Expanding the Catalytic Scope of (Cyclopentadienone)iron Complexes to the Hydrogenation of Activated Esters to Alcohols

Article abstract of DOI:10.1002/cctc.201600972

Herein, we report the application of easy-to-make and bench-stable (cyclopentadienone)iron complexes (such as 1) as precatalysts for the hydrogenation of esters. After optimization of the reaction conditions (i.e., solvent, temperature, pressure), complex 1 was tested in the hydrogenation of a range of esters. With most of the activated trifluoroacetate esters, quantitative formation of 2,2,2-trifluoroethanol was obtained at low catalyst loadings. For nonactivated esters, no reaction was observed. Trifluoroacetic acid, a common impurity in hydrolytically labile trifluoroacetate esters, was shown to act as a poison for the catalyst. However, the simple addition of Et₃N allowed the catalyst activity to be restored. Our study constitutes the first examples of ester hydrogenation with an Fe complex based on a non-pincer ligand.

Full text of DOI:10.1002/cctc.201600972

Accepted Article
Title: Expanding the catalytic scope of (cyclopentadienone)iron
complexes to the hydrogenation of activated esters to alcohols
Authors: Luca Pignataro, Piotr Gajewski, Angela Gonzalez-de-Castro,
Marc Renom-Carrasco, Umberto Piarulli, Cesare Gennari,
Johannes Gerardus de Vries, and Laurent Lefort
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To be cited as: ChemCatChem 10.1002/cctc.201600972
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Expanding the catalytic scope of (cyclopentadienone)iron
complexes to the hydrogenation of activated esters to alcohols
Piotr Gajewski,[a,b] Angela Gonzalez-de-Castro,[b] Marc Renom-Carrasco,[a,b] Umberto Piarulli,[c] Cesare
Gennari,[a] Johannes G. de Vries,[d] Laurent Lefort,[b]* and Luca Pignataro[a]*
Abstract: Herein, we report the application of the easy-to-make and
bench-stable (cyclopentadienone)iron complexes (such as 1) as pre-
catalysts for the hydrogenation of esters. After optimization of the
reaction conditions (solvent, temperature, pressure), complex 1 was
tested in the hydrogenation of a range of esters. With most of the
activated trifluoroacetate esters, a quantitative formation of TFE was
obtained at low catalyst loadings. For non-activated esters, no
reaction was observed. Trifluoroacetic acid, a common impurity in
hydrolytically labile trifluroacetate esters, was shown to act as a
poison for the catalyst. However, the simple addition of Et3N allowed
to restore the catalyst activity. Our study constitutes the first example
of ester hydrogenation with an Fe complex based on a non-pincer
ligand.
availability (2nd most abundant metal in the Earth’s crust), low
cost and scarce toxicity.[ 4 ] Accordingly, numerous Fe-based
catalysts have been developed in the last decade for the
hydrogenation of olefins,[5] ketones,[6] imines,[6c,7] and carbon
8
]
dioxide/sodium
bicarbonate,[7a,
including
several
enantioselective versions.[6i-m,7e,f] The more difficult ester
hydrogenation[9] was only recently reported for the first time by
[10]
Milstein and co-workers (Figure 1 A) .
Catalytic hydrogenations (CHs) are a reaction class of key
importance in the sustainable manufacture of both bulk
commodities and fine chemicals for several reasons. Firstly, H2
is a cheap and clean reductant, which allows to achieve a
perfect atom economy and to minimize the generation of waste.
