Benzimidazole fragment containing Mn-complex catalyzed hydrosilylation of ketones and nitriles

Article abstract of DOI:10.1016/j.tet.2020.131439

The synthesis of a new bidentate (NN)–Mn(I) complex is reported and its catalytic activity towards the reduction of ketones and nitriles is studied. On comparing the reactivity of various other Mn(I) complexes supported by benzimidazole ligand, it was observed that the Mn(I) complexes bearing 6-methylpyridine and benzimidazole fragments exhibited the highest catalytic activity towards monohydrosilylation of ketones and dihydrosilylation of nitriles. Using this protocol, a wide range of ketones were selectively reduced to the corresponding silyl ethers. In case of unsaturated ketones, the chemoselective reduction of carbonyl group over olefinic bonds was observed. Additionally, selective dihydrosilylation of several nitriles were also achieved using this complex. Mechanistic investigations with radical scavengers suggested the involvement of radical species during the catalytic reaction. Stoichiometric reaction of the Mn(I) complex with phenylsilane revealed the formation of a new Mn(I) complex.

Full text of DOI:10.1016/j.tet.2020.131439

Journal Pre-proof
Benzimidazole fragment containing Mn-complex catalyzed hydrosilylation of ketones
and nitriles
Kasturi Ganguli, Adarsha Mandal, Bidisha Sarkar, Sabuj Kundu
PII:
S0040-4020(20)30611-6
https://doi.org/10.1016/j.tet.2020.131439
TET 131439
DOI:
Reference:
To appear in: Tetrahedron
Received Date: 1 April 2020
Revised Date: 20 July 2020
Accepted Date: 21 July 2020
Please cite this article as: Ganguli K, Mandal A, Sarkar B, Kundu S, Benzimidazole fragment containing
Mn-complex catalyzed hydrosilylation of ketones and nitriles, Tetrahedron (2020), doi: https://
doi.org/10.1016/j.tet.2020.131439.
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Graphical Abstract

Benzimidazole
fragment
containing
Mn-complex
catalyzed
hydrosilylation of ketones and nitriles
Kasturi Ganguli, Adarsha Mandal, Bidisha Sarkar and Sabuj Kundu*
Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur-208016, India
Abstract
The synthesis of a new bidentate (NN)-Mn(I) complex is reported and its catalytic activity towards the
reduction of ketones and nitriles is studied. On comparing the reactivity of various other Mn(I) complexes
supported by benzimidazole ligand, it was observed that the Mn(I) complexes bearing 6-methylpyridine
and benzimidazole fragments exhibited the highest catalytic activity towards monohydrosilylation of
ketones and dihydrosilylation of nitriles. Using this protocol, a wide range of ketones were selectively
reduced to the corresponding silyl ethers. In case of unsaturated ketones, the chemoselective reduction of
carbonyl group over olefinic bonds was observed. Additionally, selective dihydrosilylation of several
nitriles were also achieved using this complex. Mechanistic investigations with radical scavengers
suggested the involvement of radical species during the catalytic reaction. Stoichiometric reaction of the
Mn(I) complex with phenylsilane revealed the formation of a new Mn(I) complex.
Keywords:
Benzimidazole; Mn(I) complex; Hydrosilylation; Nitriles; Ketones.
1
. Introduction
Transition metal-catalyzed hydrosilylation reaction has served as an attractive and important
methodology for the reduction of numerous organic molecules having unsaturated bonds.[1-5]
Among the transition metals, earth-abundant manganese catalyzed various hydrosilylation
reactions have recently received significant attention.[6] In particular, Mn(I) complexes bearing
cyclopentadiene and N-heterocyclic carbene ligands have been utilized for the hydrosilylation of
carbonyl compounds under photocatalytic conditions. [7-8] Besides, bis-N-heterocyclic carbene
and phosphine containing Mn(I) complex catalyzed hydrosilylation of carbonyl compounds also
been reported under thermal conditions.[9-10] Additionally, several Mn(0), Mn(II), Mn(III), and
Mn(V) complexes were also been investigated for the similar transformation.[11-19]
Recently, various bidentate nitrogen atom containing Mn(I) complexes have been explored in
hydrogenation and dehydrogenation reactions.[20-26] These complexes are relatively less
sensitive towards air and moisture compared to other Mn(I) complexes. However, very few
(NN)-Mn systems are known for the hydrosilylation of unsaturated organic compounds.
Recently, Leitner group reported a triazole ligand-based bidentate (NN)-Mn complex catalyzed
hydrosilylation of ketones, carboxylic acids, and esters.[27]
Hydrosilylation of nitriles is an atom-efficient process for the production of N-silylated
compounds which are useful building blocks for the synthesis of nitrogen-containing valuable
compounds and silicon-containing polymers.[28-32] Hydrosilylation of nitriles is also an
1

