AgF-mediated fluorinative homocoupling of gem -difluoroalkenes

Article abstract of DOI:10.1021/ol403083e

A novel silver(I)-fluoride-mediated homocoupling reaction of β,β-difluorostyrene derivatives is described. The transformation is initiated via nucleophilic addition of silver(I) fluoride to β,β-difluorostyrenes, which is followed by dimerization of the corresponding benzylsilver intermediates. The reaction shows good substrate scope, functional group tolerance, and represents the first report on the reactivity of (α-trifluoromethyl)benzylsilver species.

Full text of DOI:10.1021/ol403083e

Letter
pubs.acs.org/OrgLett
AgF-Mediated Fluorinative Homocoupling of gem-Difluoroalkenes
Bing Gao, Yanchuan Zhao, Chuanfa Ni, and Jinbo Hu*
Key Laboratory of Organofluorine Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Ling-Ling
Road, Shanghai 200032, China
S
* Supporting Information
ABSTRACT: A novel silver(I)-fluoride-mediated homocoupling reaction
of β,β-difluorostyrene derivatives is described. The transformation is
initiated via nucleophilic addition of silver(I) fluoride to β,β-difluoro-
styrenes, which is followed by dimerization of the corresponding
benzylsilver intermediates. The reaction shows good substrate scope, functional group tolerance, and represents the first
report on the reactivity of (α-trifluoromethyl)benzylsilver species.
Scheme 1. Variations from Trifluoromethyl Metallic
Complexes to α-Trifluoromethylated Alkyl Metallic
Complexes
electively fluorinated or fluoroalkylated organic molecules
Soften possess unique physical, chemical, and biological
properties, making them of special importance in agro-
chemicals, pharmaceuticals, as well as functional materials.1
Transition-metal-mediated fluorination and fluoroalkylation,
which provide fast and mild access to fluorine-containing
organic molecules, have become the subject of tremendous
research activity over the past decade.2 The preparation and
characterization of the corresponding Rf−M organometallic
complexes were carried out by several groups, the majority of
which mainly focused on “CF3−Pd(II/III/IV)”,3 “CF3−Cu(I/
III)”,4 “CF3−Ag(I/III)”,5 and other “CF3−M” (M = Ni, Pt, Ru,
Rh) species.6 These trifluoromethyl metallic complexes are
generally stable at low oxidation states and isolable with proper
ligands.
Despite these important examples of trifluoromethyl metallic
compounds, there are very few studies on partially fluorinated
alkyl organometallic compounds.7 The introduction of fluorine
atom(s) might dramatically change the chemical properties of
corresponding organometallic species. For instance, CH3−
PdAr(DPPBz) and PhCH2−PdAr(DPPBz) species [DPPBz:
1,2-bis(diphenylphosphino)benzene] undergo complete reduc-
tive elimination within 4 h at 40 °C, while the reductive
elimination of CF3CH2−PdAr(DPPBz) has to be conducted at
110 °C for 36 h.8a Recently, our group reported a palladium-
catalyzed 2,2,2-trifluoroethylation of organoboronic derivatives
that was accomplished within 12 h at 80 °C; a bulky phosphine
ligand (Xantphos) was employed to accelerate the crucial
reductive elimination step as well as the whole catalytic cycle
consequently.8b Encouraged by this success, we sought to
explore other related fluoroalkyl metallic complexes with
different central metal atoms. Herein, we disclose our results
on the study of an α-trifluoromethylated benzylsilver species
[CF3CH(Ar)-AgI] (Scheme 1).
MgIIXn species makes the transmetalation from the correspond-
ing Grignard reagents to silver metal center difficult (Scheme
2).1a,b Traditionally, the nucleophilic addition of AgF to
perfluoroalkenes provides an alternative approach to perfluoro-
alkylsilver compounds (Rf−AgI), taking advantage of the high
electrophilicity of perfluoroalkenes.11a gem-Difluoroalkenes, on
the other hand, although less electron-deficient, have also been
employed to synthesize 18F-labeled CF3-containing molecules
Scheme 2. Proposed Strategy for the Preparation of α-
Trifluoromethylated Alkylsilver Compounds
Currently, the development of practical approaches toward
α-trifluoromethylated alkylsilver compounds still remains a
challenging task. Unlike R−PdII (R = alkyl, alkenyl, and aryl)
species, the R−AgI compounds are inaccessible via direct
oxidative addition of alkyl halide to metal (Ag0) on the
benchtop,9,10 and a facile β-fluoride elimination of CF3CHR−
Received: October 26, 2013
© XXXX American Chemical Society
A
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Letter
for positron emission tomography (PET) with [18F]KF under
relatively harsh reaction conditions.11b Therefore, we envi-
sioned that it could be possible to use gem-difluoroalkenes as
precursors for the preparation of CF3CHR−AgI compounds
(Scheme 2).
