A Practical Synthesis of α-Fluoroketones in Aqueous Media by Decarboxylative Fluorination of β-Ketoacids

Article abstract of DOI:10.1002/adsc.201500282

A practical and novel process for the decarboxylative fluorination of β-ketoacids in water in the presence of phase transfer catalyst has been developed, affording a series of α-fluoroketones in good to excellent yields. Furthermore, a preliminary investigation for the catalytic asymmetric transformation was performed and a proposed mechanistic pathway for this catalytic process was proposed.

Full text of DOI:10.1002/adsc.201500282

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DOI: 10.1002/adsc.201500282
A Practical Synthesis of a-Fluoroketones in Aqueous Media by
Decarboxylative Fluorination of b-Ketoacids
Jian Li,a Yin-Long Li,a Nan Jin,a Ai-Lun Ma,a Ya-Nan Huang,a and Jun Denga,b,*
a
School of Pharmaceutical Science and Technology, Key Laboratory for Modern Drug Delivery & High-Efficiency, Tianjin
University, Tianjin, 300072 People’s Republic of China
Collaborative Innovation Center of Chemical Science and Engineering, Tianjin University, Tianjin, 300072 People’s
b
Republic of China
E-mail: jdeng@tju.edu.cn
Received: March 21, 2015; Revised: May 3, 2015; Published online: June 30, 2015
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201500282.
Abstract: A practical and novel process for the de-
carboxylative fluorination of b-ketoacids in water in
the presence of phase transfer catalyst has been de-
veloped, affording a series of a-fluoroketones in
good to excellent yields. Furthermore, a preliminary
investigation for the catalytic asymmetric transfor-
mation was performed and a proposed mechanistic
pathway for this catalytic process was proposed.
(Scheme 1).[8c–e] Accordingly, the exploration of prac-
tical and environmentally benign synthesis of a-fluo-
roketones is still highly desirable.
The carboxylic acids, compared to its corresponding
halohydrocarbon or organometallic reagents, have
several advantages such as easy storage, simple
handle and commercial availability, attracted great at-
tention as excellent precursor for the organic transfor-
mation,[9] and also have been shown as promising sub-
strates for the synthesis of organofluorine com-
pounds.[10] More recently, Li and co-workers devel-
oped a process through decarboxylative fluorination
of aliphatic carboxylic acids with Selectfluor under
AgNO3 catalysis, suggesting a mechanism involving
a Ag(III)-assisted single-electron transfer to generate
a putative radical intermediate prior to the fluorine
Keywords: decarboxylative fluorination; phase-
transfer catalysis; Selectfluor; a-fluoroketones; b-
ketoacids
The organofluorine compounds are widely used in atom transfer.[10d–e] Sammis and co-workers estab-
pharmaceuticals, agrochemicals and material scien- lished another protocol involved a photo-induced the
ces.[1] For its inherent importance, the development of
new and efficient methods to synthesize organic fluo-
rides has become a booming research topic. The a-
fluoroketones are important building blocks toward
a number of interesting other organofluorine deriva-
tives,[2] and frequently found in bioactive molecules
(Figure 1).[3] As a consequence, much effort has been
invested in the synthesis of the a-fluoroketones in the
past few years.[4–8] Although a diverse array of elegant
work has been established, including nucleophilic dis-
placement of a-haloketones with fluoride,[4] electro-
philic fluorination via metal enolate anion, enol ether,
enamine or imine,[5] and other processes,[6,7] some of
these methods have limitations due to using hazard-
ous and toxic chemicals, low yield or limited substrate
scope. The direct fluorination of ketones is the pref-
erable process, but only a few examples have been re-
ported,[8] such as hypervalent iodine mediated a-fluo-
rination of acetophenone derivatives with a triethyla-
mine·HF complex,[8a] as well as NFTh-Accufluor[8b]
Figure 1. Representative bioactive a-fluoroketones and its
and Selectfluor as stoichiometric fluorinating agents derivatives
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A Practical Synthesis of a-Fluoroketones
Table 2. Evaluation of the phase-transfer catalysts[a]
Entry
PTC[b]
Yield (%)[c]
1
2
3
4
5
6
7
8
TBAI
TBAB
TEBA
TBAF
TRITON
TEBA
18-crown-6
PEG-400
63
76
80
61
59
Scheme 1. Synthetic approaches to a-fluoroketones
88[d]
80[d]
75[d]
fluorination of alkyl radicals, which were generated
from the decomposition of carboxylic acids.[10f–g] How-
ever, to the best of our knowledge, the synthesis of a-
fluoroketones by decarboxylative fluorination of b-ke-
toacids, a masked enolate anion, is relatively unex-
plored.[11] Therefore, we wish to report here our suc-
cess in achieving the decarboxylative fluorination of
b-ketoacids to synthesize the a-fluoroketones under
mild conditions (Scheme 1).
