Efficient and Practical Transfer Hydrogenation of Ketones Catalyzed by a Simple Bidentate Mn?NHC Complex

Article abstract of DOI:10.1002/cctc.201900882

Catalytic reductions of carbonyl-containing compounds are highly important for the safe, sustainable, and economical production of alcohols. Herein, we report on the efficient transfer hydrogenation of ketones catalyzed by a highly potent Mn(I)?NHC complex. Mn?NHC 1 is practical at metal concentrations as low as 75 ppm, thus approaching loadings more conventionally reserved for noble metal based systems. With these low Mn concentrations, catalyst deactivation is found to be highly temperature dependent and becomes especially prominent at increased reaction temperature. Ultimately, understanding of deactivation pathways could help close the activity/stability-gap with Ru and Ir catalysts towards the practical implementation of sustainable earth-abundant Mn-complexes.

Full text of DOI:10.1002/cctc.201900882

Accepted Article
Title: Efficient and practical transfer hydrogenation of ketones
catalyzed by a simple bidentate Mn-NHC complex
Authors: Evgeny Alexandrovich Pidko, Robbert van Putten, Joeri
Benschop, Vincent de Munck, Manuela Weber, Christian
Mueller, and Georgy Filonenko
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To be cited as: ChemCatChem 10.1002/cctc.201900882
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10.1002/cctc.201900882
ChemCatChem
COMMUNICATION
Efficient and practical transfer hydrogenation of ketones
catalyzed by a simple bidentate Mn-NHC complex
Robbert van Putten[a], Joeri Benschop[a], Vincent J. de Munck[a], Manuela Weber[b], Christian Müller[b],
Georgy A. Filonenko[a], and Evgeny A. Pidko[a]*
Abstract: Catalytic reductions of carbonyl-containing compounds
are highly important for the safe, sustainable, and economical
production of alcohols. Herein, we report on the efficient transfer
hydrogenation of ketones catalyzed by a highly potent Mn(I)-NHC
complex. Mn-NHC 1 is practical at metal concentrations as low as 75
ppm, thus approaching loadings more conventionally reserved for
noble metal based systems. With these low Mn concentrations,
catalyst deactivation is found to be highly temperature dependent
and becomes especially prominent at increased reaction
temperature. Ultimately, understanding of deactivation pathways
could help close the activity/stability-gap with Ru and Ir catalysts
towards the practical implementation of sustainable earth-abundant
Mn-complexes.
consumption typically is not a large concern for academic
researchers, it likely presents critical hurdle for any
commercial application. Moreover, in order to reduce operational
cost it is desirable to replace highly complex, synthetically
challenging, and expensive phosphines for simpler and more
scalable alternatives.
a
N-Heterocyclic carbenes (NHCs) in principle meet the main
requirements to replace phosphine donors. The steric and
electronic properties of NHCs are highly tunable, their synthesis
is well established, scalable, and can conveniently be performed
in air.[6] Introduction of NHC to noble metals has been very
successful, resulting in highly active and (enantio-) selective
hydrogenation catalysts for a variety of chemistries.[7] Following
these works, several groups have reported the synthesis of
Mn(I)-NHCs[8] and their applications in reduction catalysis
The reduction of ketones to their corresponding alcohols is an
important transformation for the production of pharmaceuticals,
flavors, flagrances, and agrochemicals. Catalytic transfer
hydrogenation (TH) protocols offer an attractive and sustainable
alternative to well-known stoichiometric approaches.[1] Recently
the development of non-noble metal catalysts based on
abundant 3d transition metals such as Fe, Co, and Mn has
received significant attention.[2] This has led to an impressive
expansion of the field and the discovery of novel reactivity
patterns of 3d metals versus their noble metal analogues.