attractive strategy to produce amines.[33, 34] Compared to the hydrosilylation of carbonyl
compounds, hydrosilylation of nitriles is relatively less explored.[30, 35, 36] In literature, mostly
precious metal-based complexes were explored for the hydrosilylation of nitriles.[31, 32, 34, 35,
3
7] In the last decade a few base metals, as well as transition metal-free protocols have been
developed.[28, 30, 38-40] Notably, manganese catalyzed hydrosilylation of nitriles is scare in
literature. Recently, Mandal group disclosed hydrosilylation of primary amides to amines using a
t
Mn(III) complex in the presence of KO Bu where nitrile was formed as an intermediate.[41]
However, to the best of our knowledge to date there has been no report of a base-free Mn(I)
catalyzed dihydrosilylation of nitriles.
Benzimidazole fragment is well known as a versatile donor towards the metal center.
Additionally, it can facilitate the proton transfer process via bifunctional pathway.[22, 42-44]
Recently, our group reported benzimidazole unit containing bidentate and tridentate ligand-based
metal complexes which showed excellent reactivity towards various alcohol dehydrogenation
related reactions.[43-45] Lately, we reported the synthesis and reactivity of various
benzimidazole based Mn(I) complexes towards the transfer hydrogenation of carbonyl
compounds and imines.[43] Inspired by the remarkable reactivity of these complexes towards
various hydrogen transfer reactions, herein we report the catalytic activity of these
benzimidazole based (NN)-Mn(I) complexes towards the hydrosilylation of ketones and nitriles.
2
. Results and discussion
Mn(I) complexes A-D bearing benzimidazole moiety were synthesized following the reported
procedure.[43] The newly synthesized Mn(I) complex E bearing both 6-methylpyridine and
benzimidazole fragment was synthesized in 84% yield upon metallation of the corresponding
5
ligand with MnBr(CO) in refluxing toluene (Scheme 1). Complex E was characterized by
NMR, IR, ESI-MS, and X-ray diffraction studies. The solid-state structure of complex E
displayed a slightly distorted octahedral geometry around the Mn center with two CO ligands
trans to the nitrogen atom of the benzimidazole ring and pyridine respectively. The third CO was
trans to the bromide ligand (Fig 2). The N1-Mn-N2 bond angle was 78.5°. The benzimidazole
nitrogen coordinated to Mn was situated at a distance of 2.036 Å while the Mn-N distance with
the pyridine nitrogen was 2.150 Å.
Fig. 1 Manganese(I) complexes screened in this study
2

