Simple manganese carbonyl catalyzed hydrogenation of quinolines and imines

Article abstract of DOI:10.1016/j.cclet.2020.02.025

Manganese-catalyzed hydrogenation of unsaturated molecules has made tremendous progresses recently benefiting from non-innocent pincer or bidentate ligands for manganese. Herein, we describe the hydrogenation of quinolines and imines catalyzed by simple manganese carbonyls, Mn₂(CO)₁₀ or MnBr(CO)₅, thus eliminating the prerequisite pincer-type or bidentate ligands.

Full text of DOI:10.1016/j.cclet.2020.02.025

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Simple manganese carbonyl catalyzed hydrogenation of quinolines and
imines
Zelong Wang, Lei Chen, Guoliang Mao, Congyang Wang
PII:
S1001-8417(20)30085-1
https://doi.org/10.1016/j.cclet.2020.02.025
CCLET 5466
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Chinese Chemical Letters
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13 January 2020
11 February 2020
13 February 2020
Please cite this article as: Wang Z, Chen L, Mao G, Wang C, Simple manganese carbonyl
catalyzed hydrogenation of quinolines and imines, Chinese Chemical Letters (2020),
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Communication
Simple manganese carbonyl catalyzed hydrogenation of quinolines and imines
a,b
a,c
c,
a,b,d,
Zelong Wang , Lei Chen , Guoliang Mao * maoguoliang@nepu.edu.cn, Congyang Wang
wangcy@iccas.ac.cn

aBeijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Molecular Recognition and Function, CAS Research/Education Center for
Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
b
University of Chinese Academy of Sciences, Beijing 100049, China
c
Provincial Key Laboratory of Oil & Gas Chemical Technology, College of Chemistry and Chemical Engineering, Northeast Petroleum University, Daqing
1
d
63318, China
Physical Science Laboratory, Huairou National Comprehensive Science Center, Beijing 101400, China
Corresponding authors.
Graphical abstract
Earth-abundant manganese catalysis can now enable hydrogenation of quinolines and imines by using simple manganese
carbonyls, Mn
2
(CO)10 or MnBr(CO) , thus eliminating the previous requirements on pincer-type or bidentate ligands.
5
ABSTRACT
Manganese-catalyzed hydrogenation of unsaturated molecules has made tremendous progresses recently on the benefits of non-innocent pincer or
bidentate ligands for manganese. Herein, we describe the hydrogenation of quinolines and imines catalyzed by simple manganese carbonyls, Mn (CO)10
or MnBr(CO) , thus eliminating the prerequisite pincer-type or bidentate ligands.
2
5
Keywords: Manganese; Hydrogenation; Quinolines; Imines; Homogeneous Catalysis
Sustainable chemistry requires precise and efficient synthesis of various functional molecules and materials in an atom- and step-
economical manner. Aiming at this goal, transition metal catalysis has evolved into a powerful tool to discover new reactivity and tune
chemo-, regio- and stereoselectivity for synthetic transformations. An illustrative example is transition-metal-catalyzed hydrogenation
of unsaturated molecules, which mostly has 100% atom-economy and is potentially waste-free process [1]. Noble transition metal
catalysis has played a predominant role in this field, and very high catalytic turnovers and excellent stereoselectivity have been achieved
with structure-defined catalysts [2]. However, the rarity in the earth’s crust and intrinsic toxicity limit future wider applications of these
metals in hydrogenation reactions. As such, development of earth abundant and less toxic 3d transition metal catalysts for
hydrogenation have gained immense attention recently and are still highly desirable [3].
Manganese, as the third richest transition metal in the earth’s crust, is cheap, less toxic and diverse in oxidation states (from -3 to +7)
thus being a potential candidate for catalyst development [4,5]. In this context, Beller, Kempe, Sortais, Kirchner et al. have elegantly
reported hydrogenation of aldehydes/ketones by using manganese-pincer or -bidentate ligand complexes since 2016 (Scheme 1) [6].
Later on, Clarke, Beller, Ding and Zhong further developed asymmetric hydrogenation of ketones with chiral pincer-manganese
catalysts [7]. Other carbonyl derivatives such as nitriles [6a,6e,8], carbon dioxide [9], carbonates [10], amides [11], and esters [12]
could also be hydrogenated by adopting the related manganese-pincer or -bidentate ligand complexes. Very recently, Milstein and
coworkers have nicely demonstrated the hydrogenation of challenging carbamates and ureas with their PNN-pincer manganese catalyst
[13]. Meanwhile, Sortais, Kempe and Liu showed the hydrogenation of imines and fused N-heterocycles containing C=N bonds
respectively, again with bidentate or pincer-type ligand-manganese catalysts [14]. It is generally accepted that these non-innocent pincer
ligands cooperate with the manganese centre to enable the hydrogenation processes through an outer-sphere mechanism [5]. In our
continuous interest in MnH catalysis [15], we herein describe the hydrogenation of quinolines and imines by using simple manganese
—
——