Table 1. Optimization of Reaction Conditions
Alkyl-substituted gem-difluoroalkene 1 was first examined
with 1.0 equiv of AgF in DMSO at 80 °C for 6 h, as shown in
Scheme 3. However, only hydrofluorination product 2, rather
b
AgF
temp
time
(h)
yield
(%)
a
entry
(equiv)
solvent (0.1 M)
DMSO
(°C)
1
2
3
4
5
6
1.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
80
80
6
6
6
6
6
6
6
6
6
19
39
27
15
37
49
0
DMSO
DMSO
CH3CN
HMPA
pyridine
toluene
dioxane
Scheme 3. Impact of Substituents of gem-Difluoroalkenes
130
80
80
80
c
7
80
c
8
80
0
9
pyridine−THF
80
58
(1:1)
d
10
4.0
3.0
3.0
3.0
pyridine−THF
80
80
80
80
6
6
6
6
63
62
61
25
(1:1)
e
11
pyridine−THF
than the corresponding metallic compound, was obtained in
18% yield with 73% of 1 being recovered. Poor thermal stability
of alkylsilver compound was considered to be responsible for
the failure of 1, and the introduction of an aromatic
functionality seemed necessary for two reasons: (1) β,β-
difluorostyrene derivatives are more electrophilic and therefore
should facilitate the nucleophilic addition at relatively low
temperatures; (2) the newly formed C−AgI bond can be
stabilizied by the aromatic ring. Accordingly, 2-(2,2-difluoro-
vinyl)naphthalene (3a) was next employed as a model
precursor that was treated under the same reaction conditions.
Nevertheless, no benzylsilver compound, but homocoupling
product 4a (19%), was observed along with hydrofluorination
side product (Scheme 3).
It was reported that alkylsilver compounds could readily
undergo homocoupling to give alkyl dimers, and general
methodologies of silver-catalyzed oxidative homocoupling of
Grignard reagents have been developed.9 Although the
homocoupling products of perfluoroalkylsilver compounds
were observed during their decomposition process,11a its low
efficiency could be a hurdle for practical applications. On the
other hand, the synthetic transformations based on α-
trifluoromethylated benzylsilver species still remain unrevealed,
which stimulated us to explore such a transformation.
The optimization of the reaction conditions was commenced
using 3a as the model substrate, as shown in Table 1. The
transformation proceeded very slowly below 80 °C but turned
messy at 130 °C, giving 80 °C as a preferred option. The yield
of 4a was increased to 39% by increasing the amount of AgF to
4.0 equiv, and further screening of the solvents revealed that the
transformation could also proceed in CH3CN, HMPA, and
pyridine. It should be noted that although CH3CN was
extensively used in the preparation of perfluoroalkylsilver
metallic compounds,11a pyridine proved to be even better in
this case, which might serve as both solvent and ligand for the
organosilver intermediate (entry 7).12 Apolar solvents like
dioxane and toluene were inefficient owing to the insolubility of
AgF in these solvents (entries 8 and 9). On the other hand,
decomposition of AgF was observed in pyridine, and the
addition of THF as cosolvent (1:1 in volume) was expected to
slow such a process (entry 8). Additives like molecular sieves
gave a slightly higher yield (63%, entry 11). Finally, the reaction
was performed on both 0.5 mmol scale (0.1 M in
concentration) and 1.0 mmol scale (0.2 M in concentration)
(1:1)
f
12
pyridine−THF
(1:1)
g
13
pyridine−THF
(1:1)
a
b
All reactions were run on 0.2 mmol scale of substrate 3a. NMR yield
determined by 19F spectrospcopy with 1,3,5-trifluorobenzene as an
c
internal standard. Starting material 3a was quantitatively recovered.
Molecular sieves was added (10 mg/mL). The reaction was run on
0.5 mmol scale. The reaction was run on 1.0 mmol scale. The
d
e
f
g
reaction was run with a concentration of 0.0125 M.
with only 3.0 equiv of AgF being used, both providing
reproducible results. Attempts to lower the concentration to
0.0125 M decreased the yield to 25%. Moreover, it should be
mentioned that all reactions were conducted under the
protection from light to suppress the decomposition of AgF.
With the optimal reaction conditions in hand, the substrate
scope of the homocoupling reaction was explored. As
summarized in Table 2, a variety of β,β-difluorostyrene
derivatives bearing different substituents were examined with
satisfactory results. For precursors (3b−3d) with electron-
donating substituents, such as −NPh2, −OMe, and −SMe para
to the β,β-difluorovinyl functionality, the transformation
proceeded smoothly. Linear alkyl-substituted compound 4e
was obtained in good yield (65%, entry 5), indicating the
tolerance of ester. Halogen substituents such as iodo and
bromo were also tolerated under the current conditions (4f,
4j), which alllows further elaborations of these interesting
products (e.g., palladium-catalyzed polymerization for the
preparation of functional materials).13 Switching the position
of substituents would affect the reaction output as 4k was
obtained in 40% yield while 4g was isolated in 65%. It could be
ascribed to the steric repulsion between bromine and the β,β-
difluorovinyl group. Diastereoisomers of the majority of
products, albeit in poor diastereoselectivity, could be separated
via column chromatography. One diastereoisomer of 4m,
characterized by X-ray crystallography, was confirmed with a
syn configuration, and the syn/anti ratios of other products were
determined accordingly.
