Hydrosilylation of carbonyl and carboxyl groups catalysed by Mn(i) complexes bearing triazole ligands

Article abstract of DOI:10.1039/c9cy01738k

Manganese(i) complexes bearing triazole ligands are reported as catalysts for the hydrosilylation of carbonyl and carboxyl compounds. The desired reaction proceeds readily at 80 °C within 3 hours at catalyst loadings as low as 0.25 to 1 mol%. Hence, good to excellent yields of alcohols could be obtained for a wide range of substrates including ketones, esters, and carboxylic acids illustrating the versatility of the metal/ligand combination.

Full text of DOI:10.1039/c9cy01738k

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Hydrosilylation of carbonyl and carboxyl groups
catalysed by MnIJI) complexes bearing triazole
ligands†
Cite this: Catal. Sci. Technol., 2019,
9, 6370
Oriol Martínez-Ferraté, ‡a Basujit Chatterjee,‡a Christophe Werlé
a and
*
ab
*
Walter Leitner
Received 28th August 2019,
Accepted 13th October 2019
ManganeseIJI) complexes bearing triazole ligands are reported as catalysts for the hydrosilylation of carbonyl
and carboxyl compounds. The desired reaction proceeds readily at 80 °C within 3 hours at catalyst
loadings as low as 0.25 to 1 mol%. Hence, good to excellent yields of alcohols could be obtained for a
wide range of substrates including ketones, esters, and carboxylic acids illustrating the versatility of the
metal/ligand combination.
DOI: 10.1039/c9cy01738k
rsc.li/catalysis
second step hydrolysed to the corresponding alcohols. The
protocols represent an alternative to hydrogenation reactions
Introduction
The modern chemical industry mostly relies on catalysis for
the synthesis of bulk materials and fine chemicals.1 In the
last three decades, industrial breakthroughs in homogeneous
catalysis mainly involved catalysts based on second- and
third-row transition metals, which are rare elements, whose
mining generates waste, and is often associated with low
abundance and high costs.2 The excellent performance of the
platinum group metals has overshadowed the potential of
first-row transition metals, albeit they have been widely used
in academia and industry at the early days of homogeneous
catalysis. This interest is currently revitalized in particular
for metals which can offer potential benefits such as
biocompatibility, low toxicity, and high abundance,
where easy and safe-to-handle silanes replace the use of
hydrogen.8 Up to now, research in the area has focused
mainly on carbonyl substrates using MnIJII) complexes,9 and
only a few studies have dealt with MnIJI) or Mn(0).10 The
hydrosilylation of carboxyl compounds is even less studied.
For carboxylic acids, reduction with a MnIJ0)-complex has
been reported to yield aldehydes rather than alcohols as the
preferred products.10e This lack of knowledge is surprising
given that MnIJI) catalysts have already demonstrated a
pronounced ability for the transfer of hydrides to substrates
displaying electrophilic centres (e.g., in the case of
hydrogenation and hydroboration reactions).11 In the
previous studies, the MnIJI) complexes under scrutiny were
composed mostly of cyclopentadienyl, carbene, or phosphine
ligands (Chart 1).10a–d Nitrogen-based ligands can provide an
attractive alternative, and triazole units have received
attention as versatile donor units.1a,12 Their synthesis takes
advantage of the modularity of copper-catalysed azide–alkyne
cycloaddition (i.e., click-chemistry), which is convenient to
access structurally different ligand frameworks via this atom-
and step-economic robust synthetic method.13 Hence, the
stereoelectronic properties of the ligand architecture can be
finely adjusted. Besides, their denticity can be controlled and
thus, bidentate or tridentate variations are accessible, leading
to neutral or cationic complexes respectively.13d,e,14
constituting
greener
alternatives
towards
more
environmentally benign processes.3 In this context, iron has
arguably been the most studied candidate.4 Most recently,
manganese complexes are also gaining considerable
importance in homogeneous catalysis defining an active area
of current research.5
The hydrosilylation of CO groups is a transformation of
broad synthetic utility6 and has recently been studied with
manganese catalysts.7 This reaction allows the one-step
synthesis of protected silyl alcohols, which can be in a
a Max Planck Institute for Chemical Energy Conversion, Stiftstr. 34–36, Mülheim
an der Ruhr, 45470, Germany. E-mail: christophe.werle@cec.mpg.de,
walter.leitner@cec.mpg.de
b Institut für Technische und Makromolekulare Chemie (ITMC), RWTH Aachen
University, Worringer Weg 2, Aachen, 52074, Germany
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c9cy01738k
In the present study, we report the catalytic performance of
MnIJI) complexes bearing triazol-based ligands for the
hydrosilylation of carbonyl and in particular also for carboxyl
derivatives. The new cationic complex 3 bearing a tridentate
(PNN)-iminotriazole ligand and bidentate neutral complexes
previously reported by our group11b were investigated. We
found that the selected catalysts were able to convert a wide
‡ These authors contributed equally to this work.
