Layered double hydroxides (LDHs): As efficient heterogeneous catalyst for the cyanosilylation of aromatic aldehydes

Article abstract of DOI:10.1080/10426507.2018.1492920

We report the first application of Layered Double Hydroxides (LDHs) as efficient and novel heterogeneous catalyst for the cyanosilylation of aldehydes with excellent yields and simple work up. The reaction between different aldehydes with electron-withdra

Full text of DOI:10.1080/10426507.2018.1492920

Phosphorus, Sulfur, and Silicon and the Related Elements
ISSN: 1042-6507 (Print) 1563-5325 (Online) Journal homepage: http://www.tandfonline.com/loi/gpss20
Layered double hydroxides (LDHs): as efficient
heterogeneous catalyst for the cyanosilylation of
aromatic aldehydes
Fahimeh Afi, Javad Mokhtari & Fatemeh Tahoori
To cite this article: Fahimeh Afi, Javad Mokhtari & Fatemeh Tahoori (2018): Layered
double hydroxides (LDHs): as efficient heterogeneous catalyst for the cyanosilylation of
aromatic aldehydes, Phosphorus, Sulfur, and Silicon and the Related Elements, DOI:
10.1080/10426507.2018.1492920
To link to this article: https://doi.org/10.1080/10426507.2018.1492920
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Layered Double Hydroxides (LDHs): As Efficient Heterogeneous Catalyst for the
Cyanosilylation of Aromatic Aldehydes
Fahimeh Afi1, Javad Mokhtari1,*, Fatemeh Tahoori2
1Department of Chemistry, Science and Research Branch, Islamic Azad University, P.O. Box 14515/775, Tehran,
Iran
2Razi Vaccine and Serum Research Institute, Agricultural Research and Extension Organization (AREEO), Karaj,
Iran
*Corresponding Author Javad Mokhtari j.mokhtari@srbiau.ac.ir
Abstract
We report the first application of Layered Double Hydroxides (LDHs) as efficient and novel
heterogeneous catalyst for the cyanosilylation of aldehydes with excellent yields and simple
work up. The reaction between different aldehydes with electron-withdrawing and releasing
groups and trimethylsilyl cyanide (TMSCN) proceeds in dry CH2Cl2 at room temperature in the
presence of Mg-Al-Cu LDH. The catalyst recycled and reused for four times without loss of
catalytic activity. The structures of all compounds were corroborated spectroscopically (1H- and
13C-NMR, and elemental analysis). A plausible mechanism for this type of reaction is proposed.
Graphical Abstract
1

Keywords: Layered double hydroxide (LDH), cyanohydrin, cyanosilylation, heterogeneous
catalyst
Introduction
From a synthetic point of view, the preparation of cyanohydrins is especially significant, because
they serve as key intermediates in the synthesis of many biologically important compounds such
as β-amino alcohols, α-hydroxy acids, β-hydroxy alcohols and α-hydroxy aldehydes [1-4]. The
easiest suggested route for the synthesis of cyanohydrins is the catalytic cyanosilylation of
carbonyl compounds [5-6]. Several catalysts have been used to this aim in which cyanohydrins
have been produced from aldehydes and ketones. Some of these catalysts are; Lewis acids and
bases [7-13], synthetic peptides [14, 15], helical poly (amino acid)s [16], enzymes [17,18], chiral
metal complexes with various ligands like sulfoximines [19], Metal-Organic Frameworks
(MOFs) [20], BINOLs [21, 22], β-amino alcohol [23], and other ligands [24-27]. However, some
of these catalytic systems require multi-step preparation, high catalyst loading, inert atmosphere,
long reaction times and laborious work-up procedure [7–27]. Therefore, development of new
catalysts which operate under milder conditions is a challenge attracting much attention.
Doubled Layered Hydroxides (LDHs) are synthetic compounds that can be prepared in the
laboratory. This term is used for synthetic compounds designed with natural hydroxides with
basic layers containing two or more metal cations with anionic species in the inner layers [28].
2

