Multinuclear Zn(II)-arylhydrazone complexes as catalysts for cyanosilylation of aldehydes

Article abstract of DOI:10.1016/j.jorganchem.2020.121171

Three known multinuclear Zn(II)-arylhydrazone complexes, [Zn{(CH₃)₂SO}(H₂O)(L¹)] (1), [Zn₂(CH₃OH)₂(μ-L²)₂] (2) and [Zn₄(μ-OH)₂(1κO:2κO

Full text of DOI:10.1016/j.jorganchem.2020.121171

Journal Pre-proof
Multinuclear Zn(II)-arylhydrazone complexes as catalysts for cyanosilylation of
aldehydes
Zhen Ma, Vusala A. Aliyeva, Dilgam B. Taghiyev, Fedor I. Zubkov, Firudin I.
Guseinov, Kamran T. Mahmudov, Armando J.L. Pombeiro
PII:
S0022-328X(20)30072-3
DOI:
https://doi.org/10.1016/j.jorganchem.2020.121171
JOM 121171
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 4 December 2019
Accepted Date: 6 February 2020
Please cite this article as: Z. Ma, V.A. Aliyeva, D.B. Taghiyev, F.I. Zubkov, F.I. Guseinov, K.T.
Mahmudov, A.J.L. Pombeiro, Multinuclear Zn(II)-arylhydrazone complexes as catalysts for
cyanosilylation of aldehydes, Journal of Organometallic Chemistry (2020), doi: https://doi.org/10.1016/
j.jorganchem.2020.121171.
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Multinuclear Zn(II)-arylhydrazone complexes as catalysts for
cyanosilylation of aldehydes
3
4
5
Zhen Ma 1,2
,
,
, Vusala A. Aliyeva 2,3 Dilgam B. Taghiyev Fedor I. Zubkov , Firudin I. Guseinov ,
Kamran T. Mahmudov 2,6, and Armando J. L. Pombeiro
2,
*
*
1
School of Chemistry and Chemical Engineering, Guangxi University, Guangxi 530004, PR China
Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049–001
Lisboa, Portugal
Institute of Catalysis and Inorganic Chemistry, ANAS, AZ1143 Baku, Azerbaijan
Organic Chemistry Department, Faculty of Science, Peoples' Friendship University of Russia (RUDN University), 6
Miklukho-Maklaya St., Moscow, 117198, Russian Federation
2
3
4
5
6
National University of Science and Technology (MISIS), Moscow Leninsky Prospect 4, Russian Federation
Department of Chemistry, Baku State University, Z. Xalilov Str. 23, Az 1148 Baku, Azerbaijan
* Correspondence: kamran_chem@yahoo.com; kamran_chem@mail.ru (K.T.M.); pombeiro@tecnico.ulisboa.pt
(A.J.L.P.); Tel.: +351 218419237; Fax: (+351) 218464455
This paper is dedicated to to the 65th anniversary of INEOS and the 120th anniversary academician
Alexander N. Nesmeyanov
Abstract: Three known multinuclear Zn(II)-arylhydrazone complexes, [Zn{(CH3)2SO}(H2O)(L1)]
(1), [Zn2(CH3OH)2(µ-L2)2] (2) and [Zn4(µ-OH)2(1κO:2κO-HL3)4(κO-HL3)2(H2O)4] (3) were
prepared upon reaction of ZnCl2 or Zn(CH3COO)2·2H2O with 3-(2-(2-hydroxy-4-nitrophenyl)
hydrazineylidene) pentane-2,4-dione (H2L1), 3-(2-(2-hydroxyphenyl)hydrazineylidene)pentane-
2,4-dione (H2L2) and 2-(2-(2,4-dioxopentan-3-ylidene)hydrazineyl)benzoic acid (H2L3),
respectively, in methanol solution. Compounds 1−3 were tested as catalysts for the cyanosilylation
reaction of a diversity of both aliphatic and aromatic aldehydes with trimethylsilyl cyanide
yielding the corresponding cyanohydrin trimethylsilyl ethers in high yields (72−98 %) in methanol
and at room temperature.
Keywords: Mono-, di and tetranuclear Zn(II) complexes; catalysis; cyanosilylation of aldehydes
1. Introduction
Cyanohydrins are well-known synthetic intermediates for the synthesis of α-hydroxy acids, α-
hydroxyketones, α-aminonitriles, β-hydroxyamines, β-aminoalcohols, etc., which are also found as
components of pharmaceuticals [1,2]. In order to avoid the volatile and extremely toxic HCN in the
synthesis of cyanohydrins, the cyanation of aldehydes or ketones with trimethylsilyl cyanide
(TMSCN) as the cyanide source, is a much-explored reaction (Scheme 1).
Scheme 1. Cyanosilylation of aldehydes.
In the absence of any catalyst, the reaction of benzaldehyde with TMSCN shows a limited
conversion of 13 or 18 % after 3 or 14h [3,4], respectively, at room temperature. Thus, a great
number of organo- and metal catalysts have been applied in order to improve the reaction yield, for
example, 1,1,3,3-tetramethylguanidine [5], P(RNCH2CH2)N [6], N-methylmorpholine N-oxide [7],
1