carbonyls, Mn
Scheme 2). During the preparation of this manuscript, Beller et al. elegantly reported a very related work using MnBr(CO)
catalyst for hydrogenation of N-heterocycles at a lower temperature [16]. Of note, our results show that Mn (CO)10 can also act as an
2
(CO)10 or MnBr(CO)
5
, which eliminates the previous requirement of pincer-type or bidentate ligands as well as bases
(
5
as a
2
efficient catalyst for hydrogenation of quinolines at a higher temperature. Moreover, we demonstrate that imines could be successfully
hydrogenated by using simple manganese carbonyl catalysts.
Scheme 1. Pincer or bidentate ligand-manganese complex catalyzed hydrogenation of C=E (E = O, N) containing substrates.
Scheme 2. Simple manganese carbonyl catalyzed hydrogenation of quinolines and imines.
In 2014, we reported a manganese-catalyzed [4+2] annulation reaction of N-H imines and alkynes with the evolution of H
Mechanistic studies indicated the involvement of MnH(CO) as a true catalytic species in the reaction. Curious whether the simple
MnH(CO) species could enable the hydrogenation reactions without using previous external pincer or bidentate ligands, we
commenced our study with the hydrogenation of quinoline 1a by using simple Mn (CO)10 as a catalyst (Table 1). The expected 1,2,3,4-
tetrahydroquinoline 2a was formed quantitatively when the hydrogenation was carried out with 5 MPa of H and 10 mol% of
showed no influence
2
gas [15].
5
5
2
2
o
Mn
2
(CO)10 in THF at 150 C (entry 1). While phosphine ligands were detrimental to the reaction, NPh
3
and AsPh
3

o
on the reaction outcome (entries 2-6). Changing the reaction temperature suggested that 130 C was the optimal (entries 7 and 8).
Variations on the pressure of H showed that the reaction could proceed at lower pressure even at 1 atm, yet in decreased yields (entries
-12). The amount of Mn (CO)10 could be reduced to 5 mol%, however, 1 mol% of the catalyst failed in the reaction (entries 13-14).
2
9
2
The use of MnBr(CO)
5
instead of Mn
2
(CO)10 gave also a quantitative yield of 2a (entry 15).
Table 1
Optimization of reaction parameters.a
o
Yield (%)b
100
5
0
0
100
100
100
0
38
34
80
100
99
1
Entry
Cat. [Mn] (mol%)
Ligand
--
T ( C)
150
150
150
150
150
150
130
120
130
130
130
130
130
130
130
P (MPa)
5
5
5
5
5
5
5
5
0.1
1
2
3
3
3
3
1
2
3
4
5
6
7
8
9
Mn
Mn
Mn
Mn
Mn
Mn
Mn
Mn
Mn
Mn
Mn
Mn
Mn
Mn
2
2
2
2
2
2
2
2
2
2
2
2
2
2
(CO)10 (10)
(CO)10 (10)
(CO)10 (10)
(CO)10 (10)
(CO)10 (10)
(CO)10 (10)
(CO)10 (10)
(CO)10 (10)
(CO)10 (10)
(CO)10 (10)
(CO)10 (10)
(CO)10 (10)
(CO)10 (5)
PPh
3
DPPM
DPPE
NPh
3
AsPh
--
3
--
--
--
--
--
--
--
--
1
1
1
1
1
1
a
0
1
2
3
4
5
(CO)10 (1)
MnBr(CO)
5
(5)
100
2
Unless otherwise noted, all the reactions were carried out with 1a (0.2 mmol), [Mn] catalyst (10 mol%), Ligand (20 mol%), H (5 MPa) in
THF (1.0 mL) for 8 h.
b
1
Determined by H NMR analysis with an internal standard.
DPPM = Bis(diphenylphosphino)methane, DPPE = 1,2-Bis(diphenylphosphino)ethane.
Next, the scope of quinolines was tested with the above-obtained reaction conditions (Scheme 3). It was shown that substitutions on
various positions of quinolines 1 had no obvious effect on the reaction outcome giving the corresponding 1,2,3,4-tetrahydroquinolines
in high yields (2a-h). Both electron-donating and -withdrawing groups were well tolerated in the reactions (2i-n). Quinolines bearing
other substitution patterns were also applicable to this protocol (2o-u). The use of acridine gave 9,10-dihydroacridine (2v) smoothly in
moderate yield. Quinoxaline and 1,5-naphthyridine led to the expected products successfully (2w, 2x) with the latter only one ring
reduced (2x). Surprisingly, isoquinoline failed to afford the corresponding product (2y) under the current reaction conditions [16].
Scheme 3. Simple manganese carbonyl catalyzed hydrogenation of quinolines. Reaction conditions: 1 (0.5 mmol), MnBr(CO)
5
(5 mol%), H
2
(3
(5
o
a
b
o
c
d
e
MPa), 130 C, THF (2.5 mL), 8 h. 24 h. 150 C. 1.0 equiv. of CH
3 2
CO H was added.
H
2
(5 MPa), 24 h. MnBr(CO)
5
(10 mol%), H
2
f
MPa), 24 h. 12 h.

Encouraged by the above success on hydrogenation of quinoline derivatives, we further examined our catalytic system with
hydrogenation of imines. As shown in Scheme 4, the hydrogenation of both aldimine and ketimines 3 could give the expected amine
products 4 in excellent yields under the same reaction conditions as those of quinolines.
Scheme 4. Simple manganese carbonyl catalyzed hydrogenation of imines. Isolated yields were shown.
To explore the possible reaction mechanism, a set of experiments was conducted (Scheme 5). First, some additives were added to the
reaction in order to probe the possible radical nature of this process. It turned out that TEMPO inhibited the reaction completely and
TEMPO-H was detected by GC-MS analysis (Scheme 5a), which may result from the reaction of TEMPO with MnH(CO) . Other
5
radical scavengers such as butylated hydroxytoluene (BHT) and 1,1-diphenylethylene (1,1-DPE) affected the reaction outcome,
however, the expected product 2a could still be obtained in decreased yields. To further test whether transient radical intermediates
were generated in the reduction, cyclopropanyl substituted quinolines at 2-, or 3-, or 4-positions were subjected to the reactions and no
ring-opened products were detected in all cases (Scheme 5b), which suggested the formation of a carbon radical adjacent to the
cyclopropanyl group might not occur in the reaction. Remarkably, obvious deuterium-incorporation was found at 2-, 3-, and 4-positions
of the product with the 3-position being the most when the reaction was carried out in the presence of D
2
O (Scheme 5c). It indicated
that MnH(CO) may undergo H/D exchange with D O in the reaction. Interestingly, no D-incorporation was observed in the product
5
2
8
when the reaction was conducted in THF-d . To probe whether 1,2-reduction species 5a was the possible reaction intermediate, 5a was
synthesized and subjected to a series of different reaction conditions (Scheme 5d). It was shown that 5a completely transformed to
product 2a under the standard reaction conditions. Interestingly, disproportionation of 5a into quinoline 1a and tetrahydroquinoline 2a
took place even without MnBr(CO)
in the absence of H . Of note, the yields of 1a and 2a were not equal in these reactions, indicating a possible evolution of H
disproportionation process.
5
or any external base [14c]. Such phenomenon was also found when the reactions were carried out
2
2
during the

Scheme 5. Mechanistic studies for manganese carbonyl catalyzed hydrogenation of quinolines. Yields were determined by H NMR analysis. a
1
Detected by GC-MS analysis. ND = not detected.
Based on the above results, we prefer an ionic MnH-promoted hydrogenation to a radical mechanism for these reactions and the
tentative reaction pathways were depicted in Scheme 6. In the case of Mn
with quinoline 1 through an either 1,4-addition or 1,2-addition way. The resulting [Mn-N] species may cleave H
MnH(CO) , which could further reduce the heterocyclic intermediates 5-7 to the final tetrahydroquinoline product 2. In the case of
MnBr(CO) with H , MnH(CO) was formed together with HBr, which could activate quinoline 1 by forming a salt [16]. Two
2 2
(CO)10 with H , MnH(CO)
5
was generated first and reacted
2
to regenerate
5
5
2
5
sequential steps of reduction would eventually give the tetrahydroquinoline product 2.
Scheme 6. A proposed mechanism.
In conclusion, we have developed a simple protocol to achieve the hydrogenation of quinoline derivatives and imines by using
commercially available manganese carbonyls, Mn
2
(CO)10 and MnBr(CO) , which eliminates the need of pincer-type or bidentate
5

ligands previously commonly used. Mechanistic studies suggested the involvement of MnH(CO)
5
-promoted non-radical reduction
process with H . Further investigations on MnH(CO) -enabled catalytic reduction of other substrates and detailed mechanistic studies
2
5
are underway in our laboratory.
Conflict of interest
We herein declare that all the authors have no conflict of interest.
Acknowledgments
Financial support from the National Natural Science Foundation of China (No. 21772202, 21831008), Beijing Municipal Science &
Technology Commission (No. Z191100007219009) and Beijing National Laboratory for Molecular Sciences (No. BNLMS-CXXM-
2
01901) is gratefully acknowledged.
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