Silver-Catalyzed Decarboxylative Alkylfluorination of Alkenes

Article abstract of DOI:10.1021/acs.orglett.9b03381

A decarboxylation of alkyl carboxylic acids for alkylfluorination of alkene was developed, with the catalysis of silver(I) and Selectfluor as both the oxidant and fluorine source. This reaction is highly chemoselective, producing the decarboxylative alkyl

Full text of DOI:10.1021/acs.orglett.9b03381

Letter
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Silver-Catalyzed Decarboxylative Alkylfluorination of Alkenes
Chen-Guang Li,† ,†Qiang Xie, Xiao-Lan Xu, Feng Wang, Bei Huang, Yu-Feng Liang,*
,#
‡,#
§
†
†
,∥
and Hua-Jian Xu*
†
School of Food and Biological Engineering, Hospital of Hefei University of Technology, Hefei 230009, P. R. China
PET-CT Center, The First Affiliated Hospital of USTC, Hefei 230001, P. R. China
‡
§
∥
School of Medical Science, Anhui Medical University, Hefei 230026, P. R. China
Department of Chemistry, McGill University, 801 Sherbrooke Street West, Montreal, Quebec H3A 0B8, Canada
*
S Supporting Information
ABSTRACT: A decarboxylation of alkyl carboxylic acids for
alkylfluorination of alkene was developed, with the catalysis of
silver(I) and Selectfluor as both the oxidant and fluorine
source. This reaction is highly chemoselective, producing the
decarboxylative alkylfluorination products rather than the
competitive fluorination of aliphatic carboxylic acids. This
practical transformation proceeds efficiently in aqueous media at room temperature and exhibits a large range of functional-
group tolerance in various primary and secondary aliphatic carboxylates and alkenes.
he fluorine substituent has been increasingly applied in
pharmaceuticals and agrochemicals as well as in materials
Scheme 1. Ag-Catalyzed Decarboxylation of Aliphatic
Carboxylic Acids for the Synthesis of Organic Fluorides
T
science due to its ability to regulate the pK of molecules, alter
a
the acidity and alkalinity of nearby groups, increase the
lipophilicity of molecules, and provide metabolic stability and
1
bioavailability. Therefore, the development of efficient
approaches to synthesize fluorinated molecules has become a
2
hot topic in organic synthesis. Among the methods to
introduce fluorine atoms into a molecule, the difunctionaliza-
tion of alkenes with fluorinating reagents is a powerful
synthetic tool, as it is able to not only install a fluorine atom
but also introduce other useful groups into the molecules in an
3
,4
atom-economical fashion. As a consequence, a number of
fluorination reactions of alkenes via difunctionalization have
5
been successfully developed, including oxyfluorination,
6
7
aminofluorination, phosphonofluorination, sulfofluorina-
8
9
10
11
tion, difluorination, hydrofluorination, carbofluorination,
1
2
and others. Despite the significance of these reactions, much
undiscovered progress remains to be explored in this area. To
the best of our knowledge, it is difficult to synthesize
fluorinated compounds via decarboxylative difunctionalization
1
3
of alkenes.
Aliphatic carboxylic acids have often been used as alkyl
synthons in various types of coupling reactions in recent years,
not only because of their low cost and availability but also
because they are relatively stable and it is easy for them to
intermediates are involved in the catalytic cycle. On the basis
of the serendipitous discovery, we hypothesized that a
combination of radical acceptors, such as alkenes, might lead
the alkyl radical intermediates to attack the unsaturated double
bonds of alkenes, followed by the abstraction of the fluorine
atom in the Ag(II)−F to produce alkylfluorination products
14
generate alkyl radicals. Such decarboxylation reactions are
usually achieved by the catalysis of transition metals, including
1
5,16
17
18
19
silver,
copper, manganese, and photocatalysis, among
2
0
others. In particular, Li and co-workers innovatively
developed a silver-catalyzed decarboxylative fluorination of
aliphatic carboxylic acids that uses Selectfluor as the fluorine
15
source (Scheme 1a). Elegant mechanistic investigations of
Received: September 24, 2019
this reaction have revealed the alkyl radical and Ag(II)−F
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b03381
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Scheme 1b). As depicted in Scheme 1c, aliphatic carboxylic
Letter
a
(
Table 1. Screening of the Reaction Conditions
acids could be oxidized by the proposed Ag(III)−F
intermediate to give Ag(II)−F and an alkyl radical with one
carbon atom removed. The addition of an electrophilic alkyl
radical to olefin to form the corresponding nucleophilic
carbon-centered radical, followed by the transfer of the fluorine
atom from Ag(II)−F to the adduct radical, generates the
alkylfluorination product and regenerates the Ag(I) catalyst for
the next catalytic circle. However, we are mindful of the
chemoselectivity issue for this process to become successful:
the catalytic reaction should suppress the competitive
undesired decarboxylative fluorination. The interaction of
alkyl radicals with alkenes should be much faster than that with
Ag(II)−F intermediates, leading to the formation of
alkylfluorination products rather than fluorination byproducts.
