Transition-Metal and Solvent-Free Oxidative C-H Fluoroalkoxylation of Quinoxalinones with Fluoroalkyl Alcohols

Article abstract of DOI:10.1021/acs.orglett.9b01578

The first example of oxidative C-H fluoroalkoxylation of quinoxalinones with fluoroalkyl alcohols under transition-metal and solvent-free conditions is described. This approach provides the synthesis of fluoroalkoxylated quinoxaline derivatives with good to excellent yields under mild reactions conditions. This method can also be extended to the facile and efficient synthesis of histamine-4 receptor.

Full text of DOI:10.1021/acs.orglett.9b01578

Letter
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Transition-Metal and Solvent-Free Oxidative C−H
Fluoroalkoxylation of Quinoxalinones with Fluoroalkyl Alcohols
Jun Xu, Huiyong Yang, Heng Cai, Hanyang Bao, Wanmei Li,* and Pengfei Zhang*
College of Material Chemistry and Chemical Engineering, Hangzhou Normal University, Hangzhou 310036, China
*
S Supporting Information
ABSTRACT: The first example of oxidative C−H fluoroal-
koxylation of quinoxalinones with fluoroalkyl alcohols under
transition-metal and solvent-free conditions is described. This
approach provides the synthesis of fluoroalkoxylated quinoxa-
line derivatives with good to excellent yields under mild
reactions conditions. This method can also be extended to the
facile and efficient synthesis of histamine-4 receptor.
luorine-containing molecules have found important
Scheme 2. Present Approaches for Fluoroalkoxylation
F
applications in drug discovery, agricultural chemicals,
and materials science. Fluoroalkoxyl aryl ethers are among
those that have received much attention, because of the unique
1
properties such as good electron-withdrawing effect and the
2
significant lipophilicity of the fluoroalkoxyl group (Scheme 1).
Scheme 1. Representative Fluoroalkoxylated Compounds
The transition-metal-catalyzed coupling reactions typically
3
give access to such compounds as shown in Scheme 2a. In
2
014, Singh and co-workers reported a palladium/brettphos-
5
reactions at high temperature (Scheme 2b). In this way, the
catalyzed fluoroalkoxylation of activated aryl halides with
fluoroalkyl alcohols. Subsequently, Weng et al. synthesized
fluoroalkoxylation of aryl and heteroaryl bromides by using a
well-defined copper(I) fluoroalkoxide complex. The copper-
catalyzed oxidative trifluoroethoxylation of aryl boronic acids
with CF CH OH has also been reported. Moreover,
6
3a
use of transition-metal catalysts is avoided. Nevertheless, there
is still a substantial interest in developing simple and practical
approaches to introduce fluoroalkoxyl groups into organic
molecules.
In recent years, the transition-metal-free oxidative C−H
functionalization has received considerable attention. Espe-
cially, the hypervalent iodine(III) reagents are a highly effective
class of oxidants that have been widely used in oxidative C−H
functionalization reactions. Such reagents have high stability,
low toxicity, and ready commercial availability, making them
practically interesting for synthetic chemistry.
3b
3
c
3
2
7
palladium-catalyzed C−H trifluoroethoxylation of arenes can
3
d
be facilitated by the N atom of amide. Despite the
tremendous advances that have been made, the generation of
fluoroalkoxyl aryl ethers through transition-metal catalysis still
suffers from some instinctive drawbacks. For example, it is a
costly and challenging task to remove the trace amounts of
transition metals remaining in the final products. This is
8
7c,9
4
particularly true in the pharmaceuticals industry. Another
Received: May 5, 2019
interesting alternative is based on nucleophilic displacement
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b01578
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
For a long period of time, we have been focusing on the C−
the product yield (Table S1, entries 5−21). However, a higher
10
H functionalization of N-containing heterocycles and the
yield of 89% was obtained when 2.0 equiv of PhI(TFA) was
2
11
fluorine chemistry. In a continuation of our efforts, we herein
reported a direct C−H fluoroalkoxylation of quinoxalinones
with fluoroalkyl alcohols using hypervalent iodine(III) reagent
as an oxidant under transition-metal and solvent-free
used (Table 1, entry 3). No significant change was observed
when the dosage was further increased to 2.5 equiv (Table 1,
entry 4). After further screening of the reaction conditions, we
were surprised that the transformation could also occur under
solvent-free conditions (Table 1, entries 5−8). In this
approach, the target product 3a was isolated in 88% yield
when the dosage of 2a was enhanced to 3.0 equiv (Table 1,
entry 7).
