Selective hydrosilylation of ketones catalyzed by in situ -generated iron NHC complexes

Article abstract of DOI:10.1002/aoc.1832

Aryl alkyl-, heteroaryl alkyl- and dialkyl ketones were readily reduced to their corresponding secondary alcohols in high yields, using the commercially available and inexpensive polymeric silane polymethylhydrosiloxane (PMHS), as reducing agent. The reaction is catalyzed by an in situ-generated iron complex, conveniently generated from iron(II) acetate and the commercially available N-heterocyclic carbene (NHC) precursor IPr·HCl.

Full text of DOI:10.1002/aoc.1832

Full Paper
Received: 14 June 2011
Revised: 13 July 2011
Accepted: 13 July 2011
Published online in Wiley Online Library: 7 September 2011
(wileyonlinelibrary.com) DOI 10.1002/aoc.1832
Selective hydrosilylation of ketones catalyzed
by in situ-generated iron NHC complexes
Elina Buitragoa, Lorenzo Zanib and Hans Adolfssona∗
Aryl alkyl-, heteroaryl alkyl- and dialkyl ketones were readily reduced to their corresponding secondary alcohols in high yields,
using the commercially available and inexpensive polymeric silane polymethylhydrosiloxane (PMHS), as reducing agent. The
reaction is catalyzed by an in situ-generated iron complex, conveniently generated from iron(II) acetate and the commercially
c
available N-heterocyclic carbene (NHC) precursor IPr·HCl. Copyright ꢀ 2011 John Wiley & Sons, Ltd.
Keywords: hydrosilylation; iron; carbenes; ketones; catalysis
Introduction
as ligands in a number of catalytic applications,[12] and complexes
containingNHCshavebeenemployedinhydrosilylationprocesses
of alkynes and ketones, together with rhodium, iridium, nickel and
platinum respectiviely.[13] Iron complexes containing NHC ligands
have been prepared and characterized;[14] however, they are sel-
dom used in catalysis.[14i,j] Recently Sortais, Darcel and co-workers
reported that the neutral piano-stool complex [Cp(IMes)Fe(CO)(I)],
originally prepared by Guerchais,[15] catalyzes the hydrosilylation
of aldehydes to give the corresponding primary alcohols in high
yields after hydrolysis.[16] The reduction of ketones was also stud-
ied and moderate to good results were obtained. In addition,
Royo and co-workers showed that similar complexes with the
NHC ligand tethered to the cyclopentadienyl-moiety catalyzed
the hydrosilylation of aldehydes.[17]
Herein, we present a novel catalyst system based on iron(II)
acetate combined with N-heterocyclic carbene ligands, for the
efficient reduction of ketones using the hydrosilylation/hydrolysis
protocol. Reduction of ketones to their corresponding secondary
alcohols[1] is typically performed using stoichiometric hydride
reagents, or by the use of a catalyst in combination with molecular
hydrogen (hydrogenation)[2,3] or formic acid/2-propanol (transfer-
hydrogenation)[4,5] as reducing agents. A powerful alternative to
the above-mentioned methods for carbonyl reductions is the
catalytic two-step process involving hydrosilylation followed by
hydrolysis of the resulting silyl-ether.[6] The catalysts frequently
employed in hydrogenations, transfer-hydrogenations and hy-
drosilylations are often based on expensive and toxic transition
metals like ruthenium, rhodium or iridium.[1] However, recent dis-
coveries have demonstrated that copper[7] or iron complexes[8]
can serve as efficient and environmentally benign replacements
for the traditionally used catalysts. Iron complexes are particu-
larly attractive owing to the biocompatibility and the low cost of
the transition metal. A number of efficient hydrosilylation proto-
cols using iron-based catalysts have recently been developed.[9]
After the initial discoveries by Brunner and co-workers in the
early 1990s,[9a–d] who demonstrated that [Fe(Cp)(CO)] complexes
catalyzethehydrosilylationofacetophenone,mostrecentironcat-
alysts have been based on ligands containing nitrogen, sulfur or
phosphorous donor atoms. Nishiyama and Furuta demonstrated
that iron complexes initially containing the tmeda ligand, and
later thiophene ligands acted as efficient catalysts for the hydrosi-
lylation reaction.[9e,f] Furthermore, the groups of Nishiyama,[9g–i]
Gade[9j] and Chirik[9k,l] have successfully employed iron complexes
with imine-based ligands (pyridines or oxazolines) for the selective
reduction of ketones. Yang and Tilley demonstrated that hexam-
ethyldisilazane is a proficient ligand for iron, generating a highly
activehydrosilylationcatalystforaldehydesandketones.[9m] Beller
and co-workers reported that the catalyst combination of iron salts
and electron-rich phosphine ligands efficiently and selectively re-
duced aldehydes and ketones in the presence of the inexpensive
polymeric silane polymethylhydrosiloxane (PMHS).[10] Addition-
ally, Nikonov and co-workers used defined piano-stool iron
complexes containing phosphine ligands for the hydrosilylation of
aldehydes.[11] N-Heterocyclic carbenes (NHCs) are frequently used
Experimental
All reactions were performed under nitrogen atmosphere. All
reagents and reactants are commercially available and were
used as received. Distilled or column-dried solvents were used.
