Transfer Hydrogenation of Carbonyl Derivatives Catalyzed by an Inexpensive Phosphine-Free Manganese Precatalyst

Article abstract of DOI:10.1021/acs.orglett.7b01657

A very simple and inexpensive catalytic system based on abundant manganese as transition metal and on an inexpensive phosphine-free bidendate ligand, 2-(aminomethyl)pyridine, has been developed for the reduction of a large variety of carbonyl derivatives with 2-propanol as hydrogen donor. Remarkably, the reaction proceeds at room temperature with low catalyst loading (down to 0.1 mol %) and exhibits a good tolerance toward functional groups. High TON (2000) and TOF (3600 h^-1) were obtained.

Full text of DOI:10.1021/acs.orglett.7b01657

Letter
pubs.acs.org/OrgLett
Transfer Hydrogenation of Carbonyl Derivatives Catalyzed by an
Inexpensive Phosphine-Free Manganese Precatalyst
Antoine Bruneau-Voisine,† Ding Wang,† Vincent Dorcet,‡ Thierry Roisnel,‡ Christophe Darcel,†
and Jean-Baptiste Sortais*,†,§,∥
†Institut des Sciences Chimiques de Rennes, UMR 6226 CNRS-Universite
Catalysis, 263 avenue du General Leclerc, Rennes 35042 Cedex, France
‡Institut des Sciences Chimiques de Rennes, UMR 6226 CNRS-Universite
́
de Rennes 1, Team Organometallics: Materials and
́
́
́
de Rennes 1, Centre de Diffractometrie X, 263 avenue du
́
́ ́
General Leclerc, Rennes 35042 Cedex, France
§Institut Universitaire de France, 1 rue Descartes, Paris 75231 Cedex 05, France
S
* Supporting Information
ABSTRACT: A very simple and inexpensive catalytic system based on abundant manganese as transition metal and on an
inexpensive phosphine-free bidendate ligand, 2-(aminomethyl)pyridine, has been developed for the reduction of a large variety of
carbonyl derivatives with 2-propanol as hydrogen donor. Remarkably, the reaction proceeds at room temperature with low
catalyst loading (down to 0.1 mol %) and exhibits a good tolerance toward functional groups. High TON (2000) and TOF
(3600 h−1) were obtained.
he development of non-noble metal based catalytic
systems has been increasing for more than a decade,
bifunctional catalysts.15 We were inspired by ruthenium
catalysis and, in particular, the work of Baratta, who
demonstrated that simple bidendate aminomethylpyridine
ligand (ampy) could significantly improve the activity and the
robustness of ruthenium hydrogen transfer catalysts, reaching
very high TON and TOF.16
In the present contribution, we have demonstrated that well-
defined manganese complexes featuring a bidentate ampy
ligand are highly efficient catalysts for hydrogen transfer of
ketones and aldehydes at room temperature, with TOF up to
3600 h−1 and TON up to 2000.
We have first prepared a series of four manganese complexes,
bearing as ligand 2-(hydroxymethyl)pyridine, 2-
(aminomethyl)pyridine, 2-(N-methylaminomethyl)pyridine,
and 2-(N,N-dimethylaminomethyl)pyridine (Scheme 1). The
synthesis of the complexes is straightforward: for
(aminomethyl)pyridine derivatives, complexes 1−3 were
obtained as yellow solids with good yields (93−95%) by
reaction of 1 equiv of the ligand with 1 equiv of Mn(CO)5Br in
toluene at 110 °C for 4 h. Complex 4 was prepared in hexane at
70 °C as described by Miguel.17 Complexes 1−3 were fully
characterized by NMR, IR, and elemental analysis. The
molecular structures were confirmed by X-ray diffraction
T
notably in the field of reduction reactions or hydro-
elementation.1 Tremendous progress has been accomplished
in the case of iron2 and cobalt.3 In the case of manganese,4
which is the third most abundant transition metal after iron and
titanium, until recently the reduction of CO bond was
limited to the hydrosilylation of carbonyl derivatives,5 the
hydroboration,6 and electrochemical reduction of CO2.7
The development of efficient catalytic systems based on
manganese for reduction reactions was introduced by Beller in
2016, who reported the first examples of hydrogenation of
ketones, esters and nitriles using a tridendate aliphatic
[(iPr)2PCH2CH2]2NH ligand.8 In the case of hydrogen transfer
reactions, Beller has recently reported that dipicolylamine
manganese pincer complex could promote the hydrogen
transfer of ketones in isopropanol at 70 °C in 24 h.9 The
potential of pincer type ligands with manganese in redox
reactions was reinforced mainly by the studies of Milstein,10
Kempe,11 Kirchner,12 Boncella,13 and us,14 with pyridinyl-core
PNP and PN3P ligands and triazinyl-core PN5P ligands.
