Hydrosilylation of aldehydes and ketones catalyzed by hydrido iron complexes bearing imine ligands

Article abstract of DOI:10.1039/c4dt00944d

Two new hydrido iron complexes (2 and 4) were synthesized by the reactions of (4-methoxyphenyl)phenylketimine ((4-MeOPh)PhCNH) with Fe(PMe ₃)₄ or FeMe₂(PMe₃)₄. The molecular structures of complexes 2 and 4 were confirmed by X-ray single crystal diffraction. Using hydrido iron complexes (1-4) as catalysts, the hydrosilylations of aldehydes and ketones were investigated. The four complexes were effective catalysts for this reduction reaction. Complex 1 among them is the best catalyst. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4dt00944d

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Hydrosilylation of aldehydes and ketones
catalyzed by hydrido iron complexes bearing
imine ligands†
Cite this: Dalton Trans., 2014, 43,
11716
Zhenyu Zuo, Hongjian Sun, Lin Wang and Xiaoyan Li*
Two new hydrido iron complexes (2 and 4) were synthesized by the reactions of (4-methoxyphenyl)-
phenylketimine ((4-MeOPh)PhCvNH) with Fe(PMe3)4 or FeMe2(PMe3)4. The molecular structures of com-
plexes 2 and 4 were confirmed by X-ray single crystal diffraction. Using hydrido iron complexes (1–4) as
catalysts, the hydrosilylations of aldehydes and ketones were investigated. The four complexes were
effective catalysts for this reduction reaction. Complex 1 among them is the best catalyst.
Received 31st March 2014,
Accepted 3rd June 2014
DOI: 10.1039/c4dt00944d
www.rsc.org/dalton
for the room temperature hydrosilylation of carbonyl com-
pounds with a broad substrate scope at low loadings.2 In 2013,
Introduction
Reduction of aldehydes and ketones is one of the important Beller et al. published a series of studies on iron-catalyzed
transformations in organic synthesis and in the pharma- reduction reactions. They reported a catalytic system of air-
ceutical industry. In this process, a carbonyl group can be and water-stable iron complexes which can be used for the
reduced to a hydroxy group of an alcohol. The classic method efficient hydrogenation of aldehydes and ketones as well as
is to use a (metal) hydride of sodium, potassium, aluminum selective reduction of the carbonyl group of α,β-unsaturated
and boron as the reducing agent in the absence of a catalyst. aldehydes.3 An in situ produced hydrido iron species was used
The advantage of this method is that it is fast, while the low as a catalyst in the transformation of α,β-unsaturated alde-
selectivity and the byproduct of metal waste are its short- hydes into allylic alcohols. A mechanistic discussion sup-
comings. In order to overcome these disadvantages, catalysts have ported by DFT calculations was also provided.4 A biomimetic
been introduced in this reduction. Most of the catalysts are reduction of α-ketoesters with the iron carbonyl complex was
transition metal complexes in the homogeneous process, and explored to afford the desired α-hydroxyesters in good to excel-
in most cases they are the precious metal compounds, such as lent yields with hydrogen as the reductant, but the hydrogen
Pd, Pt, Rh, Ir etc. In recent years, there has been growing inter- pressure was 50 bar.5 Another remarkable work in this field is
est in the catalytic application of iron, cobalt and nickel com- from the group led by Chirik.6 The enantiopure iron dialkyl
plexes, which are not only inexpensive but also less toxic and complexes were studied as catalysts in the asymmetric hydro-
eco-friendly. It would be more desirable to use iron, cobalt and silylation of ketones with moderate enantioselectivity. Some
nickel complexes as catalysts in homogeneous catalytic interesting results in hydrogenation of alkenes catalysed by an
systems to replace the noble metal complexes.