N-heterocyclic carbenes [8,9], tetraethylammonium 2-(N-hydroxycarbamoyl)benzoate [10], LiCl
and LiClO4 [11−13], MgBr2·Et2O [14], Mg-Li bimetallic complex [15], Yb(CN)3 [16], Yb(OTf)3
[17], Cu(OTf)2 [18], ZnI2 [19], KCN:18-crown-6 [20], R2SnCl2 an Sn-montmorillonite [21,22], BF3
[23], VO(OTf)2 [24], InBr3 [25], FeCl3 [26], Zr(KPO4)2 [27], NbF5 [28], Fe(Cp)2PF6 [29], Cu(II)-
arylhydrazones
[30−32], lead(II)-3-aminopyrazine-2-carboxylate
[33],
lanthanide-containing
polyoxometalates intercalated layered double hydroxides [34], supported ionic liquid [35], metal
organic frameworks (MOF's) [36−41], etc. Most of these catalysts require harmful solvents,
heating, prolonged reaction times, provide rather modest yields, or recover and reuse of catalysts
are difficult. Therefore, the search the efficient catalyst for cyanosilylation of aliphatic and aromatic
aldehydes with TMSCN in high yields, under mild reaction conditions, still remains a challenge in
the field of current organic synthesis.
As other transition metals, zinc complexes have also been used as catalysts in organic synthesis
[42]. According to the mechanism of Zn(II)-catalyzed transformation of aldehydes or ketones, the
coordination of the oxygen atom of carbonyl group to metal centre increases the electrophilic
character of carbon atom at the C=O group of aldehyde or ketone towards the nucleophilic attack by
the second substrate [43]. Thus, Zn(II) complexes are able to activate C=O bonds in carbonyl
compounds as alternative catalysts instead of expensive Zr, In, V, Ln, etc. based metal catalysts.
Inspired by the previously obtained positive results with Zn(II)-arylhydrazone complexes in the
Henry reaction by our group [44], herein we expand the catalytic potential of the complexes
[Zn{(CH3)2SO}(H2O)(L1)] (1), [Zn2(CH3OH)2(µ-L2)2] (2) and [Zn4(µ-OH)2(1κO:2κO-HL3)4(κO-
HL3)2(H2O)4] (3) towards the cyanosilylation of aliphatic and aromatic aldehydes with TMSCN
under mild conditions.
Scheme 2. Schematic representations of complexes 1−3 [44,45].
2

3. Experimental
3.1. Materials and instrumentation
All the chemicals were ordered from commercial sources (Aldrich) and used as received. H2L1-3 and
1
1−3 were synthesized according to the reported procedures [42,43]. The H NMR spectra were
recorded at room temperature on a Bruker Avance II + 300 (UltraShieldTM Magnet) spectrometer
operating at 300.130 MHz for proton. The chemical shift is described in ppm using
tetramethylsilane as the internal reference.
3.2. Catalytic activity studies
In a typical cyanosilylation procedure, the solvent (CH2Cl2, THF or anhydrous MeOH; 2 mL)
was added to a flask which contained the catalyst (1−3) (1−6 mol%), aldehyde (1.0 mmol) and
trimethylsilyl cyanide (1.2 mmol), the solution was stirred continuously and the progress of the
reaction was monitored by TLC. After a certain time, the solvent was removed, 3 mL water was
added to the residue and extracted with diethyl ether (3×10 mL). After evaporation of diethyl ether
under vacuum, the extract was analyzed by 1H-NMR spectroscopy in CDCl3, to evaluate the yield
of the reaction products [46−48].
3. Results and discussion
Our initial experiments were performed using 1−3 as catalysts (Scheme 2), benzaldehyde and
TMSCN as model coupling partners in protic (methanol) and aprotic (1,2-dichloromethane and
THF) solvents. The results presented in Table 1 indicate the significant influence of the solvent
polarity on the yield of 2-phenyl-2-((trimethylsilyl)oxy)acetonitrile. Thus, the reaction is much
efficient in the protic MeOH solvent, moreover, 1−3 show a much high solubility than in CH2Cl2 or
THF (entries 1−9, Table 1). In comparison with the mononuclear 1 and dinuclear 2 complexes, the
tetranuclear 3 is the most efficient catalyst (on a molar basis) for the cyanosilylation, to give 2-
phenyl-2-((trimethylsilyl)oxy) acetonitrile in ca. 80% yield in MeOH (Table 1, entry 9) in 16 h
using 5 mol% of the catalyst. The high catalytic activity of 3 can eventually be associated to the
acidic protons of the bridged hydroxyl groups, where the −OH group(s) can play a H-bond donor
role in the activation of the C=O group of benzaldehyde (see below).
Free H2L3 bearing the −COOH group on the aromatic moiety afforded 2-phenyl-2-
((trimethylsilyl)oxy)acetonitrile in 26% yield in methanol (entry 18, Table 1). When using ZnCl2 or
Zn(CH3COO)2·2H2O as the catalysts, only 42 or 46 % yield was observed in MeOH (entries 19 and
20, Table 1), which suggests the cooperative actions of coordination (by metal centre) and weak
interactions (by ligand) in the addition of CN into benzaldehyde (see below). A low yield of 2-
phenyl-2-((trimethylsilyl)oxy)acetonitrile was under either catalyst-free or solvent-free conditions
3

