Chemo-and stereoselective iron-catalyzed hydrosilylation of ketones

Article abstract of DOI:10.1002/asia.201000064

The reduction of ketones with polymethylhydrosiloxane (PMHS) gives the corresponding alcohols in good to excellent yield applying iron-based catalyst systems. In the case of prochiral ketones, the use of Fe(OAc) ₂/(S,S)-Me-DuPhos leads to high enantioselectivity up to 99% ee. The reaction proceeds in the presence of several functional groups such as esters, halides as well as conjugated double bonds, with high chemoselectivity. The advantage of this protocol is that the reaction requires no activating agents or additives.

Full text of DOI:10.1002/asia.201000064

DOI: 10.1002/asia.201000064
Chemo- and Stereoselective Iron-Catalyzed Hydrosilylation of Ketones
Daniele Addis, Nadim Shaikh, Shaolin Zhou, Shoubhik Das, Kathrin Junge, and
Matthius Beller*[a]
Abstract: The reduction of ketones with polymethylhydrosiloxane (PMHS) gives
the corresponding alcohols in good to excellent yield applying iron-based catalyst
systems. In the case of prochiral ketones, the use of FeACTHNUTRGENNUG(OAc)2/ACHTUNGTREN(NNGU S,S)-Me-DuPhos
Keywords: asymmetric synthesis ·
enantioselectivity · hydrosilylation ·
iron · ketones
leads to high enantioselectivity up to 99% ee. The reaction proceeds in the pres-
ence of several functional groups such as esters, halides as well as conjugated
double bonds, with high chemoselectivity. The advantage of this protocol is that
the reaction requires no activating agents or additives.
Introduction
fancy compared to other transition metals (e.g. Ru, Rh, Pd
and Ir, and so forth) particularly in the field of asymmetric
catalysis.
The hydrosilylation of carbon–carbon and carbon–heteroa-
tom multiple bonds is a viable alternative to common reduc-
tions with molecular hydrogen or other hydrogen donors.[1]
Among the various substrates in the past, the hydrosilyla-
tion of ketones has especially been studied.[1e,f],[2] Since the
early reports three decades ago[3] asymmetric and non-asym-
metric hydrosilylations[4] of prochiral ketones rely on pre-
cious metal catalysts such as rhodium,[5] ruthenium,[6] and iri-
dium.[7] In addition, less expensive metal complexes based
on titanium,[8] zinc,[9] tin,[10] and copper[11] have also been ex-
plored. Clearly, each method has its advantages as well as
limitations. Often, either the cost of the catalyst, toxicity of
the residual metal in the product, operational difficulties
(such as low temperature À50 to À708C) or narrow sub-
strate scope limit their applications.
In a first report, Brunner et al. described the hydrosilyla-
tion of ketones catalyzed by [Fe(Cp)(CO)] under photo-irra-
diation.[14] More recently, Nishiyama and co-workers devel-
oped efficient iron catalysts with nitrogen ligands or thio-
phene derivatives for both asymmetric and non-asymmetric
hydrosilylation of ketones.[15] In addition, notable reports on
defined iron-catalysts for the hydrosilylation of carbon–
carbon or carbon–oxygen double bonds were reported by
the group of Chirik,[16] Gade,[17a] and Nikonov.[17b]
In 2007, we revealed a general and chemoselective
method for the hydrosilylation of aldehydes using an
FeACTHNUTRGNEUNG
(OAc)2/PCy3 catalyst in combination with PMHS[18] as an
easy to handle and inexpensive reducing agent.[19] This
simple in situ-generated catalyst system works well for aryl,
heteroaryl, and aliphatic aldehydes to afford the correspond-
ing primary alcohols in high yields. Based on this observa-
tion, an asymmetric protocol was also developed using
(S,S)-Me-DuPhos as ligand.[20] Here, we report a full account
of our work demonstrating for the first time iron-catalyzed
Recently, there has been increasing interest in the deve-
ACHTUNGTRENNUNGlopment of efficient and (stereo)selective reductions through
the application of more benign and easily available biorele-
vant metals such as iron.[12] In this respect, in the last two
years also significant efforts have been devoted to develop
iron-based catalysts for hydrosilylation of ketones. However,
despite its many advantages,[13] iron catalysis is still in its in-
reductions of ketones with FeACHTUNTRGNEUNG(OAc)2/PCy3 and extending
the substrate scope of the asymmetric version.
