Cyclopentadienyl-NHC iron complexes for solvent-free catalytic hydrosilylation of aldehydes and ketones

Article abstract of DOI:10.1002/ejic.201100762

Seven cyclopentadienyl-NHC piano-stool iron complexes were prepared and studied in the catalytic hydrosilylation of aldehydes and ketones under visible light irradiation. A significant acceleration of the rate was observed when the reactions were carried out under solvent-free conditions. Single-crystal X-ray structural analyses were performed for complexes 5 and 7. Cp-NHC piano-stool iron complexes were found to be efficient catalysts for the hydrosilylation of aldehydes and ketones under solvent-free conditions and by activation by light irradiation. An acceleration of the rate of the reaction was observed under neat conditions relative to those for the reactions in toluene or THF.

Full text of DOI:10.1002/ejic.201100762

SHORT COMMUNICATION
DOI: 10.1002/ejic.201100762
Cyclopentadienyl–NHC Iron Complexes for Solvent-Free Catalytic
Hydrosilylation of Aldehydes and Ketones
David Bézier,[a] Fan Jiang,[a] Thierry Roisnel,[b] Jean-Baptiste Sortais,*[a] and
Christophe Darcel*[a]
Keywords: Iron / Homogeneous catalysis / Hydrosilylation / Aldehydes / Ketones
Seven cyclopentadienyl–NHC piano-stool iron complexes
were prepared and studied in the catalytic hydrosilylation of
aldehydes and ketones under visible light irradiation. A sig-
nificant acceleration of the rate was observed when the reac-
tions were carried out under solvent-free conditions. Single-
crystal X-ray structural analyses were performed for com-
plexes 5 and 7.
light was accomplished under mild conditions, but its scope
for ketones was limited.[21] It also helps catalyze the re-
duction of amides to the corresponding amines.[22] Pursuing
our efforts to develop environmentally friendly systems that
use iron as the catalyst,[24] and, more particularly, for hydro-
silylation of carbonyl derivatives, it was tempting to see
whether a modification of the ligand structure (saturated
and unsaturated NHC ligands or steric effects on the N-
substituted moieties) or a modification of the reaction con-
ditions could lead to a significant improvement in the reac-
tivity. We report herein the comparative study of seven pi-
ano-stool cyclopentadienyl–NHC iron complexes 1–7 for
hydrosilylation of carbonyl derivatives under solvent or sol-
vent-free conditions (Figure 1).
Introduction
Since the pioneering reports on N-heterocyclic carbenes
(NHCs) by Öfele and Wanzlick,[1] and the early studies by
Lappert[2] on transition-metal coordination chemistry, fol-
lowed by the discovery of stable NHCs by Arduengo,[3]
these ligands have been widely applied in coordination
chemistry and catalysis over the last two decades.[4]
Furthermore, despite recent impressive advances in iron
homogeneous catalysis,[5] the number of applications in
polymerization[6] or fine chemistry[7–11] (e.g. cycloisomeriza-
tion,[8] C–C bond formation,[9] allylation,[10] or esterifica-
tion of aldehydes[11]) with iron salt/NHC ligand catalytic
systems is relatively scarce. In the area of well-defined iron
complexes, relative to other transition metals, NHC–Fe
complexes are still less represented.[12–19] Up to now, there
are few examples where they have been used as catalysts
(e.g. C–H bond activation/borylation of heterocycles).[17]
For hydrosilylation, only a few NHC–iron-catalyzed reac-
tions have been reported.[20–22]
In our recent work, we developed efficient hydrosilylation
catalysts based on iron complexes bearing diphosphane[23]
or NHC ligands.[21,22] For the latter family of catalysts, by
using the complex [CpFe(IMes)(CO)2][I] {IMes = 1,3-
bis(2,4,6-trimethylphenyl)imidazol-2-ylidene} as the pre-
catalyst, the reduction of aldehydes under activation by
Figure 1. Cationic and neutral iron piano-stool cyclopentadienyl
complexes bearing various N-heterocyclic carbene ligands.
