Homogeneous ruthenium catalyzed hydrogenation of esters to alcohols

Article abstract of DOI:10.1039/a801807c

The homogeneous catalytic hydrogenation of aromatic and aliphatic esters to the corresponding alcohols, by a catalyst generated in situ from [Ru(acac)₃] and MeC(CH₂PPh₂)₃ in an alcoholic solvent under H₂ pressure of 85 bar at 100-120°C, is described.

Full text of DOI:10.1039/a801807c

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Homogeneous ruthenium catalyzed hydrogenation of esters to alcohols‡
Herman T. Teunissen and Cornelis J. Elsevier*†
Institute of Molecular Chemistry, Universiteit van Amsterdam, Nieuwe Achtergracht 166, NL-1018 WV Amsterdam, The
Netherlands
The homogeneous catalytic hydrogenation of aromatic and
aliphatic esters to the corresponding alcohols, by a catalyst
generated in situ from [Ru(acac)3] and MeC(CH2PPh2)3 in
an alcoholic solvent under H2 pressure of 85 bar at
100–120 °C, is described.
improvement is observed when the solvent MeOH is replaced
by propan-2-ol (IPA). The combination IPA–HBF4 shows the
highest catalytic activity and gives rise to formation of BHB in
high yield (entry 5) which was not observed in any previous
experiment. Comparing our system with that of Matteoli et al.3a
(entry 6) shows that we have achieved a considerable improve-
ment with respect to turnover numbers and turnover frequencies
under relatively mild conditions.
Next, we applied our system to the hydrogenation of benzyl
benzoate (BZB, an unactivated ester) which is difficult to
hydrogenate to benzyl alcohol (BZOH) (Table 2). In the
presence of HBF4 or NEt3, we could achieve a TON of 33 (entry
1) and 105 (entry 2), respectively at 120 °C for this conversion.
Furthermore, we investigated the influence of fluorinated
alcohols on the hydrogenation of benzyl benzoate. These
investigations were based on the results of Grey et al.,2 who
found that an ester is more easily hydrogenated when electron
withdrawing substituents are present. Therefore, we postulated
that an integration of transesterification (using e.g. 2,2,2-tri-
fluoroethanol) and hydrogenation would lead to a substantial
increase in catalytic activity due to substrate activation [eqn.
(2)].
The reduction of esters to the corresponding alcohols is an
important reaction which is usually achieved by a stoichio-
metric reaction using lithium aluminium hydride,1 whereas
homogeneous catalytic routes have been scarcely explored.2–4
For aromatic esters, the only example is concerned with the
hydrogenation of dimethyl phthalate (DMP) to phthalide (PHT)
O
O
CH2OH
H2
OMe
OMe
(1)
+
O
CH2OH
O
1,2-Bis(hydroxymethyl)-
benzene (BHB)
Dimethyl
phthalate (DMP)
Phthalide (PHT)
using a ruthenium hydride complex [eqn. (1)];3a PHT was
obtained in a yield of 11.5% after 144 h at 180 °C under a
hydrogen pressure of 130 bar.
O
O
H2
F3CCH2OH
PhC
PhC
PhCH2OH
(2)
Hydrogenation of PHT to 1,2-bis(hydroxymethyl)benzene
(BHB) did not occur due to the absence of electron withdrawing
substituents which enabled the conversion of DMP to PHT.
Therefore, the hydrogenation of unactivated esters to the
corresponding alcohols is one of the most intriguing challenges
in contemporary hydrogenation catalysis. We have recently
reported an important improvement using a catalyst based on
[Ru(acac)3] and MeC(CH2PPh2)3 in methanol,4b for the homo-
geneous hydrogenation of dimethyl oxalate. Now, we wish to
report a catalytic system which is able to catalyse the
hydrogenation of unactivated aromatic and aliphatic esters with
very high turnover numbers.§
From Table 1 it is evident that the hydrogenation of DMP to
give mainly PHT is feasible under relatively mild conditions
(entry 1) employing a system consisting of [Ru(acac)3] and
MeC(CH2PPh2)3 in methanol. The catalytic activity (expressed
as turnover number, TON) appeared to be strongly influenced
by additives. Compared to entry 1, a negative effect is observed
with Zn as an additive (entry 2) while positive effects are
observed with NEt3 (entry 3) and HBF4 (entry 4). A further
OCH2Ph
OCH2CF3
PhCH2OH
F3CCH2OH
The hydrogenation of BZB to BZOH in 2,2,2-trifluoro-
ethanol (TFE) at 120 °C turned out to be very successful (Table
2) and seems to confirm our hypothesis. Compared to the
experiments in propan-2-ol (entry 2) the catalytic activity in
TFE has drastically increased reaching a turnover number of
nearly 900 in the presence of NEt3 (entry 3). At 100 °C the
catalytic activity was significantly lower, as expected (entry 4).
Hydrogenation of BZB does not occur in the presence of HBF4
(entry 5). Instead, three other products are formed: 2,2,2-tri-
fluoroethyl benzoate, benzoic acid and a polymeric material.
The formation of benzoic acid (82% isolated yield, TON 1412)
was unexpected and is not readily explained. Hydrogenolysis of
benzyl esters using heterogeneous palladium catalysts is
common5 but unprecedented for ruthenium complexes. Subse-
quently, the hydrogenation of BZB was carried out in
1,1,1,3,3,3-hexafluoropropan-2-ol (FIPA) which gave even
Table 1 Hydrogenation of DMP with the [Ru(acac)3]–MeC(CH2PPh2)3 systema
Entry
DMP/mmol
[Ru(acac)3]/mmol
Additive/mmol
Conv. (%)
Yield PHTb (%)
Yield BHBb (%)
TON
TOF/h21
1
2
3
4
5
6
1.16
1.00
1.05
1.14
1.17
30.9
19.1
18.3
15.3
17.1
18.1
—
31
25
87
91
30
18
82
79
18
12
1
0
0
0
78
0
19
10
56
53
103
51
1.2
0.6
3.5
3.4
5.6
0.4
Zn (0.82)
NEt3 (22.70)
HBF4 (0.82)
IPAc + HBF4 (0.27) 100
[Ru4H4(CO)8(PBu3)4]d
21
a Conditions: 100 °C; 85 bar H2; 16 h; 1.15–1.65 equiv. MeC(CH2PPh2)3 in MeOH (12 ml). b Yield determined by GC or NMR (entry 5). c Propan-2-ol (12
ml, instead of MeOH). d From ref. 3(a); T = 180 °C; p(H2) = 130 bar, 144 h.
Chem. Commun., 1998
1367

