Solvent-free chemoselective cyanation of α,α- dibromoacetophenones using potassium hexacyanoferrate(II) as an eco-friendly cyanide source

Article abstract of DOI:10.1002/ejoc.201000848

A chemoselective route for the synthesis of 2-aryl-3,3- dibromoacrylonitriles by solvent-free cyanation of α,α- dibromoacetophenones by using potassium hexacyanoferrate (II) as an eco-friendly cyanide source is developed. The merits of this procedure include avoiding the use of a strong toxic cyanide source, a catalyst, and volatile organic solvents and the high yields of the products.

Full text of DOI:10.1002/ejoc.201000848

FULL PAPER
DOI: 10.1002/ejoc.201000848
Solvent-Free Chemoselective Cyanation of α,α-Dibromoacetophenones Using
Potassium Hexacyanoferrate(II) as an Eco-Friendly Cyanide Source
Zhouxing Zhao[a] and Zheng Li*[a]
Keywords: Cyanides / Nucleophilic addition / Iron / Sustainable chemistry
A chemoselective route for the synthesis of 2-aryl-3,3-di-
bromoacrylonitriles by solvent-free cyanation of α,α-di-
bromoacetophenones by using potassium hexacyano-
ferrate(II) as an eco-friendly cyanide source is developed.
The merits of this procedure include avoiding the use of a
strong toxic cyanide source, a catalyst, and volatile organic
solvents and the high yields of the products.
K4[Fe(CN)6] under different conditions (Scheme 1, R = H).
Generally, there are two possible reaction paths for this sys-
tem. One pathway is the substitution of the two bromide
groups of α,α-dibromoacetophenone with the cyanide
group of K4[Fe(CN)6]. The other pathway involves an ad-
dition reaction at the carbonyl group of α,α-dibromoace-
tophenone by the cyanide group of K4[Fe(CN)6]. Fortu-
nately, it was found that α,α-dibromoacetophenone chemo-
selectively reacted with K4[Fe(CN)6] only by the addition
reaction, followed by dehydration to give 2-phenyl-3,3-di-
bromoacrylonitrile. Substitution products were not ob-
served. Furthermore, the optimal ratio of α,α-dibromo-
acetophenone to K4[Fe(CN)6] was 4:1 for the synthesis of
2-phenyl-3,3-dibromoacrylonitrile. That indicated four cy-
anide ions bound to Fe in K4[Fe(CN)6] were transferred to
2-phenyl-3,3-dibromoacrylonitrile. To select the other opti-
mal conditions, the reaction was also tested at different tem-
peratures for different times. It was found that the reaction
of α,α-dibromoacetophenone with 0.25 equiv. of
K4[Fe(CN)6] at 160 °C for 20 min gave the best yield of 2-
phenyl-3,3-dibromoacrylonitrile (Table 1, Entry 3; Table 2,
Entry 4). The reaction at higher temperature for a pro-
longed time could cause carbonization of the substrate.
Introduction
Nucleophilic addition of carbonyl compounds with cyan-
ating agents is a very important way to access nitriles, hy-
droxy acids, and amino acids, which are important interme-
diates for medicine and agrochemicals. Many cyanating
agents were used in these nucleophilic additions, such as
HCN,[1] NaCN,[2] TMSCN,[3] (R2N)2BCN,[4] cyanofor-
mate,[5] acetone cyanohydrin,[6] and acetyl cyanide.[7] How-
ever, these cyanating agents are strongly toxic chemicals,
which render nucleophilic addition of carbonyl compounds
unsafe and environmentally unfriendly. Therefore, there is a
need to explore eco-friendly cyanating agents for the corre-
sponding cyanation reactions.
Potassium hexacyanoferrate(II), K4[Fe(CN)6], is non-
toxic and is even used in the food industry for metal precipi-
tation. In addition, it has been described as an antiaggluti-
nating 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.
Very recently, K4[Fe(CN)6] has been proved to be an ef-
ficient cyanide source for the cyanation of halogenated ar-
enes and aroyl chlorides to prepare benzonitriles[8] and
aroyl cyanides.[9] In this paper, we report chemoselective nu-
cleophilic addition reactions of α,α-dibromoacetophenones
by using K4[Fe(CN)6] as an eco-friendly cyanide source un-
der solvent-free and catalyst-free conditions.
