Cp*Rh(III)-Catalyzed Directed C?H Methylation and Arylation of Quinoline N-Oxides at the C-8 Position

Article abstract of DOI:10.1002/adsc.201700484

Herein, we report a Cp*Rh(III)-catalyzed directed C?H methylation of quinoline N-oxides at the C-8 position using commercially available organotrifluoroborates as reagents. This method features perfect regioselectivity, relatively mild reaction conditions, and diverse functional group tolerance with good to excellent yields. Additionally, direct C-8 arylated quinoline N-oxides products could also be obtained under the same conditions. Preliminary mechanistic experiments were conducted and a possible mechanism was proposed. (Figure presented.).

Full text of DOI:10.1002/adsc.201700484

Title: Cp*Rh(III)-Catalyzed Directed C-H Methylation and Arylation of
Quinoline N-Oxides at the C-8 Position
Authors: Bao Wang, Chunpu Li, and Hong Liu
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201700484
Link to VoR: http://dx.doi.org/10.1002/adsc.201700484

10.1002/adsc.201700484
Advanced Synthesis & Catalysis
UPDATE
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Cp*Rh(III)-Catalyzed Directed C-H Methylation and Arylation
of Quinoline N-Oxides at the C-8 Position
Bao Wang,a,b,c,‡ Chunpu Li,b,c,‡ and Hong Liu a,b,c,*
a. School of Life Science and Technology, ShanghaiTech University, 100 Haike Road, Shanghai 201210, China.
b. State Key Laboratory of Drug Research, Shanghai Institute of Materia Medica, Chinese Academy of Sciences, 555 Zu
Chong Zhi Road, Shanghai, 201203, China.
c. Key Laboratory of Receptor Research, Shanghai Institute of Materia Medica, Chinese Academy of Sciences, 555 Zu
Chong Zhi Road, Shanghai, 201203, China.
E-mail: hliu@simm.ac.cn
‡ B. Wang and C. Li contributed equally to this work.
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
Abstract. Herein, we report a Cp*Rh(III)-catalyzed
directed C-H methylation of quinoline N-oxides at the C-
8
position
using
commercially
available
organotrifluoroborates as reagents. This method features
perfect regioselectivity, relatively mild reaction
conditions, and diverse functional group tolerance with
good to excellent yields. Additionally, direct C-8
arylated quinoline N-oxides products could also be
obtained under the same conditions. Preliminary
mechanistic experiments were conducted and a possible
mechanism was proposed.
Scheme 1. Selected examples of biologically active
compounds containing 8-methylquinoline core.
Keywords: Quinoline N-Oxides; C−H Methylation;
Rhodium
Recently, much effort has been made in the
area of C−H functionalization of quinoline at
20, 21
]
different positions.[
N-Oxide as a directing
Quinoline is an important structural motif in drug
discovery[1-4] and materials science.[5] Notably, 8-
methylquinoline derivatives are found in many drugs
and biologically active molecules (Scheme 1).[6-8]
Accordingly, the development of efficient methods to
produce substituted quinoline is important. Over the
past few decades, transition-metal-catalyzed C−H
functionalization has emerged as a powerful strategy
to synthesize complex molecules.[9-12] Recently,
rhodium(III)-catalyzed C-H activation leading to
C−C and C−X (X= O, and N) bond formation has
attracted much attention due to the high efficiency.[13-
group has attracted significant attention, which has
been used to achieve the desired regioselective
control. A significant number of methods exist for
transition-metal-catalyzed functionalization at the C2
position of quinoline N-oxides, such as olefination,[22]
[25]
sulfonylation,[23] alkylation,[24] acetoxylation,
26, 27
28, 29
]
and amination.[
]
phosphonation, arylation,[
By contrast, only a few examples have been reported
for the selective C-8 functionalization of quinoline N-
30-39
oxides.[
]
In 2014, Chang and co-workers
developed two catalytic procedures to 8-iodinated
and 8-aminated quinolines using rhodium and iridium
17
] Nevertheless, how to control the regioselectivity if
catalytic systems.[35 Rh- and Co-catalyzed C−C
]
the substrates have more than one reacting C−H bond
remains a challenge. Therefore, chelation-assisted
transformation has become a common and powerful
method for the regioselective functionalization of C–
H bonds.[18, 19]
coupling with alkynes at the C8 position of quinoline
36]
37
]
N-oxides was disclosed by Li’s,[ Chang’s,[ and
38
]
Sundararaju’s groups.[
In addition, Larionov
developed the palladium-catalyzed C8 arylation and
34, 39
]
selective homocoupling of quinoline N-oxides.[
Recently, organoboron reagents, especially alkyl
boron reagents, have been used extensively in C−H
40-44
functionalization.[
] However, related alkylation at
the C8 position of quinoline N-oxides with
1
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10.1002/adsc.201700484
Advanced Synthesis & Catalysis
45-49]
organoboron reagents remains unexplored.[
could tolerate various substrates with both electron-
donating and electron-withdrawing substituents (such
as Me, phenyl, Cl, Br, and CN) and other functional
groups in the quinoline system, and provided the
corresponding methylated products at good to
excellent yields (3a-q). Generally, quinoline N-oxides
with electron-donating substituents gave higher yields
compared with electron-withdrawing substituents
(compare compounds 3a−d with 3h, 3j and 3l). In
particular, the 4-phenyl substituted substrate 3f
showed the best reactivity under the standard reaction
conditions. The efficiency of quinoline N-oxide
methylation was affected significantly by steric
hindrance. For instance, compound 3d with 7-methyl
substituents gave only a moderate yield relative to the
50
]
Inspired by the previous work by Li[
and our
51
]
groups,[
we disclose a Rh(III)-catalyzed C8
functionalization of quinoline N-oxides with
organoboron reagents (Scheme 2).
