Direct synthesis of cyanohydrin esters from aroyl chlorides using potassium hexacyanoferrate(II) as an eco-friendly cyanide source

Article abstract of DOI:10.1007/s11164-013-1421-8

Abstract A direct synthetic method for cyanohydrin esters from aroyl chlorides using potassium hexacyanoferrate(II) as an eco-friendly cyanide source and tributylphosphine as a promoter is described. This protocol has advantages of no use of strong toxic cyanating agents, high yield, and simple work-up procedure.

Full text of DOI:10.1007/s11164-013-1421-8

Res Chem Intermed
DOI 10.1007/s11164-013-1421-8
Direct synthesis of cyanohydrin esters from aroyl
chlorides using potassium hexacyanoferrate(II)
as an eco-friendly cyanide source
•
Zheng Li Zhouxing Zhao
Received: 29 March 2013 / Accepted: 21 September 2013
Ó Springer Science+Business Media Dordrecht 2013
Abstract A direct synthetic method for cyanohydrin esters from aroyl chlorides
using potassium hexacyanoferrate(II) as an eco-friendly cyanide source and tribu-
tylphosphine as a promoter is described. This protocol has advantages of no use of
strong toxic cyanating agents, high yield, and simple work-up procedure.
Keywords Cyanation Á Cyanohydrin ester Á Eco-friendly cyanide
source Á Potassium hexacyanoferrate(II) Á Green chemistry
Introduction
Cyanohydrin esters are versatile building blocks, which can be utilized for the
synthesis of a wide range of fine chemicals, pharmaceuticals, and agrochemicals,
and they are therefore an important subject of research [1–6]. The general synthetic
methods for cyanohydrin esters include: (1) the reactions of aldehydes with aroyl
cyanides catalyzed by bases [7–10]; (2) the reactions of aldehydes with trimeth-
ylsilyl cyanide (or potassium cyanide, lithium cyanide) and acid anhydrides (or acid
chlorides) [11–15]; (3) the reduction coupling of aroyl cyanides using sodium
tetrahydroborate or trimethylphosphine [16, 17]; (4) the reactions of cyanohydrins
with benzoic acid (or benzoyl chloride) [18]; (5) the reactions of O-trimethylsilyl
cyanohydrin with aroyl halides (or acid anhydrides) [19, 20]; and (6) the reactions
of 1-(benzotriazol-1-yl)alkyl esters with potassium cyanide [21]. In these synthetic
methods, strong toxic cyanating agents, such as KCN, and volatile cyanating agents,
such as TMSCN, have been utilized. In addition, the commercially available aroyl
cyanides and cyanohydrins are limited and comparatively expensive. Especially, the
Z. Li (&) Á Z. Zhao
College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou 730070,
Gansu, People’s Republic of China
e-mail: lizheng@nwnu.edu.cn
123

