A Manganese N-Heterocyclic Carbene Catalyst for Reduction of Sulfoxides with Silanes

Article abstract of DOI:10.1002/cctc.201900662

The first reduction of sulfoxides catalysed by a well-defined manganese complex is described. A variety of sulfoxides are reduced to the corresponding sulfides in high yields using phenylsilane, diphenylsilane, and the economically feasible 1,1,3,3-tetramethyldisiloxane (TMDS) as reducing agents in the presence of a Mn-NHC complex. The reaction is performed under air and without the need of any additive. The involvement of radicals in the catalytic reaction is probed by spin-trap experiments.

Full text of DOI:10.1002/cctc.201900662

Accepted Article
Title: A Manganese N-Heterocyclic Carbene Catalyst for Reduction of
Sulfoxides with Silanes
Authors: Beatriz Royo, Carlos J. Carrasco, Sara C. A. Sousa, and
Mara F. Pinto
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10.1002/cctc.201900662
ChemCatChem
FULL PAPER
A Manganese N-Heterocyclic Carbene Catalyst for Reduction of
Sulfoxides with Silanes
Sara C. A. Sousa,[a]† Carlos J. Carrasco,[a]† Mara F. Pinto,[a] and Beatriz Royo*[a]
Abstract: The first reduction of sulfoxides catalysed by a well-defined
manganese complex is described. A variety of sulfoxides are reduced
to the corresponding sulfides in high yields using phenylsilane,
of means,[14] transition metal-based reductions using silanes
represent an interesting approach.[15] To date, the majority of the
metal/silane catalytic systems relay on expensive metals.[15,16]
Recent efforts have directed to replace those metals for more
sustainable catalysts based on 3d transition metals such as Fe,[17]
Cu,[18] and Zn.[19] Unexpectedly, manganese-catalysed reduction
of sulfoxides has been so far unexplored.
diphenylsilane,
and
the
economically
feasible
1,1,3,3-
tetramethyldisiloxane (TMDS) as reducing agents in the presence of
a Mn-NHC complex. The reaction is performed under air and without
the need of any additive. The involvement of radicals in the catalytic
reaction is probed by spin-trap experiments.
Results and Discussion
Introduction
Initially, the catalytic activity of the manganese complex [Mn(bis-
NHC)(CO)3Br] (1)[9] was studied using methyl phenyl sulfoxide as
a model substrate and phenylsilane as a reductant under solvent-
free conditions, at 100 °C with a catalyst loading of 2 mol%
(Scheme 1). Under these conditions methyl phenyl sulfoxide was
fully converted into the corresponding sulfide in 10 min. Control
experiments confirmed that no reaction took place in the absence
of catalyst.
Economic and environmental pressure for more sustainable
catalysis has provided a driving force for the incorporation of new
catalysts based on cheap and abundant metals.[1] Among them,
manganese represents an attractive candidate for catalysis due
to its nontoxic nature and natural abundance, since it is the third
most abundant transition metal in Earth´s crust. In fact, the
development of manganese catalysis has experienced
a
spectacular growth in the last few years.[2] In particular,
manganese-mediated hydrosilylation reactions[3] have proved to
be a convenient method for the reduction of carbonyl groups,
including aldehydes and ketones,[4] esters,[5] amides,[6a] carboxylic
acids,[6b] CO2,[6c] and C=C/C≡C unsaturated bonds.[7] Last year,
we disclosed the excellent reactivity of Mn(I) complexes
supported by bidentate NHC ligands (NHC = N-heterocyclic
carbene) in the catalytic reduction of carbonyl groups.[8,9]
Motivated by these findings, we became interested in further
exploring the potential of [Mn(bis-NHC)(CO)3Br] complexes in
reductions with silanes. Surprisingly, despite the enormous
success of metal NHC complexes in catalysis,[10] very few
catalytic applications of Mn-NHC complexes can be found in the
literature.[4f,4g,8,9,11,12] We report in this work the deoxygenation of
sulfoxides to sulfides with silanes mediated by the manganese
O
[1] (2 mol%)/PhSiH3 (1.2 eqv.)
