Coordination strategy-induced selective C-H amination of 8-aminoquinolines

Article abstract of DOI:10.1039/c7cc02601c

In this study, we broke through the directing function of the amide group. The coordination interaction between metal and directing-group enhanced the reactivity of the substrate. Using this strategy, we realized the selective amination of 8-aminoquinolin

Full text of DOI:10.1039/c7cc02601c

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Coordination strategy induced selective C-H
amination of 8-animoquinolines
Cite this: DOI: 10.1039/x0xx00000x
a
a
a
a
a
c
Hong Yi,‡ Hong Chen,‡ Changliang Bian, Zilu Tang, Atul K. Singh, Xiaotian Qi,
c
c
d
a,b
Xiaoyu Yue, Yu Lan, JyhꢀFu Lee and Aiwen Lei *
Received 00th January 2014,
Accepted 00th January 2014
DOI: 10.1039/x0xx00000x
www.rsc.org/
4
In this work, we broke through the directing function of the realized. In addition, the modification of the C5 position of 8ꢀ
amide group. Instead, the coordination interaction between aminoquinoline also draws much research attention in the synthetic
5
metal and “directing-group” played the role to enhance the methodology. Here, we developed a coordination interaction
reactivity of substrate. Using this strategy, we realized the promoted selective CꢀH activation strategy, which underwent an
selective amination of 8-aminoquinolins on the C5 position entirely different pathway compared to traditional directingꢀgroup
employing azoles as the source of amine. Various kinds of 8- pattern (Scheme 1B). Firstly, the interaction between substrate and
aminoquinolines and different substituted azoles were highꢀvalent metal center produced the organometallic reagent and
compatible to afford the corresponding C-N coupling subsequent singleꢀelectron transfer step afforded the radical species
products.
which was crucial to selectivity for its electron distribution. The
target product was finally obtained through nucleophilic
a
The CꢀH activation strategy provides an ideal way to the addtion/singleꢀelectron transfer/deprotonation process. Different
construction of CꢀC/CꢀX (X=N, O, P etc.) bonds for the broad from conventional strategy, in our model, the radical cation species
existence of CꢀH bonds in organic molecules. During the past which formed from singleꢀelectron transfer process was explainable
several decades, transition metal catalyzed CꢀH activation reactions for the selectivity.
have made great progress in the field of organic synthesis
1
methodology. For selective CꢀH activation, directingꢀgroup strategy
was employed widely to verify different CꢀH bonds in complex
structures and have shown great application prospect for the
2
synthesis of complicated structures. Mechanistically, coordination,
CꢀH activation and subsequently transmetallation and reductive
elimination steps were usually involved (Scheme 1A). Coordination
interaction was the key step for the selectivity and directing group
was crucial in the CꢀH activation step. In addition, this directingꢀ
group strategy went through twoꢀelectron transfer process generally.
For its strong coordination capability, the nitrogenꢀcontaining
group was usually employed as directingꢀgroup in transition metal
Scheme 1. (A) Traditional fuctionalꢀgroup strategy for selective CꢀH
activation. (B) Coordination strategy induced selective CꢀH activation.
catalyzed CꢀH activation reactions. In addition, CꢀH activation
reactions directed by amide group have shown great application
potential in the field of synthetic methodology. Different kinds of
chemical transformations including CꢀC/CꢀN/CꢀP/CꢀS etc. bonds
XAFS and DFT evidence for the single-electron transfer
process. To check the feasibility of our strategy, XAFS was initially
employed to demonstrate the electron transfer process. As shown in
3
formation have been developed. Especially, 8ꢀanimoquinoline was
Figure 1A, the XANES spectrum of the CuBr solution gave a preꢀ
2
an excellent directingꢀgroup which has two coordination sites and
has received extensive research in the past ten years. Employing
Cu/Ni/Pd/Co etc. as the catalyst, amounts of transformations
edge at 8995.0 eV (Figure 1A, red line). After the addition of 8ꢀ
aminoquinoline 1a at 80 C, a new Cu species with the edge energy
o
of 8982.6 eV was observed (Figure 1A, black line). This edge energy
2
3
6
including functionalization of Csp ꢀH and Csp ꢀH have been
was close to reported Cu(I) standard sample. The EXAFS result
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DOI: 10.1039/C C0260
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showed that no nitrogen atom coordinated to the Cu(I) center (Figure when operated at room temperature (Table 1, entry 15). In a word,
S1), which indicated that the 8ꢀanimoquinoline didn’t coordinate we could get the best result using 20 mol% CuCl or Cu(OTf) as a
2
2
with copper after the reaction. In addition, density functional theory catalyst, 2 equiv. Na S O as the oxidant, and DCE as the solvent
2
2
8
(DFT) calculation was employed to probe the reactivity of radical (Table 1, entries 1 and 5). The structure of the product 3aa was also
cation species generated via the oxidation process. As shown in confirmed by XꢀRay crystal.
