Intramolecular, Site-Selective, Iodine-Mediated, Amination of Unactivated (sp3)C-H Bonds for the Synthesis of Indoline Derivatives

Article abstract of DOI:10.1021/acs.orglett.7b00846

The Iodine-mediated oxidative intramolecular amination of anilines via cleavage of unactivated (sp³)C-H and N-H bonds for the production of indolines is described. This transition-metal-free approach provides a straightforward strategy for prod

Full text of DOI:10.1021/acs.orglett.7b00846

Letter
pubs.acs.org/OrgLett
Intramolecular, Site-Selective, Iodine-Mediated, Amination of
Unactivated (sp3)C−H Bonds for the Synthesis of Indoline Derivatives
Jinguo Long,† Xin Cao,† Longzhi Zhu,†,‡ Renhua Qiu,*,†,‡ Chak-Tong Au,§ Shuang-Feng Yin,*,†
Takanori Iwasaki,‡ and Nobuaki Kambe‡
†State Key Laboratory of Chemo/Biosensing and Chemometrics,College of Chemistry and Chemical Engineering, Hunan University,
Changsha, 410082, China
‡Department of Applied Chemistry, Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871, Japan
§College of Chemistry and Chemical Engineering, Hunan Institute of Engineering, Xiangtang, China
S
* Supporting Information
ABSTRACT: The Iodine-mediated oxidative intramolecular amina-
tion of anilines via cleavage of unactivated (sp3)C−H and N−H
bonds for the production of indolines is described. This transition-
metal-free approach provides a straightforward strategy for producing
(sp3)C−N bonds for use in the preferential functionalization of
unactivated (sp3)C−H bonds over (sp2)C−H bonds. The reaction could be performed on a gram scale for the synthesis of
functionalized indolines.
ndolines, an extensive family of N-containing heterocycles, are
structural motifs that are found in a number of biologically
Scheme 1. Synthesis of Indolines via (sp3)C−H Bond
I
Activation
active natural products and are frequently employed as a key
template in the synthesis of sophisticated structures by
assembling various molecules.1 Thus, the development of
efficient methods for the preparation of indoline derivatives is
an important issue. Intramolecular (sp2)C−N bond formation by
Buchwald−Hartwig amination2a,b was applied to the synthesis of
N-substituted indolines via C−X bond activation.2c,d As a more
elegant procedure, the transition-metal-catalyzed synthesis of N-
substituted indolines via (sp2)C−H bond activation was disclosed
by Yu,3a,b Chen,3c,d Hirano, and Miura.3e Notably, Glorius,4a
Chen,3d and Shi4b reported on the successful intramolecular
amination of anilines via the concomitant cleavage of unactivated
(sp3)C−HandN−HbondsforthesynthesisofindolinesusingPd
or Cu as catalysts. The synthesis of 2-acylindolines from ketones
via (sp3)C−H bond cleavage was disclosed by Zhang.4c
Transition-metal-catalyzed oxidative C−H bond functionaliza-
tion provides a powerful tool for step- and atom-economical
synthetic transformations,5 but sometimes suffers from undesired
chemoselectivity. For example, in the above transition-metal-
catalyzed indoline synthesis, (sp2)C−N bond formation predom-
inates over (sp3)C−N bond formation when both (sp2)C−H and
(sp3)C−H bonds are available at the appropriate positions
(Scheme 1, left).4a,b
could be used for the functionalization of only (sp)C−H bonds,
(sp2)C−H bonds, or activated (sp3)C−H bonds, except for the
case of the Hofmann−Loffler−Freytag (HLF) reaction, which
̈
affords pyrrolidines or pyrrolidones via 1,5-hydrogen radical
transfer with the cleavage of unactivated (sp3)C−H bonds by the
aid of halogenes.8l−o
Herein, during the course of our study on C−H bond
functionalization,11 we report an iodine-mediated oxidative C−
H/N−H bond coupling by the preferential functionalization of
(sp3)C−H over (sp2)C−H bonds, giving rise to the selective
formation of indolines and not carbazoles (Scheme 1, right),
which is in sharp contrast to the transition-metal-catalyzed
reactions.4a,b
We first examined the intramolecular cyclization of N-(2-tert-
butylphenyl)acetamide (1a) and optimized the reaction con-
ditions for this transformation (Table 1). The indoline 2a was not
producedintheabsenceofiodine(entry1).Theamountofiodine
used in the reaction had a strong effect on reaction efficiency, as
showninentries2−8. When1.0and1.2equivofiodinewereused,
2a was formed in high yields, i.e., 93% and 97%, respectively
(entries 6and 7). In the absence of DTBP (di-tert-butylperoxide),
2a was produced in only 43% yield, and without a base (entry 10),
As anenvironmentally benignandless expensive methodology,
the development of transition-metal-free systems has attracted
considerable attention, recently.6 Regarding C−H bonds
activation,6 iodine reagents, including hypervalent iodine
derivatives, have been successfully used in oxidative C−C bond
formation via dual C−H bond cleavage.7 Similar oxidative C−N
bond formation,8 as well as the formation of C−O9 and C−S10
bonds, would provide a straightforward route to access the
corresponding heteroatom molecules. However, these reactions
Received: March 22, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b00846
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Letter
tBu group.4a Gratifyingly, the desired products (4a−4o) were
obtained in high yields in the metal-free iodine system, as shown
in Scheme 3. Electron-donating (m-methyl 4b, p-methoxy 4c)
Table 1. Optimization of Reaction Conditions
Scheme 3. Selective (sp3)C−H Functionalization of
a
Benzanilides
a
entry
catalyst (equiv)
additive
base
yield (%)
b
1
−
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
−
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
−
ND
2
l2 (0.1)
l2 (0.2)
l2 (0.4)
l2 (0.5)
l2 (1.0)
l2 (1.2)
l2 (1.5)
l2 (1.2)
l2 (1.2)
14
31
47
57
93
97
91
43
3
4
5
6
7
8
9
b
10
DTBP
ND
a
Substrate 1a (0.2 mmol, 1.0 equiv), additive (3.0 equiv), base (2.0
b
1
equiv), 24 h, 140 °C; isolated yield. Not detected by H NMR.
no 2a was formed. Thus, the optimized conditions are substrate
1a (1.0 equiv), I2 (1.2 equiv), K2CO3 (2.0 equiv), and DTBP (3.0
equiv) in acetonitrile (1.0 mL) at 140 °C for 24 h.
We then examined the scope of substrates under the optimized
reaction conditions (Schemes 2−4). Scheme 2 summarizes the
a
Substrate 3 (0,2 mmol, 1.0 equiv), I2 (1.2 equiv), DTBP (3.0 equiv),
K2CO3 (2.0 equiv), CH3CN (1.0 mL), 140 °C, 24 h; isolated yield of
4.
Scheme 2. Effect of Substrate at the Carbonyl Group and the
a
Phenyl Ring
and electron-withdrawing (CN 4d, NO2 4e, F 4f−4h) groups
exerted little influence on the efficiency of the reaction (4b−4m).
Naphthyl and biphenyl substituted compounds, such as 4i and 4j,
were produced in 88% and 68% yields, respectively. The
heteroarene-substituted indoline 4k containing a thienyl group
was produced in 85% yield, but the reaction of a pyridyl substrate
resulted in a low yield (4l, 18%). Me and tBu substituents on the
aniline ring gave high yields of the corresponding indolines (4m,
88%; 4n, 87%). These results demonstrate that the iodine-
mediated approach issuperior to thePd system4a for the synthesis
of N-benzoyl substituted indolines. In the case of 3o, which
contains an ortho-methylbenzamide unit, there were two
competing cyclization pathways giving rise to a mixture of
indoline 4o (30%) via unactivated C−H bond cleavage and
isoindolinone 4o′ (25%) via benzylic C−H bond cleavage.
To investigate the selectivity of this method in more detail, the
ortho-arylacetoanilide 5 having both (sp2)C−H and (sp3)C−H
bonds at appropriate position were examined (Scheme 4). All of
the substrates that were examined, including those bearing H,
a
Substrate 1 (0.2 mmol, 1.0 equiv), I2 (1.2 equiv), DTBP (3.0 equiv),
K2CO3 (2.0 equiv), CH3CN (1.0 mL), 140 °C, 24 h; isolated yield of
2.
effects of substituents at the carbonyl group and the phenyl ring.
Alkyl amides with Me, tBu, nC3H7, nC5H11, or nC6H13 on the
amide group afforded the corresponding products (2a−2e) in
goodyields. It should benoted thatthe present methodcould also
be applied to a pivalamide 1c which did not produce 2c in a Pd-
catalyzed reaction.4a Nonetheless, the benzyl amide 1f afforded
low yields of 2f (21%) probably due to the weak benzylic C−H
bond (vide infra). Alkenyl groups (2g−2i) showed high
compatibility. This reaction was also compatible when various
substituents were attached to the aromatic ring such as Me (2j,
77%), tBu (2k, 97%), Br (2l, 89%), and Ph (2m, 93%).
Unfortunately, 4-pyridylindoline 2n was obtained in only 18%
yield.
Scheme 4. Competitive Experiments: (sp2)C−H versus
a
(sp3)C−H
We then examined the synthesis of N-benzoyl indolines 4 using
the present oxidative C−N coupling of 3, because this
transformation does not proceed in the case of the Pd-catalyzed
system, probably because the (sp2)C−H bond cleavage of the
benzoyl group is faster than the (sp3)C−H bond cleavage of the
a
Substrate 5 (0.2 mmol, 1.0 equiv), I2 (1.2 equiv), K2CO3 (2.0 equiv),
DTBP (3.0 equiv), CH3CN (1.0 mL), 140 °C, 24 h; isolated yield of 6.
B
DOI: 10.1021/acs.orglett.7b00846
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
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OMe, Me, F, Cl, or Ph groups, underwent (sp3)C−H bond
aminationexclusively togivethe targetproducts (6a−6g)ingood
yields (73%−92%). This is again in sharp contrast to Pd-catalyzed
reaction in which (sp2)C−N amidation is the preferred route,
forming the carbazole 6a′ (Scheme 5).4a,b This method offers a
new route to the synthesis of C-7 arylated indolines via (sp3)C−H
amidation as a supplementary method to the Ar−H arylation of
indlines.12
ligands,1d,e were successfully prepared in 83% (1.10 g) and 80%
(1.15 g) yields, respectively (Scheme 6b).
To gain additional insight into the mechanisms of this reaction,
the following experiments were performed (see Supporting
Information (SI)). When the reaction was carried out in the
presence of a radical inhibitor (2,2,6,6-tetramethylpiperidine-1-
yl)oxidanyl (TEMPO), hydroquinone (HQ), or 2,6-diisopropyl-
4-methylphenol (BHT),8d no 2a was generated, suggesting the
involvement of a radical pathway. When KI or KIO3 was used in
placeofiodine, 2awasnotformed. Thisresultsuggeststhat“I−”or
IO3− isnotlikelytobe theactual active species. When thereaction
of 1a was performed in the presence of indoline, only a trace
amount of 2a was formed. This result and the low yield of 4l
(Scheme 3) indicate that the reaction is affected by amines.
Based on the above findings and literature information,8l−o,13
we propose the mechanism shown in Scheme 7. The initial
Scheme 5. Indoline Synthesis versus Carbazole Synthesis
Scheme 7. Proposed Mechanism
A comparison of the reactivities between primary C−H bonds
andsecondary C−Hbondswasalsomade(eq1). Theproduct8a,
arising from the primary C−H bond cleavage, was formed in only
6%yieldandthedesiredproduct8a′wasproducedin87%yield. It
was reported that Cu-catalyzed intramolecular C−H bond
amidation afforded 8a′ in only moderate yield (57%).4b
We then attempted some synthetic applications, to access the
potential of the procedure as a practical tool for the synthesis of
functionalized indolines. Natural product precursors 4p and 4q
were synthesized using our iodine approach in 65% and 85%
yields from amides 3p and 3q, and subsequent oxidative
cyclization led to the formation of the anhydrolycorinone and
oxoassoanine derivatives 9a and 9b in 81% and 35% yields,
respectively (Scheme 6a).1b In addition, a large scale synthesis of
2a,aprecursorofTNNI3kinhibitors1f andproteintyrosinekinase
inhibitors,1g and 4h, with a core skeleton of Mas receptor
reaction of DTBP with iodine leads to the formation of tBuOI13
which reacts with the N−H bond to form the intermediate I and
tBuOH. Cleavage of the N−I bond affords the nitrogen-centered
radicals II at elevated temperatures. According to the Hofmann−
Loffler−Freytag reaction,8l−o the N-radical induces a 1,5-H shift
̈
to give a carbon-centered radical III which can be readily
quenched with an iodine radical to give the intermediate IV. The
subsequent intramolecular substitution gives the indoline 2
furnishing (sp3)C−N amidation. When R is an o-tolyl or benzyl
group, a 1,5-H shift also occurs at the benzylic position leading to
the formation of 4o′ as a minor product or resulting in a low yield
of 2f, respectively (Scheme 2).
Scheme 6. Application Experiments and Gram-Scale Synthesis
In summary, we report on a novel method for the synthesis of
N-acylindolines via intramolecular oxidative (sp3)C−N cross-
coupling. The reaction proceeds efficiently without a metal
catalystandshowsgoodtolerancetoavarietyofsubstituents. This
protocol opens a new avenue for the synthesis of nitrogen-
containing heterocycles via the site-selective cleavage of (sp3)C−
H bonds over (sp2)C−H bonds.
ASSOCIATED CONTENT
* Supporting Information
■
S
TheSupportingInformationisavailablefreeofchargeontheACS
Publications website at DOI: 10.1021/acs.orglett.7b00846.
Experimental procedures, screeningofreaction conditions,
and characterization data (PDF)
Crystallographic data for 4i (CIF)
Crystallographic data for 6c (CIF)
C
DOI: 10.1021/acs.orglett.7b00846
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Letter
2013, 2013, 5769. (b) Finkbeiner, P.; Nachtsheim, B. J. Synthesis 2013,
45, 979. (c) Sun, C.-L.; Shi, Z.-J. Chem. Rev. 2014, 114, 9219. (d) Liu, D.;
Lei, A. Chem. - Asian J. 2015, 10, 806.
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: renhuaqiu@hnu.edu.cn.
*E-mail: sf_yin@hnu.edu.cn.
ORCID
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Longzhi Zhu: 0000-0001-5161-9944
Renhua Qiu: 0000-0002-8423-9988
Takanori Iwasaki: 0000-0002-6663-3826
Notes
The authors declare no competing financial interest.
(8) For selected recent reports on the iodine reagent-mediated/-
catalyzed C−N formation, see: (sp)C−H amination: (a) Souto, J.;
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Becker, P.; Iglesias, A.; Muniz, K. J. Am. Chem. Soc. 2012, 134, 15505.
̃
ACKNOWLEDGMENTS
■
(sp2)C−H amination: (b) Manna, S.; Matcha, K.; Antonchick, A. P.
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The authors wish to thank the Natural Science Foundation of
China (21373003 and 21676076), the Natural Science
Foundation of Hunan Province, China (2016JJ3034), the
Hunan Youth Talent (2016RS3023), and the Chinese Scholar-
ship Council for financial support. We also wish to thank Prof.
Akihiro Orita (Okayama University of Science), Prof. Wai-Yeung
Wong (Hong Kong Polytechnic University), and Prof. Li.-Biao.
Han (AIST, Tsukuba, Japan), for helpful discussions.
C.; Bosnidou, A. E.; Allmendinger, S.; Muniz, K. Chem. - Eur. J. 2016, 22,
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