Ru(II)-Catalyzed Amidation Reactions of 8-Methylquinolines with Azides via C(sp3)-H Activation

Abstract

Ru(ii)-catalyzed amidation reactions of 8-methylquinolines with azides have been developed. They are the first examples of [(p-cymene)RuCl2]2-catalyzed C(sp(3))-H bond intermolecular amidation reactions which give quinolin-8-ylmethanamines under mild reaction conditions in good yields.

Full text

16334 |Chem. Commun., 2015, 51, 16334--16337 This journal is ©The Royal Society of Chemistry 201 5
Cite this: Chem. Commun., 2015,
51, 16334
Ru(II)-catalyzed amidation reactions of
8-methylquinolines with azides via C(sp
3
)–H
activation†
Bingxian Liu,
a
Bin Li
a
and Baiquan Wang*
abc
Ru(II)-catalyzed amidation reactions of 8-methylquinolines with azides
have been developed. They are the first examples of [(p-cymene)-
RuCl
2
]
2
-catalyzed C(sp
3
)–H bond intermolecular amidation reactions
which give quinolin-8-ylmethanamines under mild reaction condi-
tions in good yields.
Nitrogen exists widely in natural products, bioactive compounds
and materials.
1
In the context of the synthesis of compounds with
nitrogen atoms, much attention has been paid to the exploration
of efficient and selective C–N bond forming procedures. In the
last decade, with the development of transition-metal catalysis,
great efforts have been made to construct the C–N bond via
transition-metal catalyzed direct C–H amination as it alleviates
the need for prefunctionalization and is environmentally friendly.
A variety of transition-metals,
2
such as palladium,
3
rhodium,
4
iridium,
5
and ruthenium,
6–8
have been used to catalyze this class
of reactions. During the last decade, following the pioneering
research work of Oi and Inoue, Ackermann, Darses and Genet,
Maseras and Dixneuf, and Li,
9
easy to prepare [(arene)RuCl
2
]
2
catalysts became one of the hottest catalysts in C–H bond
functionalization reactions.
10
Despite many significant achieve-
ments, including the formation of C–C, C–O, and C–X (X = halogen)
bonds, having been made, there are only a few reports focused on
[( p-cymene)RuCl
2
]
2
-catalyzed direct C–H amination reactions.
6–8
Sahoo, Chang, Jiao, and Ackermann et al. have reported some
important [( p-cymene)RuCl
2
]
2
-catalyzed C–H/C–N coupling reac-
tions, in which azides were used as N atom sources for which no
oxidant would be required and the only byproduct would be
environmentally benign N
2
(Scheme 1).
7
However, all the reported
examples are limited to C(sp
2
)–H bond activation. To the best of
our knowledge, no C–H amination reaction via C(sp
3
)–H bond
activation catalyzed by [( p-cymene)RuCl
2
]
2
has been reported up to
now. Herein we report the first [( p-cymene)RuCl
2
]
2
-catalyzed
C(sp
3
)–H amidation reaction of 8-methylquinolines with azides
(Scheme 1).
8-Methylquinolines have been proved to have good cyclo-
metallation ability,
11
and many transition-metal catalyzed
C(sp
3
)–H bond activation reactions of 8-methylquinoline have
been reported.
12
However, most of these reactions are catalyzed
by palladium, little work on reactions catalyzed by other metals
has been reported, and there is not even a report on the use
of Ru(II). Our group has been continuously interested in Ru(II)-
catalyzed C–H bond activation.
13
Meanwhile, we have also
developed Rh(III)-catalyzed alkenylation and amidation reactions
of 8-methylquinolines.
12u,v
Ruthenium is not only much cheaper
than rhodium, but also often shows different reactivity from
rhodium.
6,10
In this work we have achieved the Ru(II)-catalyzed
C(sp
3
)–N bond formation reactions of 8-methylquinolines, and
higher reactivity and broader substrate scope than those of the
rhodium catalyst were observed.
Scheme 1 [(p-Cymene)RuCl
2
]
2
-catalyzed C–H bond amidation reactions
using azides as N atom sources.
a
State Key Laboratory of Elemento-Organic Chemistry, College of Chemistry,
Nankai University, Tianjin 300071, P. R. China. E-mail: bqwang@nankai.edu.cn;
Fax: +86 (22) 23504781; Tel: +86 (22) 23504781
b
Collaborative Innovation Center of Chemical Science and Engineering,
Tianjin 300071, P. R. China
c
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic
Chemistry, Chinese Academy of Sciences, Shanghai 200032, P. R. China
†Electronic supplementary information (ESI) available: Full experimental
details, characterization and NMR spectra of the target products are provided.
See DOI: 10.1039/c5cc06230f
Received 25th July 2015,
Accepted 8th September 2015
DOI: 10.1039/c5cc06230f
www.rsc.org/chemcomm
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This journal is ©The Royal Society of Chemistry 201 5 Chem. Commun., 2015, 51, 16334--16337 | 16335
At the outset of our study, 8-methylquinoline (1a) was chosen