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Chart 1 Examples of MnIJI) catalysts for hydrosilylation reactions.
range of ketones to corresponding alcohols. Notably, the
(PNN)-manganeseIJI) complex 3 showed promising results even
in the reduction of ester and acid functionalities.
at 80 °C for 20 h, produced alcohol 5a after hydrolysis in
85% yield. In order to improve the efficiency of the reaction,
a panel of different solvents were investigated. When the
reaction was carried out in tetrahydrofuran (THF) or
acetonitrile (MeCN), entry 2 and 3 respectively, yields up to
99% of 5a were obtained. When the reaction time was
reduced to 3 hours (entries 4–8), lower yields were obtained
with apolar solvents (e.g., 15% in toluene), but yields
remained high in the polar solvents (e.g., 99% in THF, 90%
in MeCN).
For comparison, neutral triazole complexes 6–8 were
prepared following previously described procedures,11b and
tested also for the hydrosilylation of acetophenone (Fig. 1).
Using 1 mol% of catalyst loading at 80 °C for 3 h, all the
studied complexes were able to reduce 4a to 5a with yields up
to 99%. To study more precisely the influence of the ligand,
the reaction times were reduced to one hour while keeping
the catalyst loading unchanged. Under these conditions,
iminotriazole complexes 3 and 8 provided the best results,
with yields reaching 99%. When complexes 6 and 7 were
used, slightly lower yields were obtained (96%). Furthermore,
when the catalyst loadings were reduced to 0.1 mol%, catalyst
8 exhibited the best performance. Finally, the activity of
Results and discussion
The synthesis of (PNN)-ligand 2 and its cationic MnIJI)
complex 3 was performed as summarised in Scheme 1.
Triazole 1 and the corresponding aldehyde were reacted in
dry toluene at 105 °C in the presence of MgSO4 serving as a
dehydrating agent. Under these reaction conditions, 2 was
obtained in 99% yield. Subsequently, triazole 2 was treated
with bromopentacarbonylmanganeseIJI) in toluene at room
temperature to provide the desired complex 3 in 46% yield.
High-resolution mass spectrometry confirmed the formation
of the expected cationic complex with bromide as
a
counterion. Furthermore, 1H- and 31P{1H}-NMR spectroscopy
indicated the diamagnetic nature of the complex which
agrees with a d6-metal centre.
The catalytic activity of complex 3 was first studied for the
hydrosilylation of ketones using acetophenone (4a) as a
benchmark substrate (Table 1). Reacting 4a with one
equivalent of PhSiH3 and 1 mol% of 3 under neat conditions
Scheme 1 Reagents and conditions for the synthesis of 2 and 3: a) MgSO4, toluene, 105 °C, 16 h, 99%; b) toluene, r.t., 60 h, 46%.
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Table
hydrosilylation
1 Optimization of reaction condition for acetophenone
obtained for all studied para-substituted substrates 5b–e.
