Two iron catalysts are better than one: A general and convenient reduction of aromatic and aliphatic primary amides

Article abstract of DOI:10.1002/anie.201108155

It takes two: For the reduction of amides to amines iron catalysts were developed. A combination of two different iron catalysts made possible the challenging reduction of primary amides (see picture). Copyright

Full text of DOI:10.1002/anie.201108155

.
Angewandte
Communications
DOI: 10.1002/anie.201108155
Homogeneous Catalysis
Two Iron Catalysts are Better than One: A General and Convenient
Reduction of Aromatic and Aliphatic Primary Amides**
Shoubhik Das, Bianca Wendt, Konstanze Mçller, Kathrin Junge, and Matthias Beller*
Substituted amines and their derivatives are of significant
importance for the synthesis of numerous products of the
pharmaceutical and agrochemical industry. Furthermore, they
represent building blocks for fine and bulk chemicals as well
[1]
as polymers and dyes. In particular, primary amines are
useful intermediates for further derivatizations. Owing to
their importance, the development of novel methods for the
synthesis of primary amines continues to be an active area of
[2]
research. The most common approaches for their synthesis
[3]
include reduction of nitriles, reductive amination of alde-
[
4]
hydes and ketones, reduction of carboxylic acids in the
[
5]
presence of ammonia, alkylation of organic halides with
[6]
ammonia or ammonia equivalents, borrowing hydrogena-
[
7]
[8]
tion methodologies, and hydroamination of alkynes. In
addition to all these achievements, the selective reduction of
primary amides represents a straightforward route to this
class of compounds because of the availability of carboxylic
acid amides.
Scheme 1. Different processes for the reduction of primary amides.
amides. Normally, primary amides are dehydrated to the
[
14]
corresponding nitriles under hydrosilylation conditions.
In recent years, iron-based complexes allowed for consid-
So far, for the reduction of primary amides traditional
metal hydride-mediated reduction reactions still prevail
[15]
erable inventions in organometallic catalysis. Apart from
the abundant availability of iron, the often low toxicity and
biomimetic pre-catalysts make iron an ideal metal for
[
9]
(
Scheme 1). Because of the air and moisture sensitivity of
these reagents and the costly purification of the products,
[10]
[16]
selective catalytic methods, especially hydrogenations, are
catalysis. Based on our previous studies using biocatalysts
[
11]
highly desired.
Unfortunately, until to date no general
for the reduction of secondary and tertiary amides, herein we
report the first general and convenient iron-based catalytic
[5]
catalytic hydrogenation of amides to amines exists.
[
17]
Complementary to reductions with molecular hydrogen,
catalytic hydrosilylations are increasingly often applied in
organic synthesis because of their operational simplicity and
often improved chemo- and regioselectivity under mild
reduction of primary amides.
At the start of our work we investigated the reaction of
benzamide (1) with inexpensive methyldiethoxysilane in
toluene as a model system to identify and optimize critical
reaction parameters (Table 1). As expected no reaction
occurred in the absence of any catalyst (Table 1, entry 1). In
agreement with previously reported results, in the presence of
ammonium hydridotriironundecarbonylate benzonitrile was
the major product instead of the desired benzyl amine
[
12]
conditions. More specifically, metal-catalyzed hydrosilyla-
tions of amides have received considerable interest in the last
decade and various catalyst systems have proven to be
[13]
effective for this reduction. Notably, the vast majority of
the known noble metal-based catalysts can be used only for
the reduction of more active tertiary or secondary amides. In
[
14b,c]
(Table 1, entry 4).
Also applying other iron pre-catalysts
[13b]
fact, there is only one report by Igarashi and Fuchikami,
who showed the possibility for the reduction of primary
benzonitrile is formed or no reactivity was observed. In
addition, we investigated the influence of several nitrogen-
based ligands (Scheme 2) together with different iron salts,
for example, Fe(OAc) as catalyst precursor in the presence of
2
[
*] Dr. S. Das, B. Wendt, Dr. K. Mçller, Dr. K. Junge, Prof. Dr. M. Beller
Leibniz-Institut fꢀr Katalyse e.V. an der Universitꢁt Rostock
Albert-Einstein-Str. 29a, 18059 Rostock (Germany)
E-mail: matthias.beller@catalysis.de
methyldiethoxysilane to improve the desired reaction (see the
Supporting Information). Unfortunately, no progress was
achieved.
However, using benzonitrile as a substrate, which is
formed from benzamide as shown in Table 1, the latter
catalysts proved to be active and provided benzylamine in
Homepage: http://www.catalysis.de
[
**] This work has been funded by the State of Mecklenburg-Western
Pomerania, the BMBF, and the DFG (Leibniz-price). We thank Dr. W.
Baumann, Dr. C. Fischer, S. Buchholz, S. Schareina, A. Kammer, A.
Koch, S. Rossmeisl, and K. Mevius (all at the LIKAT) for their
excellent technical and analytical support.
[18]
8
5% yield. The best results are observed in the presence of
phenanthroline ligands, which provided high yields of benzyl-
amine. Phenanthrolines substituted with electron-donating
groups showed the best activity (Table 2, entries 10–12).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201108155.
1
662
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Angew. Chem. Int. Ed. 2012, 51, 1662 –1666

