leads to the prolongation and
cleavage of the C−N bond, but a complete carbocation should
not be involved.6 The observed inversion of configuration
suggests that the C−C coupling mainly proceeds via a redox-
neutral SN2-type nucleophilic ring-opening pathway. The
observed regioselectivity was general; both the electron-rich
and -deficient 2-arylaziridines provided the 4-aryl dihydroiso-
quinolinones 3ag−3ai preferably (Scheme 5b). As shown in
Scheme 5c, the indene-derived aziridine 2j also underwent the
coupling reaction via the benzylic C−N cleavage to furnish the
C−H alkylated product 3aj′ as the major isomer. Although the
cyclization of 3aj′ was sluggish, the lactam product 3aj was also
isolated in 3% yield, and its trans-stereochemistry was
confirmed by X-ray crystallographic analysis (CCDC
2082069). To gain more information about the stereo-
chemistry, deuterated cis- and trans-aliphatic aziridines were
independently prepared and subjected to the standard
conditions (Scheme 5d). Intriguingly, the reaction proceeded
with ideal stereospecificity: cis-2k-d1 was converted to the
trans-3ak-d1 as single diastereomer, whereas the cis-3ak-d1 was
exclusively formed from trans-2k-d1. Consequently, the nickel
catalysis is stereospecific, and the inversion of configuration is
opposite to that observed in the reaction with epoxides,10a
which demonstrates the unique properties of a nickel catalyst
in the ring-opening reactions of small-sized heterocycles.
On the basis of the experimental results and previous
studies, a plausible reaction mechanism of 1a with 2a is
proposed in Scheme 6. The initial chelation of benzamide 1a
to Ni(OAc)2 generates Ni(II) complex A, which is followed by
the reversible C−H cleavage16 to give the nickelacycle B17 with
the generation of HOAc. Subsequent coordination of an
Scheme 6. Plausible Mechanism
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Letter
Author
aziridine nitrogen atom to the Ni(II) center leads to the
prolongation of the C−N bond and promotes the C−C
coupling to form intermediate D via an SN2-type nucleophilic
ring-opening process.12 The carbocation-involved SN1-type
pathway and single electron transfer-type4b ring opening of
aziridine could be ruled out based on the observed regio- and
stereoselectivity. The difference of Lewis basicity between N
and O atoms may be responsible for the ring-opening pathway
of aziridines and epoxides. The relatively stronger coordination
of the N atom with the metal center results in a larger
polarization of the C−N bond that favors the SN2-type
nucleophilic ring-opening process, whereas the epoxides prefer
the redox-active ring-opening pathway.10a In the reactions of
1a with para-substituted arylaziridines 2, a negative slope of ρ
= −1.09 was obtained from the Hammett plot with σp for the
conversion of 1a, suggesting that the C−C bond formation
step is probably involved in the rate-determining step.18 The
intramolecular amidation19 and simultaneous protonolysis with
HOAc deliver the final product 3aa and recovered 8-
aminoquinoline with regeneration of the starting Ni(OAc)2
to complete the catalytic cycle.
In summary, we have developed a nickel-catalyzed, amino-
quinoline-directed C−H coupling of benzamides with
aziridines. This strategy provides rapid access to functionalized
3,4-dihydroisoquinolinones via a C−H alkylation−intramolec-
ular amidation cascade process with concomitant removal of
the aminoquinoline auxiliary. The reaction is compatible with
both aryl- and alkyl-substituted aziridines, and the regiose-
lectivity is controlled by the nature of the substituents.
Additionally, the mechanistic studies reveal that the nickel
catalyst is stereospecific, and the inversion of configuration in
the C−C formation step suggests an SN2-type nucleophilic
ring-opening pathway.
Shibo Xu − Innovative Catalysis Science Division, Institute for
Open and Transdisciplinary Research Initiatives (ICS-
OTRI), Osaka University, Suita, Osaka 565-0871, Japan
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.1c01821
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by JSPS KAKENHI Grant Nos. JP
18K19078 (Grant-in-Aid for Challenging Research (Explor-
atory)) to K.H. and JP 17H06092 (Grant-in-Aid for Specially
Promoted Research) to M.M. We thank Dr. Yuji Nishii (Osaka
University) for his assistance with the X-ray analysis. S.X.
acknowledges a Japanese government (MEXT) scholarship.
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ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.1c01821.
1H, 13C{1H}, and 19F{1H} NMR spectra, detailed
optimization studies, experimental procedures, isotope-
labeling experiments, Hammett studies (PDF)
Accession Codes
CCDC 2049819 and 2082069 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Authors
Koji Hirano − Department of Applied Chemistry, Graduate
School of Engineering, Osaka University, Suita, Osaka 565-
0871, Japan; orcid.org/0000-0001-9752-1985;
Email: k_hirano@chem.eng.osaka-u.ac.jp
Masahiro Miura − Innovative Catalysis Science Division,
Institute for Open and Transdisciplinary Research Initiatives
(ICS-OTRI), Osaka University, Suita, Osaka 565-0871,
Japan; orcid.org/0000-0001-8288-6439;
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Chem. Commun. 2018, 54, 3407−3410. (c) Steiman, T. J.; Liu, J.;
Mengiste, A.; Doyle, A. G. Synthesis of β-Phenethylamines via Ni/
Email: miura@chem.eng.osaka-u.ac.jp
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