Cobalt-catalysed transfer hydrogenation of quinolines and related heterocycles using formic acid under mild conditions

Article abstract of DOI:10.1039/c7cy00437k

Herein, we report the first example of homogeneous non-noble metal-catalyzed transfer hydrogenation of N-heteroarenes. The combination of Co(BF₄)₂·6H₂O with tris(2-(diphenylphosphino)phenyl)phosphine L1 is able to selectively reduce quinolines in the presence of other sensitive functional groups, under mild conditions, using formic acid as a hydrogen source.

Full text of DOI:10.1039/c7cy00437k

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Catalytic mechanism of C–F bond cleavage: insights from QM/MM
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Cobalt-catalysed Transfer Hydrogenation of Quinolines and
Related Heterocycles Using Formic Acid under mild Conditions†
Jose R. Cabrero-Antonino,a† Rosa Adam,a† Kathrin Junge,a Ralf Jackstella and Matthias Bellera*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Herein we report the first example of homogeneous non-noble formic acid has been intensively investigated as a hydrogen
metal catalyzed transfer hydrogenation of N-heteroarenes. The carrier due to its safety, accessibility and high stability.7
combination
of
Co(BF4)2·6H2O
with
tris(2-
In the last years, many efforts have been done for
(diphenylphosphino)phenyl)phosphine L1 is able to selectively developing heterogeneous8 as well as homogeneous catalytic
reduce quinolines in the presence of other sensitive functional systems for transfer hydrogenations of N-heteroarenes.
groups, at mild conditions, using formic acid as hydrogen source.
Generally, homogeneous systems have the advantage of acting
at milder conditions. However, apart from organocatalytic
systems,9 they are mainly based on precious metals such as
Ru,10 Pd,11 Ir12 or Rh.13 Nowadays, there is a strong tendency of
replacing noble metals by earth abundant base metals,14 due
to their lower toxicity and abundance. In this direction,
recently several catalysts based on Fe,15 Co16 and Mn17 have
been described for the transfer hydrogenation of C-O, C-N and
C-C multiple bonds. Interestingly, cobalt and manganese metal
nanoparticles in the presence of Li and traces of H2O have
been reported to efficiently reduce quinoline and other
arenes.18 Despite these important achievements, there is still
no general methodology available for transfer hydrogenation
of N-heterocycles in the presence of non-noble metal
catalysts. In this regard, here we report the first molecularly-
defined base metal catalysed transfer hydrogenation of N-
heterocycles. Key for the success is the use of a specific
tetradentate phosphine. Notably, the reaction proceeds in the
absence of any additives.
N-heterocycles are interesting molecules with important
applications, mainly as bioactive compounds.1 In particular
1,2,3,4-tetrahydroquinolines constitute a widespread scaffold
present in many natural products (i. e. galipinine) and drugs.2
Indeed,
oxamniquine3
or
flumequine,4
containing
tetrahydroquinoline moiety, are currently in use as
antihelmintic or antibacterial agents (Figure 1).
The most straightforward approach for the synthesis of
tetrahydroquinolines and related compounds, is the reduction
of the parent arene. In terms of sustainability, catalytic
hydrogenation5 and transfer hydrogenation6 are preferred
methodologies for these reductions. Although hydrogenation
is the most atom-efficient approach, transfer hydrogenation
has also advantages, as it avoids the use of high pressures and
allows an easy operational set up. Among the possible
hydrogen donors available for transfer hydrogenations,
alcohols and formic acid are the most desirable. Specifically
Previous experience of our group involved formic acid
dehydrogenation as well as transfer hydrogenations using iron
salts in combination with tetradentante phosphines.19 Very
recently, we have described the combination of Co(BF4)2·6H2O
with tris(2-(diphenylphosphino)phenyl)phosphine L1 as an
active catalyst for the hydrogenation of N-heteroarenes.20, 21
Inspired by these results, we started to explore the transfer
hydrogenation of quinoline 1a using formic acid as hydrogen
source and Co(BF4)2·6H2O/L1 as catalyst (Table 1). In a first
approach, we performed the reaction at 60 °C, with iPrOH as
F
HO
O2N
N
H
N
O
N
N
H
O
O
COOH
(S)ꢀFlumequine
(R)ꢀGalipinine
Oxamniquine
Fig.
1 Examples of natural products and drugs with 1,2,3,4-tetrahydroquinoline
structure.
solvent and during 20
h in the presence of different
combinations of Co(BF4)2·6H2O, ligand L1 and formic acid (4.5
eq) (Table 1, entries 1-4). Interestingly, only when
Co(BF4)2·6H2O, L1 and formic acid were present, moderate
yields of 1,2,3,4-tetrahydroquinoline 2a were detected. This
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Table 1. Transfer hydrogenation of quinoline (1a) using formic acid: Initial optimization
conditions the corresponding hydrogenated product 2a was
obtained in excellent yield. Even at lower catalyst loadings (3
mol% ) excellent conversions and selectivities were achieved
(Table 1, entries 5-8). By adding a larger excess of formic acid
(10 eq.), it was possible to successfully carry out the reaction
using 2 mol% of catalyst (Table 1, entry 10).
of the reaction conditions.
