Manganese N-Heterocyclic Carbene Complexes for Catalytic Reduction of Ketones with Silanes

Article abstract of DOI:10.1002/cctc.201800241

Well-defined manganese(I) carbonyl complexes bearing bis-N-heterocyclic carbene (NHC) ligands are shown to be effective catalysts for the reduction of carbonyl groups through hydrosilylation reactions. A wide variety of ketones are selectively reduced to the corresponding alcohols by using phenylsilane, and the cheap and readily abundant polymethylhydrosiloxane (PMHS) in the presence of catalytic amounts of Mn^I organometallic complexes. Interestingly, α,β-unsaturated ketones and dialkyl ketones are selectively reduced. Mechanistic studies based on radical scavengers suggest the involvement of radical species in the catalytic reaction.

Full text of DOI:10.1002/cctc.201800241

Accepted Article
Title: Manganese N-Heterocyclic Carbene Complexes for Catalytic
Reduction of Ketones with Silanes
Authors: Mara Pinto, Beatriz Royo, Sofia Friães, Federico Franco, and
Julio Lloret-Fillol
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10.1002/cctc.201800241
ChemCatChem
FULL PAPER
Manganese N-Heterocyclic Carbene Complexes for Catalytic
Reduction of Ketones with Silanes
Mara Pinto,[a] S. Friães,[a] Federico Franco,[b] J. LLoret-Fillol,[b,c] Beatriz Royo*[a]
Abstract: Well-defined manganese(I) carbonyl complexes bearing
bis-N-heterocyclic carbene ligands are shown to be effective
catalysts for the reduction of carbonyl groups through hydrosilylation
reactions. A wide variety of ketones are selectively reduced to the
corresponding alcohols using phenylsilane, and the cheap and
readily abundant polymethylhydrosiloxane (PMHS) in the presence
of catalytic amounts of MnI organometallic complexes. Interestingly,
α,β-unsaturated ketones and dialkyl ketones are selectively reduced.
Mechanistic studies based on radical scavengers suggest the
involvement of radical species in the catalytic reaction.
in hydrosilylation reactions, the only examples of well-defined
Mn-NHC complexes were recently reported by Lugan, Sortais
and co-workers.[5g,5h]
In line with our interest in developing the organometallic
chemistry and catalysis of 3d metals with NHC ligands,[12] we
decided to develop a new family of manganese carbonyl
complexes bearing bis-NHCs and explore their potential in
catalysis. In a recent work, we demonstrated the high catalytic
efficiency of the manganese complex MnBr(CO)3(bis-NHCMe) for
the electrocatalytic reduction of CO2 to CO.[13] Here, we report
our studies on the reduction of ketones using manganese(I)
compounds bearing bis-NHC, mixed NHC-pyridyl, and bypiridyl
ligands, aiming to explore the effect of the introduction of NHCs
in the first coordination sphere of metal.
Introduction
In recent years, the development of catalysts based on
abundant first-row transition metals has become a central topic
in catalysis.[1] Among 3d metals, iron and manganese are
particularly attractive candidates for catalysis due to their natural
abundance, and their unique features of being non-toxic and
biocompatible metals. In comparison with the spectacular growth
of catalysis based on iron,[2] manganese has remained relatively
unexploited. However, the last few years have witnessed a
remarkable growth of interest in manganese-based catalysis.[3,4]
In particular, a number of efficient manganese catalysts have
been reported for the reduction of carbonyl compounds through
hydrosilylation reactions.[4,5] The hydrosilylation reaction
represents a fundamental method for the preparation of silyl
ethers, and can be employed as a convenient alternative to
hydrogenation when combined with a hydrolysis protocol.[6] So
far, the majority of the Mn complexes developed for catalytic
reduction of carbonyl groups are coordination compounds
bearing N- , O-, and P- ligands; [3a,4,5,7-9] pure organometallic Mn-
based catalysts are scarce.[3d,3f]
Results and Discussion
The synthesis of the manganese bis-NHC complexes fac-
[Mn(bis-NHCR)(CO)3Br] [R = Me (1); Mes (2)] was achieved by
treatment of Mn(CO)5Br with the corresponding imidazolium
salts in the presence of potassium tert-butoxide (Schemes 1-2).
Complexes 1 and 2 are air stable; they can be stored in air for
weeks without noticeable decomposition. Complexes [M(NHC-
py)(CO)3Br] (3) and [Mn(bpy)(CO)3Br] (4) were prepared
following the procedure reported in the literature.[14,15] The
identity of all complexes was established by analytical and
spectroscopic methods. The synthesis and characterisation of
complex 1, including the X-ray diffraction studies, has been
recently described by us.[13] The 13C NMR of 2 shows a
resonance at 191 ppm, indicative of
a Mn-Ccarbene bond,
confirming that coordination of the NHC has occurred. The IR
spectrum of 2 displays the characteristic carbonyl stretching
bands (at 2007, 1923, and 1887 cm-1) for three CO ligands
coordinated to Mn in a facial fashion, two carbonyls located
trans to the NHC ligands and the third CO situated trans to the
bromide ligand.[16] As expected, complex 2 exhibited lower
energy carbonyl bands than those of 3 and 4, consistent with the
expected effect of the strong donating NHC ligand versus the
mixed pyridyl-NHC and the bipyridyl ligand. In addition, the
weaker donating character of the NHC bearing a mesityl wingtip
in 2 compared to the methyl group in 1 is reflected in their IR
spectra (2004, 1912, and 1881 cm-1).