Driven by the above hypothesis, we set out to explore this
possibility by evaluating the reaction of styrene (1a) with 4-
phenylbutanoic acid (2a) and Selectfluor under a silver-
catalyzed reaction system at room temperature [eq 1]. We
b
yield
entry
[Ag]
additive
solvent
(%)
1
2
3
4
5
6
7
8
AgNO3
AgOA3
AgBF4
AgNO3
AgNO3
AgNO3
AgNO3
AgNO3
KHCO3 CH CN/H O
KHCO3 CH CN/H O
KHCO3 CH CN/H O
15
13
11
0
27
25
47
0
3
2
3
2
3
2
KHCO3 CH CN/H O
3 2
KHCO3 acetone/H O
KHCO3 DMF/H O
KHCO3 1,4-dioxane/H O
KHCO3 CH CN, CH Cl , acetone, DMF,
2
2
2
3
2
2
1
,4-dioxane
9
AgNO3
AgNO3
AgNO3
AgNO3
AgNO3
AgNO3
AgNO3
AgNO3
-
K CO3
1,4-dioxane/H O
31
23
26
30
19
69
76
0
2
2
1
1
1
1
1
1
1
1
0
1
2
3
4
5
6
7
tBuOK
1,4-dioxane/H O
2
Cs CO3 1,4-dioxane/H O
2
2
DBU
CsF
KOAc
KOAc
KOAc
KOAc
1,4-dioxane/H O
2
1,4-dioxane/H O
2
1,4-dioxane/H O
2
c
1,4-dioxane/H O
2
d
1,4-dioxane/H O
2
1,4-dioxane/H O
0
2
a
Reaction conditions: 1b (0.2 mmol), 2b (0.6 mmol), Selectfluor (0.6
mmol), AgNO (0.04 mmol), additive (0.5 mmol), solvent (v/v = 1/
3
b
c
1
(
, 2 mL), at room temperature for 12 h. Isolated yield. With KOAc
0.6 mmol) in the absence of acetic acid (2b). With NFSI instead of
were delighted to observe the expected decarboxylative
alkylfluoroation product 3aa in 37% yield, while the
unexpected decarboxylative alkylfluorination product 4a, as
well as the possible hydrogenation and hydroxylation by-
product, were not detected on the basis of the NMR and GC-
MS analyses. This positive outcome suggested the feasibility of
our proposed assumption. Herein, we wish to report the
decarboxylative alkyl carboxylic acids for alkylfluorination of
unsaturated double bonds in aqueous solution with the
Selectfluor reagents as oxidants and fluorine sources, which
was achieved under the silver-catalyzed conditions. The
reaction conditions are mild, operate simply, and can be
achieved directly at room temperature.
d
Selectfluor. DBU = 1,8-diazabicyclo[5.4.0]undec-7-ene. NFSI = N-
fluorobis(benzenesulfonyl)imide.
1, entry 15). In contrast, no product formed when the fluorine
source was switched to NFSI (Table 1, entry 16), and no
reaction occurred in the absence of silver catalyst (Table 1,
entry 17).
With the optimal conditions established, we investigated the
scope and limitation of this decarboxylative alkylfluorination
method with different aliphatic carboxylic acids (Scheme 2).
Primary aliphatic carboxylic acids underwent efficient alkyl-
fluorination, affording the corresponding products (3aa−3ae)
in moderate to good yields. The transformation of secondary
aliphatic carboxylic acids also proceeded smoothly, furnishing
products 3af−3ah in moderate yields. Unfortunately, the
reactions with tertiary aliphatic carboxylic acids deliver the
desired alkylfluorination product in trace amounts, with the
decarboxylative fluorination as the main side reaction.
Aromatic acids did not react under the standard reaction
conditions, with the acids being fully recovered.
Next, the scope of the alkylfluorination reaction for alkenes
was examined (Scheme 3). A range of styrenes containing an
electron-donating or electron-withdrawing group underwent
the alkylfluorination to give the corresponding desired
products (3cc−3jc) ranging from 43% to 81% yield where
various para-, meta-, or ortho-substitutents were well tolerated.
A variety of functional groups including nitrile, fluoro, chloro,
and bromo were compatible under these mild reaction
conditions. The reaction of naphthyl ethylene exhibited
moderate reactivity (3kc). The 1,1-disubstitued alkene was
also a suitable substrate (3lb). Moreover, the nonstyrene
olefins could also react smoothly to afford the desired
alkylfluorination products (3mb, 3nc) in moderate yields.