Results and Discussion
[a] D. Addis, N. Shaikh, S. Zhou, S. Das, Dr. K. Junge,
Prof. Dr. M. Beller
Leibniz-Institut fꢀr Katalyse e.V. an der Universitꢁt Rostock
Albert-Einstein-Straße 29a
D-18059 Rostock (Germany)
Fax : (+49)381-1281-5000
E-mail: matthias.beller@catalysis.de
While the combination of FeACHTNUGRTNEUNG(OAc)2/PCy3 has been shown to
work well in the hydrosilylation of aldehydes,[19] reactions
with aromatic and aliphatic ketones have not been described
with this catalyst system to the best of our knowledge. To
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FULL PAPERS
Table 2. Iron-catalyzed non-asymmetric hydrosilylation of ketones.
broaden the substrate scope of this low-cost catalyst, we in-
vestigated the racemic reduction of ketones. As model
system, the reaction of benzophenone with inexpensive
PMHS as reducing agent was studied. Because of steric hin-
drance this substrate is more challenging compared to the
originally studied aldehydes. In a first set of experiments,
the ratio of iron source to phosphine to silane was opti-
mized. Selected results are shown in Table 1. The corres-
Entry Ketone[a]
Yield[b]
[%]
Entry Ketone
Yield
[%]
1
2
84
92
10
11
82
78
Table 1. Iron-catalyzed hydrosilylation of benzophenone.
3
4
5
95
96
87
12
13
14
82
58
60
Entry
Fe
[mol%]
(OAc)2
PCy3
[mol%]
Cat.
PMHS
[equiv]
Conv. Yield[b]
[%][a]
[%]
1
2
3
4
5
5
5
10
8
8
10
10
15
20
20
20
Fe
Fe(OAc)2
Fe(OAc)2
Fe(OAc)2
Fe(OAc)2
Fe(OAc)2
G
4
8
4
4
6
6
27
68
67
75
88
95
20
60
56
63
80
82
6
7
94
92
15
16
71
87
5
6[c]
Reaction conditions: [a] Ketone (1 mmol), FeACTHNUTRGENUG(N OAc)2 (0.05–0.1 mmol),
PCy3 (0.1–0.2 mmol), PMHS (4–8 mmol), THF (3 mL), 16 h, 658C.
[b] Yield of isolated pure product. [c] 24 h.
8
9
94
81
17
78
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
Reaction conditions: [a] Ketone (1 mmol), FeACHTUNTGRNEUNG(OAc)2 (0.05 mmol), PCy3
(0.12 mmol), PMHS (3 mmol), THF (3 mL), 20 h, 658C. [b] Yield of iso-
lated pure product.
similarly in good to very good yield after basic cleavage of
the silylether (Figure 1). No significant electronic influence
of substituents was observed in these reactions.
In contrast to aromatic ketones, dialkyl ketones were
somewhat less reactive and gave yields between 58 and
87% (Table 2, entries 13–17). Notably, a,b-unsaturated ke-
tones reacted with high chemoselectivity to give the corre-
sponding allylic alcohols (Table 2, entries 12 and 14). Here,
only traces (<5%) of the saturated alcohol were detected.
Next, we turned our attention to the stereoselective re-
duction of ketones in the presence of chiral phosphines. As
shown in our original communication, the combination of
FeACTHUNTRGENNG(U OAc)2 and electron rich bidentate phosphines such as
(S,S)-Me-DuPhos catalyzed the asymmetric hydrosilylation
of prochiral ketones with moderate to excellent enantiose-
lectivities.[20] In the meantime, a number of additional sub-
strates have been reduced, which we report here for the first
time.
In general, aryl ketones were reacted either at room tem-
perature or at 658C. Interestingly, reactions at 1008C gave
similar yield of the desired alcohol within 1 h with only mar-
ginal loss in ee. In contrast to previous studies, formation of
the corresponding silyl enol ether was not detected after full
conversion. Under our reaction conditions, the silyl ether of
the corresponding secondary alcohol was the only product
obtained before hydrolysis.
Figure 1. Hydrosilylation of substituted diarylketones.
After that, various substituted aryl alkyl ketones were
subjected to the optimized reaction conditions (Table 2). In
the case of elongated alkyl chains (Table 2, entry 2) or sub-
stitutions on the phenyl ring of acetophenone (Table 2, en-
tries 3–9), good to excellent yields (81-96%) of the corres-
ACHTUNGTRENNUNGponding benzylic alcohols were obtained. In addition, heter-
ocyclic (pyridyl) ketones were converted smoothly to the
corresponding alcohols in 78–82% yield (Table 2, en-
tries 10–11).
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Ketone Hydrosilylation
Table 3. Asymmetric hydrosilylation of ketones.