Secondly, CHs are operationally simple and generally require
minimal workup operations for the isolation of the product. Last
but not least, as CHs are a very mature research field, countless
heterogeneous[ 1 ] and homogeneous[ 2 ] catalysts have been
developed, which allow to carry out these reactions with high
chemo-, regio- and/or stereoselectivity. However, most of these
catalysts rely on precious metals (e.g., Ru, Rh, Ir, Pd, Pt), with
associated problems of toxicity, high cost and limited stock
which prevent, in some cases, their industrial use. Therefore,
replacing precious metals with cheap base metals such as Fe,
Co or Ni is currently considered a task of primary scientific and
industrial relevance.[3] From the point of view of sustainability, Fe
is certainly the most appealing of these metals, owing to its wide
Figure 1. Previous examples of Fe-catalyzed ester hydrogenation (A, B), and
the new methodology described in this paper (C).
Shortly after the Milstein’s report, other examples of Fe-
catalyzed ester hydrogenation were described by the groups of
Beller[11] and Guan-Fairweather[12] (Figure 1 B), expanding the
substrate scope to non-trifluoroacetate esters.[13] However, all
these methodologies rely on costly and air-sensitive PNP pincer
ligand Fe-complexes,[10-12] which hampers their possible
industrial use. In sharp contrast, (cyclopentadienone)iron
complexes[14] (Figure 1 C, Figure 2) are ideal pre-catalysts for
industrial applications, as they are easy-to-make and stable
compounds.[ 15 ] Under suitable experimental conditions, these
complexes can be converted in situ into catalysts for the
hydrogenation of ketones,[6g,h,k-m] imines[7a-d] and carbon
dioxide/sodium bicarbonate.[7a,8a] However, to the best of our
knowledge, no application of (cyclopentadienone)iron complexes
in ester CH has been reported so far.
[a]
P. Gajewski, M. Renom-Carrasco, Dr. L. Pignataro, Prof. Dr. C.
Gennari
Università degli Studi di Milano, Dipartimento di Chimica
Via C. Golgi, 19, I-20133, Milan (Italy)
P. Gajewski, Dr. A. Gonzalez-de-Castro, M. Renom-Carrasco, Dr. L.
Lefort
DSM Innovative Synthesis BV,
P.O. box 18, 6160 MD Geleen (The Netherlands)
E-mail: laurent.lefort@dsm.com
[b]
[c]
[d]
Prof. Dr. U. Piarulli
Università degli Studi dell’Insubria,
Dipartimento di Scienza e Alta Tecnologia,
Via Valleggio, 11, I-22100, Como (Italy)
E-mail: umberto.piarulli@uninsubria.it
Prof. Dr. J. G. de Vries
Leibniz-Institut für Katalyse e. V.
Albert-Einstein-Str., 29 a 18059 Rostock (Germany)
Building on our expertise in the synthesis and catalytic use of
(cyclopentadienone)iron complexes,[6k,l] we set to investigate
whether they could be employed as pre-catalysts for the
hydrogenation of esters. Thus, the “classical” complex 1 (Figure
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2) was tested, after in situ activation with Me3NO,[6i-m,7a-d] in the
hydrogenation of activated trifluoroacetate substrates S1-S3
(Table 1). The first experiments were carried out under 70 bar of
H2 at 110 °C for 17 h.
catalyst being operating. With the less activated substrates S2
and S3, full conversion was not obtained (Table 1, entries 8, 9).
However, when using pure 1,4-dioxane as a solvent and
therefore adding Me3NO as a solid, S2 was fully hydrogenated
(Table 1, entry 10). Uncomplete conversion was obtained in
pure DCE (Table 1, entry 11), indicating that this solvent may
have been the cause of the previously observed catalyst stalling.
Toluene also allowed to obtain 100% yield for both S2 and S3
(Table 1, entries 12-13) and was selected as solvent for the rest
of our study.
Figure 2. Examples of (cyclopentadienone)iron complex.
Further optimization of the reaction parameters was performed
with substrate S3 in toluene (Table 2). Decreasing the amount of
pre-catalyst 1 from 2.5 to 1 mol% did not affect the yield, which
remained quantitative both at 110 °C (Table 2, entry 2) and at
90 °C (Table 2, entry 3), corresponding to a TON of 100. Nearly
quantitative yields were obtained also when temperature (Table
2, entry 4) or pressure (Table 2, entry 5) were further decreased
to 70 °C and 35 bar, respectively. At 90 °C and under 70 bar of
H2, the catalyst loading could be further reduced (Table 2,
entries 6-8), leading to a decrease in conversion but an increase
in TON.