as the model substrate. As shown in Table 1, upon treatment of
1a (0.3 mmol) with 4-(trifluoromethyl)benzenesulfonylazide (2a)
(0.6 mmol) in the presence of [( p-cymene)RuCl
2
]
2
(0.015 mmol,
5mol%)andAgSbF
6
(0.06 mmol) in DCE (2 mL) at 80 1C for 12 h,
no desired product was detected (entry 1). The use of catalytic
amounts of the acetate additive always led to a dramatic improve-
ment in the reaction efficiency.
5d,7a–c
Thus various acetate ions
were screened (entries 3–6), among which Zn(OAc)
2
2H
2
Owasthe
most efficient in leading to good product yield (entry 5). Other
solvents were tested in this system giving deficient or negative
results(entries7–9).Raisingthe reaction temperature was not
effective in increasing the product yield (entry 10). A smaller
amount of Zn(OAc)
2
2H
2
O (25 mol%) yielded a poor amount of
3aa (entry 11). By changing the ratio of 1a :2a,wewerepleasedto
observe that a higher yield was obtained when two equivalents
of 1a were used (entries 16–18). Finally, we chose the reaction
conditions of entry 18 as the standard conditions.
With the optimal reaction conditions in hand, various sub-
stituted 8-methylquinolines (1a–p) were treated with an azide
(2a) and the corresponding C(sp
3
)-amidated products (3aa–na)
were obtained in moderate to good yields (Table 2). When
5-substituted or 7-substituted substrates were reacted with 2a,
higher yields were obtained for the electron-withdrawing groups
(3ea,3ka, and 3na) than for the electron-donating groups
(3ba and 3la). When the substituent groups were located at
the 6-position, the electronic effect was not obvious. Both of
the electron-rich and electron-deficient substrates (1f–j) gave
moderate to good yields (55–82%). It is noteworthy that both
6-OMe (1g) and 7-OMe (1l) substrates with strong electron-rich
groups can give the desired products which are not effective in
the Rh(III) system.
12v
The effect of steric hindrance was also
investigated. When 8-methylquinoline was replaced by 8-ethyl-
quinoline (1o) or quinolin-8-ylmethyl acetate (1p) no product
was detected, probably due to the steric effect of the substrate.
In addition to 2a, different sulfonyl azides were also tested
under the standard reaction conditions (Table 3). All the para-,
ortho-, and meta-substituted arenesulfonylazide substrates with
electron-withdrawing groups provided good yields (3ea,3ee,
3ef, and 3eg). Similarly to the substrates of 8-methylquinolines,
azides bearing electron-donating groups also afforded the
corresponding products in high yields (3eb and 3ec). Besides
the arenesulfonylazides, aliphatic sulfonyl azides (2h and 2i)
were also viable to give the desired products in moderate to
good yields.
To gain more insight into the mechanism of this reaction,
KIE (kinetic isotope effect) experiments were performed in two
independent reactions (Scheme 2). The KIE was found to be
k
H
/k
D
= 1.9, which indicated that the cleavage of the methyl C–H
bond may be involved in the rate-determining step.
Based on the known Ru(II)-catalyzed C(sp
2
)–H bond amid-
ation reactions,
7
a possible mechanism is proposed for the
present catalytic reaction (Scheme 3). The first step is likely to
be a C(sp
3
)–H activation process affording a five-membered
intermediate B. The coordination of an azide with Bgives the
intermediate C. The sulfonamido moiety of intermediate Csub-
sequently inserts into the Ru–C bond either directly or by involving
aRu(
IV)–nitrenoid intermediate Dto form intermediate E. Finally,
protonolysis of Edelivers the desired product.
Table 1 Optimization of reaction conditions
a
Entry Additive (50%) Solvent 1a :2a Yield (%)
1 No additive DCE 1 : 1 n.r.
2 No [Ag] and additive DCE 1 : 1 n.r.
3 AgOAc DCE 1 : 1 16
4 Cu(OAc)
2
H
2
O DCE 1 : 1 16
5 Zn(OAc)
2
2H
2
O DCE 1 : 1 51
6 Zn(CF
3
SO
3
)
2
DCE 1 : 1 n.r.
7 Zn(OAc)
2
2H
2
OCH
2
Cl
2
1:1 22
8 Zn(OAc)
2
2H
2
O THF 1 : 1 23
9 Zn(OAc)
2
2H
2
Ot-AmOH 1 : 1 n.r.
10
b
Zn(OAc)
2
2H
2
O DCE 1 : 1 44
11 Zn(OAc)
2
2H
2
O 25% DCE 1 : 1 40
12 Zn(OAc)
2
2H
2
O 75% DCE 1 : 1 49
13
c
Zn(OAc)
2
2H
2
O DCE 1 : 1 44
14
d
Zn(OAc)
2
2H
2
O DCE 1 : 1 46
15
e
Zn(OAc)
2
2H
2
O DCE 1 : 1 51
16 Zn(OAc)
2
2H
2
O DCE 1 : 2 41
17 Zn(OAc)
2
2H
2
O DCE 1.5 : 1 59
18 Zn(OAc)
2
2H
2
O DCE 2 : 1 61
19 Zn(OAc)
2
2H
2
O DCE 3 : 1 61
a
Conditions: 1a or 2a (0.3 mmol), [(p-cymene)RuCl
2
]
2
5 mol%, AgSbF
6
20 mol%, additive 50 mol%, solvent 2 mL, at 80 1C for 12 h, under Ar,
isolated yield.
b
Temp. 120 1C.
c
6h.
d
24 h.
e
Solvent 3 mL.
Table 2 Substrate scope of 8-methylquinolines
a
a
Conditions: 1(0.6 mmol), 2a (0.3 mmol), [(p-cymene)RuCl
2
]
2
5 mol%,
AgSbF
6
20 mol%, Zn(OAc)
2
2H
2
O 50 mol%, DCE 2 mL, at 80 1C for 12 h,
under Ar, isolated yield.
Communication ChemComm