No significant difference in reactivity was observed on
comparison of the various substrates. On the other hand,
the enantioselectivity was considerably influenced by steric
and electronic changes in the substrate. While acetophenone
as the benchmark substrate led to 70% ee, the introduction
of electron-withdrawing groups such as CF3 or Cl in the 4-
position reduced the selectivity to 35 and 48%, respectively
(Table 3, entries 3 and 5). On the other hand, electron-do-
nating substituents (Table 3, entry 12) as well as ortho-sub-
stituted acetophenones (Table 3, entries 13–15) increased
considerably the enantioselectivity up to 99% ee. As shown
by the reaction of 1 f, methoxy substitution on the alkyl
group led to 90% of isolated yield with modest ee. Exten-
sion of the methyl group to the five-membered ring in com-
pound 1d had a significant negative influence on the ee
(Table 3, entry 4). Para- and ortho-acetylpyridine, 1g and 1h
respectively, showed comparable yields and enantioselectivi-
ties (Table 3, entries 7–8). The a,b-unsaturated ketone 1j
and the benzyl-alkyl ketone 1k reacted at room temperature
in good yield but with disappointingly low enantioselectivi-
ties (Table 3, entries 10–11).
Entry Ketone
1
T [8C] t [h] Yield[b] [%] ee[c] [%]
RT
1a 65
48
2
80
79
75
75
85[d]
82[d]
100
1
2
1b RT
32
68
77
3
4
5
1c
RT
45
45
45
94
85
90
35
16
48
1d RT
1e RT
Finally, we were interested in the aggregation state of the
active catalyst system under these hydrosilylation conditions.
Hence, experiments were performed in which the enantio-
purity of the ligand was varied. Acetophenone (0.5 mmol)
was reacted under the typical reaction conditions (658C,
6
7
8
1 f
85
2
34
45
90
78
68
56
20
28
1g RT
1h 65
16 h) with FeACHTUNRGTNEUNG(OAc)2, Me-DuPhos (different ee) and
(EtO)2MeSiH. As known from the seminal study of Kagan
and Girard,[21] the ee value of the product (eeprod) might not
be linearly correlated to the ee value of the chiral auxiliary
(eeaux) if bimetallic or oligomeric catalyst species are active.
However, in our case we found no deviations from a linear
behavior (Figure 2), which was indicative of an active mono-
meric catalyst.
9
1i
1j
65
20
52
25
10
11
RT
32
42
68
86
22
22
1k RT
12
1l
RT
RT
32
84
89[f]
48
20
20
76
94
>99
99
13
14
1m 65
98
65
99[e]
1n 65
1o 65
24
24
78
45
99[g]
15
99[g]
[a] General reaction conditions: Ketone (1 mmol), FeACTHNUGTRENUNG(OAc)2 (0.05 mmol,
Figure 2. Plot of observed ee of product alcohols as a function of optical
purity of (S,S)-Me-DuPhos. Reaction conditions: Ketone (1 mmol),
FeACTHNUTRGEN(NUG OAc)2 (0.05 mmol, 8.7 mg), [(S,S)+(R,R)]-Me-DuPhos (0.1 mmol,
28 mg), (EtO)2MeSiH (2 mmol, 0.18 mL), THF (3 mL), 658C, 16 h. The
absolute configuration of the secondary alcohols was determined to be
(R) by comparison of optical rotation with reported values.
8.7 mg), (S,S)-Me-DuPhos (0.1 mmol, 28 mg), (EtO)2MeSiH (2 mmol,
0.18 mL), THF (3 mL). The absolute configuration of the secondary alco-
hols was determined to be (R) by comparison of the optical rotation with
reported values. [b] Yield of isolated pure product. [c] Determined by
GC/HPLC with chiral column. [d] Reaction in toluene. [e] Using PMHS
(4 equiv). [f] Work up with sat. NaHCO3. [g] Using PMHS (4 equiv).
Chem. Asian J. 2010, 5, 1687 – 1691
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M. Beller et al.
FULL PAPERS
ing with argon (argon-vacuum three cycles) anhydrous THF (3 mL) fol-
lowed by ketone (1 mmol) were added and the reaction mixture was
stirred in a preheated oil bath at 658C for 10–15 min to obtain an
orange-colored solution. The reaction Schlenk was removed from the oil
bath and (EtO)2MeSiH (0.18 mL, 2 equivalents) or PMHS (0.360 mL, 6
equivalents) was added with a syringe under argon. The reaction mixture
was further stirred for the duration as mentioned in Table 3 at room tem-
perature (unless stated otherwise). The reaction mixture was then cooled
to 08C followed by the sequential addition of diglyme (80 mL) as a GC
standard (in case of analysis by GC), MeOH (1 mL), 2m NaOH (1 mL)
or saturated aqueous solution of NaHCO3 (2 mL, in case of methoxy sub-
stituted compounds) with vigorous stirring (Caution: The reaction mix-
ture bubbled briefly but vigorously upon the addition of the base). The re-
action mixture was stirred further for one hour (or until the organic layer
was colorless/pale yellow) at room temperature and was extracted with
diethyl ether (3ꢃ10 mL). The combined organic layers were washed with
brine, dried over anhydrous Na2SO4, filtered (an aliquot was removed for
GC analysis) and concentrated in vacuo. The residue was purified by
silica gel column chromatography using ethyl acetate-n-hexane mixture
(20 to 40%) as eluent to afford the desired product.)
