Mild and efficient method for-thiocyanation of ketones and-dicarbonyl compounds using bromodimethylsulfonium bromide-ammonium thiocyanate

Article abstract of DOI:10.1080/00397910902838938

An efficient and convenient method for-thiocyanation of ketones and-dicarbonyl compounds has been developed using a reagent combination of bromodimethylsulfonium bromide (BDMS) and ammonium thiocyanate in acetonitrile. The developed method is mild and gave good yield of the products at room temperature.

Full text of DOI:10.1080/00397910902838938

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Mild and Efficient Method for
α-Thiocyanation of Ketones
and β-Dicarbonyl Compounds
Using Bromodimethylsulfonium
Bromide–Ammonium Thiocyanate
Dinesh S. Bhalerao a & Krishnacharya G. Akamanchi a
a Department of Pharmaceutical Sciences and Technology, Institute
of Chemical Technology, Matunga, Mumbai, India
Published online: 22 Feb 2010.
To cite this article: Dinesh S. Bhalerao & Krishnacharya G. Akamanchi (2010) Mild and Efficient
Method for α-Thiocyanation of Ketones and β-Dicarbonyl Compounds Using Bromodimethylsulfonium
Bromide–Ammonium Thiocyanate, Synthetic Communications: An International Journal for Rapid
Communication of Synthetic Organic Chemistry, 40:6, 799-807, DOI: 10.1080/00397910902838938
To link to this article: http://dx.doi.org/10.1080/00397910902838938
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Synthetic Communications1, 40: 799–807, 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910902838938
MILD AND EFFICIENT METHOD FOR
a-THIOCYANATION OF KETONES AND b-DICARBONYL
COMPOUNDS USING BROMODIMETHYLSULFONIUM
BROMIDE–AMMONIUM THIOCYANATE
Dinesh S. Bhalerao and Krishnacharya G. Akamanchi
Department of Pharmaceutical Sciences and Technology,
Institute of Chemical Technology, Matunga, Mumbai, India
An efficient and convenient method for a-thiocyanation of ketones and b-dicarbonyl
compounds has been developed using a reagent combination of bromodimethylsulfonium
bromide (BDMS) and ammonium thiocyanate in acetonitrile. The developed method is
mild and gave good yield of the products at room temperature.
Keywords: Ammonium thiocyanate; BDMS; b-dicarbonyl compounds; ketones; a-thiocyanation
Organosulfur chemistry of thiocyanates and their derivatives is of considerable
importance. a-Thiocyanato carbonyl compounds are valuable precursors for
synthesis of heterocyclic ring systems such as 2-amino-1,3-thiazines, thiazoles,
and their derivatives,[1] some of which are associated with herbicidal and other
biological activities.[2] Thiocynato group plays an important function in certain
anticancer natural products formed by deglycosylation of glucosinolates derived
from cruciferous vegetables.[3]
Thiocyanation is usually carried out via a nucleophilic reaction using the
thiocyanate anion. Generally a-thiocyanato carbonyl compounds are prepared from
a-halocarbonyl compounds[4] or a-tosyloxycarbonyls[5] or via nucleophilic epoxide
ring opening using the thiocyanate anion.[6] Electrophilic or radical reactions using
thiocyanogen and thiocyanogen chloride constitute other useful methods for
thiocynations.[1c,7] a-Thiocyanation of enolizable carbonyl compounds can be
effected using thiocyanatotrimethylsilane and SO2Cl2,[8] which may be considered
to be an electrophilic method. a-Thiocyanation of carbonyl and b-dicarbonyl com-
pounds are also carried out by using (dichloroiodo)benzene-lead(II) thiocyanate,[9]
K2S2O8-Cu=ammonium thiocyanate,[10] iodine=ammonium thiocyanate,[11a] and
FeCl3=ammonium thiocyanate.[11b]
Received November 25, 2008.
Address correspondence to Krishnacharya G. Akamanchi, Department of Pharmaceutical Sciences
and Technology, Institute of Chemical Technology, Matunga, Mumbai 400 019, India. E-mail: kgap@
rediffmail.com
799

