Efficient and selective aldehyde cyanosilylation catalyzed by Mg-Li bimetallic complex

Article abstract of DOI:10.1016/j.jorganchem.2018.08.019

A NCN-pincer ligand-based Mg-Li bimetallic complex [NCN-MgBr₂][Li(THF)₄] 1 has been employed as an efficient catalyst for cyanosilylation of a wide range of aldehydes with trimethylsilyl cyanide (TMSCN) at room temperature in CDCl

Full text of DOI:10.1016/j.jorganchem.2018.08.019

Journal of Organometallic Chemistry 874 (2018) 83e86
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Efficient and selective aldehyde cyanosilylation catalyzed by Mg-Li
bimetallic complex
,
*
Jia Li a, Ting Yu a, Man Luo a, Qian Xiao a, Weiwei Yao b, Li Xu a, Mengtao Ma a
a College of Science, Nanjing Forestry University, Nanjing, 210037, China
b College of Pharmacy, Nanjing University of Chinese Medicine, Nanjing, 210023, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A NCN-pincer ligand-based Mg-Li bimetallic complex [NCN-MgBr2][Li(THF)4] 1 has been employed as an
efficient catalyst for cyanosilylation of a wide range of aldehydes with trimethylsilyl cyanide (TMSCN) at
room temperature in CDCl3. In addition, catalyst 1 shows a good tolerance towards various functional
groups such as amide, acid, ester, heterocyclic and alkene etc. The chemoselective cyanosilylation of
aldehydes over ketones was also achieved under these conditions.
Received 3 August 2018
Received in revised form
20 August 2018
Accepted 23 August 2018
Available online 24 August 2018
© 2018 Elsevier B.V. All rights reserved.
Keywords:
Cyanosilylation
Bimetallic complex
Aldehyde
1. Introduction
cyanosilylation reaction of a wide range of carbonyl compounds
under mild conditions [10].
The reductions of unsaturated organic compounds are the
important transformations in organic chemistry. They generally
involve the hydroamination, hydrophosphination, hydroboration
and hydrosilylation of C¼C, C¼O or C¼N etc unsaturated bonds
[1e4]. The catalytic cyanosilylation reaction of carbonyl com-
pounds was one of these high-value transformations [5]. Up to
now, a number of catalyst have been employed for the cyanosi-
lylation reaction of carbonyl compounds to obtain the cyanohy-
It was well known that tridentate ligands such as NCN-pincer
ligands have been proved as an appropriate choice of ligands to
accomplish a well-defined metal-ligand bonding in the catalytic
system [11]. In contrast to the wide application in transition metal
complexes, main group compounds supported by pincer ligands
received less attention and relatively few examples of catalytic
application have been reported [12]. In 2008, Cui and co-workers
prepared the first aryldiimine NCN-pincer stabilized rare earth
metal dichlorides which can be used as good co-catalysts for the
highly cis-1,4 selective polymerization of dienes [13]. Recently, our
group also reported a NCN-pincer ligand-based magnesiate com-
plex [NCN-MgBr2][Li(THF)4] 1 as an efficient and selective catalyst
for hydroboration of carbonyl compounds [14].
To our surprise, few bimetal catalyzed cyanosilylation of
carbonyl compounds has been reported hitherto. To the best of our
knowledge, there is only one example of bimetal catalyzed cya-
nosilylation, that is, Nagendran et al. explored the catalytic cya-
nosilylation of carbonyl compounds using germylene stabilized
platinum(II) dicyanide lately [6d]. The bimetallic complexes
generally have a synergistic effect on catalysis. Based on the pre-
viously obtained positive results of bimetal complexes in the
drins derivatives which can be further converted into
a-hydroxy
acids, -amino acids, -amino alcohols etc. The catalytic carbonyl
a
b
cyanosilylation is usually achieved by the use of transition metal
and lanthanide metal complexes [6,7]. Recently, the main group
metal catalyzed cyanosilylation of carbonyl compounds has
attracted increasing attention, particularly for aluminium, alkali
and alkaline earth metal complexes [8]. For example, Sen group
successively reported that an organocalcium iodide complex and
three simple lithium compounds have been used as efficient cat-
alysts for cyanosilylation reaction of a variety of aldehydes and
ketones with TMSCN under ambient conditions without the need
of any co-catalyst quite recently [9]. Meanwhile, our group also
independently reported a low-valent magnesium(I)-catalyzed
* Corresponding author.
E-mail address: mengtao@njfu.edu.cn (M. Ma).
https://doi.org/10.1016/j.jorganchem.2018.08.019
0022-328X/© 2018 Elsevier B.V. All rights reserved.

