Visible-light-promoted selective C-H amination of heteroarenes with heteroaromatic amines under metal-free conditions

Article abstract of DOI:10.1039/c7ob02504a

The regioselective C-H amination of quinoline amides (C5) and imidazopyridines (C3) under transition-metal-free conditions at room temperature with a high degree of functional group tolerance is reported. The C-H amination promoted by visible light in the

Full text of DOI:10.1039/c7ob02504a

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Takeharu Haino et al.
Solvent-induced emission of organogels based on tris(phenylisoxazolyl)
benzene
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Visible- Light-Promoted Selective C–H Amination of Heteroarenes
with Heteroaromatic Amines under Metal-Free Conditions
Received 00th January 20xx,
Accepted 00th January 20xx
Supravat Samanta, Chitrakar Ravi, Sadu Nageswara Rao, Abhisek Joshi, and Subbarayappa
Adimurthy*
DOI: 10.1039/x0xx00000x
www.rsc.org/
The regioselective C−H aminaꢀon of quinoline amides (C5) and continuing interest in the development of synthetic methods for
imidazopyridines (C3) under transition-metal-free conditions at the chemical modification of quinolines. In particular C–H amination
room temperature with
a
high degree of functional group at C5 position of quinolones6 is an important scaffold found in many
tolerance is reported. The C−H aminaꢀon promoted by visible light bioactive compounds and exhibit various biological activities.
in presence of photo catalyst with wide range of heteroamines including antimalarial and anticancer properties.7 However, the
makes the present protocal more sustainable.
above transformations usually require transition metal catalysts.
Beyond the metal catalyzed methods, stoichiometric hypervalent
iodine reagents such as PhI(OAc)2, DMP, and IBX provided metal-
free direct amination avenues.8 Considering the importance of
quinolines and amines, combining these two components into a
single entity would be very fascinating for the discovery new lead
compounds. Therefore, it is still an attractive to develop simple and
efficient methods through the direct C–H amination of quinoline
scaffolds at the C5 position under metal-free conditions were rarely
explored.1f
H
Y
X
N
H
N
N
N
Ar
HN
O
X
N
Y
N
N
R
ClO4-
N
HN
O
R = aryl; R =alkyl
N
X=Y= N,CH
X
Y
R
Ar
visible light
K2S2O8
Upto 82%Yield
12 example
Upto 84% Yield
34 examples
N
H
Visible light-promoted direct functionalization of C–H bonds is an
important transformation in organic synthesis due to its
environmentally benign nature.1 Particularly, C–H amination of
heteroarenes is the hot topic of current research, because
heteroarylamines are widely exist in many natural products,
pharmaceuticals, and functional materials, and much attention has
been paid by the synthetic and medicinal chemists.2 Selective C–H
amination is a challenging task to the synthetic and medicinal
chemists due to the presence of multiple C–H bonds in heteroarene
moiety. This method does not require the use of prefunctionalized
substrates, and thus provides an attractive alternative to traditional
transition metal-catalysed C–H amination of heteroarenes.3 In the
field of transition metal-catalyzed amination reactions such as
Buchwald−Hartwig and Ullmann-type coupling reactions have been
emerged as powerful tools.4
Previous report
R2
N
R1
metal or
R1
+
R2
NH
N
metal free
N
HN
O
HN
O
R
R
Present work
Y
N
H
N
X
Y
X
N
N
N
HN
O
Ar
Ar
N
N
R
N
Y
visible light
N
X
Acr+ -Mes, K2S2O8
HN
O
R = aryl; R =alkyl
R
= Acr+-Mes
N
ClO4-
Scheme 1. Selective C–H amination
Among the nitrogen heterocycles, quinoline scaffold is a privileged
and prominent chemical entity found in numerous natural products,
bioactive molecules, drugs, and materials.5 Hence there is
In continuation of our research on the development of visible light
induced oxidative reactions9 and metal-free C–H activation for the
synthesis of heterocycles, 10-11 herein we report visible light induced
selective C−H aminaꢀon of quinoline amides (C5) and
imidazopyridines (C3) under metal-free conditions (Scheme 1).
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At the outset of our investigation, we selected N-(quinolin-8- oxidants like, H2O2, DTBP, KHSO5, K2S2O8, (entries 3−6) among these
yl)pivalamide(1a) and 1H-benzo[d][1,2,3]triazole (2a) as model oxidants tested, 3a was obtained in 71% yield with K2S2O8 as
substrates to optimize the reaction conditions. Initially, we oxidant (entry 6). While decreasing the amount of K2S2O8 (entries 7
evaluated the reaction conditions by subjecting 1a and 2a at room and 8) and reaction time (entries 9−11) the yield of the product was
temperature using 12W blue LED strips under argon atmosphere also decreased. Keeping the optimum amount of oxidant K2S2O8 as
and the results are summarized in Table 1. In the presence of 5 two equivalents and fixing the reaction time 28 h, we checked the
mol% 9-mesityl-10-methylacridinium perchlorate (Acr+-Mes) as effect of different solvents (THF, toluene, CH3CN, DMF, DMSO, 1,4-
photo catalyst with two equivalents of TBHP(aq) as oxidant in dioxane, EtOH, H2O, acetone and DCM) for the reaction, but the
dichloroethane at room
yield was not improved (entries 12−21). Performing the reaction
without oxidant and photo catalyst, the yield of 3a was enormously
dropped (Table 1, entries 22 and 23). No reaction was observed
while conducting the reaction under dark conditions (entry 24)12
and only 30% yield was obtained with oxygen balloon as oxidant
(entry 25).
