Platinum on carbon-catalyzed hydrodefluorination of fluoroarenes using isopropyl alcohol-water-sodium carbonate combination

Article abstract of DOI:10.1002/adsc.201100927

We have developed a platinum on carbon-isopropyl alcohol-catalyzed and widely applicable defluorination method for fluoroarenes, and the addition of water and sodium carbonate efficiently accelerated the reaction. The defluorination readily occurred under the reaction conditions in comparison with the dehalogenation of other aromatic halides (fluorine>chlorine> bromine≤laquo;iodine). Copyright

Full text of DOI:10.1002/adsc.201100927

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DOI: 10.1002/adsc.201100927
Platinum on Carbon-Catalyzed Hydrodefluorination of
Fluoroarenes using Isopropyl Alcohol-Water-Sodium Carbonate
Combination
a
a
a
a
Yoshinari Sawama, Yuki Yabe, Masahiro Shigetsura, Tsuyoshi Yamada,
a
a
a,b
a
Saori Nagata, Yuta Fujiwara, Tomohiro Maegawa, Yasunari Monguchi,
a,
and Hironao Sajiki *
a
Laboratory of Organic Chemistry, Gifu Pharmaceutical University, 1-25-4 Daigaku-nishi, Gifu 501-1196, Japan
Fax : (+81)58-230-8109; e-mail: sajiki@gifu-pu.ac.jp
b
Current address: Graduate School of Pharmaceutical Sciences, Osaka University, 1–6 Yamada-oka, Suita, Osaka
65-0871, Japan
5
Received: November 29, 2011; Published online: March 13, 2012
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201100927.
[
6]
[7]
Abstract: We have developed a platinum on
carbon-isopropyl alcohol-catalyzed and widely ap-
plicable defluorination method for fluoroarenes,
and the addition of water and sodium carbonate ef-
ficiently accelerated the reaction. The defluorina-
tion readily occurred under the reaction conditions
in comparison with the dehalogenation of other ar-
omatic halides (fluorine>chlorine>bromine@
iodine).
fluorinated by homogeneous or heterogeneous
transition metal-catalyzed hydrogenations under
harsh conditions such as high pressure and tempera-
ture and/or over a long reaction time together with
side-reactions (reduction of aromatic nuclei and re-
ducible functionalities). Herein, we report a heteroge-
neous Pt/C-catalyzed HDF of mono- and polyfluoro-
AHCTUNGTRENNUNG
2
2
3
tives.
Keywords: chemoselectivity; defluorination; haloar-
enes; heterogeneous catalysis; platinum
We were fascinated by the utility of heterogeneous
catalysts in terms of ease of handling and recoverabil-
ity of the catalyst without metal leaching into the
products and recently reported the efficient dechlori-
nation of chloroarenes including poisonous poly-
chlorinated biphenyls (PCB) by the platinum metal
[
8]
The activation of carbon (C)-fluorine (F) bonds, on activated carbon-catalyzed hydrogenation of aro-
[
9]
which are the thermally and chemically most stable matic nuclei. During the investigation of the Pt/C-
[
1]
bonds in organic compounds, is an attractive and catalyzed hydrogenation, bis(4-fluorophenyl)methane
[
2]
challenging subject in organic chemistry. The hydro- was found to be reduced to give defluorinated cyclo-
defluorination reaction (HDF) of aromatic fluorides hexane derivatives in i-PrOH at a high temperature
is a kind of CÀF bond activation which classically re- (1208C) and hydrogen pressure (10 atm) by arene hy-
quires harsh reaction conditions using stoichiometric drogenation and concomitant HDF (Scheme 1).
[
3]
metal hydrides (e.g., LiAlH and DIBAL) or one-
Further examination to confirm the essential fac-
4
[
4]
electron reductants (e.g., Zn and Mg) accompanied tors for HDF revealed that the reductive defluorina-
by a large quantity of metal sludge. On the other tion proceeds without H gas conditions by the combi-
2
hand, transition metal-catalyzed HDFs using various
hydrogen donors (hydrogen, alcohols, silanes etc.)
[
5–7]
were also eagerly examined.
Mono- or didefluori-
nation of multi-fluorinated C F or C F H possessing
6
6
6 5
a strongly electron-deficient property proceeded com-
[
5]
paratively easily to form C F H and/or C F H , and
6
5
6
4
2
the mechanistic considerations were actively investi-
[5,6d]
gated.
Meanwhile, monofluoroarenes could be de- Scheme 1. Pt/C-catalyzed arene hydrogenation.
Adv. Synth. Catal. 2012, 354, 777 – 782
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
777