[a]
Reaction conditions: b-ketoacids (0.30 mmol), Selectfluor
(0.36 mmol), PTC (10 mol%), solvent (2.0 mL), K2CO3
(0.36 mmol), 48 h at r.t. unless otherwise noted
TBAI=tetrabutylammonium iodide; TBAB =tetrabuty-
lammonium bromide; TEBA=benzyltriethylammonium
TBAF=tetrabutylammonium
TRITON=benzyltrimethylammonium hydroxide.
Isolated yield.
[b]
chloride;
fluoride;
[c]
[d]
The reaction was performed at 408C for 14 h.
Among the different solvent alternatives, water is
extremely cheap and nontoxic, and has emerged as
a versatile solvent in organic chemistry.[12] Therefore, ate salts successfully provided the desired products,
we commenced our studies in water using 3-oxo-3- and K2CO3 appeared preferable to afford a-fluoroa-
phenylpropanoic acid 1a as the model substrate, Se- cetophenone with 60% yield. Other carbonates such
lectfluor as the fluorine source, and various bases as Na2CO3, NaHCO3 and Cs2CO3 showed different
have been investigated to find out the most suitable degrees of activities and furnished the products with
conditions for this transformation. The results are 42, 32 and 30% yields, respectively. Subsequent
summarized in Table 1. To our delight, most carbon- screening of organic bases such as Et3N and DBU to
improve the yields proved unsuccessful. Et3N gave
the desired product 2a in 13% yield (entry 6), and
DBU provided the product with 27% yield (entry 7).
Table 1. Optimization of conditions[a]
Nevertheless, efforts to replace Selectfluor with other
fluorine source such as NFSI, NFPT did not bring any
improvement (entries 8, 9) with the acetophenone
and byproducts were obtained; only NFSI can afford
the desired product with 36% yield under the similar
reaction conditions.
Entry
Fluorine source[b]
Base
Yield (%)[c]
1
2
3
4
5
6
7
8
9
Selectfluor
Selectfluor
Selectfluor
Selectfluor
Selectfluor
Selectfluor
Selectfluor
NFSI
Na2CO3
K2CO3
Cs2CO3
NaHCO3
NaOH
Et3N
DBU
K2CO3
K2CO3
42
60
30
32
11
13
27
36
Encouraged by the aforementioned results, our ef-
forts then focused on improving the reaction efficien-
cy. Notably, phase-transfer catalysis is a well-known
and practical strategy for promoting the reactions be-
tween reagents with opposite solubility preferences,
and has been widely applied in organic synthesis.[13]
We consequently supposed that using phase-transfer
catalyst might improve the yield of decarboxylative
fluorination reaction. With this in mind, several
phase-transfer catalysts were investigated under the
standard reaction conditions. The results are summar-
ized in Table 2. Most the phase-transfer catalysts such
as TBAI, TBAF, and TRITON did not show a signifi-
cant acceleration effect in this reaction (entries 1, 4
and 5). Gratifyingly, the yield of this reaction can be
improved to 80% while using 5 mol% of TEBA as
NFPT
trace
[a]
Reaction conditions: b-ketoacids (0.30 mmol), fluorine
reagent (0.36 mmol), water (2.0 mL), base (0.36 mmol),
12 h at r.t.
Selectfluor=[(1-chloromethyl-4-fluoro-1,4-diazoniabicy-
clo-[2.2.2]octane bis(tetrafluoroborate); NFSI=N-fluoro-
benzenesulfonimide; NFPT=N-fluoropentachloropyridi-
nium triflate.
[b]
[c]
Isolated yield.
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Table 3. Scope of the b-ketoacids[a]
86% and 94% yields. In addition, using 2- and 3-
naphthyl b-ketoacids as the substrates, products 2 f
and 2g can be obtained in moderate to good yields.
However, for substrates with an electron-withdrawing
substituent on the aromatic ring, including F, Cl, Br
and NO2, the reaction did not occur (not shown). We
also found that introducing a substituent to the a-po-
sition of the b-ketoacid could enhance the reactivity.
After introducing a methyl group to the substrate,
which had an electron-withdrawing substituent (F, Cl
or Br) on the aromatic ring, the decarboxylative fluo-
rination reactions proceeded smoothly and lead to the
corresponding products 2n, 2o and 2p in 29, 20 and
32% yields, respectively. It is noteworthy that the
yields of these reactions can be further improved
when DMF was used as solvent, and in addition the
a,a-difluoroketones 2n’, 2o’ and 2p’ were obtained in
14%, 29% and 22% yields, respectively. Similarly,
the substrates with an electron-rich aromatic ring af-
forded the products 2l and 2m with 44% and 40%
yield in water, and the corresponding propiophenones
were provided in these cases; compared to the reac-
tion in water, higher yields of products 2l and 2m
(65% and 82%) can be obtained using DMF as sol-
vent.