Z. Li, Z. Zhao
synthesis of aroyl cyanides and cyanohydrins have also had to use strong toxic
reagents as original cyanide sources, such as HgCN [22], NaCN [23, 24], CuCN
[25], KCN [26, 27], and TMSCN [28, 29], which render the reactions unsafe and
environmentally unfriendly. Therefore, there is a need to explore environmentally
benign cyanide sources and simple procedures for the synthesis of cyanohydrin
esters.
Potassium hexacyanoferrate(II), K4[Fe(CN)6, is mainly used as a carburizing
agent in the iron and steel industry, and it is also used in the food industry for metal
precipitation. In addition, it has been described as an anti-agglutinating auxiliary for
table salt (NaCl). K4[Fe(CN)6] is a by-product of the coal chemical industry and
commercially available on a ton scale, and it is even cheaper than KCN. Recently,
K4[Fe(CN)6] has been used as a cyanide source for some substitution reactions to
synthesize benzonitriles [30–47], aroyl cyanides [48], benzyl cyanides [49, 50], and
cinnamonitriles [44]. Our current research interests focus on the cyanation of
unsaturated compounds including C=O, C=N and/or C=C by nucleophilic addition
reactions using K4[Fe(CN)6] as an eco-friendly cyanide source [51–56].
Here, we report an efficient method for the direct synthesis of cyanohydrin esters
from aroyl chlorides using K4[Fe(CN)6] as an eco-friendly original cyanide source.
Experimental
IR spectra were recorded using KBr pellets on an Alpha Centauri FTIR
1
spectrophotometer. H NMR and 13C NMR spectra were recorded on a Mercury-
400BB instrument using CDCl3 as solvent and Me4Si as internal standard.
Elemental analyses were performed on a Vario E1 Elemental Analysis instrument.
Melting points were observed in an electrothermal melting point apparatus.
Potassium hexacyanoferrate(II) was dried at 80 °C under vacuum for 24 h and
finely powdered prior to use.
General procedure for the preparation of cyanohydrin esters
The mixture of aroyl chloride (10 mmol) and potassium hexacyanoferrate(II)
(0.92 g, 2.5 mmol) was heated at 160 °C under stirring for 3 h. Then, the mixture
was cooled to room temperature, and tributylphosphine (2.02 g, 10 mmol) and THF
(10 mL) were added. The resulting mixture was further refluxed for 1 h. Then, the
solid was removed by filtration, the filtrate was evaporated off the solvent, and the
residue was subjected to silica gel flash column chromatography (ethyl acetate,
petroleum ether, 1:25) to obtain the pure product. The analytical and spectral data
for products are given below.
Cyano(phenyl)methyl benzoate (1)
1
White solid. Mp 56–58 °C. H NMR (CDCl3, 400 MHz): d 6.67 (s, 1H, CH),
7.44–7.49 (m, 6H, Ar), 7.60–7.64 (m, 2H, Ar), 8.05–8.08 (m, 2H, Ar); 13C NMR
(CDCl3, 100 MHz): d 63.2, 116.1, 127.7, 128.0, 128.5, 129.1, 130.0, 130.3,
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Direct synthesis of cyanohydrin esters from aroyl chlorides
131.7,134.0, 164.5; IR (KBr) mmax/cm-1: 3,054, 2,959, 2,244, 1,727, 1,600, 1,495,
1,452, 1,269, 1,101, 752, 712; Anal. Calcd. for C15H11NO2: C, 75.94; H, 4.67; N,
5.90; Found: C, 75.88; H, 4.66; N, 5.89.
Cyano(2-chlorophenyl)methyl 2-chlorobenzoate (2)
1
White solid. Mp 50–52 °C. H NMR (CDCl3, 400 MHz): d 6.95 (s, 1H, CH),
7.32–7.50 (m, 6H, Ar), 7.80–7.83 (m, 1H, Ar), 7.90 (d, J = 7.6 Hz, 1H, Ar); 13C
NMR (CDCl3, 100 MHz): d 61.0, 115.0, 126.7, 127.3, 127.6, 129.1, 129.6, 130.2,
131.4, 131.8, 132.0, 133.4, 133.7, 134.6, 163.0; IR (KBr) mmax/cm-1: 3,064,
2,969, 2,256, 1,740, 1,591, 1,476, 1,438, 1,311, 1,232, 1,097, 1,036, 767, 739;
Anal. Calcd. for C15H9Cl2NO2: C, 58.85; H, 2.96; N, 4.58; Found: C, 58.97; H,
2.96; N, 4.60.
Cyano (3-chlorophenyl)methyl 3-chlorobenzoate (3)
1
Light yellow oil. H NMR (CDCl3, 400 MHz): d 6.62 (s, 1H, CH), 7.40–7.50 (m,
4H, Ar), 7.57–7.61 (m, 2H, Ar), 7.93–7.97 (m, 1H, Ar), 8.01–8.03 (m, 1H, Ar); 13
C
NMR (CDCl3, 100 MHz): d 62.8, 115.3, 126.0, 128.0, 128.1, 129.3, 130.0, 130.1,
130.6, 130.7, 133.1, 134.2, 134.8, 135.2, 163.2; IR (KBr) mmax/cm-1: 3,058, 2,959,
2,247, 1,737, 1,601, 1,484, 1,376, 1,243, 1,089, 1,042, 756, 732; Anal. Calcd. for
C15H9Cl2NO2: C, 58.85; H, 2.96; N, 4.58; Found: C, 58.78; H, 2.95; N, 4.59.
Cyano (4-chlorophenyl)methyl 4-chlorobenzoate (4)
Light yellow solid. Mp 64–65 °C. 1H NMR (CDCl3, 400 MHz): d 6.62 (s, 1H, CH),
7.43–7.47 (m, 4H, Ar), 7.54 (d, J = 8.4 Hz, 2H, Ar), 7.97 (d, J = 8.4 Hz, 2H, Ar);
13C NMR (CDCl3, 100 MHz): d 62.7, 115.5, 126.1, 129.0, 129.2, 129.4, 130.0,
131.2, 136.6, 140.7, 163.5; IR (KBr) mmax/cm-1: 3,076, 2,954, 2,236, 1743, 1598,
1465, 1369, 1234, 1096, 798, 773; Anal. Calcd. for C15H9Cl2NO2: C, 58.85; H,
2.96; N, 4.58; Found: C, 58.80; H, 2.97; N, 4.57.
Cyano (3-nitrophenyl)methyl 3-nitrobenzoate (5)
Yellow solid. Mp 65–67 °C.1H NMR (CDCl3, 400 MHz): d 6.25 (s, 1H, CH),
7.43–7.76 (m, 4H, Ar), 7.89–8.12 (m, 2H, Ar), 8.32–8.49 (m, 1H, Ar), 8.62–8.79
(m, 1H, Ar); 13C NMR (CDCl3, 100 MHz): d 61.9, 118.4, 120.9, 124.2, 124.8,
125.4, 128.3, 129.3, 131.2, 134.7, 135.8, 136.2, 147.3, 148.9, 166.3; IR (KBr)
m
746, 712; Anal. Calcd. for C15H9N3O6: C, 55.05; H, 2.77; N, 12.84; Found: C,
55.14; H, 2.78; N, 12.80.
max/cm-1: 3,048, 2,929, 2,242, 1,768, 1,601, 1,492, 1,363, 1,226, 1,096, 1,022,
Cyano(4-nitrophenyl)methyl 4-nitrobenzoate (6)
White solid. Mp 72–74 °C. 1H NMR (CDCl3, 400 MHz): 6.32 (s, 1H, CH), 7.26 (d,
J = 8.4 Hz, 2H, Ar), 8.14 (d, J = 8.4 Hz, 2H, Ar), 8.23–8.34 (m, 4H, Ar); 13C
123