N-Heterocyclic carbenes (NHCs) are a fundamental class of
ligands in organometallic chemistry and catalysis. It is well
recognised that their use as supporting ligands in transition
metal chemistry can provide metal complexes with enhanced
catalytic performances and higher stability.[10] However, until now,
catalysis with Mn-NHC has been poorly explored.[11] In particular,
[a]
M. Pinto, S. Friães, Dr. B. Royo
ITQB NOVA, Instituto de Tecnologia Química e Biológica António
Xavier
Av. da República, 2780-157 Oeiras, Portugal
E-mail: broyo@itqb.unl.pt
[b]
[c]
Dr. F. Franco, Dr. J. Lloret-Fillol
Institute of Chemical Research of Catalonia (ICIQ), The Barcelona
Institute of Science and Technology, Avinguda Països Catalans 16,
43007 Tarragona, Spain
Dr. J. Lloret-Fillol
Catalan Institution for Research and Advances Studies (ICREA),
Passeig Lluïs Companys 23, 08010 Barcelona, Spain
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10.1002/cctc.201800241
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Me
Mes
Br
identical conditions did not yield any detectable conversion of
benzaldehyde (entry 9, Table 1).
N
N
Br
CO
CO
CO
CO
N
N
N
Mn
Mn
CO
Table 1. Hydrosilylation of benzaldehyde catalysed by complexes 1-4.[a]
N
CO
N
N
Entry
Catalyst
t (h)
6
Yield (%)[b]
Me
Mes
1
2
1
2
3
4
5
6
7
8
9
78
98
62
86
56
96
31
53
0
[BF4]
1
Me
N
Br
Br
N
16
6
NCMe
CO
CO
CO
N
N
CO
CO
N
Mn
N
N
Mn
Mn
N
2
3
CO
CO
CO
16
6
CO
Me
3
N
4
5
16
6
Scheme 1. Manganese NHC complexes 1-5 explored as catalysts in the
hydrosilylation of carbonyl groups.
4
24
24
-----
[a] All reactions were carried out with 1 mmol of benzaldehyde and 1.2
mmol of phenylsilane using 1 mol% of catalysts 1-4 at 80 °C in benzene
(0.4 mL). [b] Yields were determined by 1H-NMR spectroscopy using n-
tetradecane as internal standard after basic hydrolysis.
R
N
Br
2Br-
CO
N
N
R
N
MnBr(CO)5/KOBut
THF, reflux, 16 h
Mn
R
CO
N
N
CO
R
Encouraged by the efficiency of complex 1 in the hydrosilylation
of benzaldehyde, we decided to investigate its catalytic activity
in the reduction of acetophenone and evaluate the effect of the
solvent in the catalysis. Full conversion of acetophenone was
observed using 1 and PhSiH3 in benzene after 20 h at 80 °C
(entry 1, Table 2). Interestingly, the reaction proceeded similarly
in THF and neat conditions (entries 2 and 3, Table 2), but a
dramatic increase in the reaction rate was observed when the
reaction was performed in acetonitrile, affording quantitative
yield in 5 h. Attempts to reduce the temperature of the reaction
and catalyst loading resulted in a decrease in the yield of 1-
phenylethanol (53% yield at 60 °C, and 37% yield using 0.5
mol% of 1 in 5 h).
N
N
R = Me (1), Mes (2)
Scheme 2. Synthesis of manganese NHC complexes 1 and 2.
The hydrosilylation of benzaldehyde was selected as
a
benchmark reaction for an initial screening of the MnI catalysts
1-4 using phenylsilane as reducing agent. Initially, reactions
were carried out in benzene at 80 °C using 1 mol% of catalyst.
Conversion of benzaldehyde to the corresponding silyl ether was
monitored by 1H-NMR. When the reaction was completed,
hydrolysis with aqueous NaOH (2 M) solutions, followed by
extraction with dichloromethane allowed the isolation of the
corresponding alcohols. Results are summarised in Table 1.
Excellent aldehyde conversions were achieved for complexes 1-
3 bearing NHC ligands (entries 2, 4, 6, Table 1). Interestingly,
the presence of the strong donating NHC ligands improves the
catalytic efficiency (1-3 versus 4). Complex 4, bearing a bipyridyl
ligand resulted in the least active catalyst affording a moderate
yield of benzyl alcohol (53%) after 24 h (entry 8, Table 1).
Complexes 1-3 displayed comparable activity achieving high
yield of benzyl alcohol in 16 h; although a slight decrease in
activity in the series 1 > 2 > 3 was observed after 6 h of reaction
(entries 1, 3, 5, 7, Table 1). These results suggest that
Table 2. Hydrosilylation of acetophenone catalysed by 1.[a]
Entry
Solvent
benzene
THF
t (h)
Yield (%)[b]
1
2
3
4
20
20
24
5
93
94
83
99
neat
acetonitrile
[a] All reactions were carried out with 1 mmol of acetophenone and 1.2
mmol of phenylsilane using 1 mol% of catalyst 1 at 80 °C in 0.4 mL of
solvent. [b] Yields were determined by 1H-NMR spectroscopy using n-
tetradecane as internal standard after basic hydrolysis.
increasing
the
donor
properties
of
the
chelating
CNHC,CNHC/CNHC,Nbyp fragment in [LMn(CO)3Br] (L = bis-NHC,
NHC-py) complexes, makes the manganese(I) center more
electron rich, facilitating the activation of silane. A blank
experiment in the absence of a manganese complex under
Taking into consideration our previous results on iron-based
hydrosilylation reactions, in which we observed that addition of
catalytic amounts of silver tetrafluoroborate activated the
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10.1002/cctc.201800241
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catalyst,[12d] we decided to explore the reaction of 1 with AgBF4.