Angewandte
Chemie
Table 1: Catalytic reduction of benzamide in the presence of iron
catalysts.
Table 2: Iron-catalyzed reduction of benzonitrile.
[
a]
[
a]
Entry
Catalyst [mol%]
Ligand [mol%]
Yield [%]
[
b]
[b]
Entry Catalyst
mol%]
Solvent Yield of 2
Yield of 3
[%]
1
2
3
4
5
6
7
8
9
Fe(OAc) (10)
L1 (20)
L2 (20)
L3 (20)
L4 (20)
L5 (20)
L6 (20)
L7 (20)
L8 (20)
L9 (20)
L10 (20)
L11 (20)
L12 (20)
L11 (15)
L11 (20)
L11 (20)
L11 (20)
L11 (20)
L11 (20)
L11 (20)
L11 (20)
L11 (20)
L11 (20)
5
7
–
–
–
–
25
4
40
50
85
75
62
50
–
2
[
[%]
Fe(OAc) (10)
2
Fe(OAc) (10)
1
2
3
4
5
6
7
8
9
0
1
–
toluene
toluene
toluene
–
3
1
4
–
–
–
–
–
–
–
–
2
Fe(OAc) (10)
Fe(OAc) (10)
Fe (CO) (10)
[Et NH][HFe (CO) ] (5) toluene
Fe(acac) (10)
FeO (10)
Fe(OTf) (10)
Fe(NO ) (10)
Fe(acac) (10)
83
83
90
–
–
–
–
65
–
2
2
Fe(OAc) (10)
2
3
12
Fe(OAc) (10)
2
3
3
11
Fe(OAc) (10)
toluene
toluene
toluene
toluene
toluene
toluene
dioxane
2
2
Fe(OAc) (10)
2
Fe(OAc) (10)
2
2
10
11
12
13
14
15
16
17
18
19
20
21
22
Fe(OAc) (10)
2
3
2
Fe(OAc) (10)
2
3
Fe(OAc) (10)
1
1
FeBr (10)
Fe(OAc) (10)
2
3
Fe(OAc) (10)
–
2
2
Fe(OAc) (7.5)
2
[
a] Reaction conditions: Benzamide (1.0 mmol), silane (3.0 mmol),
solvent (3 mL), 24 h. [b] Determined by GC with hexadecane as an
internal standard.
Fe(acac) (10)
2
FeO (10)
–
Fe(BF ) (10)
10
15
10
5
6
4
4
2
FeF (10)
2
Fe (CO) (10)
3
12
Fe(OTf) (10)
2
Fe(NO ) (10)
3
2
[Et NH][HFe (CO) ] (2)
3
3
11
[a] Determined by GC with hexadecane as an internal standard.
Table 3: Diiron-catalyzed reduction of benzamide.
Scheme 2. Structures of different ligands for hydrosilylation of primary
amides.
[a]
Entry
Fe Catalyst 1
2 mol%]
Fe Catalyst 2
[mol%]
Ligand
[mol%]
Yield
[%]
[
1
2
3
4
5
6
7
8
9
[Et NH][HFe (CO) ]
Fe(OAc) (10)
L1 (20)
L2 (20)
L3 (20)
L4 (20)
L5 (20)
L6 (20)
L7 (20)
L8 (20)
L9 (20)
L10 (20)
L11 (20)
L12 (20)
L11 (15)
L11 (20)
L11 (20)
L11 (20)
L11 (20)
L11 (20)
L11 (20)
L11 (20)
L11 (20)
1
3
–
–
–
–
15
2
15
35
70
60
45
42
–
3
3
11
2
Surprisingly, oxazoline and terpyridine ligands did not
show any activity at all (Table 2, entries 5–6). Furthermore,
other iron precursors were also not active including ammo-
nium hydridotriironundecarbonylate (Table 2, entries 15–22).
Next, we tried to combine the two different iron-catalyzed
reactions. To our delight, the chemoselective reduction to
benzylamine is possible in a consecutive manner in the same
solvent and with the same silane. As shown in Table 3, this
[Et NH][HFe (CO) ]
Fe(OAc) (10)
3
3
11
2
[Et NH][HFe (CO) ]
Fe(OAc) (10)
3
3
11
2
[Et
3
NH[HFe
(CO)11
]
Fe(OAc)
(10)
3
2
[Et NH][HFe (CO) ]
Fe(OAc) (10)
3
3
11
2
[Et NH][HFe (CO) ]
Fe(OAc) (10)
3
3
11
2
[Et NH][HFe (CO) ]
Fe(OAc) (10)
3
3
11
2
[Et NH][HFe (CO) ]
Fe(OAc) (10)
3
3
11
2
[Et NH][HFe (CO) ]
Fe(OAc) (10)
3
3
11
2
10
[Et NH][HFe (CO) ]
Fe(OAc) (10)
3
3
11
2
diiron catalysis is performed with 2 mol% of [Et NH]-
11
12
[Et NH][HFe (CO) ]
Fe(OAc) (10)
3
3
3
3
3
11
2
[
HFe (CO) ] and 10 mol% of Fe(OAc) in the presence of
[Et
NH][HFe
(CO)11
]
Fe(OAc) (10)
2
3
11
2
13
14
15
16
17
[Et NH][HFe (CO) ]
Fe(OAc) (10)
nitrogen-based ligands. Again, the best result (70% of
benzylamine) is obtained applying ligand L11 (Table 3,
entry 11). The use of other iron precursors did not increase
the yield (Table 3, entries 15–21).
3
3
11
2
[Et NH][HFe (CO) ]
Fe(OAc) (7.5)
3
3
11
2
[Et NH][HFe (CO) ]
Fe(acac) (10)
3
3
11
2
[Et NH][HFe (CO) ]
FeO (10)
–
5
2
6
5
4
3
3
11
[Et NH][HFe (CO) ]
Fe(BF ) (10)
3
3
11
4 2
In addition to (EtO) MeSiH, other silanes such as PhSiH ,
18
19
[Et NH][HFe (CO) ]
FeF (10)
2
3
3
3
11
2
Ph SiH , (EtO) SiH, PMHS, and TMDS were also tested for