84
J. Li et al. / Journal of Organometallic Chemistry 874 (2018) 83e86
catalytic hydrosilylation and hydroboration of carbonyl compounds
by our group [14,15], to expand the catalytic potential of the bimetal
complex, we extend Mg-Li bimetallic complex 1 to the catalytic
cyanosilylation of carbonyl compounds. Herein, we report a NCN-
pincer ligand-based Mg-Li bimetallic complex (1) catalyzed cya-
nosilylation of a variety of aldehydes under mild conditions.
(entries 17e18, Table 2). The N-(4-formylphenyl) acetamide, 4-
formylphenyl acetate and 2-formylbenzoic acid were selectively
and exclusively cyanosilylated on the aldehyde substituent and
retained the amide, ester and acid group intact (entries 19e21,
Table 2). For dicarbonyl substrate 2-bromoisophthalaldehyde,
when it was treated with equimolar TMSCN at room temperature, a
mixture of the monocyanosilylated and dicyanosilylated products
were observed in 7:3 M ratio with the full consumption of trime-
thylsilyl cyanide in 6 h monitored by 1H NMR spectroscopy. Hence
2.5 equiv. TMSCN was used to get dicyanosilylated product in 99%
yield within 2 h (entry 22, Table 2).
Encouraged by the above aldehyde cyanosilylation results, we
further investigate the cyanosilylation of ketones. As expected, the
cyanosilylation of aromatic ketones catalyzed by bimetallic com-
plex 1 displayed a lower reactivity than that of aldehydes due to
the steric effect. For example, an initial reaction of acetophenone
and 1.5 equiv. TMSCN with 5 mol% catalyst loading of 1 at room
temperature for 48 h generated only 70% conversion. The cyano-
silylation of aromatic ketones with electron-withdrawing sub-
stituents such as 4-nitroacetophenone and 4-acetylbenzonitrile
also showed a similar reactivity (79% and 73% yield, respectively)
under same reaction conditions (5 mol% catalyst 1, rt, 48 h). To
further investigate the selectivity of catalyst 1, an intramolecular
reaction of 4-acetyl benzaldehyde with 1.5 equivalent of TMSCN in
the presence of 2 mol% catalyst 1 was performed. Only the alde-
hyde moiety of 4-acetyl benzaldehyde was selectively cyanosily-
lated in quantitative yield and the ketone group retained intact
(entry 23, Table 2).
2. Results and discussion
Initially the catalytic cyanosilylation of benzaldehyde with
TMSCN was investigated. Reaction of benzaldehyde (1.0 equiv.) and
TMSCN (1.0 equiv.) with bimetallic catalyst 1 (2 mol % and 5 mol %)
at room temperature for 15 min in CDCl3 resulted in 73% and 87%
yields, respectively (entries 1e2, Table 1). Increasing the reaction
time to 30 min lead to a slightly high yield (77% and 91%) (entries
3e4, Table 1). When TMSCN was increased from 1.0 to 1.5 equiva-
lent, however, the quantitative conversion was observed in 15 min
with 2 mol% catalyst loading of 1 (entry 5, Table 1). Compared with
the aforementioned Ge-Pt bimetallic complex catalyzed cyanosi-
lylation of benzaldehyde [6d], the Mg-Li bimetallic complex 1
showed a slightly high activity (Mg-Li: 15 min, 25 ꢀC, 2 mol% cata-
lyst, 99% yield VS Ge-Pt: 90 min, 50 ꢀC, 1 mol% catalyst, 99% yield). A
blank reaction of benzaldehyde and 1.5 equiv. of TMSCN without
catalyst only gave trace yield after 48 h (entry 6, Table 1). We also
tested the solvent effect (entries 7e8, Table 1). The protic MeOH
only gave 83% yield. The corresponding solvent-free experiment
has been done as well (entry 9, Table 1).
With the optimized reaction conditions in hand, a variety of
aliphatic and aromatic aldehydes were screened (Table 2). The
cyanosilylation of 1-pentanal, 5-chloropentanal and cyclohexyl
aldehydes with 2 mol% catalyst loading of complex 1 all afforded
the corresponding cyanohydrin trimethylsilyl ethers in 99% yields
in 15 min at room temperature (entries 1e3, Table 2). Compared
with benzaldehyde, however, the cyanosilylation of aromatic al-
dehydes with electron-donating or electron-withdrawing groups
such as -Me, -NMe2, -OMe, -NO2, -F or -Cl required a slightly longer
reaction time (ꢁ3 h) to get the quantitative conversion (entries
3. Conclusions
In summary, we have demonstrated that the Mg-Li bimetallic
complex 1 was employed as an efficient and chemoselective
catalyst for the cyanosilylation of a wide range of aldehydes in a
relative cheaper and unpurified CDCl3 at room temperature
although less effective to ketones. Catalyst 1 showed high func-
tional group tolerance towards amide, ester and acid ect. The
chemoselectivity of the aldehyde over ketone was achieved
through the intramolecular cyanosilylation of 4-acetyl
benzaldehyde.
5e14, Table 2). Similarly, the reaction of a, b-unsaturated cinna-
maldehyde or even sterically bulky 9-anthraldehyde can also be
completed to give 99% yield in 3 h (entries 15e16, Table 2). Under
the same reaction conditions, heterocyclic aldehyde 2-
thiophenecarboxaldehyde could be cleanly converted into the
corresponding cyanohydrin trimethylsilyl ether in 2 h, however, the
cyanosilylation of 3-pyridinecarboxaldehyde took only half an hour
4. Experimental section
All air-sensitive compounds were carried out using standard
Table 1
Optimization of cyanosilylation of benzaldehyde.
Entry
n
Cat (mol%)
Solvent
Time
Yield (%)a
1
2
3
4
5
6
7
8
9
1.0
1.0
1.0
1.0
1.5
1.5
1.5
1.5
1.5
2
5
2
5
CDCl3
CDCl3
CDCl3
CDCl3
CDCl3
CDCl3
MeOD
THF-d8
e
15 min
15 min
30 min
30 min
15 min
48 h
2 h
2 h
2 h
73
87
77
91
99
trace
83
95
97
2
none
2
2
2
a
The reaction was monitored by 1H NMR spectroscopy.

