New catalyst systems for iron-catalyzed hydrosilane reduction of carboxamides

Article abstract of DOI:10.1039/c1cc10636h

A heptanuclear iron carbonyl cluster, [Fe₃(CO) ₁₁(μ-H)]₂Fe(DMF)₄ (4), is found to be a highly efficient catalyst for the reduction of various carboxamides by 1,2-bis(dimethylsilyl)benzene (BDSB), which makes possible reducing the amount of the catalyst, shortening the reaction time, and lowering the reaction temperatures.

Full text of DOI:10.1039/c1cc10636h

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COMMUNICATION
New catalyst systems for iron-catalyzed hydrosilane reduction of
carboxamidesw
Hironori Tsutsumi,a Yusuke Sunadaab and Hideo Nagashima*ab
Received 1st February 2011, Accepted 26th April 2011
DOI: 10.1039/c1cc10636h
A
heptanuclear iron carbonyl cluster, [Fe3(CO)11(l-H)]2-
Table 1 Reduction of N,N-dimethyl-p-methoxybenzamide (1a) with
various hydrosilane catalyzed by Fe(CO)5 (5 mol%) in toluene
at 80 1C
Fe(DMF)4 (4), is found to be a highly efficient catalyst for the
reduction of various carboxamides by 1,2-bis(dimethylsilyl)-
benzene (BDSB), which makes possible reducing the amount
of the catalyst, shortening the reaction time, and lowering the
reaction temperatures.
In increasing concern to environmentally benign chemical
processes for production of organic molecules, homogeneous
catalysis by iron compounds has become more important for a
solution to the residual metal problems in the product.1,2 One of
the solutions to this issue is the use of iron catalysts, and we and
Beller’s group have recently reported Fe(CO)5 or Fe3(CO)12 to be
a catalyst for hydrosilane reduction of secondary or tertiary
amides.3,4 The process has synthetic merits in the successful use
of iron catalysts and shows unique chemoselectivity different
from the analogous ruthenium or platinum-catalyzed hydro-
silane reduction of amides; however, it is desirable to improve
because the reaction generally requires long time (24 h) at high
temperature (100 1C) even in the presence of relatively large
amounts of iron carbonyls (6–30 mol% for Fe).z In this paper,
we wish to report a solution to this problem, in which a new iron
carbonyl catalyst makes possible the efficient reduction of amides
with an appropriate hydrosilane, 1,2-bis(dimethylsilyl)benzene
(BDSB). Discovery of the new catalyst actually shortens the
reaction time even with lower catalyst concentration at lower
temperatures compared with the previous iron-catalyzed
hydrosilane reduction of amides.
Yield of 2aa (%)
0.5/h
3/h
24/h
Entry
‘‘Si–H’’
1
2
3
4
Me2PhSiH
(EtO)3SiH
TMDS
o1
o1
6
o1
o1
o1
o1
52
17
BDSB
o1
499
—
Determined by 1H NMR analysis with ferrocene as an internal
standard.
a
80 1C showed that PhMe2SiH and (EtO)3SiH did not work,
whereas TMDS gave the product in 52% yield after 24 h. The
best result so far obtained is 1,2-bis(dimethylsilyl)benzene
(BDSB); after the induction period (0.5–1 h), the reaction
proceeded quickly to give the amine quantitatively after 3 h.
Appropriate catalyst design provided us further improvement
of the reaction conditions. As shown in Scheme 1,
coordinatively unsaturated Fe(CO)3 species is considered to
be involved in the catalytic cycle, which is generated by
thermolysis or photolysis of Fe(CO)5 or Fe3(CO)12. There
are two possible reaction pathways from Fe(CO)3; one is the
double oxidative addition of Si–H groups in BDSB to form the
disilaferracyclic intermediate A,5 whereas the other is the
reaction with amides producing the iron-amide species B.
Fink’s disilaferracycle [Me2SiC6H4SiMe2]Fe(CO)4 (3) has an
analogous structure to A and can be synthesized as a colorless
crystal with ease.6 This is one of the candidates for the
improved catalyst. A new heptanuclear iron carbonyl cluster
4, which could produce the intermediate B by thermal
disproportionation, was obtained by treatment of Fe(CO)5
with DMF at 65 1C in 48% yield as a dark purple crystal.
Identification of paramagnetic 4 was carried out by X-ray
diffraction analysis. The ORTEP drawing was depicted
in Fig. 1, and the selected bond distances and angles are
summarized in the ESI.w The molecular structure of 4 is
As described above, a typical thermal reaction process of
hydrosilane reduction of amides is treatment of p-MeOC6H4-
CONMe2 with TMDS in the presence of Fe(CO)5 (10 mol%
for Fe) at 100 1C for 24 h. The corresponding amine was
obtained in 81% yield.4 Our first trial to mitigate the reaction
conditions is exploration of appropriate hydrosilanes. As
shown in Table 1, the reaction with 5 mol% of Fe(CO)5 at
a Graduate School of Engineering Sciences, Kyushu University,
6-1 Kasugakoen, Kasuga, Fukuoka, 816-8580, Japan.
E-mail: nagasima@cm.kyushu-u.ac.jp; Fax: +81 92-583-7819;
Tel: +81 92-583-7819
b Institute for Materials Chemistry and Engineering, Kyushu
University, 6-1 Kasugakoen, Kasuga, Fukuoka, 816-8580, Japan
w Electronic supplementary information (ESI) available: Experimental
procedure and details of crystallographic data for 4. CCDC 808120.
For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/c1cc10636h
analogous to
a
heptanuclear iron carbonyl cluster,
Fe(THF)4[Fe3(H)(CO)11]2, reported by Brook and coworkers.7
c
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Chem. Commun., 2011, 47, 6581–6583 6581

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Table 2 Reduction of N,N-dimethyl-p-methoxybenzamide (1a) with
BDSB catalyzed by various iron carbonyls (5 mol% for Fe)
Yield of 2aa (%)
0.5/h 1/h 3/h 5/h 12/h 19/h
Entry Fe cat.
Temp/1C
1
2
3
4
5
6
7
8
Fe(CO)5
Fe(CO)5
Fe(CO)5
Fe(CO)5
Fe3(CO)12 50
Fe3(CO)12 60
Fe3(CO)12 70
Fe3(CO)12 80
3
3
3
3
4
4
4
4
50
60
70
80
o1
o1
o1
o1
o1
o1
o1
o1
o1
o1
o1 o1 o1 o1 o1
o1 o1 o1 o1 o1
o1 o1
31 499
14
20
24
—
—
—
o1 o1 o1 o1 o1
o1 o1 o1
o1 20 28
42 499
6
32
—
8
29
—
—
47 499
—
—
—
6
34
—
9
30
—
—
—
9
50
60
70
80
50
60
70
80
o1 o1 o1
10
11
12
13
14
15
16
13
58
15
85 499
29
25
75
o1
o1
88 499
o1 o1
—
4
32 499
o1
—
—
—
82 499
499
—
—
—
—
—
a
Scheme 1 Proposed reaction mechanism.
Determined by 1H NMR analysis with ferrocene as an internal
standard.
The high catalytic efficiency of 4 is also shown in the
reduction of several tertiary and secondary amides in the
presence of 0.5 mol% (based on one Fe atom) of 4 as
summarized in Table 3. With BDSB (2.2 equivalent to 1) in
toluene at 100 1C, the reactions shown in the table proceeded
smoothly, and the amide was converted to the corresponding
amine in high yields within 30 min. A sterically crowded amide
1g and a secondary amide 1j required prolonged reaction time
for completion of the reduction, but the reaction time is no
longer than 3 h (Table 3, entry 7, 10). Amides containing a
chloride, bromide, or ester as the functional group also
underwent the reduction with these reducible functional
groups remaining intact, giving the corresponding amines 2c,
2d, and 2e in high yields (entries 3, 4, and 5). Reduction of
benzylic chloride was seen as a side reaction for the reduction
of 1i; 1i was converted to a mixture of 2i and dehalogenated
product (entry 9). Attempted reduction of primary amides was
not successful. For instance, treatment of benzamide with
BDSB at 100 1C for 3 h resulted in formation of benzonitrile
by dehydration in 15% yield with a recovery of benzamide.8,9
Success of the reaction using a small amount of catalyst
(0.5 mol%) led to effective removal of the iron species in the
product. In a typical example, reduction of 1a (10 mmol) with
BDSB in the presence of 4 (0.05 mmol of Fe) is followed by the
work up described in the ESI.w The crude product (95% yield)
contained no spectroscopically-detectable silicon wastes and
0.7 ppm of Fe residue (ICP-mass); this means almost all of the
silicon residue and the Fe species (499.6%) charged can be
removed without purification of the crude product by chromato-
graphy or distillation. Efficiency to remove the iron residue is
much better than that of the previously reported Fe(CO)5-
catalyzed process with PMHS, in which 95% of iron species
was soaked up to a siloxane resin produced by the reaction.4,10
Fig. 1 Molecular structure of 4 with 50% probability ellipsoid.
Hydrogen atoms except for the bridging hydride atoms were omitted
for clarity.
IR spectrum of 4 [nCO(terminal) = 1950–2070 cmꢀ1, nCO(bridging)
=
1592 cmꢀ1, nCO of DMF = 1654 cmꢀ1] and elemental analysis
are in accord with the molecular structure of 4.
Superior catalytic activity of 3 and 4 to Fe(CO)5 and
Fe3(CO)12 is clearly demonstrated in the reduction of
p-MeOC6H4CONMe2 (1a) with BDSB at 50–80 1C as shown
in Table 2. The reaction in the presence of 5 mol% of the
catalyst at 80 1C for 30 min showed that the yield of
p-MeOC6H4CH2NMe2 (2a) is increased in the order, Fe(CO)5 o
Fe3(CO)12 o 3 (75%) o 4 (99%). At lower temperatures than
70 1C, conversion of 1a was lower than 40% in the reaction
with Fe(CO)5 and Fe3(CO)12 after 19 h. Fink’s complex 3
showed good activity at 70 1C; however, the yield of the
product did not exceed ca. 30% at 60 1C, and deactivation of
the catalyst was observed after a few hours. It is noteworthy that
the reaction catalyzed by 4 proceeded even at 50 1C. No catalyst
deactivation was observed, and the yield of the product reached
99% after 19 h.
c
6582 Chem. Commun., 2011, 47, 6581–6583
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Table 3 Reduction of various carboxamides catalyzed by 4a
complex with the number of CO ligands close to 3 is a key to find
a good catalyst for this reduction. In this sense, it is noteworthy
that the ratio of CO to the Fe is decreased in the order, Fe(CO)5
(CO/Fe = 5) 4 Fe3(CO)12 and Fink’s complex (3) (CO/Fe = 4) 4
4 (CO/Fe = 3.1). Apparently, 4 efficiently generates catalytically
active species Fe(CO)3 easily reacting with amide and BDSB,
but does not produce CO, which deactivates the catalysis by
contact with the catalytically active Fe(CO)3.
Entry Amide
Time/min Yieldb (%)
In conclusion, we have found a new efficient iron catalyst
and a better hydrosilane for reduction of carboxamides. The
discovery makes possible to lower the catalyst concentration
and reduce the reaction time.z The use of iron is a target for
development of environmentally benign organic processes.
Even if iron is indeed a harmless element, facile removal of
the residual iron is desirable. Discovery of 4 provides one clear
solution to the residual metal problem for the hydrosilane
reduction of amides, and also affords a strategy for the catalyst
design, i.e. pursuing the lower CO/Fe ratio.
1
2
3
4
5
1a: X=OMe
1b: X=H
1c: X=Cl
1d: X=Br
30
30
30
30
92
89
94
96
93
1e: X=COOMe 30
6
7
8
1f
30
180
30
86c
94
1g
1h
Notes and references
z Improvement in the efficiency of the iron catalyst is a problem recently
pursued: for example, Tilley have reported hydrosilylation of ketones
occurring at room temperature with a small amount of a catalyst.11
y For the experimental evidence suggesting the importance of a lower
CO/Fe ratio in the catalytic species, we examined the reduction of 1a with
BDSB catalyzed by 3 or 4 under a CO atmosphere (1 atm) at 80 1C for
3 h. In both cases, application of CO completely inhibited the reduction.
z Although the reaction at lower than 100 1C is also possible, we
recommend the application at 100 1C for obtaining reproducible
results when less than 1 mol% of 4 is used. When concentration of
the catalyst is low, only a small amount of CO existing in the reaction
medium easily kills the catalytically active species. Conversion of the
starting amide sometimes does not reach 499%. At 100 1C, the
deactivated species can be reactivated by dissociation of CO.