Results and Discussion
[a] UMR 6226 CNRS – Université de Rennes 1 “Sciences
Chimiques de Rennes” “Catalysis and organometallics” –
Campus de Beaulieu, Bat 10C,
According to the methodology described by Guer-
chais,[13,14a] the cationic N-heterocyclic carbene complexes
1, 3, 5, and 7 with iodide as the counterion, were prepared
by mixing the corresponding NHC ligands, prepared from
the hydrochloride salt by deprotonation with freshly sub-
limed tBuOK, and [CpFe(CO)2(I)] in toluene. The neutral
complexes 2, 4, and 6 were then obtained by photoirradia-
tion of 1, 3, and 5, respectively (Scheme 1). Surprisingly,
263 avenue du Général Leclerc 35042 Rennes Cedex, France
E-mail: jean-baptiste.sortais@univ-rennes1.fr
christophe.darcel@univ-rennes1.fr
[b] UMR 6226 CNRS – Université de Rennes 1 “Sciences
Chimiques de Rennes” “Centre de Diffractométrie X” –
Campus de Beaulieu, Bat 10B,
263 avenue du Général Leclerc 35042 Rennes Cedex, France
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201100762.
Eur. J. Inorg. Chem. 2012, 1333–1337
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1333

D. Bézier, F. Jiang, T. Roisnel, J.-B. Sortais, C. Darcel
SHORT COMMUNICATION
an attempt to obtain the neutral complex by irradiation of action proceeded perfectly in THF (Table 2, Entries 1 and
complex 7 failed, certainly because of the steric effect of the 2). Surprisingly, the reaction was performed with excellent
DIPP carbene ligand, which disfavors the coordination of conversions under solvent-free conditions (Table 2, Entries
iodine.
1 and 2). For an economic and environmentally friendly
reaction, these solvent-free conditions can be favorable. We
then investigated the activity of different NHC–iron cata-
lysts 3–7 bearing different NHC ligands. With the 1,3-
bis(2,4,6-trimethylphenyl)imidazolidin-2-ylidene (SIMes)
cationic iron complex 3, the result showed poor activity
when the hydrosilylation was performed in THF (Table 2,
Entry 3) as only 47% conversion was obtained after 3 h at
30 °C. However, when the SIMes neutral complex 4 was
used as the catalyst in THF under the same conditions, 96%
conversion was observed (Table 2, Entry 4). This difference
is mainly due to the difference in solubility of the catalysts
in THF. Interestingly, when the reaction was performed for
3 h at 30 °C under neat conditions, a full conversion was
obtained with the use of catalysts 3 and 4. By using N-
isopropyl-substituted NHC complexes 5 and 6, the hydro-
silylation reaction in THF led to poor conversions (Table 2,
Entries 5 and 6), mainly because of a problem of solubility
of these complexes. However, when the reaction was per-
formed under neat conditions, better results were obtained
(76 and 92% conversions were observed, respectively). Fi-
nally, we tested the iron cationic complex 7, which led to
moderate conversions in THF or solvent-free conditions
(Table 2, Entry 7). This result seems to indicate that steric
hindrance has a deleterious effect on the activity.
Scheme 1. Preparation of the Cp–NHC iron complexes 1–7.
Single crystals of the new complex 7 and complex 5 suit-
able for X-ray diffraction studies were obtained, and their
molecular structures are confirmed by X-ray crystallogra-
phy (Table 5). ORTEP drawings with the corresponding
atom labels are shown in Figure 2. Both structures show a
typical piano-stool geometry. The structure of 5 is very
similar to the structure described with BF4 as the
counteranion,[14a] with CO–Fe–CO angle of 92.33° and C1–
Fe–CO angles of 94.64° and 94.03° (Table 1). However, 7
shows a slightly more distorted geometry (with a CO–Fe–
CO angle of 90.97° and C1–Fe–CO angles of 98.70° and
94.10° and N2–C1–Fe–C3 and N1–C1–Fe–C2 torsion
angles of 83.2(2)° and 5.6(2)°, respectively) probably as a
result of the bulkiness of the DIPP–NHC ligand.