Consequently, to explore the general reactivity of this
transformation, acetic acid (2b) (a challenging primary alkyl
carboxylic acid due to its low reactivity) was selected as the
model substrate for decarboxylative fluoromethylation of 1-
chloro-4-vinylbenzene (1b). Intriguingly, the target product
3
bb was obtained in 15% yield under Ag-catalyzed conditions
(Table 1, entry 1). Encouraged by this outcome, we examined
different silver salts and found that AgNO showed slightly
3
higher reactivity compared with other Ag(I) salts (Table 1,
entries 2−3). Among the different solvents tested, the mixed
solvents 1,4-dioxane/H O (v/v = 1/1) demonstrated superior
2
results compared to other solvents (Table 1, entries 4−6),
leading to the formation of the alkylfluorination product 3bb in
4
7% yield (Table 1, entry 7). The reactions in pure organic
solvents such as CH CN, CH Cl , acetone, DMF, or 1,4-
3
2
2
dioxane failed to give any desired product (Table 1, entry 8),
indicating the essential role of water. Then, the introduction of
other additives was evaluated, and KHCO was found to be the
3
most effective (Table 1, entries 9−13). To our delight, the
yield was improved to 69% when KOAc was employed as the
addictive (Table 1, entry 14). Then, KOAc was used instead of
acetic acid (2b) as the methyl source for the decarboxylative
fluoromethylation, further improving the yield to 76% (Table
B
DOI: 10.1021/acs.orglett.9b03381
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 2. Decarboxylation of Aliphatic Carboxylic Acids
for Alkylfluorination
A gram-scale reaction of 4-fluoro-styrene (3d) and
potassium hexanoate (2c) was then performed [eq 2].
a
Delightfully, the yield was similar to that in the small-scale
reaction. The above relative reactivities of carboxylic acids and
15,16
the literature
suggested that the reaction proceeds via a
radical mechanism. When 1 equiv of TEMPO (2,2,6,6-
tetramethyl-1-oxylpiperidine), the radical trapper, was added
in the reaction, the desired alkylfluorination product was
reduced in a trace amount, with the adduct of the alkyl radical
with TEMPO detected by GC-MS [eq 3], further indicating
the single electron transfer pathway as shown in Scheme 1c.
To conclude, the decarboxylative alkylfluorination of
unsaturated CC double bonds has been successfully
developed, which was achieved under the silver-catalyzed
conditions in aqueous media at room temperature, with the
Selectfluor reagent as both the oxidant and fluorine source.
Thus, the reactions of a number of primary and secondary alkyl
carboxylic acids afforded the corresponding alkylfluorination
compounds with good chemoselectivity in an operationally
simple and mild approach. This transformation provides a
practical method for the synthesis of fluorinated molecules.
a
Reaction conditions: 1 (0.2 mmol), 2 (0.6 mmol), Selectfluor (0.6
mmol), AgNO (0.04 mmol), 1,4-dioxane/H O (v/v = 1/1, 2 mL), at
room temperature for 12 h. Isolated yield. CH CN/H O (v/v = 1/
3
2
b
3
2
c
1
) as the solvent for 24 h. Alkyl acids as the starting materials with
KHCO (0.5 mmol) as the additive in CH CN/H O (v/v = 1/1)
3
3
2
solvent for 24 h.
a
Scheme 3. Decarboxylative Alkylfluorination of Alkenes
ASSOCIATED CONTENT
Supporting Information
■
*
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b03381.
Experimental procedures and NMR spectra (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
*
E-mail: yfliang2008@gmail.com.
E-mail: hjxu@hfut.edu.cn.
*
ORCID
Yu-Feng Liang: 0000-0002-9920-741X
Hua-Jian Xu: 0000-0002-0789-3484
Author Contributions
#
C.L. and Q.X. contributed equally to this work.
Notes
a
Reaction conditions: 1 (0.2 mmol), 2 (0.6 mmol), Selectfluor (0.6
The authors declare no competing financial interest.
mmol), AgNO (0.04 mmol), 1,4-dioxane/H O (v/v = 1/1, 2 mL), at
3
2
b
room temperature for 12 h. Isolated yield. CH CN/H O (v/v = 1/
3
2
c
ACKNOWLEDGMENTS
1
) as the solvent for 24 h. KHCO (0.2 mmol) was added and in
■
3
CH CN/H O (v/v = 1/1) solvent for 24 h.
We are grateful to the National Natural Science Foundation of
China (No. 21472033), Key Research and Development
Program Projects in Anhui Province (201904a07020069), and
3
2
C
DOI: 10.1021/acs.orglett.9b03381
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
the Fundamental Research Funds for the Central Universities
for financial support.
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