In general, the introduction of electron-deficient substituents
should facilitate the electron polarization of the π-obital of gem-
difluoroalkenes and thus favor the nucleophilic addition of
AgF.1b However, when 3m was treated under our standard
B
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Letter
Table 2. Survey of Substrate Scope
Scheme 4. Unexpected Result of Alkene 3m
cleavage of the C−Ag bond followed by random radical
reactions, and (2) concerted process involving both C−Ag
bond breaking and C−C bond formation.9 In our reaction,
when 3.0 equiv of TEMPO (2,2,6,6-tetramethyl-1-oxyl-
piperidine) was added under standard reaction conditions
with 3a as substrate, homocoupling was completely inhibited
and 6 was isolated in 87% yield (Scheme 5), which might
a
In all cases, the reactions were run on 1.0 mmol scale using 3.0 equiv
of AgF and 50 mg of molecular sieves in cosolvent of pyridine and
THF (2.5 mL:2.5 mL) without light. Isolated yield. Diastereose-
lectivity determined by 19F spectroscopy is given in parentheses.
b
c
Scheme 5. Mechanistic Study of the Homocoupling Reaction
reaction conditions, compounds 5a and 5b were formed instead
of the expected homocoupling product 4m (Scheme 4). 5b was
further confirmed by X-ray crystallography with a Z
configuration. We assumed that 5a and 5b might derive from
the oxidation of 4m by excess AgF in the reaction system since
4m, 5a, and 5b were all observed when the amount of AgF was
minimized to 1.5 equiv.
To gain further mechanistic insights, 4m was separately
prepared14 and treated with standard homocoupling conditions.
A full conversion of 4m into 5a and 5b with a ratio of 1.0:1.6
was observed. However, 4c could not be oxidized under
identical conditions. Although the detailed oxidation mecha-
nism was still unclear, our experiments indicate that AgF acts as
both an oxidant and a base throughout the transformation.15
The different performances between 4c and 4m was ascribed to
the high acidity of the benzylic C−H bond of 4m caused by the
−CN group.
support a free radical pathway. However, a concerted pathway
could not be ruled out since, in pyridine, an aggregation state of
perfluoroalkylsilver species has also been proposed based on
NMR study.12
Moreover, we were wondering whether the incorporation of
F− source and other silver salts could replace AgF to promote
the same transformation.16 When 3b was treated with
conditions A (CsF), B (AgF), and C (CsF and AgBF4), the
hydrofluorination product 7, homocoupling product 4b, and
unreacted 3b were observed, the yields of which were
determined by 19F NMR (as illustrated in Scheme 5).
According to these experimental results, gem-difluoroalkene
According to previous reports,9 perfluoroalkylsilver com-
pounds were relatively stable at room temperature in polar
solvents and were therefore capable of participating in synthetic
transformations. In sharp contrast, although the addition of
salts (LiBr, MgBr2) could increase their stability, neat alkylsilver
species were extremly unstable and would undergo quick
decomposition even below 0 °C. This might explain why our
endeavor to directly observe a benzylsilver intermediate proved
unsuccessful.
Two plausible mechanistic pathways have been proposed for
the homocoupling of alkylsilver compounds: (1) homolytic
C
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Letter
3b should be inert to the nucleophilic addition of F− anion at
80 °C, and a synergetic addition of AgF to double bond proved
crucial for the generation of benzylsilver intermediate. The
combination of CsF and AgBF4 provided an alternative
approach to AgF.
In conclusion, we have developed an efficient fluorinative
homocoupling reaction of gem-difluoroalkenes. Both the
generality of substrate scope and functional group tolerance
are demonstrated. The dimers possess unique chemical
structure, which might find applications in life science- and
materials-science-related fields.17 Although the key intermediate
is unstable at the temperature necessary for its formation, to
our knowledge, it is the first example in which the chemistry of
α-trifluoromethylated benzylsilver species is demonstrated.
Further development of new methodologies based on this
unique species is underway in our laboratory.
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ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures, compound characterization data.
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: jinbohu@sioc.ac.cn.
Notes
(7) (a) Burton, D. J.; Hartgraves, G. A. J. Fluorine Chem. 2007, 128,
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43, 747.
(8) (a) Culkin, D. A.; Hartwig, J. F. Organometallics 2004, 23, 3398.
(b) Zhao, Y.-C.; Hu, J.-B. Angew. Chem., Int. Ed. 2012, 51, 1033.
(9) Michael, H. Silver in Organic Chemistry; Wiley: Hoboken, NJ,
2010 and references therein.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Basic Research
Program of China (2012CB821600, 2012CB215500), the
National Natural Science Foundation of China (21372246,
20825209), and the Chinese Academy of Sciences.
(10) Klabunde, K. J. J. Fluorine Chem. 1976, 7, 95.
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