All products obtained and presented in Table 2 are known
compounds, and their characterization data (1H NMR) are in
agreement with published data.
GeneralProcedureforIron-catalyzedHydrosilylationReaction
The iron source, Fe(OAc)2 (4.3 mg, 0.025 mmol), and 1,3-bis(2,6-
diisopropylphenyl)imidazolium chloride (12.8 mg, 0.03 mmol)
were treated under vacuum for 10 min. Tetrahydrofuran (THF)
(3 ml) was added under nitrogen atmosphere followed by addi-
tion of nBuLi (30 µl, 0.03 mmol, 1 M in hexane) and the substrate
∗
Correspondence to: Hans Adolfsson, Department of Organic Chemistry,
Stockholm University, the Arrhenius Laboratory, SE-10691 Stockholm, Sweden.
E-mail: hansa@organ.su.se
a
Department of Organic Chemistry, Stockholm University, the Arrhenius
Laboratory, SE-10691 Stockholm, Sweden
b
Institute for the Chemistry of Organometallic Compounds (CNR-ICCOM), Via
Madonna del Piano 10, I-50019, Sesto Fiorentino, Florence, Italy
c
Appl. Organometal. Chem. 2011, 25, 748–752
Copyright ꢀ 2011 John Wiley & Sons, Ltd.

Iron–NHC-catalyzed hydrosilylation
Table 1. Optimization for the reduction of acetophenonea
Entry
Fe(OAc)2 (mol%)
Ligand (mol%)
Base (mol%)
Silane (equiv.)
Conversion (%)b
1
2
1
IMes·HCl (2)
IMes·HCl (2)
IMes·HCl (2)
IMes·HCl (5)
IMes·HCl (5)
IAd·HBF4 (5)
SIMes·HCl (5)
IPr·HCl (5)
IPr·HCl (3)
IPr·HCl (3)
–
tBuONa (2)
tBuONa (2)
tBuONa (2)
tBuONa (5)
tBuOK (5)
tBuOK (5)
tBuOK (5)
tBuOK (5)
tBuOK (3)
tBuOK (3)
tBuOK (3)
nBuLi (3)
PMHS (4)
(EtO)2MeSiH (4)
Ph2SiH2 (4)
PMHS (4)
PMHS (4)
PMHS (4)
PMHS (4)
PMHS (4)
PMHS (3)
PMHS (3)
PMHS (3)
PMHS (3)
PMHS (3)
PMHS (3)
PMHS (3)
61
63
22
78
87
93
69
94
94
<5
99
99
<5
<5
99
1
3
1
4
2.5
2.5
2.5
2.5
2.5
2.5
–
5
6
7
8
9
10
11
12
13
14
15c
–
2.5
–
IPr·HCl (3)
IPr·HCl (3)
–
nBuLi (3)
–
nBuLi (3)
2.5
IPr·HCl (3)
nBuLi (3)
a General conditions: acetophenone (1 mmol), Fe(OAc)2 NHC-precursor, base and silane according to the table, THF (3 ml), 65 ◦C, 16–18 h. Hydrolytic
workup according to Experimental section. b Conversion determined by GLC.c FeOAc2 99.995% based on trace metal analysis was employed.