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Table 2 Hydrogenation of esters in fluorinated alcoholsa
Entry
Substrate/mmol
[Ru(acac)3]/mmol
Additive/mmol
Solvent
Conv. (%)
Productb
Yieldc (%)
TON
1
2
3
4d
5
6
7f
8
9
10
BZB (1.3)
BZB (4.2)
BZB (30.5)
BZB (17.3)
BZB (29.5)
BZB (28.4)
BZB (25.9)
BZB (31.0)
DMM (14.2)
MP (8.3)
21.3
18.3
18.1
18.1
17.1
13.1
20.1
10.8
14.1
13.1
HBF4 (0.42)
NEt3 (0.34)
NEt3 (2.60)
NEt3 (2.46)
HBF4 (0.47)
NEt3 (2.58)
NEt3 (2.55)
NEt3 (2.72)
NEt3 (2.76)
NEt3 (2.66)
IPA
IPA
TFE
TFE
63
87
65
43
87
97
7
75
100
94
BZOH
BZOH
BZOH
BZOH
BZA
BZOH
BZOH
BZOH
BDO
56
82
53
23
82e
95
1
67
100
94
33
105
896
219
TFE
1412
2071
12
1909
2019g
596
FIPA
FIPA
FIPA–TFE
FIPA
FIPA
HDO
a Conditions: 120 °C; 85 bar H2; 16 h; [Ru(acac)3]; 1.15–1.65 equiv. MeC(CH2PPh2)3. b Apart from the alcohol, significant formation of transesterification
c
1
d
product was observed: entry 3 (8%); entry 4 (16%); entry 5 not determined; entry 7 (1%); entry 8 (4%). Yield determined by H NMR. T = 100 °C.
e Isolated yield. f p(H2) = 0 bar. g The hydrogenation of the CNC bond is not included in the turnover number.
d –
d +
better results. Hydrogenation of BZB in FIPA occurs with a
turnover number of > 2000 (entry 6), which is an enormous step
forward in the homogeneous catalytic hydrogenation of un-
activated esters. This result was completely unexpected and
contradicts the hypothesis of transesterification prior to hydro-
genation. In principle, a primary alcohol such as TFE should
give rise to transesterification more easily than a secondary
alcohol like FIPA.6 In view of these facts we suggest that the
high catalytic activity in FIPA is due to ionic hydrogenation
(vide infra) rather than to an integration of transesterification
and hydrogenation. Compared to the experiment in FIPA, a
mixture of TFE and FIPA exhibited a slightly lower activity
(entry 8), which also raises objection to our initial hypothesis.
Finally, we investigated the possibility of transfer hydro-
genation (entry 7). The low catalytic activity in this case
emphasizes that a substantial hydrogen pressure is necessary for
successful catalysis. Possibly, the ruthenium catalyzed oxida-
tive transformation of BZOH to BZB7 becomes feasible in the
absence of hydrogen pressure thus obstructing a successful
transfer hydrogenation.
W
H
H
OPh
Ph
C
H
O
A
A comparable complex has been reported recently for a
ruthenium hydride complex and FIPA.8c The unique features of
our catalytic system are ascribed to ionic hydrogenation of the
ester. Investigations pertaining to the mechanism of the
hydrogenation of esters are currently in progress in our
laboratory.
We gratefully acknowledge stimulating discussions with Dr
J. G. de Vries (DSM, Geleen, NL) and Dr M. J. Doyle (SRTCA,
Amsterdam, NL).
Notes and References
† E-mail: else4@anorg.chem.uva.nl
‡ NIOK publication UvA 98-03-03. This work was supported by the
Innovation Oriented Research Programme (IOP-katalyse) under the aus-
pices of the Netherlands’ Ministry of Economic Affairs.