16336 |Chem. Commun., 2015, 51, 16334--16337 This journal is ©The Royal Society of Chemistry 2015
Quinolin-8-ylmethanamine was reported to be a building
block in enormous areas involved in medicinal chemistry, organic
synthesis, and analytical chemistry.
14
Its derivatives have been
studied for their medicinal properties, as exemplified by the
potent and selective melanin concentrating hormone (MCH)
antagonists.
14a
They are also building blocks in inorganic
synthesis like the synthesis of the imine–amine type of chiral
ligands (Scheme 4).
14b
Our work provides a new simple route to
synthesize this class of compounds followed by a simple depro-
tection process.
12v
To further demonstrate the synthetic utility of
the products 3, one more derivatization reaction was carried
out. The amidation product 3aa could be reduced selectively by
NaBH
4
/NiCl
2
6H
2
O, giving the product with exposed amino
groups (Scheme 4).
12v,15
In conclusion, we have developed a Ru(II)-catalyzed amidation
reaction of 8-methylquionlines with azides to obtain quinolin-
8-ylmethanamine derivatives in good yields. This is the first
[( p-cymene)RuCl
2
]
2
-catalyzed amidation reaction of C(sp
3
)–H
bond with azides. Further application of this method in the
synthesis of other targets and a detailed mechanistic investiga-
tion are in progress.
The authors wish to thank the National Natural Science
Foundation of China (Grant No. 21372122, 21372121, and
21421062) for financial support.
Notes and references
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a
a
Conditions: 1e (0.6 mmol), 2(0.3 mmol), [(p-cymene)RuCl
2
]
2
5 mol%,
AgSbF
6
20 mol%, Zn(OAc)
2
2H
2
O 50 mol%, DCE 2 mL, at 80 1C for 12 h,
under Ar, isolated yield.
Scheme 2 The KIE experiments.
Scheme 3 Proposed mechanistic pathway of the amidation reaction.
Scheme 4 Utility and derivatization reactions of 3.
ChemComm Communication

This journal is ©The Royal Society of Chemistry 201 5 Chem. Commun., 2015, 51, 16334--16337 | 16337
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