A General and Highly Selective Cobalt-Catalyzed Hydrogenation of N-Heteroarenes under Mild Reaction Conditions

Article abstract of DOI:10.1002/anie.201612290

Herein, a general and efficient method for the homogeneous cobalt-catalyzed hydrogenation of N-heterocycles, under mild reaction conditions, is reported. Key to success is the use of the tetradentate ligand tris(2-(diphenylphosphino)phenyl)phosphine). This non-noble metal catalyst system allows the selective hydrogenation of heteroarenes in the presence of a broad range of other sensitive reducible groups.

Full text of DOI:10.1002/anie.201612290

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201612290
German Edition: DOI: 10.1002/ange.201612290
Homogeneous Catalysis
A General and Highly Selective Cobalt-Catalyzed Hydrogenation of
N-Heteroarenes under Mild Reaction Conditions
+
+
Rosa Adam , Jose R. Cabrero-Antonino , Anke Spannenberg, Kathrin Junge, Ralf Jackstell, and
Matthias Beller*
Dedicated to Professor Belꢀn Abarca on the occasion of her 70th birthday
Abstract: Herein, a general and efficient method for the
homogeneous cobalt-catalyzed hydrogenation of N-hetero-
cycles, under mild reaction conditions, is reported. Key to
success is the use of the tetradentate ligand tris(2-(diphenyl-
phosphino)phenyl)phosphine). This non-noble metal catalyst
system allows the selective hydrogenation of heteroarenes in
the presence of a broad range of other sensitive reducible
groups.
H
omogeneous hydrogenation of arenes is a challenging
transformation because of the stability derived from the
aromaticity. In the particular case of N-heteroarenes, this
process not only has to overcome the resonance stability, but
also the possible poisoning of the catalyst by either the
Figure 1. Reported examples of quinoline hydrogenation using: a) het-
erogeneous catalysts; b) homogeneous catalysts of Ir, Ru, or Rh;
c) homogeneous catalysts of either Fe- or Co-PNP complexes;
d) [Co]/L2 homogeneous catalyst reported in this work.
[
1]
starting material or the product. Nevertheless, significant
developments took place in the past decades and many
applications are known for the hydrogenation of model N-
heterocycles. Among the various N-heterocycles, compounds
containing a 1,2,3,4-tetrahydroquinoline structure and related
ones are ubiquitous in natural products. Hence, many
bioactive compounds based on this scaffold are used as
[
8]
[9]
[9a,10]
on either Ir, Rh, or Ru,
and some of them require the
[
8a,11]
use of additives, such as I (Figure 1b).
2
The replacement of precious metals by earth-abundant,
first-row metals, is contemporary and highly desirable in
[2]
[3]
[12]
drugs and agrochemicals. In fact, oxamniquine and
terms of sustainability.
In the last decade important
[
4]
[13]
flumequine are currently in use as antihelmintic and
antibacterial agents, respectively. Moreover, the (de)hydro-
genation of N-heterocycles is of interest for hydrogen
advances in the scientific community have been reported.
Major works are concerned with the use of iron-based
catalysts for the hydrogenation of alkenes, aldehydes,
[
5]
[14]
storage.
ketones, imines, nitriles, and carboxylic acid derivatives.
In general, heterogeneous catalysts have been developed
for this transformation and most of them are based on
precious metals, which sometimes have selectivity issues
In addition, important work for the development of new
[
15]
[16]
cobalt and manganese hydrogenation catalysts has been
done. In the particular case of N-heteroarenes, elegant work
[
6]
[17]
[18]
(
Figure 1a). More recently, non-noble metal catalysts have
by Jones and co-workers showed that iron and cobalt
been applied: For example, supported cobalt nanoparticles
have been shown to efficiently hydrogenate N-heteroarenes
at > 1008C, but the chemoselectivity in the presence of
additional reducible groups is still a limitation (Figure 1a).
In contrast, homogeneous catalysts allow hydrogenation at
milder reaction conditions. Most of these systems are based
catalysts are able to perform acceptorless catalytic dehydro-
genation and hydrogenation of N-heterocycles (Figure 1c).
Herein, we report the first general protocol for the
chemoselective hydrogenation of quinolines and related
heterocycles, based on a nonprecious metal, under mild
reaction conditions and with a high tolerance for other
reducible groups (Figure 1d).
[7]
Encouraged by the previous experiences within our group
regarding cobalt- and iron-catalyzed hydrogenation and
dehydrogenation reactions using tetradentate phosphine-
type ligands, we became interested in the cobalt-catalyzed
hydrogenation of quinoline (1a). In a first approach, several
[
+]
[+]
[
*] Dr. R. Adam, Dr. J. R. Cabrero-Antonino, Dr. A. Spannenberg,
Dr. K. Junge, Dr. R. Jackstell, Prof. M. Beller
Leibniz-Institut fꢀr Katalyse e.V.