Results and Discussion
Initially, α,α-dibromoacetophenone was selected as a
substrate to examine the feasibility of cyanation by using
[a] Key Laboratory of Polymer Materials of Gansu Province,
College of Chemistry and Chemical Engineering, Northwest
Normal University,
Lanzhou, Gansu 730070, P. R. China
E-mail: lizheng@nwnu.edu.cn
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201000848.
Scheme 1. The reactions of α,α-dibromoacetophenones with
K4[Fe(CN)6].
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Eur. J. Org. Chem. 2010, 5460–5463

Solvent-Free Chemoselective Cyanation of α,α-Dibromoacetophenones
Table 1. The reactions of α,α-dibromoacetophenone with
Table 3. Synthesis of 2-aryl-3,3-dibromoacrylonitriles under sol-
vent-free and catalyst-free conditions.
K4[Fe(CN)6] at different temperatures.[a]
Entry
Temperature [°C]
Yield [%][b]
1
2
3
4
5
6
120
140
160
180
190
200
NR[c]
30
88
70
62
34
[a] All reactions were carried out by using α,α-dibromoacetophen-
one (10 mmol) and K4[Fe(CN)6] (2.5 mmol). [b] Isolated yield.
[c] No reaction.
Table 2. The reactions of α,α-dibromoacetophenone with
K4[Fe(CN)6] for different times.[a]
Entry
Reaction time [min]
Yield [%][b]
1
2
3
4
5
6
5
30
50
65
88
70
62
10
15
20
25
30
[a] All reactions were carried out by using α,α-dibromoacetophen-
one (10 mmol) and K4[Fe(CN)6] (2.5 mmol). [b] Isolated yield.
Under these optimal conditions, various substituted α,α-
dibromoacetophenones were examined for the reactions.
The results are summarized in Table 3. It was found that
α,α-dibromoacetophenones bearing electron-withdrawing
substituents such as chloride and nitro groups on the aro-
matic ring gave the corresponding products in higher yield
(Table 3, Entries 2–4, 8, and 9). In contrast, α,α-dibromo-
acetophenones bearing electron-donating substituents such
as methyl and methoxy groups on the aromatic ring gave
the corresponding products in slightly lower yield under
similar conditions (Table 3, Entries 5–7, 10). For ortho-sub-
stituted α,α-dibromoacetophenones (Table 3, Entries 2 and
5), the corresponding products were obtained in slightly
lower yield than para-substituted α,α-dibromoacetophen-
ones (Table 3, Entries 4 and 7), presumably due to steric
effects. The substrate including a heterocycle such as 2,2-
dibromo-1-(furan-2-yl)ethanone was also effective for the
nucleophilic addition reactions (Table 3, Entry 12).
To examine the effect of the α-bromo group of aceto-
phenone on the reaction rate and reaction yield, the reac-
tions of acetophenone, α-bromoacetophenone, and α,α-di-
bromoacetophenone with K4[Fe(CN)6] were investigated.
The results showed that the bromide group played a very
important role in the reaction rate and yield. Acetophenone
failed to react with K4[Fe(CN)6] under the studied condi-
[a] Isolated yield.
Eur. J. Org. Chem. 2010, 5460–5463
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5461

Z. Zhao, Z. Li
FULL PAPER
tions, even with a prolonged reaction time (Table 4, En-
try 1). α-Bromoacetophenone could react with K4[Fe-
(CN)6] to give a mixture of (E)- and (Z)-2-phenyl-3-bromo-
acrylonitriles in moderate yield (E:Z = 87:13; Table 4, En-
try 2). In contrast, α,α-dibromoacetophenone could react
with K4[Fe(CN)6] more quickly than α-bromoacetophenone
to give 2-phenyl-3,3-dibromoacrylonitrile in higher yield
Experimental Section
General Methods: IR spectra were recorded by using KBr pellets
with an Alpha Centauri FTIR spectrophotometer, 1H NMR and
13C NMR spectra were recorded with a Mercury-400BB instrument
by using CDCl3 as the solvent and Me4Si as the internal standard.
