Selectively catalytic hydrodefluorination of perfluoroarenes by Co(PMe 3)4 with sodium formate as reducing agent and mechanism study

Article abstract of DOI:10.1039/c3dt50409c

Successful selective hydrodefluorinations of aryl fluorides were carried out in the presence of a cobalt catalyst supported by trimethylphosphine and with sodium formate as a reducing agent in acetonitrile or DMSO. Octafluorotoluene (1), pentafluoropyridine (2), hexafluorobenzene (3), pentafluorobenzene (3a) and perfluorobiphenyl (4) were studied to investigate the scope of this catalytic system. It was found that the fluorinated compounds 1, 2 and 4 with electron-withdrawing groups are more active than 3 and 3a. The catalytic hydrodefluorination mechanism is proposed and discussed with the support of the experimental results of the stoichiometric reactions and the in situ IR and NMR data.

Full text of DOI:10.1039/c3dt50409c

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Selectively catalytic hydrodefluorination of
perfluoroarenes by Co(PMe ) with sodium formate as
Cite this: Dalton Trans., 2013, 42, 13048
3 4
reducing agent and mechanism study†
a
a,b
a
a
Junye Li,‡ Tingting Zheng,‡ Hongjian Sun and Xiaoyan Li*
Successful selective hydrodefluorinations of aryl fluorides were carried out in the presence of a cobalt
catalyst supported by trimethylphosphine and with sodium formate as a reducing agent in acetonitrile or
DMSO. Octafluorotoluene (1), pentafluoropyridine (2), hexafluorobenzene (3), pentafluorobenzene (3a)
and perfluorobiphenyl (4) were studied to investigate the scope of this catalytic system. It was found that
the fluorinated compounds 1, 2 and 4 with electron-withdrawing groups are more active than 3 and 3a.
The catalytic hydrodefluorination mechanism is proposed and discussed with the support of the experi-
mental results of the stoichiometric reactions and the in situ IR and NMR data.
Received 12th February 2013,
Accepted 17th June 2013
DOI: 10.1039/c3dt50409c
www.rsc.org/dalton
C–F bond in unreactive monofluoroarenes with rhodium
1
. Introduction
2
0
complexes. Braun showed the first activation of an aromatic
C–F bond activation has attracted more and more attention C–F bond at a metal center in the presence of a C–Cl bond in
2
1
owing to the growth of the application of fluoro compounds in the same aromatic ring. The hydrodefluorination of fluori-
2
2
advanced materials, pharmaceuticals, agricultural chemicals nated alkenes proceeded at the rhodium center. Braun also
1
–15
and some other fields,
but catalytic C–F bond activation realized a stoichiometric and catalytic route to fluorinated pyri-
and functionalization with transition metal complexes as cata- dine derivatives based on C–F bond activation reactions of
16–24
23
lysts are still rare.
The first example of homogeneously cata- pentafluoropyridine at palladium.
The N-heterocyclic
lytic C–F bond activation by transition metal complexes was carbene complexes of ruthenium have shown to be precursors
the catalytic reaction between polyfluorobenzenes and hydro- for the catalytic hydrodefluorination of aromatic fluorocarbons
26
silanes with Rh(I) silyl coordination compounds as catalysts by in the presence of alkyl silanes. Hydrodefluorination of per-
2
5
Milstein. C–F bonds of C
6 6 6 5
F and C F H can be selectively fluorinated aromatic compounds with silane is catalyzed by the
hydrogenolyzed in a simple reaction with H by trimethylpho- addition of iron(II) fluoride complexes, giving mainly the singly
2
2
7
sphine complexes of rhodium as catalysts in the presence of a hydrodefluorinated products. Zhang and co-workers have found
base. Aqueous ammonia as the reaction medium was used that gold(I) complexes and copper(I) salts have good catalytic reac-
in the reductive defluorination of polyfluoroarenes by zinc in a tivity towards the hydrodefluorination of fluoroarenes using
highly selective manner. Rhodium hydrides with a penta- silanes as a hydrogen source. However, in most of the studies,
methylcyclopentadienyl ligand were used as potential catalysts either noble metals or high pressure hydrogen were used. In some
in C–F bond activation of polyfluoroaromatics. With a ketone cases, more expensive silanes were added as reducing agents.