H
N
H
N
N
N
MnBr(CO)5
Mn
OC
OC CO
Br
N
N
toluene, reflux, 8 h
E
84% yield
Scheme 1 Synthesis of manganese complex E
Fig. 2 Solid-state structure of complex E drawn with 30% thermal ellipsoids. Selective bond
distances (Å) and angles (°): Mn-N1 2.151(14), Mn-N2 2.036(14), Mn-Br 2.548(3), N1-Mn-N2
7
8.52(5).
Initially, acetophenone was selected as the benchmark substrate to find out the optimum reaction
conditions for the hydrosilylation of ketones. The reactivities of complexes A-E were screened
using 1 mol% catalyst loading in THF at 100 °C. Complexes A, B, and C showed very poor
reactivity towards the reduction of acetophenone using phenylsilane (Table 1, entries 1-3)
although, they exhibited moderate reactivity towards the transfer hydrogenation of acetophenone
i
in PrOH.[43] Notably, complexes D and E presented quantitative yield of the silyl ether in the
presence of 1.2 equivalent phenylsilane after heating for 3 hours at 100 °C in THF, which upon
basic hydrolysis produced 1-phenylethanol (Table 1, entries 4-5). On decreasing the catalyst
loading to 0.3 mol% the activity of complex D dropped to 73%, whereas complex E showed
quantitative conversion to 1-phenylethanol (Table 1, entries 6-7). Further decreasing the loading
of complex E, time, temperature, or amount of phenylsilane led to lower yields of 1-
phenylethanol (Table 1, entries 8, 10-12 and 15). Notably, 0.1 mol% complex E produced 96%
yield of 1-phenylethanol after 20 hours (Table 1, entry 9). Apart from phenylsilane, other silanes
such as diphenylsilane and polymethoxyhydrosiloxane were also tested. However, they were
ineffective under the present catalytic system (Table 1, entries 13-14). Other solvents such as
acetonitrile, benzene, and toluene were also effective in carrying out this transformation (Table
1
, entries 16-18). Overall, 0.3 mol% complex E in the presence of 1.2 equiv. PhSiH in THF at
3
3

1
00 °C for 3 hours was selected as the optimized condition for the hydrosilylation of
acetophenone (Table 1, entry 6).
a
Table 1 Optimization of the reaction parameters for the hydrosilylation of acetophenone
Entry Catalyst (mol%) Silane (equiv.)
Solvent
Temp.
Time
1-phenylethanol
%)
(
1
2
3
4
5
6
7
8
9
A (1.0)
B (1.0)
C (1.0)
D (1.0)
E (1.0)
E (0.3)
D (0.3)
E (0.1)
E (0.1)
E (0.3)
E (0.3)
E (0.3)
E (0.3)
E (0.3)
E(0.3)
E (0.3)
E (0.3)
E (0.3)
PhSiH (1.2)
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
100
100
100
100
100
100
100
100
100
100
�
�
�
�
�
�
�
�
�
�
3 h
3 h
3 h
3 h
3 h
3 h
3 h
3 h
20 h
2 h
3 h
3 h
3 h
3 h
3 h
3 h
3 h
3 h
<5
7
3
PhSiH (1.2)
3
PhSiH
3
(1.2)
<5
>99
>99
>99
73
PhSiH (1.2)
3
PhSiH (1.2)
3
3
PhSiH (1.2)
PhSiH (1.2)
3
PhSiH (1.2)
56
3
PhSiH
3
(1.2)
96
86
1
0
1
2
3
PhSiH (1.2)
3
1
1
1
PhSiH
3
(1.2)
90
�
92
PhSiH (1.2)
80
�
84
5
3
Ph SiH (1.2)
100
100
100
100
100
100
�
�
�
�
�
�
2
2
b
14
PMHS
n.d.
10
>99
>99
90
1
1
1
1
5
6
7
8
PhSiH (0.6)
3
PhSiH (1.2)
CH CN
3
3
PhSiH
3
(1.2)
6 6
C H
PhSiH (1.2)
Toluene
3
a
Reaction Conditions: Acetophenone (0.5 mmol), silane (equiv.), Mn complex (x mol%),
refluxed in solvent (0.5 mL) at temp. ( ) for time (h) followed by basic hydrolysis in the
presence of 10% NaOH solution; GC yields ( -dodecane was used as an internal standard);
�
n
b
PMHS = polymethoxyhydrosiloxane (0.3 mL) was used; n.d. = not detected.
Next, under the optimized conditions, hydrosilylation of a wide range of aromatic and aliphatic
ketones were carried out (Scheme 2). Electron donating groups such as methyl, methoxy, amino
and 2,4,6-trimethyl substituted acetophenones furnished excellent yields of the corresponding
alcohols (2b-e). The methoxy group at ortho and meta positions gave slightly lower yields
probably due to steric reasons (2k-l). Electron withdrawing groups such as nitrile, fluoro, nitro,
and trifluoromethyl bearing acetophenones also led to quantitative yields of the alcohols (2f-g,
4