The general formula proposed for LDH is M(II)1-x M(III) x(OH)2]+ x [(A-n)x/ n.mH2O, where M(II)
and M(III) metal species are di- and tri- valent and A is an anion (Figure 1) [29].
Figure 1.General formula for LDHs [29b]
Such compounds have brucite-like structure in the center of the octahedral oxygen and two
dimensional layers that are formed from sharing octahedral edges, organic or inorganic anions
are placed between the layers in a way that balance is established between the charges [30, 31].
There are numerous applications for this class of compounds, such as good supporting for the
different catalysts [32-34] and in organic synthesis including epoxidation reaction of styrene
[35], Knoevenagel condensation [36, 37], Heck-Suzuki reaction [38], hydroxylation of phenol
[39] and carbonylation of methanol to methyl acetate [40]. On the other hand this class of
compound was used as environmental catalysis [41-43], ion exchange/adsorption [44]
pharmaceutics [45-47], photochemistry [48-50] and electrochemistry [51-53].
Considering the catalytic activity of these compounds as mentioned above and as part of our
ongoing work on the synthesis of cyanohydrins as a pharmacologically interesting intermediates
by different salt of N-hydroxyl phthalimides [54], herein we report the first application of LDHs
as efficient and novel heterogeneous catalyst for the cyanosilylation of carbonyl compounds.
Results and discussion
The goal of the present work was to propose simple and convenient method for the synthesis of
Mg-Al-Cu LDH, and its application in the synthesis of cyanosilylethers from aldehydes and
TMSCN. Mg-Al-Cu LDH has been used as a catalyst in different organic synthesis in recent
years [55]. In the present work, the Mg-Al-Cu LDH was synthesized by ball-mill at room
temperature under solvent-free condition. The process was completed within 0.5h and resulting
in a white powder. To confirm the mechanochemical preparation of LDH, X-ray powder
diffraction was performed. The observed peaks at 2Theta of 11, 22, 34, 38, 44, and 61 (Fig. S 1
Supplemental Materials) are in a good agreement with reported in literature [56]. Also in FT-IR
3

spectrum, strong and broad peak at 3450 cm-1 indicates the presence of OH group. The peaks in
2-
1639 cm-1 and 1365 cm-1 are related to CO3 [57] and peaks from 400 to 1000 cm-1 were
attributed to the stretching and bending vibrations of M-O and M-OH [58] (Fig. S 1 and S 2).
The synthesized Mg-Al-Cu LDH was employed as a heterogeneous catalyst in the
cyanosilylation of aldehydes. Initially, for the optimization of reaction condition, the reaction of
benzaldehyde with TMSCN was selected as a model reaction. The effect of various parameters
such as catalyst loading and nature of the solvent on its cyanosilylation at room temperature was
studied. The results have been summarized in Table 1. As shown in Table 1, LDH catalyst (20
mol%) was found to facilitate the cyanosilylation in quantitative yield within 30 min in CH2Cl2
(table 1, entry 2), while by increasing the catalyst up to 30 mg it still affords the desired products
in quantitative yields within short reaction times (Table 1, entry 5). With decreasing the catalyst
loading to 10 mg the yield decreased to 85%. Furthermore, no reaction was observed under
similar reaction conditions in the absence of any LDH catalyst (Table 1, entry 6). Between
CH3CN and CH2Cl2 as a solvent, the best was CH2Cl2 and provided the product of the
cyanosilylation in high isolated yield (Table 1, Entry 3, 4, 5).
Table 1. Optimization and comparison of the cyanosilylation of benzaldehyde in various conditions
Having optimal conditions of reaction (20 mg of LDH, TMSCN/aldehyde = 1.2 equiv., CH2Cl2,
r.t.) in hand, the reactivity of different aldehydes were investigated in the next step. The results
have been summarized in Table 2. Trimethylsilylcyanation of several aromatic aldehydes
including electron-withdrawing and releasing groups was investigated and the results showed
that type of substitution no significant change in the yields (Table 2, Entry 2-10), but steric
hindrance at the ortho position results in lower yields (Table 2, Entry 6, 9, 10).
Table 2. Results of derivatives synthesis
One of the important properties of the heterogeneous catalyst is the reusability, so to confirm this
for our catalyst, we performed recycling experiment; so the cyanosilylation of benzaldehyde was
4