carbonyl group as the directing group, rhodium-catalyzed C–F Recently, we have reported the selective C–F/C–H bond
bond activation via a Si–F exchange reaction between fluoro- activation of fluoroarenes by a cobalt complex supported with
1
6
1
7
28
1
8
1
9
29
benzenes and a disilane was reported by Murai. Grushin dis- phosphine ligands. The C–H bond activation product, (C F )
6
5
closed his study results on the catalytic hydrogenolysis of a Co(PMe ) , was obtained as the main product through the
3
4
3 4
reaction of Co(PMe ) with pentafluorobenzene. The hydro-
defluorination products, 1,2,4,5-C F H and F PMe , were also
a
6
4
2
2
3
School of Chemistry and Chemical Engineering, Key Laboratory of Special
observed as the byproducts. The reaction mechanism includ-
ing a hydrido cobalt(II) intermediate (A) and ligand exchange
intermediate, tris(trimethylphosphine)pentafluorophenylcobalt(II)
fluoride (B), was proposed and partly-experimentally verified
Functional Aggregated Materials, Ministry of Education, Shandong University,
Shanda Nanlu 27, 250100 Jinan, People’s Republic of China.
E-mail: xli63@sdu.edu.cn; Fax: +86 531 88361350
b
Department of Chemistry, Capital Normal University, 100037 Beijing, People’s
Republic of China
(
Scheme 1).
†
1
‡
Electronic supplementary information (ESI)
0.1039/c3dt50409c
available. See DOI:
Based on the above research on the stoichiometric reactions
and mechanism study, we selected Co(PMe ) as the catalyst
3 4
These authors contributed equally to this work.
1
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under nitrogen and are uncorrected. GC-MS were recorded on
a TRACE-DSQ.
2
.2. Catalytic study
In a typical reaction, the substrate (0.5 mmol) was dissolved in
dried DMSO or CH CN (3 mL) together with HCOONa
0.75 mmol), stirring for 1 minute. After the addition of
Co(PMe (0.05 mmol), the suspension was stirred for 3 h at the
3
(
3 4
)
given temperature under oxygen-free nitrogen. The solution
turned to orange from brown. The products and their yields
1
9
were determined from the catalytic solutions by F NMR
1
9
spectra. The F NMR was measured with trifluoromethyl-
toluene as the external standard.
2.2.1. Isolation of 1a. The reaction mixture was quenched
with dilute HCl (1 M) and extracted by diethyl ether, dried over
MgSO4 and filtered. Compound 1a as a colorless liquid was
Scheme 1
obtained by distillation (boiling range 110–115 °C). Yield:
1
0
.076 g (69.7%). H NMR (CDCl
3
, 300 MHz): δ 7.12–7.42 (m,
1
9
4
Aryl-H). F NMR (CDCl , 282 MHz): δ −56.62 (t, 3F, J = 22.5
3
FF
for the catalytic hydrodefluorination of aryl fluorides. The reac-
tion of the aryl fluorides (compounds 1, 2, 3, 3a and 4) with
sodium formate in the presence of the cobalt catalyst selec-
tively leads to hydrodefluorination products at para-
positions (exception of 3). In the case of 3, the conversion
to pentafluorobenzene was poor, while 1,2,4,5-tetrafluoro-
benzene (3b) was obtained from 3a in a high yield. From
compound 4, 2,3,5,6,2′,3′,4′,5′,6′-nonafluorobiphenyl (4a) or
+
Hz), −136.3 (m, 2F), −140.2 (m, 2F). GC-MS (m/z): M : 217.9.