Only a minor impact related to the electronic properties was
observed. The yields decreased slightly in the presence of
electron donating groups where 95% and 93% of 5b and 5c
were obtained, respectively. Introducing electron-donating
groups in meta- and para-position of substrate 4h, provided
only 14% of 5h when 0.25 mol% of catalyst loading was used.
However, this could be increased to 73% yield with 1 mol%
of catalyst loading under otherwise identical conditions. Low
yields (2% of 5f and 10% of 5g) were observed also for the
ortho-substituted compounds at low catalyst loading. Again,
moderate yields (49% for 5g) could be achieved when 1
mol% of catalyst 8 was used. It is conceivable that the lower
yields are due to an increased steric hindrance around the
Entry
Solvent
T (h)
Yielda (%)
1
2
3
4
5
6
7
8
Neat
THF
MeCN
THF
20
20
20
3
3
3
85
99
99
99
90
15
36
95
MeCN
Toluene
1,4-Dioxane
Dimethoxyethane
3
3
carbonyl
functionality
for
these
substrates.
The
0.5 mmol acetophenone, 0.5 mmol phenylsilane, 1 mol% 3, 0.2 mL
a
of solvent, 80 °C. Quantified by 1H NMR using mesitylene as an
heteroaromatic ketone 4i were not reduced under these
conditions. The naphthyl derivatives 4j and 4k could be
converted with good yields showing different reactivities
depending on the relative position of the ketone. In the case
of 2-acetonaphtone 4j, 94% of isolated yield could be
obtained with only 0.25 mol% of 8. On the other hand,
1-acetonaphthone 4k, required higher catalyst loadings (1
mol%) to furnish 5k in 77% yield. This result can be
rationalized by the ortho-substituted nature of ketone 4k
following the same trend as for 4f–i. Benzophenone 4l was
internal standard.
MnIJCO)5Br was also verified. When subjected to the standard
set of conditions, 56% of conversion and 21% of the
corresponding alcohol were obtained.
Based on the standard set of reaction conditions, we
examined the substrate scope. These results are summarized
in Scheme 2. For acetophenone derivatives, high yields were
Fig. 1 Screening of reaction times and MnIJI) complexes for the hydrosilylation of 4a. Reaction conditions: 0.5 mmol acetophenone, 0.5 mmol
phenylsilane, 0.2 mL THF, 80 °C. Quantified by 1H NMR using mesitylene as internal standard.
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of PhSiH3 and 2 mol% of complex 8 in THF at 80 °C for 3
hours led to moderate conversions (53%), with preferential
formation of ethyl benzyl ether 11 (94%) relative to benzyl
alcohol 10 (6%, entry 1). Catalyst 6 gave slightly higher
conversion than 8 forming a nearly 1 : 1 mixture of ether and
alcohol (entry 2). When complex 7 was used, only 18%
conversion was obtained with alcohol 10 being the preferred
product in this case (entry 3). The highest activity for
reduction was observed with complex 3 that fully converted 9
to a roughly 60 : 40 mixture of 10 and 11 under the given
conditions (entry 4).
To investigate whether the two products 10 and 11 are
interconverted under the given reaction conditions, two
control experiments were carried out (Scheme S1†). Firstly,
reductive ether cleavage was investigated. Only 1% of 11 was
converted to alcohol 10 under the optimized reaction
conditions established for the hydrosilylation of
9 in
presence of excess phenylsilane. Similarly, no etherification
was observed when alcohol 10 was reacted with ethanol in
the presence of 3 and phenylsilane. These results indicate
that the selectivity is controlled through competing pathways,
presumable branching from
a
common intermediate
RC(OR′)IJOSiH2Ph), rather than by secondary interconversion.
Having established the principle ability for carboxyl
reduction, the variation of reaction conditions was
investigated (Table 2). The rate of reduction increased at
elevated temperatures reaching full conversion above 120 °C
for catalysts 8 (entries 1, 5–7). The ether 11 remained the
preferred product, but selectivity decreased from 94% to ca.