With the optimal reaction conditions in hand, the substrate
scope of the C−H fluoroalkoxylation was subsequently
explored by employing various quinoxalinones (1) with
trifluoroethanol (2a) (Scheme 3). First, the compatibility of
1
2
conditions (Scheme 2c). We chose quinoxaline derivatives
as substrates for two reasons. First, substituted quinoxalines
and their analogues are important organic compounds in
1
3
14
pharmaceuticals, natural products, and materials chem-
1
5
istry. Second, although many C−H functionalization
16
reactions have been reported for the construction of C−C,
1
7
18
C−N, and C−P bonds of quinoxalines, the development of
C−O bond formation reaction remains scarce.
To the best of our knowledge, this is the first sample on the
oxidative C−H fluoroalkoxylation of quinoxalinones with
fluoroalkyl alcohols under transition-metal and solvent-free
conditions.
Scheme 3. Substrate Scope of Quinoxalinones with
Trifluoroethanol
,
a b
In a first set of experiments, we investigated the C−H
fluoroalkoxylation of quinoxalinone (1a) with trifluoroethanol
(
2a) as the fluoroalkoxylating reagent. The reaction to form
product (3a) gave 65% yield within 1 h by using 1 mol % of
Eosin Y as the photocatalyst, 1.5 equiv of PhI(TFA) as the
2
external oxidant, and 0.2 mL of MeCN as the solvent under the
illumination of blue light-emitting diode (LED) (see Table S1,
entry 1, in the Supporting Information). Other photocatalysts
such as Ir(ppy) and Rhodamine B could also promote this
3
transformation and give the target product in similar yield
(Table S1, entries 2 and 3). These results prompted us to
further explore if the photocatalyst was indispensable. To our
surprise, 64% yield of product 3a was still isolated even without
any photocatalyst (Table S1, entry 4). Instead, it was found
that the reactivity of the reaction was the result of thermal
activation, because of the actual fact that the C−H
fluoroalkoxylation reaction also occurred without light at 35
°
C, giving the product 3a in 63% yield (see Table 1, entry 1).
a
Reaction conditions: 1 (0.2 mmol), 2a (3.0 equiv), PhI(TFA) (2.0
2
In this case, we could confirm that the heat generated from the
blue LEDs utilized in our experiment (at a temperature of ∼35
b
c
equiv), open flask, 35 °C, 1 h. Isolated yields. Reaction was
performed on a 1 mmol scale.
°C) was sufficient to initiate the reaction. This hypothesis was
further proved when the reaction was performed at room
temperature (∼15 °C) without light (19% yield; see Table 1,
entry 2). Changing the oxidants or solvents did not enhance
different N-substituted quinoxalinones in the present trans-
formation was examined. To our delight, a wide range of N-
protecting groups including methyl, ethyl, butyl, cyclo-
hexylmethyl, esteryl, keto, and allyl groups were well
compatible in this transformation, giving access to the desired
products in good to excellent yields (3a−3g). It should be
mentioned that the N-free protecting quinoxalinone was also
suitable for the transformation, the product (3h) was obtained
in 80% yield. Interestingly, when using Boc as the N-protecting
group, the deprotection step also happened to provide product
a b
Table 1. Optimization of Reaction Conditions ,
b
entry
1
catalyst
oxidant
solvent
yield (%)
3
h in 75% yield (see the Experimental Section in the
−
−
−
−
−
−
−
−
PhI(TFA)2
PhI(TFA)2
PhI(TFA)2
PhI(TFA)2
PhI(TFA)2
PhI(TFA)2
PhI(TFA)2
PhI(TFA)2
MeCN
MeCN
MeCN
MeCN
−
−
−
−
63
19
89
87
42
52
88
86
c
Supporting Information, subsection 1.3). Substrates with
various N-benzyl groups were also tolerated in this reaction,
as demonstrated with 3i−3u. The molecular structure of
product 3l was further confirmed by X-ray crystallographic
analysis (CCDC No. 1911863). In addition, quinoxalinones
bearing the methyl, methoxyl, chloro, and bromo groups on
the benzene ring were also investigated under the optimal
reaction conditions, and the corresponding products (3v−3aa)
were isolated in good yields.