However, one major drawback of these phosphorus pincer-
type ligands used in catalysis with nonprecious metals is the
cost of the ligand which, overwhelms the price of the metal
precursors.
In the quest for simple and inexpensive catalytic systems, we
Received: June 1, 2017
were looking for alternative ligands suitable for promoting
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b01657
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
very low catalyst loading of 0.01 mol %, a TON of 2000 was
achieved after 16 h (entry 5). Encouraged by these results, the
temperature was then decreased to room temperature. With 0.5
mol % of catalyst, after 3 h, 50% of the ketone was reduced, and
a full conversion was obtained after 16 h (entry 7). Even with a
substrate/catalyst ratio of 1000, the reduction of acetophenone
was still achieved in 90% conversion (entry 8). Control
experiments (entries 9−12) demonstrated that all of the
components of the catalytic system were crucial to obtain high
activities.19 Compared to catalyst 1, catalyst 2, bearing one
methyl substituent on the amine moieties, was found to be
slightly less active (see the SI) than 1, and catalysts 3 and 4
showed very low activities (entries 14 and 15). These results
show that the presence of NH moieties on the ligand is crucial
for the hydrogen transfer to occur.
Scheme 1. Well-Defined Manganese Complexes Used in
This Study
studies on single crystals (Figure 1).18 These complexes are
fairly stable under air but slowly decompose under exposure to
light.
The generality of the reaction was then probed using two
optimal conditions: 80 °C, 20 min or 30 °C, 16 h, 0.5 mol % of
1, and 1 mol % of tBuOK (Table 2). In general, the reaction
proceeded well with a large range of substrates. Steric hindrance
was well tolerated as from acetophenone to pivalophenone or
2-methylacetophenone high yields of the corresponding
alcohols were obtained (entries 1−6). Aromatic ketones
bearing electron-donating groups (entries 6−9) were reduced
with high yield. Interestingly, aromatic aldehydes were also
converted nicely to the corresponding benzyl alcohols (entries
10 and 11) with almost no side reactions16d (in the case of 4-
phenylbenzaldehyde, less than 5% of the product of the aldol
condensation of the aldehyde with acetone was detected in the
crude NMR). Halogen-containing substrates are suitable for
this reduction without formation of dehalogenated products
(entries 12−17). Functional group tolerance was then
evaluated: cyano, nitro, amino, amido, and ester were tolerated
(entries 18−24). In the case of ethyl levunilate (entry 24, 98%
conversion), a mixture of cyclic γ-valerolactone (84%) and
linear isopropyl-4-hydroxypentanoate (16%), resulting from
trans-esterification, was obtained. Aliphatic and cyclic ketones
were fully converted to the corresponding alcohols (entries
25−29). Conjugated α,β-unsaturated ketones were reduced to
the corresponding unsaturated alcohols with high selectivity, as
the sole reduction of the carbonyl moieties was observed in the
case of β-ionone (entry 31), and only 5% of the saturated
alcohol was detected in the case of cinnamyl ketone (entry 30).