Scheme 2. Hydrosilylation of benzaldehyde 8 and acetophenone 9.
Table 2. Optimization for the reduction of benzaldehyde 8 and
acetophenone 9 with catalysts 1–7.
Entry
Catalyst
Benzaldehyde[a,b]
Acetophenone[c,b]
THF
Neat
Toluene
Neat
1
2
3
4
5
6
7
1
2
3
4
5
6
7
Ͼ97
90
47
96
28
11
46
Ͼ97
97
Ͼ97
Ͼ97
76
92
50
78
67
60
5
Ͼ97
Ͼ97
Ͼ97
22
Figure 2. ORTEP-style plot of compound 5 and 7. Thermal ellip-
soids are drawn at the 50% probability level, and hydrogen and
iodide atoms are omitted for clarity.
0
82
7
74
Table 1. Selected bonds [Å] and angles [°] for complexes 5 and 7.
40[d]
95[d]
5
7
[a] Typical conditions for aldehydes: benzaldehyde (0.5 mmol), cat-
alyst 1–7 (1 mol-%), 1.0 equiv. PhSiH3, 30 °C, 3 h, under visible
light irradiation. [b] Conversion determined by 1H NMR spec-
troscopy after methanolysis. [c] Typical conditions for ketones: ace-
tophenone (0.5 mmol), catalyst 1–7 (2 mol-%), 1.2 equiv. PhSiH3,
50 °C, 16 h, under visible light irradiation. [d] 70 °C, 16 h.
Fe–CO
Fe–C1
Fe–centroid Cp
C2–Fe–C3
C1–Fe–C2
C1–Fe–C3
1.7797(19), 1.7801(19)
1.9742(16)
1.7195(8)
92.33(8)
94.64(7)
1.780(3), 1.781(3)
1.990(2)
1.7327(13)
90.97(14)
98.70(12)
94.10(12)
94.03(7)
Encouraged by the efficiency of the reaction protocol de-
In our preliminary report,[21] the reduction reaction for scribed above with cationic complexes 1 or 3, the scope of
benzaldehyde 8 was carried out in THF, with cationic com- the reaction was examined under solvent-free conditions at
plex 1 and neutral complex 2 (1 mol-%) as the catalysts, 30 °C as illustrated in Table 3. Firstly, various benzaldehyde
under light irradiation at 30 °C for 3 h (Scheme 2). The re- derivatives were reduced in the presence of 1.0 equiv. phen-
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Cyclopentadienyl–NHC Fe Complexes for Catalytic Hydrosilylation
ylsilane and 1 mol-% of the cationic complex 1 for 3 h at study of complexes 1–7 (2 mol-%) in the hydrosilylation of
30 °C (Table 3, Entries 1–5). When the reaction was carried acetophenone was undertaken at 50 °C for 16 h under vis-
out with benzaldehydes bearing a methyl substituent in the ible light irradiation. In toluene, the best system was the
ortho position, a full conversion was observed, which shows cationic complex 1 (Table 2, Entries 1–7). An impressive im-
that ortho substituents do not hamper the reaction (Table 3, provement was observed when the reactions were per-
Entries 2 vs. 3). The electronic effects on the reactivity are
limited: electron-donating as well as electron-withdrawing
groups on the aryl ring do not show any significant influ- Table 4. Scope of the iron-catalyzed hydrosilylation of ketones with
cationic complex 1.[a]
ence on the activity of the iron catalyst. Interestingly, the
chemoselectivity of the reaction did not change under sol-
vent free conditions: functional groups such as CN or al-
kene were not hydrosilylated under such reaction conditions
(Table 3, Entries 4, 6–8). Furthermore, the catalyzed hydro-
silylation of enal derivatives such as (–)-myrtenal and cinna-
maldehyde led exclusively to the corresponding allyl
alcohols, which resulted in exclusive 1,2-addition with no
GC-detectable amounts of 1,4-addition products (Table 3,
Entries 6 and 7).