Remarkably, of the Mn-catalysts currently discussed in the open
literature[3], several of the most active systems do not rely on
9]
(Scheme 1,
D
&
E).[3c,
Taking notice of the recent
developments in the field of Mn(I)-NHC catalysis, we
hypothesized that the combination of a bidentate ligand bearing
a strongly donating NHC group and an amine donor function
could lead to a highly active and stable catalytic system. At the
same time such a complex would maintain attractive ligand
simplicity. Based on prior work with NHCs in our group[7d, 7e], we
sought to synthesize and test two simple bidentate Mn-NHC
complexes 1 and 2 (Scheme 1).
OH
Ph2 Br
P
Mn
A
B
D
H2
N
C
Br
Mn
CO
Br
Ph
Ph
NH
NH
CO
CO
N
N
CO
CO
CO
CO
phosphine ligands, but instead are based on simple bidentate N-
Mn
N
3h, 3i, 4]
N
donors (Scheme 1, A - C).[3b,
Sortais and co-workers
CO
Mes
Ketone
CO
OH
N
+
Mn(CO)5Br
reported on the use of 2-picolylamine as a ligand for Mn, while
also disclosing a series of in-situ systems bearing bidentate
0.5 mol%
30 oC, 16 h
0.3 mol%
80 oC, 24 h
1.0 mol% in-situ
80 oC, 3 h
hydrogenation
3h]
diamines as ligands.[3b,
Additionally, the group of
Sortais3b
Sortais3h
Khusnutdinova3i
Sortais9c
Khusnutdinova very recently disclosed a Mn-bipyridine-derived
complex which showed good activity for the TH of ketones,
aldehydes, and imines at 0.3 mol% Mn (3000 ppm Mn; 330
TON).[3i]
However, several challenges remain to be addressed
before earth-abundant catalytic systems can be practically
utilized in (industrial) organic synthesis. For example, compared
to noble metals, reported metal loadings for 3d transition metals
(TM) remain up to four orders of magnitude higher at several
thousands of ppm (i.e., 0.1 – 1.0 mol%).[1, 5] Although catalyst
N
E
R
N
R = H
TON up to 17.300
TOF0 = 11.800 h-1
Highly active
Br
Br
1
N
N
CO
CO
Mn
CO
Mn
CO
N
CO
CO
Efficient catalysis down to 75 ppm
N
N
Mes
R = Me Inactive
2
CO2 Reduction
Ketone hydrosilylation
This work
Royo9a,b
Scheme 1. Mn-complexes for sustainable reduction catalysis.
Herein, we report on the catalytic transfer hydrogenation of
a broad scope of aromatic and aliphatic ketones with novel
[a]
R. van Putten, J. Benschop, V.J. de Munck, Dr. G.A. Filonenko,
Prof. Dr. E.A. Pidko
Inorganic Systems Engineering group, Department of Chemical
Engineering, Faculty of Applied Sciences
Delft University of Technology
Van der Maasweg 9, 2629 HZ, Delft, The Netherlands
E-mail: E.A.Pidko@tudelft.nl
M. Weber, Prof. Dr. C. Müller
Mn(I)-NHC complex
1 (Scheme 1). The catalyst shows
unprecedented activity and turnover numbers (TONs) and
enables quantitative alcohol yields at Mn concentrations as low
as 75 ppm (0.0075 mol%), thereby reducing the activity-gap with
noble metal based systems[1, 10] by two orders of magnitude.
[b]
Institute of Chemistry and Biochemistry
Freie Universität Berlin
Fabeckstraße 34/36, D-14195 Berlin, Germany
Supporting information for this article is given via a link at the end of
the document.
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10.1002/cctc.201900882
ChemCatChem
COMMUNICATION
Ligands L1 and L2 were prepared by reaction of 1-mesityl-
1H-imidazole with the corresponding 2-bromoethylamine
hydrobromide salt (Scheme 2). We found that the removal of
HBr and subsequent ion metathesis to the hexafluorophosphate
analogues of L1/L2 was required to obtain the target complexes
in their pure form. Compounds 1 and 2 were further obtained by
reaction of the in-situ generated free carbene with Mn(CO)5Br in
THF. Recrystallization from DCM/Et2O produced analytically
pure 1 and 2 in 67 % and 89 % yield, respectively. The isolated
materials were fully characterized with 1H/13C NMR, FTIR,
single-crystal X-ray structure determination, and elemental
analysis (see Supporting Information).
investigated critical reaction parameters such as base loading,
solvent, and the nature of the hydrogen donor to optimize
catalysis with 1 (see SI). Presence of 2 eq. KOtBu to Mn was
insufficient to quantitatively activate 1 at this low catalyst
concentration of 0.05 mol% Mn (Table S2). We believe this
effect is caused by traces impurities in the solvent that react with
the base. Generally, highest conversions were observed when
hydrogen donors were used as the co-solvent (Table S3).
Strongly coordinating solvents such as MeOH and EtOH led to
(almost) fully diminished performance, while aliphatic media
gave intermediate results. When used neat, secondary alcohol
donors 2-propanol, 2-butanol, and 3-methyl-2-butanol all
resulted in virtually identical alcohol yields (Table S4). Reaction
with the formic acid/NEt3 mixture or potassium formate as the
alternative hydrogen donor unfortunately did not lead to any
activity. Thus, all subsequent reactions were performed in neat
iPrOH with 1 mol% KOtBu as the base relative to the reaction
substrate.
Table 1. TH of acetophenone with 1 and 2.[a]
Scheme 2. Synthetic procedure for Mn(I)-NHCs 1 and 2.
The FTIR spectra of 1 and 2 each feature three sharp
peaks around 2004, 1919, and 1892 cm-1, indicative of an
octahedral complex in which the three CO ligands are bound in
cis-fashion. Single-crystal X-ray analysis confirmed the
octahedral geometry and revealed the formation of Mn(I)-NHC
bonds with lengths of 2.053 Å in 1, and 2.055 Å in 2,
respectively (Figure 1). Coordination of the ligand to Mn
effectively locks the configuration of the ethylene linker in place.
As a consequence, equivalency of the geminal protons within
the ethylene linker fragment is lost and they appear as four
independent resonances in 1H NMR (see SI).
Catalyst
Temp.
[°C]
50
50
80
80
80
80
80
Yield
[%]
92
1
93
94
94
85
47
TON
[-]
Entry
([mol% / ppm])
1 (0.5 / 5000)
2 (0.5 / 5000)
1 (0.5 / 5000)
1 (0.1 / 1000)
1 (0.05 / 500)
1 (0.02 / 200)
1 (0.01 / 100)
1
2
3
4
5
6
7
184
2
186
940
1880
4250
4700
[a] Conditions: 1.0 mmol acetophenone, 0.01 – 0.5 mol% Mn, 2 eq. KOtBu to
i
Mn, 2.5 ml PrOH, T = 50 – 80 °C, 1 h. Yield determined by GC-FID using n-
C12 as IS.
Following the discrete optimization, we sought to study the
influence of reaction temperature and catalyst concentration.
Increased
temperature
induced
pronounced
catalyst
deactivation, which became particularly noticeable at low metal
loadings below ~100 ppm 1 (Figure 2 and SI). At 70 °C and 50
ppm Mn (0.005 mol%), 1 reacts with a high initial TOF0 of 11.800
h-1 and completes 11.100 turnovers in 6 h (56 % yield). Reaction
at 60 °C is slower (TOF0 6100 h-1), but surpasses the integral
TON of reaction at 70 °C after approximately 2.5 h (TON = 9600).
After 6 h the reaction is 76 % complete, after having undergone
15.100 turnovers. Further reduction of catalyst concentration to
25 ppm expedites the deactivation process and TON-crossover
takes place after only 20 min (1800 TON). Under these
conditions the reaction progress halts at 8 %. In contrast, at
60 °C 1 achieves a very high TON of 17.300 in 6 h and,
importantly, at that time does not show signs of imminent
reaction termination. These observations indicate that catalyst
deactivation is independent of total catalytic turnover and must
be caused by another, as of yet unknown, phenomenon.
Ultimately, very good alcohol yield could be obtained in 6 h at 75
ppm 1 and 60 °C (Table S6).
Figure 1. ORTEP diagram of 1 (left) and 2 (right). Thermal ellipsoids are
drawn at 30 % probability. Hydrogen atoms (except bound to N and N-Me) and
co-crystallized solvent are omitted for clarity.
Complex 1 is a highly active pre-catalyst for TH of ketones
with PrOH. Fully methylated analogue 2 is inactive under the
i
selected conditions (Table 1, entries 1-2). Initial experiments
were performed using acetophenone as a model substrate at
50 °C and 0.5 mol% Mn. Increased reaction temperature
allowed a tenfold reduction of catalyst loading down to 0.05
mol% (500 ppm) and resulted in quantitative yields of 1-
phenylethanol within 1 h (Table 1, entries 3-5). Further reduction
of catalyst concentration led to incomplete reaction progress,
while achieving a high turnover number of 4700 at 100 ppm
loading (Table 1, entries 6-7). This indicates 1 is catalytically
active and remarkably stable over a large number of turnovers
and potentially allows for additional optimization. Next, we
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10.1002/cctc.201900882
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COMMUNICATION
cyano (E8), or nitro-group (E9). Thus, our results show that 1 is
a potent catalyst for a broad scope of substrates. Strongly
coordinating substrates are a limitation in the scope and lead to
complete loss of catalytic activity.
TON 15.100
60 oC
70 oC
25 ppm
50 ppm
@
TON 11.100
TON 17.300
TON Crossover
TON 9.600
TOF0
11.800 h-1
TON 3.100
Figure 2. Kinetic traces of acetophenone TH with 1. Conditions: 0.5 mmol
acetophenone, 25 & 50 ppm 1, 1 mol% KOtBu to substrate, 3.82 ml iPrOH, 60
& 70 °C. Yield determined by GC-FID using n-C12 as the internal standard.
Having established critical performance parameters, we
performed a substrate scope to investigate the effect of several
functional groups and varied electronic and steric configurations
on catalysis with 1 (Scheme 3). Reactions were performed at
40 °C and 500 ppm Mn (0.05 mol%) to minimize potential
detrimental effects caused by irreversible catalyst deactivation.
These conditions were estimated to provide a reasonable trade-
off between reaction rate, catalyst consumption, and the
temperature-influenced rate of deactivation. Aromatic ketones
A1 to A9 were all reduced in good to quantitative yield, except
for sterically demanding 2,4,6-trimethylacetophenone A4.
Aliphatic ketones were significantly less reactive towards
reduction. The corresponding alcohols from ketones B1 – B4
were all obtained in fair yield of 56 %. Bulky 1-adamantyl methyl
ketone B5 was only partially reduced under the selected
conditions. Evidently, steric accessibility is an important factor
for catalysis with 1. Long-chain aliphatic alcohols from B7 – B9
were obtained in good to excellent yield, depending on the exact
location of the functional group. Dicyclohexyl methyl ketone B10
was not reduced, while reaction with B11 was not selective and
resulted in extensive (cyclic) side product formation.
Chemoselectivity for carbonyl versus alkene reduction was
probed with aliphatic and aromatic examples C1 and C2. 6-
Methyl-5-hepten-2-one C1 was selectively reduced to the
corresponding unsaturated alcohol in 67 % yield. Aromatic
benzylideneacetone C2 showed good chemoselectivity and
enabled production of the unsaturated alcohol in 82 % yield
(90 % selectivity C=O vs. C=C; 9 % fully reduced product was
observed). GC-MS suggests TH of heterocyclic compounds D1
and D2 was quantitatively followed by dehydration to 2-
vinylfuran (92 % yield) and 2-vinylthiophene (68 % yield).
