Reduction of carbonyl compounds via hydrosilylation catalyzed by well-defined PNP-Mn(I) hydride complexes

Article abstract of DOI:10.1007/s00706-021-02774-y

Reduction reactions of unsaturated compounds are fundamental transformations in synthetic chemistry. In this context, the reduction of polarized double bonds such as carbonyl or C=C motifs can be achieved by hydrogenation reactions. We describe here a highly chemoselective Mn(I)-based PNP pincer catalyst for the hydrosilylation of aldehydes and ketones employing polymethylhydrosiloxane (PMHS) as inexpensive hydrogen donor. Graphic abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s00706-021-02774-y

Monatshefte für Chemie - Chemical Monthly
https://doi.org/10.1007/s00706-021-02774-y
ORIGINAL PAPER
Reduction of carbonyl compounds via hydrosilylation catalyzed
by well‑defned PNP‑Mn(I) hydride complexes
Stefan Weber1 · Dina Iebed1 · Mathias Glatz1 · Karl Kirchner1
Received: 15 March 2021 / Accepted: 20 April 2021
© The Author(s) 2021
Abstract
Reduction reactions of unsaturated compounds are fundamental transformations in synthetic chemistry. In this context, the
reduction of polarized double bonds such as carbonyl or C=C motifs can be achieved by hydrogenation reactions. We describe
here a highly chemoselective Mn(I)-based PNP pincer catalyst for the hydrosilylation of aldehydes and ketones employing
polymethylhydrosiloxane (PMHS) as inexpensive hydrogen donor.
Graphic abstract
Keyword Manganese · Pincer complexes · Reduction · Silanes · Ketones
Introduction
abundance [2]. Manganese-based complexes play a major
role in sustainable oxidation [3–10] and hydrogenation
[11–18] reactions by now. Although, the use of dihydrogen
displays advantages such as low costs and easy removal from
the reaction mixture, several drawbacks should be taken in
account. Dihydrogen is explosive and expensive reactors
have to be used. An attractive alternative to that is the use
of hydrogen donors such as silanes [19, 20]. Since the frst
report on manganese-catalyzed hydrosilylation reactions by
Yates and coworkers in 1982 [21], several protocols for the
hydrosilylation of polarized double bonds such as ketones
[22–27], esters [28, 29], amides [30–32] and acids [33] as
well as alkenes [34–36] and alkynes [37] were developed.
Our group recently reported on the chemoselective hydro-
genation of aldehydes, catalyzed by a well-defned PNP-
Mn(I) complex [15]. We wondered if the same complex
is also capable of undergoing hydrosilylation reactions of
carbonyl compounds and whether the substrate scope could
be extended to ketones (Scheme 1). Here, we describe the
chemoselective hydrosilylation of aldehydes and ketones
The reduction of polarized double bonds such as carbonyl
motifs is among the most important transformations in
organic synthesis. To increase atom efciency and avoid
massive production of waste, catalytic reactions should
be employed. Within this context, precious metals are fre-
quently used. However, their production shows high envi-
ronmental impact and their amount is limited. The usage
of earth abundant metals would decrease the environmental
impact and could be an interesting alternative to noble met-
als [1].
Within this context, manganese is an interesting can-
didate for investigations due to its low toxicity and high
* Karl Kirchner
karl.kirchner@tuwien.ac.at
1
Institute of Applied Synthetic Chemistry, Vienna University
of Technology, Getreidemarkt 9/163-AC, 1060 Vienna,
Austria
Vol.:(0123456789)
1 3