.2.2. Isolation of 2a. A similar method as that for 1a was
used to isolate 2a (boiling range 98–105 °C). Yield: 0.046 g
61.3%). H NMR (CDCl
CDCl , 282 MHz): δ −90.7 (m, 2F), −139.7 (m, 2F). C NMR
CDCl , 75.5 MHz): δ 143.7 (m, J = 215.1 Hz, J = 15.1 Hz),
41.7 (m, JCF = 264.2 Hz), 119.1 (tt, JCF = 20.3 Hz, JCF = 2.64
Hz). GC-MS (m/z): M : 150.9.
.2.3. Confirmation of 3a. Compound 3a could not be iso-
2
1
19
(
(
(
1
3
, 300 MHz): δ 7.63 (m). F NMR
1
3
3
1
2
3
CF
CF
1
2
3
+
2
,3,5,6,2′,3′,5′,6′-octafluorobiphenyl (4b) could be selectively
2
obtained depending on the different reaction conditions.
A reaction mechanism of this hydrodefluorination process is
proposed and supported by in situ IR experiments. At the
present time, the hydrodefluorination reaction is one of
the important synthetic tools for the generation of partially-
fluorinated building blocks from readily available perfluori-
1
9
lated due to the low yield. F NMR: δ −139.0 (m, 2F), −153.8
t, 1F), −162.7 (m, 2F). GC-MS (m/z): M : 167.9.
.2.4. Confirmation of 3b. Compound 3b could not be
+
(
2
1
9
isolated due to the low yield. F NMR: δ −140.1 (m). GC-MS
+
(
m/z): M : 150.1.
3
0
2.2.5. Isolation of 4a. The reaction mixture was quenched
with dilute HCl (1 M), extracted by diethyl ether, dried over
MgSO4 and filtered. 4a as a white solid was obtained by
nated bulk chemicals. Until now, there has been only one
example of a cobalt mediated hydrodefluorination reaction,
1
4
reported by Holland. In this paper, several derivatives of
polyfluoroarenes were selectively obtained through cobalt-cata-
lyzed hydrodefluorination under mild conditions. It is not easy
to prepare some of these products directly with classic routes
column chromatography (silica) using petroleum ether
1
(
3
60–90 °C) as an eluent. Yield: 0.12 g (75.9%). H NMR (CDCl
3
,
1
9
3
00 MHz): δ 7.13–7.02 (m). F NMR (CDCl , 282 MHz):
3
1
δ −137.4 (m, 4F), −138.2 (m, 2F), −150.2 (t, 1F), −160.5 (m, 2F).
of organic synthesis.
1
3
C NMR (CDCl
3
, 75.5 MHz): δ 147.6 (m), 145.8 (m), 144.3 (m),
2
3
1
42.3 (m), 139.5 (m), 108.1 (tt, JCF = 97.3 Hz, JCF = 5.2 Hz).
+
GC-MS (m/z): M : 316.0. M.p.: 80 °C.
.2.6. Isolation of 4b. A similar method as that for 4a was
2. Experimental
2
1
2
.1. General procedures and materials
3
used to isolate 4b. Yield: 0.12 g (80.5%). H NMR (CDCl ,
00 MHz): δ 7.20–7.31 (m). F NMR (CDCl , 282 MHz):
1
9
3
Standard vacuum techniques were used in the manipulations
of volatile and air sensitive materials. Solvents were dried by
known procedures and distilled under nitrogen before use. Lit-
3
1
3
δ −137.7 (m, 4F), −138.2 (m, 4F). C NMR (CDCl
3
, 75.5 MHz):
1
1
δ 146.2 (m, JCF = 272.5 Hz), 144.4 (m, JCF = 236.3 Hz), 107.8
2
+
3
2
(
m, J = 22.6 Hz). GC-MS (m/z): M : 298.0. M.p.: 84 °C.
CF
erature methods were used in the preparation of Co(PMe
3
)
4
.