Table 3. Scope of iron-catalyzed hydrosilylation of aldehydes with
complex 1.[a]
[a] Typical conditions: substrate (0.5 mmol), complex 1 (1 mol-%),
1.0 equiv. PhSiH3, 30 °C, 3 h, under visible light irradiation. [b]
Conversion determined by 1H NMR spectroscopy after meth-
anolysis, isolated yield in parenthesis after purification by column
chromatography.
The reduction of ketones was then investigated, using
acetophenone as the model substrate (Table 2). In a pre-
vious contribution,[21] we have shown that catalyst 1 was
[a] Typical conditions: ketone (0.5 mmol), catalyst 1 (2 mol-%),
PhSiH3 (1.2 equiv.), neat, 16 h, under visible light irradiation. [b]
only effective for the hydrosilylation of electron-deficient
ketones, but the yields were moderate for arylketones bear-
Conversion determined by 1H NMR spectroscopy after meth-
anolysis, isolated yield in parenthesis after purification by column
ing electron-donating substituents at 70 °C. A systematic chromatography.
Eur. J. Inorg. Chem. 2012, 1333–1337
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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1335

D. Bézier, F. Jiang, T. Roisnel, J.-B. Sortais, C. Darcel
SHORT COMMUNICATION
formed under solvent-free conditions. Indeed, with catalyst catalytic hydrosilylation reaction, and (iii) all types of
1, the conversion increased to Ͼ97% (instead of 78% in ketones could be reduced under solvent-free conditions. It
toluene). The best improvements were observed for cata- must be also underlined that such complexes are usually
lysts 2 and 3 with full conversion at 50 °C after 16 h of stable under solvent-free conditions and at temperatures of
reaction (instead of 67 and 60% in toluene, respectively) up to 100 °C. Further investigations into the mechanism of
(Table 2, Entries 2 and 3). It must be underlined that for this reaction are underway in our laboratory.
complex 7, to obtain a good conversion (95%), the reaction
has to be warmed at 70 °C for 16 h.
Experimental Section
By taking advantage of this improvement by working un-
Typical Procedure for Cationic Complex Synthesis: A mixture of
freshly sublimed tBuOK (290 mg, 2.60 mmol) and the NHC·HCl
salt (2 mmol) in THF (10 mL) was stirred at room temperature
for 30 min. After evaporation of the solvent, the free carbene was
extracted in warm toluene (2ϫ 30 mL), and to this solution was
added [CpFe(CO)2(I)] (550 mg, 1.80 mmol) at room temperature.
Overnight stirring at room temperature allows precipitation of the
product, and the solid was filtered and washed with 3ϫ 20 mL
toluene and 2ϫ 20 mL ethyl ether. Crystallization in CH2Cl2/Et2O
gave yellow microcrystals. For complex 7, yield: 77% (0.96 g).
der solvent-free conditions, the scope for the reaction of
ketones was re-examined by using the cationic complex 1
(2 mol-%) at 50 °C (Table 4). Firstly, various arylketones
were reduced in the presence of 1.2 equiv. phenylsilane.
Electron-poor arylketones were found to be suitable sub-
strates for hydrosilylation as full conversion was obtained
(Table 4, Entries 2–4). Interestingly, when using electron-
rich arylketones such as p-methylacetophenone, a 97% con-
version was observed (Table 4, Entry 5). It must be pointed
out that when this reaction was performed in toluene at
70 °C for 16 h, only 66% conversion was obtained.[21] By
using p-methoxyacetophenone as starting material led to a
86% conversion (vs. 61% in toluene). When the reaction
was carried out with the acetophenone derivative bearing a
methyl substituent in the ortho position, the corresponding
alcohol was obtained with a 43% conversion when the reac-
tion was performed at 50 °C for 16 h (Table 4, Entry 7) and
with a 59% conversion after 16 h at 70 °C, which shows
that the ortho substituents did not hamper the reaction, but
had a significant impact on the yield (Table 4, Entries 7 and
8). On the other hand, this iron-catalyzed transformation
can also be extended to benzophenone; however, to reach
full conversion, the reaction needs to be performed at 70 °C
for 16 h, because at 50 °C, only 44% conversion was ob-
tained (Table 4, Entries 9 and 10). Similarly, reduction of
cyclic ketones, namely α-tetralone and 1-indanone, was
achieved with good yields (Table 4, Entries 12 and 13).