Figure 1B, the calculated Mulliken atomic spin densities (ASD) of
a
Table 1. Optimization of Reaction Conditions
radical cation 1a’ suggested that the spin density on C5 position was
.43, which was much higher than that on other carbon positions or
0
nitrogen positions. Frontier molecular orbital (FMO) analysis also
revealed that the βꢀLUMO of 1a’ was mainly located on C5 position.
Therefore, both ASD results and FMO analysis supported that the
C5 position has higher reactivity and nucleophiles would
preferentially react at C5 position in subsequent transformation.
b
Entry
Copperer
CuCl2
Oxidant
Na S O
Solvent
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
MeCN
DMF
DMSO
DCE
DCE
DCE
Yield(%)
c
1.2
0.8
0.4
0.0
1
2
3
4
5
6
7
8
9
72 (72)
2
2
8
8
8
8
8
8
CuBr2
Na S O
57
30
2
2
Cu(OAc)2
Cu(acac)2
Cu(OTf)2
CuBr
Na S O
2 2
Na S O
16
2
2
Na S O
72
2
2
Na S O
37
2
2
8970
8980
8990
9000
9010
CuCl
PhI(OAc)
n.d.
trace
n.d.
57
2
2
CuCl2
CuCl2
CuCl2
CuCl2
CuCl2
ꢀ
Ag CO3
2
DDQ
1
1
1
1
1
0
1
2
3
4
Na S O
2
2
8
8
8
8
Na S O
trace
trace
15
2
2
Na S O
2
2
Na S O
2
2
CuCl2
CuCl2
ꢀ
n.d.
trace
d
1
5
Na S O
2 2
8
a
Reaction conditions: 1a (0.25 mmol), 2a (1.0 mmol), copper catalyst (0.05
o
Figure 1. (A) XANES evidence for the singleꢀelectron transfer process mmol) and oxidant (0.5 mmol) was stirred in 2.0 mL solvent at 80 C under
b
between Cu(II) and 1a. (B) Calculated Mulliken atomic spin densities (ASD)
and lowest unoccupied molecular orbital (βꢀLUMO) of radical cation 1a’.
N atmosphere for 24 h. The yield was determined by GC using biphenyl as
an internal standard. Isolated yield in parentheses. Room temperature.
2
c
d
Optimization of Reaction Conditions. Next, we turned our
attention to the application of this oxidative induction strategy. From
the aspect of synthetic chemistry, it is a very useful transformation to
Scope of 8-aminoquinolines. With the optimized conditions in
realize the construction of aryl CꢀN bonds through the direct crossꢀ hand, we turned our attention to the exploration of substrate scope.
7
coupling of CꢀH/NꢀH. Herein, we employed acylꢀsubstituted 8ꢀ As shown in Scheme 2, we firstly evaluated different substituted 8ꢀ
aminoquinoline 1a and pyrazole 2a as model substrates to construct aminoquinolines employing pyrazole 2a as coꢀpartner. Various kinds
the CꢀN bonds in selective manner and screened a series of reaction of alkylamides showed excellent reactivity, affording the
8
conditions. As our continuous interest in copper chemistry, several corresponding amination product in high yields (3aa-3fa). In
available copper salts were employed as catalyst precursors (Table 1, particular, cycloalkane amide was also a good reaction partner to
entries 1ꢀ6). Different copper salts including Cu(I) and Cu(II) were produce the CꢀN structure in 68% yield (3ea). It is worth mentioning
able to afford the desired product, CuCl and Cu(OTf) could get the that halogen atom which could provide a further functionalization
2
2
same result (Table 1, entries 1 and 5). Different kinds of oxidants site was well tolerated under the reaction conditions (3fa).