70% at higher temperature. For catalyst 3, the selectivity
towards the alcohol product 10 also increased with
decreasing temperature, albeit at the expense of conversion
(entry 4, 8–10). The reaction was not very solvent dependent
Scheme
2 Substrate screening for the hydrosilylation of ketones
catalysed by 8. Reaction conditions: 0.5 mmol substrate, 0.25 mol% of
8, 1 mmol of silane, 0.2 mL THF, 80 °C, 1 h. Quantified by 1H NMR
using mesitylene or tetradecane (0.5 mmol) as the internal standard. a1
mol% catalyst loading. bIsolated yield.
Table 2 Optimization of the catalyst loading and temperature for the
hydrosilylation of ethyl benzoate
reduced to the corresponding alcohol 5l in high yield, similar
to other substrates containing unsubstituted phenyl rings
(4m, n). For the aliphatic substrate cyclohexanone 4o and
1-cyclohexylethanone 4p, good yields (43% and 63%
respectively) could be achieved with 1 mol% of catalyst
loading. The linear aliphatic ketones 2- and 3-octanone (4q
and 4r) were hydrosilylated in high yields, whereby 4q
required again higher catalyst loading (1 mol%). Even the
sterically congested tert-butyl methyl ketone 4s was readily
reduced providing 5s in good yields (79%).
After the successful hydrosilylation of ketones, the
catalytic competence of the MnIJI) catalysts was probed for the
hydrosilylation of more challenging carboxyl groups in esters
and acids. In the case of esters as substrates, the reduction
and hydrolysis can lead to either the corresponding alcohols
or ethers as products. Ethyl benzoate 9 was chosen as
prototypical substrate for the screening of catalysts and
reaction conditions (Table 2). Reacting 9 with 2 equivalents
Entry t (°C) Cat. (mol%) Conv.a (%) Sel. 10a (%) Sel. 11a (%)
1
2
3
4
5
6
7
8
80
80
80
80
105
120
140
r.t.
60
100
80
80
8 (2)
6 (2)
7 (2)
3 (2)
8 (2)
8 (2)
8 (2)
3 (2)
3 (2)
3 (2)
3 (2)
3 (2)
53
67
18
100
88
100
100
18
6
94
52
33
38
65
68
75
17
23
43
16
100
48
67
62
35
32
25
83
77
57
84
—
9
64
10
11b
12c
100
81
78d
0.5 mmol ethyl benzoate, 1 mmol phenylsilane, 3 h, 0.2 mL of
a
THF. Quantified by 1H NMR using tetradecane as an internal
b
standard. Slow addition of phenyl silane, 0.33 mmol per hour.
c
Hydrolysis with Me4NF, and further addition of 10 equivalents of
d
sodium hydride and 5 equivalents of ethyl bromide. Yield after
workup.
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Table 3 MnIJI) catalysed hydrosilylation of esters
Subsequently, various esters were hydrosilylated using
cationic catalyst
3 under the standard set of reaction
conditions to assess the scope and limitation of the reduction
(Table 3). When benzoates were used as substrates,
comparable results in yield and selectivity for methyl-, ethyl-
and benzyl-benzoates were observed (entry 1–3, Table 3). For
the bulky tert-butyl ester a low conversion was obtained (14%)
with the selectivity favouring formation of ether-type product
11 over the alcohol 10 (60% versus 40%, entry 4). Interestingly,
phenyl benzoate (entry 5), provided satisfactory results with
80% conversion and high selectivity 82% for corresponding
benzyl alcohol. When ethyl heptanoate was used as substrate
we observed a full reduction of the ester, with the ether being
the major product (60%, entry 6). Finally, para-substituted
methyl-benzoates substrates could be fully converted to the
alcohols as the major product (entry 7–8).