Initially, our studies focused on the rhodium-
catalyzed cross coupling between 4-methylquinoline
1-oxide 1a and readily available potassium
methyltrifluoroborate 2a (Table 1). In the presence of
5 mol% [RhCp*Cl2]2, 20 mol% AgSbF6, and 1.5
equivalents of Ag2O under argon at 65 °C, an 18%
yield of product 3a was detected by 1H NMR analysis
(entry 1). After a series of oxidants were screened,
AgOAc was found to be the most effective, giving
3a at 36% yield (entry 3); Ag2CO3 and AgF
Table 1. Optimization of reaction condition.[a,b]
3a
en
try
Catalyst
(mol %)
Additive Oxidant
Solve
nt
Yield
(mol %)
(equiv)
(%)b
[RhCp*Cl2]2
(5)
[RhCp*Cl2]2
(5)
[RhCp*Cl2]2
(5)
AgSbF6
(20)
AgSbF6
(20)
AgSbF6
(20)
Ag2O
(1.5)
Ag2CO3
(1.5)
AgOAc
(1.5)
1
2
3
DCE
DCE
DCE
18
35
36
AgBF4
[RhCp*Cl2]2
(5)
AgSbF6
(20)
4
DCE
ND
(1.5)
AgTFA
(1.5)
AgOTf
(1.5)
AgF
(1.5)
AgOAc
(1.5)
AgOAc
(1.5)
AgOAc
(1.5)
AgOAc
(1.5)
AgOAc
(1.5)
AgOAc
(1.5)
AgOAc
(1.5)
AgOAc
(2)
AgOAc
(2)
[RhCp*Cl2]2
(5)
[RhCp*Cl2]2
(5)
[RhCp*Cl2]2
(5)
[IrCp*Cl2]2
(5)
[RuCl2(p-
Cymene)]2 (5)
[RhCp*Cl2]2
(5)
[RhCp*Cl2]2
(5)
[RhCp*Cl2]2
(5)
[RhCp*Cl2]2
(5)
AgSbF6
(20)
AgSbF6
(20)
AgSbF6
(20)
AgSbF6
(20)
AgSbF6
(20)
AgSbF6
(20)
AgSbF6
(20)
AgSbF6
(20)
5
6
DCE
DCE
DCE
DCE
DCE
ND
ND
25
Scheme 2. C8-Selective Functionalization of quinoline N-
oxides.
7
8
ND
ND
65
showed comparable efficiency (entries 2 and 7),
while other oxidants gave no product (entries 4-6).
On the other hand, no product was formed with
[IrCp*Cl2]2 or [RuCl2(p-Cymene)]2 (entries 8 and 9).
The efficiency of this catalytic C-H methylation was
9
Tolue
ne
10
11
12
13
14
15
dependent
on
the
solvents
employed.
MeCN
DMF
DME
THF
55
Dimethoxyethane (DME) was the most effective
solvent among those examined (entries 10-14).
Encouraged by these results, a survey of the amount
of 2a was conducted, and with the use of 2
equivalents of 2a, the reaction proceeded more
smoothly to afford 89% yield of 3a upon isolation
(entry 15). By contrast, the reaction gave poor
conversion when AgSbF6 was absent (entry 16).