Elemental analyses were performed with a Vario El Elemental
Analysis instrument. Melting points were observed in an electro-
(Table 4, Entry 3). This indicated that the more bromide thermal melting point apparatus. Potassium hexacyanoferrate(II)
was dried at 80 °C under vacuum for 24 h and finely powdered
prior to use. All reactions were monitored by TLC. Flash column
chromatography was carried out by using 200–300 mesh silica gel
at increased pressure. α,α-Dibromoacetophenones were synthesized
according to standard methods.[11]
substituents on the methyl group of acetophenone, the more
favorable the reactions. This is possibly because the elec-
tron-withdrawing effect of the bromide group renders the
carbon atom of the carbonyl group more active to the cy-
anide reagent.
General Procedure for the Preparation of 2-Aryl-3,3-dibromoacrylo-
nitriles: A round-bottomed flask equipped with an air condenser
was charged with the (un)substituted α,α-dibromoacetophenone
(10 mmol) and K4[Fe(CN)6] (2.5 mmol). Then, the mixture was
heated at 160 °C for 20 min. The progress of the reaction was moni-
tored by TLC. After completion of the reaction, the resulting mix-
ture was extracted with dichloromethane (3ϫ10 mL), and the ex-
tract was concentrated under reduced pressure. The residue was
subjected to silica gel flash column chromatography (petroleum
ether/EtOAc, 30:1) to obtain the pure product.
Table 4. The reactions of acetophenone, α-bromoacetophenone,
and α,α-dibromoacetophenone with K4[Fe(CN)6].[a]
Entry R1
R2
Reaction time [min]
Yield [%][b]
1
2
3
H
H
Br
H
Br
Br
120
120
20
NR
42 (E:Z = 87:13)
88
2-Phenyl-3,3-dibromoacrylonitrile (Table 3, Entry 1): White solid.
M.p. 62–64 °C. IR (KBr): ν = 3062, 2923, 2216, 1579, 1487, 1247,
˜
842, 752, 692 cm–1. 1H NMR (400 MHz, CDCl3): δ = 7.48–7.45
(m, 5 H, Ph-H) ppm. 13C NMR (100 MHz, CDCl3): δ = 133.3,
130.0, 128.9, 128.5, 122.6, 117.0, 109.7 ppm. C9H5Br2N (286.95):
calcd. C 37.67, H 1.76, N 4.88; found C 37.71, H 1.77, N 4.86.
[a] All reactions were carried out by using the substrate (10 mmol)
and K4[Fe(CN)6] (2.5 mmol). [b] Isolated yield.
The possible mechanism for the nucleophilic addition of
α,α-dibromoacetophenones with K4[Fe(CN)6] might in-
volve thermal decomposition of K4[Fe(CN)6] to form metal
cyanides; then, a cyanide anion can selectively attack the
carbon atom of the carbonyl of α,α-dibromoacetophenone
to yield a cyanohydrin intermediate, which can undergo de-
hydration to produce 2-aryl-3,3-dibromoacrylonitrile
(Scheme 2).
2-(2-Chlorophenyl)-3,3-dibromoacrylonitrile (Table 3, Entry 2):
White solid. M.p. 84–86 °C. IR (KBr): ν = 3069, 2924, 2218, 1554,
˜
1467, 1435, 1287, 1038, 853, 750 cm–1. 1H NMR (400 MHz,
CDCl3): δ = 7.50–7.25 (m, 4 H, Ph-H) ppm. 13C NMR (100 MHz,
CDCl3): δ = 132.8, 132.4, 131.5, 130.4, 130.2, 127.4, 120.3, 115.5,
113.9 ppm. C9H4Br2ClN (321.40): calcd. C 33.63, H 1.25, N 4.36;
found C 33.69, H 1.25, N 4.34.
2-(3-Chlorophenyl)-3,3-dibromoacrylonitrile (Table 3, Entry 3):
White solid. M.p. 78–80 °C. IR (KBr): ν = 3061, 2924, 2218, 1570,
˜
1469, 1254, 852, 785 cm–1. 1H NMR (400 MHz, CDCl3): δ = 7.48–
7.35 (m, 4 H, Ph-H) ppm. 13C NMR (100 MHz, CDCl3): δ = 134.9,
134.7, 130.3, 130.2, 128.6, 126.7, 121.2, 116.6, 111.1 ppm.