Unfortunately, introduction of nitrogen to the heterocyclic ring led
to complete loss of catalytic activity (D3 – D5). Conclusively,
functional group tolerance was studied using para-substituted
acetophenones E1 to E9. Incorporation of halides or a phenyl
moiety had no effect on performance (E1 – E3), nor did
presence of an ester (E4). Similar to the products of D1 and D2,
4-acetylaniline E5 and 4-acetylanisole E6 were reduced to the
corresponding vinylic products. Catalytic performance was
severely decreased for substrates containing a hydroxyl (E7),
Scheme 3. Substrate scope with 1. Conditions: 0.5 mmol substrate, 0.05
i
mol% 1 (500 ppm), 1 mol% KOtBu to substrate, 3.82 ml PrOH, 40 °C, 24 h.
Yields determined by GC-FID using n-C12 as internal standard. [a]
Corresponding vinyl (ID by GC-MS). [b] Corresponding isopropyl ester (ID by
GC-MS).
To gain mechanistic insight into catalysis with 1, we
investigated the pre-catalyst activation in a series of 1H NMR
stoichiometric reactivity experiments (Scheme 4). Addition of 2
i
eq. PrOH in presence of 2 eq. KOtBu in benzene-d6 led to the
formation of two new resonances at δ = -4.09 ppm and δ = -3.59
ppm. We attribute these peaks to the two expected cis and trans
Mn(I)-H species (relative to N-H), which are formed in an
approximate 5:1 ratio. Under these conditions Mn-NHC 1 is
quantitatively converted into the corresponding Mn-Hs upon
i
dehydrogenation of PrOH. No hydrides were be observed in
i
absence of base and PrOH. In contrast, only ~2 % hydride
signal was observed with 2, suggesting that loss of the reactive
N-H functionality in 2 prevents metal-ligand cooperativity and
formation of the active hydride species from iPrOH. These
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10.1002/cctc.201900882
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catalysis with
1 likely operates via the well-established
[3]
bifunctional protonation/deprotonation mechanism.[5b, 11]
Scheme 4. Stoichiometric 1H NMR studies into pre-catalyst activation.
In conclusion, we have prepared the novel Mn(I)-NHC
complex 1 and reported on its remarkable catalytic activity for
i
transfer hydrogenation of ketones with PrOH. Catalysis with 1
[4]
[5]
[6]
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proceeds with very high maximum TON of at least 17.000. The
complex is useful at unprecedented catalyst loadings and
enables quantitative alcohol yields at only 75 ppm Mn. Such
loadings approach those more conventionally utilized for Ru and
Ir catalysts, thus highlighting the high potential of 1 and other 3d
TM-catalyzed processes for sustainable catalysis. Significant
catalyst deactivation was observed at elevated temperatures,
which inhibited further reduction of metal content. Therefore, we
believe thorough understanding of the deactivation processes
will ultimately enable a leap forward in rational catalyst design
towards improved catalytic systems.
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Acknowledgements
This work has received funding from the European Research
Council (ERC) under the European Union’s Horizon 2020
research and innovation programme (grant agreement #725686).
M.W. and C.M. gratefully acknowledge the Deutsche
Forschungsgemeinschaft (DFG) for financial support.
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Hydrogenation • Ketones • Alcohols
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COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Mn(I)-NHC complex 1 is a highly
active and robust catalyst for the
transfer hydrogenation of ketones.
Catalysis with 1 proceeds efficiently at
unprecedented catalyst loadings as
low as 75 ppm, thus approaching
levels more conventionally reserved
for privileged Ru-based catalysts.
Robbert van Putten, Joeri Benschop,
Vincent J. de Munck, Manuela Weber,
Christian Müller, Georgy A. Filonenko,
Evgeny A. Pidko*
Page No. – Page No.
Efficient and practical transfer
hydrogenation of ketones catalysed
by a simple bidentate Mn-NHC
complex
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