800
D. S. BHALERAO AND K. G. AKAMANCHI
In recent years, the use of bromodimethylsulfonium bromide (BDMS) has
gained attention in synthetic organic chemistry.[12] BDMS has been utilized for the
transformation of alcohols to corresponding bromides,[13] oxidation of thiols to
disulfides,[14] deprotection of dithioacetals,[15] preparation of a-bromoenones from
the corresponding enones,[16] and regioselective a-bromination of b-keto esters and
1,3-diketones.[17] The reagent BDMS is readily accessible, can be conveniently
stored, is less hazardous, and is easy to handle. Recently we have developed BDMS
as an efficient and regioselective reagent for aromatic and heteroaromatic thio-
cyanation.[18] We have now explored its utility for a-thiocyanation of ketones and
b-dicarbonyl compounds in combination with ammonium thiocyanate.
BDMS was prepared by following the literature procedure.[15] To optimize
reaction conditions, acetophenone was taken as study substrate, and results are
summarized in Table 1.
When acetophenone was added to a stirred mixture of BDMS and NH4SCN
in acetonitrile, it gave a mixture of a-bromoacetophenone (2a) and a-thiocynatoa-
cetophenone (3a) with recovery of a small amount of unreacted starting material
(Table 1, entry 1). By increasing the molar ratio of BDMS–NH4SCN to 1:2, only
a-thiocynatoacetophenone (3a) was obtained with a substantial amount of
unreacted acetophenone (Table 1, entry 2). The sequence of addition was very
critical for getting good yields and conversion; for example, when ammonium
thiocynate was added to the stirred mixture of acetophenone and BDMS, only
a-thiocynatoacetophenone (3a) was obtained in 90% yield with practically no
recovery of unreacted acetophenone (Table 1, entry 4).
Table 1. Optimization of reaction conditions for a-thiocyanationa
Product (%)f
Recovered (%)f
Molar ratiob
wrt. 1a
Entry
Solvent
Time
2a
3a
1a
1
2
3
4
5
6
7
8
1.1:1.1c
1.1:2.2c
1.5:3.0c
1.1:2.2d
1.1:2.2c
1.1:2.2c
1.1:2.2c
1.1:2.2e
CH3CN
CH3CN
CH3CN
CH3CN
MeOH
CHCl3
5 h
5 h
70
—
—
—
—
70
60
—
10
40
50
90
80
25
30
95
15
55
5 h
5 h
45
Nil
15
5 h
5 h
—
—
CH2Cl2
CH2Cl2
5 h
20 min
Nil
aReaction was performed on 5 mmol scale.
b1a (1 equiv): BDMS–NH4SCN.
cReaction carried out using a mixture of BDMS and NH4SCN.
dAddition sequence is acetophenone, BDMS, and NH4SCN.
eNH4SCN was added after complete consumption of 1a to form 2a.
fBy gas chromatography.