Using the optimized catalytic system for the conversion of
BZB (FIPA, entry 6), we explored the scope of this system using
dimethyl maleate (DMM) [eqn. (3)] and methyl palmitate (MP)
[eqn. (4)] as substrates. The results show that our system is able
§ General procedure: first, a solution was prepared of [Ru(acac)3],
MeC(CH2PPh2)3, the appropriate ester and additive (see Tables 1 and 2) in
the appropriate solvent (15 ml) under N2. Separately, a home-built stainless
steel autoclave equipped with a magnetic stirring bar was flushed with dry
nitrogen after which the dark red pre-catalyst solution was introduced via a
needle. The autoclave was flushed with H2 (at 50 bar), pressurized with
hydrogen (85 bar at 20 °C) and heated for 16 h at the indicated temperature.
The reaction products were characterized by GC–MS, the yields were
determined with GC (internal standard) or 1H NMR.
O
CH2OH
OMe
OMe
H2
(3)
CH2OH
O
1 S. N. Ege, Organic Chemistry, D. C. Heath and Company, Lexington,
1989, p. 596.
2 R. A. Grey, G. P. Pez and A. Wallo, J. Am. Chem. Soc., 1981, 103,
7536.
3 (a) U. Matteoli, M. Bianchi, G. Menchi, P. Frediani and F. Piacenti,
J. Mol. Catal., 1984, 22, 353; (b) U. Matteoli, G. Menchi, M. Bianchi and
F. Piacenti, J. Organomet. Chem., 1986, 299, 233; (c) U. Matteoli, G.
Menchi, M. Bianchi, F. Piacenti, S. Ianelli and M. Nardelli,
J. Organomet. Chem., 1995, 498, 177 and refs. therein.
4 (a) Y. Hara, H. Inagaki, S. Nishimura and K. Wada, Chem. Lett., 1992,
1983; (b) H. T. Teunissen and C. J. Elsevier, Chem. Commun., 1997,
667.
5 J. S. Bajwa, Tetrahedron Lett., 1992, 33, 2299; W. H. Hartung and R.
Simonoff, Org. React., 1953, 7, 263.
6 J. Otera, Chem. Rev., 1993, 93, 1449.
7 S. I. Murahashi, T. Naota, K. Ito, Y. Maeda and H. Taki, J. Org. Chem.,
1987, 52, 4319; Y. Blum and Y. Shvo, J. Organomet. Chem., 1985, 282,
C7.
8 (a) H. Berke and P. Burger, Comments Inorg. Chem., 1994, 16, 279; (b)
H. Berke, Book of abstracts, XIIth FECHEM Conference on Organo-
metallic Chemistry, Prague 1997, PL 9; (c) J. A. Ayllon, C. Gervaux, S.
Sabo-Etienne and B. Chaudret, Organometallics, 1997, 16, 2000.
Butane-1,4-diol (BDO)
Dimethyl
maleate (DMM)
O
H2
CH2OH
C
15H31
(4)
C
15H31
OMe
Methyl
palmitate (MP)
Hexadecan-1-ol (HDO)
to catalyze the hydrogenation of these substrates to the
corresponding alcohols with, compared to BZB, a high activity
for DMM (entry 9) and a slightly smaller activity for MP (entry
10). Altogether we can conclude that efficient hydrogenation of
unactivated aromatic and aliphatic esters is possible using
[Ru(acac)3] and MeC(CH2PPh2)3 in FIPA at 120 °C. Possibly,
the remarkable activity in TFE and FIPA compared to IPA is
related to the pKa of the alcohols and not to transesterification.
Berke and Burger showed that phenol8a and FIPA8b drastically
influence the rate of insertion of aldehydes into the W–H bond
in tungsten nitrosyl complexes. This influence was explained by
ionic hydrogenation (via A).
Received in Basel, Switzerland, 5th March 1998; 8/01807C
1368
Chem. Commun., 1998