Practical selective hydrogenation of α-fluorinated esters with bifunctional pincer-type ruthenium(II) catalysts leading to fluorinated alcohols or fluoral hemiacetals

Article abstract of DOI:10.1021/ja403852e

Selective hydrogenation of fluorinated esters with pincer-type bifunctional catalysts RuHCl(CO)(dpa) 1a, trans-RuH₂(CO)(dpa) 1b, and trans-RuCl₂(CO)(dpa) 1c under mild conditions proceeds rapidly to give the corresponding fluorinated alcohols or hemiacetals in good to excellent yields. Under the optimized conditions, the hydrogenation of chiral (R)-2-fluoropropionate proceeds smoothly to give the corresponding chiral alcohol without any serious decrease of the ee value.

Full text of DOI:10.1021/ja403852e

Communication
pubs.acs.org/JACS
Practical Selective Hydrogenation of α‑Fluorinated Esters with
Bifunctional Pincer-Type Ruthenium(II) Catalysts Leading to
Fluorinated Alcohols or Fluoral Hemiacetals
Takashi Otsuka,† Akihiro Ishii,† Pavel A. Dub,‡ and Takao Ikariya*,‡
†Chemical Research Center, Central Glass Co. Ltd., 2805 Imafuku-Nakadai Kawagoe, Saitama, 350-1151, Japan
‡Department of Applied Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1-E4-1
O-okayama, Meguro-ku, Tokyo 152-8552, Japan
S
* Supporting Information
Scheme 1. Selective Hydrogenation of Fluorinated Esters with
Ruthenium Catalysts 1
ABSTRACT: Selective hydrogenation of fluorinated
esters with pincer-type bifunctional catalysts RuHCl(CO)-
(dpa) 1a, trans-RuH2(CO)(dpa) 1b, and trans-
RuCl2(CO)(dpa) 1c under mild conditions proceeds
rapidly to give the corresponding fluorinated alcohols or
hemiacetals in good to excellent yields. Under the
optimized conditions, the hydrogenation of chiral (R)-2-
fluoropropionate proceeds smoothly to give the corre-
sponding chiral alcohol without any serious decrease of the
ee value.
n incorporation of fluorine atoms into organic molecules is
Aof particular interest because of their unique biological,
pharmaceuticals, and optoelectronic properties.1,2 To access this
important class of compounds, hydrogenation of fluorinated
esters is one of the most straightforward and clean processes in
organic synthesis. However, contrary to ketones, a limited
progress in the hydrogenation of carboxylic esters despite of its
tremendous potential for synthetic organic chemistry has been
made during the last few decades.3 The reduction of carboxylic
acid derivatives still relies mostly on the stoichiometric use of
metal hydrides. Since discovery in 2006 of the hydrogenation of
esters with Milstein’s pincer-type Ru catalysts,4 a rapid advance of
particularly ester hydrogenation has been achieved by utilization
of bifunctional catalysts based on the metal−ligand cooperation.5
Newly developed bifunctional catalysts can efficiently hydro-
genate both activated and/or nonactivated esters. However, most
catalyst systems still need relatively forced reaction conditions for
the smooth reaction except for a limited number of catalyst
systems.6 Herein we disclose practical and selective hydro-
genation of α-fluorinated esters with commercially available
pincer-type bifunctional complex, RuHCl(CO)(dpa)5e 1a (dpa
= bis-(2-diphenylphosphinoethyl)amine) under mild conditions
and its application to synthesis of fluorinated alcohols and fluoral
hemiacetal intermediates from fluorinated esters. The present
hydrogenation is a practical and environmentally benign access
to fluorinated alcohols or hemiacetals, which are key compounds
in synthetic organic chemistry.1,2,7
relatively forced conditions. These results prompted us to quest
for practical catalyst systems for the hydrogenation of fluorinated
esters to valuable fluorinated alcohols. We found that bifunc-
tional catalyst RuHCl(CO)(dpa) 1a efficiently effects hydro-
genation of methyl difluoroacetate 2a with S/C = 5000 in
methanol containing sodium methoxide under 10 atm of H2 at 40
°C giving fluorinated alcohols in almost quantitative yields as
listed in Table 1. The reaction of methyl trifluoroacetate 2b also
gave satisfactory result. Notably, nonfluorinated methyl acetate
and methyl trifluoropropionate bearing fluorine atoms at the β-
position were hydrogenated inefficiently. Hence, the introduc-
tion of the fluorine atom at α-position of esters caused a marked
increase in the rate of the reaction. Methanol is the solvent of
choice for the present hydrogenation. THF and toluene gave
unsatisfactory results. The catalyst 1a is active enough at lower
temperatures 5−30 °C to hydrogenate 2a, 2b giving the
corresponding alcohols in reasonably good yields. The catalyst
trans-RuH2(CO)(dpa) 1b, which is derived from 1a (vide infra),
exhibits similar catalytic activity within experimental accuracy.