(entries 21-26). After checking the catalytic activities of 1−3, H2L1-3 and Zn(II) salts, 3 and
methanol were chosen as preferable catalyst and solvent, respectively, for further studies.
Table 1. Selection of catalyst for the cyanosilylation of benzaldehyde with TMSCN.a.
Entry
1
Catalyst
Time (h)
16
Catalyst amount (mol%) Solvent Yieldb (%)
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
-
CH2Cl2
CH2Cl2
CH2Cl2
THF
36
34
38
37
36
39
66
66
80
16
15
17
17
16
15
24
22
26
42
46
17
45
14
22
13
20
1
2
16
2
3
16
3
4
16
1
5
16
THF
2
6
16
THF
3
7
16
MeOH
MeOH
MeOH
CH2Cl2
CH2Cl2
CH2Cl2
THF
1
8
16
2
9
16
3
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
H2L1
16
H2L2
16
H2L3
16
H2L1
16
H2L2
16
THF
H2L3
16
THF
H2L1
16
MeOH
MeOH
MeOH
MeOH
MeOH
CH2Cl2
CH2Cl2
THF
H2L2
16
H2L3
16
ZnCl2
16
Zn(CH3COO)2·2H2O
16
-
-
-
-
-
-
48
6 days
24
-
-
16
-
MeOH
-
16
-
24
-
-
a Reaction conditions: solvent (2 mL), TMSCN (1.2 mmol) and benzaldehyde (1.0 mmol), in air at
room temperature. b Determined by 1H NMR.
Shortening the reaction time from 16 h to 1 h results in a significant yield decrease from 80 to
63 % (entries 1-5, Table 2), but shortening to 3h leads to almost the same yield as after 16h (entry 3
versus entry 5). Hence, 3h is the most adequate reaction time. When the catalyst loading increased
from 1.0 mol% to 4.0 mol%, the yield of product increased from 58% to 79 % (entries 6-9, Table
2), respectively, but when the catalyst loading increased further (to reach 6 mol%), the yield
o
increased by only 0.1 % (entry 11, Table 2). Lowering the temperature to 15 C led to an yield
decrease (entry 12, Table 2), while the increase of temperature (up to 55 oC) has only a small effect
on the yield (entries 13−15, Table 2) and hence room temperature was typically used.
4

Table 2. Evaluation of reaction conditions of cyanosilylation of benzaldehyde with
TMSCN catalyzed by 3.a.
Entry Time (h) Catalyst amount (mol%) T (˚C) Yieldb (%)
1
2
3
4
5
1
2
3
5
16
5
5
5
5
5
25
25
25
25
25
63
76
79
80
80
6
7
8
9
10
11
3
3
3
3
3
3
1
2
3
4
5
6
25
25
25
25
25
25
58
65
78
79
79
79
12
13
14
15
3
3
3
3
4
4
4
4
15
35
45
55
73
80
80
81
a
b
Reaction conditions: MeOH (2 mL), TMSCN (1.2 mmol) and benzaldehyde (1.0 mmol), in air.
Determined by 1H NMR.
The activities of catalyst 3 for other aldehydes in TMSCN reactions were compared under the
optimized reaction conditions. The corresponding derivatives of cyanohydrin were generated in
yields between 72 and 98 % (Table 3). In the case of aliphatic aldehydes, the yields were rather high
(in the 93-98% range) and decreased with an increase of the chain length (entries 1-5, Table 3). For
instance, propionaldehyde gave a higher yield than the other longer aliphatic aldehydes. Under the
same conditions, the reaction of aromatic aldehydes with TMSCN afforded the cyanohydrin
trimethylsilyl ethers in a lower yield (72−83%, entries 6-14, Table 3). Aromatic aldehydes with
para electron-withdrawing substituents (such as −Br, −Cl or –NO2) revealed the highest reactivities
(entries 10-12, Table 3) as compared to those having electron-donor substituents [−CH3, −OCH3 or
−N(CH3)2] (entries 6−8, Table 3). Both sterically hindered 2-methylbenzaldehyde and 2,4,6-
trimethylbenzaldehyde gave the corresponding cyanohydrin trimethylsilyl ethers in relative low
yields, 74.1 and 71.8%, respectively (entries 13 and 14, Table 3).
A comparison of the activity of our catalyst 3 with other reported catalytic systems for the
cyanosilylation of benzaldehyde with TMSCN shows several advantages. For example: i) 3 shows a
higher catalytic activity (79%) in methanol medium in comparison to a Zn(II)-MOF catalyst (32%)
in CH2Cl2 [49] and a Cu(II) coordination polymer (27%) in n-pentane [50]; ii) a shorter reaction
time (3 h) and room temperature are required in our system in comparison to the higher (40 oC) [50]
or lower (-50 oC) [51] temperatures, and a longer reaction time (96 h) [52].
5