Treatment of 1 with one equivalent of AgBF4 in acetonitrile,
resulted in abstraction of the bromide ligand and formation of the
cationic species [Mn(CO)3(bis-NHCMe)(NCMe)][BF4] (5) (Scheme
3). Complex 5 was isolated as a brown stable solid, which can
be handled in air. As expected, the infrared spectrum of 5 shows
the characteristic pattern of a tricarbonyl complex, displaying the
symmetric CO stretching frequency at 2020 cm-1, at higher value
than that displayed by the neutral complex 1 (2004 cm-1).[13]
[a]
Table 3. Hydrosilylation of ketones catalysed by 1 and 5 with PhSiH3
Yield
(%)[b]
Entry
1
Substrate
Catal.
t (h)
5
O
1
97 (92)
O
O
O
5
1
5
1
2
3
4
5
1
5
1
3
93
[BF4]
Me
Br
Me
NCMe
CO
N
N
97
CO
CO
N
N
AgBF4/NCMe
N
N
Mn
Mn
CO
F3C
CO
Me
CO
Me
N
N
85
1
5
F3C
Scheme 3. Synthesis of the cationic complex 5.
O
96
F
As anticipated, complex 5 displayed higher catalytic activity than
1 in the hydrosilylation of acetophenone using phenylsilane as a
reducing agent. Under similar conditions, quantitative conversion
of acetophenone was obtained after 1 h, while 5 h were needed
for complex 1 to reach full conversion (entries 1 and 2, Table 3).
To determine the highest attainable TOF and TON numbers for
5-catalysed ketone hydrosilylation, the catalyst loading was
reduced to 0.1 mol%, reaching a TOF number of 320 h-1
(calculated at 90 min of reaction, kinetic profile in SI, Figure S8),
and a TON number of 1000 (in acetonitrile at 80 °C). The TON
value is comparable to those obtained by the scarce Mn-NHC
complexes known to catalyse this reaction (TON up 960
reported in the literature for [CpMn(CO)2(NHC)]),[5h] but is lower
than the best performance Mn catalysts reported in the literature
(TOF of up to 76,800 h-1 reported by Trovitch and co-workers).[4]
The scope of the reaction was explored using 1 and 5
complexes. All ketone substrates were converted within 1-5 h
when using 5, while 1 needed slightly longer reaction times (2-8
h), depending on the substrates. Both catalysts 1 and 5 tolerated
fluoro, chloro, and bromo functionalities (entries 3-10, Table 3),
in contrast with other Mn-based catalysts, reported to be inactive
in the presence of chloro- and bromo-substituents.[5g] For the p-
iodo-benzaldehyde, reductive dehalogenation occurred and
formation of 1-phenyethanol was detected by NMR. Ketones
bearing electrondonating substituents such as methoxy, and
methyl groups in the para-position were fully reduced and the
corresponding alcohols obtained in high yields (entries 15-18,
Table 3). Reduction of 4-nitro-acetophenone was also selective,
achieving good yield of the corresponding alcohol after 3-4 h.
Interestingly, both 1 and 5 were capable to reduce aliphatic
ketones, cyclic and lineal, including the selective reduction of
trans-4-phenyl-3-buten-2-one (entries 19-28, Table 3). All yields
were consistently very good to excellent, ranging between 72
and 99% isolated yields. Unfortunately, our catalytic system was
inactive towards the reduction of esters under similar reaction
conditions.
5
1
O
6
7
2
2
97 (93)
96
F
O
Cl
5
O
8
2
95
Cl
Br
Br
I
O
1
5
1
5
9
8
5
78
90
0
O
10
11
12
O
24
24
O
0
I
O
1
5
13
14
4
3
91
83
O2N
O2N
O
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O
1
5
4
3
93
95
in 8 h, using 1 and 5, respectively), achieving moderate to good
yields and selectivities for the reduction of a wide variety of
substrates, including both aromatic and aliphatic ketones.
Results are summarised in Table 4.
15
16
Me
O
Table 4. Hydrosilylation of ketones catalysed by 1 with PMHS[a]
Me
Entry
1
Substrate
Yield (%)[b]
80
1
5
6
3
72
86
O
17
18
O
MeO
MeO
O
O
2
3
88 (82)
57
F3C
1
5
1
5
4
8
72
92 (85)
91
O
19
20
21
O
O
F
O
4
65 (58)
O
Cl
Br
I
5
5
93 (93)
O
O
22
5
6
7
56
0
O
1
5
1
5
2
8
94
99
93
23
24
25
O
O
O
O
O
O
O
75 (72)
O2N
5
1
5
5
8
4
91 (88)
26
27
28
O
8
53
69
Me
89
O
9[c]
MeO
93
10[c]
11
69
65
O
O
[a] All reactions were carried out with 1.0 mmol of substrate and 1.2 mmol of
PhSiH3 in acetonitrile (0.4 mL) at 80 °C and 1 mol% of catalyst. [b] Yields were
determined by 1H-NMR spectroscopy using n-tetradecane as internal standard
after basic hydrolysis. Isolated yields are indicated in parenthesis.
O
12
13
91 (88)
63
O
We have also explored the applicability of the method using
other silanes, such as Ph2SiH2 and Ph3SiH. Both silanes
afforded the reduction of acetophenone in good yields, 94% and
89%, respectively in 8 h in the presence of catalyst 5.