2
d
3
e
4
5
d
d
d
d
,
,
,
f
6
7
8
g
h
a
Reaction conditions: 1a (0.2 mmol), 2a (1.5 equiv), oxidant (1.5
b
After the successful implementation of C−H fluoroalkox-
ylation of quinoxalinones with trifluoroethanol, the trans-
formation was further studied using other fluoroalkyl alcohols
equiv), solvent (0.2 mL), open flask, 35 °C, 1 h. Isolated yields.
c
d
e
Stirred at room temperature. PhI(TFA) (2.0 equiv). PhI(TFA)
2
2
f
g
h
(
2.5 equiv). 2a (2.0 equiv). 2a (3.0 equiv). 2a (4.0 equiv).
B
DOI: 10.1021/acs.orglett.9b01578
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
as fluoroalkoxylating reagents (Scheme 4). The present
method was also suitable for C−H fluoroalkoxylation of
In addition to the special bioactivity, the trifluoroethoxyl
group could also be used as an available leaving group in S Ar
N
19
reactions. In this case, the reactivity of trifluoroethyl ethers 3
toward nucleophilic substitution with morpholine was
investigated and the aminated products 12 were obtained in
good yields (Scheme 6a). Subsequently, the two-step synthesis
of histamine-4 receptor antagonist 13 was successfully
achieved in 68% yield (Scheme 6b).
Scheme 4. Substrate Scope of Quinoxalinones with
,
a b
Fluoroalkyl Alcohol
Scheme 6. Application of Products in S Ar Reactions and
N
Synthesis of Histamine-4 Receptor Antagonist
a
Reaction conditions: 1 (0.2 mmol), 2 (3.0 equiv), PhI(TFA) (2.0
2
b
equiv), open flask, 35 °C, 1 h. Isolated yields.
As reported previously, most of C−H functionalization
quinoxalinones with a variety of fluoroalkyl alcohols, such as
fluoroethanol (4a−4f), difluoroethanol (5a−5d), pentafluor-
opropanol (6a−6g), tetrafluoropropanol (7a−7e), heptafluor-
obutanol (8a−8e), and trifluorobutanol (9a−9e), which
provided the corresponding products in good to excellent
yields. Furthermore, the molecular structure of product 7d was
confirmed by X-ray crystallographic analysis (CCDC No.
reactions that contain hypervalent iodine reagents involve
9
radical mechanisms. To explain the reaction mechanism, we
performed a preliminary mechanism investigation (Scheme 7).
Scheme 7. Control Experiments for Preliminary Mechanism
Study
1
913960). An exception is that the hexafluoroisopropanol
could not be transformed to the corresponding product. We
suspected that the effect of steric hindrance prevented the
transformation.
To further display the scalability and synthetic utility of the
transformation, we performed the C−H fluoroalkoxylation of
trifluoroethanol with quinoxalinones on a 6.0 mmol scale, and
the desired product 3h was obtained in 75% yield (Scheme 5).
Scheme 5. Gram-Scale Synthesis and Product
a
Transformations
First, the transformation was suppressed when an amount of 2
equiv of TEMPO (2,2,6,6-tetramethyl-piperidin-1-oxyl) or
DPE (diphenylethlene) was added as a radical inhibitor,
respectively. Meanwhile, a considerable amount of quinox-
alinones (1a) was recovered in 83% or 65% yields, respectively.
Furthermore, the adducts A and B were also detected by
HRMS. These experimental results clearly revealed that a
radical pathway was involved in the reaction.
Based on the above-mentioned results and previous
a
For the detailed reaction conditions, see the Supporting Information.
7,9,16−18
reports,
a single electron transfer (SET) mechanism
was proposed (Scheme 8). Initially, trifluoroethoxyl radical was
formed from trifluoroethanol through the oxidation of
In addition, the fluoroalkoxylated quinoxalinone could be
transformed to various quinoxaline derivatives, which are
challenging reactions using traditional methods (Scheme 5).
For instance, the above product 3h could be first converted to
the chlorinated quinoxaline 10 in the presence of POCl3.
Further transformations of chlorinated quinoxaline 11 with
various reactants under different conditions gave the
corresponding products in excellent yields.
PhI(TFA) . The obtained radical then attacked substrate 1a
2
to produce nitrogen radical intermediate C (path A). However,
an alternative route for the formation of nitrogen radical
intermediate C cannot be excluded. In this case, the substrate
1a first coordinated with PhI(TFA) to generate intermediate
2
A, and the reactive radical cation B was then generated through
a single electron transfer (SET) process, which was trapped by
C
DOI: 10.1021/acs.orglett.9b01578
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 8. Plausible Mechanism
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Pengfei Zhang: 0000-0001-9859-0237
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We thank the National Natural Science Foundation of China
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Innovation Project of Zhejiang (No. 2019C01081) for
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DOI: 10.1021/acs.orglett.9b01578
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