C9H4Br2ClN (321.40): calcd. C 33.63, H 1.25, N 4.36; found C
33.59, H 1.26, N 4.35.
Scheme 2. Possible mechanism for the reaction of α,α-dibromo-
acetophenones with K4[Fe(CN)6].
2-(4-Chlorophenyl)-3,3-dibromoacrylonitrile (Table 3, Entry 4):
White solid. M.p. 103–104 °C. IR (KBr): ν = 2922, 2213, 1590,
˜
1482, 1396, 1095, 831 cm–1. 1H NMR (400 MHz, CDCl3): δ = 7.43
(s, 4 H, Ph-H) ppm. 13C NMR (100 MHz, CDCl3): δ = 136.2,
131.6, 129.9, 129.2, 121.5, 116.7, 110.4 ppm. C9H4Br2ClN (321.40):
calcd. C 33.63, H 1.25, N 4.36; found C 33.65, H 1.25, N 4.37.
Conclusions
An efficient and convenient method for the chemoselec-
tive cyanation of various substituted α,α-dibromoaceto-
phenones using inexpensive and nontoxic K4[Fe(CN)6] as
an environmentally benign cyanide source under solvent-
free and catalyst-free conditions has developed. A series of
2-aryl-3,3-dibromoacrylonitriles were efficiently synthesized
by these nucleophilic additions. These compounds are im-
portant intermediates for the further synthesis of various
fine chemicals because of their many functional groups,
which include cyano and bromide groups, a double bond,
and an aromatic ring.[10]
2-(2-Methylphenyl)-3,3-dibromoacrylonitrile (Table 3, Entry 5):
White solid. M.p. 74–76 °C. IR (KBr): ν = 3062, 2920, 2214, 1546,
˜
1453, 1251, 852, 735 cm–1. 1H NMR (400 MHz, CDCl3): δ = 7.37–
7.18 (m, 4 H, Ar-H), 2.33 (s, 3 H, CH3) ppm. 13C NMR (100 MHz,
CDCl3): δ = 136.1, 133.3, 130.8, 130.2, 128.8, 126.6, 122.4, 116.1,
111.9, 19.3 ppm. C10H7Br2N (300.98): calcd. C 39.91, H 2.34, N
4.65; found C 39.86, H 2.35, N 4.63.
2-(3-Methylphenyl)-3,3-dibromoacrylonitrile (Table 3, Entry 6):
White solid. M.p. 40–41 °C. IR (KBr): ν = 3018, 2921, 2214, 1600,
˜
1554, 1095, 850, 789 cm–1. 1H NMR (400 MHz, CDCl3): δ = 7.34–
7.24 (m, 4 H, Ar-H), 2.39 (s, 3 H, CH3) ppm. 13C NMR (100 MHz,
5462
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Eur. J. Org. Chem. 2010, 5460–5463

Solvent-Free Chemoselective Cyanation of α,α-Dibromoacetophenones
CDCl3): δ = 138.8, 133.2, 130.8, 128.9, 128.8, 125.6, 122.7, 117.1,
Supporting Information (see footnote on the first page of this arti-
1
109.4, 21.3 ppm. C10H7Br2N (300.98): calcd. C 39.91, H 2.34, N cle): Copies of the IR, H NMR, and 13C NMR spectra of the 2-
4.65; found C 39.95, H 2.34, N 4.67.
aryl-3,3-dibromoacrylonitriles.
2-(4-Methylphenyl)-3,3-dibromoacrylonitrile (Table 3, Entry 7):
White solid. M.p. 62–64 °C. IR (KBr): ν = 3028, 2917, 2220, 1613,
˜
Acknowledgments
1551, 1505, 1260, 849, 823, 742 cm–1. 1H NMR (400 MHz, CDCl3):
δ = 7.38 (d, J = 8.0 Hz, 2 H, Ar-H), 7.25 (d, J = 8.0 Hz, 2 H, Ar-
H), 2.38 (s, 3 H, CH3) ppm. 13C NMR (100 MHz, CDCl3): δ =
140.4, 130.4, 129.6, 128.4, 122.6, 117.1, 108.9, 21.4 ppm.
C10H7Br2N (300.98): calcd. C 39.91, H 2.34, N 4.65; found C 39.96,
H 2.35, N 4.66.