which were usually employed in copper chemistry were screened Amazingly, alkene which existed in many natural products showed
and we found the Na S O was the best partner (Table 1, entries 7ꢀ9). good tolerance under our catalytic system, giving the selective
2
2
8
Screening of solvents showed that using DCE as solvent could amination product in 53% yield (3ga). Apart from alkylamides,
afford the highest yield (Table 1, entries 10ꢀ12). Control experiments arylamides were also good reaction partners (3ha-3la). Methylꢀ
implied that oxidant was essential for the conversion (Table 1, entry substituted aryl showed similar reactivity (3ia-3ka). Notably,
1
1
4) and copper catalyst played the role as a promoter (Table 1, entry
3). In addition, the reaction afforded trace amount of product only
2
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halogen substituted phenyl afforded 44% yield which could be when the reaction was conducted under the addition of 2 equiv.
further functionalized (3la).
TEMPO or BHT, the reaction was totally inhibited which suggested
the radical pathway (Scheme 4A). Finally, an isotope labelling
a
Scheme 2. Substrate scope of 8ꢀaminoquinolines.
experiment employing equivalent 1a and D ꢀ1a was conducted under
2
standard conditions, and the result showed the cleavage step of CꢀH
bond was not involved in the rateꢀdetermining step (Scheme 4B).
a
Scheme 3. Substrate scope of azoles.
a
Reaction conditions: 1 (0.25 mmol), 2a (1.0 mmol), CuCl
2
(0.05 mmol) and
(0.5 mmol) was stirred in 2.0 mL DCE at 80 C under N atmosphere
for 24 hours.
o
Na
2
S
2
O
8
2
Scope of azoles. It is of great significance to develop efficient
methods to the synthesis of functionalized azoles because they are
important structural moieties in many kinds of pharmacologically
active compounds and usually act as directing groups in transitionꢀ
a
Reaction conditions: 1a (0.25 mmol), 2 (0.5 mmol), CuCl
2
(0.05 mmol) and
o
7
b, 9
metal catalyzed reactions.
Therefore, we investigated a series of
Na
2
S
2
O
8
(0.5 mmol) were stirred in 2.0 mL solvent at 80 C under N
2
2 2
azoles using 1a as coꢀpartner. As shown in Scheme 3, methyl groups atmosphere for 24 hours. b, Cu(OTf) instead of CuCl . c, MeCN instead of
DCE.
were tolerated well for pyrazoles to afford the corresponding
molecules in approximately 60% yield (3ab-3ad). 3ꢀPhenylꢀ1Hꢀ
pyrazole and 3,5ꢀdiphenylꢀ1Hꢀpyrazole were also good reaction
partners (3ae, 3af). Notably, halogen atoms including bromine and
iodine which could be further functionalized were compatible under
our catalytic conditions (3ag-3ai). Apart from pyrazoles, various
kinds of triazoles were good reaction partners under our system.
1
,2,3ꢀTriazole and 4ꢀphenyl triazoles showed similar reactivity to
afford the desired products in 66ꢀ68% yield (3aj-3al). In addition,
,2,4ꢀtriazole was successfully employed in our system and
1
produced the CꢀN structure in 60% yield. And benzotriazoles which
were used as corrosion inhibitor of copper and its oxides were also
Scheme 4. (A) Radical inhibition experiments. (B) Isotope labelling
compatible and afforded the target product in moderate yield under experiment.
1
0
our conditions (3an).
Finally, on the basis of our experimental results, we proposed a
mechanism for the selective CꢀH activation of 8ꢀanimoquinolines
on the C5 position (Scheme 5). Initially, coordination of Cu(II)
Mechanistic studies. To further confirm our coordination strategy,
a series of control experiments were conducted. Firstly, when the
substrate 4 or 5 was employed under our conditions, no CꢀN
coupling product was detected which demonstrated the importance
of the formed organometallic reagents. (Scheme S1). In addition,
with
1 afforded the corresponding organometallic species I which
subsequently went a singleꢀelectron transfer process, affording
the radical cation II. The radical cation II was a crucial
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DOI: 10.1039/C C0260
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National Synchrotron Radiation Research Center, Hsinchu 30076,
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†
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here. See
DOI: 10.1039/b000000x/
‡
Hong Yi and Hong Chen contributed equally
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