After successfully synthesized a series of a-fluoro-
ketones via the decarboxylative nucleophilic fluorina-
tion, the development of an asymmetric version for
this transformation was investigated.[13] N-(Benzyl)-
cinchoninium chloride, a typical chiral phase-transfer
catalyst, was selected for this attempt.[14] The results
are shown in Scheme 2. Unfortunately, racemic prod-
uct was obtained when the reaction was carried out at
308C in CH2Cl2/H2O, and low enantioselectivity (7%
ee) was observed even at a lower reaction tempera-
ture (08C).
To gain more insight about the mechanism of this
reaction, several control experiments were conducted.
When acetophenone or propiophenone were used as
substrate, no fluorination products were detected (Eq
1 in Scheme 3), which indicated that the fluoroke-
tones were not generated directly from the corre-
[a]
Reaction conditions: b-ketoacid (0.30 mmol), Selectfluor
(0.36 mmol),
TEBA
(0.015 mmol),
and
K2CO3
(0.36 mmol) in water (2.0 mL) at 408C, isolated yield.
The reaction was performed with 3.0 mmol of 1a.
The reaction was performed in DMF(2.0 mL) at 108C.
Mixture of a-monofluoroketone and a-difluoroketone
was obtained.
[b]
[c]
[d]
the phase-transfer catalyst (entry 3). Furthermore, the
yield of the 2a can be increased to 88% by heating
the reaction at 408C (entry 6).
With the optimized conditions in hand, we proceed-
ed to evaluate the generality of this transformation
(Table 3). Firstly, the effect of substituents residing on
the aromatic moiety of the b-ketoacids was investigat-
ed. We found that in the presence of an electron-do-
nating substituent (e. g., Me, OMe), the reaction rate
was accelerated considerably, providing the corre-
sponding a-fluoroketones 2b–e with excellent yields.
Furthermore, electron-rich heterocyclic substrates af-
Scheme 2. Asymmetric decarboxylative fluorination of b-ke-
forded the desired a-fluoroketones 2h and 2i with toacid
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A Practical Synthesis of a-Fluoroketones
sis of chiral a-fluoroketones is currently ongoing in
our laboratory.
Experimental Section
General procedure for synthesis of a-fluoroketones in
aqueous media
To a solution of b-ketoacid (0.30 mmol) in water (2.0 mL)
was added Selectfluor (0.36 mmol, 127 mg), TEBA
(0.015 mmol, 3.4 mg), and K2CO3 (0.36 mmol, 50 mg). The
resulting solution was stirred at 408C, after the TLC showed
that the starting material was exhausted, the reaction was
quenched by water (5 mL), extracted with CH2Cl2 (3”
20 mL), then dried by Na2SO4, and concentrated under re-
duced pressure. The crude residue was purified by flash
column chromatography (silica gel, gradient eluent: 0% to
10% EtOAc in hexanes) to provide the a-fluoroketones.
2-Fluoro-1-phenylethanone (2a): 36.4 mg, 88% yield, col-
orless oil; Rf =0.65 (10% EtOAc/petroleum ether);
1H NMR (400 MHz, CDCl3) d 5.54 (d, J=46.9 Hz, 2H), 7.50
(t, J=7.9 Hz, 2H), 7.63 (t, J=7.4 Hz, 1H), 7.90 (d, J=
7.6 Hz, 2H); 19F (564 MHz, CDCl3) d À230.8; 13C NMR
(100 MHz, CDCl3) d 83.6 (d, J=182.0 Hz), 127.9 (d, J=
2.6 Hz), 129.0, 133.7, 134.2, 193.4 (d, J=15.2 Hz); IR (KBr)
cmÀ1 691, 795, 804, 969, 1090, 1234, 1452, 1596, 1704, 2938;
mass spectrum (ESI): m/e (% relative intensity) 139.7 (100)
(M+H)+.
Scheme 3. Control experiments
Scheme 4. A proposed mechanistic pathway
Acknowledgements
sponding ketones. Furthermore, when 2-fluoro-3-oxo-
3-phenylpropanoic acid was used as substrate under
the standard reaction conditions, this reaction afford-
ed the 2-fluoro-1-phenylethanone in 65% yield with
a trace of difluoroketone (Eq 2 in Scheme 3).
The authors thank Professor Richard P. Hsung [University of
Wisconsin at Madison] for invaluable discussions. The proj-
ect sponsored by the Scientific Research Foundation for the
Returned Overseas Chinese Scholars, State Education Minis-
try, the National Basic Research Project (No. 2014CB932201)
and the National Natural Science Foundation of China (No.
21172168).
Based on the above results and the literature re-
ports,[11,13] a plausible mechanistic pathway for this de-
carboxylative fluorination of b-ketoacids to synthesize
a-fluoroketones was proposed in Scheme 4. Initially,
the b-ketoacid was deprotonated by the potassium
carbonate in the presence of phase transfer catalyst to
form the dianion intermediate I via an electrostatic
interaction between the lone pair of the substrate and
the ammonium cation; the intermediate I subsequent-
ly reacted with Selectfluor to initiate the nucleophilic
fluorination to form the a-fluoroketocarboxylate
anion III, which undergo a decarboxylation reaction
to furnish the a-fluoroketone IV.
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