Table 3. Addition of TMSCN to aliphatic and aromatic aldehydes catalysed by catalyst 3
in methanol.a.
Entry
Substrate
Yieldb (%)
1
2
3
4
5
CH3CH2CHO
98
97
96
96
93
CH3(CH2)2CHO
CH3(CH2)3CHO
CH3(CH2)4CHO
CH3(CH2)5CHO
6
7
76
77
78
79
80
81
83
8
9
10
11
12
13
74
14
72
a Reaction conditions: 4 mol% of catalyst 3, MeOH (2 mL), TMSCN (1.2 mmol) and aldehyde (1.0
mmol); reaction time: 3 h. b Determined by 1H NMR.
The mechanism of cyanosilylation reaction should be similar to those proposed for other metal
complexes catalyzed systems, involving activation, upon coordination to the metal centre, of the
aldehyde towards nucleophilic attach by TMSCN [1]. In our system, the catalytic transformation
possibly can be assisted by cooperation of hydrogen bonding (provided by ligand and MeOH) with
coordination, with an increase of the electrophilic character of carbon atom at the C=O group of
benzaldehyde towards the nucleophilic addition of the C≡N moiety, also with the support of a tetrel
bonding between the Si atom of TMSCN and a carboxylate oxygen of a ligand (Scheme 3)
[1,53,54]. This would be followed by migration of the silyl group to the oxygen and subsequent
liberation of the cyanohydrin trimethylsilyl ether.
6

Scheme 3. Proposed cooperation of coordination and noncovalent interactions in the
postulated intermediate.
4. Conclusions
Three previously reported Zn(II)-arylhydrazone complexes [44,45] have been prepared and
used as catalysts in the cyanosilylation of both aromatic and aliphatic aldehydes with TMSCN. On
account of the acidic proton(s) of the bridged hydroxyl group(s), the catalytic activity of complex 3
(tetranuclear) is higher than those of 1 (mononuclear) and 2 (dinuclear) in methanol at room
temperature, producing cyanohydrin trimethylsilyl ethers in high yields (72−98%) in 3 h. As
substrates, both aliphatic and aromatic aldehydes are employed effectively, and the electron-donor
or withdrawing character of a substituent in the para position of the aromatic aldehydes has an
inhibiting or promoting effect, respectively, on the reaction yield.
Not only a conventional coordination bond, but also hydrogen and tetrel noncovalent bonds are
postulated in the proposed mechanism for the addition of TMSCN to the aldehyde. Further studies
focusing on the modification of arylhydrazone ligands in the design of coordination compounds and
their use as catalysts for cyanosilylation and other C−C coupling reactions promoted by
noncovalent interactions are currently under study and will be reported in due course.
Acknowledgements
This research was founded by the Foundation for Science and Technology (FCT), project
UID/QUI/00100/2019. KTM acknowledges the FCT and Instituto Superior Técnico (DL 57/2016
and L 57/2017 Program, Contract no: IST-ID/85/2018). Authors are thankful to the Portuguese
NMR Network (IST-UL Centre) for access to the NMR facility and the IST Node of the Portuguese
Network of mass-spectrometry for the ESI-MS measurements. This work also was supported by the
''RUDN University Program 5-100''. This work was supported by the Ministry of Education and
Science of the Russian Federation in the framework of Increase Competitiveness Program of NUST
«MISiS» (grant no. К2-2019-005).
7

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10

Highlights
► Multinuclear zinc(II) complexes with arylhydrazones of active methylene compounds
► A tetranuclear zinc(II) complex shows high catalytic activity in cyanosilylation of aldehydes
► The yield of cyanohydrin trimethylsilyl ethers depends on the amounts of catalyst and nature of
solvents

Conflicts of Interest: The authors declare no conflicts of interest.