a-THIOCYANATION OF CARBONYL COMPOUNDS
801
Scheme 1. a-Thiocyanation of acetophenone.
The results of solvent study revealed that dichloromethane and methanol were
also viable for this transformation; however, in dichloromethane, a-bromination was
the major product (Table 1, entries 5–7).
With these results in hand, the reaction was conducted in two stages (Scheme 1)
using dichloromethane as solvent. Initially acetophenone 1a was reacted with 1.1
equiv of BDMS and the reaction mixture was stirred at room temperature until com-
plete consumption of starting material and formation of a-bromoketone 2a as
observed by thin-layer chromotography (TLC). This was followed by addition of
2.2 equiv of NH4SCN. By this modification, the reaction was completed within an
overall time of 20 min and gave of a-thiocyanatoacetophenone 3a in 95% yield
(Table 1, entry 8).
This developed protocol was applied to a wide range of acetophenones 1,
which underwent a-thiocyanation smoothly to afford a-thiocyanato products in
87–95% yields (Table 2, entries 1–8). The protocol was extended to cyclic ketones
such as cyclohexanone and cyclopentanone and corresponding a-thiocyanato
ketones were obtained in good yields (Table 2, entries 9 and 10). In the case of
1-phenylpropan-2-one 3k, thiocyanation occurred as expected, predominantly at
the benzylic position (Table 2, entry 11). The thiocyanato products 3a–k were char-
acterized by 1H NMR and infrared (IR) spectroscopy and are in agreement with the
reported values.
After establishing the reaction with ketones, a-thiocyanation of b-dicarbonyl
compounds was studied. The reaction was carried out on acetylacetone in acetoni-
trile and gave 96% of 3-thiocyanatopentan-2,4-dione as a solid product in 10 min
1
(Table 3, entry 1). The H NMR and IR spectral data were in agreement with the
reported values. Being relatively unstable, its shelf life at room temperature was
short, and slow decomposition was observed.
Under the same conditions, a-thiocyanation of benzoylacetone, ethyl acetoa-
cetate, and ethyl benzoylacetate proceeded to completion (as indicated by TLC and
IR of the crude product) (Table 3, entries 2–4); however, it was not possible to
isolate pure products because of their ready decomposition into a complex
mixture.[9,20]
In contrast, a-substituted b-dicarbonyl compounds reacted smoothly to give a-
thiocyanato derivatives (Table 3, entries 5 and 6). These are nonenolizable and quite
stable crystalline solids.
In summary, we have developed a simple, fast, and mild method for a-thiocya-
nation of ketones and b-dicarbonyl compounds to give corresponding a-thiocyanato
products in excellent to good yields. The reaction can be conducted at room tem-
perature. The reagents used are not hazardous and are easy to handle.

802
D. S. BHALERAO AND K. G. AKAMANCHI
Table 2. a-Thiocyanation of ketones
Product 3
Entry
1
Substrate 1
Time (min)
Yield (%)a
20
95
2
20
92
3
4
5
6
7
25
20
25
25
35
88
91
87
87
88
8
9
40
30
88
65
(Continued )

a-THIOCYANATION OF CARBONYL COMPOUNDS
Table 2. Continued
803
Entry
10
Substrate 1
Product 3
Time (min)
Yield (%)a
30
35
60
11
80
aIsolated yield by column chromatography.
EXPERIMENTAL
General Procedures
a-Thiocyanato ketones (3a–3k). Substrate ketone (5 mmol) was added to a
yellow suspension of BDMS (1.22 g, 5.5 mmol) in CH2Cl2 in one portion. The reac-
tion mixture was stirred at room temperature. After complete consumption of the
starting material as observed by TLC (about 10 min), NH4SCN (0.83 g, 11 mmol)
was added and stirring continued. After completion of reaction (monitored on
TLC), the solid was removed by filtration and washed with ethyl acetate (50 ml).
Table 3. a-Thiocyanation of b-dicarbonyl compoundsa
Entry
1
Substrate
Product
Time (min)
Yieldb (%)
10
96
—
—
—
c
2
3
4
180
120
150
—
c
—
c
—
5
6
300
300
90
92
aReactions were carried out on 5-mmol scale at rt using 1.5 equiv of BDMS and 3.0 equiv of NH4SCN
in acetonitrile.
bYield by column chromatographically isolated compounds.
cA mixture of decomposed product was obtained.