Yoshinari Sawama et al.
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Table 1. Pt/C-catalyzed hydrogen-transfer hydrodefluorina-
tion of 4-fluorobiphenyl (1a).
Table 2. Catalyst efficiencies of various carbon-supported
metals.
[b]
[b]
Entry
Solvent
Co-solvent
1a:2a
Entry
Catalyst
1a:2a
1
2
3
4
5
6
7
8
9
i-PrOH
n-PrOH
MeOH
t-BuOH
–
–
–
–
–
H O
H O
H O
H O
63:37
1
2
3
4
5
6
7
10% Pt/C
10% Ru/C
10% Rh/C
10% Ir/C
10% Pd/C
10% Ni/C
10% Au/C
13:87
89:11
94:6
97:3
no reaction
no reaction
no reaction
no reaction
no reaction
no reaction
no reaction
13:87[
H O
2
i-PrOH
i-PrOH
i-PrOH
i-PrOH
i-PrOH
i-PrOH
i-PrOH
i-PrOH
2
c]
51:49
2
[d]
15:85[
2
[a]
e]
Refluxed in a 1008C heating bath.
0:100
2
[b]
1
The ratio was determined by H NMR and GC/MS.
10
11
12
13
toluene
1,4-dioxane
DMF
no reaction
no reaction
no reaction
no reaction
DMA
Table 3. Effects of the amounts of i-PrOH and H O.
2
[
a]
Refluxed in a 1008C heating bath unless otherwise
noted.
[
[
[
[
b]
1
The ratio was determined by H NMR and GC/MS.
c]
d]
e]
Heated in an 808C heating bath.
Refluxed in a 1208C heating bath.
For 30 h.
[b]
Entry
i-PrOH (X mL)
H O (Y mL)
2
1a:2a
1
2
3
4
5
6
2.0
2.0
2.0
2.0
2.0
1.0
0
63:37
54:46
21:79
13:87
17:83
31:69
0.1
0.5
1.0
2.0
0.5
nation of Pt/C and i-PrOH widely utilized as a hydro-
[
10]
gen donor in hydrogen transfer reactions. 4-Fluoro-
biphenyl (1a) could be defluorinated under 3 mol%
of Pt/C-catalyzed reflux conditions in i-PrOH in
a 1008C heating bath for 12 h to give the desired bi-
phenyl (2a), although the reaction ratio was unsatis-
factory (Table 1, entry 1). On the other hand, primary
alcohols such as n-PrOH and MeOH inefficiently re-
leasing a-hydrogen and tert-BuOH bearing no a-hy-
drogen were found to play no role in the reduction
[
[
a]
b]
Refluxed in a 1008C heating bath.
The ratio was determined by H NMR and GC/MS.
1
Table 4. Effect of various bases.
(
(
entries 2–4). The addition of H O as a co-solvent
2
1 mL of H O for 0.25 mmol of 1a in 2 mL of i-
2
PrOH) dramatically accelerated the defluorination
(
(
entries 1 vs. 6), while independent use of H O
entry 5) and the addition of other co-solvents (e.g.,
2
[b]
toluene, 1,4-dioxane, DMF, DMA, entries 10–13) re- Entry
Base
1a:2a
sulted in no reaction. The reaction at 808C or 1208C
on the outside heating device temperature never im-
1
2
–
79:21
no reaction
18:82
NEt3
K CO
Na CO
Na CO
proved the reaction ratios (entries 7 and 8), whereas
3
2
3
prolonging the reaction time up to 30 h in i-PrOH
4
8:92 [
2
3
3
c]
[d]
(
2 mL) and H O (1 mL) led to the complete reduction
2
5
6
7
8
9
0:100 (94%)
2
(
entry 9).
Li CO
32:68
45:55
63:37
66:34
2
3
Catalyst efficiencies were investigated as described
Cs
CH
CH CO K
2
CO
CO
3
in Table 2. Ru/C, Rh/C, Ir/C, Pd/C, Ni/C and Au/C
only, except for Pt/C (entry 1), gave an extremely low
yield or no catalyst activities for the hydrodefluorina-
3
Na
2
3
2
[a]
Refluxed in a 1008C heating bath.
The ratio was determined by H NMR and GC/MS.
[
[
[
b]
c]
d]
1
tion in i-PrOH-H O (entries 2–7).
2
The reactivity was strongly influenced by the ratio
For 6 h.
Isolated yield.
of H O and substrate concentration (Table 3). Addi-
2
778
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ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2012, 354, 777 – 782