S
S
neat, 100 0C, 10 min
Me
97% yield
N
Br
CO
N
Mn
CO
N
CO
N
Me
1
Scheme 1. Reduction of methyl phenyl sulfoxide with PhSiH3 catalysed by Mn
complex 1.
complex [Mn(bis-NHC)(CO)3Br] bearing
a bis-N-heterocyclic
carbene ligand. The deoxygenation of sulfur-containing
compounds is a fundamental reaction in organic synthesis and
biochemistry.[13] Moreover, organic sulfides are valuable
intermediates in the synthesis of pharmaceutical, biological, and
natural active molecules. While they may be prepared by a variety
The course of the catalytic reaction was monitored following both
the formation of methyl phenyl sulfide and the consumption of
methyl phenyl sulfoxide by 1H NMR spectroscopy using
diphenylmethane as internal standard. The kinetic profile of the
reaction showed that the reduction reaction is very fast—no
induction period was observed—indicating that the active species
are formed rapidly upon addition of the silane (Figure 1). A
turnover frequency, TOF50 of 788 h-1 was obtained (measured at
2 min of reaction), which represents the highest TOF value
reported so far for a metal-based catalyst, surpassing the activity
of its third row counterpart, rhenium, in reduction of sulfoxides with
silanes.[16a] When the catalyst loading was lowered to 1 mol%, a
significant decrease in its activity was observed (Table 1, entry 2,
Figure S1). In addition, the reaction temperature plays an
[a]
Dr. S. C. A. Sousa, Dr. C. J. Carrasco, M. F. Pinto, Dr. B. Royo
ITQB NOVA, Instituto de Tecnologia Química e Biológica,
Universidade Nova de Lisboa
Av. da República, 2780-157 Oeiras, Portugal
E-mail: broyo@itqb.unl.pt
†Dr. S. C. A. Sousa and Dr. C. J. Carrasco contributed equally.
Supporting information for this article is given via a link at the end of
the document.
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10.1002/cctc.201900662
ChemCatChem
FULL PAPER
important role in the catalytic reaction. The activity of 1 is
substantially reduced when the reaction was performed under
100 °C; at 90 °C, the conversion yield drops to 75% in 16 h (Table
1, entry 3, Figure S6). No reaction occurred at ambient
temperature. Thus, the optimal reaction conditions were proposed
to be 2 mol% of catalyst 1 in neat conditions at 100 °C.
The scope and limitations of the catalytic reaction was studied for
a variety of sulfoxides, including aryl-alkyl, aryl-aryl, and alkyl-
alkyl sulfoxides under the optimal reaction conditions (2 mol% of
1, 1.2 eqv. of PhSiH3, 100 °C in neat conditions or in toluene,
depending on the substrate). When the reaction was performed
in THF, chloroform, or acetonitrile lower conversions were
obtained (Table S1). As shown in Table 2, symmetric and
asymmetric diaryl and dialkyl, both acyclic and cyclic sulfoxides
were readily reduced affording excellent yields of the
corresponding sulfides (84-98% isolated yields, entries 1, 8-13
and 16) in short times (5 min-4 h). Notably, asymmetric aliphatic
sulfoxides, including the most sterically hindered tert-butyl methyl
sulfoxide were also reduced in high-good yields (entries 14 and
15). In addition, the reduction of sulfoxides containing functional
groups such as halogens, alkyl, ester, cyano, and nitro were
reduced with excellent selectivities and good to moderate yields
(entries 2-6 and 9), whereas olefinic groups were not tolerated.
Reduction of the alkenyl group occurred in large extent (entry 7).
In addition, methionine sulfoxide could not be reduced by the
Mn(1)/PhSiH3 catalytic system, probably due to the poor solubility
of the substrate in organic solvents, and methionine sulfoxide was
recovered intact at the end of the reaction (entry 17).