S. Weber et al.
Table 1 Catalyst
screening
of
for
hydrosi-
Scheme 1
lylation
4-fuoroacetophenone
Entry
Catalyst
Conversiona/%
78
70
–
Yieldb/%
1
2
3
4
1
2
3
4
65
63
–
–
–
Reaction conditions: 0.35 mmol ketone, 2 mol% catalyst, 0.1 mmol
PhSiH3, 2 cm3 ACN, 18 h, 80 °C
aConversion determined via 19F{1H} NMR-spectroscopy
bYield determined via 19F{1H} NMR-spectroscopy and fuorobenzene
as standard
catalyzed by well-defined PNP-Mn(I) complexes based
upon the 2,6-diaminopyridine scafold (Scheme 2). As inex-
pensive silane source polymethylhydrosiloxane (PMHS) is
utilized.
According to the results represented in Table 1, catalyst
1 was chosen for further investigations and optimization of
the reaction parameters. Since the use of phenylsilane as
hydrogen-source is connected to high costs, we wondered if
substitution by the inexpensive siloxane polymethylhydrosi-
loxane (PMHS) is possible. PMHS is a byproduct in indus-
trial siloxane product and, therefore, displays an interesting
candidate as hydrogen-source in hydrosilylation reactions
[38]. To our delight, the substitution of phenylsilane with
PMHS even increased the conversion to 92% (Table 2, entry
1). Increasing the reaction temperature to 110 °C led to full
conversion of substrate at only 1 mol% catalyst (Table 2,
entry 4). However, the diference between determined con-
version and yield was attributed to formation of a hemiacetal
as side product.
Results and discussion
To evaluate the potential use of PNP-based Mn(I) com-
plexes, the hydrosilylation of 4-fuoroacetophenone with
phenylsilane was chosen as model reaction utilizing com-
plexes 1–4 as pre-catalysts. Conversions and yields were
determined by 19F{1H} NMR-spectroscopy [13, 18]. While
complexes 1, 3, and 4 are capable of metal–ligand bifunc-
tionality, metal–ligand cooperation is blocked in the case of
complex 2 due to methylation of the N-linker. As shown in
Table 1, complexes 1 and 2 gave moderate to good conver-
sion whereas compounds 3 and 4 did not show any reac-
tivity for the hydrosilylation of 4-fuoroacetophenone. The
hydride ligand seems to be vital for the activity of the sys-
tem. Interestingly, complex 1 and 2 gave similar conversions
indicating that metal–ligand cooperativity is not crucial for
the hydrosilylation of ketones.
Therefore, a screening of solvents was done to circum-
vent the formation of undesired byproduct. Toluene and THF
gave low conversion, but no formation of hemiacetal could
be detected. Employing isopropanol as solvent led only to
traces of product formation. 1,2-Dimethoxyethane (DME)
revealed to be the best solvent for this transformation, giving
a clean reaction. To achieve good conversions, the catalyst
loading was reinvestigated, whereas 2.5 mol% gave the best
results (Table 2, entry 10).
Scheme 2
HN
N
NH
N
N
N
HN
N
NH
HN
R2P Mn PR2
OC
N
NH
H
H
Br
X
R2P Mn PR2
R2P Mn PR2
R2P Mn PR2
OC
OC
OC
CO
CO
CO
CO
X = OCOH
3
1
2
4
R = iPr
1 3