Co(BF4) —6H2O
2
Ligand (M:L/1:1)
HCO2H (4.5ꢀ10 eq)
T (°C), 20 h
dry iPrOH
N
N
H
1a
2a
To investigate in more detail the effect of the phosphine
ligand, several bi-, tri- and tetradentate phosphines were
tested under the optimized reaction conditions (Table 1,
entries 13-17). No activity was detected for any of the ligands,
apart from L1, indicating that this tetradentate phosphine is
unique for forming an active cobalt catalyst. In addition, we
also explored the effect of different metal salts (Table S1).
Among the cobalt salts tested in the benchmark reaction
(Table S1, entries 1-9), Co(BF4)2·6H2O was the most active and
only CoSO4·7H2O afforded tetrahydroquinoline 2a in moderate
PPh2
PCy2
PCy2
PPh2
PPh2
L1
P
P
PPh2
PPh2
L2
P
PCy2
Ph2P
L3
PPh2
PPh2
PPh2
L4
PPh2
Ph2P
Ph2P
PPh2
L5
L6
[Co]
(mol%)
HCO2H
(eq)
Conv.
(%)b
2a
Entrya
T (°C)
Ligand
(%)b
yields (Table S1, entries
1
and 7, respectively).
Tetrafluoroborate salts of iron, copper and zinc did not show
any activity under the tested conditions (Table S1, entries 10-
12). The reaction was also performed using the well-defined
[CoF(L1)][BF4] complex20 (Table 1, entry 18) with excellent
results, comparable to the ones with the situ system. To
confirm the homogeneous nature of our catalyst, poisoning
experiments22 were performed in the presence of varying
amounts of Hg (Table S2). In all the performed tests no
significant effects on the transfer hydrogenation of quinoline
were observed, indicating that a molecularly defined complex
is the catalyst of this reaction.
1
2
60
60
60
60
80
80
80
80
80
80
80
80
80
80
80
80
80
80
5
-
-
-
-
-
-
5
5
5
5
4
3
2
2
2
2
2
2
2
2
2
2
-
-
4.5
-
3
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
L2
L3
L4
L5
L6
-
-
-
4
4.5
4.5
4.5
4.5
4.5
8
52
>99
>99
95
73
85
>99
10
-
48
>99
95
84
68
84
97
9
5
6
7
8
With regard to the solvent effect, several polar and apolar
solvents were explored (Table S3). In general, alcohol type
solvents afforded the best conversions and selectivities with a
few exceptions (Table S3, entries 1-9), being iPrOH the best
one (Table S3, entry 1). The addition of water to the reaction
media had a detrimental effect in the activity, giving 2a in low
yields (Table S3, entry 2). Polar aprotic solvents, such as ether
type, DMF or DMSO, neither gave good results (Table S3,
9
10
11c
12
13
14
15
16
17
18 d
10
10
-
-
10
10
10
10
10
10
-
-
entries
10-13).
In
contrast,
toluene
afforded
-
-
tetrahydroquinoline 2a in good yields, whereas in m-xylene
the catalyst was not active (Table S3, entries 14 and 15,).
At this point the reducing agent was also the subject of further
investigations (Figure 2). This study showed that the reaction
only proceeded with formic acid. Formates or other acids such
as acetic acid were completely inactive. Surprisingly, the
addition of base inhibited totally the reaction
2
-
-
-
3
-
>99
96
aStandard reaction conditions: quinoline (1a) (29.6 µL, 0.25 mmol), Co(BF4)2·6H2O
(2-5 mol%), ligand (M:L/1:1), HCO2H (2.5-10 eq) and dry iPrOH (2 mL) at 80 °C
during 20 h. bConversion of 1a and yield of product 2a were calculated by GC
using hexadecane as internal standard. cRun at 2 h. (Cy = cyclohexyl). dReaction
performed with [CoF(L1)][BF4] (2 mol%).
Once determined the optimized conditions, the general
applicability of the protocol was studied through the reaction
of several substituted quinolines (Table 2). The Co/L1 complex
demonstrated to be tolerant to several electron-donating and
electron-withdrawing substituents. Most of the substrates
were hydrogenated at 80 °C, using 10 equivalents of formic
acid and catalyst loadings of 2.5-5 mol%. 6-Substituted
quinolines 1b-i, containing electronically different groups,
were successfully converted (Table 2, entries 2-9). Gratifyingly,
the studied cobalt catalyst demonstrated to be selective to the
hydrogenation of the N-heteroarene in the presence of
carboxylic acid, ester and alkene groups (Table 2, entries 7-9).
