Convenient synthesis of alkyl thioacetate from alkyl halide using a polymer-supported sodium thioacetate

Article abstract of DOI:10.1007/s13738-012-0093-4

Alkyl halides are efficiently converted to their corresponding S-alkyl thioacetates under mild and nonaqueous conditions, using polymer-supported sodium thioacetate as a new polymeric reagent at room temperature in high yields and purity. The spent polymeric reagent can be removed quantitatively by filtration and pure products can be obtained by evaporation of the solvent. The spent polymeric reagent can be regenerated and reused several times without its activity changing appreciably. Iranian Chemical Society 2012.

Full text of DOI:10.1007/s13738-012-0093-4

J IRAN CHEM SOC (2012) 9:767–774
DOI 10.1007/s13738-012-0093-4
ORIGINAL PAPER
Convenient synthesis of alkyl thioacetate from alkyl halide
using a polymer-supported sodium thioacetate
•
Mohammad Ali Karimi Zarchi Mojgan Nejabat
Received: 3 November 2011 / Accepted: 2 March 2012 / Published online: 14 June 2012
Ó Iranian Chemical Society 2012
Abstract Alkyl halides are efficiently converted to their
corresponding S-alkyl thioacetates under mild and non-
aqueous conditions, using polymer-supported sodium
thioacetate as a new polymeric reagent at room temperature
in high yields and purity. The spent polymeric reagent can
be removed quantitatively by filtration and pure products
can be obtained by evaporation of the solvent. The spent
polymeric reagent can be regenerated and reused several
times without its activity changing appreciably.
ketones [14], acids [15], esters [16], lactones [17], amides
[18], lactams and related heterocycles [19], and utilized as
a protecting group of thiols [20].
Until the mid-1980s, thioesters were prepared by con-
ventional methods, i.e., condensation of thiols with the
parent carboxylic acids in the presence of an activating
agent or substitution of acid chlorides or acid anhydrides
with metal thiolates [21–23]. However, in proportion with
growing interests in organic transformation of thioesters,
the following synthetic methods are now available: tran-
sition metal-catalyzed carbonylation [24, 25], reactions of
acyllithiums with disulfides [26], hydration of thioacetyl-
enes [27, 28], Tishchenko-type reaction [29] and coupling
of thiol chloroformate with organotin compounds [30].
However, these methods implicate toxic and hazardous
reagents, harsh conditions or uncommon starting materials.
Although polymer-supported reagents, especially anion
exchange resins, have been widely applied in organic
synthesis [31–50], there are only a few reports in the lit-
erature for the synthesis of organic thioesters based on
polymeric reagents [50, 51].
Keywords S-alkyl thioacetate Á Alkyl halide Á
Polymeric reagent Á Thioester
Introduction
Thioesters represent organic derivatives of wide interest
due to their wide range of biological activity and consid-
erable applications in drug development [1–4] and industry
[5–7].
Thioesters are activated carboxylic acid derivatives
which exhibit acylating properties similar to those of acid
anhydrides [8, 9]. Thioesters show higher reactivity and
selectivity toward nucleophiles than their oxygen analogs
and play important roles in biological systems such as acyl
coenzyme A and S-acetyl dihydrolipoic acid [10, 11].
Thioesters are important intermediates in organic syn-
thesis. They have been used as mild acyl transfer reagents,
[12] as intermediates in the synthesis of aldehydes [13],
Cainelli et al. [50] reported the synthesis of alkyl thio-
esters from alkyl halides or tosylates by using the amber-
lyst A-26 thioacetate form. Another report in the literature
is the synthesis of thiols from alkyl halides or tosylates by
using a supported thioacetate reagent [51]. In this strategy,
displacement of the leaving group with thioacetate ion
afforded the intermediate thioesters. Using polymeric
quaternary ammonium thiocarboxylate, the reaction time is
very long (many hours) and some products remain adsor-
bed on the polymer.
