Manganese-Catalyzed Transfer Hydrogenation of Aldimines

Article abstract of DOI:10.1002/cctc.201900314

The reduction of imines to amines via transfer hydrogenation was achieved promoted by phosphine-free manganese(I) catalyst. Using isopropanol as reductant, in the presence of tBuOK (4 mol %) and manganese complex [Mn(CO)₃Br(κ²N,N-PyCH₂NH₂)] (2 mol %), a large variety of aldimines (30 examples) were typically reduced in 3 hours at 80 °C with good to excellent yield.

Full text of DOI:10.1002/cctc.201900314

Accepted Article
Title: Manganese-catalyzed transfer hydrogenation of aldimines
Authors: Duo Wei, Antoine Bruneau-Voisine, Maxime Dubois,
Stéphanie Bastin, and Jean-Baptiste Sortais
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10.1002/cctc.201900314
ChemCatChem
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Manganese-catalyzed transfer hydrogenation of aldimines
Duo Wei,[a,b]# Antoine Bruneau-Voisine,[a,b]# Maxime Dubois, [b] Stéphanie Bastin[b] and Jean-Baptiste
Sortais*[b,c]
Dedication ((optional))
Abstract: The reduction of imines to amines via transfer
hydrogenation was achieved promoted by phosphine-free
manganese(I) catalyst. Using isopropanol as reductant, in the
presence of tBuOK (4 mol%) and manganese complex
[Mn(CO)3Br(2N,N-PyCH2NH2)] (2 mol%), a large variety of aldimines
(30 examples) were typically reduced in 3 hours at 80 °C with good to
excellent yield.
tridendate ligand, namely dipicolamine, for the reduction of
ketones.[18a] Zirakzadeh and Kirchner reported the application of
a chiral catalyst supported by a tridendate PNP ligand bearing a
ferrocenyl moiety for the enantioselective production of
alcohols.[18b] Similar catalytic system, developed by Clarke,
performed the reduction of ketones in the presence of alcohols,
but required hydrogen pressure to exhibit enantioselectivity.[8f]
More recently, the groups of Leitner[18c] and Morris,[18d] reported
respectively an aminotriazole and a chiral tridentate manganese
catalyst efficient in transfer hydrogenation reactions. Our group
demonstrated that simple manganese complex based on a
bidendate aminomethylpyridine ligand could perform with high
efficiency the reduction of ketones and aldehydes, even at room
temperature.[18e] We also showed that the combination of
[Mn(CO)5Br], as commercial manganese precursor, and a
diamine ligand, for example ethylenediamine or a chiral diamine,
namely (1R,2R)-N,N-dimethyl-1,2-diphenylethane-1,2-diamine,
could promote the (asymmetric-) transfer hydrogen of carbonyl
compound.[18f]
So, to date, manganese-catalyzed transfer hydrogenation
reactions were limited to the reduction of ketones and aldehydes.
Furthermore, examples of reduction of imines via this
methodology involving non-noble metals such as iron,[19] nickel,[20]
and cobalt[21], are quite scarce. Starting from this statement, and
taking into account the importance of amines in organic
synthesis,[22]we found relevant to explore the manganese-
catalyzed reduction of imines by hydrogen transfer[23] as a
complementary method in the chemist tool box to
hydrogenation,[24] hydrosilylation[25] or hydrogen borrowing,[2b] for
example.