2
i-j). Halogen containing substrates such as 4-fluoro and 4-chloroacetophenone presented
excellent yields of the corresponding alcohols without the formation of any dehalogenated side
product (2g-h). Reactivity was slightly lower with heteroaromatic ketones such as 3-acetyl
pyridine and 2-acetylthiophene which gave moderate to good yields of the corresponding
alcohols (2n-o).[15] Sterically hindered 2,2,2-trimethylacetophenone and aromatic long chain
ketones such as propiophenone and octanophenone were also presented moderate to good yields
of the corresponding alcohols (2p-r). Aliphatic cyclic ketones such as 1-tetralone,
cyclohexanone and straight chain 4-heptanone were required longer time (6 hours) to provide
good yields of the corresponding cyclic or straight-chain alcohols (2s-u).[9] In the case of 6-
methyl-5-hepten-2-one, the carbonyl group was selectively reduced over the olefinic group and
produced the unsaturated alcohol (2v) efficiently after 6 hours.[43]
Scheme 2 Substrate scope for the hydrosilylation of ketones. Reaction conditions: Substrate (0.5
mmol), PhSiH (0.6 mmol), Comp. E (0.0015 mmol) refluxed in THF at 100
� for 3 h followed
3
a
by basic hydrolysis in the presence of 10% NaOH solution; isolated yields; GC yields (
n
-
b
dodecane was used as an internal standard); isolated yields in parentheses; 6 h.
After investigating the hydrosilylation of ketones, the potential of this catalytic system was
further extended for the reduction of nitriles. For this purpose, the reaction conditions were
optimized using benzonitrile as a model substrate. With 5 mol% complex E, quantitative yield of
benzylamine was observed after heating for 24 h in THF at 120 °C followed by hydrolysis
(Table 2, entry 3). Upon lowering the reaction temperature or duration of reaction the yields as
well as selectivity of benzylamine were dropped considerably and aldimine (5) was produced as
a side product (Table 2, entries 1-2, 4). When complex D was used as the catalyst, the conversion
5