Therefore, the standard conditions for the Rh-
catalyzed C-H functionalization reaction are: 5 mol%
[RhCp*Cl2]2, 20 mol% AgSbF6, 2 equiv of AgOAc,
and 2 equiv of MeBF3K in DME at 65 °C for 18 h
under argon.
58
AgSbF6
(20)
AgSbF6
(20)
AgSbF6
(20)
78
[RhCp*Cl2]2
(5)
[RhCp*Cl2]2
(5)
[RhCp*Cl2]2
(5)
74
DME 95(89)
DME 16
16
--
[a]
Reaction conditions: 1a (0.4 mmol), 2a (1.2 mmol),
catalyst (5 mol %), additive (20 mol %) and oxidant
in solvent (2.0 mL) under argon at 65 °C for 18 h.
NMR yields using CH2Br2 as an internal standard,
isolated yields in parentheses.
[b]
Having identified the optimal reaction conditions,
we first turned our attention to examining the scope
and limitation of quinoline N-oxide derivatives, as
illustrated in Table 2. It was found that this reaction
2
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10.1002/adsc.201700484
Advanced Synthesis & Catalysis
2-, 3- and 4-methyl-substituted quinoline N-oxides
(3d vs 3a−3c). It should be mentioned that the
substituents of quinoline N-oxide at different
positions of its aromatic ring did not alter the reaction
efficiency; the methylated substrates (3a−c) provided
the desired products in fair yields. Halogenated
quinoline N-oxides could work without difficulty to
provide the desired products such as, 4-chloro-8-
yields are isolated ones.
Next, in order to expand the utility of this reaction,
we investigated variety of potassium
alkyltrifluoroborate derivatives (such as potassium
a
butyltrifluoroborate,
cyclopropyltrifluoroborate,
allyltrifluoroborate,
potassium
potassium
potassium
methylquinoline
dimethylquinoline
methylquinoline
N-oxide,
N-oxide,
4-chloro-6,8-
3-bromo-8-
6-bromo-2,8-
and
benzyltrifluoroborate) as the nucleophilic reagent to
obtain the corresponding alkylated products (see
Scheme S1 in the supporting information for details).
Unfortunately, none of the tested potassium
alkyltrifluoroborates provided the desired alkylated
products under the optimal reaction conditions and
the reactant 1a can be recovered in 90% amount
approximately. GC-MS results show that no self-
coupling products or uncontrolled beta-hydride
eliminations products of 2 were formed (see the
supporting information for details). However, some
of the 8-arylated quinoline N-oxides were produced
N-oxide,
and
dimethylquinoline N-oxide at 88% (3h), 78% (3i),
70% (3j), and 91% (3k) yields, respectively. This
finding indicated that the products of this reaction
were compatible in transition-metal-catalyzed
coupling reactions, thus significantly expanding the
synthetic utility of this reaction for further
transformation. Notably, Various functional groups
commonly used in synthetic chemistry, such as cyan
(3l), silyloxy (3m), esters (3n), carbamate (3o), and
acetal (3p), were all compatible with the standard
reaction conditions to generate the desired products in
moderate to good yields, which guaranteed the
synthetic application of the current methylation
protocol. In addition, the 8-methyl derivative of
selective agrochemical fungicide quinoxyfen could be
obtained by this directed C-H methylation (3q).
using
the
corresponding
potassium
aryltrifluoroborates, but only in acceptable yields
(23%-41%) (scheme 3). This quinoline N-oxide C-8
arylation transformation tolerated either electron-
donating or with-drawing substituents in the para- or
meta- positions of the aryltrifluoroborates. In addition,
steric hindrance played an important role in the yield
of this transformation. For example, methyl
substituent at the less sterically hindered para-
position of the trifluoroborate gave a higher yield
than the meta-methyl substituted trifluoroborate,
while the ortho-methylated trifluoroborate was
detrimental (3s-3u), which indicated the influence of
the steric hindrance.
Table 2. Scope of Quinoline N-oxide Derivatives.[a]
Scheme 3. Preliminary study of C-8 arylation
transformation.
To demonstrate the synthetic utility of the current
methylation protocol, a reaction of 1a and 2a was
performed on a 5 mmol scale of quinoline N-oxide
(Scheme 4). Gratifyingly, the methylation efficiency
was maintained at a high level in this large scale
reaction to give the desired product 3a at 82% yield
when the reaction time was prolonged to 36 hours.
[a] all the reactions were carried out with 0.4 mmol of 1, 1.2
mmol of 2a, 0.02 mmol of [Cp*RhCl2]2, 0.08 mmol of
AgSbF6, 0.8 mmol of AgOAc, 2 mL of DME as solvent
o
under argon atmosphere at 65 C for 16 h. All listed
3
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10.1002/adsc.201700484
Advanced Synthesis & Catalysis
Scheme 4. Gram-scale reaction.