(1 mmol). The temperature was increased to 65 ◦C and PMHS
Cl
(0.18 ml, 3 mmol) was added. The reaction was stirred at 65 ◦C for
16–18 h. The reaction mixture was cooled to room temperature.
MeOH (1 ml) was added followed by NaOH 2 M (10 ml). The result-
ing mixture was stirred for 3 h and subsequently extracted with
CH2Cl2 (3 × 20 ml). The combined organic layer was dried with
MgSO4 and the solvent was removed under reduced pressure.
The crude product was treated with 2-propanol and purified by
column chromatography. The products were analyzed by 1H NMR,
and conversions were determined by GLC analysis.
R1
R1
N
N
R2
R2
R1
R1
IMes HCl: R1 = R2 = Me
IPr HCl: R1 = i-Pr; R2 = H
BF4
N
N
Results and Discussion
Thestrongσ-donorabilityofNHCligandsrendersthemparticularly
useful in catalytic processes where electron-rich metal complexes
are required.[12] In carbonyl reductions it is important that the
hydrogen being transferred to the substrate has a typical hydridic
nature. The use of strong σ-donating ligands such as NHCs would
result in the formation of intermediate M–H complexes where the
metal hydrogen bond has a stronger ionic character, and hence is
more hydridic. To investigate the possibility of generating simple
iron-basedcatalystscontainingNHCligandsforketonereductions,
we initiated the following study. In an initial experiment we
combined iron acetate with the imidazolium salt IMes·HCl. Our
intention was to generate the Fe–NHC complex in situ, which
required the addition of base to assist in deprotonation of
the azolium salt, thereby generating the carbene. Consequently,
starting with 1 mol% of Fe(OAc)2 combined with IMes·HCl, and
tBuONa in a ratio of 1 : 2 : 2 in THF, using acetophenone as model
substrate and the inexpensive PMHS (4 equiv.) as silane precursor,
resulted in 61% conversion of acetophenone obtained after basic
workup (entry 1, Table 1). Further experiments revealed that the
use of (EtO)2MeSiH gave essentially identical results, whereas the
reaction with diphenylsilane as terminal reductant resulted in
significantly lower conversion (entries 2 and 3, Table 1). Increasing
the catalyst loading to 2.5 mol% resulted in higher conversion
(entry 4), and changing the base to potassium tert-butoxide
gave even better results (entry 5). Different azolium salts (Fig. 1)
IAd HBF4
Cl
N
N
SIMes HCl
Figure 1. NHC-precursors evaluated as ligands in the iron-catalyzed
hydrosilylation of acetophenone.
were employed and small variations were observed using the
IAd- and IPr-based ligands, respectively, whereas the saturated
SIMes gave lower conversion (entries 6–8). Interestingly, reducing
the amounts of ligand, base, and silane, as illustrated for the
reaction in entry 9 using the IPr ligand, did not reduce the catalyst
activity.
When the hydrosilylation was performed without addition of
iron acetate, exceptionally poor conversion was observed (entry
10), which demonstrates the need for the iron source in the
reaction. On the other hand, when both iron acetate and the NHC
precursor were omitted, the reaction generated the hydrosilylated
c
Appl. Organometal. Chem. 2011, 25, 748–752
Copyright ꢀ 2011 John Wiley & Sons, Ltd.
