Highly chemoselective and efficient Strecker reaction of aldehydes with TMSCN catalyzed by MgI2 etherate under solvent-free conditions

Article abstract of DOI:10.1080/10426507.2019.1577846

Strecker reaction of various substituted aromatic aldehydes, heteroaromatic aldehydes, aliphatic aldehydes and α,β-unsaturated aldehydes with trimethylsilyl cyanide (TMSCN) was realized in the presence of 5 mol % of MgI₂ etherate in a mild, efficient and highly chemoselective manner under solvent-free conditions.

Full text of DOI:10.1080/10426507.2019.1577846

Phosphorus, Sulfur, and Silicon and the Related Elements
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Highly chemoselective and efficient Strecker
reaction of aldehydes with TMSCN catalyzed by
MgI2 etherate under solvent-free conditions
Huijun Liu, Kailun He, Jing Zhou & Xingxian Zhang
To cite this article: Huijun Liu, Kailun He, Jing Zhou & Xingxian Zhang (2019): Highly
chemoselective and efficient Strecker reaction of aldehydes with TMSCN catalyzed by MgI2
etherate under solvent-free conditions, Phosphorus, Sulfur, and Silicon and the Related Elements,
DOI: 10.1080/10426507.2019.1577846
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PHOSPHORUS, SULFUR, AND SILICON AND THE RELATED ELEMENTS
https://doi.org/10.1080/10426507.2019.1577846
Highly chemoselective and efficient Strecker reaction of aldehydes with TMSCN
catalyzed by MgI2 etherate under solvent-free conditions
Huijun Liu, Kailun He, Jing Zhou, and Xingxian Zhang
College of Pharmaceutical Sciences, Zhejiang University of Technology, Hangzhou, P. R. China
ABSTRACT
ARTICLE HISTORY
Received 20 December 2018
Accepted 30 January 2019
Strecker reaction of various substituted aromatic aldehydes, heteroaromatic aldehydes, aliphatic
aldehydes and a,b-unsaturated aldehydes with trimethylsilyl cyanide (TMSCN) was realized in the
presence of 5 mol % of MgI2 etherate in a mild, efficient and highly chemoselective manner under
solvent-free conditions.
KEYWORDS
Aldehyde; trimethylsilyl
cyanide; Strecker reaction;
MgI2 etherate
GRAPHICAL ABSTRACT
Introduction
conditions and tedious work-up procedure and give unsatis-
factory yields. Therefore, the development of less expensive,
environmentally benign, and easily handled promoters for
the synthesis of cyanohydrins under neutral, mild, and con-
venient conditions is still highly desirable.
In our previous paper, we reported the use of MgI2 etherate
as a catalyst for the cyanosilylation of carbonyl compounds,
primary amines with TMSCN under solvent-free conditions.[26]
Herein, we will report a mild, efficient, and highly chemoselec-
tive Strecker reaction of aromatic aldehydes, aliphatic alde-
hydes and unsaturated aldehydes with TMSCN catalyzed by
5mol% of MgI2 etherate under solvent-free conditions.
Cyanosilylation of carbonyl compounds is an efficient pro-
cedure for the synthesis of silylated cyanohydrins or
a-hydroxy nitriles and has considerable synthetic potential
in organic synthesis.[1] Moreover, cyanohydrins are useful
starting materials for the synthesis of biologically active
compounds such as a-amino alcohols, a-hydroxy aldehydes,
and a-hydroxy acids.[2] For these reasons, cyanosilylation
methodologies are attracting increasing interest in organic
synthesis.[3] Strecker reaction has been widely utilized into
the construction of carbon-carbon bond formation in gener-
ating cyanohydrins in the presence of cyanide sources such
as HCN,[4] KCN,[5] (EtO)2P(O)CN,[6] Et2AlCN[7] and
Bu3SnCN[8] under various reaction conditions. The handling
of gaseous HCN, KCN and liquid Bu3SnCN, Et2AlCN and
(EtO)2P(O)CN is cumbersome due to their hazardous
nature, while special equipment and care are needed for
transfer of these materials on large scale. However, TMSCN
is a safer, more effective, and more easily handled cyanide
anion source compared to other cyanating reagents for the
nucleophilic addition reactions of aldehydes under mild con-
ditions.[9] It is noteworthy that TMSCN is able to transfer
the cyanide ion only in the presence of a catalyst. Therefore,
the Strecker reaction has been promoted by a variety of
Lewis acid catalys.[10–16] These catalysts all homogeneously
catalyze the Strecker reaction. Recently, some recyclable
heterogenous catalysts and organocatalysts are found to be
utilized into the cyanosilylation of carbonyl compounds with
TMSCN.[17–25] However, most of these methods require
Results and discussion
Initially, we have chosen benzaldehyde and TMSCN as the
model substrate for surveying the reaction parameters. The
results are summarized in Table 1. By screening various sol-
vents we have found that CH2Cl2 and CHCl3 are the best
solvents for this reaction (Table 1, entries 1 and 2).
Moderate yields were given in toluene, THF, Et2O and
MeCN (Table 1, entries 3–6). Low yields were found in
DMF and methanol (Table 1, entries 7 and 8). Gratifyingly,
we have found that the Strecker reaction of benzaldehyde
with TMSCN was carried out smoothly in the presence of
5 mol% of MgI2 etherate under solvent-free conditions and
provided the desired product in 95% yield. (Table 1, entry
9). To examine the halide anion effect, halogen analogs of
MgBr2 etherate, MgCl2 and Mg(ClO4)2 were compared
under similar reaction conditions (Table 1, entries 10–12),
expensive reagents, long reaction times, harsh reaction respectively. A good yield was also given by using 5 mol% of
CONTACT Xingxian Zhang
zhangxx@zjut.edu.cn
Supplemental data for this article can be accessed at https://doi.org/10.1080/10426507.2019.1577846.
ß 2019 Taylor & Francis Group, LLC

2
H. LIU ET AL.
Table 1. Optimization of reaction conditions for MgX2-catalyzed
Table 2. The cyanosilylation of various aldehydes with TMSCN catalyzed by
MgI2ꢀ(OEt2)n under solvent-free conditions.a
Strecker reaction.a
Entry
MgX2
Solvent
Time (h)
Yield (%)b
Entry
1
Aldehyde
Product
1a[27]
Time (h)
1
Yield(%)b
95
1
2
3
4
5
6
7
8
MgI2
MgI2
MgI2
MgI2
MgI2
MgI2
MgI2
MgI2
CH2Cl2
CHCl3
PhMe
THF
Et2O
MeCN
DMF
MeOH
neat
neat
neat
3.0
4.0
4.0
4.0
4.0
5.0
6.0
5.0
1.0
5.0
8.5
8.0
93
92
86
80
83
83
60
65
95
85
70
75
2
3
1b[28]
1c[29]
1
1
94
92
9
MgI2
4
5
6
1d[30]
1e[31]
1f[29]
1
1
1
89
93
93
10
11
12
MgBr2
MgCl2
MgClO4
neat
aThe reaction was carried out by addition of benzaldehyde (1.0 mmol) with
TMSCN (1.2 mmol) in the presence of 5 mol% of MgX2 at room temperature.
bIsolated yield by silica gel flash chromatography.
MgBr2 etherate (Table 1, entry 10). MgCl2 and MgClO4 pro-
vided the moderate yields (Table 1, entries 11–12).
7
8
1g[32]
1
1
94
90
With these optimal conditions in hand, we chose a var-
iety of structurally divergent aldehydes possessing a wide
range of functional groups to understand the scope and gen-
erality of this MgI2ꢀ(Et2O)n-catalyzed Strecker reaction to
afford cyanohydrins under solvent-free conditions. The
results are listed in Table 2. The reaction proceeded
smoothly at room temperature and provided cyanohydrins
1h[33]
9
1i[27]
1j[31]
1k[33]
1l[30]
1m[34]
0.5
0.5
0.5
1
99
98
97
95
94
in good to excellent yields in
a
short time. 1-
Naphthaldehyde is also effective and gave the corresponding
adduct in 94% yield (Table 2, entry 2). Aromatic aldehydes
bearing electron-donating groups afforded high yields of the
desired products (Table 2, entries 3–8). Furthermore, nearly
quantitative yields were obtained for aromatic aldehydes
possessing electron-withdrawing groups at the para position
(Table 2, entries 9–11) and excellent yields were afforded
with ortho-substituted aromatic aldehydes (Table 2, entries
12–13). Heteroaromatic aldehydes such as furan-2-carbalde-
hyde, thiophene-2-carboxaldehyde and indole-2-carbalde-
hyde gave high yields (Table 2, entries 14–16). Moreover, an
a, b-unsaturated aldehyde such as cinnamaldehyde also gave
1,2-adduct product in good yield (Table 2, entry 17).
Aliphatic aldehydes, especially bearing a bulkier substituent
such as tert-butyl group, also afforded good yields (Table 2,
entries 18–19). However, the condensation of aliphatic and
aromatic ketones, such as cyclohexanone and acetophenone,
with TMSCN were found to be unsuccessful due to its lower
reactivity in this catalysis process.
The delicate chemoselectivity of Strecker reaction was
evaluated by crossover experiments of various aldehydes
with TMSCN, respectively. MgI2 etherate shows high levels
of aromatic aldehydes discrimination in the competitive
reactions with TMSCN (Table 3). Firstly, MgI2ꢀ(Et2O)n can
uniquely recognize the delicate difference in electronic effect
involved in aromatic aldehydes. p-Anisaldehyde is less react-
ive than benzaldehyde and the cyanosilylated product of
10
11
12
13
0.5
14
15
1n[31]
1o[35]
1
1
88
89
16
17
1p[31]
1q[32]
1
1
88
86
18
19
1r[34]
1s[28]
2
2
89
87
aThe reaction was carried out by addition of aldehyde (0.5 mmol) with TMSCN
(0.6 mmol) catalyzed by 5 mol % of MgI2ꢀ(OEt2)n under solvent-free condi-
tions at room temperature.
bIsolated yield by silica gel flash chromatography.
was exclusively obtained over benzaldehyde (Table 3, entry
2). In crossover reactions of p-bromobenzaldehyde with p-
benzaldehyde was predominately afforded (Table 3, entry 1). nitrobenzaldehyde or p-fluorobenzaldehyde, respectively, the
Similarly, the cyanosilylated product of p-nitrobenzaldehyde reaction exclusively gave the cyanosilylated product of p-

PHOSPHORUS, SULFUR, AND SILICON AND THE RELATED ELEMENTS
3
Table 3. Crossover cyanosilylation of aldehydes with TMSCN.a
Entry
R
R’
Ratio (1:1’)b
Overall Yield (%)c
1
2
3
4
5
6
7
C6H5
4-MeOC6H4
C6H5
4-BrC6H4
4-BrC6H4
4-MeOC6H4
(CH3)3C
>99/<1
>99/<1
>99/<1
>99/<1
>99/<1
>99/<1
>99/<1a
93
98
98
97
93
98
89
4-NO2C6H4
4-NO2C6H4
4-FC6H4
3-MeOC6H4
4-NO2C6H4
C2H5
(CH3)3C
aReactions were run with a mixture of 1.0 mmol of each aldehyde, 1.0 mmol of TMSCN and 0.05 mmol of MgI2ꢀ(Et2O)n under solvent-free conditions at room
bThe ratio was determined by isolated yield on silica gel column.
cIsolated overall yields.
temperature.
nitrobenzaldehyde or p-fluorobenzaldehyde (Table 3, entries aldehydes, unsaturated aldehydes and aliphatic aldehydes
3–4). More significantly, MgI2 etherate shows the remark- with TMSCN catalyzed by MgI2 etherate with highly chemo-
able preference for p-anisaldehyde over m-anisaldehyde selective manner. This magnesium-catalyzed Strecker reac-
(Table 3, entry 5). As well, the reactivity of p-nitrobenzalde- tion is mild, efficient and operationally simple. Further
hyde is better than that of pivalaldehyde in this MgI2 ether- investigation on the asymmetric Strecker reaction catalyzed
ate catalysis (Table 3, entry 6). In addition, propionaldehyde
is much more reactive than pivalaldehyde due to its steric
hinderance (Table 3, entry 7). Apparently, the relative
reactivity of aromatic aldehydes in the MgI2 etherate-pro-
moted process is determined almost solely by electrophilicity
of carbonyl group of aromatic aldehydes.
by chiral Mg(II)-complex is ongoing in our lab.
Disclosure statement
No potential conflict of interest is reported by the authors.
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Experimental
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