Table 1. Optimization of the reaction conditions for 3aa
Scheme 2. Scope for C–H amination of 1a with heteroarenesa
aReaction conditions otherwise stated: 1 (0.20 mmol), 2 (0.40) mmol, (0.40
mmol) K2S2O8, Acr+ -Mes (5 mol %) and DCE (2.5 mL), at room temperature,
argon atmosphere, under irradiation with 12W blue LED strip, isolated
yields. bGram scale synthesis of 3a (1a; 1.00 g, 2a; 1.04 g).
On the basis of the results obtained from the screening of the
reaction conditions, the optimized conditions were fixed as 0.2
mmol of 1a, 0.40 mmol of 2a, 2.0 equivalents of K2S2O8 and 5 mol%
of photo catalyst, DCE as solvent at room temperature, 28 h
reaction time for further selective C−H aminaꢀon of 1a with other
heteroarenes (Scheme 2).
aReaction conditions otherwise stated: 1a (0.20 mmol), 2a (0.40) mmol, Acr+
-Mes (5 mole %) and solvent (2.5 mL), at room temperature, argon
atmosphere, under irradiation of 12 W blue LED strips for 28 h, isolated
Under the optimized conditions the reaction of 1a with 5-methyl-
1H-benzo[d][1,2,3]triazole 2a, the selective C5 aminated product 3b
was obtained in 67% yield of two inseparable regioisomers.
Purification by column chromatography, these two regioisomers
obtained together as confirmed by NMR. Then, the reaction of 1a
with 1H-pyrazole and its halogen, methyl and phenyl substituted
derivatives including di- and tri- substituted pyrazoles were
subjected to optimized conditions, they all reacted well and
provided the corresponding C5 aminated products 3c−3j in good
yields (60-84%). Further, the reaction of 1a is compatible with 1H-
1,2,3-triazole and arylated triazoles and gave the corresponding
c
yields. bReaction performed without Acr+ -Mes. Reaction performed under
under dark condition condition. dReaction performed without any oxidant
under argon atmosphere.
temperature; after 24 h, the desired aminated product N-(5-(1H-
benzo[d][1,2,3]triazol-1-yl)quinolin-8-yl)pivalamide
(3a)
was
obtained in 20% yield (Table 1, entry 1). Upon performing the same
reaction using TBHP in dacane, 3a was isolated in 27 % yield (Table
1, entry 2). Then, the reaction was performed using different
2 | J. Name., 2012, 00, 1-3
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aminated products 3k−3n in good yields. It may be noted that, the
halogen (Br, Cl, F and I) substituted derivatives were well tolerated
and which could be further applied in traditional cross-coupling
reactions. Under the optimized conditions other hetero arenes such
1H-imidazole, 1H-1,2,4-triazole, 1H-benzo[d]imidazole and 1H-
indazole were also reacted and afford the products 3o−3r in
moderate yields. More over to validate the present protocol, the
reaction of 1a was carried out at gram scale and obtained 3a in 62%
yield.
Figure 1. Crystal structure of 4j (CCDC:1571571).
Further, to expand the scope of the methodology, we extended C−H
amination of important but representative imidazopyridines
5
Further, the present strategy was extended to different 8-
amidoquinolines other heteroarene amines, with the optimized
conditions to ascertain the scope of our methodology (Scheme 3).
under the optimized conditions with different heteroarenes such as
benzotriazole, pyrazole, imidazole, 1H-1,2,4-triazole, 1H-
benzo[d]imidazole and 1H-indazole and the corresponding C3
aminated products 6a−k were obtained in moderate to good yields
(Scheme 4). It may be further noted that, the presence of
halogenated products were unaffected under the present
conditions for the C−H amination.
