Silver-catalyzed radical phosphonofluorination of unactivated alkenes

Article abstract of DOI:10.1021/ja408031s

We report herein a mild and catalytic phosphonofluorination of unactivated alkenes. With catalysis by AgNO₃, the condensation of various unactivated alkenes with diethyl phosphite and Selectfluor reagent in CH ₂Cl₂/H₂O/HOAc at 40 C led to the efficient synthesis of β-fluorinated alkylphosphonates with good stereoselectivity and wide functional group compatibility. A mechanism involving silver-catalyzed oxidative generation of phosphonyl radicals and silver-assisted fluorine atom transfer is proposed.

Full text of DOI:10.1021/ja408031s

Communication
pubs.acs.org/JACS
Silver-Catalyzed Radical Phosphonofluorination of Unactivated
Alkenes
Chengwei Zhang, Zhaodong Li, Lin Zhu, Limei Yu, Zhentao Wang, and Chaozhong Li*
Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, P.R. China
S
* Supporting Information
diazoniabicyclo[2.2.2]octane bis(tetrafluoroborate) (Selectfluor
reagent, Air Products and Chemicals)12 and Ag(I) catalyst
ABSTRACT: We report herein a mild and catalytic
phosphonofluorination of unactivated alkenes. With
catalysis by AgNO3, the condensation of various
unactivated alkenes with diethyl phosphite and Selectfluor
reagent in CH2Cl2/H2O/HOAc at 40 °C led to the
efficient synthesis of β-fluorinated alkylphosphonates with
good stereoselectivity and wide functional group compat-
ibility. A mechanism involving silver-catalyzed oxidative
generation of phosphonyl radicals and silver-assisted
fluorine atom transfer is proposed.
served not only as an oxidant but also as a fluorine atom
transfer agent in fluorodecarboxylation of aliphatic carboxylic
acids and in intramolecular radical aminofluorination of
unactivated alkenes, presumably via Ag(III)F and Ag(II)F as
intermediates. We envisioned that such a combination might be
utilized to allow the catalytic oxidative generation of
phosphonyl radicals.13,14 As depicted in Figure 1, diethyl
he increasing importance of fluorine in agrochemicals and
1
18F-labeled organic
Tpharmaceuticals as well as the use of
compounds as contrast agents for positron emission tomog-
raphy (PET)2 has spurred a vigorous research for the
development of new methods for C−F bond formation under
mild conditions.3 In particular, fluorination of unsaturated
carbon−carbon bonds, such as oxyfluorination,4 aminofluori-
nation,5 hydrofluorination,6 and even carbofluorination7 of
alkenes, allenes, and alkynes, has emerged as a powerful tool for
C(sp3)−F bond formation. For example, electrophilic fluo-
rocyclization offers a convenient synthesis of fluorinated O- or
N-heterocycles.4,5 The fluorocyanation of enamines provides a
facile entry to β-fluoro-α-amino acid derivatives.7c The Fe(III)/
NaBH4-mediated radical hydrofluorination of unactivated
alkenes allows the incorporation of fluorine into organic
substrates with an excellent functional group tolerance.6a
However, to the best of our knowledge, no example of
phosphonofluorination of any kind has ever been reported.
Phosphonates as phosphate mimics also have enormous
significance in agrochemicals, pharmaceuticals, and materials.8
Fluorinated phosphonates have thus found wide applications,
especially in biomedical studies.9 Specifically, β-fluoroalkyl-
phosphonates exhibit a diverse range of biological activities.10
For example, β-fluoro-α-aminoalkylphosphonates are inhibitors
of alanine racemase,10c and the β-fluorinated phosphonate
analogue of lysophosphatidic acid is an autotoxin inhibitor.10d
However, methods for the synthesis of this important class of
compounds are extremely limited.9,10 The concomitant
introduction of a fluorine atom and a phosphonyl moiety
into one molecule, the phosphonofluorination of alkenes,
should serve as an ideal entry to β-fluorinated phosphonates.
Herein we report the first example of phosphonofluorination of
unactivated alkenes.
Figure 1. Proposed mechanism of radical phosphonofluorination of
unactivated alkenes.
phosphite might be oxidized by the proposed intermediate
Ag(III)F to generate Ag(II)F and a phosphonyl radical. The
electrophilic phosphonyl radical then adds intermolecularly to a
CC double bond to afford the corresponding nucleophilic
carbon-centered radical. Subsequent fluorine atom transfer
from Ag(II)F to the adduct radical produces the phosphono-
fluorination product and regenerates Ag(I), which enters into
the next catalytic circle. Driven by our interest in Ag(I)-
catalyzed radical reactions,11,15 we set out to explore this
possibility.