Entry
R
R′
Conv.a (%) Sel. 10a (%) Sel. 11a (%)
1
2
3
4
5
6
7
Ph
Ph
Ph
Ph
Ph
C6H13
Me
Et
Bn
tBu
Ph
Et
97
100
100
14
80
100
100
69
62
70
40
82
40
73
31
38
30
60
18
60
27
Me
8
Me
100
78
22
0.5 mmol ethyl benzoate, 2 mol% of 3, 1 mmol phenylsilane, 0.2 mL
of THF. Quantified by 1H NMR using tetradecane or mesitylene as
a
an internal standard.
Then, we explored the hydrosilylation of acids with
leading complexes
3 and 8 and benzoic acid as the
benchmark substrate (Table 4). Gratifyingly, conversion of
50% with 98% selectivity towards the corresponding alcohol
was observed with catalyst 3 at 80 °C (entry 2). At lower (60
°C, entry 1) as well as higher temperatures (140 °C, entry 3)
the conversions towards 10 decreased to 26% and 31%
respectively. At 60 °C (entry 1), significant amounts of
aldehyde 13 were formed (38%). Control experiments at 80
°C for 2 hours in absence of catalyst 3 in solution (entry 8)
and neat (entry 9) revealed no conversion. The neutral
complex 8 showed lower catalytic activity than 3 (entries 4–6).
When catalyst loading for 3 was increased to 2 mol%, 94%
conversion (entry 7) with nearly perfect selectivity for the
alcohol could be obtained even with lower reaction time of 2
h, setting these conditions as standard conditions for
exploring the substrate scope.
and toluene, as well as neat conditions, provide potential
alternatives to THF (Fig. S1†). The reactivity of silanes
followed the hydricity strength in the order PhSiH3
>
Ph2SiH2 ≫ Ph3SiH with only moderate influence on product
distribution (Table S1†). A significant improvement could be
achieved when PhSiH3 was added slowly (0.33 mmol per
hour) to the reaction mixture at 80 °C. This protocol
combined high conversion (81%) with good selectivity
towards the alcohol 10 (84%) (entry 11).
Examples for the synthesis of ethers via hydrosilylation of
esters are limited and ample scope prevails for further
development.10e,g We, therefore, investigated the possibility
to combine the reduction step directly with a workup under
etherification conditions to provide access to 11 from 9. After
reduction of 9 with PhSiH3 using catalyst 3 under standard
conditions, the reaction mixture was treated with
tetramethylammonium fluoride to remove the silyl group,
followed by addition of ethyl bromide and base. The ether 11
was isolated in 78% yield directly from this method after
standard workup procedure (Table 2, entry 12).
Based on the standard set of reaction conditions, we
examined the substrate scope of carboxylic acids as reported
in Scheme 3. It is shown that the para-substituted substrates
15b and 15c could be converted in high yields, albeit with a
noticeable influence of the electron-withdrawing substituents.
Introducing a NO2 group in the meta-position in substrate
Table 4 Optimization of the catalyst loading, time and temperature for the hydrosilylation of benzoic acid
Entry
t (°C)
Time (h)
Cat. (mol%)
Conv.a (%)
Sel. 10a (%)
Sel. 13a (%)
1
2
3
4
5
6
60
80
140
80
120
140
80
3
3
3
3
3
3
2
2
2
3 (1)
3 (1)
3 (1)
8 (1)
8 (1)
8 (1)
3 (2)
—
26
50
31
28
10
2
94
0
0
62
98
38
<1
—
—
20
—
—
—
—
100
100
80
100
100
—
7
8b
9c
80
80
—
—
a
0.5 mmol acid, 1 mmol phenylsilane, 0.2 mL of THF. Quantified by 1H NMR using mesitylene or tetradecane as an internal standard.
b
Reaction was performed in the absence of catalyst. c Reaction was performed in absence of catalyst under neat condition.
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hydride complex [Mn-H] may be formed under concomitant
formation of a silyl cation.16 This mode of activation is
supported by ex situ reaction of complex 3 with PhSiH3.