Platinum on Carbon-Catalyzed Hydrodefluorination of Fluoroarenes
[
11]
tion of H O up to 1 mL into the reaction mixture in- Pt/C, the addition effect of various bases as a scav-
2
cluding 0.25 mmol of 1a and 2 mL of i-PrOH en- enger of HF was investigated (Table 4). Reaction effi-
hanced the reaction efficiency, and the highest conver- ciencies were compared after 3 h reaction to clarify
sion was achieved when using 1 mL of H O (entry 4), the influence of the bases. Although NEt as an or-
2
3
although further increments of H O up to 2 mL pro- ganic base completely blocked the defluorination,
2
vided no effective result. Because a higher substrate presumably due to the catalyst poisoning effect by the
concentration (0.16M) caused a significant suppres- lone electron pair on the nitrogen atom (entry 2), in-
sive effect on the reaction rate due to the lower solu- organic bases effectively accelerated the defluorina-
bility of 1a into the aqueous solvent (entry 6), further tion of 1a (entries 3, 4 and 6–9). Especially, Na CO3
2
reactions were carried out in ca. 0.08M solution was found to be the most efficient base despite its
(
2 mL of i.PrOH and 1 mL of H O for 0.25 mmol of mild basicity (entry 4), and the defluorination was
2
1
a, entry 4).
completed by extension of the reaction time to 6 h to
Because hydrogen fluoride (HF) generated by the afford 2a in excellent isolated yield (entry 5).
reaction progress could be a catalyst poison for
Table 5. Scope of substrates.
Entry
1
Substrate
Product
Time [h]
6
Yield [%]
94
2
3
15
3
88
88
4
5
9
9
98
80
6
7
12
6
83
85
8
9
15
15
89
87
Adv. Synth. Catal. 2012, 354, 777 – 782
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
779

Yoshinari Sawama et al.
COMMUNICATIONS
Table 5. (Continued)
Entry
Substrate
Product
Time [h]
6
Yield [%]
89
10
11
12
6
6
89
91
[
b]
13
16
88
[
[
a]
Refluxed in a 1008C heating bath.
b]
3
equiv. of Na CO were used.
2 3
Next, we investigated the general applicability of catalyzed hydrogenation of halobenzenes under
the present defluorination method using the reaction a high temperature-induced gas phase reaction, which
conditions indicated in Table 4, entry 5 (Table 5). 2- is adaptable only to easily vaporized substrates.
Fluorobiphenyl (1b) possessing a relatively sterically Therefore, the present method is the first example of
hindered fluorine atom, 4,4’-bisfluorinated biphenyl such an unusual order of halogen reactivity on aro-
(
1c), 1-fluoronaphthalene (1d) and 4-fluorodiphenyl- matic nuclei in the commonly used liquid phase re-
methane (1e) were easily defluorinated in excellent duction.
isolated yields (entries 1–4). 4-Fluorinated phenol
Preceding reports indicated that CÀF cleavage
1f), anisole (1g) and aniline (1h) were also smoothly could be initiated by the electron transfer into the ar-
(
[
5b,c]
defluorinated in spite of the presence of their heter- omatic nuclei from a metal,
insertion of metal into
[
5i]
oatoms which would serve as catalyst poisons for Pt a CÀF or adjacent CÀH bond, or nucleophilic aro-
[
5e,f,h,j 6c]
metal (entries 5–7). Acetophenone and benzoic acid matic substitution by a metal hydride.
HDF
derivatives (1i–m) bearing carbonyl groups, such as using the Pt/C-iPrOH/H O-Na CO combination was
2
2
3
ketone, ester and carboxylic acid, selectively under- not affected by steric hindrance of the substrates (1a
went only defluorination without reduction of the car- vs. 1b or 1l vs. 1m), which should suppress the reac-
bonyl moieties (entries 8–12). Furthermore, multi-de- tion efficiencies in the cases of nucleophilic aromatic
fluorination of 1n also proceeded smoothly and com- substitution and oxidative addition pathways. There-
pletely to afford the totally non-fluorinated benzoic fore, we suggested an electron-transfer-mediated
acid (2n) in 88% yield (entry 13).
mechanism (Scheme 2), although reaction mecha-
A time-course study of the Pt/C-catalyzed hydrode- nisms via oxidative addition or nucelophilic substitu-
fluorination using the i-PrOH-H O-Na CO combina- tion could not be completely ruled out. Pt activated
2
2
3
[
13]
tion of 1-halogenated naphthalenes as substrates re- by H (A) generated from Pt/C and i-PrOH is coor-
2
vealed an interesting trend in the halogen-dependent dinated by H O to form the active species (B). A
2
dehalogenation properties. The fluorine atom was single electron transfer from B into fluoroarene and
À
preferentially removed under the reaction conditions subsequent F elimination gives a radical intermedi-
in comparison with other aromatic halides, and the ate (E) via an anion-radical species (D). Further
hierarchy of the ease of dehalogenation was F>Cl> single electron transfer followed by protonation or hy-
Br@I (Figure 1), although aromatic halides are well- drogen atom abstraction from the platinum surface
known to be reduced (dehalogenated) in the order of affords the defluorinated product and Pt(II), which is
I>Br>Cl>F under conventional hydrogenation reused after the reduction to Pt(0) by i-PrOH. The
(
e.g., Pd/C-H ) and transition metal-catalyzed reduc- first single electron transfer step may preferentially
2
[6e 7b,d]
tion conditions.
was accomplished only by the Mo C
Reactivity similar to our results proceed when using more electron-deficient sub-
[12a] [12b]
or Ni/SiO2
-
strates such as fluoroarenes because of the strongest
2
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Adv. Synth. Catal. 2012, 354, 777 – 782