of benzonitrile was lower compared to complex E and a significant amount of aldimine was
formed (Table 2, entry 5). Upon lowering the catalyst loading or the amount of phenylsilane the
yield of benzylamine was decreased (Table 2, entries 6-7). Hence, for the hydrosilylation of
nitriles, 5 mol% complex E in the presence of 3 equivalents of phenylsilane at 120 °C for 24
hours was selected as the optimized conditions.
a
Table 2 Optimization of the reaction parameters for the hydrosilylation of benzonitrile
Entry Catalyst Temp. (°C) Time (h) % Conv. 4 (%) 5(%)
1
2
3
4
5
E
E
E
E
D
E
E
100
110
120
120
120
120
120
�
�
�
�
�
�
�
24
24
24
12
24
24
24
90
90
82
90
8
-
>99
60
>99
46
-
14
25
16
-
80
55
b
6
7
96
72
80
72
c
a
Reaction Conditions: Benzonitrile (0.5 mmol), PhSiH (1.5 mmol), Mn complex (0.025 mmol),
3
refluxed in THF at temp. (
� ), for time (h) followed by basic hydrolysis in the presence of 10%
b
NaOH solution; GC yields (
n-dodecane was used as an internal standard); Comp. E (0.02
c
mmol); PhSiH (1.0 mmol) was used.
3
Utilizing the optimized conditions reduction of numerous nitriles to their corresponding amines
were carried out and they were further isolated as the corresponding amine salts (Scheme 3).
Substituted benzonitriles having electron-donating groups such as methyl, methoxy, thiomethyl,
and tert-butyl groups gave moderate to excellent yields of the corresponding amines (6b-c, e-f).
However, 2-methoxybenzonitrile provided a slightly lower yield of 2-methoxybenzylamine (6d)
probably due to steric reasons.[40] Electron withdrawing groups such as chloro, fluoro, and
trifluoromethyl substituted benzonitriles also led to excellent yields of the corresponding amines
(6g-i). Noticeably, hydrosilylation of halogen-containing substrates proceeded smoothly without
the formation of any dehalogenated side products (6g, i). Heteroaromatic substrates such as
thiophene-2-carbonitrile gave a good yield of the corresponding amine (6l). However, 3-
pyridinecarbonitrile provided a negligible amount of the desired product (4k) probably due to
coordination of the pyridine nitrogen to the Mn(I) center.[38] Next, some aliphatic nitrile
substrates were tested and they required longer periods of heating to give a good yields of the
6

corresponding amines (6m, 4n-o).[34] However, the aliphatic straight-chain valeronitrile could
not be reduced to pentan-1-amine even heating for a longer time (4p).
Scheme 3 Substrate scope for the hydrosilylation of aliphatic and aromatic nitriles. Reaction
3
conditions: Nitriles (0.5 mmol), PhSiH (1.5 mmol), Comp. E (0.025 mmol) was refluxed in
THF at 120 °C for 24 h followed by hydrolysis with 10% NaOH; isolated yields based on the
a
b
HCl salts of the amines; 36 h; GC yields of the amines (
standard).
n-dodecane was used as an internal
3
. Mechanistic Investigation
To understand the mechanism of complex E catalyzed hydrosilylation reaction several control
experiments were carried out. At first, radical trapping experiments were performed with this
system for the hydrosilylation of acetophenone. In the presence of 0.3 mol% radical quenchers
such as TEMPO, diphenylamine, and DPPH the catalytic activity of complex E was drastically
quenched and the yields of 1-phenylethanol was dropped to less than 5% (Table 3).[9] On
increasing the reaction time to 20 hours, the yields of 1-phenylethanol did not increase in the
presence of radical scavengers (Table 3, entries 4-6).
a
Table 3 Effect of radical scavengers on the hydrosilylation of acetophenone
Entry
Radical scavenger (mol%)
TEMPO (0.3)
Time (h)
% Yield of 1-phenylethanol
1
2
3
3
3
3
<5
<5
<5
DPPH (0.3)
2
Ph NH(0.3)
7