100
1
80
2
60
40
20
0
0
2
4
6
8
10
12
14
16
time (min)
Figure 1. Kinetic profile of the deoxygenation of methyl phenyl sulfoxide (1
mmol) with PhSiH3 (1.2 mmol) catalysed by [Mn(bis-NHC)(CO)3Br] (1) and [Mn
(CO)5Br] (2) (2 mol%) in neat conditions at 100 °C.
Table 2. Reduction of sulfoxides to sulfides with PhSiH3 catalysed by 1.[a]
Table 1. Reduction of methyl phenyl sulfoxide with PhSiH3 catalysed by 1.[a]
Yield
Entry
1
Substrate
Product
Time
(%)[b]
Catal. loading
(mol%)
T
(°C)
Yield
(%)[b]
TOF50
(h-1)[c]
Entry
Time
10 min
97
1
2
3
4
5
2
1
100
100
90
10 min
30 min
16 h
97
95
75
0
788
466
141
0
2
3[c]
4
7 min
4 h
92
71
2
2
25
24 h
-----
100
1 h
0
0
[a] Reaction conditions: methyl phenyl sulfoxide (1 mmol), catalyst 1, PhSiH3
(1.2 mmol), neat conditions. [b] Yields determined by 1H NMR using Ph2CH2
as internal standard. [c] TOF50 calculated at 50% of conversion.
16 h
16 h
16 h
16 h
1 h
70
To evaluate the impact of the N-heterocyclic carbene ligand, we
compared the catalytic activity of 1 to that of [Mn(CO)5Br] (2)
under the same reaction conditions. As shown in Figure 1, the
reaction catalysed by 2 was very fast in the first 2 minutes,
reaching 58% yield of the corresponding sulfide. Then, the
reaction slowed down and stopped at 69% conversion, indicating
the rapid deactivation (7 min) of the active species. These findings
showed that the presence of the NHC ligand in the coordination
sphere of Mn stabilises the intermediate active species. To
confirm the robustness of 1, we explored the reuse of the catalyst
by adding new charges of methyl phenyl sulfoxide and
phenylsilane in the same amount used in the first catalytic run.
Interestingly, the catalyst could be reused up to 9 cycles, although
longer reaction times (up to 6 h) were needed after the second
cycle to reach quantitative conversion of the sulfoxide. The total
turnover attained was 450 (after 24 h, Figure S3).
5
50
6
73
7
12[d]
8[c]
98
9[c]
3 h
84
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10.1002/cctc.201900662
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2
3
4
5
6
7
8
9
Ph2SiH2 (1.2)
PMHS (1.2)
PMHS (5)
3
5
5
5
5
5
3
3
neat
toluene
NCMe
MeOH
THF
3 h
90
0
10[c]
4 h
4 h
86
85
84
96
89
74
86
24 h
24 h
24 h
24 h
16 h
24 h
24 h
11
20
0
PMHS (5)
12[c]
13
4 h
PMHS (5)
17
91
0
<5 min
4 h
TMDS (1.2)
Et3SiH (1.2)
(EtO)3SiH (1.2)
toluene
neat
14[c]
15[c]
16
neat
10
16 h
15 min
[a] Reaction conditions: methyl phenyl sulfoxide (1 mmol), catalyst, silane,
solvent (0.4 mL) at 100 ºC. [b] Isolated yields.
Interestingly,
a variety of sulfoxides were reduced using
diphenylsilane and TMDS; results are summarised in Table 4.