The complex trans-RuCl2(CO)(dpa) 1c also worked well albeit
under longer reaction times. A substoichiometric amount of the
base, NaOCH3, is crucial for smooth reaction, although the role
of the base is still unclear.5d After optimization of reaction
conditions, a large-scale hydrogenation of 2a with catalysts 1a
under an S/C = 20 000 at 40 °C proceeded completely for 23 h.
After direct distillation, pure 2,2-difluoroethanol was obtained in
77% isolated yield.
Our early works on the hydrogenation of esters and lactones
with Cp*RuCl(L-N)3,5c,d (L-N: (C6H5)2P(CH2)2NH2, P-N), 2-
C5H4NCH2NH2, N-N) as precatalysts clearly showed that
fluorinated esters can be rapidly hydrogenated albeit under
Received: April 18, 2013
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ja403852e | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
chiral monofluorinated alcohol without any serious decrease of
the ee value after 25 h reaction.
Table 1. Hydrogenation of Fluorinated Esters Catalyzed by
RuHCl(CO)(dpa) or trans-RuX2(CO)(dpa) (X = H, Cl)
a
Thanks to excellent reactivity of the catalysts under mild
conditions, fluorinated esters, including CF2HCO2CH3 2a,
CF3CO2CH3 2b, CClF2CO2CH3 2o, and C2F5CO2CH3 2q,
were selectively hydrogenated with 1a or 1b in the reaction with
S/C = 5000 to hemiacetal intermediates, which are among the
most important synthons for organic synthesis.7 As shown in
Table 2, the outcome of the reaction was delicately influenced by
the reaction conditions. In fact, a decrease of the reaction
temperature to 15 °C in the hydrogenation of 2a caused a drastic
change in the product ratio of 3/4, giving preferentially the
corresponding hemiacetals 4 although in moderate yield. When
hydrogen pressure was decreased under <10 atm at 40 °C, the
hemiacetals 4b, 4o, and 4q were obtainable preferentially. These
results suggest that the highly fluorinated hemiacetal is stable
enough to resist further conversion to alcohols under the
reaction conditions. On the other hand, the reaction of
bromodifluoroacetate 2p provided unsatisfactory results in
terms of the reactivity probably due to the steric effect of
bromine atom. Notably, a decrease of the amount the base
caused a preferential formation of hemiacetal (runs 4,9),
although the precise role of the base is unclear. After optimization
of hydrogenation conditions, a preparative scale reaction of
methyl trifluoroacetate 2b, 128 g with S/C = 20 000 using the
catalyst 1a gave fluoral hemiacetal 4b in 89% yield and with 96%
selectivity. After simple distillation 87 g of 4b was obtained in
67% isolated yield.
temp,
H2, 3 yield,
b
run cat ester S/C
solvent
time, h
°C
atm
%
1
2
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1b
1c
1c
2a
2b
2c
2d
2a
2a
2a
2a
2a
5000
2000
5000
2000
5000
5000
5000
5000
5000
CH3OH
CH3OH
CH3OH
CH3OH
THF
6
24
18
22
19
22
6
40
40
40
40
40
40
30
15
5
10
25
10
30
10
10
10
10
10
10
10
10
10
>99
98
3
17
4
5
5
39
6
toluene
CH3OH
CH3OH
CH3OH
31
7
87
8
6
58
9
6
24
10
11
12
13
2a 10 000 CH3OH
8
40
40
40
40
>99
>99
65
2a
2a
2a
5000
5000
1000
CH3OH
CH3OH
CH3OH
6
21
23
>99
a
Standard reaction conditions: substrate (40 mmol), ester:base =
b
1:0.25, solvent (20 mL). Determined by GC or 19F NMR.
Various α-fluorinated esters were smoothly hydrogenated to
the corresponding fluorinated alcohols in methanol containing
the catalyst 1a and the base under mild conditions described in
Chart 1. Aliphatic and aromatic difluoro esters were reducible
Chart 1. Scope of Substrates Including Chiral Ester
The catalyst precursor, RuHCl(CO)(dpa) 1a, was found to be
readily converted to the real catalyst, trans-RuH2dpa(CO) 1b (δ
1H −5.84 ppm, m, 2H, overlapped; 1H{31P} −5.81 ppm, brs, 1H;
−5.86 ppm, brs, 1H; δ 31P{1H} 66.9 ppm, s) by treatment of 1a
with 1 equiv of tert-BuOK at −80 °C in THF-d8 and following 10
min stirring at room temperature under the dihydrogen
atmosphere. The resulting trans-RuH2(CO)(dpa) 1b has an
almost identical catalytic performance to RuHCl(CO)(dpa) 1a
in the ester hydrogenation as shown in Tables 1 and 2.
Noticeably, contrary to the complex 1a, the complex 1b readily
reacted with methanol to evolve vigorously H2 gas. Similarly, the
complex 1b readily reacted with hemiacetal 4b with H2 release
albeit less vigorously. Recently, methanol dehydrogenation to
hydrogen gas8a or transformation of ethanol to ethyl acetate8b,c in
the presence of complex 1a/base was reported. These
experimental data suggest existence of the dihydrogen complex
on the reaction coordinate between 1b with methanol or 4b.
To gain the insights into the mechanism of selective
hydrogenation of fluorinated esters to hemiacetals, theoretical
studies on the hydrogenation of methyl trifluoroacetate as a
model system were performed. The hydrogenation of CF3C-
with good to excellent conversion and selectivity. In the case of
aliphatic monofluoro esters, the reaction proceeded smoothly in
high yields but less selectivity probably because of formation of
the epoxide generated by cyclization from fluorinated primary
alcohols.
(O)OCH3 into CF3CH(OH)OCH3 is exergonic (ΔG°298K
=
−3.4 kcal·mol−1) in continuum methanol reaction field under
ωB97X-D/6-311++G**/SMD(CH3OH) level of theory, where-
as further dissociation of the hemiacetal into fluoral and CH3OH
is endergonic by 2.4 kcal·mol−1. The α-fluorinated hemiacetals
are known to be stabilized by strong electron-withdrawing
groups giving stable compounds.9 On the other hand, the
hydrogenation of fluoral into CF3CH2OH is thermodynamically
favorable, ΔG°298K = −24.6 kcal·mol−1, thus the present
optimized conditions for the catalytic formation of the
hemiacetal are a pure kinetic phenomenon. This is further
supported by the experimental results: the molar ratio of
CF3CH(OH)OCH3/CF3CH2OH diminishes upon increasing
the temperature or hydrogen pressure.
Other functionalized groups in the esters, such as olefinic (2i,
2j), α-pyridyl (2l), α-thienyl (2m) groups, are intact during the
reaction. The carbonyl selective hydrogenation is applicable to
synthesis of α-fluorinated unsaturated alcohols. For example,
ethyl 2,2-difluoropentenoate (2j) was reduced to unsaturated
alcohol with 99.6% selectivity (Chart 1), in which the E/Z ratio
(94/6) of the substrate was invariable during the reaction.
Noticeably, the hydrogenation of chiral (R)-2-fluoropropionate
(2n) proceeded smoothly to give the corresponding alcohol in
almost quantitative yield albeit with a low ee due to the basic
conditions. To avoid long exposure of chiral esters to the strong
base, the slow addition of 2n during 14 h under otherwise
identical condition was tried. It provided the corresponding
B
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Journal of the American Chemical Society
Communication
a
Table 2. Hydrogenation of Fluorinated Esters Catalyzed by RuHCl(CO)(dpa) or trans-RuX2(CO)(dpa) (X = H, Cl)
b
run
cat
ester
S/C
time, h
temp, °C
H2, atm
yield, %
3
4
1
2
3
1a
1a
1a
1a
1a
1a
1a
1a
1a
1b
1c
2a
2a
2b
2b
2o
2o
2p
2q
2q
2b
2b
5000
5000
2000
5000
2000
2000
3000
2000
5000
5000
3000
6
6
40
15
40
40
40
40
40
40
40
40
40
10
10
20
10
20
10
30
10
5
>99
58
100
8
0
92
15
91
0
6
99
85
9
c
4
6
92
5
6
7
8
22
22
22
22
22
8
>99
83
100
11
−
100
20
8
89
<10
58
−
0
c
9
84
80
92
96
c
10
10
10
89
11
21
84
4
a
b
c
Standard reaction conditions: substrate (40 mmol), ester:base = 1:0.25, CH3OH solvent (20 mL). Determined by GC or 19F NMR. Ester:base
1:0.1
We propose the mechanism of fluoral methyl hemiacetal
formation as shown in Figure 1. Continuum methanol reaction
amido complex. No stationary point was found when a
constrained potential energy surface (PES) scan was performed
along N−H or H···−O coordinate along the vector N−H···O− in
Int2. This is due to a very high basicity of the putative amido
complex. The O− anion reorientation via TS3 (i95 cm−1) affords
hemiacetaloxo complex A (ΔG≠298K = 3.6 kcal·mol−1). Because
of the intrinsic nature of the hemiacetal anion, Int2 is also
intermediate to afford further fluoral and CH3OH. The
possibility for neutralizing the produced anion, −OCH(OCH3)-
(CF3) by the media, rather than by N−H proton transfer from
the Ru cationic complex, was also probed via mixed continuum/
discrete solvation model containing up to 4 explicit methanol
molecules. All the attempts to transfer the proton from methanol