Thus, N-(pent-4-en-1-yl)phthalimide (A-1) was chosen as
the model substrate for the optimization of reaction conditions
(Table 1). The initial screening of solvents (entries 1−5)
indicated that, under catalysis with AgNO3 (20 mol %), the
condensation of A-1 with (EtO)2P(O)H and Selectfluor in
CH2Cl2/H2O at reflux (∼40 °C) gave the expected
phosphonofluorination product 1 in 18% yield, while most of
the alkene A-1 (>70%) was recovered. Note that no product 1
could be observed in the absence of water. Switching the
catalyst AgNO3 to AgOAc or AgOTf did not show much
difference. The reaction was completely inhibited by the
Our idea was based on our recent finding11 that the
c o m b i n a t i o n o f 1 - c h l o r o m e t h y l - 4 - fl u o r o - 1 , 4 -
Received: August 3, 2013
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ja408031s | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
Table 1. Optimization of Reaction Conditions
Scheme 1. Silver-Catalyzed Phosphonofluorination of
Unactivated Alkenes
a
b
entry
solvent
additive (equiv)
yield (%)
1
CH3CN/H2O (1:1)
Me2CO/H2O (1:1)
C6H6/H2O (1:1)
none
0
0
2
none
3
none
0
4
CH2Cl2/H2O (1:1)
none
18
20
0
5
CH2Cl2/H2O (1:2)
none
6
CH2Cl2/H2O (1:2)
Na2CO3 (1)
HOAc (2)
TFA (5)
none
7
CH2Cl2/H2O (1:2)
20
22
90
90
0
8
CH2Cl2/H2O (1:2)
9
CH2Cl2/H2O/TFA (1:2:1)
CH2Cl2/H2O/HOAc (1:2:1)
H2O/HOAc (2:1)
10
11
12
none
none
CH2Cl2/HOAc (1:1)
CH2Cl2/H2O/HOAc (1:2:1)
none
0
c
13
none
0
a
Conditions: A-1 (0.2 mmol), diethyl phosphite (0.4 mmol),
Selectfluor (0.4 mmol), AgNO3 (0.04 mmol), solvent (2 mL), 40
b
c
°C, 12 h. Isolated yield based on A-1. The reaction was run without
AgNO3.
addition of a base such as Na2CO3 (entry 6). We then turned to
acids for help. The addition of a small amount (2−5 equiv) of
acetic acid or trifluoroacetic acid (TFA) did not offer any
improvement (entries 7 and 8). However, when acetic acid or
TFA was used as the co-solvent, we were delighted to see that
the phosphonofluorination proceeded cleanly, and fluoride 1
was isolated in 90% yield (entries 9 and 10). The subsequent
control experiments revealed that all three co-solvents as well as
the catalyst AgNO3 were required for successful implementa-
tion of the reaction (entries 11−13). In addition, no reaction
occurred when the fluorine source was changed to N-
fluorobis(benzenesulfonyl)imide (NFSI). While the role of
acetic acid or TFA remains unclear, it might be that the stability
of the proposed Ag(III)F and/or Ag(II)F intermediates is
enhanced in acidic media.
Acetic acid is obviously superior to TFA as the co-solvent in
terms of the mildness of reaction conditions. Thus, the
optimized conditions (entry 10, Table 1) were employed to
explore the scope and limitations of the above phosphono-
fluorination of unactivated alkenes. The results are summarized
in Scheme 1. The alkene substrate scope proved quite general,
as not only various unactivated mono- and disubstituted
terminal alkenes but also di- and trisubstituted internal alkenes
participated in the phosphonofluorination reaction effectively.
In addition, substituted styrenes could also be used as
substrates, as exemplified by the synthesis of product 14.
Nevertheless, electron-deficient alkenes such as methyl acrylate
failed to give the desired products. A wide range of functional
groups were well tolerated, including unprotected and
protected alcohol, protected amine, alkyl chloride, ether,
sulfonate, ester, amide, ketone, and nitrile. However, easily
oxidized groups such as phenol and unprotected amine are not
compatible due to the oxidative nature of phosphonofluorina-
tion. Notably, the reaction of cyclohexene afforded the expected
phosphonate 25 as a 1:3 mixture of stereoisomers in favor of
the cis-isomer, indicating that the stereoselective phosphono-
fluorination could be achieved. Indeed, the reaction of 1-
substituted cyclohexene A-28 gave predominantly (∼10:1) the
a
Reaction conditions: alkene (0.2 mmol), diethyl phosphite (0.4
mmol), Selectfluor (0.4 mmol), AgNO3 (0.04 mmol), CH2Cl2 (0.5
b
mL), H2O (1 mL), HOAc (0.5 mL), 40 °C, 12−48 h. Isolated yield
based on the substrate alkene. 30 mol % of AgNO3 was used. The
substrate alkene was recovered in 47% yield.
c
d
cis-addition product 28 (eq 1). A high diastereoselectivity (8:1)
was also observed in the phosphonofluorination of 17-
methylene steroid A-29 (eq 2).