iron [NNN] pincer complex were also disclosed.7–9 Milstein
Although many results with iron complexes as catalysts et al. synthesized the iron borohydride pincer complexes for
have been disclosed, successful examples are rare in the field the efficient hydrogenation of ketones under mild, base-free
of iron-catalyzed reduction reaction of carbonyl compounds.1 conditions with turnover numbers up to 1980.10
In most cases, the catalytic effect of iron catalysts is weaker
Hydrido iron complexes formed as products of C–H bond
than that of noble metal catalysts. Recently, Turculet reported activation in some cases have been widely used as catalysts in
a tri-coordinate iron(II) complex with excellent catalytic activity the reduction of unsaturated organic substrates (e.g. alkynes,
alkenes or carbonyl groups)11,12 and suggested to be inter-
mediates in the catalysis by hydrogenase enzymes.13–15 Bian-
chini’s group hydrogenated alkynes to afford trans-alkenes by
an iron hydride with multidentate phosphorus ligands16 and
also realized catalytic reduction of α,β-unsaturated ketones
using the η2-dihydrogen iron(II) complex by transfer hydrogen-
ation.17 Casey et al. disclosed the hydrogenation of ketones by
a well-defined iron catalyst under low hydrogen pressure with
School of Chemistry and Chemical Engineering, Key Laboratory of Special Functional
Aggregated Materials, Ministry of Education, Shandong University, Shanda Nanlu
27, 250199 Jinan, PR China. E-mail: huqingping@sdu.edu.cn, xli63@sdu.edu.cn;
Fax: +86 531 88361350
†Electronic supplementary information (ESI) available. CCDC 959625 and
959626. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c4dt00944d
11716 | Dalton Trans., 2014, 43, 11716–11722
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high chemoselectivity at room temperature.18 Guan et al. also extracted with pentane (65 mL). Crystallization at −17 °C
achieved catalytic hydrosilylation of aldehydes and ketones afforded purple crystals of 2. Yield: 1.01 g (86%). Dec. >94 °C.
with hydrido [PCP]-pincer iron complexes.19
IR (Nujol mull, cm−1): 3290 ν(vN–H), 1749 ν(Fe–H), 1595,
Using a C–H activation strategy, Klein reported an example 1569 ν(CvC), 942 cm−1. ρ1(PCH3); 1H NMR (300 MHz, C6D6,
of ortho-metalated hydrido iron(II) complexes through the reac- 298 K, ppm): 8.78 (s, 1H, CvNH), 7.90–6.78 (m, 8H, Ar–H),
tion of ketimine with Fe(PMe3)4 and FeMe2(PMe3)4.20,21 Regret- 3.80 (s, 3H, –OCH3), 1.19 (d, J = 6 Hz, 9H, PCH3), 1.07 (t, J =
tably, until now the catalytic performance of this kind of 6 Hz, 18H, PCH3), −17.11 (dt, J = 81 Hz, J = 24 Hz, 1H, Fe–H).
hydrido iron complexes has not been studied. Recently, we 31P NMR (121.5 MHz, C6D6, 298 K, ppm): 24.8 (t, J = 38.8 Hz,
found that a hydrido(2-mercaptobenzoyl)tris(trimethylphos- 1P), 20.7 (d, J = 38.8 Hz, 2P). Anal. Calc. for C23H40FeNOP3
phine)cobalt(III) complex22 and hydrido silyl iron complexes (495.32 g mol−1): C, 55.77; H, 8.14. Found: C, 55.35; H, 7.57.
supported by trimethylphosphine ligands bearing a [PSiP]-
Synthesis of complex 4
pincer ligand23 are effective catalysts for hydrosilylation of
aldehydes and ketones. In this paper, as a continuation of our FeMe2(PMe3)4 (0.94 g, 2.40 mmol) in THF (30 mL) was
study on hydrido metal complexes and their catalytic activities, combined with (4-methoxyphenyl)phenylketimine (0.50 g,
two new hydrido iron complexes (2 and 4) were synthesized by 2.37 mmol) in THF (20 mL) at −78 °C. The resulting mixture
the reactions of (4-methoxyphenyl)phenylketimine ((4-MeOPh)- was warmed to 25 °C and stirred for 24 h to obtain a violet
PhCvNH) with Fe(PMe3)4 and FeMe2(PMe3)4. The molecular solution. The solvent was removed under vacuum and the
structures of complexes 2 and 4 were confirmed by X-ray single residue was extracted with pentane (65 mL). Crystallization at
crystal diffraction. Complexes 1 and 3 were obtained according −17 °C afforded purple crystals of 4. Yield: 0.92 g (76%). Dec.
to the literature.20,21 Using hydrido iron complexes (1–4) as >98 °C. IR (Nujol mull, cm−1): 3286 ν(vN–H), 1745 ν(Fe–H),
catalysts, the hydrosilylation of the carbonyl compounds was 1604, 1588 ν(CvC), 930 ρ1(PCH3); 1H NMR (300 MHz, C6D6,
investigated. It was found that hydrido iron complexes 1–4 are 298 K, ppm): 8.47 (s, 1H, CvNH), 7.86–6.81 (m, 7H, Ar–H),
effective catalysts for the hydrosilylation reaction under mild 3.76 (s, 3H, –OCH3), 2.34 (s, 3H, Ar–CH3), 1.11–1.14 (m, 27 H,
conditions. Among the four hydrido iron complexes (1–4), PCH3), −17.90 (dt, J = 81 Hz, J = 24 Hz, 1H, Fe–H); 31P NMR
complex 1 is the best catalyst.