Aryl fluorides 1, 2, 3, 3a and 4 were obtained from ABCR. All
other chemicals were used as purchased. The in situ IR ana-
lysis was carried out on a METTLER TOLEDO React IR IC 15.
3
. Results and discussion
1
13
19
3.1. Cobalt-catalyzed selective C–F bond activation and
hydrodefluorination of perfluoroarenes
H, C and F NMR spectra (300, 75 and 282 MHz, respectively)
1
3
were recorded on a Bruker Avance 300 spectrometer. C NMR
resonances were obtained with broadband proton decoupling. In this cobalt-catalyzed C–F bond activation system, the
The melting points were measured in capillaries sealed hydrodefluorination of 1, 2, 3, 3a and 4 as substrates was
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Table 1 Hydrodefluorination of 1 in different solvents
ð2Þ
Solvents
DMSO
NMP
DMF
3
CH CN
Toluene
Dioxane
Conversion (%)
Yield (%)
100
46.5
100
53.7
100
68.5
100
85.6
70.0
34.9
61.4
37.7
a
a
19
The yields were determined by F NMR spectroscopy with trifluoromethyltoluene as the external standard.
investigated with tetrakis(trimethylphosphine)cobalt(0) as a
catalyst and sodium formate as a reducing agent (eqn (1)).
The catalytic mono-hydrodefluorinations of the other fluori-
nated arenes with Co(PMe ) are listed in Table 3. All of these
3
4
transformations have good chemo- and regioselectivity. The
hydrodefluorination of 1 took place selectively at position
3
. Therefore, 1,2,4,5-tetrafluoro-3-(trifluoromethyl)benzene (1a)
ð1Þ
was obtained in a yield of 85.6%. Under the same conditions
the hydrodefluorination of 2 proceeds rapidly, affording
2
,3,5,6-tetrafluoropyridine (2a) in a yield of 72.7%. With per-
Initially, compound 1 was selected as a model substrate to
scan the catalytic conditions (eqn (2)). The experimental
results are summarized in Tables 1 and 2. From Table 1 it can
be concluded that the best solvent among the five tested sol-
vents for this catalysis is acetonitrile. Although the conversions
in some other solvents can also reach 100%, the selectivity is
poor. We considered two reasons for the results. For DMSO,
NMP and DMF, which have a larger polarity than that of
fluorobiphenyl (4) as the substrate, the catalytic reaction leads
to the mono-hydrodefluorinated product, 2,3,5,6,2′,3′,4′,5′,6′-
nonafluorobiphenyl (4a), with a conversion of 100% in a yield
of 82.6%. When the reaction time was increased from 3 h to
8
h, with a catalyst loading of 20% and 3 eq. of HCOONa, the
selective hydrodefluorination in DMSO took place to give rise
to the di-hydrodefluorinated product, 2,3,5,6,2′,3′,5′,6′-octa-
fluorobiphenyl (4b), in a yield of 85.7% and with a conversion
of 100%. In contrast to the above results, the catalytic trans-
formation of 3 was poor, with a yield of only 9.2% for 3a. Even
in DMSO, the yield is 11%, although the selectivity is 100%.
However, 3b was obtained in a yield of 51.2% when using 3a
as a substrate in DMSO. The para-(C–F) bond to the C–H bond
in pentafluorobenzene 3a was exclusively activated.
3
CH CN, the catalytic reactions are very fast and more side reac-
tions occur. For 1,4-dioxane and toluene, which have a smaller
polarity than that of CH CN, the catalytic reactions are slow. It
3
is suggested that a strong polarity of the reaction medium
would be beneficial for increasing the reaction rate. It is
important for this catalytic system to have a solvent with a suit-
able polarity, like acetonitrile. Table 2 shows that the reaction
yield increases from 35.6% to 85.6% when raising the tempera-
ture from 30 °C to 80 °C with CH CN as the solvent (entries 3.2. Mechanistic study of the catalytic hydrodefluorination
3
1
–3). At 80 °C, the reaction yield increases from 82.2% to reaction
8
5.6% when the reaction time is prolonged from 0.5 h to 3 h
According to the results of the hydrodefluorination of perfluoro-
arenes and the reported experimental results of the stoichio-
(entries 5–3).