The development of catalysts based on earth abundant 3d
transition metals, as sustainable and economical alternative to
precious transition metals, is one of the current challenge in the
field of homogeneous catalysis.[1] In the case of reduction
reactions,[2] despite its natural abundance, the potential of
manganese[3] was disregarded until recently compared to iron,[4]
cobalt,[5] and nickel.[6]
Indeed, the first efficient manganese based catalyst for
hydrogenation of polar bonds was reported latterly, in 2016, by
the group of Beller.[7] Since then, hydrogenation catalysts were
developed for the reduction of carbonyl derivatives,[8] nitriles,[9]
esters,[10] amides,[11] carbonates,[12] and CO2.[13] To our
knowledge, only two examples of hydrogenation of imines are
reported, one with a manganese nano-sheet[14] and one with an
aminophosphinepyridine manganese catalyst developed by our
group.[15] As an alternative to molecular dihydrogen (or
hydrosilanes[16]) as reductants, secondary alcohols, typically
isopropanol, are convenient source of hydrogen.[17] Advantages
of transfer hydrogenation over hydrogenation are that reduction
reactions can be performed in conventional equipments, i.e.
without requiring the use of high-pressure autoclaves, and that
alcohols are usually benign chemicals, safer to handle than
dihydrogen.
Compared to hydrogenation, only a few catalytic systems based
on manganese were developed for transfer hydrogenation
reactions, including the asymmetric version (Scheme 1).[8f, 18] The
first system was developed by the group of Beller, with a
[a]
[b]
D. Wei, Dr. A. Bruneau-Voisine
Univ Rennes, CNRS, ISCR – UMR 6226, F-35000, Rennes, France
D. Wei, Dr. A. Bruneau-Voisine, M. Dubois Dr. S. Bastin and Prof.
Dr. J.-B. Sortais
LCC-CNRS, Universite´ de Toulouse, CNRS, UPS, Toulouse,
France
E-mail: jean-baptiste.sortais@lcc-toulouse.fr
Prof. Dr. J.-B. Sortais
Institut Universitaire de France 1 rue Descartes, F-75231 Paris
Cedex 05, France
[c]
#
Equal contribution
Scheme 1. Manganese catalysts used in transfer hydrogenation.
Supporting information for this article is given via a link at the end of
the document.
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In line with our research program to develop manganese
complexes for reduction reactions,[8d, 26] we have selected the
well-defined Mn(I) complex 1 (Scheme 1) featuring an 2-
(aminomethyl)-pyridine ligand for this study as it was the most
efficient catalyst for the reduction of carbonyl derivatives.[18e]
Besides, the synthesis of this complex is very straightforward, as
1 can be obtained in high yield in only one step from commercially
available reactants.[18e]
lower conversion than the former bases (65%, entry 4) The
concentration of the reaction mixture is often crucial for the
reduction of carbonyl derivatives, as transfer hydrogenation is a
reversible reaction.[27] Therefore, the influence of the
concentration of a1 was examined ranging from 0.5 to 0.05 mol.L-
1. An optimal concentration of 0.1 mol.L-1 (entries 1, 7-9) was
identified. With 1 mol% catalyst, lengthen the reaction time (3 h)
allowed to improve the conversion to 88% (entry 10). However,
lowering the temperature had a detrimental effect on the -activity
as the conversion dropped to 39% at 50 °C and to 7% at 30 °C
(entries 11 and 12). A satisfying yield (93%) was obtained using
2 mol% of catalyst and 4 mol% of tBuOK at 80 °C after 3 h (entry
13). Finally, lowering the catalyst loading to 0.5 mol% resulted in
a moderate TON of 120 (1, 3 h, 60% conversion, entry 14). It is
worth noting that under the optimized conditions, but in the
absence of manganese complex 1, no reaction occurred (entry 15,
Table 1. Optimization of the parameters of the reduction of imine with catalyst
1. [a]
Table S2), demonstrating
a
metal-catalyzed reduction.[28]
Additionally, the reaction proceeded in the presence of Hg (300
equiv.) without observing any significant changes in reactivity
(Table 1, entry 16), suggesting a homogeneous catalytic system.
Given the promising activity of catalyst 1 for the reduction of
aldimine 1a, we then investigated its potential for the more
challenging reduction of ketimines, with N-[1-phenylethylidene]-
aniline as a model subtrate (See Table S1). Even under harsch
conditions (1 (5 mol%), tBuOK (10 mol%), 130 °C, 18 h), the
corresponding amine was obtained in low yield (20%),
demonstrating that this catalytic system is not suitable for the
reduction of ketimines.