chosen as a model reaction with recycled catalyst; the reaction was completed after 0.5 h and the
conversion and isolated yield of the reaction was 100 and 94 %, respectively. This experiment
was performed for four more times and the result showed a negligible change in catalyst activity
and yield of the product.
The mechanism of the LDH-catalyzed cyanosilylation probably involves the initial activation of
carbonyl group in aldehyde at the unsaturated within M (II) or M (III) (Mg2+, Cu2+, Al3+) sites
and then activation of TMSCN by exposed OH- of the LDH and subsequent release of the
nucleophilic cyanide ion to add to the carbonyl moiety in a rate-determining step followed by
silylation and formation of the product 3 (Scheme 1). Therefore, it is likely that both TMSCN
and carbonyl compounds are activated by surface oxygen atoms (nucleophilically) and the Cu
(II) (electrophilically), respectively [59].
Scheme 1. Postulated mechanism of the reaction
Experimental
All solvents and chemical materials were purchased from Sigma–Aldrich and Merck. Progress of
reactions was monitored by thin layer chromatography while purification was effected by
column chromatography, using silica-gel (Merck 230-240 mesh). X-ray powder diffraction
(XRD) measurements were performed using an X’pert MPD. Philips diffractometer with Cu
radiation source (λ = 1.54050 Å) at 40 kV voltage and 40 mA current. FT-IR spectra were
1
recorded with a Shimadzu 8400s FT-IR spectrometer using potassium bromide pellets. H and
13C NMR spectra were measured with a Bruker DRX-500 AVANCE spectrometer at 500.13 and
125.8 MHz, with CDCl3 as the solvent, respectively. Elemental analyses for C, H, and N:
Heraeus CHN-O-Rapid analyzer. The Supplemental Materials contains sample 1H and 13C NMR
spectra of the known products (Figures S 3 – S 9).
5

Synthesis of Mg-Al-Cu-NO3 LDH
A mixture of Mg(OH)2 ( 2.45 g, 42 mmol) and Al(OH)3 (0.55 g, 7 mmol) with molar ratio of 6:1
were ball-milled at 28 Hz, for 5h at room temperature in solvent-free conditions. Then, resulting
powder was added to 30 ml Cu(NO3)2·3H2O (0.17mM) and the mixture was transferred to an
autoclave and placed in the oven at 80 oC for 12 h. After that, the resulting precipitate was
filtered or centrifuged and washed with water. The resulting cake was dried at oven for 12h in
60°C to yield 2.5 gr of Mg-Al-Cu-NO3 LDH. The structure of synthesized LDH characterized
and confirmed by comparison with the literature reports [56] (Fig. S 1 and S 2).
General procedure for cyanosilylation of carbonyl compounds catalyzed by LDH:
TMSCN (1.2 mmol, 0.15 ml) was added dropwise to the stirred solution of 1.0 mmol of a
carbonyl compounds and 20 mg of LDH in dry CH2Cl2 (5 ml). The mixture was stirred at room
temperature for 30 min. The reaction was monitored by TLC. Then, the reaction mixture was
quenched by water (5 mL) and LDH filtered. The organic compound was extracted with CH2Cl2
(2×5.0 mL). The obtained organic phase was washed with water (5.0 mL) and dried over
Na2SO4. The solvent was evaporated to afford the desired products which in some cases were
essentially pure cyanohydrin TMS ethers. Further purification of the products was performed by
silica gel column chromatography (EtOAc-Hexane, 1:8).
Conclusions
In summary, Mg-Al-Cu Layered double hydroxide (LDH) was found to be an good heterogenous
catalyst for cyanosilylation of various aromatic aldehydes with electron-withdrawing and
releasing group under mild conditions. Attractive features of the method include the general
application, mild conditions and simplicity in the preparation, operation and separation of the
catalyst by simple filtration. The catalytic ability of LDHs has been developed for the addition of
TMSCN to carbonyl compounds for the first time. Further studies on preparation of other useful
LDHs and development of its catalytic scope are in progress and will be presented in duecourse.
6

Acknowledgements
The authors gratefully acknowledge from Science and Research Branch of Islamic Azad University and
Razi Vaccine and Serum Research Institute for partial financial support of this work.
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14

Figure 1.General formula for LDHs [30b]
Table 1. Optimization and comparison of the cyanosilylation of benzaldehyde in various conditions a
Entry
Catalyst (mg)
Solvent
Time (h)
Yield (%)
1
2
3
4
5
6
10
20
30
20
40
-
CH2Cl2
CH2Cl2
CH2Cl2
CH3CN
CH3CN
CH2Cl2
0.5
0.5
0.5
8
8
8
85
95
95
65
75
0
a Reaction conditions: 20 mol% of catalyst (Mg-Al-Cu LDH), solvent (5
mL), TMSCN (1.2 mmol) and aldehyde (1 mmol).
15

Table 2.Results of derivatives synthesis a
Entry
Aldehyde
Product
Yield (%)b
Lit. Ref.
1
95
60
3a
2
3
4
5
6
96
92
93
95
89
60
61
61
61
60
3b
3c
3d
3e
3f
7
90
62
3g
8
9
92
88
90
54b
60
3h
3i
10
54b
3j
a Reaction conditions: 20 mol% of catalyst (Mg-Al-Cu LDH), CH2Cl2 (5 mL), TMSCN (1.2 mmol) and aldehyde (1 mmol). Time: 0.5 h.
b Isolated yield
16

Scheme 1. Postulated mechanism of the reaction
17