Gratifyingly, we observed that hydrosilylation could be
performed using the cheap, and readily available
polymethylhydrosiloxane (PMHS) as reducing agent. When the
reactions were carried out with PMHS, catalyst 1 displayed
higher activity than 5 (80% versus 64% yield of 1-phenylethanol
64 (58)
O
14
[a] All reactions were carried out with 1.0 mmol of substrate and 1.2 mmol of
PhSiH3 using 1 mol% of 1 in acetonitrile at 80 °C in 8 h. [b] Yields were
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10.1002/cctc.201800241
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determined by 1H-NMR spectroscopy using n-tetradecane as internal standard
after basic hydrolysis. Isolated yields are indicated in parenthesis.
provide the open chain phenyl propyl ketones B and C as final
products (Scheme 4). In our studies with complex 5, the
hydrosilylation reaction of cyclopropyl phenyl ketone afforded,
after hydrolysis, a complex mixture in which the ring-opening
product β-ethyl styrene (C), and cyclopropyl phenyl methanol (A)
were detected in a C:A ratio of 1:2.8 (SI, Figure S9) . The
presence of C, unequivocally indicate the involvement of a
radical pathway. Our findings differ from the Mn-based catalysts
reported in the literature, in which the classical Ojima-like
mechanism has been proposed to be operative.[5a,5d,5e] Trovitch
elegantly proved that the concurrent modified Ojima and Mn-H
insertion mechanisms were responsible for catalysis with a
Mn(0) catalyst.[5a,5d] Identification of Mn-H species was also
achieved by Lugan, Sortais, and co-workers with
CpMn(NHC)(CO)2 systems.[5g,5h] In contrast, a radical transfer
mechanism has been proposed by Trovitch and co-workers with
a Mn complex bearing a redox-active ligand.[5f]
Prompted by the observation that acetonitrile accelerates the
hydrosilylation reaction rate, we decided to investigate the
influence of radical traps on the catalytic rates. The catalytic
reduction of acetophenone with PhSiH3 (1.2 equiv.) using 5 (1
mol%) was performed in the presence of a variety of spin-traps.
Interestingly, the presence of Ph2NH, CBrCl3, and 2,2,6,6-
tetramethylpiperidin N-oxide (TEMPO), radical scavengers of
oxygen- carbon- and silyl-, respectively,[17] strongly inhibited the
reaction. As shown in Table 5, hydrosilylation of acetophenone
in the presence of 1 mol% of Ph2NH, CBrCl3, and TEMPO
afforded only 11, 14, and 26% conversion in 1 h, respectively,
while in the absence of radical scavengers full conversion was
obtained (entries 1, 4, 7 and 10, Table 5). However, the reaction
resumed and reached quantitative conversions after 8 h when
TEMPO and Ph2NH were present, and it afforded 44% of
conversion in the presence of CBrCl3. These observations
indicated the involvement of radical species as intermediates in
the catalytic reaction.
Heterolytic pathway
OH
A
O
[Cat]
Ph
Ph
PhSiH3
Table 5. Effect of addition of spin-traps in the hydrosilylation of
acetophenone catalysed by 5.[a]
Entry
Spin-trap
t (h)
Yield (%)[b]
Homolytic pathway
O
O
1
2
1
3
11
30
[Cat]
+
Ph
Ph
Ph
PhSiH3
Ph2NH
C
B
3
8
>99
14
Scheme 4. Heterolytic and homolytic pathways for the reduction of cyclopropyl
phenyl ketone with phenylsilane.
4
1
CBrCl3
5
3
28
6
24
1
44
Further experiments were performed in order to get an insight
into the mode of activation of the Mn-NHC catalyst. The
stoichiometric reaction of 5 and acetophenone in a molar ratio
1:1 did not display any observable change neither by NMR nor
by UV-Vis spectroscopy. Treatment of complex 5 with two
equivalents of PhSiH3 in acetonitrile at 80 °C during 2 h,
produced a rapid color change from brownish to grey. Formation
of a new Mn complex featuring a 1H NMR signal at -6.8 ppm was
detected (SI, Figure S10). The signal at -6.8 ppm could be
indicative of the formation of Mn-H or Mn(η2-H-SiH2Ph)
species.[5g,5h] In an attempt to characterise Mn-H species, the
reaction of 5 with LiEt3BH4 was performed without success.
Finally, the stoichiometric reaction of 5 with phenylsilane in THF
was monitored in a high-pressure sealed reactor by FTIR
spectroscopy. The temperature of the reactor was controlled
externally with a thermocouple and progressively increased. At
room temperature (20 °C), the typical CO stretches of 5 at 2025,
1935, 1925 cm-1 are observed in the spectrum, together with the
νSi-H band at 2158 cm-1 and a new set of signals (2007, 1925,
1879 cm-1), most likely corresponding to a new carbonyl Mn
species. Increasing the temperature favors the formation of the
new species at the expense of 5 being predominant at 80 °C.
7
26
TEMPO
-----
8
3
64
9
8
93
10
1
>99
[a] All reactions were carried out with 1 mmol of acetophenone and 1.2
mmol of phenylsilane using 1 mol% of catalyst 5, and 1 mol% of radical
scavenger at 80 °C in acetonitrile (0.4 mL). [b] Yields were determined by
1H-NMR spectroscopy using n-tetradecane as internal standard after basic
hydrolysis.