via different constrained PES scans were unsuccessful; all these
scans resulted in the O− anion coordination to afford the
hemiacetaloxo complex A. These results suggest that the anion is
unlikely to be neutralized by the media, this is further supported
by the very basic reaction conditions. The located stationary
point of A is 5.6 kcal·mol−1 (ΔG°298K) below Int2. Hence this
complex could be a possible candidate for the resting state “off
catalytic cycle”. Among two outlined proton sources available in
the media, there is a third possibility under hydrogenation
condition, namely dihydrogen complex Int3 obtained from Int2
via the H2 coordination. The located stationary point for Int3 is
only 3.0 kcal·mol−1 (ΔG°298K) higher than Int2. The dH‑H of 0.82
Å for the coordinated hydrogen places Int3 into the category of
the “true H2 complexes”.16 The acidity of such η2-H2 compounds
sometimes is as strong as that of sulphuric or triflic acid.16 Indeed
deprotonation of dihydrogen in Int3 via TS2 (i935 cm−1)
Figure 1. A possible catalytic cycle for the formation of trifluor-
oacetaldehyde methyl hemiacetal. All the free energies (ΔG°298K
,
kcal·mol−1) are calibrated relative to Int1 (R = CH3).
field DFT10 analysis of unambridged molecules under ωB97X-
D11/6-311++G**(C,H,N,O,F)/SDD(Ru) level of theory with
the SMD12 (solvation model based on density) reveals a very
facile outer-sphere hydride transfer (ΔG≠298K = 7.6 kcal·mol−1)
from the trans-RuH2dpa(CO) 1b to CF3C(O)OCH3 through H-
bonded complex Int1 via TS1 (i611 cm−1) via a two-step process
to afford the inner-sphere ion pair13 intermediate Int2. The
intermediate Int2 is stabilized via two hydrogen bonds, the
strong ionic NH···−OCH(OCH3)(CF3) and very weak “non-
classical” Ru···(H)C bonds.14 The presence of the later is
suggested by the structural analysis: Slightly elongated C−H
bond of the anion that is directed toward Ru atom with the dC‑H
of 1.1136 Å, dRu···(H)C 3.41 Å (cf. 3.73 Å, the sum of the van der
Waals of radii of Ru and carbon),15 angle Ru···H−C 127°.
The intermediate Int2 is a branching point of the reaction. The
N−H proton transfer from the Ru cation to the hemiacetal anion
could afford directly the hemiacetal 4b and the putative 16e Ru
C
dx.doi.org/10.1021/ja403852e | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
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= 6.6 kcal·mol−1). The dihydrogen bonding in the
298K
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catalytic cycle is the outer-sphere hydride transfer (TS1) and not
H2 cleavage.19
In conclusion, selective hydrogenation of fluorinated esters 2
with highly efficient bifunctional RuHCl(CO)(dpa) 1a or trans-
RuH2(CO)(dpa) 1b catalysts proceeded rapidly under mild
conditions to give the corresponding fluorinated alcohols in
almost quantitative yields. Under the optimized conditions, the
hemiacetals intermediates are obtainable from the reaction of α-
fluorinated esters in good to excellent yields. DFT analysis of the
selective hydrogenation of 2b suggests that the hydride transfer
from the dihydride complex 1b to the ester occurs in the outer-
sphere to produces a contact ion pair intermediate Int2. The
latter further coordinates the molecular hydrogen and affords the
final hemiacetal via intramolecular deprotonation, a step during
which H-H bond is cleaved.
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ASSOCIATED CONTENT
* Supporting Information
Experimental procedure and analysis details. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
S
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AUTHOR INFORMATION
Corresponding Author
tikariya@apc.titech.ac.jp
■
Notes
(13) Macchioni, A. Chem. Rev. 2005, 105, 2039.
The authors declare no competing financial interest.
(14) Dub, P. A.; Ikariya, T. J. Am. Chem. Soc. 2013, 135, 14960.
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ACKNOWLEDGMENTS
■
Research was financially supported by Grants-in-Aid for
Scientific Research (S) (22225004) from the Ministry of
Education, Culture, Sports, Science and Technology, Japan,
and partly supported by the GCOE Program. We acknowledge
Takasago Inter Corp for a gift of RuHCl(CO)(dpa) catalyst.
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