For very challenging cinnamaldehyde (entry 32), the chemo-
selectivity was perfect toward the allylic alcohol, and less than
7% of aldol consendation byproducts were detected after 2 h at
80 °C. Finally, heteroaromatic ketones were tested: the
thiophene-yl derivative was fully reduced to the corresponding
alcohol (entry 33), while 4-acetylpyridine led to moderate
conversion at 80 °C and almost no conversion at 30 °C (entry
34). The bidentate 2-acetylpyridine poisoned the catalyst, and
little conversion was observed under both conditions (entry
35). The same limitation was observed in the case of 2-
acetylfuran and several 1,3-diketones.20
Figure 1. Perspective views of the molecular structures of complexes 1
(left), 2 (middle), and 3 (right) with thermal ellipsoids drawn at 50%
probability. Hydrogens, except the NH, were omitted for clarity.
We then explored the catalytic activities of complex 1 for the
reduction of acetophenone a1 in the presence of 2-propanol as
the hydrogen source. To our delight, with 1 mol % of 1 and 2
mol % of tBuOK as the base, at 80 °C, within 5 min, we
observed a full reduction of the ketone to the corresponding 1-
phenylethanol b1 (Table 1, entry 1). The catalyst loading was
then decreased to 0.1 mol %, leading to a conversion of 31%
and 58% after 5 and 10 min, respectively (TOF = 3720 and
3480 h−1, respectively). An average TOF of 3600 h−1 was
obtained, demonstrating the high activity of complex 1. With a
a
Table 1. Optimization of the Reaction Parameters
tBuOK
temp
conv
(%)
entry
cat. (mol %)
1 (1)
(mol %)
(°C)
time
1
2
3
2
80
80
80
5 min
15 min
5 min
10 min
2 h
>97
>97
31
58
>97
80
20
96
50
97
90
0
1 (0.5)
1 (0.1)
1
0.2
4
5
6
7
1 (0.05)
1 (0.01)
1 (0.5)
1 (0.5)
0.1
0.02
1
80
80
60
30
2 h 30
16 h
1 h
1
3 h
16 h
8
1 (0.1)
1 (1)
0.2
30
80
80
80
80
80
80
80
19 h
9
1 h
10
11
12
13
14
15
10
2
1 h
5
Ampy (1)
Mn(CO)5Br (1)
2 (0.5)
1 h
0
Then, to gain insight into the nature of the catalytic active
species, we performed stoichiometric reactions between
complex 1 and the base.21 After 24 h at room temperature,
the dimeric manganese complex 5, resulting from the
deprotonation of the NH moieties, could be isolated in 42%
yield (Scheme 2).5k A similar dimer was obtained by Miguel
starting from 2-pyridinemethanol.17 The molecular structure of
5 was confirmed by X-ray diffraction studies (Figure 2).22
Unfortunately, further reactions of 5 with 2-propanol or 1 with
2
1 h
<1
97
8
1
15 min
30 min
20 min
3 (0.5)
1
4 (0.5)
1
6
a
Typical conditions: To a Schlenk tube, under argon, were added in
this order: catalyst precursor, iPrOH (2 mL), acetophenone (0.5
1
mmol), and tBuOK. The conversion was determined by GC and H
NMR.
B
DOI: 10.1021/acs.orglett.7b01657
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Table 2. Scope of the Hydrogen Transfer of Ketones To
Give Alcohols under the Catalysis of 1
Scheme 2. Synthesis of Dimer 5
a
Figure 2. Perspective view of the molecular structure of complex 5
with thermal ellipsoids drawn at 50% probability. Hydrogens, except
the NH, were omitted for clarity.
Finally, complex 5 was tested as catalyst without addition of
external base under standard conditions: with 0.5 mol % of 5, at
80 °C after 90 min, acetophenone was fully reduced to 1-
phenylethanol. The corresponding deprotonated monomer,
formed by the dissociation of the dimer, is likely the
intermediate of the catalytic cycle, which is in line with the
mechanism of ligand-assisted hydrogen-transfer reactions.