J. Li et al. / Journal of Organometallic Chemistry 874 (2018) 83e86
85
Table 2
Cyanosilylation of aldehydes catalyzed by 1.
Entry
1
Product
Time
Yield (%)a
99(85d)
Entry
2
Product
Time
Yield (%)a
15min
15min
99
3
15min
99
4
15min
99(90d)
5
7
15min
30min
99
99
6
8
30min
3 h
99
99
9
3 h
1 h
99
99
10
12
30min
1 h
99
99
11
13
15
17
19
1 h
99
99
99
99
14
16
18
20
1 h
99
99
99
99
3 h
3 h
2 h
30min
2 h
30min
21b
23
3 h
2 h
96
99
22c
2 h
99
a
The reaction was monitored by 1H NMR spectroscopy.
TMSCN (1 equiv.).
TMSCN (2.5 equiv.).
b
c
d
Isolated yield.
Schlenk-line or glovebox techniques under high-purity argon.
Diethyl ether, toluene, THF and hexane were dried and distilled
from molten sodium. 1H and 13C{1H} NMR spectra were recorded at
25 ꢀC with a Bruker Avance III 600 MHz spectrometer and were
referenced to the resonances of the solvent used. Catalyst 1 was
prepared according to literature procedure [14]. Other reagents
were used as received.
4.1. General procedure for catalytic cyanosilylation of aldehydes
In a glove box, catalyst 1 (2 mol%) was added to a solution of
aldehydes (0.25 mmol) and Me3SiCN (1.5 equiv.) at room temper-
ature in a common NMR tube, which was charged with CDCl3
(0.5 mL). The progress of the reaction was monitored by 1H NMR
and 13C NMR. Two examples were selected to give the isolated

86
J. Li et al. / Journal of Organometallic Chemistry 874 (2018) 83e86
(d) M.K. Sharma, D. Singh, P. Mahawar, R. Yadav, S. Nagendran, Dalton Trans.
47 (2018) 5943;
(e) A.V. Gurbanov, G. Mahmoudi, M.F.C. Guedes da Silva, F.I. Zubkov,
K.T. Mahmudov, A.J.L. Pombeiro, Inorg. Chim. Acta. 471 (2018) 130;
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yield: after evaporation of CDCl3 solvent, the resulting viscous
liquid was treated with acidic water (HCl) and was purified by flash
column chromatography to give pure products.
Acknowledgements
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We thank financial supports from the National Natural Science
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Practice Innovation Program of Jiangsu Province (KYCX17_0844).
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