metric reactions of the selective C–F/C–H bond activation of
2
9
fluoroarenes and the mechanism study in Scheme 1, a mech-
anism of the catalytic hydrodefluorination is proposed in
Scheme 2. The first step is the oxidative addition of the C–F
Table 2 Hydrodefluorination of 1 in CH
different times
3
CN at different temperatures and in
a
Entry
Time (h)
Temperature (°C)
Conversion (%)
Yield (%) bond of the aryl fluoride at the cobalt(0) center to afford a Co(II)
fluoride a. A similar complex to complex a was isolated in
1
2
3
4
5
6
3
3
3
1.5
0.5
3
30
50
80
80
80
80
42.5
52.7
100
95.1
90.1
8.0
35.6
36.3
85.6
83.3
82.2
33
−
our previous work.
F ligand substitution of intermediate a
by a formate anion delivers intermediate b with the co-
ordinated carboxyl group. Decarboxylation of intermediate b
gives rise to the hydrido cobalt(II) intermediate, c, with the
b
0
escape of CO . The subsequent ligand exchange of the hydrido
2
a
1
3 4
mmol substrate, 10 mol% Co(PMe ) , 1.5 eq. HCOONa. The yields
19
H atom by the F atom of the perfluoroarenes yields the hydro-
defluorination product along with the recovery of the staring
Co(II) fluoride a to complete this catalytic cycle.
were determined by F NMR spectroscopy with trifluoromethyltoluene
as the external standard. The experiment was carried out without the
b
catalyst, 1.5 eq. HCOONa.
1
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Table 3 Cobalt-catalyzed hydrodefluorination of polyfluoroarenes
a
a
Substrate
Product
Solvent
Time (h)
3
Conversion (%)
100
Yield (isolated) (%)
TON
CH
CH
CH
3
CN
3
CN
3
CN
85.6 (69.7)
8.56
3
84.6
72.7 (61.3)
7.27
3
3
19.0
11.1
9.2
11.1
0.09
1.11
DMSO
CH
DMSO
3
CN
3
3
12.7
73.3
7.3
51.2
1.27
5.12
CH
3
CN
3
8
100
100
82.6 (75.9)
8.26
8.57
b
DMSO
85.7 (80.5)
a
19
1
3 4
mmol substrate, 10 mol% cat. Co(PMe ) , 1.5 eq. HCOONa, 80 °C. The data were determined by F NMR spectroscopy with trifluoromethyl
b
3 4
toluene as the external standard. 1 mmol substrate, 20 mol% cat. Co(PMe ) , 3.0 eq. HCOONa, 80 °C.
Scheme 3 The in situ IR spectra of the hydrodefluorination of 1 (all the IR
spectra of the reaction with CH CN as background).
3
and then the temperature was increased to 80 °C. The color of
the reaction solution turned to orange from brown.
Scheme 2 Proposed mechanism of the catalytic hydrodefluorination.
According to the results from the in situ IR spectra
Schemes 3 and 4), the peak at 1606 cm− indicates the pres-
ence of intermediate b with a coordinated carboxyl group,
1
(
This mechanism was also supported by the experimental while this peak is at 1595 cm− for the free formate. The Co–H
1
34
1
in situ IR and H NMR spectroscopy. In the in situ IR spec- bond vibration of the hydrido intermediate c appears at
troscopy, the model reaction of the hydrodefluorination of 1906 cm− . The hydrido resonance of the Co–H bond in the H
1
1
compound 1 was taken to track the reaction process. The sub- NMR spectra in DMSO-D6 was observed at −15.1 ppm as a
strate 1 was dissolved in dried CH CN together with HCOONa. broad peak with the reaction of compound 4 and a stoichio-
3
After the addition of Co(PMe
3 4 3 4
) , the suspension was stirred for metric amount of Co(PMe ) as well as 3.0 eq. of HCOONa. In
5
minutes at room temperature under oxygen-free nitrogen Scheme 4, the developing trends of the specific peak intensity
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3
4
K. Fuchibe and T. Akiyama, J. Am. Chem. Soc., 2006, 128,
1434.