The authors thank the National Natural Science Foundation of
China (20772096) for financial support of this work.
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2-(3-Nitrophenyl)-3,3-dibromoacrylonitrile (Table 3, Entry 8): White
solid. M.p. 158–160 °C. IR (KBr): ν = 3088, 2924, 2218, 1615,
˜
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8.41 (s, 1 H, Ar-H), 8.33 (d, J = 8.4 Hz, 1 H, Ar-H), 7.84 (d, J =
8.4 Hz, 1 H, Ar-H), 7.69 (t, J = 8.4 Hz, 1 H, Ar-H) ppm. 13C NMR
(100 MHz, CDCl3): δ = 148.3, 134.7, 134.5, 130.3, 124.8, 123.8,
120.3, 116.2, 112.8 ppm. C9H4Br2N2O2 (331.95): calcd. C 32.56, H
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2-(4-Nitrophenyl)-3,3-dibromoacrylonitrile (Table 3, Entry 9): White
solid. M.p. 130–132 °C. IR (KBr): ν = 3104, 2924, 2216, 1600,
˜
1518, 1350, 1294, 858 cm–1. 1H NMR (400 MHz, CDCl3): δ = 8.32
(d, J = 7.2 Hz, 2 H, Ar-H), 7.71 (d, J = 7.2 Hz, 2 H, Ar-H) ppm.
13C NMR (100 MHz, CDCl3): δ = 148.3, 139.1, 129.9, 124.2, 120.6,
116.2, 112.7 ppm. C9H4Br2N2O2 (331.95): calcd. C 32.56, H 1.21,
N 8.44; found C 32.49, H 1.21, N 8.41.
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2-(4-Methoxyphenyl)-3,3-dibromoacrylonitrile (Table 3, Entry 10):
White solid. M.p. 136–138 °C. IR (KBr): ν = 2999, 2939, 2214,
˜
1605, 1508, 1259, 1182, 1021, 829 cm–1. 1H NMR (400 MHz,
CDCl3): δ = 7.44 (d, J = 8.8 Hz, 2 H, Ar-H), 6.94 (d, J = 8.8 Hz,
2 H, Ar-H), 3.84 (s, 3 H, CH3) ppm. 13C NMR (100 MHz, CDCl3):
δ = 160.6, 130.1, 125.4, 122.2, 117.1, 114.2, 108.0, 55.3 ppm.
C10H7Br2NO (316.98): calcd. C 37.89, H 2.23, N 4.42; found C
37.95, H 2.24, N 4.40.
2-(4-Biphenyl)-3,3-dibromoacrylonitrile (Table 3, Entry 11): Brown
solid. M.p. 74–76 °C. IR (KBr): ν = 3063, 2922, 2223, 1600, 1479,
˜
1
1400, 842, 767 cm–1. H NMR (400 MHz, CDCl3): δ = 7.74–7.42
(m, 9 H, Ar-H) ppm. 13C NMR (100 MHz, CDCl3): δ = 145.6,
139.1, 132.5, 132.4, 129.0, 128.6, 127.7, 127.2, 122.1, 118.9,
110.8 ppm. C15H9Br2N (363.05): calcd. C 49.62, H 2.50, N 3.86;
found C 49.69, H 2.49, N 3.88.
2-(Furan-2-yl)-3,3-dibromoacrylonitrile (Table 3, Entry 12): White
solid. M.p. 52–54 °C. IR (KBr): ν = 3153, 2227, 1634, 1534, 1476,
˜
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1
1030, 850, 750 cm–1. H NMR (400 MHz, CDCl3): δ = 7.58 (d, J
= 1.6 Hz, 1 H, Fu-H), 7.08 (d, J = 4.0 Hz, 1 H, Fu-H), 6.54 (dd,
J = 3.6, 1.6 Hz, 1 H, Fu-H) ppm. 13C NMR (100 MHz, CDCl3): δ
= 145.9, 144.2, 115.3, 114.5, 113.5, 112.1, 103.7 ppm. C7H3Br2NO
(276.91): calcd. C 30.36, H 1.09, N 5.06; found C 30.29, H 1.08, N
5.08.
Received: June 14, 2010
Published Online: August 16, 2010
Eur. J. Org. Chem. 2010, 5460–5463
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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5463