In addition to the spin trap experiments, cyclopropyl phenyl
ketone was employed as a mechanistic probe of intermediate
radical species involvement. In fact, formation of the cyclopropyl
ring-opening products would indicate a radical mechanism,
allowing to differentiate between heterolytic and homolytic
pathways for reduction.[18] If cyclopropyl phenyl ketone is
reduced by the hydride transfer mechanism, it yields
phenylcyclopropylcarbonol A (Scheme 4), while the homolytic
pathway, based on the formation of a ketyl intermediate, would
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10.1002/cctc.201800241
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12 Hz, 1H, NCH2N), 2.28 (s, 6H, CH3), 2.07 (s, 6H, CH3), 1.82 (s, 6H,
CH3). 13C NMR (DMSO-d6, 25 °C): δ 223.99 (CO), 217.91 (CO), 191.63
(Mn-Ccarbene), 138.32 (CHMes), 136.64 (CHMes), 136.00 (CHMes), 135.16
(CHMes), 128.69 (CHimid), 128.50 (CHimid), 123.33 (CHimid), 123.04 (CHimid),
61.79 (NCH2N), 20.64 (CH3), 18.03 (CH3), 17.46 (CH3). IR (KBr): υ(CO)
2007 (s), 1923 (s), 1887 (s) cm-1. HRMS ESI-MS (positive mode): 523
[M-Br]+. Anal. Calcd. for C28H28MnBrN4O3Br: C, 55.73; H, 4.68; N, 9.28.
Found: C, 55.40; H, 4.61; N, 8.90.
The new compound evidence the typical pattern of a facial
tricarbonyl compound without loss of any CO ligand. This finding
contrasts with CpMn(NHC)(CO)2 compounds, in which
dissociation of a CO ligand precedes activation of silane.[5g,5h]
Conclusions
Preparation and characterisation of 5
In conclusion, we have reported Mn-NHC complexes for the
effective catalytic reduction of ketones groups using silanes as
reducing agents, including polymethylhydrosiloxane, which is a
cheap, easy to handle, and environmentally friendly by-product
of the silicone industry. Interestingly, the Mn-NHC complexes
are air stable and catalytic experiments are performed under
atmospheric air. The reduction reaction is compatible with
aromatic and aliphatic ketones bearing a wide variety of
functional groups. We have demonstrated that the presence of
NHC ligands in the coordination sphere of Mn produces an
enhancement of the catalyst performance. It is expected that the
enormous flexibility of NHC ligands will allow a significant
optimisation of these promising catalysts. Ongoing studies in our
group are focusing on the synthesis of Mn-NHC compounds with
improved catalytic efficiencies, and further development of the
methodology to other applications.
To a solution of 1 (0.26 g, 0.66 mmol) in NCMe (15 mL) was added
AgBF4 (0.14 g, 0.73 mmol), and the reaction mixture was stirred at room
temperature overnight. The solution was filtered, and the filtrate was
concentrated to dryness under vacuum. The residue was washed with
diethyl ether and dried under vacuum to afford 0.20 g (72% yield) of
crystalline brown solid. 1H NMR (CD3CN, 25 °C): δ 7.41 (s, 2H, CHimid),
7.21 (s, 2H, CHimid), 6.11 (d, 2JH-H = 12 Hz, 1H, NCH2N), 6.01 (d, 2JH-H
=
12 Hz, 1H, NCH2N), 3.93 (s, 6H, NCH3). 13C NMR (CD3CN, 25 °C): δ
220.99 (CO), 218.29 (CO), 188.28 (Mn-Ccarbene), 124.93 (CHimid), 123.13
(CHimid), 63.07 (NCH2N), 38.37 (NCH3). IR (KBr): υ (CO) 2020 (s), 1928
(s) cm-1. HRMS ESI-MS (positive mode): 356.0 [M-BF4]+ and 315.0 [M-
NCMe-BF4]+. Anal. Calcd. for C14H15MnN5O3: C, 37.95; H, 3.41; N, 15.81.
Found: C, 37.81; H, 3.21; N, 15.90.
Typical procedure for the hydrosilylation of benzaldehyde and
ketones catalysed by complexes 1-5.
An open-air flask was charged with catalyst (1 mol%) and 0.4 mL solvent.
Then, silane (1.2 equiv.) and the corresponding substrate (1 mmol) were
added. The reaction mixture was stirred and heated at 80 °C. The
progress of the reactions was monitored by taking aliquots of the reaction
mixtures and subjecting them to 1H NMR in chloroform-d. Conversion
was determined by 1H NMR using n-tetradecane as an internal standard.
At the end of the reaction, MeOH (1 mL) and 2 M NaOH (10 mL) were
added consecutively under vigorous stirring. The reaction mixture was
stirred overnight at room temperature and was extracted with
dichloromethane (3 x 10 mL). The combined organic layers were dried
over anhydrous Na2SO4, filtered and concentrated under vacuum to
afford the isolated product. For some representative samples, the final
products were isolated following same experimental procedure (reactions
were performed in the absence of internal standard), dried and weight to
calculate the isolated yields.
