Manganese-Catalyzed Hydrogenation of Ketones under Mild and Base-free Conditions

Article abstract of DOI:10.1021/acs.organomet.1c00161

In this paper, several Mn(I) complexes were applied as catalysts for the homogeneous hydrogenation of ketones. The most active precatalyst is the bench-stable alkyl bisphosphine Mn(I) complex fac-[Mn(dippe) (CO)₃(CH₂CH₂CH₃)]. The reaction proceeds at room temperature under base-free conditions with a catalyst loading of 3 mol % and a hydrogen pressure of 10 bar. A temperature-dependent selectivity for the reduction of α,β-unsaturated carbonyls was observed. At room temperature, the carbonyl group was selectively hydrogenated, while the C=C bond stayed intact. At 60 °C, fully saturated systems were obtained. A plausible mechanism based on DFT calculations which involves an inner-sphere hydride transfer is proposed.

Full text of DOI:10.1021/acs.organomet.1c00161

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Article
Manganese-Catalyzed Hydrogenation of Ketones under Mild and
Base-free Conditions
̈
Stefan Weber, Julian Brunig, Luis F. Veiros, and Karl Kirchner*
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sı Supporting Information
ABSTRACT: In this paper, several Mn(I) complexes were applied as catalysts for the homogeneous hydrogenation of ketones. The
most active precatalyst is the bench-stable alkyl bisphosphine Mn(I) complex fac-[Mn(dippe) (CO) (CH CH CH )]. The reaction
3
2
2
3
proceeds at room temperature under base-free conditions with a catalyst loading of 3 mol % and a hydrogen pressure of 10 bar. A
temperature-dependent selectivity for the reduction of α,β-unsaturated carbonyls was observed. At room temperature, the carbonyl
group was selectively hydrogenated, while the CC bond stayed intact. At 60 °C, fully saturated systems were obtained. A plausible
mechanism based on DFT calculations which involves an inner-sphere hydride transfer is proposed.
INTRODUCTION
insertions to form highly reactive acyl intermediates (a well-
12
■
known reaction in organometallic chemistry ) which are able
to activate dihydrogen, thereby forming the 16e− Mn(I)
hydride catalysts (Scheme 2). Accordingly, bisphosphine
manganese tricarbonyl complexes containing alkyl ligands
could be employed for the additive-free hydrogenation of
The catalytic reduction of polar multiple bonds via molecular
hydrogen plays a significant role in modern synthetic organic
chemistry. Within this context, the use of catalytic procedures
in combination with hydrogen gas displays an attractive option
1
to develop efficient and cleaner processes. In the last few
13,9c
alkenes and nitriles.
years, well-defined Mn(I) complexes were introduced as
powerful players in the field of sustainable hydrogenation
Here, we describe an additive-free hydrogenation of ketones
at room temperature, utilizing Mn(I) alkyl carbonyl complexes
fac-[Mn(dpre) (CO) (CH )] (dpre = 1,2-bis(di-n-
2
chemistry, being active for the hydrogenation of not only
3
4
5
6
7
3
3
aldehydes, ketones, esters, CO , and carbonates but also
2
p r o p y l p h o s p h i n o ) e t h a n e , f a c - [ M n ( d p r e )
8
9
nitrogen-containing compounds such as imines, nitriles,
(
(
CO) (CH CH CH )] (2) and fac-[Mn(dippe)
1
0
11
3 2 2 3
amides, and heterocycles.
CO) (CH CH CH ) (dippe = 1,2-bis(di-iso-
3
2
2
3
It is interesting to note that many of these transition-metal-
catalyzed hydrogenations rely on metal−ligand bifunctional
catalysis (metal−ligand cooperation), where complexes con-
tain electronically coupled hydride and acidic hydrogen atoms.
An effective way of bond activation by metal−ligand
cooperation involves aromatization/dearomatization of the
ligand in pincer complexes in which a central pyridine-based
backbone is connected with −CH PR and/or −CH NR
propylphosphino)ethane) (3).