As in the Fe-catalyzed hydrogenation of S1 reported by Milstein
and co-workers,[10] we intended to use 1,4-dioxane as solvent.
However, Me3NO – i.e. the catalyst activator – was poorly
soluble in 1,4-dioxane. Therefore, it was added to the catalytic
mixture from
a 1,2-dichloroethane (DCE) stock solution.
Consequently our initial trials were done using
dioxane/DCE solvent mixture.
a 1,4-
Table 1. Test of pre-catalyst 1 in the hydrogenation of trifluoroacetate esters
S1-S3.[a]
Table 2. Optimization of reaction parameters in the hydrogenation of n-hexyl
trifluoroacetate S3 promoted by complex 1.[a]
Yield TFE
Entry
Sub.
Deviation from above
(%)[b]
100[c]
7[d]
Entry mol% 1
T (°C) P (bar) Yield (%)[b]
TON
40
100
100
99
1
2
3
4
5
6
7
8
9
10
11
12
13
S1
S1
S1
S1
S1
S1
S1
S2
S3
S2
S2
S2
S3
-
without 1
1
2
3
4
5
6
7
8
2.5
1
1
1
1
0.5
0.25
0.2
110
110
90
70
70
70
70
35
70
70
70
100
100
100
99
without Me3NO
+ Hg(0) [38 mol%]
+ P(OMe)3 [1.3 mol%]
+ P(OMe)3 [2.5 mol%]
+ P(OMe)3 [5.0 mol%]
-
17[d]
100
100
36
70
90
98
98
90
97
194
336
310
16
77
90
84
90
62
-
82[e]
100
89
[a] Reaction conditions: substrate (1 mmol), 1 (x mol%), Me3NO (2x mol%),
1,4-dioxane
DCE
toluene
[b]
toluene (0.75 mL), reaction time = 17 h.
Determined by 19F{1H}-NMR
100
100
analysis of the reaction crude. No substrate hydrolysis observed in any case.
toluene
[a] Reaction conditions: substrate (1 mmol), 1 (2.5 mol%), Me3NO (5 mol%),
The known (cyclopentadienone)iron complexes 2 and 3 (Figure
2), screened in the hydrogenation of substrate S3 under the
optimized conditions, also showed catalytic activity (Scheme 1).
PH2 = 70 bar, solvent (0.75 mL), T = 110 °C, reaction time = 17 h. DCE = 1,2-
[b]
dichloroethane; TFE = 2,2,2-trifluoroethanol.
Determined by 19F{1H}-NMR
[c]
analysis of the reaction crude.[10]
Mole balance (by 19F{1H}-NMR, using
α,α,α-trifluorotoluene as internal standard) = 89%, due to the low boiling point
[d]
of S1 and consequent loss during handling.
Formed by hydrolysis of S1,
[e]
together with an equimolar amount of trifluoroacetic acid (TFA).
balance = 99%.
Mole
Scheme 1. Screening of (cyclopentadienone)iron complexes 2 and 3 in the
hydrogenation of n-hexyl trifluoroacetate (S3).
As shown in Table 1 (entry 1), substrate S1 was quantitatively
hydrogenated to 2,2,2-trifluoroethanol (TFE) with no sign of
hydrolysis to trifluoroacetic acid (TFA). In the absence of either
complex 1 or Me3NO, no reduction took place (Table 1, entries 2
and 3), but TFE was detected due to hydrolysis of S1, as
confirmed by the presence of equimolar amounts of TFA.