(121.5 MHz, C6D6, 298 K, ppm): 25.0 (t, J = 38.8 Hz, 1P), 19.9
(d, J = 38.8 Hz, 2P). Anal. Calc. for C24H42FeNOP3 (509.35 g
mol−1): C, 56.59; H, 8.31. Found: C, 57.02; H, 7.89.
Experimental section
General procedures and materials
General procedure for catalytic hydrosilylation of aldehydes
and ketones
All reactions were carried out under an atmosphere of nitrogen
using the standard Schlenk technique. This standard vacuum
technique was used in manipulations with volatile and air-
sensitive materials. All solvents were distilled from Na/benzo-
phenone under nitrogen. Fe(PMe3)4, FeMe2(PMe3)4,24 and
complexes 1 and 320,21 were prepared by the literature
methods. All other chemicals were purchased from Aldrich or
Acros and used as received without further purification. Infra-
red spectra, as obtained from Nujol mulls between KBr disks,
were performed within the 4000–400 cm−1 region on a Bruker
To a solution of complex 1 (2.8 mg, 0.006 mmol) in THF
(2 mL) were added the aldehyde or ketone (1 mmol), (EtO)3SiH
(1.2 mmol) and n-dodecane (1 mmol) as internal standard
under a N2 atmosphere. The reaction mixture was stirred at
55 °C until there was no aldehyde or ketone left (monitored by
TLC and GC). The reaction was then quenched using MeOH
(1 mL) and a 10% aqueous solution of NaOH (5 mL) with vig-
orous stirring at 50 °C for 2 days. The organic product was
extracted with Et2O (10 mL × 3), dried over anhydrous MgSO4,
and concentrated under vacuum. The alcohol product was
further purified by silica column chromatography (eluted with
15–35% ethyl acetate in petroleum). Finally, all products were
confirmed by 1H NMR.
X-ray structure determination: Intensity data were collected
on a Bruker SMART diffractometer with a graphite-monochro-
mated Mo-Kα radiation (λ = 0.71073 Å). The structure was
solved by direct methods and refined with the full-matrix least-
squares method on all F2 (SHELXL-97)25 with anisotropic
approximation for non-hydrogen atoms. CCDC 959625 (2) and
CCDC 959626 (4) contain the supplementary crystallographic
data for this paper.
1
ALPHA FT-IR spectrometer. H and 31P NMR spectra (300 and
121 MHz, respectively) were recorded on a Bruker Avance 300
spectrometer with C6D6 as the solvent at room temperature.
31P NMR resonance was obtained with a broad band proton
decoupling. Elemental analyses were carried out on an
Elementar Vario ELIII. Melting points were measured in
capillaries sealed under argon and are uncorrected. X-ray
crystallography was performed with a Bruker Smart 1000
diffractometer. GC was recorded on a Fuli 9790.
Synthesis of complex 2
Fe(PMe3)4 (0.86 g, 2.40 mmol) in THF (30 mL) was combined
with (4-methoxyphenyl)phenylketimine (0.50 g, 2.37 mmol) in
Crystallographic data of complex 2
THF (20 mL) at −78 °C. The resulting mixture was warmed to Purple crystals, C23H40FeNOP3 = 495.32 g mol−1, monoclinic,
25 °C and stirred for 24 h to obtain a violet solution. The I2/a, a = 17.694(4), b = 9.5613(19), c = 33.263(7) Å, β = 91.29(3)
solvent was removed under vacuum and the residue was degrees, V = 5626.1(19) Å3, Z = 8, Dx = 1.170 g cm−3, 4895 reflec-
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tions measured, 3085 observed, θmax = 25.00°, R1 (I > 2σ(I)) =
0.0830, wR2 (all data) = 0.2612.
Crystallographic data of complex 4
Purple crystals, C24H42FeNOP3 = 509.35 g mol−1, monoclinic,
C2/c, a = 36.1399(10), b = 9.5531(4), c = 17.9590(5) Å, β =
107.528(2) degrees, V = 5912.4(3) Å3, Z = 8, Dx = 1.144 g cm−3
,
6029 reflections measured, 5353 observed, θmax = 26.37°,
R1 (I > 2σ(I)) = 0.0546, wR2 (all data) = 0.2106.
Results and discussion
Synthesis of hydrido iron complexes 1–4
Fig. 1 Molecular structure of complex 2.
Four hydrido iron complexes 1–4 were synthesized according
to the method reported by Klein (eqn (1) and (2)).20,21 Among
them, complexes 2 and 4 were new and were completely
characterized.
ð1Þ
ð2Þ
The typical signals for ν(Fe–H) vibration in the IR spectra of
complexes 2 and 4 were found at 1749 cm−1 (2) and 1745 cm−1
(4). In the 1H NMR spectra, the H(Fe–H) resonance appears at
−17.11 ppm (2) and −17.90 ppm (4) as a doublet of triplet due
to the coupling with three meridional-positioned trimethyl-
phosphine ligands. As expected, in the 31P NMR spectra the
phosphorus signals of the two types of PMe3 ligands appear at
24.8 (t, J = 38.8 Hz, 1P), 20.7 ppm (d, J = 38.8 Hz, 2P) for 2 and
at 25.0 (t, J = 38.8 Hz, 1P), 19.9 ppm (d, J = 38.8 Hz, 2P) for 4.
Complexes 2 and 4 should have structures similar to those of 1
and 3.21 X-ray diffraction study confirmed the proposed struc-
tures (Fig. 1 and 2).
Fig. 2 Molecular structure of complex 4.
Based on the report by Klein,21 the mechanism of for-
mation of complexes 2 and 4 should be once again discussed
because of the introduction of the para-MeO group. For reac-
tion (1), there are two possibilities for C–H activation: on the
phenyl ring or on the para-(MeO)-phenyl ring. The C–H bond
activation occurred on the para-(MeO)-phenyl ring because of
the steric effect. Another possible reason is that the MeO
group exerts an electron-withdrawing inductive effect at the
meta-position. This makes the ortho-carbon atoms more electro-
philic. In the mechanism of the reaction between FeMe2-
(PMe3)4 and (4-methoxyphenyl)phenylketimine (Scheme 1), Scheme 1 Proposed mechanism of formation of complex 4.
11718 | Dalton Trans., 2014, 43, 11716–11722
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the first step is the elimination of methane produced from one whether these hydrido iron complexes can catalyze the hydro-
methyl ligand of FeMe2(PMe3)4 and the H atom of the phenyl silylation of aldehydes and ketones.
group to form the chelate iron(II) complex A through ortho-
cyclometalation. We do not know why the first step occurs in
hydrido iron complex
PhCHO þ ðEtOÞ3SiH ꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀ! PhCH2OSiðOEtÞ3 ð3Þ
the phenyl ring. Obviously, the experiments confirmed that
With benzaldehyde as a substrate the catalytic conditions
the activation of the C–H bond in the phenyl ring is easier
were optimized (eqn (3), Table 1). The preliminary survey
than that in the para-(MeO)-phenyl ring in reaction (2). This
(entry 1) was carried out in THF using 1 mol% of complex 1 as
might be caused by the different properties between
a catalyst and H2 as a hydrogen source. After 1 h at 55 °C, no
FeMe2(PMe3)4 and Fe(PMe3)4. It is certain that the cleavage of
conversion was observed. When the reaction time was pro-
the C–H bond by FeMe2(PMe3)4 is not an oxidative addition
longed to 24 h or the catalyst loading was increased to 10 mol%,
pathway. This means that a hydrido iron(IV) intermediate in
there was still no conversion (entries 2 and 3). These results
the first step is not formed. Otherwise, the first C–H bond acti-
illustrated that complex 1 could not activate the H–H bond
vation should occur in the para-(MeO)-phenyl ring. The
under the reaction conditions. Interestingly, with (EtO)3SiH as
methylated intermediate iron(II) complex A is transformed into
a hydrogen source a complete conversion was observed within
the intermediate B through reductive elimination via C,C-
1 h (entry 4). From entry 4 to 9, it is seen that the optimal cata-
coupling with the formation of unsaturated coordinate iron(0)
lyst loading for this reaction is 0.3 mol%. With less loading of
complex, Fe(PMe3)3. As in reaction (1), the oxidative addition
complex 1 the conversions are very low, even though the
of the ortho-(C–H) bond of B occurred on the para-(MeO)-
reaction time was prolonged to 24 h (entries 8 and 9). If the
phenyl ring to afford the hydrido iron(II) complex 4.
hydrogen source was changed to Et3SiH or Ph2MeSiH, the con-
versions are decreased to less than 1% (entries 10 and 11).
These changing trends are consistent with the results reported
by Guan.19 More experiments verified that THF is the best
Catalytic application of hydrido iron complexes 1–4 in
hydrosilylation of aryl aldehydes and ketones
solvent among the five tested reaction media (entries 6,
It has been reported that iron complexes could be used as cata- 12–15). The optimized reaction temperature is 55 °C (entries 6,
lysts in place of some noble metal complexes. Especially, 19 and 20). The most appropriate reaction time is one hour
hydrido iron complexes with high reactivity have been success- (entries 6, 17 and 18). In addition, complex 1 has the best cata-
fully used in the reduction of unsaturated species, such lytic activity and complex 4 demonstrated the poorest catalytic
as nitro compounds,26 carbonyl compounds,11,12,22,23 imides, activity among the four hydrido iron complexes (entries 6,
alkenes and alkynes.16 Currently, there is a growing interest in 21–23). There are two reasons to explain these observations.
seeking low-cost transition metal catalysts for the reduction of For the anchoring CvN group in complexes 2–4, both methyl
carbonyl compounds. Therefore, it is necessary for us to find and methoxy are electron-donating groups because they are in
Table 1 Catalytic activity of hydrido iron complexes for hydrosilylation of PhCHOa
Entry
Complex
Loading (mol%)
Hydrogen source
Solvent
Temp. (°C)
Time (h)
Conversionb (%)
1
2
3
4
5
6
7
8
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
2
4
3
1
1
10
1
H2
H2
H2
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
1,4-Dioxane
DMSO
DMF
Toluene
Toluene
THF
55
55
55
55
55
55
55
55
55
55
55
55
55
55
55
55
55
55
45
35
55
55
55
1
24
24
1
1
1
0
0
0
(EtO)3SiH
(EtO)3SiH
(EtO)3SiH
(EtO)3SiH
(EtO)3SiH
(EtO)3SiH
Et3SiH
Ph2MeSiH
(EtO)3SiH
(EtO)3SiH
(EtO)3SiH
(EtO)3SiH
(EtO)3SiH
(EtO)3SiH
(EtO)3SiH
(EtO)3SiH
(EtO)3SiH
(EtO)3SiH
(EtO)3SiH
(EtO)3SiH
>99
>99
>99
27
<5
0
<1
<1
8
0.5
0.3
0.2
0.1
0.05
0.3
0.3
0.3
0.3
0.3
0.3
0.3
0.3
0.3
0.3
0.3
0.3
0.3
0.3
1
24
24
1
1
1
1
1
1
1.5
0.75
0.5
1
1
1
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
82
6
75
87
92
68
70
64
85
72
75
THF
THF
THF
THF
THF
THF
1
1
a Catalytic reaction conditions: PhCHO (1.0 mmol), (EtO)3SiH (1.3 mmol) and n-dodecane (internal standard) (1.0 mmol), 2 mL THF, 55 °C.
b Determined by GC analysis.
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Table 2 Catalytic hydrosilylation of aldehydes with complex 1a
Entry
1
Substrate
Loading (mol%)
0.3
Time (h)
1
Conversionb (%)
>99
GC yield (%)
96
Isolated yield (%)
88
2
3
0.6
0.6
2
3
>99
>99
93
87
90
85
4
0.6
4
>99
84
77
5
6
0.6
0.6
3
3
>99
>99
90
92
88
89
7
8
0.6
0.6
4
4
>99
92
92
85
90
75
9
0.6
2
>99
92
85
a Reaction conditions: PhCHO (1.0 mmol), (EtO)3SiH (1.2 mmol) and internal standard (n-dodecane) (1.0 mmol), 2 mL THF, 55 °C. b Determined
by GC analysis.
the ortho- and para-positions. This makes the coordination of 1-naphthaldehyde (entry 8) as the substrates, the hydrosilyl-
the CvN group stronger. Therefore, the hydrido complexes ations proceed relatively slow and the GC yields of reduction
2–4 are more stable than complex 1 and have lower catalytic products are relatively lower. This might be caused by the
activity. On the other hand, the electron density on central steric hindrance of the neighboring position(s) of the aldehyde
iron atom increases with the increase of the electron density group in the molecular structures. Comparing the result of
on the imine nitrogen atom. Therefore, the hydride ligand entry 1 with that of entry 9, it can be concluded that
becomes less hydridic and the ability of the hydrido ligands to the heterocyclic aromatic aldehyde can also be reduced in
be added to the carbonyl group as a nucleophile is reduced. excellent yield with this catalytic system.
The final conclusion is that complex 1 is the best catalyst and
complex 4 is the worst catalyst among the four complexes.
For complexes 3 and 4, the electron-donating methyl group
makes the coordination ability of the CvN group stronger,
ð5Þ
The reduction reaction with ketones as substrates was
studied (eqn (5)) using complex 1 as a catalyst (Table 3). It was
found that the transformation of acetophenone to 1-phenyl-
ethanol is almost complete in this catalytic system after 4 h
(entry 1). The hydrosilylations of 2-acetylpyridine (entry 7) and
2-acetylthiophene (entry 6) are relatively fast and the related
isolated yields of reduction products are high. As for 2-methyl-
and therefore the hydrido complexes with the methyl group
are more stable and have a lower catalytic activity. Complex 1
is the best catalyst among the four hydrido iron complexes.
complex 1
10% NaOH
ArCHO þ ðEtOÞ3SiH ꢀꢀꢀꢀꢀꢀ! ꢀꢀꢀꢀꢀꢀ! ArCH2OH
ð4Þ
THF; 55 °C MeOH; 50 °C
To further expand the scope of the application of complex 1 acetophenone (entry 2) and benzophenone (entry 8), a long
as a catalyst, the reduction of different aromatic aldehydes was reaction time (>12 h) is needed for the ideal conversions. This
explored (eqn (4)). From Table 2 it can be seen that the elec- possibly results from the steric hindrance effect. However, the
tron-withdrawing groups (entries 2, 3, 5 and 6) appear to make hydrosilylation of 2-acetylnaphthalene (entry 5) was complete
the hydrosilylation relatively fast (except for entry 4), while an within 2 h and the isolated yield was up to 86% despite the
electron-donating group (entry 7) seems to render the reaction large molecular size. Nevertheless, the conversion from
sluggish. With 2,6-dichlorobenzaldehyde (entry 4) and acetone to 2-propanol is only 85% after 12 h at 55 °C and the
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Table 3 Catalytic hydrosilylation of ketones with complex 1a
Entry
1
Substrate
Loading (mol%)
0.6
Time (h)
4
Conversionb (%)
>99
GC yield (%)
95
Isolated yield (%)
91
2
0.6
12
87
84
79
3
4
5
0.6
0.6
0.6
4
4
4
>99
>99
>99
92
85
89
90
77
86
6
7
0.6
0.6
2
2
>99
>99
96
89
92
85
8
9
0.6
0.6
12
12
94
85
90
76
87
65
a Reaction conditions: R1COR2 (1.0 mmol), (EtO)3SiH (1.2 mmol) and internal standard (n-dodecane) (1.0 mmol), THF, 55 °C. b Determined by
GC analysis.
A molecule of the carbonyl compound coordinates to the
iron center to afford the intermediate D. After nucleophilic
addition of the hydrido ligand to the CvO bond, a new coordi-
nately unsaturated intermediate E is formed. The reaction of
(EtO)3SiH with the intermediate E gives rise to the hydrosilyl-
ation product via ligand exchange with the recovery of the
catalyst C. A similar mechanism was reported by Nikonov27
and Schreiber.28
Conclusions
Four hydrido iron complexes (1–4) were synthesized. Two of
them (2 and 4) are new and structurally characterized by X-ray
single crystal diffraction. The four complexes could be used as
catalysts in the hydrosilylation of carbonyl compounds. Among
them complex 1 is the best catalyst for reduction of ketones
and aldehydes.
Scheme 2 Mechanism of catalytic hydrosilylation with complex 1.
isolated yield is moderate (65%). All in all, the hydrido iron
complex 1 is an effective catalyst for the hydrosilylations of
Acknowledgements
aldehydes and ketones.
We gratefully acknowledge the financial support by the NSF
The proposed reaction mechanism of the hydrosilylation is China (no. 21372143/21172132) and the support from Prof.
shown in Scheme 2. The first step is ligand dissociation from Dr Dieter Fenske (Karlsruhe Nano-Micro Facility) for the deter-
complex 1 to give a coordinately unsaturated intermediate C. mination of the crystal structures.
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