Scheme 3. Substrate scope for C–H amination of
heteroarenesa
1
with different
Y
X
N
H
N
blue LED, DCE
X
+
Acr+ -Mes, K2S2O8
N
Y
N
HN
O
2
HN
O
Scheme 4. Substrate scope for C−H amination of imidazopyridinesa
4
1
R
R
N
N
N
N
N
N
N
N
N
N
N
N
H
N
N
Ar
N
N
N
N
visible light
N
Ar
X
Acr+ -Mes, K2S2O8
N
Y
N
Y
X
N
N
N
N
N
H
HN
62%
O
HN
O
HN
65%
O
HN
O
HN
O
6
5
2
4e;
63%
4c;
4b;
4d;
4a;
68%
68%
[b]Ph
Ph (3-Me/5-Me; 1:1)
N
N
N
N
Cl
N
Cl
Ph
N
N
N
Cl
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
6b; 55%
N
6d; 52%
6a; 60%
6e; 82%
6c; 52%
N
N
N
N
N
N
N
N
N
N
O
HN
Cl
HN
O
HN
O
HN
O
N
N
N
HN
O
N
Cl
N
N
4g;
4h;
4j;
83%
4f;
63%
62%
60%
N
4i;
58%
N
N
N
6g,68%
6h; 63%
6f, 65%
N
N
Br
Ph
Ph
N
Ph
N
Ph
Ph
N
Ph
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
6j; 72%
6k; 72%
6i; 50%
N
N
N
N
N
N
N
HN
4k; 79%
O
HN
O
HN
O
HN
62%
O
HN
81%
O
HN
4n;
65%
O
aReaction conditions otherwise stated: 5 (0.20 mmol), 2 (0.40) mmol, (0.40
mmol) of K2S2O8, Acr+-Mes (5 mole %) and DCE (2.5 mL), at room
temperature, atmosphere under irradiation of 12 W blue LED strips for 28 h,
isolated yields.
4o;
53%
4p; 71%
Ph
4m;
4l;
F
aReaction conditions otherwise stated: 1 (0.20 mmol), 2 (0.40) mmol, (0.40
mmol) of K2S2O8, Acr+-Mes (5 mol %) and DCE (2.5 mL), at room
temperature, argon atmosphere, under irradiation with 12W blue LED strip,
isolated yields. bPurification by column chromatography, two inseparable
regioisomers were obtained as confirmed by NMR.
As evident from the products of scheme 3, variety of amides amides
such as acetamide, isobutyramide, 2,2-dimethylbutanamide,
cyclopentanecarboxamide and ben-zamide including adamantane-
1-carboxamide substituted quinolines underwent selective C−H
amination with different arenes (benzotriazole and pyrazole
derivatives) and afforded the products 4a−4p in moderate to good
yields (53-83%). One of the products, 4j, was further confirmed by
single-crystal X-ray diffraction (Figure 1).
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intermediates B and C may furnish another intermediate D and its
isomerization afford the desired product 3.
Scheme 5. Control experiments for mechanistic study
To understand the reaction mechanism, some selective and control
experiments were performed (Scheme 5). Initially, the reaction of
1a and 2a was conducted under the optimized conditions with
radical trapping agents TEMPO and BHT, no reaction was observed
(Scheme 5, eq. 1). This reaction indicates that, the reaction
proceeds via radical pathway. To check the selectivity of C−H
amination, the C5 substituted substrates 7 and 8 were subjected to
the reaction of 2a under the optimized conditions, no reaction was
observed (Scheme 5, eq. 2).
Scheme 6. Plausible mechanism
Conclusions
In conclusion, we have developed an efficient visible light induced
regioselective C−H aminaꢀon of quinoline amides under transition-
metal-free conditions at room temperature with a high degree of
functional group tolerance. The scope of the methodology has been
extended to imidazo[1,2-a]pyridines. Control experiments confirm
the applicability for wide range of heteroamines (such as
benzotriazoles, benzoimidazoles, triazoles, pyrazoles, imidazoles
and indazoles) use of light at room temperature for C−H aminaꢀon
makes the transformation more sustainable.
Further, the substrates 9, 10 and 11 were subjected to the reaction
of 2a under the optimized conditions, no reactions were observed
(Scheme 5, eq. 3). These control experiments suggest that, if the C-
5 position of quinoline amide 1 is substituted by any group, no
reaction takes place to yield the C−H aminated product. Similarly to
check the selective C−H aminaꢀon for imidazopyridines, under the
optimized conditions, the reaction of unsubstituted imidazopyridine
12 was subjected with 2a, the C3 aminated product 13 was
obtained in 55% yield (Scheme 5, eq. 4). Further, the C3 substituted
imidazopyridine 14 was subjected to the reaction of 2a under
similar conditions, no reaction was observed (Scheme 5, eq. 5). The
control experiments (eqs.2−4) suggests the regioselecꢀve C−H
amination of quinoline amides and imidazopyridines. Based on the
above observations and the literature reports,1d-f, 1j, 3a,13 a plausible
reaction mechanism has been proposed for the present protocol
(Scheme 6). Initially, photocatalyst, Acr+-Mes (PC) under irradiation
with visible light and may reach its excited state (PC*), it further
undergoes single electron transfer excited state (PC*), it further
undergoes single electron transfer by abstracting an electron from
2a which generates anion radical (PC•−) and radical intermediates C.
The ground state of PC was regenerated from PC•− by donating an
Acknowledgement
CSIR-CSMCRI Communication No.121/2017. S.S. C.R. and A. J.
are thankful to AcSIR for their Ph.D. enrolment and also
thankful to CSIR, New Delhi for their fellowships. Authors are
thankful to “Analytical Discipline and Centralized Instru-mental
Facilities” for providing instrumentation facilities. We thank
DST, Government of India (EMR/2016/000010), and CSIR-
CSMCRI (OLP-0087) for financial support.
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