90
9
1i
1j
30
42d
80
10
180
a
All reactions were carried out with 1 (1 mmol), 4 (0.5 mol% for Fe),
b
and BDSB (2.2 mmol) in toluene (0.5 mL) at 100 1C. Isolated
yield. c 1H NMR spectrum of the crude product revealed the
d
formation of enamine in ca. 3% yield. Dehalogenated product was
1 For the reviews of iron-catalyzed organic reactions, see: (a) B. Plietker,
in Iron Catalysis in Organic Chemistry, ed. B. Plietker, Wiley-VCH,
Weinheim, Germany, 2008; (b) S. Enthaler, K. Junge and M. Beller,
Angew. Chem., Int. Ed., 2008, 47, 3317; (c) E. Nakamura and
N. Yoshikai, J. Org. Chem., 2010, 75, 6061.
isolated in 47% yield.
A question is why the combination of the iron cluster 4 and
BDSB reduces the catalyst loading and shortens the reaction
time. Several findings provide clues for solving this question,
which is related to a strong affinity of CO ligands to the iron
center. First, addition of PPh3 to the solution in which the
catalytic reaction by 4 (5 mol% for Fe) is smoothly running at
60 1C instantly terminated the reaction, and Fe(CO)3(PPh3)2 was
recovered from the reaction mixture. This suggests Fe(CO)3 is
involved in the catalytic cycle. Second, the catalytic reduction of
1a with BDSB in the presence of 5 mol% of Fink’s complex 3
afforded a slightly red-violet solution; its NMR spectrum showed
that 495% of 3 remained unreacted, whereas UV-vis spectrum
of the reaction mixture indicates the existence of a homologue of
4, of which amount is ca. 1.8% from the e of lmax = 499 nm.
This indicates a reaction pathway from 3 to Fe(CO)3, which is
trapped by amides in the catalytic reaction to form 4. The cluster
4 behaves as a dormant species which reversibly generates a
catalytically active Fe(CO)3(k1-amide) (B). Third, the reaction
catalyzed by Fink’s complex 3 at 60 1C showed catalyst
deactivation after a while. The catalyst was able to reactivate
by heating the reaction mixture at 100 1C. A reasonable inter-
pretation of this result is that the net catalytic activity of Fe(CO)3
is facilely killed by coordination of CO.y In other words, the iron
2 Recent examples of iron-catalyzed hydrosilylation reactions, see:
(a) H. Nishiyama and A. Furuta, Chem. Commun., 2007, 760;
(b) N. S. Shaikh, K. Junge and M. Beller, Org. Lett., 2007, 9, 5429;
(c) B. K. Langlotz, H. Wadepohl and L. H. Gade, Angew. Chem.,
Int. Ed., 2008, 47, 4670; (d) S. Enthaler, K. Junge and M. Beller,
Angew. Chem., Int. Ed., 2008, 47, 3317; (e) J. Y. Wu, B. N. Stanzl
and T. Ritter, J. Am. Chem. Soc., 2010, 132, 13214.
3 S. Zhou, K. Junge, D. Addis, S. Das and M. Beller, Angew. Chem.,
Int. Ed., 2009, 48, 9507.
4 Y. Sunada, H. Kawakami, T. Imaoka, Y. Motoyama and
H. Nagashima, Angew. Chem., Int. Ed., 2009, 48, 9511.
5 For catalytic reduction of carbonyl compounds through
disilametallacyclic intermediates: (a) Y. Sunada, Y. Fujimura and
H. Nagashima, Organometallics, 2008, 27, 3502; (b) H. Tsutsumi,
Y. Sunada and H. Nagashima, Organometallics, 2011, 30, 68.
6 W. Fink, Helv. Chim. Acta, 1975, 58, 1464.
7 D. J. R. Brook, V. Lynch and T. H. Koch, Inorg. Chem., 1995, 34,
5691.
8 S. L. Zhou, D. Addis, K. Junge, S. Das and M. Beller, Chem.
Commun., 2009, 4883.
9 S. Hanada, Y. Motoyama and H. Nagashima, Eur. J. Org. Chem.,
2008, 4097.
10 (a) Y. Motoyama, K. Mitsui, T. Ishida and H. Nagashima, J. Am.
Chem. Soc., 2005, 127, 13150; (b) S. Hanada, Y. Motoyama and
H. Nagashima, Tetrahedron Lett., 2006, 47, 6173; (c) S. Hanada,
E. Tsutsumi, Y. Motoyama and H. Nagashima, J. Am. Chem. Soc.,
2009, 131, 15032.
11 J. Yang and T. Don Tilley, Angew. Chem., Int. Ed., 2010, 49, 10186.
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 6581–6583 6583