Experimental Section
General Methods
The synthesis of manganese complexes was carried out under a nitrogen
atmosphere using standard Schlenk techniques. Solvents were purified
from appropriate drying agents and distilled under nitrogen. Deuterated
solvents were degassed and stored over molecular sieves. The
imidazolium salt 1,1′-methylene-3,3′-di-mesitylimidazolium dibromide,[19]
and the manganese complexes [Mn(bpy)(CO)3Br],[15] [Mn(NHC-
py)(CO)3Br],[14] and [Mn(bis-NHCMe)(CO)3Br][13] were synthesised
according to the methods described in the literature. All other reagentes
were purchased from commercial suppliers and used without further
purification. 1H and 13C NMR spectra were recorded on a Bruker Avance
III 400 MHz. Assignment of resonances was made from HMQC and
HMBC experiments. Electrospray mass spectra (ESI-MS) were recorded
on a Micromass Quatro LC instrument; nitrogen was employed as a
drying and nebulising gas. Elemental analyses were performed in our
laboratories at ITQB.
Acknowledgements
We would like to thank FCT from Portugal for projects
UID/Multi/04551/2013, and RECI/BBB-BQB/0230/2012 (NMR
facilities), and for grant PD/BD/105994/2014 (M.P.) and contract
IF/00346/2013 (B.R). We also thank UniMS-Mass Spectrometry
Unit, ITQB/IBET. The NMR spectrometers are part of the
national NMR facility supported by FCT (RECI/BBB-
BQB/0230/2012). We also thank CERCA Programme
(Generalitat de Catalunya), the MINECO CTQ2016-80038-R
project, ICIQ-IPMP program (F.F.) and Severo Ochoa
Excellence Accreditation 2014–2018 (SEV-2013-0319).
Preparation and chacterisation of 2
Solid MnBr(CO)5 (0.19 g, 0.7 mmol) was suspended in THF (15 mL) and
potassium tert-butoxide (0.14 g, 1.2 mmol) was added first, followed by
addition of 1,1′-methylene-3,3′-di-mesitylimidazolium dibromide (0.30 g,
0.55 mmol) at 60 °C. The resulting suspension was heated at 60 °C for
16 h under stirring. All volatiles were removed under vacuum and the
resulting residue was washed with Et2O (4 x 15 mL), and dissolved in
dichloromethane (80 mL). The dichloromethane solution was washed
with water (80 mL), and the organic extract was dried with Na2SO4. The
solution was filtered and concentrated to dryness under vacuum to yield
a yellow crystalline powder. Yield: 0.15 g (45 % yield). 1H NMR (DMSO-
d6, 25 °C): δ 7.83 (s, 2H, CHimid), 7.36 (s, 2H, CHimid), 7.01 (s, 2H, CHPh),
Keywords: manganese• N-heterocyclic carbenes •
hydrosilylation • reduction carbonyls
7.01 (s, 2H, CHPh), 6.78 (d, 2JH-H = 12 Hz, 1H, NCH2N), 6.43, (d, 2JH-H
=
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10.1002/cctc.201800241
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FULL PAPER
[1]
a) R. M. Bullock in Catalysis without precious metals, (Eds.: H. Baltes,
Junge, S. Walker, Z. Wei, H. Jiao, A. Spanneberg, S. Bachmann, M.
Scalone, M. Beller, Angew. Chem. Int. Ed. 2017, 56, 11237-11241.
Mn in transfer hydrogenation reactions: a) M. B. Widegren, G. J.
Harkness, A. M. Z Slawin, D. B. Cordes, M. L. Clarke, Angew. Chem.
Int. Ed. 2017, 56, 5825-58284; b) A. Zirakzadeh, S. R. M. M. de Aguiar,
B. Stoger, M. Widhalm, K. Kirchner, ChemCatChem 2017, 9, 1744-
1748; c) M. Perez, S. Elangovan, A. Spannenberg, K. Junge, M. Beller,
ChemSusChem 2017, 10, 83-86; d) A. Bruneau-Voisine, D. Wang, V.
Dorcet, T. Roisnel, C. Darcel, J.-B. Sortais, Org. Lett. 2017, 19, 3656-
3659.
W. Göpel, J. Hesse), Wiley-VCH, Weinheim, 2010; b) V. Charra, P. de
Frémont, P. Braunstein, Coord. Chem. Rev. 2017, 341, 53-176; c) P. J.
Chirik, T. B. Gunnoe, ACS Catal. 2015, 5, 5584-5585; d) M. Albrecht, R.
Bedford, B. Plietker, Organometallics 2014, 33, 5619-5621.
[8]
[2]
a) S. Z. Tasker, E. A. Standley, T. F. Jamison, Nature 2014, 509, 299-
309; b) R. Shang, L. Ilies, E. Nakamura, Chem. Rev. 2017, 117, 9086-
9139; c) A. Fürstner , ACS Cent. Sci. 2017, 2, 778-789; d) I. Bauer, H.-
J. Knölker, Chem. Rev. 2015, 115, 3170-3387; d) C. Johnson, M.
Albrecht, Coord. Chem. Rev. 2017, 352, 1-14; e) K. Riener, S.
Haslinger, A. Raba, M. P. Högerl, M. Cokoja, W. A. Herrmann, F. E.
Kühn, Chem. Rev. 2014, 114, 5215-5272; f) D. Bézier, J.-B. Sortais, C.
Darcel, Adv. Synth. Catal. 2013, 355, 19-33; g) M. J. Ingleson, R. A.
Layfield, Chem. Commun. 2012, 48, 3579-3589; h) R. Lopes, B. Royo,
Isr. J. Chem. 2017, 57, 1151-1159.
[9]
Mn in hydroboration reactions: a) G. Zhang, H. Zeng, J. Wu, Z. Yin, S.
Zheng, J. C. Fettinger, Angew. Chem. Int. Ed. 2016, 55, 14369-14369;
b) V. Vasilenko, C. K. Blasius, H. Wadepohl, L. H. Gade, Angew. Chem.