The demonstrated selectivity can be a key issue in the
indicates that Co(BF4)2·6H2O and L1 form an active complex
able to catalyse the transfer hydrogenation from formic acid to
quinoline 1a without the need of a base present. Moreover,
the lack of activity in the absence of formic acid discards that
isopropanol acts as hydrogen source (Table 1, entry 3). To
further improve this reaction, we increased the temperature
to 80 °C (Table 1, entry 5). To our delight, under these
2 | J. Name., 2012, 00, 1-3
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Co(BF4)2—6H2O (2 mol%)
7
2.5
8
>99
>99
2g [82]
2h [98]
L1 (M:L/1:1)
Reductor (10 eq)
80 °C, 20 h
dry iPrOH
N
H
N
2a
1a
8
9c
4
4
>99
97
2i [89]
2j [93]
10
11
2.5
5
>99
88
2k [93]
2l [82]
Fig. 2 Yield of product 2a in the transfer hydrogenation of quinoline (1a) using
different reducing agents. Standard reaction conditions: quinoline (1a) (29.6 µL,
0.25 mmol), Co(BF4)2·6H2O (1.70 mg, 0.005 mmol, 2 mol%), ligand L1 (4.0 mg,
0.005 mmol, 2 mol%), reductor (2.5 mmol, 10 eq) and dry iPrOH (2 mL) at 80 °C
during 20 h. [b] Conversion of 1a and yield of product 2a were calculated by GC
using hexadecane as internal standard.
12
13
14
2.5
2.5
99
2m [90]
2n [97]
potential application of this methodology to the synthesis of
further functionalized tetrahydroquinolines. In addition, 8-
substituted quinolines 1j-n were smoothly hydrogenated
affording 1,2,3,4-tetrahydroquinolines in good yields (Table 2,
entries 10-14). Finally, more challenging quinaldines 1o and 1p
could also be converted, although higher temperature (100 °C)
and catalyst loadings were required (Table 2, entries 12 and
13).
>99
15d
16d
8
5
84
94
2o [78]
2p [84]
Table 2. [Co/L1]-catalyzed transfer hydrogenation of different substituted quinolines
using formic acid.
aStandard reaction conditions: quinoline (0.25 mmol), Co(BF4)2·6H2O (2.5-8
mol%), ligand L1 (M:L/1:1), HCO2H (95.0 µL, 2.5 mmol, 10 eq) and dry iPrOH (2
mL) at 80 °C during 20 h. bConversion of the starting material was calculated by
GC using hexadecane as internal standard. Isolated yields of the products are
given between brackets. cSmall amounts (< 5%) of product containing the double
bond hydrogenated were detected. dRun at 100 °C.
Co(BF4)2—6H2O
L1 (M:L/1:1)
R
R'
R
R'
HCO2H (10 eq)
80 °C, 20 h
dry iPrOH
N
N
H
2a-p
1a-p
Entrya
1
Substrate 1
[Co] (mol%)
2.5
Conv.
(%)b
2
(%)b
With the aim of broadening the scope of the Co/L1 catalysed
method, the transfer hydrogenation of other N-heterocycles
was tested at 100 °C, using 5 mol% of catalyst and 10
equivalents of the reducing agent (Scheme 1). Under these
conditions naphthyridine 1q, quinoxaline 1r or acridine 1s
were converted to the corresponding reduced N-heterocycles.
In conclusion, we have described the first methodology
>99
2a [90]
2
3
2.5
2.5
>99
2b [90]
>99
2c [88]
based on
a homogeneous non-noble metal catalyst for
performing the transfer hydrogenation of quinolines and other
N-heteroarenes. The developed defined cobalt catalyst system
uses formic acid as an inexpensive and convenient source of
hydrogen. Compared to most other transfer hydrogenations
this novel protocol works in the absence of basic additives.
Interestingly, the catalyst shows a high selectivity for the
reduction of quinoline in the presence of other sensitive
groups. The easy operational set up, mild conditions and
selectivity of the methodology are significant advantages for
its future application in synthetic strategies for obtaining
valuable N-heterocycles.
4
5
5
5
85
2d [78]
2e [76]
>99
6
5
89
2f [80]
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Scheme 1. [Co/L1]-catalyzed transfer hydrogenation of N-heterocycles using formic
acid. Standard reaction conditions: substrate (0.25 mmol), Co(BF4)2·6H2O (4.25 mg,
0.0125 mmol, 5 mol%), ligand L1 (M:L/1:1), HCO2H (95.0 µL, 2.5 mmol, 10 eq), dry
iPrOH (2 mL) at 100 °C, 20 h. Isolated yields of the products are given.
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Financial support from the state of Mecklenburg-
Vorpommern and the BMBF is gratefully acknowledged. J.R. C.-
A. and R.A. thank the Ramon Areces Foundation for a
postdoctoral fellowship. The analytic department of LIKAT is
gratefully acknowledged for their support.
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