Conclusions
In summary, we have demonstrated that the inexpensive
and convenient Fe(OAc)2/PCy3 catalyst system is also useful
ACHTUNGTRENNUNG
for the reduction of ketones with broad functional group tol-
erance. The system provides the corresponding alcohols in
good yield and selectivity with PMHS as reducing agent.
Furthermore, the combination of iron acetate with (S,S)-
Me-DuPhos has been applied to the stereoselective reduc-
tion of a number of novel substrates. It was demonstrated
that high enantioselectivities can only be obtained with ace-
tophenones with electron-rich substituents and sterically
hindered substrates.
Experimental Section
Materials and Methods: All reactions were conducted under argon at-
mosphere using standard Schlenk techniques. All the substrates were pur-
chased from commercial sources and were distilled over CaH2 and stored
under argon. Solid substrates were used as received. FeACHTNUTRGNEUNG(OAc)2 was pur-
chased from Aldrich with 99.995% purity. Phosphine ligands were pur-
chased from Strem Chemical Company. (EtO)2MeSiH and other silanes
used were purchased from Aldrich and distilled under vacuum and
stored under argon. PMHS was purchased from Aldrich and degassed
before use. (PMHS: MW 1900, a 29-mer, with one hydride per monomer-
ic unit. Monomer weight=60). THF and toluene were dried over sodium
benzophenone, distilled and stored under argon. Melting points were de-
termined on a Leica galen III melting point apparatus and were uncor-
rected. Optical rotations were measured on a Perkin–Elmer 343 polarim-
eter at 258C in a 1-dm cell. Absolute configuration of the product alco-
hols were assigned by comparison of the sign of optical rotation with re-
ported values. NMR spectra were recorded on a Bruker ARX 300 spec-
trometer, operating at 300 MHz for 1H NMR, and 75 MHz for 13C NMR.
NMR spectra were reported downfield from CDCl3 (d: 7.27 ppm) for
1H NMR. For 13C NMR, chemical shifts were reported in the scale rela-
tive to the solvent CDCl3 (d: 77.0 ppm). Mass spectra were recorded on
an AMD 402/3 or a HP 5989 A mass selective detector. GC analyses
were performed with a Hewlett Packard HP 6890 spectrometer. Column
chromatography was performed with silica gel Merck 60 (70–230 mesh
ASTM) using solvents of commercial grade. Isolated products were char-
acterized by comparison of NMR data with reported values and GC-MS
(showed>95% purity).
Acknowledgements
This work has been funded by the State of Mecklenburg-Western Pomer-
ania, the BMBF, and the DFG (Leibniz Prize). The authors thank Dr. C.
Fischer, S. Buchholz, S. Schareina, and A. Kammer (all at the Leibniz-In-
stitut fꢀr Katalyse e.V.) for analytical and technical support.
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oven-dried Schlenk tube containing a stirring bar, was charged with Fe-
(OAc)2 (8.7 mg, 0.05 mmol) and PCy3 (28 mg, 0.1 mmol). After purging
A 10 mL
ACHTUNGTRENNUNG
with argon (argon-vacuum three cycles) anhydrous THF (4 mL) followed
by ketone (1 mmol) were added and the reaction mixture was stirred in a
preheated oil bath at 658C for 10–15 min to obtain an orange-colored so-
lution. The reaction Schlenk was removed from the oil bath and PMHS
(0.18 mL, 3 equivalents) was added with a syringe under argon. The reac-
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lowed by the sequential addition of diglyme (80 GL) as a GC standard
(in case of analysis by GC), MeOH (1 mL), 2m NaOH (1 mL) with vigo-
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for one hour (or until the organic layer was colorless/pale yellow) at
room temperature and was extracted with diethyl ether (3ꢃ10 mL). The
combined organic layers were washed with brine, dried over anhydrous
Na2SO4, filtered (an aliquot was removed for GC analysis) and concen-
trated in vacuo. The residue was purified by silica gel column chromatog-
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Ketone Hydrosilylation
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Received: January 21, 2010
Published online: May 14, 2010
Chem. Asian J. 2010, 5, 1687 – 1691
ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
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