804
D. S. BHALERAO AND K. G. AKAMANCHI
The combined organic layer was washed with saturated solution of sodium bicarbon-
ate (50 ml) and brine (50 ml), dried over anhydrous sodium sulfate, and concentrated
under reduced pressure to give crude thiocyanato product. Pure product was
obtained after column chromatography (silica gel, mesh size 60–120, eluent ethyl
acetate–hexane 10:90).
Caution: Reaction should be carried out in an efficient fume hood.
Thiocyanation of b-dicarbonyl compounds. A substrate (5 mmol) was
added immediately to a yellow suspension of BDMS (1.66 g, 7.5 mmol) and
ammonium thiocyanate (1.14 g, 15 mmol) in 50 ml of dry acetonitrile in one portion.
The reaction mixture was stirred at room temperature until complete consumption
of starting material as observed by TLC. After completion of the reaction, solid
residue was removed by filtration, and the residue was washed with ethyl acetate.
The combined organic layer was washed with saturated solution of sodium bicarbon-
ate (50 ml) and brine (50 ml), dried over anhydrous sodium sulfate, and concentrated
under reduced pressure to give crude thiocyanato product. Pure product was
obtained after column chromatography (silica gel, mesh size 60–120, eluent ethyl
acetate–hexane 10:90).
Caution: Reaction should be carried out in an efficient fume hood.
Spectral Data for Selected Compounds
1-Phenyl-2-thiocyanatoethanone (3a). Solid. Mp 70–72ꢀC (lit.[11b]
67–69ꢀC). 1H NMR (60 MHz, CDCl3): d 4.73 (s, 2H), 7.26–7.62 (m, 3H),
7.88–8.01 (m, 2H) ppm. IR (KBr): nmax 2926, 2154 (–SCN), 1677, 1591, 1201,
996 cmꢁ1
.
2-Thiocyanato-p-tolylethanone (3b). Solid. Mp 103–106ꢀC (lit.[19a]
105–107ꢀC). 1H NMR (60 MHz, CDCl3): d 2.45 (s, 3H), 4.71 (s, 2H), 7.32 (d,
J ¼ 7.8 Hz, 2H), 7.84 (d, J ¼ 7.8 Hz, 2H) ppm. IR (KBr): nmax 2936, 2148 (ꢁSCN),
1677, 1591, 1201, 996 cmꢁ1
.
1-(4-Hydroxyphenyl)-2-thiocyanatoethanone (3c). Solid. Mp 159–161ꢀC
1
(lit.[11b] 160–162ꢀC). H NMR (60 MHz, CDCl3): d 4.68 (s, 2H), 6.94 (d, J ¼ 7.8 Hz,
2H), 7.88 (d, J ¼ 7.8 Hz, 2H) ppm. IR (KBr): nmax 3500, 2156 (–SCN), 1678, 1596,
1205, 997 cmꢁ1
.
1-(4-Methoxyphenyl)-2-thiocyanatoethanone (3d). Solid. Mp 121–125ꢀC
1
(lit.[19a] 122–125ꢀC). H NMR (60 MHz, CDCl3): d 3.91 (s, 3H), 4.70 (s, 2H), 6.98
(d, J ¼ 9.0 Hz, 2H), 7.93 (d, J ¼ 9.0 Hz, 2H) ppm. IR (KBr): nmax 2930, 2153
(–SCN), 1666, 1598, 1208, 829 cmꢁ1
.
1-(4-Bromophenyl)-2-thiocyanatoethanone[19b]
(3e). Solid.
Mp
1
123–125ꢀC. H NMR (60 MHz, CDCl3): d 4.67 (s, 2H), 7.66 (d, J ¼ 10.2 Hz, 2H),
7.82 (d, J ¼ 10.2 Hz, 2H) ppm. IR (KBr): nmax 2926, 2155 (–SCN), 1669, 1584,
1202, 998 cmꢁ1
.
1-(4-Chlorophenyl)-2-thiocyanatoethanone (3f). Solid. Mp 132–134ꢀC
(lit.[19a] 132–135ꢀC). 1H NMR (60 MHz, CDCl3): d 4.69 (s, 2H), 7.35 (d, J ¼ 10.2 Hz,
2H), 7.92 (d, J ¼ 10.2 Hz, 2H) ppm. IR (KBr): nmax 2978, 2148 (–SCN), 1669, 1588,
1203, 998 cmꢁ1
.