[
19]
Albert-Einstein-Strasse 29a, 18059 Rostock (Germany)
E-mail: matthias.beller@catalysis.de
+
[
phosphines were tested in the presence of Co(BF ) ·6H O
] These authors contributed equally to this work.
4 2 2
(
5 mol%), under 50 bar of H , at 1008C in THF (tetrahy-
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
http://dx.doi.org/10.1002/anie.201612290.
2
drofuran) during 15 hours (Figure 2). Commercially available
tridentate (L4 and L5), as well as bidentate (L6 and L7)
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Figure 2. Bi-, tri-, and tetradentate phosphines screened in this work.
Figure 3. Molecular structure of the cation of [CoF(L2)][BF ] (3) in the
4
[
23]
crystal. Hydrogen atoms have been omitted for clarity. Displacement
ellipsoids correspond to 30% probability.
ligands did not show any activity under the studied reaction
conditions (see Table S1, entries 4–7 in the Supporting
Information). Unfortunately, L1, which is also useful for
Ru-, Fe-, and Co-catalyzed hydrogenation of polar bonds,
showed very little activity (Table S1, entry 1). However in
contrast, the modified tetradentate ligand tris(2-(diphenyl-
phosphino)phenyl)phosphine (L2) afforded the desired
[
20]
complexes of L2 have been analytically characterized, to
the best of our knowledge, no catalytic experiments have been
reported. The new complex 3 afforded excellent results for
the hydrogenation of 1a comparable to the in situ system
(Table S1, entry 18).
1
,2,3,4-tetrahydroquinoline (2a) in high yield (98%;
Table S1, entry 2). Interestingly, the related tetraphos-type
ligand L3 gave no desired product (Table S1, entry 3).
Remarkably, the [Co]/L2 system works in the absence of
any acid additive, which was needed in previously described
With the aim of studying the generality of the protocol,
several substituted quinolines were reacted using the [Co]/L2
system (Table 1). Both, electron-releasing and electron-with-
drawing groups were well tolerated. In the case of 6-
substituted and 8-substituted quinolines, hydrogenation pro-
ceeded smoothly under mild reaction conditions (entries 2–9
and 11), and the corresponding 1,2,3,4-tetrahydroquinolines
[
19e,f]
[
Fe]/L2 hydrogenation systems.
A systematic investigation of the pressure and temper-
ature parameters using the [Co]/L2 system revealed the
efficient hydrogenation of quinoline at 608C under only
1
0 bar of H2 (Table S1, entries 8–12). To the best of our
[a]
Table 1: Cobalt-catalyzed hydrogenation of substituted quinolines.
knowledge this class of hydrogenations have not been
achieved under such mild reaction conditions using base
metal catalysts. Next, several metal precursors were tested for
the benchmark reaction (Table S2). Among the cobalt
precursors studied, Co(BF ) ·6H O resulted the most active
[b]
4
2
2
Entry
Quinoline 1
T [8C]
Conv. [%]
2
[
b]
one, although Co(ClO ) ·6H O also presented a remarkable
Yield [%]
4
2
2
activity (> 99 and 79% conv., Table S2, entries 1 and 2). In
addition, tetrafluoroborate salts of other nonprecious metals
were explored, but only little activity (12% conv.) was
observed in the case of the iron salt (Table S2, entries 11–13).
At this point, the influence of several solvents on the
reaction was assessed (Table S3). In general, good yields were
obtained with polar solvents (Table S3, entries 1–6), with the
best results in the case of THF (> 99% conv., Table S3,
entry 1). Notably, the catalyst system is tolerant to water.
Hence, its addition resulted only in a slight drop of the activity
1
2
3
60
60
60
>99
>99
>99
2a, 89
2b, 85
2c, 91
4
5
6
60
80
80
>99
>99
95
2d, 95
2e, 82
2 f, 90
(
86% conv., Table S3, entry 2), while in toluene, the system
was inactive (entry 7). Finally, the reaction was tested using
different catalyst loadings (Table S1, entries 16 and 17), and it
was discovered that 3 mol% of the cobalt catalyst is sufficient
for full conversion.
