Green synthesis of methyl trifluoropyruvate catalyzed by solid acids

Article abstract of DOI:10.1016/j.jfluchem.2008.01.001

Solid acids - NiSO₄/Al₂O₃, Fe₂(SO₄)₃/Al₂O₃ and TiO₂/SO₄^2- - appeared to be effective catalysts for the acid catalyzed synthesis of met

Full text of DOI:10.1016/j.jfluchem.2008.01.001

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Journal of Fluorine Chemistry 129 (2008) 332–334
www.elsevier.com/locate/fluor
Green synthesis of methyl trifluoropyruvate catalyzed by solid acids
*
Sergey A. Lermontov , Lyudmila L. Ushakova, Nina V. Kuryleva
Institute of Physiologically Active Compounds, Russian Academy of Sciences, Chernogolovka, Moscow Region 142432, Severny proezd 1, Russia
Received 8 November 2007; received in revised form 24 December 2007; accepted 6 January 2008
Available online 15 January 2008
Abstract
Solid acids – NiSO4/Al2O3, Fe2(SO4)3/Al2O3 and TiO2/SO42À – appeared to be effective catalysts for the acid catalyzed synthesis of methyl
ester of trifluoropyruvic acid. They are active at 150–180 8C.
# 2008 Elsevier B.V. All rights reserved.
Keywords: Methyl ester of trifluoropyruvic acid; Solid acids; Superacids; Hexafluoropropylene oxide
1. Introduction
2. Results and discussion
The methyl ester of trifluoropyruvic acid (CF3COCOOMe,
MTFP) is a valuable C3 fluorinated synthon for organofluorine
chemistry [1,2]. As a rule, MTFP is synthesized via a two-step
procedure starting from hexafluoropropylene oxide [1,3]
(Scheme 1).
In order to investigate the possible catalytic properties of
these solid acids in the reaction in Scheme 1, we have prepared
a number of catalyst samples with varied amount of the
inorganic sulfate on supporting Al2O3. A heterogeneous
mixture of the ester A and a catalyst was heated in a steel
bomb and products were investigated by 19F NMR. The results
are summarized in Table 1.
The key step of this transformation is a decomposition of 2-
alkoxytetrafluoropropionic acid ester (A) catalyzed by con-
centrated sulfuric acid (reactions of this type are widely used in
organofluorine chemistry [4]). This step is usually performed at
elevated temperature (150–170 8C) and very large amount of
the acid catalyst (up to 3.8 moles of H2SO4 for 1 mole of the
starting ester) is used. To minimize the amount of acid wastes
from this reaction we paid attention to a novel class of solid
The acid TiO2/SO42À had the highest activity of all catalysts
investigated (entry 5). Aluminium oxide sulfated by the same
2À
procedure as TiO2/SO4 was significantly less active (entry
6). It is seen from Table 1 that there exists an optimal
concentration of the sulfate salt in the catalyst—10% for
NiSO4 and 2–5% for Fe2(SO4)3, the latter being more active.
Pure Al2O3 was absolutely inactive (entry 1). At 180 8C the
reaction yield was much higher than at 150 8C (entries 8, 9).
Regenerated catalyst was a little bit less active than a freshly
prepared one (entries 8, 12).
acids – nickel (II) and iron (III) sulfates supported on Al2O3,
2À
TiO2 and ZrO2 [5–9] and sulfated titania [10] – TiO2/SO4
.
These acids are stronger than 100% H2SO4 [11], and, hence,
they are considered to possess superacidic properties [12].
Their catalytic properties for the purposes of organic synthesis
are practically not investigated—ethylene dimerization [5–8],
cumene dealkylation, 2-propanol dehydration [9] and some
condensations [10] are described in a literature.
The mechanism of the sulfuric acid catalyzed decom-
position of A has been recently investigated thoroughly
[1]. The authors have found that the reaction begins
with a protonation of an ether oxygen, followed by methyl
sulfate formation as a primary CH3-containing by-product.
Gaseous reaction products were supposed to consist mainly
of carbon oxide CO, though no evidence of its formation was
given.
We have investigated the gaseous products of our reaction
and have found that they contained a significant amount of
methyl fluoride CH3F together with traces of the starting ester
* Corresponding author. Tel.: +7 496 524 95 08; fax: +7 496 524 95 08.
E-mail address: lermon@ipac.ac.ru (S.A. Lermontov).
0022-1139/$ – see front matter # 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jfluchem.2008.01.001

S.A. Lermontov et al. / Journal of Fluorine Chemistry 129 (2008) 332–334
333
Table 1
Solid acids catalyzed synthesis of methyl trifluoropyruvate
Entry
Catalysta,b
MTFP content (%)c
1
2
Al2O3
0
18
29
16
88
28
63
58
95
35
46
54
5% NiSO4/Al2O3
3
10% NiSO4/Al2O3
20% NiSO4/Al2O3
(NH4)2SO4/TiO2
4
5
6
(NH4)2SO4/Al2O3
2% Fe2(SO4)3/Al2O3
5% Fe2(SO4)3/Al2O3
5% Fe2(SO4)3/Al2O3
10% Fe2(SO4)3/Al2O3
20% Fe2(SO4)3/Al2O3
5% Fe2(SO4)3/Al2O3 regenerated
7
8
9
Scheme 1.
10
11
12
a
All samples were calcined at 500 8C during 7 h before a reaction.
Reaction time was 7 h in all cases. Reaction temperature was 150 8C except
b
for entry 9 (180 8C).
c
Determined by 19F NMR (see Section 4).
centers [12] (Scheme 3), whereas NiSO4/Al2O3 is considered to
have mainly Lewis acid centers on the surface [13].
As a consequence a hindered ether oxygen is much easier
reached by a small proton than by a Lewis metal atom on the
surface.
Scheme 2.
A, MTFP and unidentified CF3-containing compound, probably
CF3COOMe.
In a special experiment we have analyzed the gas phase from
an H2SO4-catalyzed reaction (like in [1]) and have found no
traces of methyl fluoride.
3. Conclusion
This method of methyl trifluoropyruvate synthesis signifi-
cantly reduces the amount of acid wastes during the preparation
procedure. For example, only 9.3 Â 10À5 moles of a sulfate salt
is used for 5.3 Â 10À3 moles of the starting ester A (in the case
of 5% Fe2(SO4)3), compared with 2 Â 10À2 moles of con-
centrated H2SO4 in a literature procedure.
The obtained data show that the mechanism and products
distribution can vary depending on the catalyst used. In a case
of sulfated metal oxides an immobilized sulfate cannot serve as
a nucleophile and, hence, a methyl group ‘‘finds’’ another
nucleophile (fluoride) to form CH3F.
In all cases the reaction probably begins with a proton attack
on an ether oxygen followed by CH3–O bond splitting. Then in
our case HF releases from the intermediate a-fluoroalcohol B
forming methyl fluoride (Scheme 2).
4. Experimental
The difference in a catalytic activity of titanium, iron and
nickel sulfates can be explained by the different chemical
nature of their acidity. Solid superacids, namely sulfated metal
4.1. General experimental procedure
1H and 19F NMR spectra were recorded using Bruker DPX-
200 spectrometer with TMS and CFCl3 as external standards
respectively. Methyl ester of 2-methoxytetrafluoropropionic
acid A and MTFP were synthesized by a literature procedure
[3], their 1H and 19F NMR spectra coincided with those
described in [1]. NiSO4Á7H2O, Fe2(SO4)3Á5H2O (Acros) and g-
Al2O3 (Engelhardt E-800, 200 m2/g) were used as purchased.
+
+
˝
oxides, usually contain both Lewis (M ) and Bronsted (H ) acid
4.2. Catalyst preparation
A known amount of a metal sulfate was dissolved in water,
Al2O3 (small sticks) was added to this solution and the solvent
was removed using a rotary evaporator till dryness. The
prepared catalyst was then dried in an oven at 120 8C during
1 h. TiO2/SO42À was prepared by a procedure described in [10].
Scheme 3.

334
S.A. Lermontov et al. / Journal of Fluorine Chemistry 129 (2008) 332–334
4.3. Catalytic experiment
separated from the catalyst by centrifugation and distilled using
a high, vacuum jacketed column. Methyl trifluoropyruvate,
The catalyst (2 g) was calcined in a quartz tube reactor under
conditions shown in Table 1. A reactor was periodically flushed
by a stream of dry air for removing of water vapor. A catalyst
sample was cooled to 200–250 8C and then was quickly
transferred to a stainless steel bomb, which was then sealed to
let a catalyst cool under water-protected conditions. A bomb
was opened; a sample of the ester A (2.7 g, 1.42 Â 10À2 mole)
was added to a catalyst, a bomb was sealed again and heated as
shown in Table 1 with periodical shaking. After cooling the gas
from a bomb was passed through CDCl3, a bomb was opened
and a reaction mixture was analyzed by 19F NMR (188 MHz;
CDCl3). The content of MTFP in reaction mixtures was
determined by comparison of the ester A. À80.5 (3F, d, CF3,
3JF-F = 3 Hz), À133.5 (1F, m, CF) and MTFP. À74.0 (s, CF3)
signals. A 19F NMR spectrum of a gas phase contained a signal
of methyl fluoride (À266.5, q, 2JF-H = 46.4 Hz [14]) as a main
product and signals of the ester A, MTFP and À82.0 ppm,
probably CF3COOMe.
MTFP, was obtained in a 50% yield (5.5 g, 3.5 Â 10À2 mole).
1
b.p. 85–87 8C. H NMR (200 MHz, CDCl3): 4.0 (s).
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