In conclusion, we have developed an highly efficient and
simple catalytic system for the reduction by hydrogen transfer
of ketones and aldehydes, including α,β-unsaturated aldehydes,
at room temperature, based on phosphine-free bidendate
aminomethylpyridine ligand. High TON (2000) and TOF
(3600 h−1) were achieved with this catalytic system.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.7b01657.
Experimental procedures and full characterization data,
including X-ray data and NMR spectra of all the
compounds (PDF)
X-ray data for compound 1 (CIF)
X-ray data for compound 2 (CIF)
X-ray data for compound 3 (CIF)
X-ray data for compound 5 (CIF)
a
Typical conditions: To a Schlenk tube, under argon, were added in
this order: catalyst 1, iPrOH (0.25 mol L−1), ketone or aldehyde (0.5
or 2 mmol), and tBuOK. The conversion was determined by 1H NMR
b
on the crude mixture. Isolated yield in parentheses. Selectivity
alcohol: aldol byproducts of 67:33. 5% of aldol condensation
byproduct was detected in the crude mixture. 1 week, due to low
solubility of the starting material. 72 h. 1 h. Selectivity γ-
valerolactone/Isopropyl-4-hydroxypentanoate 84/16 at 80 °C and
c
d
AUTHOR INFORMATION
e
f
g
■
Corresponding Author
*E-mail: jean-baptiste.sortais@lcc-toulouse.fr.
ORCID
h
i
77/23 at 30 °C. 2 h, iPrOH 4 mL. Enol 95%, 4-phenylbutan-2-ol 5%.
j
2 h, 7% aldol condensation byproducts were detected in the crude
mixture.
Jean-Baptiste Sortais: 0000-0003-1178-8588
Present Address
NaBH4 did not allow the characterization of any manganese
hydride intermediate.
∥(J.-B.S.) LCC-CNRS, Universite
́
de Toulouse, INPT, UPS,
205 route de Narbonne, 31077 Toulouse Cedex 4, France.
C
DOI: 10.1021/acs.orglett.7b01657
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
C.; Beller, M. Nat. Commun. 2016, 7, 12641. (d) Pena-Lop
́
ez, M.;
Piehl, P.; Elangovan, S.; Neumann, H.; Beller, M. Angew. Chem., Int.
Ed. 2016, 55, 14967−14971. (e) Anderez-Fernandez, M.; Vogt, L. K.;
Notes
̃
The authors declare no competing financial interest.
́
́
Fischer, S.; Zhou, W.; Jiao, H.; Garbe, M.; Elangovan, S.; Junge, K.;
Junge, H.; Ludwig, R.; Beller, M. Angew. Chem., Int. Ed. 2017, 56, 559−
562. (f) Neumann, J.; Elangovan, S.; Spannenberg, A.; Junge, K.;
Beller, M. Chem. - Eur. J. 2017, 23, 5410−5413. (g) Papa, V.; Cabrero-
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Capet, F.; Paul, J.-F.; Dumeignil, F.; Gauvin, R. M. ACS Catal. 2017, 7,
2022−2032.
ACKNOWLEDGMENTS
■
We thank the Centre National de la Recherche Scientifique
(CNRS), the Universite
́
de Rennes 1, Fonds Europee
́
ns de
dev
́
eloppement ec
́
onomique regional (FEDER founds), IN-
́
CREASE FR-CNRS 3707, la Fondation Rennes 1 (grant to
D.W.), and the Agence National de la Recherche (ANR agency,
program JCJC ANR-15-CE07-0001 “Ferracycles”).
(9) Perez, M.; Elangovan, S.; Spannenberg, A.; Junge, K.; Beller, M.
ChemSusChem 2017, 10, 83−86.
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DOI: 10.1021/acs.orglett.7b01657
Org. Lett. XXXX, XXX, XXX−XXX