Reduction of carbonyl compounds via hydrosilylation catalyzed by well‑defned PNP‑Mn(I)…
Table 2 Optimization
reactions
for
hydros-
yields could be detected in case of helional (24), citronel-
lal (26), and citral (27) as substrates. Unfortunately, only
low conversion could be detected in case of dartanal (29)
(Table 3).
ilylation
of
4-fuoroacetophenone
Entry
Catalyst load- Solvent
ing/mol%
Conver- Yieldb/%
siona/%
Conclusion
1c
2
2
2
2
1
2
2
2
2
ACN
ACN
ACN
ACN
Toluene
THF
i-PrOH
DME
DME
DME
DME
92
>99
>99
>99
13
26
<5
74
86
85
88
13
26
<5
36
>99
>99
25
In sum, we have described an efcient manganese-catalyzed
hydrosilylation of aldehydes and ketones with the inexpen-
sive siloxane PMHS, which is a byproduct in industry. High
chemoselectivity for the reduction of carbonyl groups in the
presence of (conjugated) C–C double bonds or other reduc-
ible groups such as nitro or nitrile functionalities could be
reported. The scope of the introduced protocol covered a
broad variety of aromatic ketones. In the case of aldehydes,
challenging substrates featuring (conjugated) C–C double
bonds were chosen which are important in the fragrance
industry.
3d
4
5
6
7
8
9
10
11
36
3
2.5
1
>99
>99
25
Reaction conditions: 0.35 mmol ketone, 2.5 mol% 1, 0.1 mmol
PhSiH3 or 0.035 mmol PMHS (average MW 1850 g/mol), 2 cm3 sol-
vent, 18 h, 110 °C
aConversion determined via 19F{1H} NMR-spectroscopy
Experimental
bYield determined via 19F{1H} NMR-spectroscopy and fuorobenzene
as internal standard
All manipulations were performed under an inert atmos-
phere of argon using Schlenk techniques or in an MBraun
inert-gas glovebox. The solvents were purifed according to
standard procedures [39]. The deuterated solvents were pur-
chased from Aldrich and dried over 4 Å molecular sieves.
Complexes [Mn(PNP-iPr)(CO)2H] (1), [Mn(PNPMe-iPr)-
(CO)2H] (2), [Mn(PNP-iPr)(CO)2Br] (3), and [Mn(PNP-
iPr)(CO)2(κ1-O-OCHO)] (4) were prepared according to
the literature [15, 40, 41]. 1H NMR spectra were recorded
c80 °C
d0.05 equiv. PMHS
Having established the optimized reaction conditions, the
scope and limitation was investigated and a broad variety
of diferent (hetero)aromatic substrates was examined. The
introduced procedure tolerated halides (7) as well as coordi-
nating groups such as amine (10), ether (9), and nitrile (11)
functionalities. It should be noted, that high chemoselectiv-
ity towards the reduction of the keto-group in the presence
of a nitrile moiety could be detected, whereas the nitrile
functionality stays unaltered. Lower conversion could be
achieved in the presence of a nitro-group (8). Sterically more
demanding ketones (14 and 15) gave excellent yields. Furane
(18)- and pyridine (17)-based systems gave good yields.
Lower reactivity could be observed in case of aliphatic sys-
tems (19 and 20). However, no reduction of the conjugated
C–C double bond could be detected in case of 20.
1
on Bruker AVANCE-250 and 400. H NMR spectra were
referenced internally to residual protio-solvent, and solvent
resonances, respectively, and are reported relative to tetra-
methylsilane (δ=0 ppm). 19F{1H} NMR spectra are reported
relative to trichlorofluoromethane (CFCl3) (δ = 0 ppm).
GC–MS analysis was conducted on an ISQ LT Single quad-
rupole MS (Thermo Fisher) directly interfaced to a TRACE
1300 Gas Chromatographic systems (Thermo Fisher), using
a Rxi-5Sil MS (30 m, 0.25 mm ID) cross-bonded dimethyl
polysiloxane capillary column.
General procedure for hydrosilylation reactions
The substrate scope was further extended to aldehydes.
The challenging substrate salicylaldehyde (21) gave excel-
lent yield. Due to the high chemoselectivity of catalyst 1,
a variety of challenging aldehydes containing C–C double
bonds was investigated. Cinnamon aldehyde (22) as well as
1-methyl-1-butene-carbaldehyde (25) gave good to excellent
conversion, without any reduction of the conjugated C–C
double bond. Finally, aldehydes, which are important com-
pounds in fragrance industry were investigated. Excellent
Inside an Ar-fushed glovebox, an 8 cm3 microwave vial
was charged with complex (0.01–0.03 mol%), carbonyl
substrate (0.35 mmol), 2 cm3 solvent, and silane (0.035–0.1
mmol) in this order. A stirring bar was added, and the
vial was sealed. The closed vial was removed from the
glovebox and stirred for 18 h at the indicated tempera-
ture in a heated aluminum block. The vial was allowed to
reach room temperature and the reaction was quenched by
1 3

S. Weber et al.
Table 3 Scope and limitation of
the hydrosilylation of carbonyls
catalyzed by 1
Reaction conditions: 0.35 mmol substrate, 2.5 mol% 1, 0.035 mmol PMHS (average MW 1850 g/
mol), 2 cm3 DME, 18 h, 110 °C in closed microwave vial; isolated yields
aConversion determined via GC–MS
exposure to air. In case of screening reactions, fuoroben-
zene (0.35 mmol) was added and the reaction mixture was
analyzed by 19F{1H} NMR.
Isolation of the product
To the reaction mixture 2 cm3 of a 20 wt% NaOH-solution
were added and the solution was stirred for 18 h at room
1 3

Reduction of carbonyl compounds via hydrosilylation catalyzed by well‑defned PNP‑Mn(I)…
16. Weber S, Stöger B, Veiros LF, Kirchner K (2019) ACS Catal
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Spectroscopic data of all isolated products are in line with
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Open Access This article is licensed under a Creative Commons Attri-
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