a-THIOCYANATION OF CARBONYL COMPOUNDS
805
1-(4-Nitrophenyl)-2-thiocyanatoethanone (3g). Solid. Mp 119–120ꢀC
1
(lit.[21] 118.5ꢀC). H NMR (60 MHz, CDCl3): d 4.71 (s, 2H), 8.11 (d, J ¼ 9.0 Hz,
2H), 8.40 (d, J ¼ 9.0 Hz, 2H) ppm. IR (KBr): nmax 2980, 2153 (–SCN), 1677, 1601,
1531, 1348, 1198, 1000 cmꢁ1
.
1-(2,4-Dichlorophenyl)-2-thiocyanatoethanone[1b]
(3h). Solid.
Mp
1
88–90ꢀC. H NMR (60MHz, CDCl3): d 4.73 (s, 2H), 7.46–7.98 (m, 3H) ppm. IR
(KBr): nmax 2985, 2152 (–SCN), 1674, 1594, 1200, 996 cmꢁ1
.
2-Thiocyanatocyclohexanone (3i). Oil (lit.[9] oil). 1H NMR (60 MHz,
CDCl3): d 1.59–2.87 (m, 8H), 4.13–4.39 (m, 1H) ppm. IR (neat): nmax 2948, 2155
(–SCN), 1713, 1449, 1302, 1127 cmꢁ1
.
2-Thiocyanatocyclopentanone (3j). Oil (lit.[9] Oil). 1H NMR (60 MHz,
CDCl3): d 1.65–2.87 (m, 6H), 3.75–3.89 (m, 1H) ppm.
IR (neat): nmax 2954, 2156 (–SCN), 1750, 1458, 832 cmꢁ1
.
1-Phenyl-1-thiocyanatopropan-2-one (3k). Oil. 1H NMR (60 MHz,
CDCl3): d 2.14 (s, 3H), 5.26 (s, 1H), 7.23–7.45 (m, 5H) ppm.
IR (neat): nmax 2924, 2156 (–SCN), 1713, 1595, 1254 cmꢁ1. Anal. calcd. for
C10H9NOS: C, 62.80; H, 4.74; N, 7.32. Found: C, 62.70; H, 4.63; N, 7.25.
4-Hydroxy-3-thiocyanatopent-3-en-2-one. Solid. Mp 78–80ꢀC (lit.[9]
1
79–81). H NMR (60 MHz, CDCl3): d 2.51 (s, 6H), 17.18 (s, 1H, enol OH) ppm.
IR (KBr): nmax 3414, 2159 (–SCN), 1711, 1586, 1409, 1365, 1000 cmꢁ1
.
2-Methyl-1-phenyl-2-thiocyanatobutane-1,3-dione. Oil.
(60 MHz, CDCl3): d 1.23 (s, 3H), 2.24 (s, 3H), 7.12–7.85 (m, 5H) ppm.
IR (neat): nmax 2938, 2156 (–SCN), 1722, 1674, 1596, 1449, 1211, 952 cmꢁ1
1H
NMR
.
Anal. calcd. for C12H11NO2S: C, 61.78; H, 4.75; N, 6.0. Found: C, 61.67; H, 4.79;
N, 6.1.
Ethyl 2-methyl-3-oxo-3-phenyl-2-thiocyanatopropanoate. Oil. 1H NMR
(60 MHz, CDCl3): d 1.21 (t, J ¼ 6.6 Hz, 3H), 1.57 (s, 3H), 4.20 (q, J ¼ 6.6 Hz, 2H),
7.32–7.96 (m, 5H) ppm. IR (neat): nmax 2980, 2159 (–SCN), 1738, 1670, 1594, 1449,
1281, 1021 cmꢁ1. Anal. calcd. for C13H13NO3S: C, 59.39; H, 4.98; N, 5.32. Found:
C, 59.35; H, 4.87; N, 5.42.
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