Selective Oxidative Cleavage of 3-Methylindoles with Primary Amines Affording Quinazolinones

Article abstract of DOI:10.1021/acs.orglett.0c00271

A selective functionalization of C-C-C bonds toward N-C-O bonds is realized by an n-Bu₄NI-catalyzed reaction of 3-methylindoles with primary amines using TBHP as the unique oxidant. The systematic process involves oxygenation, nitrogenation, ring-opening, and recyclization, affording a broad range of quinazolinones in good to excellent yields.

Full text of DOI:10.1021/acs.orglett.0c00271

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Letter
Selective Oxidative Cleavage of 3‑Methylindoles with Primary
Amines Affording Quinazolinones
Junhui He, Jianyu Dong, Lebin Su, Shaofeng Wu, Lixin Liu, Shuang-Feng Yin, and Yongbo Zhou*
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ABSTRACT: A selective functionalization of C−CC bonds toward
N−CO bonds is realized by an n-Bu4NI-catalyzed reaction of 3-
methylindoles with primary amines using TBHP as the unique oxidant.
The systematic process involves oxygenation, nitrogenation, ring-opening,
and recyclization, affording a broad range of quinazolinones in good to
excellent yields.
Scheme 1. Selective Functionalization of Multiple C−C
Bonds
he C−C bond is the most fundamental unit in organic
Tmolecules. Comparable to C−H bond activation, the
selective functionalization of C−C bonds allows the formation
of new C−heteroatom bonds directly from inert feedstocks
and is a very important topic in organic synthesis.1 Due to
thermodynamic and kinetic stability, the selective functional-
ization (cleavage along with their transformation) of un-
strained C−C bonds is a very challenging task in modern
organic chemistry. To meet such a challenge, transition-metal
complexes and harsh reaction conditions are commonly
required.1−4
Over the past two decades, the functionalization of C−C
single bonds,2 double bonds,3 and triple bonds4 has been
extensively explored,1 inter alia, by Milstein,1a,2a Dong,2k−m
Bower,1g,2i,j and Jiao.1c,3g−i,4d,f In contrast, the selective
functionalization of multiple C−C bonds is very limited.5
For example, Shi and Jiao5a have reported a selective
functionalization of C−CC bonds (sp2C−spC−spC) of
aryl alkynes by Au-catalyzed nitrogenation of alkynes (Scheme
1a), and the carbon atoms are all incorporated in the desired
products. Another C−CC bond (spC−spC−spC) cleavage
has been reported by Yamamoto5b for hydroamination of
diynes with substituted aminophenols via Ru or Pd catalysis;
however, the middle carbon atom is transferred to the
byproduct.
Herein, we report a highly selective oxidative cleavage of
multiple C−C bonds of 3-methylindoles and incorporation of
primary amines to yield quinazolinones (Scheme 1b). By the
treatment of 3-methylindoles and primary amines with tert-
butyl hydroperoxide (TBHP) using n-Bu4NI (TBAI) as the
catalyst under transition-metal-free conditions, oxidative C2−
C3 bond cleavage of indoles occurs along with oxygenation,
nitrogenation, ring-opening and recyclization, and various
quinazolinones,6 an important scaffold in numerous pharma-
ceuticals and naturally occurring alkaloids,7 are produced in
good to excellent yields.
We commenced the investigation with the treatment of 3-
methylindole (1a, 0.2 mmol) and benzylamine (2a, 2.0 equiv)
in the presence of n-Bu4NI (20 mol %) and TBHP (70% in
H2O, 6.0 equiv) in CH3CN at 100 °C for 12 h, the formation
of 3-benzylquinazolin-4(3H)-one (3a) was observed in 74%
GC yield (Table 1, entry 1). This finding encouraged us to
further optimize the reaction conditions. Tetrabutylammonium
iodide was essential for the reaction (Table 1, entries 2−15).
Other iodide sources, such as NaI, KI, CuI, NIS, and I2, were
inferior to n-Bu4NI (Table 1, entries 2−6). In the absence of
the iodide catalyst, the reaction did not take place (Table 1,
entry 7). Increasing the use of TBAI to 30 mol % did not
improved the yield of the product (Table 1, entry 8). TBHP
Received: January 18, 2020
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© XXXX American Chemical Society
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A

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a
a
Table 1. Optimization of the Reaction Conditions
Scheme 2. Substrate Scope
b
entry
catalyst
oxidant
solvent
CH3CN
yield (%)
1
2
3
4
5
6
7
n-Bu4NI
NaI
KI
CuI
NIS
I2
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBPB
DTBP
K2S2O8
H2O2
74
31
52
trace
30
31
0
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
DMSO
DMF
toluene
H2O
CH3CN/H2O
CH3CN/H2O
CH3CN/H2O
CH3CN/H2O
c
8
n-Bu4NI
n-Bu4NI
n-Bu4NI
n-Bu4NI
n-Bu4NI
n-Bu4NI
n-Bu4NI
n-Bu4NI
n-Bu4NI
n-Bu4NI
n-Bu4NI
n-Bu4NI
n-Bu4NI
n-Bu4NI
n-Bu4NI
n-Bu4NI
71
0
9
10
11
12
13
0
0
0
0
O2
d
14
e
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
58
73
11
12
10
21
92
82
80
84
15
16
17
18
19
f
20
f g
21 ,
f h
22 ,
f i
23 ,
a
Reaction conditions: 1a (0.2 mmol), 2a (0.4 mmol), catalyst (20
mol %), and TBHP (6.0 equiv) in solvent (1.5 mL) at 100 °C under
b
c
N2 for 12 h. GC yield using n-tridecane as an internal standard. n-
d
e
Bu4NI (30 mol %). TBHP (4.0 equiv). TBHP (8.0 equiv).
CH3CN/H2O (1.5 mL, v/v = 2:1). 80 °C. 120 °C. Under air.
f
g
h
i
showed a unique effect in this reaction, and other oxidants
such as tert-butyl peroxybenzoate (TBPB), di-tert-butyl
8
peroxide (DTBP), K2S2O8, H2O2 (30% in water), and O2
were ineffective (Table 1, entries 9−13). When the amount of
TBHP was decreased to 4.0 equiv, the yield of 3a was reduced
to 58% (Table 1, entry 14); while an increased amount of
TBHP exhibited little influence on the yield of 3a (Table 1,
entry 15). The investigation of the solvent showed that
CH3CN was the best choice for the reaction, other solvents
such as DMSO, DMF, toluene, and H2O gave low yields of 3a
(10−21%; Table 1, entries 16−19). The yield of the desired
product was significantly improved using a mixture of CH3CN
and H2O as solvent (92% yield, Table 1, entry 20). Either
increase or decrease in temperatures had negative effect on the
yields (Table 1, entries 21 and 22). The reaction also took
place under real conditions, producing 3a in 84% yield (Table
1, entry 23).
Amines are easily oxidized to an array of products under
oxidative conditions, which often results in limited generality
in their oxidative transformation reactions.9 Thus, we first
examined the compatibility of amines. As shown in Scheme 2,
this tandem reaction displayed a broad substrate scope with
good functional group tolerance, selectively producing various
quinazolinones in good to excellent yields. Arylmethanamines
with either electron-donating or electron-withdrawing groups
on any position (ortho-, meta-, and para-positions) of the
a
Reaction conditions: 1 (0.2 mmol), 2 (0.4 mmol), n-Bu4NI (20 mol
%), TBHP (6.0 equiv), CH3CN/H2O (1.5 mL, v/v = 2:1), N2, 100
°C, 12 h. Isolated yields are provided.
phenyl ring were good substrates for the reaction, yielding the
corresponding products (3a−l) in 70−92% yields. A variety of
functional groups, such as alkyl, halogens (fluoro, chloro,
bromo, and iodo), methoxy, and trifluoromethyl, were well
tolerated. The structure of 3a was further confirmed by single-
B
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crystal X-ray structure analysis.10 In addition, the more
sterically encumbered naphthyl group was well suited for this
transformation, giving the desired product (3m) in a 91%
yield. Expectedly, heteroarylmethanamines participated in the
metal-free reaction, yielding products 3n and 3o in 78% and
79% yields, respectively. Common aliphatic amines, such as n-
butylamine (3r), n-hexylamine (3s), long chain n-hexadecyl-
amine (3t), branched amines (3v and 3x−za), cyclic amines
(3zb and 3zc), and heteroatom-containing amines (3zd−zf
and 3zi) were also good substrates, and the corresponding
quinazolinones were produced in 58−94% yields. Interestingly,
aliphatic amine containing an internal alkenyl group, which is a
reactive radical acceptor, was also well tolerated, producing the
desired product in excellent yield (3zg, 92%). Whereas,
terminal alkynyl group resulted in lower yield (60%) of the
desired product (3zh). The presence of substituents on the α-
carbon atom of the arylmethanamines resulted in lower yield,
and the products 3p and 3q were obtained in 62% and 50%
yields, respectively. Amines containing β-tert-carbon atom also
showed low reactivity (3za, 60% yield; 3zj, 45% yield). Due to
the low nucleophilicity of aniline (vide infra), its reaction with
1a produced the corresponding product (3zk) in a 35% yield.
The procedure was applicable to various indoles substituted
with methyl, methoxy, halogen groups and trifluoromethyl at
the C4, C5, C6, and C7 positions and the desired products
(3zl−zs) were obtained in 60−89% yields. The reactivity of 6-
azaindole was also investigated, and 3-benzylpyrido[3,4-
d]pyrimidin-4(3H)-one (3zt) was isolated in a 28% yield.
A scale-up reaction was performed to investigate the
potential practicality of this method (Scheme 3, eq 1), and
Scheme 4. Control Experiments
Scheme 3. Synthetic Utility
adduct C′. Radical adduct C′ could also be obtained by the
direct reaction of 1a with BHT. These results suggested that a
free radical process was involved in the reaction. It was
initiated by hydrogen abstraction of N−H bonds of indoles
with tert-butoxyl or tert-butylperoxy radicals (vide infra).12 The
addition of 15 equiv of H218O to the reaction produced 3a in
68% yield without observation of 18O-labeled product (Scheme
4, eq 2), which suggested that the carbonyl oxygen atom of the
product came from TBHP.13 The oxidation of indole could
produce N-(2-acetylphenyl)formamide (7) via cleavage of its
C2−C3 double bond,14 However, only a 20% amount of 7 was
observed by the treatment of 1a under the oxidative system
(Scheme 4, eq 3); indeed, the reaction of 7 with 2a only gave
the desired product (3a) in a 7% yield (Scheme 4, eq 4). These
results indicated that 7 did not serve as the reaction
intermediate, i.e., nitrogenation of C2 atom was prior to the
oxidative cleavage of the C2−C3 bond (for details, see the SI).
The reaction of 3-benzylindole (8) and 3-diphenylmethy-
lindole (9) with 4-fluorobenzylamine (2e) could also produce
the desired product (3e, 66% and 15% yield, respectively) with
the concomitant generation of benzaldehyde (50% yield) and
1.01 g of 3a (72% yield) was successfully obtained by the
treatment of 6 mmol of 1a with 12 mmol of 2a under similar
conditions. The halogenated aryl compounds are important
organic feedstocks for the synthesis of complex molecules by
cross-coupling reactions.11 For example, the reaction of 3zr
and 3j with phenylacetylene (4) could yield more function-
alized compounds 5 and 6 in 86% and 94% yields, respectively,
by the Pd-catalyzed cross-coupling (Scheme 3, eq 2; for details,
see the Supporting Information). These results verified that
this reaction was practical for the construction of complex
organic molecules.
To understand the reaction mechanism, several control
experiments were conducted (for details, see the SI). Initially,
the reaction could be completely suppressed by the radical
inhibitor BHT (Scheme 4, eq 1) with observation of radical
C
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diphenyl ketone (11% yield), respectively, (Scheme 4, eq 5).
The replacement of 1a with indole or 3-phenylindole without
sp3C−H bonds did not produce the desired product at all
(Scheme 4, eq 6).14b These results demonstrated that the
oxygenation of sp3C−H bonds was involved and was
indispensable for the reaction. Indeed, another O-radical
adduct G′, analogous to G (vide infra), was observed upon the
addition of BHT to the reaction system (Scheme 4, eq 7) (for
details, see the SI), which also suggested that G was the
reaction intermediate.
3-methylindoles are highly selectively transformed into N−
CO bonds with amines via C−CC bond cleavage. This
reaction takes place under metal-free conditions using n-Bu4NI
as the catalyst and TBHP as the unique oxidant, and the
indoles and easily oxidizable amines are highly selectively
incorporated in quinazolinone scaffolds in good to excellent
yields with broad substrate scope and good functional group
tolerance. This mild procedure allows the cleavage of the
sp3C−sp2C single bond and sp2C−sp2C double bonds, along
with the formation of three C−N bonds and one CO double
bond. Mechanistic investigation revealed that the systematic
process is initiated by the oxygenation of C3 atoms of indoles
with tert-butylperoxy radicals, and probably involves the
nitrogenation of the C2 atoms, ring-opening by the cleavage
of the C2−C3 bonds, recyclization by nitrogenation of C2
atoms, and oxidative cleavage of C3−sp3C single bonds, which
are readily controllable under oxidative conditions. This
reaction not only represents a new type of functionalization
of multiple C−C bonds but also provides a mild, efficient, and
straightforward method to develop valuable quinazolinones.
Further studies on the synthetic applications and detailed
mechanism of the reaction are ongoing in our group.
Based on the above results and literature reports,12,15−17
a
plausible mechanistic pathway is illustrated in Scheme 5. First,
Scheme 5. Possible Mechanistic Pathway
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c00271.
Experimental procedures, full spectroscopic data, and
1
copies of H, 13C and 19F spectroscopies (PDF)
Accession Codes
CCDC 1938061 contains 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 Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
tert-butoxyl and tert-butylperoxy radicals are easily formed by
the exposure of TBHP to n-Bu4NI,12b,15 which initiates
hydrogen abstraction of 1 to afford indolyl radical A and its
resonance species B. According to the persistent radical effect,
the adduction of longer-lived tert-butylperoxy radical to radical
B affords intermediate C.12 Subsequently, the nucleophilic
addition of amine to C forms D.12a,b An intramolecular C−C
bond cleavage of D with elimination of tert-butyl alcohol then
produces intermediate E,12c,16 which can isomerize to
intermediate F by intramolecular nucleophilic addition. Finally,
hydrogen abstraction of F occurs to form peroxide
intermediate G, followed by rearrangement of G, which leads
to C−C bond cleavage,17a producing the desired product
(3).17
AUTHOR INFORMATION
■
Corresponding Author
Yongbo Zhou − State Key Laboratory of Chemo/Biosensing and
Chemometrics, College of Chemistry and Chemical Engineering,
Hunan University, Changsha 410082, China; orcid.org/
0000-0002-3540-8618; Email: zhouyb@hnu.edu.cn
Authors
Junhui He − State Key Laboratory of Chemo/Biosensing and
Chemometrics, College of Chemistry and Chemical Engineering,
Hunan University, Changsha 410082, China
According to the proposed reaction pathway, electron-
donating groups on the indoles favor the formation of the
radical intermediates A and B; however, they also favor the
undesired oxidation of indoles. Similarly, electron-donating
groups on amines favor the nucleophilic attack of amines to
intermediate C, however, it results in the undesired oxidation
of amines. Indeed, both lower and higher yields (compared
with 87% of 3a without the substituents) of the products were
observed for the substrates with electron-donating groups, as
well as with the electron-withdrawing groups.
Jianyu Dong − Department of Educational Science, Hunan First
Normal University, Changsha 410205, China; orcid.org/
0000-0003-2161-1372
Lebin Su − State Key Laboratory of Chemo/Biosensing and
Chemometrics, College of Chemistry and Chemical Engineering,
Hunan University, Changsha 410082, China
Shaofeng Wu − State Key Laboratory of Chemo/Biosensing and
Chemometrics, College of Chemistry and Chemical Engineering,
Hunan University, Changsha 410082, China
Lixin Liu − State Key Laboratory of Chemo/Biosensing and
Chemometrics, College of Chemistry and Chemical Engineering,
Hunan University, Changsha 410082, China
In summary, we have developed a novel and highly selective
functionalization of multiple C−C bonds in which C3 atoms of
D
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(6) For selected examples on the synthesis of quinazolinones, see:
(a) Xie, F.; Chen, Q.-H.; Xie, R.; Jiang, H.-F.; Zhang, M. ACS Catal.
2018, 8, 5869. (b) An, J.; Wang, Y.; Zhang, Z.; Zhao, Z.; Zhang, J.;
Wang, F. Angew. Chem., Int. Ed. 2018, 57, 12308. (c) Wang, Q.; Lv,
M.; Liu, J.; Li, Y.; Xu, Q.; Zhang, X.; Cao, H. ChemSusChem 2019, 12,
3043. (d) Liang, Y.; Tan, Z.; Jiang, H.; Zhu, Z.; Zhang, M. Org. Lett.
2019, 21, 4725.
Shuang-Feng Yin − State Key Laboratory of Chemo/Biosensing
and Chemometrics, College of Chemistry and Chemical
Engineering, Hunan University, Changsha 410082, China
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c00271
(7) (a) Mhaske, S. B.; Argade, N. P. Tetrahedron 2006, 62, 9787.
(b) Wang, Z.; Wang, M.; Yao, X.; Li, Y.; Tan, J.; Wang, L.; Qiao, W.;
Geng, Y.; Liu, Y.; Wang, Q. Eur. J. Med. Chem. 2012, 53, 275.
(c) Kshirsagar, U. A. Org. Biomol. Chem. 2015, 13, 9336. (d) Bollu, R.;
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(8) An example for transformation of 1a to 3a as one of several
byproducts in trace amount had been reported under O2/photo-
irradiation. This high yield of 3a demonstrated the excellent selectivity
and controllability of the new reaction; see: Karlsson, I.; Persson, E.;
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support from the NSF of China (Grant Nos.
21706058, 21878072, and 21573065) and the NSF of
Hunan Province (Nos. 2016JJ1007 and 2018JJ3031) is much
appreciated.
̈
Ekebergh, A.; Mårtensson, J.; Borje, A. Chem. Res. Toxicol. 2014, 27,
1294.
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