4
5
6
TEMPO (0.3)
DPPH (0.3)
20
20
20
<5
<5
<5
Ph NH(0.3)
2
a
Reaction Conditions: Acetophenone (0.5 mmol), silane (0.6 mmol), Comp. E (0.0015 mmol),
radical scavengers (0.0015 mmol), refluxed in THF at 100 °C for 3 h followed by basic
hydrolysis in the presence of 10% NaOH solution; GC yields (n-dodecane was used as an
internal standard).
In addition to the spin trap experiments, hydrosilylation of cyclopropyl phenyl ketone was tested
as a mechanistic probe to study the involvement of radical intermediates. [9] Under the standard
catalytic conditions cyclopropyl phenyl ketone was converted quantitatively to cyclopropyl
phenyl methanol (A) and β-ethyl styrene (B) which were formed in an A/B ratio of 1.5:1 (Fig S5,
SI). This result suggested the involvement of radical intermediates in this catalytic reaction.
Scheme 4 Hydrosilylation of cyclopropylphenylketone using phenylsilane.
In most of the transition metal-catalyzed hydrosilylation reactions, in situ generated metal
hydrides have been proposed as the active catalyst. Upon reacting complex E with 3 equivalents
phenylsilane in THF a sharp color change from yellow to dark red was observed within 15
1
minutes of heating at 100 °C. On analyzing the H-NMR spectrum of the crude reaction mixture
two peaks (~ 1:1 ratio) were observed at
δ -12.5 and -16.2 ppm respectively (Fig S6, SI). These
peaks clearly indicated the formation of a new Mn(I) complex. This complex could be either Mn-
2
H or the silane bound
2
σ-complex Mn(H)(η -H-SiH Ph).[7, 9, 46] In an attempt to isolate the Mn-
H species, complex E was treated separately with LiHBEt in THF and was also refluxed in
3
i
i
PrOH in the presence of NaO Pr. However, both these attempts to isolate the hydride species
were unsuccessful.
In literature, different mechanistic pathways have been proposed for the manganese catalyzed
hydrosilylation reactions. Sortais and co-workers proposed a Mn(I)/Mn(III) catalytic cycle for
CpMn(NHC)(CO)
Trovitch group suggested a radical transfer mechanism along with a modified Ojima mechanism
for their Mn(0) complex bearing a redox non-innocent ligand.[13] [16] For the bis-
heterocyclic carbene containing Mn(I) complex, Pinto group observed the formation of either
Mn(I)-H or a -silane complex and intermediacy of radical species in their catalytic reaction.[9]
2
system and they identified a σ-silane complex.[7] On the other hand,
N
-
σ
Recently, Leitner group reported involvement of (NNP)Mn(I)-H intermediate during the
hydrosilylation of carboxyl and carbonyl substrates.[27] Based on the mechanistic investigations
and previous literature reports, we believe complex E bears much resemblance with the bis-
8

NHC-Mn(I) complex[9] and Leitner’s triazole ligand-based (NN)-Mn(I) complexes[27] and it
may followed the similar mechanism. However, the exact mode of activation of both the catalyst
and silane are yet to be explored and detailed mechanistic investigations are currently underway
in our laboratory.
4
. Conclusion
In summary, we report the synthesis of a new Mn(I) complex bearing benzimidazole and 6-
methylpyridine fragments containing ligand. This complex (E) exhibited superior reactivity
compared to the other benzimidazole based Mn(I) towards the hydrosilylation of ketones and
nitriles. Utilizing this methodology, a wide range of aliphatic, aromatic ketones and nitriles were
reduced to the corresponding silyl ethers and disilylated amines which upon hydrolysis produced
the corresponding alcohols and amines respectively. This protocol was chemoselective towards
the reduction of the carbonyl group over the olefinic bond. To the best of our knowledge, this is
the first report of a base-free Mn(I) catalyzed dihydrosilylation of nitriles. The mechanistic
investigation suggested the involvement of radical intermediate. Further mechanistic
investigations are required to understand this mechanism more clearly which are currently
ongoing in our laboratory.
5
. Acknowledgements
We are grateful to the Science and Engineering Research Board (SERB), India and Council of
Scientific & Industrial Research, India (CSIR) for financial support. K.G. thanks IITK and B.S.
thanks INSPIRE for their fellowships.
6
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Highlights
•
Synthesis and characterization of nitrogen containing bidentate Mn(I) complexes
and investigating their catalytic activity towards the catalytic hydrosilylation of
ketones.
•
•
This is the first report of base free Mn(I) complex catalyzed dihydrosilylation of
nitriles.
Mechanistic studies revealed the intermediacy of radical species.

Declaration of interests
☒
The authors declare that they have no known competing financial interests or personal relationships
that could have appeared to influence the work reported in this paper.
☐
The authors declare the following financial interests/personal relationships which may be considered
as potential competing interests: