Homogeneous catalytic hydrogenation of perfluoro methyl esters

Article abstract of DOI:10.1039/c3ra43410a

The first example of perfluoroalkyl methyl ester R_fC(O)OMe (R_f = C₃F₇, C₅F₁₁) reduction by homogeneous catalytic hydrogenation with the ruthenium catalyst Ru-MACHO is herein reported. The hydrogenation process leads to the corresponding perfluorinated alcohols thus replacing sodium borohydride that has so far represented the state of art in perfluoro ester reduction.

Full text of DOI:10.1039/c3ra43410a

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COMMUNICATION
Homogeneous catalytic hydrogenation of perfluoro
methyl esters3{
Cite this: DOI: 10.1039/c3ra43410a
Dario Lazzari,*a Maria Cristina Cassani,*b Maurizio Bertola,a Francisco
Casado Morenoa and Damiano Torrentea
Received 5th July 2013,
Accepted 8th July 2013
DOI: 10.1039/c3ra43410a
www.rsc.org/advances
The first example of perfluoroalkyl methyl ester RfC(O)OMe (Rf =
C3F7, C5F11) reduction by homogeneous catalytic hydrogenation
with the ruthenium catalyst Ru-MACHOTM is herein reported. The
hydrogenation process leads to the corresponding perfluorinated
alcohols thus replacing sodium borohydride that has so far
represented the state of art in perfluoro ester reduction.
number of carbon atoms, alcohols with an odd number of
fluorinated carbons can be synthesised via reduction of perfluor-
oesters.
The production of fluorinated alcohols of the type RfCH2OH
where Rf = C3F7 and C5F11 is part of the core competence of Miteni
S.p.A.3 They are obtained from reduction of the corresponding
perfluoro methyl esters whose preparation is depicted in
Scheme 1. In the first step the butanoyl and hexanoyl chlorides
undergo an halide exchange with anhydrous HF, then, by
Electrochemical Fluorination (ECF),4 the acyl fluorides are
transformed in the perfluoro acyl derivatives CF3(CF2)nC(O)F
(under n.c.: n = 2, gas; n = 4 liquid) that after methanolysis leads
to the formation of the perfluoromethyl esters.
The traditional method reported in the literature for the
reduction of perfluoro esters to alcohols of formula RfCH2OH is
based on the use of NaBH4.5 The yields of hydrogenation are in
the range of 80%, and although sodium borohydride is a versatile
reagent, its hazardous nature, complex postreaction workup
procedures, and high level of residual waste are matters of
concern in industrial operations. Moreover the postreaction
The existing concern towards long perfluorinated alkyl chains
owing to their bioaccumulation, toxicity and environmental
persistence is making more and more important the availability
of fluorinated building blocks with shorter perfluorinated carbon
chains. In general, the reduction of the perfluorinated chain
lengths on one hand decreases the toxicity and bioaccumulation
potential but, on the other, leads to inferior performances in the
applications where a large content of fluorine atoms is mandatory.
Therefore, the synthetic strategies adopted by academic and
industrial research groups working in this field are not simply
consisting in a shortening of the fluorocarbon chains but also
include a complete molecule redesign and a common approach is
to replace a single long chain with branched structures containing
short fluorocarbon alkyl groups.1 In this context the need for short
fluorinated alcohols is becoming a pressing request for several
industries in the development of new fluorinated additives,
surfactants and reagents for surface treatment.2 While
a
Table 1 Reduction of C3F7C(O)OMea
telomerisation approach delivers only alcohols with an even
Entry
T/uC
pH2 (bar)
Cat. (mol%)
Yieldb
TON
a
`
Miteni S.p.A., Localita Colombara 91, I-36070 Trissino (VI), Italy.
1
2
3
4
5
6
7
8
40
40
40
20
60
60
60
60
80
100
50
60
10
10
10
10
10
10
10
10
10
30
10
10
0.1
0.05
0.045
0.05
0.05
0.025
0.0125
0.005
0.0125
0.05
0.005
0.005
99
89
65
17
97
89
74
46
66
—
43
36
990
1780
1444
340
1940
3560
5920
9200
5280
—
E-mail: dario.lazzari@miteni.com; Fax: +39 0445 499507; Tel: +39 0445 499560
bDipartimento di Chimica Industriale ‘‘Toso Montanari’’, Viale Risorgimento 4, I-
40136 Bologna, Italy. E-mail: maria.cassani@unibo.it; Fax: +39 051 2093694;
Tel: +39 0512093700
Electronic supplementary information (ESI) available: Experimental details,
3
characterisation data, 1H and 13C NMR spectra. See DOI: 10.1039/c3ra43410a
{ Typical procedure for hydrogenation of a perfluorinated methyl ester in methanol
(Table 1, entry 6): a 100-mL stainless steel autoclave equipped with a Teflon-coated
stirring bar and previously filled with nitrogen (three purging cycles) was charged
with methanol (7.0 g, 0.22 mol), NaOCH3 (0.62 g, 0.011 mol), C3F7C(O)OMe (10.0 g,
0.044 mol) and Ru-Macho (0.007 g, 0,011 mmol). The autoclave was closed and
purged three times with nitrogen. Successively the vessel was purged three times
with hydrogen gas then the mixture was stirred (500 rpm), heated up to 60 uC and
pressurized with hydrogen (10 bar). After stirring for 23 h, the system was cooled to
25 uC, and then the hydrogen gas released. The mixture was analyzed by GC.
9
10c
11d
12e
8600
7200
a
Reaction conditions: MeOH, pH2 = 10 bar, 23 h, ester/NaOCH3
b
molar ratio = 0.25, in a 100 mL autoclave. Determined by GC
c
analysis (internal standard: decane). pH2 = 30 bar, 5 h.
d
e
Perfluorobutanol. Isopropyl alcohol.
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Scheme 2 Hydrogenation of perfluorinated methyl esters with Ru-MACHO.
Scheme 1 Synthesis of perfluorinated methyl esters (n = 2, 4).
By decreasing the catalyst loading by half (0.05 mol%) the
yields decreases of ca. 10% but the TON is nearly doubled whilst a
further reduction of the cat. loading leads to a significant decrease
of the yield (entries 2 and 3 respectively). The reaction temperature
also has a significant influence as demonstrated in the entries 4 to
10 with the optimal working conditions temperatures found at 60
uC and turn over frequencies higher than those generally reported
for this catalyst.9b At higher temperature we observed a drastic
reduction of the catalyst performances even in the presence of an
H2 pressure of 30 bar (entry 10). We also performed the
hydrogenation in different solvents such perfluorobutanol (PFB)
and isopropanol alcohol (IPA) (entries 11, 12) with similar results
whilst in THF we constantly observed a significantly lower catalytic
activity (data not reported).
The reduction of C5F11C(O)OMe, carried out in the same
conditions (pH2 = 10 bar), led to the corresponding alcohol
although in lower yields. Contrary to what observed in the
previous case, we found that the hydrogenation reaction is
particularly sensitive to the ester/NaOMe molar ratio. As shown
in Table 2 entry 1, when the hydrogenation is carried out at 40 uC
with a cat. loading of 0.05 mol% and an ester/NaOMe molar ratio
equal to 0.25 the alcohol is obtained with just a 10% yield that
raises to nearly 50% when the ester/NaOMe molar ratio is
doubled.
workup generates boric acid and borates that according to the
IUCLID Dataset published by the European Commission, in high
doses show significant developmental toxicity and teratogenicity
in rabbits, rats, and mouse fetuses as well as cardiovascular
defects, skeletal variations, mild kidney lesions.6,7
Replacing hydride reductions with hydrogenations enables
companies to implement simple and convenient processes. For
this purpose, catalysts which work under mild conditions would
have significant industrial advantages. The catalytic hydrogenation
of perfluoroalkyl esters to fluoroalkyl alcohols is a very exotic
research area and to the best of our knowledge very few examples
have been recently reported on this topic.8 In particular in the
hydrogenation (pH2 = 50 bar, 140 uC, 24 h) of CF3(CF2)2CO2Me
reported by Clarke et al., the ruthenium catalyst bearing a chiral
tridendate amine functionalized phosphine (0.5%) must be
activated by a three-fold excess (1.5%) of LiHBEt3.8a
Our first attempts were done carrying out the hydrogenation
reaction under heterogeneous conditions: several systems such as
Pd, Pt and Ru on a variety of supports (activated carbon, alumina,
titania) were tested with, at least under relatively harsh
experimental conditions, negligible or no results.§ In view of
these findings we have therefore decided to focus our efforts on
homogenous catalytic hydrogenation and we considered the
ruthenium(II) tri-chelated complex carbonylchlorohydrido[bis(2-
(diphenylphosphinoethyl)amino]ruthenium(II), a commercial cat-
alyst known as Ru-MACHO produced in bulk by the Takasago
Company. This catalyst is active in the hydrogenation of esters9
although no information were available regarding his activity in
the hydrogenation of perfluorinated methyl esters. The hydro-
genation of fully fluorinated methyl esters CF3(CF2)nC(O)OMe (n =
Once again we found that the reaction temperature has a
significant impact on the final outcome (entry 6) with the best
conditions found in the temperature range 40–60 uC as described
in entries 3 and 4.
2, R = C F ; n = 4, R = C F ) was carried out in autoclave under a
variety of reaction conditions in the presence of Ru-MACHO and
NaOMe in methanol (Scheme 2).
3
f
3
7
f
5 11
Table 2 Reduction of C5F11C(O)OMea
Entry T/uC Cat. (mol%) ester/NaOMe molar ratio Yieldb TON
A set of catalytic entries have been proposed with the aim of
investigating the influence of the catalyst loading, solvent,
temperature and reaction set-up on the reaction output. Table 1
summarizes the results collected by varying the catalyst loading,
temperature and solvents for the reduction of C3F7C(O)OMe. The
reduction carried out in methanol at 40 uC, pH2 = 10 bar using a
cat. loading of 0.1 mol% led to the formation of alcohol with a
1
2
3
4
5
6
40
40
40
60
60
80
0.05
0.05
0.065
0.075
0.041
0.059
1/0.25
1/0.50
1/0.50
1/0.50
1/0.50
1/0.50
10
48
81
86
9
200
960
1246
1146
220
17
288
a
Reaction conditions: MeOH, pH2 = 10 bar, 23 h, in a 100 mL
b
autoclave. Determined by GC analysis (internal standard: decane).
99% yield (GC, IS) in an highly reproducible manner (entry 1).
3
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In conclusion, in this communication we have shown that the
ruthenium catalyst Ru-MACHO is an active catalyst for the
hydrogenation of perfluorinated methyl esters under very mild
conditions and represents a valid alternative to the use of metal
hydrides.
M.C.C. wishes to thank the University of Bologna and the
`
Ministero dell’Universita e della Ricerca (MUR) (project: ‘‘New
strategies for the control of reactions: interactions of molecular
fragments with metallic sites in unconventional species’’, PRIN
2007) for financial support.
Notes and references
§ Under the following conditions: 10% Ru on Carbon with hydrogen
pressures up to 50 bar and temperature up to 120 uC a 1% conversion (GC)
was observed.
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(International Chemical Investors Group, http://www.
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chemical companies, operating worldwide in base chemicals,
fine chemicals and polymers.
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