When diphenylsilane was used as reducing agent, the reactions
were performed under neat conditions or toluene (to solubilise the
solid substrates) using 3 mol% of catalyst and 1.2 eqv. of silane
at 100 °C. Under these conditions, excellent yields (88-93%) were
obtained for a variety of sulfoxides (Table 4, entries 2, 4, 6, 8, 10,
12, and 14). Notably, TMDS displayed remarkable activity in the
reduction of aryl-alkyl, aryl-aryl, and alkyl-alkyl sulfoxides
mediated by 1, considering the common lower reactivity of TMDS
compared to other silanes such as PhSiH3 and Ph2SiH2. As
shown in Table 4 (entries 1, 3, 5, 7, 9, 11, and 13), sulfides were
obtained in high yields (72-93%) using 1.2 eqv of TMDS in the
presence of 1 (5 mol%) at 100°C. The choice of the reaction
solvent (THF, toluene or acetonitrile) was done depending on the
solubility of the substrates. This work represents the first
application of TMDS as a reducing agent in Mn-mediated
reactions.[2,20] Noteworthy, PMHS and TMDS have been
successfully applied in reduction reactions with several Earth
abundant metals eg. Fe, Ni, Co, and Zn.[17a,20,21]
17
------
24 h
0[e]
[a] Reaction conditions: substrate (1 mmol), 1 (2 mol%), PhSiH3 (1.2 mmol),
100ºC, neat conditions. [b] Isolated yields. [c] Substrate (0.5 mmol), 1 (2 mol%),
PhSiH3 (0.6 mmol), toluene (0.4 mL) at 100 ºC. [d] along with phenyl vinyl sulfide,
products with reduction of the alkenyl group were detected by GC. [e] no reaction
occurred, starting material was recovered after 24 h.
The major drawbacks of the use of phenylsilane as reducing
agent is its high price and toxicity, which prevents its use in large
scale applications. Thus, replacement of phenylsilane by cheaper
and safer silanes is highly desirable. Among commercially
available silanes, diphenylsilane (Ph2SiH2) and triethylsilane
(Et3SiH) represent a good choice for a small and medium scale
processes,
while
tetramethyldisiloxane
(TMDS)
and
polymethylhydrosiloxane (PMHS) are the best options for large
scale synthesis.[20] Other cheap silanes, such as triethoxysilane
present safety problems since can disproportionate partially to
SiH4, which is a pyrophoric and explosive gas.[20] As shown in
Table 3, triethylsilane, triethoxysilane, and PMHS resulted
inactive in the catalytic reaction mediated by 1 (residual activity,
20% and 10% conversion, was obtained with PMHS and
triethoxysilane, respectively, entries 4 and 9). Gratifyingly,
diphenylsilane and TMDS displayed excellent reactivity in the
reduction of methyl phenyl sulfoxide, affording 90 and 91% of the
corresponding sulfide, respectively (entries 2 and 7), although
higher catalyst loading was required (3 and 5 mol%, respectively).
In the case of TMDS, toluene was added to increase the solubility
of the substrate. A further advantage of the use of TMDS is the
benign silicon-based byproducts that are formed (mainly
octamethylcyclotetrasiloxane), which are essentially non-toxic
and can be readily removed under reduced pressure or
chromatographically.[20]
Table 4. Reduction of sulfoxides to sulfides with TMDS and Ph2SiH2 catalysed
by 1.[a]
Yield
Entry
Substrate
Silane
TMDS
Solvent
toluene
neat
Time
16 h
3 h
(%)[b]
1
2
91
Ph2SiH2
90
74
88
72
90
75
87
93
93
3
4
TMDS
Ph2SiH2
TMDS
toluene
neat
16 h
8 h
5
NCMe
toluene
THF
22 h
8 h
6
Ph2SiH2
TMDS
7
16 h
9 h
Table 3. Reduction of methyl phenyl sulfoxide with silanes catalysed by 1.[a]
8
Ph2SiH2
TMDS
toluene
THF
Silane
(eqv.)
Catal. loading
(mol%)
Yield
Entry
1
Solvent
neat
Time
(%) [b]
9
22 h
7 h
10
Ph2SiH2
toluene
PhSiH3 (1.2)
2
10 min
97
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10.1002/cctc.201900662
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Experimental Section
11
12
TMDS
Ph2SiH2
TMDS
toluene
neat
22 h
8 h
77
82
82
93
General methods
All compounds were used as received without further purification. The
[Mn(bis-NHC)(CO)3Br] complex was synthesised following the procedure
already published by us.[9] Solvents were dried according to standard
procedures. 1H and 13C NMR spectra were recorded with a Bruker Avance
III 400 MHz. Infrared spectra were obtained in a Mattson 7000 FTIR
spectrometer.
13[c]
14[c]
neat
24 h
3 h
Ph2SiH2
neat
[a] Reaction conditions: substrate (1 mmol), 1 (2 mol%), silane (1.2 mmol)
and 100 ºC. [b] Isolated yields. [c] Yields determined by 1H NMR using
Ph2CH2 as internal standard.
General procedure for the reduction of sulfoxides with silanes
catalysed by complex 1
An open-air flask was charged with complex 1 and the corresponding
substrate. Then, silane was added and the reaction mixture was stirred
and heated at 100 ºC. With solid substrates, the addition of the appropriate
solvent was needed to facilitate the solubilisation of the reagents. The
progress of the reaction was monitored by TLC and by 1H NMR in
chloroform-d. Upon completion, the product was purified by silica gel
column chromatography to afford the corresponding pure sulfides, which
are all known compounds. For some samples, conversion was determined
by 1H NMR spectroscopy using diphenylmethane as internal standard.
Yields were calculated by integration ratios of the methyl resonance of the
product and the CH2 resonance of the standard Ph2CH2.
In order to get an insight into the mechanistic details of the
catalytic reduction, the stochiometric reaction of
1 with
phenylsilane was investigated. Complex 1 was treated with one
equivalent of PhSiH3 in a J Young valve NMR tube in deuterated
1
benzene, and the reaction was monitored by H NMR. At room
temperature, no reaction occurred. The temperature was then
gradually increased to reach 80 °C. After 15 min at 80 °C, partial
conversion of 1 to a new Mn complex featuring a hydride signal
at -6.9 ppm was observed in the 1H NMR spectrum.[8] The
identification of Mn-H species as catalytically relevant
intermediates has also been described by Trovitch in the
hydrosilylation of carbonyl
groups mediated by
a
bis(imino)pyridine manganese complex.[4c] In addition, we
explored the reaction of 1 with the substrate. Complex 1 was
treated with a toluene solution of methyl phenyl sulfoxide (in a 1:1
ratio) and heated to 100 °C for 1 h. The recorded NMR and IR
spectra of the reaction mixture indicated that no reaction occurred.
Based on these experimental observations, initial coordination of
the sulfoxide to Mn is excluded. It is proposed that the reaction
initiates by reaction of complex 1 with silane, which readily
Acknowledgements
We would like to thank FCT from Portugal for project PTDC/QUI-
QIN/28151/2017, the grant PD/BD/105994/2014 (MP), and
contract IF/00346/2013 (BR). The NMR spectrometers at
CERMAX are integrated in the National NMR Network (PTNMR)
supported by Project 022161 (FEDER through COMPETE 2020,
POCI, PORL, FCT).
generates
a catalytically relevant Mn-H species. Spin-trap
experiments indicated the involvement of radical intermediates in
the reduction of sulfoxides. When the catalytic reaction is
performed in the presence of stoichiometric amounts of the
persistent radical TEMPO, a significant decrease in the activity of
1 was observed. Other radical scavengers such as Ph2NH and
CBrCl3 also slowed down the reaction to a great extent (see
kinetic profiles in Figure S4). Similar results have been described
by us in the reduction of ketones with silanes catalysed by 1.[8]
Keywords: manganese • reduction • sulfoxides • silanes •
TMDS
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In conclusion, the first Mn-catalysed reduction of sulfoxides to
sulfides has been developed using a Mn-NHC complex as
catalyst and silanes as reducing agents. Noteworthy, the catalytic
reaction is performed under non-inert conditions and without the
need of any additive. The utility of the method is outlined by the
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catalyst, and the cost-efficient and easy to handle TMDS as a
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catalytic system to the reduction of other functional groups is in
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10.1002/cctc.201900662
ChemCatChem
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An air stable manganese NHC
complex displayed excellent
catalytic activity in the reduction
of a variety of sulfoxides to
sulfides using cheap and safe
tetramethyldisiloxane as
Sara C. A: Sousa, Carlos J.
Carrasco, Mara F. Pinto, Beatriz
Royo*
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A manganese N-heterocyclic
carbene catalyst for reduction
of sulfoxides with silanes
reducing agent.
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