Platinum on Carbon-Catalyzed Hydrodefluorination of Fluoroarenes
Figure 1. Dehalogenation reactivity of 1-halonaphthalenes. Reaction conditions: substrate (0.25 mmol), Pt/C (3 mol%) and
Na CO (1.1 equiv.) in i-PrOH (2 mL) and H O (1 mL) in a 1008C heating bath under argon.
2
3
2
electronegativity of the fluorine atom in comparison gas-free hydrodefluorination method can provide an
with other halogen species. A detailed elucidation of easy-to-handle, safe and practical protocol for the in-
the reaction mechanism including the role of H O dustrial as well as laboratory scale removal of fluorine
2
and Na CO is under intense investigation.
atoms from fluorinated arenes. Furthermore, the un-
2
3
In summary, we have developed a mild and effi- precedented dehalogenation reactivity of halogen spe-
cient hydrodefluorination method for various fluo- cies (F>Cl>Br@I) in the liquid phase reaction
roarenes using the Pt/C-i-PrOH-H O-Na CO combi- should attract much attention from organic chemists.
2
2
3
nation. The Pt/C catalyst was easily recovered by
[
14,15]
a simple filtration without metal leaching
into not
only the defluorinated product but also the reaction Experimental Section
mixture, and analytically pure products could be ob-
tained merely by a simple extraction. The present H2 General Procedure for the Hydrodefluorination
A mixture of fluoroarene (0.25 mmol), 10% Pt/C (14.6 mg,
7
.5 mmol), Na CO (29.1 mg, 0.27 mmol), i-PrOH (2 mL)
2 3
and H O (1 mL) in a 15-mL test tube was refluxed in
2
a 1008C heating bath under an argon atmosphere (balloon)
using
a
personal organic synthesizer (Chemistation,
EYELA, Tokyo). After the reaction was completed, the
mixture was passed through a membrane filter (Millipore,
Millex-LH, 0.45 mm) to remove Pt/C. The filtrate was ex-
tracted with Et O (10 mL) and H O (10 mL), and then the
2
2
aqueous layer was further extracted with Et O (10 mLꢁ2).
2
The combined organic layer was dried over anhydrous
MgSO , filtered and concentrated under vacuum to give the
4
pure defluorinated product.
Pt-Leaching Test (ICP-AES Analysis)
A mixture of 4-fluorobiphenyl (1a, 861 mg, 5.0 mmol), 10%
Pt/C (292 mg, 0.15 mmol), Na CO (583 mg, 5.5 mmol), i-
2
3
PrOH (40 mL) and H O (20 mL) in a 200-mL flask was re-
2
fluxed for 12 h in a 1008C heating bath under an argon at-
Scheme 2. Proposed reaction mechanism.
mosphere (balloon). The mixture was passed through a glass
Adv. Synth. Catal. 2012, 354, 777 – 782
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Yoshinari Sawama et al.
COMMUNICATIONS
filter loaded with a celite pad to remove Pt/C. The filtrate
was extracted using Et O (40 mL) and H O (40 mL). Fur-
thermore, each layer was passed through two types of mem-
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Mahon, M. K. Whittlesey, J. Am. Chem. Soc. 2009, 131,
1847–1861; j) J. A. Panetier, S. A. Macgregor, M. K.
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2
2
brane filters (Millipore, Millex-LH, 0.45 mm and Millex-LH,
0
.20 mm), respectively. The resulting aqueous and Et O
2
layers were respectively diluted with water or Et O in a 100-
mL volumetric flask. No Pt-leaching (<1 ppm) into either
layer was observed by ICP-AES.
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Acknowledgements
We are deeply grateful to Professor David Milstein, the Weiz-
mann Institute of Science, Rehovot, Israel for the useful sug-
gestion associated with Pt-leaching test. We also thank the
N.E. Chemcat Corporation for the kind gift of the catalysts.
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1
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[15] The further defluorinaton of 1a in the filtrate obtained
by the filtration of the reaction mixture to remove Pt/C
never proceeded. Therefore, we obtained further assur-
ance that the Pt-leaching never occurred during the re-
action. See Supporting Information.
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ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2012, 354, 777 – 782