Int. Ed. 2017, 56, 8393-8397.
[10] a) F. E. Hahn, Angew. Chem. Int. Ed. 2006, 45, 1348-1352; b) E. Peris,
Chem. Rev. 2018, DOI:10.1021/acs.chemrev.6b00695; c) M. N.
Hopkinson, C. Richter, M. Schedler, F. Glorius, Nature 2014, 510, 485-
496; d) H. D. Velazquez, F. Verpoort, Chem. Soc. Rev. 2012, 41, 7032-
7060; e) D. Martin, M. Soleilhavoup, G. Bertrand, Chem. Sci. 2011, 2,
389-399; f) S. Díez-González, N. Marion, S P. Nolan, Chem. Rev. 2009,
109, 3612-3676.
[3]
a) B. Maji, M. K. Barman, Synthesis 2017, 49, 3377-3393; b) W. Liu, L.
Ackermann, ACS Catal. 2016, 6, 3743-3752; c) R. Carney, B. R. Dillon,
S. P. Thomas, Eur. J. Org. Chem. 2016, 3912-3929; d) D. A. Valyaev,
G. Lavigne, N. Lugan, Coord. Chem. Rev. 2016, 308, 191-235; e) R. I.
Khusntdinov, A. R. Bayguzina, U. M. Dzhemilev, Russ. J. Org. Chem.
2012, 48, 309-348; f) R. A. Layfield, Chem. Soc. Rev. 2008, 37, 1098-
1107.
[11] a) S. J. Hock, L.-A. Schaper, W. A. Hermann, F. E. Kühn, Chem. Soc.
Rev. 2013, 42, 5073-5089; b) S. Bellemin-Laponnaz, S. Dagorne,
Chem. Rev. 2014, 114, 8747-8774.
[4]
[5]
a) R. J. Trovitch, Acc. Chem. Res. 2017, 50, 2842-2852; b) R. J.
Trovitch, Synlett 2014, 25, 1638-1642.
a) T. K. Mukhopadhyay, C. Ghosh, M. Flores, T. L. Groy, R. J. Trovitch.
Organometallics 2017, 36, 3477-3483; b) C. M. Kelly, R. McDonald, O.
L. Sydora, M. Stradiotto, L. Turculet, Angew. Chem. Int. Ed. 2017, 56,
15901-15094; c) S. Vijamarri, V. K. Chidara, J. Rousova, G. Du, Catal.
Sci. Technol. 2016, 6, 3886-3892; d) T. K. Mukhopadhyay, C. L. Rock,
M. Hong, D. C. Ashley, T. L. Groy, M.-H. Baik, R. J. Trovitch. J. Am.
Chem. Soc. 2017, 139, 490-4915; e) V. K. Chidara, G. Du,
Organometallics 2013, 32, 5034-5037; f) T. K. Mukhopadhyay, M.
Flores, T. L. Groy, R. J. Trovitch. J. Am. Chem. Soc. 2014, 136, 882-
885; g) J. Zheng, S. Elangovan, D. A. Valyaev, R. Brousses, V. César,
J.-B. Sortais, C. Darcel, N. Lugan, G. Lavigne, Adv. Synth. Catal. 2014,
356, 1093-1097; h) D. A. Valyaev, D. Wei, S. Elangovan, M. Cavailles,
V. Dorcet, J.-B. Sortais, C. Darcel, N. Lugan, Organometallics 2016, 35,
4090-4098; i) J. Zheng, S. Chevance, J C. Darcel, J-B. Sortais, Chem.
Commun. 2013, 49, 10010-10012; j) C. Ghosh, T. K. Mukhopadhyay, M.
Flores, T. L. Groy, R. J. Trovitch. Inorg. Chem. 2015, 54, 10398-10406;
k) S. Vijamarri, V. K. Chidara, G. Du, ACS Omega, 2017, 2, 582-591; l)
S. U. Son, S.-J. Paik, I. S. Lee, Y.-A. Lee, Y. K. Chung, W. K. Seok, H.
N. Lee, Organometallics 1999, 18, 4114-4118; m) S. U. Son, S.-J. Paik,
Y. K. Chung, J. Mol. Catal A: Chem. 2000, 151, 87-90.
[12] a) V. V. K. M. Kandepi, J. M. S. Cardoso, E. Peris, B. Royo,
Organometallics 2010, 29, 2777-2782; b) J. M. S. Cardoso, B. Royo,
Chem. Commun. 2012, 48, 4944-4946; c) L. Postigo, B. Royo, Adv.
Synth. Catal. 2012, 354, 2613-2618; d) R. Lopes, J. M. S. Cardoso, L.
Postigo, B. Royo, Catal. Lett. 2013, 143, 1061-1066; e) S. Warratz, L.
Postigo, B. Royo, Organometallics, 2013, 32, 893-897; f) J. M. S.
Cardoso, A. Fernandes, B. de P. Cardoso, M. D. Carvalho, L. P.
Ferreira, M. J. C. Calhorda, B. Royo, Organometallics 2014, 33, 5670-
5677; g) L. Postigo, R. Lopes, B. Royo, Dalton Trans. 2014, 43, 853-
858; h) J. M. S. Cardoso, R. Lopes, B. Royo, J. Organomet. Chem.
2015, 775, 173-177; i) B. de P. Cardoso, B. Royo, M. J. C. Calhorda, J.