S. A. Garratt, R. P. Hughes, I. Kovacik, A. J. Ward,
S. Willemsen and D. Zhang, J. Am. Chem. Soc., 2005, 127,
15585.
5
6
R. P. Hughes, R. B. Laritchev, L. N. Zakharov and
A. L. Rheingold, J. Am. Chem. Soc., 2005, 127, 6325.
U. Jaeger-Fiedler, P. Arndt, W. Baumann, A. Spannenberg,
V. V. Burlakov and U. Rosenthal, Eur. J. Inorg. Chem., 2005,
2842.
Scheme 4 The peak intensity trend of the characteristic peaks.
7
8
9
I. M. Piglosiewicz, S. Kraft, R. Beckhaus, D. Haase and
W. Saak, Eur. J. Inorg. Chem., 2005, 938.
A. Steffen, M. I. Sladek, T. Braun, B. Neumann and
H.-G. Stammler, Organometallics, 2005, 24, 4057.
E. L. Werkema, E. Messines, L. Perrin, L. Maron,
O. Eisenstein and R. A. Andersen, J. Am. Chem. Soc., 2005,
are illustrated. In addition, the peak from the C–H bond at
031 cm− gradually appeared with a decrease in the intensity
1
1
−1
of the peak at 1002 cm from the C–F bond of the substrate.
The formation of a white precipitate of NaF, confirmed by the
1
9
F NMR spectra (δ: −114 ppm), in the solution also supports
this putative reaction mechanism.
127, 7781.
10 D. H. Shen, Q. L. Yu and L. Lu, J. Org. Chem., 2012, 77, 1798.
1 J. A. Hatnean and S. A. Johnson, Organometallics, 2012, 31,
1
1
361.
2 (a) A. D. Sun and J. A. Love, Org. Lett., 2011, 13, 2750;
b) T. G. Wang, L. Keyes, B. O. Patrick and J. A. Love, Organo-
4. Conclusions
1
(
In summary, the successful selective hydrodefluorination of
3 4
aryl fluorides was carried out in the presence of a Co(PMe )
metallics, 2012, 31, 1397.
1
1
1
1
1
3 P. Fischer, K. Götz, A. Eichhorn and U. Radius, Organome-
tallics, 2012, 31, 1374.
4 T. R. Dugan, J. M. Goldberg, W. W. Brennessel and
P. L. Holland, Organometallics, 2012, 31, 1349.
5 Y. Nakamura, N. Yoshikai, L. Llies and E. Nakamura, Org.
Lett., 2012, 14, 3316.
catalyst and with sodium formate as a reducing agent. Aryl flu-
orides 1, 2, 3, 3a and 4 as substrates were studied to investigate
the scope of this catalytic system. It was found that the fluori-
nated compounds 1, 2 and 4 with electron-withdrawing groups
are more active than 3 and 3a. The catalytic hydrodefluorina-
tion mechanism is proposed and discussed with the support
of the experimental results of the stoichiometric reactions and
the in situ IR and NMR data.
6 M. Aizenberg and D. Milstein, J. Am. Chem. Soc., 1995, 117,
8
674.
7 S. S. Laev and V. D. Shteingarts, Tetrahedron Lett., 1997, 38,
765.
18 B. L. Edelbach and W. D. Jones, J. Am. Chem. Soc., 1997,
19, 7734.
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998, 157.
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94.
3
Acknowledgements
1
We gratefully acknowledge the support of the NSF China, No.
1
2
2
2
1172132.
1
2
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