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E. Buitrago, L. Zani and H. Adolfsson
Table 2. Substrate evaluation for the iron-IPr-catalyzed hydrosilylation of ketonesa
Entry
1
Substrate
Yieldb (%)
Entry
11
Substrate
Yieldb (%)
55
>99c
2
53c
12
58
3
4
96
13
14
87d
89c
70d
5
6
7
99c
94c
92
15
16
17
89
72
75c
8
9
97
83
18
19
66
94
10
91
20
81e
a General conditions: ketone (1 mmol), nBuLi (3 mol%), Fe(OAc)2 (2.5 mol%), IPr•HCl (3 mol%), THF (3 ml), PMHS (3 equiv.), 65 ◦C, 16–18 h. For
hydrolytic workup, see Experimental section. b Isolated yields. c Conversion determined by GLC. d Hydrolytic workup using TBAF.[21] e Diastereomeric
ratio 8 : 1 determined by 1H-NMR.
product in high conversion (entry 11). The fact that simple alkoxide
salts catalyze the hydrosilylation reaction with tri-substituted
silanes at room or elevated temperatures has previously been
reported.[18,19] However, this background reaction is not present
when the base initially reacts with the azolium salts, as seen in the
result presented in entry 10. Since the same conditions also apply
to entries 1–9 in Table 1, we can conclude that the reaction is
indeed catalyzed by an iron–NHC complex. Nevertheless, to avoid
the background reaction observed in entry 11, we examined other
bases for the in situ deprotonation of the NHC precursor, and we
c
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Copyright ꢀ 2011 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2011, 25, 748–752

Iron–NHC-catalyzed hydrosilylation
methyl ketone was reduced in an excellent 94% yield (entry 19). In
the reduction of (+)-camphor, a preference for hydride addition
from the least hindered face was found, and a diastereomeric
mixture of isoborneol and borneol in an 8 : 1 ratio was obtained
(entry 20).
Fe(OAc)2 (2.5 mol%)
[Si]
O
O
IPr•HCl (3 mol%)
nBuLi (3 mol%)
PMHS (3 equiv)
THF, 65 °C, 16-18 h
Conclusions
OH
In summary, we have presented an efficient and general protocol
fortheiron–NHC-catalyzedhydrosilylationofketones.Thecatalyst
is easily generated in situ by treatment of an azolium salt with base
in the presence of an iron source. Efficient ketone reductions were
obtained using low catalyst loading, and with the polymeric PMHS
as a nontoxic and stable reducing agent. The iron source and the
reagents are readily available and rather inexpensive. The catalyst
system works well for aromatic ketones, heteroaromatic ketones
and alkyl ketones, and the corresponding alcohols are obtained
in high yields. Furthermore, this mild protocol demonstrates good
functional group tolerance, giving chemoselective reductions
on substrates with more than one reducible group. We are
currently investigating further use of iron complexes containing
N-heterocyclic carbene ligands in reductive processes.
NaOH (aq)
MeOH
Scheme 1. The optimized conditions for the iron catalyzed hydrosilylation
of acetophenone.
found that n-BuLi was favorable and that full conversion to the
alcohol product was obtained (entry 12). In control experiments
using butyllithium without iron acetate and ligand precursor, poor
conversion of acetophenone was observed (entries 13 and 14).
To verify that the reaction is not catalyzed by trace elements of
other metal sources found as contaminants in the employed iron
source, we performed hydrosilylations using salts with different
levels of Fe purity. We found that the same result was obtained
using Fe(OAc)2 with an Fe purity of 95 or 99.995% (based on
trace metal analysis), respectively (entries 12 and 15).[20] The use
of other iron salts such as FeCl2, FeCl3, and Fe(acac)2 resulted in
inferior conversions (<25%). Moreover, the use of other solvents,
suchasdichloromethane, diethyletherortoluene, resultedinpoor
conversion, and performing the hydrosilylation in THF at ambient
temperature gave no product formation. The found optimized
conditions for the iron-catalyzed hydrosilylation of acetophenone
are presented in Scheme 1.
Acknowledgments
This work was financially supported by The Swedish Research
Council, The Carl Trygger Foundation, The Wenner-Gren Founda-
tions, The K & A Wallenberg Foundation, Ångpannefo¨reningen’s
FoundationforResearchandDevelopment,andtheMagn.Bergvall
Foundation.
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screen was conducted using the optimized conditions shown in
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yields (entries 1 and 3), while the meta-methoxy acetophenone
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alcohols (entries 17 and 18). The sterically hindered adamantyl
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[20] The purity analysis of this particular Fe(OAc)2 showed that it
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extracted according to the experimental procedure given above.
c
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Copyright ꢀ 2011 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2011, 25, 748–752