Organomet. Chem. 2015, 792, 167-176; j) S. Pottabathula, B. Royo,
Tetrahedron Lett. 2012, 53, 5156-5158; k) M. Pinto, B. de P. Cardoso,
S. Barroso, A. M. Martins, B. Royo, Dalton Trans. 2016, 45, 13541-
13546; l) R. Lopes, M. Pereira, B. Royo, ChemCatChem. 2017, 9,
3073-3077.
[13] F. Franco, M. Pinto, B. Royo, J. Lloret-Fillol, Angew. Chem. Int. Ed.
2018, DOI: 10.1002/anie.201800705.
[14] J. Agarwal, T. W. Shaw, C. J. Stanton III, G. F. Majetich, A. B. Bocarsly,
H. F. Schaefer III, Angew. Chem. Int. Ed. 2014, 53, 5152.
[6]
[7]
Reduction (Chapter 8) in Comprehensive Organic Synthesis: Selectivity,
Strategy Efficiency in Modern Organic Chemistry 1991. Ed B. M. Trost,
I. Fleming, Elsevier, Pergamon Press.
[15] L. H. Staal, A. Oskam, K. Vrieze, J. Organomet. Chem. 1979, 170, 235-
245.
[16] a) O. Hiltner, F. J. Boch, L. Brewitz, P. Härter, M. Drees, E. Herdtweck,
W. A. Herrmann, F. E. Kühn, Eur. J. Inorg. Chem. 2010, 5284-5293; b)
V. Yempally, S. J. Kyran, R. K. Raju, W. Y. Fan, E. N. Brothers, D. J.
Darensbourg, A. A. Bengali, Inorg. Chem. 2014, 53, 4081-4088; c) V.
Yempally, S. Moncho, F. Hasanayn, W. Y. Fan, E. N. Brothers, A. A.
Bengali, Inorg. Chem. 2017, 56, 11254-11253; d) T. A. Martin, C. E.
Elul, M. F. Mahon, M. E. Warren, D. Allan, M. K. Whittlesey,
Organometallics 2011, 30, 2200-2211.
Selected examples of Mn in hydrogenation reactions: a) M. B.
Widegren, G. J. Harkness, A. M. Z. Slawin, D. B. Cordes, M. L. Clarke,
Angew. Int. Ed. 2017, 56, 5825-5828; b) S. Elangovan, C. Topf, S.
Fischer, H. Jiao, A. Spannenberg, W. Baumann, R. Ludwig, K. Junge,
M. Beller, J. Am. Chem. Soc. 2016, 138, 8809-8814; c) S. Elangovan,
M. Garbe, H. Jiao, A. Spannenberg, R. Ludwig, K. Junge, M. Beller,
Angew. Chem. Int. Ed. 2016, 55, 15364-15368; d) F. Kallmeier, T.
Irrgang, T. Dietel, R. Kempe, Angew. Chem. Int. Ed. 2016, 55, 11806-
11809; e) C. M. Kelly, R. McDonald, O. L. Sydora, M. Stradiotto, L.
Turculet, Angew.Chem. Int. Ed. 2017, 56, 15901-15904; f) F. Kallmeier,
T. Irrgang, T. Dietel, R. Kempe, Angew. Chem. Int. Ed. 2016, 55,
11806-7534; g) R. van Putten, E. A. Uslamin, M. Garbe, C. Liu, A.
Gonzalez-de-Castro, M. Lutz, K. Junge, E. J. M. Hensen, M. Beller, L.
Lefort, E. A. Pidko, Angew. Chem. Int. Ed. 2017, 56, 7531-7534; h) F.
Bertini, M. Glatz, N. Gorgas, B. Stoger, M. Peruzzini, L. F. Veiros, K.
Kirchner, L. Gonsalvi, Chem. Sci. 2017, 8, 5024-5029; h) M. Garbe, K.
[17] a) P. A. McFaul, D. D. M. Wayner, K. U. Ingold, Acc Chem. Res. 1998,
31, 159; b) S. V. Dvinskikh, A. V. Yurkovskaya and H.-M. Vieth, J. Phys.
Chem. 1996, 100, 8125–8130; c) Organosilanes in Radical Chemistry,
Ed. C. Chryssostomos, 2004, John Wiley & Sons, Ltd
[18] a) D. D. Tanner, G. E. Diaz, A. Potter, J. Org. Chem. 1985, 50, 2149-
2154; b) D. Yang, D. D. Tanner, J. Org. Chem. 1986, 5, 2267-2270; c)
S. Abbina, S. Bian, C. Oian, G. Du, ACS Catal. 2013, 3, 678-684.
[19]
F. A. Westerhaus, B. Wendt, A. Dumrateh, G. Wienhöfer, K. Junge, M.
Beller, ChemSusChem 2013, 6, 1001-1005.
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Entry for the Table of Contents (Please choose one layout)
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O
[Mn]
OH
Well-defined Mn(I)-NHC complexes
displayed excellent catalytic activity in
the reduction of ketones with silanes.
High yields and selectivities were
obtained for a wide range of ketones
using phenylsilane and the cheap and
readily available
Mara Pinto, Sofia Friães, Federico
Franco, Julio LLoret-Fillol, Beatriz Royo*
R1
R2
R1
i) PMHS
ii) H+
R2
Page No. – Page No.
Me
Manganese N-heterocyclic carbene
complexes for catalytic reduction of
ketones with silanes
N
Br
CO
CO
N
N
Mn
polymethylhydrosiloxane as reducing
agents.
CO
Me
N
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