In connection with our previous work on cross-linked
poly (4-vinylpyridine), [38–49], now we wish to report
an efficient and easy method for preparation in high
yields of alkyl thioesters from alkyl halides by using a
M. A. Karimi Zarchi (&) Á M. Nejabat
Department of Chemistry, College of Sciences, Yazd University,
P.O. Box 89195-741, Yazd, Iran
e-mail: makarimi@yazdun.ac.ir
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J IRAN CHEM SOC (2012) 9:767–774
polymer-supported sodium thioacetate under mild, non-
aqueous and heterogeneous conditions.
solution of silver nitrate and 1.0 mmol of thioacetate ion
per gram of polymer was found.
General procedure for the conversion of alkyl halides
to the corresponding S-alkyl thioesters by using
[P4-VP]SCOCH3
Experimental
General
To a mixture of alkyl halide (1 mmol) and acetonitrile
The chemicals were either prepared in our laboratory or
purchased from Fluka (Buchs, Switzerland) Aldrich
(Milwaukee, WI) or Merck chemical companies. Poly
(4-vinylpyridine) cross-linked with 2 % divinyl benzene
(100–200 mesh), [P4-VP] 2 % DVB, was purchased from
Fluka (Milwaukee, WI). Cross-linked poly (N-methyl-4-
vinylpyridinium) iodide, [P4-VP]I, was synthesized
according to the reported procedure [40–42], (Scheme 1).
Progress of the reaction was followed by thin layer chro-
matography (TLC) using silica gel Poly Gram SIL G/UV
254 plates.
(5 mL) in a round-bottomed flask (50 mL), 1.5 g
(1.5 mmol) of cross-linked poly (4-vinylpyridine)-sup-
ported sodium thioacetate was added, and the mixture was
slowly stirred magnetically at room temperature for
9–200 min (Table 1). Reaction monitoring was accom-
plished by TLC. After completion of the reaction, the
suspension was filtered and washed with acetonitrile
(2 9 5 mL). On evaporation of the solvent, the S-alkyl
thioacetate esters were obtained in high to excellent yields
(73–91 %) (Table 1). If further purification is needed, flash
chromatography on silica gel [eluent: n-hexane–ethyl
acetate (90/10)] provides highly pure products.
Products were characterized by comparison of their
melting point, FT-IR and 1H-NMR spectral data with those
of known samples and all yields refer to the isolated pure
products. Melting points were determined with a Buchi
melting point B-540 B.V. CHI apparatus. FT-IR spectra
were obtained by using a Bruker, Equinox (model 55), and
NMR spectra were recorded on a Bruker AC 400, Avance
DPX spectrophotometer in CDCl3 solutions.
Preparation of S-(4-nitrobenzyl thioacetate) (entry 1
in Table 1) using [P4-VP]SCOCH3: a typical procedure
To a solution of p-nitrobenzyl chloride (172 mg, 1 mmol)
in acetonitrile (5 mL), 1.5 g (1.5 mmol) of cross-linked
poly (4-vinylpyridine)-supported sodium thioacetate was
added, and the mixture was slowly stirred magnetically at
room temperature for 10 min. The suspension was filtered
and washed with acetonitrile (2 9 5 mL). On evaporation
of the solvent, the pure S-(4-nitrophenylmethyl) thioacetate
ester in 91 % yield (192 mg) was obtained. m.p.:
52–54 °C; FT-IR (neat) mmax (cm-1): 3,040 (C–H, aro-
matic), 2,980 (C–H, aliphatic), 1,681 (C=O), 1,601–1,508
(C=C), 1,136 (C–S); 1,019, 954, 847, 804, 710, 627; 1H
NMR (400 MHz, CDCl3), d (ppm) = 2.385 (3H, CH3, s),
4.179 (2H, CH2, s), 7.272 (CDCl3), 7.471 (2H, d,
J = 8.8 Hz), 8.167 (2H, d, J = 8.8 Hz).