Z. Li, Z. Zhao
NMR (CDCl3, 100 MHz): d 65.2, 118.7, 121.7, 125.3, 129.1, 130.6, 136.2,139.6,
147.9, 154.6, 166.2; IR (KBr) mmax/cm-1: 3,039, 2,918, 2,247, 1,736, 1,601, 1,489,
1,345, 1,218, 1,122, 774, 734; Anal. Calcd. for C15H9N3O6: C, 55.05; H, 2.77; N,
12.84; Found: C, 54.98; H, 2.77; N, 12.81.
Cyano(4-methylphenyl)methyl 4-methylbenzoate (7)
1
White solid. Mp 112–114 °C. H NMR (CDCl3, 400 MHz): d 2.41 (s, 3H, CH3),
2.43 (s, 3H, CH3), 6.63 (s, 1H, CH), 7.27 (d, J = 6.4 Hz, 4H, Ar), 7.51 (d,
J = 8.4 Hz, 2H, Ar), 7.95 (d, J = 8.4 Hz, 2H, Ar); 13C NMR (CDCl3, 100 MHz): d
21.3, 21.8, 63.1, 116.5, 125.4, 127.9, 129.0, 129.4,129.9, 130.1, 140.6, 144.9, 164.7;
IR (KBr) mmax/cm-1: 3,069, 2,938, 2,241, 1,756, 1,611, 1,479, 1,375, 1,242, 1,102,
787, 754; Anal. Calcd. for C17H15NO2: C, 76.96; H, 5.70; N, 5.28; Found: C, 76.84;
H, 5.68; N, 5.26.
Cyano(3,5-dimethylphenyl)methyl 3,5-dimethylbenzoate (8)
Light yellow oil. 1H NMR (CDCl3, 400 MHz): d 2.36 (s, 6H, CH3), 2.37 (s,
6H, CH3), 6.61 (s, 1H, CH), 7.11 (s, 1H, Ar), 7.22 (s, 2H, Ar), 7.25 (s, 1H,
Ar), 7.68 (s, 2H, Ar). 13C NMR (CDCl3, 100 MHz): d 21.1, 21.2, 63.3, 116.5,
125.7, 127.7, 127.9, 131.8, 131.9, 135.7, 138.3, 138.9, 164.9; IR (KBr) mmax/cm-1
:
3,010, 2,922, 2,859, 2,242, 1,727, 1,609, 1,462, 1,301, 1,241, 1,121, 792, 748;
Anal. Calcd. for C19H19NO2: C, 77.79; H, 6.53; N, 4.77; Found: C, 77.85; H,
6.51; N, 4.75.
Cyano(3,4,5-trimethoxyphenyl)methyl 3,4,5-trimethoxybenzoate (9)
Yellow solid. Mp 136–138 °C. 1H NMR (CDCl3, 400 MHz): d 3.87 (s, 3H, OCH3),
3.90 (s, 6H, OCH3), 3.91 (s, 6H, OCH3), 3.92 (s, 3H, OCH3), 6.62 (s, 1H, CH), 6.81
(s, 2H, Ar), 7.30 (s, 2H, Ar); 13C NMR (CDCl3, 100 MHz): d 56.0, 56.1, 60.6, 60.7,
63.4, 105.0, 107.1, 116.0, 122.6, 127.0, 139.3, 143.0, 152.8, 153.5, 164.1; IR (KBr)
m
Anal. Calcd. for C21H23NO8: C, 60.43; H, 5.55; N, 3.36; Found: C, 60.35; H, 5.56;
N, 3.35.
max/cm-1: 3,044, 2,942, 2,239, 1,728, 1,597, 1,469, 1,332, 1,216, 1,143, 765, 743;
Cyano(furan-2-yl)methyl furan-2-carboxylate (10)
1
Yellow solid. Mp 120–122 °C. H NMR (CDCl3, 400 MHz): d 6.47–6.49 (m, 1H,
Fu), 6.54–6.56 (m, 1H, Fu), 6.72 (s, 1H, CH), 6.75–6.77 (m, 1H, Fu), 7.33–7.34 (m,
1H, Fu), 7.52–7.54 (m, 1H, Fu), 7.65–7.67 (m, 1H, Fu); 13C NMR (CDCl3,
100 MHz): d 55.7, 111.1, 112.2, 113.0, 116.8, 120.6, 142.4, 145.1, 146.3, 147.7,
156.2; IR (KBr) mmax/cm-1: 3,038, 2,923, 2,228, 1,732, 1,496, 1,354, 1,280, 1,098,
782, 743; Anal. Calcd. for C11H7NO4: C, 60.83; H, 3.25; N, 6.45; Found: C, 60.89;
H, 3.24; N, 6.47.
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Direct synthesis of cyanohydrin esters from aroyl chlorides
Results and discussion
Initially, the reaction of benzoyl chloride with K4[Fe(CN)6] was selected to examine
the feasibility for the synthesis of corresponding cyanohydrin ester under different
conditions (Scheme 1, R=H). It was found that no corresponding product was
observed in the absence of a promoter (Table 1, entry 1). In later research, some
Lewis bases, such as pyridine, triethylamine, triphenylphosphine, and tributylphos-
phine, were selected as promoters. Pyridine and triethylamine had no effect on the
reaction, and no cyanohydrin ester was produced (Table 1, entries 2–3). However,
triphenylphosphine and tributylphosphine can promote the reactions in different
solvents under refluxing conditions. Among them, the best yield for cyanohydrin
ester was obtained when tributylphosphine was used as a promoter and THF as
solvent under refluxing condition (Table 1, entry 9). In addition, the optimal mole
ratio of benzoyl chloride to potassium hexacyanoferrate(II) was 4:1 for the reaction,
which indicated that an excess of K4[Fe(CN)6] (based on the number of CN-) was
needed in the reaction. In fact, the reaction may undergo a one-pot two-step
CN
O
O
n
-Bu3P
THF
O
Cl +
K
]
4[Fe(CN)6
R
R
R
Scheme 1 Synthesis of cyanohydrin esters
Table 1 The effect of the reaction conditions on the yield of cyanohydrin ester
CN O
O
O
Cl
+
K
]
4[Fe(CN)6
Entry
Promoter
Solvent
Reaction time (h)
Yield (%)a
1
2
3
4
5
6
7
8
9
–
THF
6
6
6
3
3
3
1
1
1
0
0
Pyridine
Et3N
THF
THF
0
Ph3P
CH2Cl2
Toluene
THF
11
36
40
66
78
88
Ph3P
Ph3P
n-Bu3P
n-Bu3P
n-Bu3P
CH2Cl2
Toluene
THF
All reactions were carried out first using benzoyl chloride (10 mmol) and K4[Fe(CN)6] (2.5 mmol) at
160 °C, then at refluxing condition after addition of promoter (10 mmol) and different solvents (10 mL)
a
Isolated yields
123