Analysis of the reaction mixture revealed the formation of a
hydride complex 16 based on HR-MS data indicating a
molecular formula [C31H25MnN4O2P]+ and a hydride signal
in the 1H-NMR spectrum at −8.56 ppm (see ESI† for further
details). The [Mn-H] species are expected to catalyse hydride
transfer from PhSiH3 to the silyl-activated substrate. The
resulting intermediate A can be converted by hydrolysis or
desilylation to aldehyde B or it is reduced further via a
second Mn-catalysed hydride transfer to the alkyl- or silyl-
ether
derivatives
affording
after
hydrolysis
the
corresponding organic product alcohol C or ether D. While
the exact nature of the catalytically active [Mn-H] species
and how it enables the hydride transfer still needs to be
elucidated, the proposed pathway is in line with the
currently available experimental data and may thus serve as
a working hypothesis for future studies.
In summary, we have shown that manganeseIJI) complexes
based on iminotriazole ligands constitute efficient catalysts
for the hydrosilylation of carbonyl and carboxyl groups.
Aromatic and aliphatic ketones can be reduced with good to
excellent yields at low catalyst loadings within 1 h reaction
time, using either neutral or cationic complexes comprising
bidentate or tridentated ligands, respectively. Extending the
reaction to carboxyl groups, esters were reduced with very
high conversions, whereby the bidentate-ligated complex 8
favoured formation of ether products while in complex 3
Scheme 3 Substrate screening for the hydrosilylation of carboxylic
acids catalysed by 3. Reaction conditions: 0.5 mmol substrate, 2 mol%
of 3, 1 mmol of silane, 0.2 mL THF, 80 °C, 2 h. Quantified by 1H NMR
using mesitylene as an internal standard. b2 mmol of silane.
14d, reduced the yield to 12% for 15d. The aliphatic
substrates 14e–h could be converted in good yields (67% and
66% for 14e and 14h, respectively) to excellent yields (95%
and 86% for 14f and 14g, respectively). In addition, oxalic acid
14i remarkably provided the corresponding diol (15i) with a
high yield of 91%. These results indicate a broad portfolio of
potential target substrates.
On the basis of the experimental observations and
current reports in the literature,7b,15 a tentative mechanism
for the hydrosilylation of the carboxylic CO units using
MnIJI) complexes can be proposed (Scheme 4). In the
bearing
a
tridentate (PNN)-iminotriazole ligand lead
preferentially to alcohols. Most significantly, complex 3
provides the first example for effective manganeseIJI) catalysed
hydrosilylation of carboxylic acids to alcohols, we believe that
the insights provided herein will encourage further
investigations into the dynamic field of manganeseIJI)
catalysis in the prospect of finding greener alternatives to
existing catalytic methodologies.
presence of phenylsilane,
a catalytically active neutral
Scheme 4 A tentative reaction mechanism for MnIJI)-catalysed hydrosilylation of carboxylic groups in acids or esters.
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Experimental section
General procedure for the catalytic hydrosilylation of
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carbonyl and carboxyl substrates
Selected ketone/ester/acid (0.5 mmol), phenylsilane (0.5 mmol
for ketone and 1–2 mmol for ester/acid), and mesitylene or
tetradecane (0.5 mmol) were added to a stock solution (0.2 mL)
of the MnIJI) catalyst. The reaction mixture was stirred at 80 °C
for the required time (1 to 3 h). After this time, the reaction
was cooled to room temperature and the corresponding
hydrolysis was performed (see ESI†). After hydrolysis, the
1
sample was diluted with CDCl3 (0.6 mL), and subjected to H-
NMR spectroscopy to determine the yield of the product.
Conflicts of interest
There are no conflicts to declare.
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We gratefully acknowledge generous financial support by the Max
Planck Society and the RWTH Aachen University. We thank all
analytical departments involved in this project for their excellent
support. This work was supported by the Bundesministerium für
Bildung und Forschung as part of the project “MANGAN”
(03SF0508). Additional support by the Cluster of Excellence
“The Fuel Science Center” is gratefully acknowledged. Open
Access funding provided by the Max Planck Society.
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