Apart from acetophenone-type derivatives, α,β-unsaturated
ketones were also investigated; however, lower isolated
yields were observed (38 and 58%) starting from β-ionone
and (3E)-4-phenyl-3-buten-2-one, respectively (Table 4, En-
tries 14 and 15). Finally, an alkyl-ketone such as cyclohep-
tanone was reduced, but a higher temperature is necessary
to observe a 65% conversion after 16 h at 70 °C (Table 4,
Entry 16).
Typical Procedure for the Iron-Catalyzed Hydrosilylation of Alde-
hydes: A 10-mL oven-dried Schlenk tube containing a stirring bar
was charged with [CpFe(CO)2(IMes)]I (1, 3.0 mg, 0.005 mmol,
1 mol-%). After purging with argon (argon-vacuum three cycles),
the aldehyde (0.5 mmol) was added followed by PhSiH3 (62 μL,
0.5 mmol). The reaction mixture was stirred in a preheated oil bath
at 30 °C for 3 h under light irradiation (with a 24-watt compact
fluorescent lamp). MeOH (1 mL) was then added, followed by
NaOH aqueous solution (1 mL of a 2 m solution) whilst stirring
vigorously. The reaction mixture was further stirred for 1 h at room
temperature and was extracted with diethyl ether (2ϫ 10 mL). The
combined organic layers were washed with brine (3ϫ 10 mL), dried
with anhydrous MgSO4, filtered, and concentrated under vacuum.
The conversion was determined by 1H NMR spectroscopy. The
residue was then purified by silica gel column chromatography by
using an ethyl acetate/petroleum ether mixture (10 to 50%) to af-
ford the desired product.
Typical Procedure for the Iron-Catalyzed Hydrosilylation of
Ketones: The same procedure was used for the hydrosilylation of
ketones, with [CpFe(CO)2(IMes)]I (1, 6.0 mg, 0.010 mmol, 2 mol-
%) and PhSiH3 (74 μL, 0.6 mmol) and with heating at 50 °C for
16 h.
Crystallographic data for complexes 5 and 7 are summarised in
Table 5. CCDC-824559 (for 5) and -824559 (for 7) contain the sup-
Table 5. Selected crystallographic data for complexes 5 and 7.
5
7
Empirical formula
Crystal system
Space group
C16H21FeIN2O2 C34H41FeIN2O2
triclinic
monoclinic
P21/n
¯
P1
Conclusions
a [Å]
b [Å]
c [Å]
α [°]
β [°]
γ [°]
9.0497(2)
10.2873(3)
10.4346(3)
87.4870(10)
79.3400(10)
72.5920(10)
2
1.663
12926, 4128
0.0287
12.5709(3)
23.4892(4)
12.7467(3)
90
97.7000(10)
90
We have developed an efficient solvent-free iron-cata-
lyzed hydrosilylation of aldehydes and ketones under visible
light activation. When well-defined cyclopentadienyl–NHC
iron complexes were used as the pre-catalysts (1–2 mol-%),
in most cases the reactions proceed with higher conversions
and yields than those obtained by similar reactions in THF
or toluene solution. It must be pointed out that (i) the reac-
tions were performed at lower temperatures under solvent-
free conditions (50 °C) than that in toluene (70 °C), (ii) the
visible-light activation is crucial for the promotion of the
Z
4
ρcalcd. [gcm–3]
1.233
26740, 8522
0.0397
Reflections collected, unique
Rint
GooF
1.068
1.086
R1, wR2 [IϾ2σ(I)]
0.0209, 0.0491 0.0400, 0.1023
0.593/ –0.521 1.249/ –0.743
Largest diff. peak/hole [eÅ–3]
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Cyclopentadienyl–NHC Fe Complexes for Catalytic Hydrosilylation
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Received: July 22, 2011
Published Online: October 14, 2011
Eur. J. Inorg. Chem. 2012, 1333–1337
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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