Preparation of [P4-VP]SCOCH3
Poly (4-vinylpyridine) cross-linked with 2 % DVB (white
powder, 100–200 mesh) (1.0 g) was treated with methyl
iodide (20 mmol, 3.24 g) in acetonitrile (10 mL) and
slowly stirred for 24 h at room temperature. The yellow
quaternized polymer, [P4-VP]I, was filtered and washed
with distilled water and acetonitrile. It was then dried under
vacuum in the presence of P2O5 at 40 °C overnight.
The obtained [P4-VP]I (2.3 g) was added to an excess
aqueous solution of sodium thioacetate and slowly stirred
for 24 h at room temperature. The mixture was filtered and
washed with distilled water (2 9 10 mL). It was then dried
under vacuum at 40 °C in the presence of P2O5 overnight
to produce a green powder polymeric reagent, [P4-VP]
SCOCH3 (0.65 g). The capacity of the polymeric reagent
was determined by potentiometric titration with a 0.1 M
Regeneration of [P4-VP]SCOCH3
To a solution of sodium thioacetate (0.50 g) in distilled
water (20 mL), cream-colored spent polymeric reagent
(1.00 g) was added and the mixture was slowly stirred at
room temperature for 24 h. The mixture was filtered and
Scheme 1 Mechanism of the
reaction and regeneration of
polymeric reagent
1)
2)
[P4-VP] 2 % DVB
+
Me-I
[P4-VP]I
NaI (aq)
+ NaSCOCH3 (aq)
[P4-VP]I
[P4-VP]SCOCH3
+
RS-CO-CH3
[P4-VP]X
+
Acetonitrile, rt
[P4-VP]SCOCH3
[P4-VP]X
R-X
+
3)
4)
[P4-VP]SCOCH3
NaX(aq)
(Regeneration)
+
NaSCOCH3 (aq)
+
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J IRAN CHEM SOC (2012) 9:767–774
769
Table 1 Conversion of alkyl halides to corresponding S-alky thioacetates using [P4-VP]SCOCH3 in acetonitrile at room temperature
Entry
1
Substrate
Producta
Time (min)
55
Yieldb (%)
O
O
O
O
O
O
85
Cl
Br
CH3
CH3
CH3
CH3
CH3
CH3
S
S
S
S
S
S
2
40
25
30
35
37
87
89
88
89
89
3
I
4c
5c
6c
I
I
I
7
8
Cl
O
65
9
86
89
Br
Br
Cl
S
CH3
O
S
CH3
H3C
H3C
9
O
25
20
15
65
84
79
91
73
Br
CH3
S
Br
Br
10
11
12
O
Br
Br
CH3
S
Br
Br
O
CH3
S
O2
N
O2N
Ph2CH-Cl
CH3
Ph
S
C
O
Ph
Br
13
14
160
180
82
74
CH3
S
O
CH3-I
O
H3C
CH3
S
15
200
87
S
C8H17
CH3
Br
n-
n-C7H15
O
a
Products were characterized by comparison of their IR and NMR spectra and physical data with those of known samples
Yields of isolated compounds
b
c
The entries 4,5 and 6 refer to the use of [P4-VP]SCOCH3 that is recycled first, second and third time, respectively, under identical conditions
washed with distilled water several times. It was then
washed with ether and dried overnight at 4 °C under vac-
uum to give [P4-VP]SCOCH3 (1.20 g). Determination of
its capacity shows the same capacity of the original poly-
meric reagent and the activity of regenerated polymer is
nearly of the original form (entries 4–6 in Table 1).