Once we had the optimal reaction conditions in hand, we
tried our method with the isolated complex. Following the
previously described protocol for a related iron complex
7
8
9
60
80
80
>99
95
2g, 95
2h, 84
2i, 91
[19d]
(
(
see the Supporting Information), we obtained the [CoF-
L2)][BF ] complex 3, having one fluorine ligand derived
4
À
from the original BF4 anion. X-ray analysis revealed that the
cobalt center presents a distorted trigonal bipyramidal
coordination sphere (Figure 3). Although different cobalt
>99
2
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Table 1: (Continued)
Entry Quinoline 1
[
b]
T [8C]
Conv. [%]
2
[
b]
Yield [%]
1
0
1
120
60
>99
>99
2j, 98
1
2k, 95
[
c]
c]
c]
1
1
1
1
1
2
60
60
98
>99
>99
92
2l, 79
2m, 91
2n, 85
2o, 88
2p, 82
[
3
Scheme 1. [Co/L2]-catalyzed selective hydrogenation of different quino-
lines containing reducible groups. Standard reaction conditions:
[
4
5
6
60
substrate (0.25 mmol), Co(BF ) ·6H O (3 mol%), L2 (M/L, 1:1), THF
4
2
2
(
2 mL), and H (10 bar) at the noted temperature, 15 h. Yield of
2
80
product given. [a] Co(BF ) ·6H O (4 mol%). [b] Traces of 6-styryl-
4
2
2
1
,2,3,4-tetrahydroquinoline were detected. The moderate yield of
isolated 2y is due to degradation problems. [c] Traces of 6-phenethyl-
,2,3,4-tetrahydroquinoline were detected. [d] Co(BF ) ·6H O
100
90
1
4
2
2
(5 mol%).
[
c]
1
1
1
7
100
100
100
97
>99
84
2q, 94
2r, 96
2s, 78
highly sensitive groups such as alkene, alkyne, and aldehyde.
To the best of our knowledge such latter heteroarene
hydrogenations have not been described. Apparently, this
method constitutes a powerful tool for the further function-
alization of the corresponding substituted 1,2,3,4-tetrahydro-
quinolines.
[
c,d]
c]
8
[
9
From the viewpoint of organic synthesis, it is interesting
that the protocol could also be extended to the hydrogenation
of other important N-heteroarenes (Scheme 2). Thus, several
heterocycles such as naphthyridine (4a), quinoxaline (4b), or
acridine (4c), were hydrogenated at 608C, while higher
temperatures were required for isoquinoline (4d) and the
benzoquinolines 4e–g. In contrast, only moderate or low
yields were obtained for various indole and pyridine deriv-
atives (Scheme S5).
[
e]
2
0
130
98
2t, 85
[
(
[
a] Standard reaction conditions: Substrate (0.25 mmol), Co(BF ) ·6H O
3 mol%), L2 (M/L, 1:1), THF (2 mL), H (10 bar), 60–1308C, 15 h.
2
4 2 2
b] Conversion of the starting material was calculated by GC using
hexadecane as internal standard. Yield of isolated product given.
c] Co(BF ) ·6H O (5 mol%). [d] Run at 40 h. Total hydrogenation of
both rings was detected obtaining a d.r. (racemic/meso)=(69:31).
e] Co(BF ) ·6H O (8 mol%).
[
4
2
2
[
4
2
2
were isolated in good to excellent yields. The only exception
was 8-aminoquinoline (1j), which required 1208C, probably
because of its more favorable coordination to the metal
center. Remarkably, the quinaldines 1l–o were also success-
fully hydrogenated (entries 12–15). Furthermore, the 2-sub-
stituted more sterically hindered quinolines 1p and 1q were
hydrogenated at 1008C (entries 16 and 17). 2,2-Biquinoline
(1r) reacted simultaneously at both pyridine rings at 1008C
(
entry 18) using 5 mol% of cobalt and a longer reaction time.
3
- and 4-methyl quinolines (1s and 1t), wherein the latter
[
7a,21]
example is commonly known as a challenging substrate,
were successfully hydrogenated in good to excellent yield,
albeit with higher catalyst loadings and temperatures.
Then, we explored the selectivity of this cobalt system in
the hydrogenation of several quinolines bearing other reduci-
ble groups (Scheme 1). Gratifyingly, the [Co]/L2 system
hydrogenated the heteroarene selectively in the presence of
not only esters, a carboxylic acid, or a cyclic imide, but also
Scheme 2. [Co/L2]-catalyzed selective hydrogenation of N-hetero-
arenes. Standard reaction conditions: substrate (0.25 mmol),
Co(BF ) ·6H O (5 mol%), L2 (M/L, 1:1), THF (2 mL), and H (10 bar)
4
2
2
2
at the noted temperature, 15 h. Yield of isolated product given.
[a] Co(BF ) ·6H O (8 mol%).
4
2
2
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with other methodologies and the demonstrated chemo-
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valuable heterocyclic products.
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Manuscript received: December 19, 2016
L. Avram, Y. Ben-David, D. Milstein, Chem. Eur. J. 2016, DOI:
Final Article published: && &&, &&&&
Angew. Chem. Int. Ed. 2017, 56, 1 – 6
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
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These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Homogeneous Catalysis
R. Adam, J. R. Cabrero-Antonino,
A. Spannenberg, K. Junge, R. Jackstell,
M. Beller*
&&&& — &&&&
A General and Highly Selective Cobalt-
Catalyzed Hydrogenation of N-
Heteroarenes under Mild Reaction
Conditions
Reduction! Quinolines and related N-
heteroarenes are hydrogenated using
[Co]/L. The reactions proceed under mild
conditions in a highly selective manner.
6
www.angewandte.org
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2017, 56, 1 – 6
These are not the final page numbers!