Z. Li, Z. Zhao
procedure to give product: firstly, benzoyl chloride reacted with K4[Fe(CN)6]
under solvent-free condition at 160 °C to give benzoyl cyanide as an intermediate,
then 2 mol of benzoyl cyanide, through reductive coupling in THF at refluxing
condition in the presence of tributylphosphine, afforded cyanohydrin ester as a final
product.
Based on the above promising findings, and to explore the generality and scope
of the synthetic method, various substituted aroyl chlorides were examined for the
reactions under the optimal conditions (Scheme 1). The results are summarized in
Table 2. It was found that aroyl chlorides bearing electron-withdrawing substitu-
ents, such as chloro and nitro groups, on the aromatic rings gave the corresponding
products in higher yield (Table 2, entries 2–6). In contrast, aroyl chlorides bearing
electron-donating substituents, such as methyl and methoxy groups, on the aromatic
rings afforded the corresponding products in slightly lower yield under similar
conditions (Table 2, entries 7–9). In addition, for ortho-substituted aroyl chlorides
(Table 2, entry 2), the corresponding products were obtained in slightly lower yield
than para-substituted ones (Table 2, entry 4), presumably due to the steric
hindrance (Table 2, entry 2). Heteroaroyl chloride, such as furoyl chloride, was also
very efficient for the reaction (Table 2, entry 10). Aliphatic acyl chlorides, such as
acetyl chloride and oxalyl chloride, and vinylic acyl chloride, such as acryloyl
chloride, were also tested for the similar reactions. However, no corresponding
cyanohydrin esters were observed because the high volatilities and low stabilities of
acyl chlorides made the reactions with K4[Fe(CN)6] difficult to proceed at high
temperature.
A plausible mechanism for these reactions is shown in Scheme 2. Initially,
potassium hexacyanoferrate(II) reacted with aroyl chlorides to form aroyl cyanides
[51], which were probably activated by 0.5 equiv. of tributylphosphine to yield
intermediates A. Intermediates A were in equilibrium with the corresponding
intermediates B [57]. Then, the nucleophilic attack of aroyl cyanides by
intermediates B in the following step gave intermediates C. The subsequent one
hydride transfer from tributylphosphine to the carbon connected with the CN
group took place to give cyanohydrin esters by ambient moisture through the loss
of tributylphosphine oxide and hydrogen cyanide. In addition, the hydrogen
cyanide produced was quenched by other 0.5 equiv. of tributylphosphine during
the reaction.
Conclusion
An efficient direct synthetic route from aroyl chlorides to cyanohydrin esters using
potassium hexacyanoferrate(II) as an eco-friendly cyanide source has been
developed. The protocol has the advantages of no use of toxic cyanating agents,
high yield, and simple work-up procedure.
123