[Et
3
NH][HFe
3
(CO)11
]
Fe
3
(CO)12 (10)
2
2
3
2
2
0
1
[Et NH][HFe (CO) ]
Fe(OTf) (10)
this reduction. However, in none of the cases appreciable
activity was found (see the Supporting Information). The
reaction can be performed also in 1,4-dioxane or di-n-
3
3
11
2
[Et NH][HFe (CO) ]
Fe(NO ) (10)
3
3
11
3 2
[a] Determined by GC with hexadecane as an internal standard.
Angew. Chem. Int. Ed. 2012, 51, 1662 –1666
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Angewandte
Communications
Table 4: Diiron-catalyzed hydrosilylation of primary amides: substrate
scope.
Table 4: (Continued)
Entry Substrates
[b]
[Et NH][HFe (CO) ]
Yield
[%]
3
3
11
[mol%]
18
4
53
[
a] Isolated yields are given in parenthesis for entries 5, 6, and 7. In all
[
b]
Entry
Substrates
[Et NH][HFe (CO) ]
Yield
[%]
3
3
11
other cases GC yields are reported. [b] After the end of the reaction, the
mixture was stirred with 4 mL of a 1m TBAF solution for 3 h and then
measured with GC using hexadecane as a standard (for 1 mmol
substrate we added 1 mmol of hexadecane).
[mol%]
1
2
3
4
2
2
2
2
70
70
68
68
butylether; however a slower reaction took place in the latter
solvents.
Regarding the mechanism (Scheme 3), we propose an
initial dehydration of benzamide to benzonitrile with subse-
quent reduction. Clearly, the different reactivity of the two
iron species for the dehydration of benzamide and for
hydrosilylation of the corresponding nitrile is surprising.
Obviously, benzonitrile is not efficiently hydrosilylated by the
iron carbonyl complex (see Table 2, entry 22), which might be
explained by the lower coordination ability compared to the
65
5
6
5
2
[
[
a]
a]
(
(
59)
II
Fe complex. On the other hand, the amide is not activated by
Fe(OAc)2.
68
Next, the scope and limitations of this first iron-catalyzed
reduction of primary amides with methyldiethoxysilane were
explored. As shown in Table 4, 18 different carboxylic acid
amides were reduced smoothly to the corresponding amines.
Substrates included various substituted benzamides and
higher aromatic amides as well as linear and cyclic aliphatic
amides.
In none of the cases any secondary or tertiary amines were
observed as side products. With respect to benzamides, both
electron-donating and electron-withdrawing substituents
behaved in the same manner (Table 4, entries 5–6). In
addition, the hydrosilylation took place in case of ortho-
substituted benzamide (Table 4, entry 2). Noteworthy, the
reaction tolerated different halides and amine substituents
60)
62
7
2
[
a]
(58)
8
9
5
4
5
5
63
60
65
60
1
1
0
1
(
Table 4, entries 8–10, and 13). Expediently, several aliphatic
amides including adamantanecarboxamide were also reduced
to the amines in moderate yield (Table 4, entry 18).
1
2
5
60
1
1
3
4
4
5
58
52
1
1
1
5
6
7
CH
3
(CH
2
)
7
CONH
2
2
2
4
4
4
48
51
49
CH (CH ) CONH
CH (CH ) CONH
3
2 4
3
2 8
Scheme 3. Proposed mechanism for the diiron-catalyzed reduction of
benzamide.
1
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In conclusion, we developed the first iron-catalyzed
reductions of primary amides to amines. Notably, the desired
transformation does not proceed in the presence of a single
metal catalyst, but using two different iron complexes in a
consecutive manner allowed for the formation of the primary
amine. The generality of this protocol is shown in the
hydrosilylation of 18 different primary amides with inexpen-
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[
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.
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