Catalyst poisoning experiments were carried out in operando:[16]
while the addition of an excess of Hg(0) did not affect the
hydrogenation of S1 (Table 1, entry 4), P(OMe)3 led to a drop of
conversion only when its amount was ≥ 1 equiv. to 1 (Table 1,
entries 5-7). These data are consistent with a homogeneous
The 3,4-ethylenediamino-substituted (cyclopentadienone)iron
complex 3, recently reported as very active in the hydrogenation
of imines and NaHCO3,[7a] appeared to be slightly less active
than pre-catalysts 1 and 2.
Trifluoroacetates are prone to undergo hydrolysis with
adventitious water, releasing TFA. Thus, before determining the
substrate scope of pre-catalyst 1, we decided to investigate
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10.1002/cctc.201600972
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whether traces of TFA could exert a detrimental effect on the
catalytic reaction (Table 3).
Table 4. Substrate screening for the hydrogenation of esters in the
presence of pre-catalyst 1 [a]
.
Table 3. Study of effect of TFA on the hydrogenation activity of 1.[a]
Yield
(%)[b]
Yield
(%)[b]
#
1
Substrate
#
Substrate
S11
100[c]
11
100[d]
Entry mol% TFA mol% TEA Yield (%)[b]
S1
1
2
3
4
5
6
7
-
1.3
5
-
5
-
-
-
5
1
5
20
100
4
0
100
0
7
2
3
4
100
12
13
14
0[d]
S12
S2
S3
5
5
85
100
0[d]
Reaction conditions: substrate S4 (1 mmol), 1 (1 mol%), Me3NO (2 mol%),
[b]
toluene (0.25 mL), reaction time = 17 h.
analysis of the reaction crude.
Determined by 19F{1H}-NMR
S13
S14
100
100
0[d]
For this purpose, a fresh batch of benzyl trifluoroacetate S4,
totally exempt from TFA (as verified by 19F{1H}-NMR), was
prepared. As shown in Table 3, the hydrogenation of S4
proceeded smoothly in the absence of any additive (Table 3,
entry 1). The addition of 1.3 mol% of TFA (Table 3, entry 2)
caused an almost complete deactivation of the catalyst – further
confirmed when 5 mol% of TFA was used (Table 3, entry 3). We
considered that addition of a base such as triethylamine (TEA)
could prevent catalyst poisoning by neutralizing TFA. First, it
was verified that TEA alone (5 mol%) does not impair the
reaction (Table 3, entry 4). Gratifyingly, the addition of an excess
of TEA restored to a large extent the activity of the catalyst
(Table 3, entries 5-7).
S4
S5
S6
5
6
15
16
0[d]
S15
S16
87,
0
100[d]
50,
7
8
17
18
0[d]
100[d,e]
S7
S8
S17
S18
100
0[f]
A substrate screening with 20 different esters was carried out
under optimized reaction conditions (Table 4). When traces of
TFA were detected in the starting material, TEA (20 mol%) was
added prior to the hydrogenation. With the exception of S12 and
S13 – bearing electron-poor aryl groups – all trifluoroacetates
S1-11 gave full conversion (entries 1-13). In contrary to the
seminal report of Milstein and co-workers on the Fe-catalyzed
hydrogenation of trifluoroacetate esters,[10] the yields were
scarcely influenced by the steric bulk of the substituents at the
alkoxy group, and relatively bulky substrates such as S2, S6 and
S8 were quantitatively hydrogenated. Unfortunately, all the non-
trifluoroacetate substrates S14-20 (Table 4, entries 14-20)
turned out to be totally unreactive, including CF2HCOOEt (S14)
and CF2BrCOOEt (S15) which were expected to be only slightly
“less activated” than CF3COOEt (S5).
60,
9
19
20
0[f]
100[d]
S9
S19
S20
0,
10
0[f]
100[d]
S10
Reaction conditions: substrate (1 mmol), 1 (1 mol%), Me3NO (2 mol%),
[b]
toluene (0.25 mL), reaction time = 17 h.