Direct synthesis of cyanohydrin esters from aroyl chlorides
Table 2 Synthesis of cyanohydrin esters from aroyl chlorides
CN O
O
O
n
-Bu3P
THF
Cl +
K
]
4[Fe(CN)6
R
R
R
Entry Aroyl chloride
1
Product
Yield (%)b
CN O
O
88
Cl
Cl
CN O
O
2
3
80
82
CN O
O
Cl
Cl
Cl
Cl
CN O
O
4
86
82
87
CN O
O
O2N
NO2
5
CN O
O
O
Cl
6
O2N
O2N
NO2
CN O
O
7
79
75
CN O
O
8
CN
O
O
O
O
O
O
O
O
Cl
9
73
78
O
O
O
O
CN O
O
O
O
10
a
All reactions were carried out first using benzoyl chloride (10 mmol) and K4[Fe(CN)6] (2.5 mmol) at
160 °C, then at refluxing condition after addition of n-Bu3P (10 mmol) and THF (10 mL)
b
Isolated yields
123

Z. Li, Z. Zhao
Bu3P
O-
O
O
O
-CN
+
Ar
CN
K
Ar
PBu3
]
4[Fe(CN)6
Ar
Ar
CN
Cl
+PBu3
KCl
FeCl2
A
B
O
Ar
CN
H
2O
CN
O
-CN
Ar
CN
O
Ar
O
Bu2P
Ar
Ar
O
HCN
Bu3P=O
H
H7
C3
C
Scheme 2 The mechanism for the synthesis of cyanohydrin esters from aroyl chlorides
Acknowledgment The authors thank the National Natural Science Foundation of China (21162024) for
the financial support of this work.
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