Entry
Loading
(mol%)
Base
Conc.
Temp.
(°C)
Time
(h)
Conv.
(%)
(mol%)]
(mol.L-
1
)
1
1
1
1
1
1
1
1
1
1
1
1
1
2
0.5
0
2
tBuOK (2)
tBuONa (2)
KHMDS (2)
KOH (2)
0.16
0.16
0.16
0.16
0.16
0.16
0.5
80
80
80
80
80
80
80
80
80
80
50
30
80
80
80
80
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
3
77
63
77
65
15
12
58
82
83
88
29
7
2
3
4
5
Na2CO3 (2)
K3PO4 (2)
tBuOK (2)
tBuOK (2)
tBuOK (2)
tBuOK (2)
tBuOK (2)
tBuOK (2)
tBuOK (4)
tBuOK (2)
tBuOK (4)
tBuOK (4)
6
Having optimized the conditions for the reduction of aldimines
(complex 1 (2 mol%), tBuOK (4 mol%), iPrOH (C = 0.1 mol.L-1),
80 °C, 3 h), we then explored the scope of this transformation
(Table 2). In general, a large variety of aldimines, prepared by the
condensation of benzaldehyde with aniline derivatives, was
reduced in high yield. Steric hindrance has an influence as
increasing the number of ortho substituents on both the
benzaldehyde (a3, a4) or the aniline (a5, a6) moieties, from one
to two, led to the sterically hindered amines b4 and b6 in lower
yield than b3 and b5. The steric hindrance on the aniline moieties
has a more drastic influence than on the benzaldehyde part (b4
vs b7). Halogen substituents (F, Cl, Br, I) were well tolerated (b8-
b11, isolated yield > 90%), and no product resulting from
dehalogenation was detected. Interestingly, heteroaromatic
7
8
0.1
9
0.05
0.1
10
11
12
13
14
15
16[b]
0.1
3
0.1
3
0.1
3
93
60
<1
94
0.1
3
imines
(a12-a15)
affording
N-substituted-
0.1
3
aminomethylheterocycles (b12-b15) were also surprisingly
reduced without significant loss of activity. It should be noticed
that the structure of the reaction products are very similar to the
one of the ligand used to support the manganese center, namely
2-(aminomethyl)-pyridine. Under the conditions of the catalysis, a
dynamic exchange of ligands can not be ruled out. Since diverse
diamines proved to be active ligands in transfer hydrogenation of
ketones, such an exchange may not alter the reduction.[18f]
Likewise, organometallic 4-methyl-N-(ferrocenylmethylidene)-
aniline a16 was totally reduced and led to the corresponding
amine b16 in excellent isolated yield (88%). The tolerance toward
various functional groups such as amino (a17), methoxy (a18),
cyano (a19), nitro (a20), ester (a21), amido (a22), acetal (a23),
vinyl (a24) and alkynyl (a25) groups was tested.
0.1
3
[a] Reaction conditions: Under inert atmosphere, a 20 mL Schlenk tube was
filled sequentially with the imine a1 (0.5 mmol), anhydrous 2-propanol, complex
1, and the base. The reaction was heated in an oil bath. Conversion was
determined by 1H NMR spectroscopy [b] In the presence of Hg (300 equiv.).