RESULTS AND DISCUSSION
The catalytic performance of manganese(I) alkyl complexes
−3 for the hydrogenation of ketones was evaluated. The
experiments were performed using Et O as the solvent at 25
■
1
2
°
C and 50 bar H pressure and 4-fluoroacetophenone as the
model substrate to find the most active catalyst and optimal
2
2
2
2
2
substituents. This has resulted in the development of novel
and unprecedented iron and manganese catalysis, where this
type of cooperation plays a key role in the heterolytic cleavage
Received: March 15, 2021
Published: April 22, 2021
of H . An overview of well-defined manganese complexes for
2
hydrogenation reactions is depicted in Scheme 1.
An alternative way to activate dihydrogen was recently
described by our group. We took advantage of the fact that
Mn(I) alkyl carbonyl complexes are known to undergo
©
2021 The Authors. Published by
American Chemical Society
https://doi.org/10.1021/acs.organomet.1c00161
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Scheme 1. Selected Mn(I) Precatalysts for Hydrogenation Reactions
Scheme 2. Formation of the Catalytically Active Species
Upon Reaction With Dihydrogen
Table 1. Optimization Reaction for the Hydrogenation of 4-
Fluoroacetophenone
a
hydrogenation reaction conditions (Table 1). In the cases of 1
and 2, negligible reactivity was observed (Table 1, entries 1
and 2), while with 3, excellent conversion to the desired
product was achieved. The drastic increase in reactivity may be
addressed to the increased steric demand of the ligand in
comparison to complexes 1 and 2. The importance of the steric
demand of the bisphosphine ligand for the reactivity of alkyl
complexes was also demonstrated previously for the hydro-
entry
catalyst (mol %)
solvent
conversion (%)
1
2
3
4
5
6
7
1 (3)
2 (3)
3 (3)
3 (3)
3 (3)
3 (3)
3 (3)
3 (3)
3 (3)
3 (2)
3 (3)
Et O
2
Et O
traces
95
31
30
69
2
Et O
2
13
MeOH
DCM
THF
genation of alkenes. The stability of the active species may be
preserved due to increased steric hindrance. It should be noted
that the hydrogenation of ketones at room temperature is
4f,g
DME
83
comparingly rare in the field of manganese(I) chemistry. So
b
8
9
Et O
2
>99
>99
69
far, Mn(I)-catalyzed base-free hydrogenation reactions are only
c
3
a
9a
11b,c
Et
Et
2
O
O
known for aldehydes, nitriles, N-heterocycles,
and
c
1
3
10
1
2
alkenes.
In other solvents such as MeOH, CH Cl , or dimethoxy-
,
c d
1
Et O
2
22
2
2
a
Reaction conditions: 4-fluoroacetophenone (0.38 mmol), 5 mL
anhydrous solvent, 25 °C, 50 bar H , 24 h, conversion determined via
ethane (DME), lower reactivities were observed. Interestingly,
lowering the hydrogen pressure from 50 to 10 bar resulted in
full conversion (Table 1, entry 9), which is comparatively low
for manganese-based catalysts. A shorter reaction time (8 h)
led to a drastic decrease in conversion (Table 1, entry 11),
which might be attributed to an induction period required for
catalyst activation.
Having determined 3 as the most active catalyst and to
prove its general applicability, various substrates have been
tested to establish scope and limitations (Table 2). The
catalytic experiments were conducted in the presence of 3 mol
2
1
9
1
b
c
d
F{ H}-NMR spectroscopy. 30 bar H . 10 bar H . 8 h.
2
2
context, halide-containing substrates (Table 2, entries 4−7) as
well as substrates with electron-donating groups (Table 2,
entries 11 and 12) gave excellent yields. Lower reactivity could
be detected for substrates containing a coordinating amine or
pyridine (Table 2, entries 13 and 19). No conversion could be
detected for substrate 9, bearing the strongly coordinating
nitrile functionality. Furthermore, no reaction was observed in
the presence of a nitro group (Table 2, entry 10), presumably
due to the possible undesired redox reactions with the catalyst.
%
of catalyst at 25 °C and 10 bar hydrogen pressure, a reaction
time of 24 h, without the addition of any additives. Within this
1
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a
Table 2. Scope and Limitation for the Hydrogenation of Ketones Catalyzed by 3
a
b
Reaction conditions: ketone (0.38 mmol), 3 (3 mol %), 5 mL anhydrous Et O, 10 bar H , 25 °C, 24 h; isolated yields. Conversion determined
2
2
c
via GC−MS. 36 h.
Table 3. Temperature Dependence of the Hydrogenation of α,β-Unsaturated Carbonyls Catalyzed by 3
a
b
Reaction conditions: ketone (0.38 mmol), 3 (3 mol %), 5 mL anhydrous Et O, 10 bar H , 25 °C, 36 h; isolated yields. Ketone (0.38 mmol), 3 (1
2
2
c
mol %), 5 mL anhydrous Et O, 10 bar H , 60 °C, 36 h; isolated yields. Conversion determined via GC−MS.
2
2
4
f,g
In the case of sterically more demanding substrate 15, only a
moderate conversion could be achieved. Aliphatic ketones
were very efficiently reduced to the corresponding alcohols
rare,
and to the best of our knowledge, an additive-free
hydrogenation of ketones has not been reported.
Furthermore, a potential temperature-dependent selectivity
for the hydrogenation of α,β-unsaturated carbonyls was
investigated (Table 3). At room temperature, the high
selectivity for the reduction of the carbonyl group could be
(Table 2, entries 21−23). However, the reaction time had to
be increased to achieve high conversions. Manganese-catalyzed
hydrogenations of ketones at room temperature are relatively
1
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Figure 1. Free-energy profile calculated for the hydrogenation of acetophenone. Free energies (kcal/mol) are referred to intermediate A.
Scheme 3. Simplified Catalytic Cycle for the Hydrogenation of Ketones
detected, whereas the CC bond stays unaltered (Table 3,
4−27). Interestingly, if hydrogenation was carried out at 60
C, fully saturated systems (Table 3, 28−30) were received as
A mechanistic investigation of the introduced system
revealed that the reactivity of 3 was drastically lowered upon
2
°
the addition of 1 equiv of PMe (with 4-fluoroacetophenone as
3
products. Additionally, the catalyst loading could be decreased
to 1 mol %. The reaction barrier for the hydrogenation of 1,2-
disubstituted C−C double bonds is generally higher than for
ketones, requiring a higher reaction temperature, as demon-
the substrate). This finding indicates the presence of an inner-
sphere reaction, as the strong donor PMe apparently blocks
3
the vacant coordination site of the active catalyst for the
incoming substrates. The homogeneity of the system was
proven by the Hg drop test as no significant decrease in
reactivity could be detected.
The mechanism of hydrogenation of ketones by 3 was
investigated in detail by DFT calculations using acetophenone
as the model substrate. The resulting free-energy profile is
13
strated previously. In the case of citral as the substrate, solely
the CO and not the trisubstituted CC bond was
hydrogenated (Table 3, 25). This temperature-dependent
selectivity for the reduction of α,β-unsaturated carbonyl
moieties may be interesting for synthetic applications.
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represented in Figure 1 while Scheme 3 depicts a summary of
the catalytic cycle.
GC−MS analysis was conducted on an ISQ LT single quadrupole MS
system (Thermo Fisher) directly interfaced to a TRACE 1300 gas
chromatographic system (Thermo Fisher), using a Rxi-5Sil MS (30 m,
0.25 mm ID) cross-bonded dimethyl polysiloxane capillary column.
General Procedure for the Hydrogenation of Ketones.
Inside an Ar-flushed glovebox, ketone substrate (0.38 mmol, 1 equiv)
Catalyst initiation, starting from 3, has been reported
1
3
previously. Acetophenone coordination to the 16-electron
1
hydride intermediate forms intermediate A, a κ -(O) complex
2
that rearranges to a η -coordination mode in B. This is a facile
and 3 (3 mol %) were dissolved in 5 mL of Et O and taken up in a
2
process with a barrier of only 4 kcal/mol (TS ). From B,
AB
syringe. The mixture was injected into a steel autoclave, and the
there occurs an attack of the hydride on the carbonyl C atom,
resulting in C, an alkoxide complex stabilized by an agostic
interaction involving the recently formed C−H bond. The
formation of C, from B, is also easy with a barrier of only 3
reaction vessel was flushed three times with 10 bar H . The reaction
2
was stirred for the indicated time. The autoclave was depressurized
and the sample was taken for GC−MS analysis. The reaction mixture
was passed through a pad of silica. The silica pad was rinsed with
Et O, and the solvent was gently removed.
kcal/mol (TS ), being a favorable step, from the thermody-
2
BC
Computational Details. The computational results presented
have been achieved in part using the Vienna scientific cluster. All
calculations were performed using the Gaussian 09 software
namic point of view with ΔG = −6 kcal/mol. The path
proceeds with the dihydrogen addition to the alkoxide
intermediate, from D to E, overcoming a barrier of 9 kcal/
14
package. Geometry optimizations were obtained using the Perdew,
mol, measured from the pair of molecules (H + alkoxide
2
Burke, and Ernzerhof (PBE)0 functional without symmetry
constraints, a basis set consisting of the Stuttgart/Dresden ECP
intermediate) in D to TS . This is an endergonic step with
DE
15
ΔG = 9 kcal/mol. Finally, in the last step of the cycle, there
basis set to describe the electrons of Mn, and a standard 6-31G(d,p)
16
occurs H transfer from the H ligand to the alkoxide O atom,
basis set for all other atoms. The PBE0 functional uses a hybrid
generalized gradient approximation, including 25% mixture of
2
regenerating the hydride and forming the O-coordinated
1
7
18
Hartree−Fock exchange with DFT exchange−correlation, ob-
alcohol product in F. This is a clearly favorable process (ΔG =
19
tained by the PBE functional. Transition-state optimizations were
performed with the synchronous transit-guided quasi-Newton method
developed by Schlegel et al., following extensive searches of the
−
7 kcal/mol) with a barrier of 4 kcal/mol (TS ), from E to F.
EF
The cycle is closed by the release of the product (1-
phenylethanol) and the coordination of a new acetophenone
molecule, from F back to A, a process with a free energy
balance of 5 kcal/mol. The least stable transition state is the
one associated with the hydride attack on the carbonyl C atom
20
potential energy surface. Frequency calculations were performed to
confirm the nature of the stationary points, yielding one imaginary
frequency for the transition states and none for the minima. Each
transition state was further confirmed by following its vibrational
mode downhill on both sides and obtaining the minima presented on
the energy profiles. The electronic energies were converted to free
energy at 298.15 K and 1 atm using zero-point energy and thermal
energy corrections based on the structural and vibration frequency
data calculated at the same level. The free-energy values presented
were corrected for dispersion by means of the Grimme DFT-D3
(
TS ), and the overall barrier for the catalytic cycle is 14
BC
kcal/mol, measured from the most stable intermediate (D) to
TSBC of the following cycle.
CONCLUSIONS
In conclusion, the hydrogenation of aromatic and aliphatic
ketones using a bench-stable Mn(I) alkyl complex is described.
■
2
1
22
method, with the Becke and Johnson short-distance damping.
Solvent effects (Et O) were considered in all the calculations using
the polarizable continuum model initially devised by Tomasi and co-
2
The reaction proceeds under mild conditions (10 bar H , 25
2
2
3
workers, with the radii and nonelectrostatic terms of the SMD
°C) and notably without the addition of any additives. Under
24
solvation model developed by Truhlar et al.
these conditions, chemoselective hydrogenation of the carbon-
yl moiety of α,β-unsaturated carbonyls could be achieved.
Interestingly, if the reaction was carried out at 60 °C, 1,2-
disubstituted CC bonds are additionally reduced, whereas a
trisubstituted CC bond stays intact. A detailed reaction
mechanism based on DFT calculations is presented. The
precatalyst is activated by dihydrogen upon the migratory
insertion of the alkyl group into the adjacent CO ligand and
consecutive split of the coordinated dihydrogen. The catalytic
reaction proceeds via an inner-sphere reaction upon substrate
coordination, insertion, dihydrogen activation, and regener-
ation of the active species due to product release.
ASSOCIATED CONTENT
sı Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.organomet.1c00161.
■
*
1H NMR and 13C{ H} NMR spectra of all compounds
PDF)
Cartesian coordinates for DFT-optimized structures
XYZ)
1
(
(
AUTHOR INFORMATION
Corresponding Author
■
EXPERIMENTAL SECTION
■
General Information. All reactions were performed under an
inert atmosphere of argon using Schlenk techniques or in a MBraun
inert gas glovebox. The solvents were purified according to standard
procedures. The deuterated solvents were purchased from Aldrich and
dried over 3 Å molecular sieves. Complexes fac-[Mn(dpre)
Karl Kirchner − Institute of Applied Synthetic Chemistry,
Vienna University of Technology, Vienna A-1060, Austria;
orcid.org/0000-0003-0872-6159; Email: karl.kirchner@
tuwien.ac.at
(
CO) (Me)] (dpre = 1,2-bis(di-n-propylphosphino)ethane) (1), fac-
3
Authors
[
Mn(dpre) (CO) (Pr)] (2), and fac-[Mn(dippe) (CO) (Pr)] (dippe
3
3
Stefan Weber − Institute of Applied Synthetic Chemistry,
Vienna University of Technology, Vienna A-1060, Austria;
orcid.org/0000-0002-1777-0971
̈
Julian Brunig − Institute of Applied Synthetic Chemistry,
Vienna University of Technology, Vienna A-1060, Austria
Luis F. Veiros − Centro de Química Estrutural and
Departamento de Engenharia Química, Instituto Superior
=
1,2-bis(di-iso-propylphosphino)ethane) (3) were synthesized
13
1
13
1
according to the literature. H- and C{ H}-NMR spectra were
recorded on Bruker AVANCE-250 and AVANCE-400 spectrometers.
1
13
1
H and C{ H}-NMR spectra were referenced internally to residual
protio-solvent and solvent resonances, respectively, and are reported
relative to tetramethylsilane (δ = 0 ppm). Hydrogenation reactions
were carried out in a Roth steel autoclave using a Tecsis manometer.
1
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Técnico, Universidade de Lisboa, Lisboa 1049-001,
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Non-Classical Metal-Ligand Cooperative H
Activation Mode. Angew.
2
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Z.; Ding, K. Lutidine-Based Chiral Pincer Manganese Catalysts for
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Portugal; orcid.org/0000-0001-5841-3519
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.organomet.1c00161
2019, 58, 4973−4977. (g) Zhang, L.; Wang, Z.; Han, Z.; Ding, K.
Manganese-Catalyzed anti-Selective Asymmetric Hydrogenation of α-
Substituted β-Ketoamides. Angew. Chem., Int. Ed. 2020, 59, 15565−
15569. (h) Zeng, L.; Yang, H.; Zhao, M.; Wen, J.; Tucker, J. H. R.;
Zhang, X. C1-Symmetric PNP Ligands for Manganese-Catalyzed
Enantioselective Hydrogenation of Ketones: Reaction Scope and
Enantioinduction Model. ACS Catal. 2020, 10, 13794−13799.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support by the Austrian Science Fund (FWF) is
gratefully acknowledged (project no. P 33016-N). Centro de
Química Estrutural acknowledges the financial support of
(i) Yang, W.; Chernyshov, I. Y.; van Schendel, R. K. A.; Weber, M.;
Muller, C.; Filonenko, G. A.; Pidko, E. A. Robust and efficient
̈
hydrogenation of carbonyl compounds by mixed donor Mn(I) pincer
complexes. Nat. Commun. 2021, 12, 12. (j) Buhaibeh, R.; Duhayon,
C.; Valyaev, D. A.; Sortais, J.-B.; Canac, Y. Cationic PCP and PCN
NHC Core Pincer-Type Mn(I) Complexes: From Synthesis to
Catalysis. Organometallics 2021, 40, 231−241.
Fundaca
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̃
̂
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