The above reactions could be extended to other types of
phosphonyl radicals as well. As demonstrated in eq 3,
diisopropyl phosphite, ethyl butylphosphinate, and even
dibutylphosphine oxide all participated nicely in the phospho-
nofluorination under the same conditions as above without
B
dx.doi.org/10.1021/ja408031s | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
12 h also led to the hydrophosphorylation of A-1 (∼50% yield)
only. The reaction of A-1 with (EtO)2P(O)H (2 equiv) and
AgF2 (2 equiv) in acetonitrile or CH2Cl2/H2O/HOAc also
failed to give fluoride 1. These results, in combination with our
previous finding in decarboxylative fluorination and in radical
aminofluorination,11 support the proposed mechanism in
Figure 1.
In conclusion, we have successfully developed the catalytic
phosphonofluorination of unactivated alkenes via condensation
with diethyl phosphite and Selectfluor, leading to the
convenient and efficient synthesis of β-fluorinated alkylphosph-
onates. The reaction proceeds under mild conditions in
aqueous media and enjoys a broad substrate scope and wide
functional group compatibility as well as good stereoselectivity.
The above results further expand the scope of radical
fluorination,6a,11,18,19 which is emerging as a versatile and
powerful tool for C(sp3)−F bond formation. Furthermore, the
silver-catalyzed oxidative generation of electrophilic radicals and
the silver-assisted fluorine atom transfer to nucleophilic alkyl
radicals illustrated in Figure 1 will allow the development of
more new radical fluorination methods. This is being actively
pursued in our laboratory.
ASSOCIATED CONTENT
* Supporting Information
■
S
Full experimental details, characterizations of new compounds,
1H, 13C and 19F NMR spectra, and complete ref 10e. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
clig@mail.sioc.ac.cn
■
Notes
further optimization. These results further expanded the
substrate scope of the reaction.
The authors declare no competing financial interest.
It is worth mentioning that in the synthesis of 10 and 13 no
competitive electrophilic fluorocyclization4 was observed. This
also sheds light on the mechanism of phosphonofluorination,
indicating it does not proceed through a carbocationic
intermediate. A more direct evidence for the radical mechanism
was the reaction of diene A-33 under the above optimized
conditions, which gave the cyclized products 33 (53% yield)
and 34 (26% yield), both in predominantly cis-configuration16
(cis/trans > 10:1) (eq 4). This result supports the radical
mechanism, as the cyclized radical may abstract a fluorine atom
(to give 33) or a hydrogen atom (to give 34). Furthermore,
vinyl cyclopropane A-35 was designed as the radical probe.17
Indeed, the reaction of A-35 afforded cleanly the ring-opened
product 35 in 47% yield in E-configuration, along with the
recovery of A-35 in 37% yield. This experiment provides solid
evidence for the intermediacy of carbon-centered radicals (the
adduct radicals in Figure 1) in the phosphonofluorination.
To gain more insight into the role of silver catalyst in the
above radical reactions, the following experiments were carried
out. When divalent silver complex Ag(Phen)2S2O8 (2 equiv)
was used as the oxidant, the reaction of A-1 with diethyl
phosphite (2 equiv) and Selectfluor (2 equiv) in CH2Cl2/H2O/
HOAc at reflux gave only the hydrophosphorylation14h product
in ∼30% yield, while no fluoride 1 could be detected. With
K2S2O8 (2 equiv) as the oxidant and AgNO3 (20 mol %) as the
catalyst, the treatment of alkene A-1 with (EtO)2P(O)H (2
equiv) and KF (2 equiv) in CH2Cl2/H2O/AcOH at reflux for
ACKNOWLEDGMENTS
■
Dedicated to Prof. Chengye Yuan on the occasion of his 90th
birthday. This project was supported by the NSFC (grants nos.
21072211, 21228202, 21272259, and 21290180) and by the
National Basic Research Program of China (973 Program, grant
no. 2011CB710805).
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