The conditions of the reaction were optimized for the reduction of
N-benzylidene-4-methylaniline a1 in 2-propanol as solvent (Table
1). At 80 °C, in the presence of tBuOK (2 mol%) and manganese
complex 1 (1 mol%), the corresponding amine b1, namely N-
benzyl-4-methylaniline, was formed in 77% yield. Various bases
were first screened (entries 1-6), showing that tBuOK and
KHMDS gave the best results (77%, entries 1 and 3). Interestingly,
common base KOH could also be used, albeit affording a slightly
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[a] ] Reaction conditions: Under inert atmosphere, a 20 mL Schlenk tube was
filled sequentially with the imine (0.5 mmol), anhydrous 2-propanol (5 mL),
complex 1 (3.3 mg, 2 mol%), and tBuOK (2.2 mg, 4 mol%). The reaction was
heated in an oil bath at 80 °C for 3 h. Conversion was determined by 1H NMR
spectroscopy. Isolated yield in parenthesis. [b] Isolated as mixture containing
20% of saturated amine. [c] complex 1 (6.6 mg, 4 mol%), tBuOK (4.4 mg, 8
mol%)
Table 1. Generality of the reduction of aldimines catalyzed by complex 1.[a]
Except for the nitro-substituted imine (a20),[29] all the functional
groups did not affect the catalytic system and remained intact
after the reduction. In the case of the conjugated imine (a26), the
unsaturated amine b26 was obtained as major product (80%)
along with 20% of the corresponding saturated amine.
The unability of our catalytic system to reduce ketimines allowed
the selective reduction of substrates displaying both ketimine and
aldimine moieties. Gratifyingly, for the diimine a27 derived from 4-
formylacetophenone, only the aldimine moiety was reduced
affording the corresponding amino-ketimine b27. Finally, imines
bearing aliphatic substituents were difficult to convert to the
desired amines (b28-b31, yield 17%-38%).
In conclusion, we reported the first application of Mn-based
catalyst in transfer hydrogenation of imines. We have shown that
manganese (I) pre-catalyst bearing a 2-aminomethylpyridine
ligand catalyzes efficiently the transfer hydrogenation of aldimines
in the presence of iPrOH as reductant under mild conditions (2
mol% catalyst, 4 mol% base, 80 °C, 3 to 18 h). The catalytic
process displayied a high tolerance towards a large variety of
functional groups. This protocol enlarges the scope of reactions
catalyzed by manganese, highlighting the rising potential of this
transition metal in homogeneous catalysis.
Experimental Section
Typical catalytic reduction. A 20 mL Schlenk tube was charged with
imine (0.5 mmol), anhydrous isopropanol (5.0 mL), manganese complex 1
(3.3 mg, 2 mol%), and tBuOK (2.2 mg, 4 mol%) under argon, in that order.
Then the mixture was stirred at 80 oC in an oil bath for 3 h. After cooling to
room temperature, the solution was diluted with ethyl acetate (2.0 mL) and
filtered through a small pad of celite (2 cm in a Pasteur pipette). The celite
was washed with ethyl acetate (2×2.0 mL). The filtrate was evaporated
and the crude residue was purified by column chromatography (SiO2,
mixture of petroleum ether/ethyl acetate as eluent) to afford the desired
product.
Acknowledgements
We thank the Centre National de la Recherche Scientifique
(CNRS), the Université de Rennes 1, the Université Toulouse III,
Paul Sabatier, the Institut Universitaire de France (IUF) and the
Agence National de la Recherche (ANR agency, program JCJC
ANR-15-CE07-0001 “Ferracycles”).
Keywords: manganese • imines • reduction • transfer
hydrogenation • amines
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presence
of
p-nitrotoluene,
N,N-dimethylaniline
and
N-
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methylbenzylamine, see Table S2, demontrated that the moderate yield
obtained for amines b17 and b29-31 are probably due to the intrinsic
electronic properties of the substrates (electrodonating substituents)
while p-nitrotoluene has a deleterious effect on the reduction of 1a.
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10.1002/cctc.201900314
ChemCatChem
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Duo Wei, Antoine Bruneau-Voisine,
Maxime Dubois, Stéphanie Bastin and
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Manganese-catalyzed transfer
hydrogenation of aldimines
Using isopropanol as reductant, in the presence of tBuOK and manganese complex
[Mn(CO)3Br(2N,N-PyCH2NH2)], a large variety of aldimines (30 examples) were
reduced to the corresponding amines with good to excellent yield.
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