Determined by 19F{1H}-NMR
[c]
analysis of the reaction crude.
Pre-treatment with TEA (20 mol%) was
necessary at this stage due to the formation of TFA in our ageing batch of S1.
[d]
[e]
Substrate pre-treated with TEA (20 mol%) before hydrogenation.
Our
analytical method did not allow us to determine whether the olefin was
hydrogenated or not. [f] Determined by GC with dodecane as internal standard.
In both cycles A and B, the active complex act-1a (initially
generated from 1 in the presence of Me3NO) splits H2 forming
the (hydroxycyclopentadienyl)iron hydride act-1b, which was
firstly isolated and characterized by Knölker et al.[17] The two
above-mentioned C=O reduction steps are sequentially
performed by act-1b through the pericyclic transition states TS-
A and TS-B, similar to those commonly accepted for the
hydrogenation of aldehydes and ketones.[6e,f,18] As esters are
much less reactive towards hydride attack compared to
aldehydes, the formation of the hemiacetal A should be the rate-
limiting step of the process. This expectation is supported by the
We elaborated a tentative mechanism (Scheme 2) which,
similarly to the mechanisms proposed for other catalytic ester
hydrogenation methodologies,[9-12] involves two steps performed
by the same catalyst: i) reduction of the ester C=O group leading
to the hemiacetal A (catalytic Cycle A); ii) reduction of the
aldehyde B (catalytic Cycle B), formed by hydrolysis of the
hemiacetal A.
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were flushed with nitrogen (10 bar). After adding α,α,α-trifluorotoluene
fact that conversion is observed only with trifluoroacetate esters,
possessing the strongly electron-withdrawing group CF3.
(about 0.5 mmol), the crudes were analyzed by 19F{1H} NMR.
Acknowledgements
We thank the European Commission [ITN-EID “REDUCTO”
PITN-GA-2012-316371] for financial support and for predoctoral
fellowships (to P. Gajewski and M. Renom-Carrasco).
Keywords: iron • hydrogenation • homogeneous catalysis •
ester reduction • (cyclopentadienone)iron complexes
References
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In conclusion, we have demonstrated that air-stable and easy-
to-make (cyclopentadienone)iron complexes such as 1 can be
used as pre-catalysts for the hydrogenation of trifluoroacetate
esters. The reaction, which occurs quantitatively at low catalyst
loading (TON up to 336), has a broad substrate scope in terms
of alkoxy residues, and bulky substituents are also tolerated.
This study represents a remarkable expansion of the application
scope of (cyclopentadienone)metal complexes in general (metal
= Fe or Ru, in most cases), which did not include so far any
ester substrate.[14,19] Moreover, to the best of our knowledge, this
is the first example of Fe-catalyzed ester hydrogenation
involving a cheap and stable pre-catalyst instead of costly and
air-sensitive pincer complexes.[9,11b]
Experimental Section
General procedure for the ester hydrogenation screening. In a
nitrogen-filled mBraun glovebox, a solution of pre-catalyst (0.01 mmol, 1
mol% in 0.25 mL solvent) was dispensed to a glass vial containing solid
Me3NO (0.02 mmol, 2 mol%). The vials were capped and the obtained
solutions were stirred for 1 h at 30 °C. After this time, each vial was
opened and a mixture of substrate (1 mmol) and TEA (0.2 mmol, 20
mol%) was added. The vials were capped again and put inside the
Premex 96er Multireactor. The system was purged three times with
nitrogen (10 bar) and three times with hydrogen (10 bar). The reaction
vessels were pressurized at 70 bar, heated at 90 °C and stirred overnight.
The reactions were cooled down and then, after releasing hydrogen, they
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Filonenko, R. van Putten, E. J. M. Hensen, E. A. Pidko, Chem. Soc. Rev.
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COMMUNICATION
2015, 44, 3808-3833; b) P. A. Dub, T. Ikariya, ACS Catalysis 2012, 2,
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