As a versatile structural motif, the diversification
of quinoline N-oxide has been investigated actively.
We have performed several transformations of the
methylated product 3a. As depicted in Scheme 5,
selective deoxygenation of 3a was mediated by zinc
reagent, leading to the reductive quinoline compound
4 at 54% yield. A 2-Quinolone skeleton was also
readily achieved by a rearrangement reaction using
trifluoroacetic anhydride (TFAA), gaving a yield of
69%. Furthermore, the N-O moiety was used to
incorporate a p-methoxyphenyl group at the C2
position at 73% yield by means of a palladium-
catalyzed direct arylation reaction. Compound 7 was
accomplished at 59% yield via a regioselective
conversion of the corresponding quinoline N-oxide
with thionyl chloride.
Scheme 6. Preliminary mechanistic studies.
A plausible mechanism is proposed on the basis of
preliminary mechanistic experiments and previous
reports.[28,29] As shown in Scheme 7, the five-
membered rhodacycle A intermediate was formed via
coordination of the rhodium catalyst with the O atom
from the N-oxide moiety, and undergoes electrophilic
C-H bond cleavage at the C8 position. Next, the
complex A could convert into intermediate B through
the transmetalation with the organoboron reagent.
Finally, the C-C reductive elimination of intermediate
B gives rise to the desired product 3a and the Rh(I)
species. The active cationic Cp*Rh(III) species could
regenerated by Ag(I)-mediated reoxidization of the
Rh(I) species for the next cycle.
Scheme 5. Derivatization of C8-methylated products.
To obtain more insight into the mechanism, a series
of control experiments were performed (Scheme 6).
First, radical scavenger experiments were conducted
in the presence of a typical radical scavenger 2,2,6,6-
tetramethylpiperidyl-1-oxyl (TEMPO) under standard
reaction conditions (Scheme 6a). No significant effect
on the performance of the reaction was observed,
which suggested the irrelevancy of the organic radical
process in this reaction. Next, we synthesized a five-
membered rhodacycle A by treating 1e with
[Cp*RhCl2]2 and NaOAc (Scheme 6b).[52] Complex A
was submitted to the reaction to replace the rhodium
catalyst and successfully catalyzed the reaction of 1e
with 2a to afford the desired product 3e at 84% yield
(Scheme 6c). These experiments suggested that the
five-membered rhodacycle A could be the possible
active intermediate species in the catalytic cycle of
this reaction.
Scheme 7. Proposed mechanism.
In summary, we have developed a simple, and
highly efficient Rh(III)-catalyzed direct site-selective
C8-H-methylation and arylation of quinoline N-
oxides methylation and arylation using potassium
trifluoroborates under mild conditions. This reaction
features high regioselectivity at the C8 position of
quinolines with broad substrate scopes. This concise
protocol and further chemical transformations of the
4
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10.1002/adsc.201700484
Advanced Synthesis & Catalysis
products might have important applications in the
synthesis of biologically active compounds. Future
exploration of the Rh(III)-catalyzed C8-H activation
of quinoline N-oxides are underway in our laboratory.
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Experimental Section
Typical Procedure
To a 25 mL of Schlenk tube were added quinoline N-oxide
1 (0.4 mmol), potassium trifluoroborate 2 (1.2 mmol),
[RhCp*Cl2]2 (5.0 mol %), AgSbF6 (20.0 mol %), and
AgOAc (2.0 equiv) under air. The mixture was then
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dimethoxyethane (DME, 2.0 mL) were added subsequently.
o
The Schlenck tube was screw capped and stirred at 65 C
for 16 h. After that, the crude product was filtered through
a kieselguhr pad and washed with EA/MeOH (10:1, 30
mL). Then, the solvents were removed under vacuo, and
the residue was purified by
chromatography (EA/MeOH = 10:1) to give the desired
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Acknowledgements
We gratefully acknowledge financial support from the National
Natural Science Foundation of China (81620108027, 21632008,
91229204, 81220108025 and 81602975), the Major Project of
Chinese National Programs for Fundamental Research and
Development (2015CB910304), and National S&T Major
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10.1002/adsc.201700484
Advanced Synthesis & Catalysis
UPDATE
Cp*Rh(III)-Catalyzed Directed C-H Methylation
and Arylation of Quinoline N-Oxides at the C-8
Position
Adv. Synth. Catal. Year, Volume, Page – Page
Bao Wang,‡ Chunpu Li,‡ and Hong Liu *
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