Results and discussion
During our investigation of multiple phase techniques in
organic synthesis, it was observed that cross-linked poly
(4-vinylpyridine)-supported sodium thioacetate, [P4-VP]
SCOCH3, can be easily prepared and used as a mild
123

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J IRAN CHEM SOC (2012) 9:767–774
and efficient polymeric reagent for the conversion of
alkyl halides to the corresponding S-alkyl thioacetates
(Scheme 1). Poly (4-vinylpyridine) cross-linked with 2 %
DVB, [P4-VP] 2 % DVB, was purchased from Fluka
company and in reaction with methyl iodide was converted
to cross-linked poly (N-methyl-4-vinylpyridinium) iodide,
[P4-VP]I (Eq. 1 in Scheme 1); then, [P4-VP]SCOCH3 was
prepared by an exchange reaction between [P4-VP]I and a
slight excess of aqueous solution of sodium thioacetate
(Eq. 2 in Scheme 1). Using this heterogeneous reagent,
alkyl halides were converted to S-alkyl thioacetate esters in
acetonitrile (Eq. 3 in Scheme 1). Optimization of the
reaction conditions showed that for most of the reactions,
among other solvents, acetonitrile at room temperature and
reagent/substrate ratio of 1.5 were the best. This method
represents an extremely convenient procedure for obtaining
a wide variety of alkyl thioacetates in high to excellent
yields (71–95 %) and of sufficient purity. The reactions
were performed under mild and completely heterogeneous
conditions in an organic solvent (acetonitrile) at room
temperature, and S-alkyl thioacetates were obtained simply
by filtration and evaporation of the solvent. The results and
reaction conditions are summarized in Table 1.
Table 1 reveals that the order of reactivity of alkyl
halides is benzyl [ alkyl (entries 1–12 vs. 13–15 in
Table 1); also, it was observed that in this procedure, steric
hindrance is important and the larger R group in alkyl
halide decreased its activity (entries 12–15 in Table 1).
Also, based on the leaving group effect, the order of
reactivity of alkyl halides is RI [ RBr [ RCl (entries 1–3
in Table 1).
The reaction is believed to follow the typical pathway
shown in Scheme 1 (Eq. 1–3 in Scheme 1). The spent
polymeric reagent was easily regenerated by treatment with
an aqueous solution of sodium thioacetate (Eq. 4 in
Scheme 1). The pure S-alkyl thioacetates were identified
by FT-IR, 1H-NMR spectra and physical data with those of
known samples. In this respect, the appearance of a strong
peak at 1,651–1,689 cm-1 for –S–CO– and a strong peak
at 1,132–1,208 cm-1 for C–S, stretching in their FT-IR
spectra, indicate the formation of S-alkyl thioacetate esters.
Some spectral data of thioacetate ester products and FT-IR
Table 2 Characteristic spectral data of some S-alkyl thioacetate products and polymeric reagents
Entry Product
1
mmax (cm-1
)
1H NMR d (ppm)
m.p. (°C)
3,070 (C–H, ArH), 2,960 (C–H, CH2 and CH3),
1,687 (C=O), 1,454–1,496 (C=C), 1,353 (CH2 and
CH3, bending) 1,132 (C–S), 948, 724, 698, 624
2.347 (3H, CH3, s), 2.884 (2H, CH2, t), Yellow oil
3.139 (2H, CH2, t), 7.222–7.271 (3H,
ArH), 7.301–7.339 (2H, ArH)
S
CH3
O
2
3
3,059 (C–H, ArH), 2,950 (C–H, CH2 and CH3),
1,687 (C=O), 1,447 and 1,595 (C=C), 1,353 (CH2
and CH3, bending), 1,132 (C–S), 956, 700, 625
–
Yellow oil
O
S
CH3
3,040 (C–H, ArH), 2,980, (C–H, CH2 and CH3),
1,681 (C=O), 1,601, 1,508, 1,406 (C=C), (CH2
and CH3, bending) 1,136, (C–S), 1,019, 954, 847,
804, 710, 627
2.385 (3H, CH3, s), 4.179 (2H, CH2, s), 52–54
7.272 (CDCl3), 7.471 (2H, d,
J = 8.8 Hz), 8.167 (2H, d,
J = 8.8 Hz)
O
S
CH3
O2N
4
3,055 (C–H, ArH), 2,920 (C–H, CH2 and CH3),
1,658 (C=O), 1,578, 1,485, 1,445 (C=C), 1,401,
1,305, 1,205, 1,172, 1,067, 1,010, 830, 771, 738,
682, 645
2.329 (3H, CH3, s), 4.061 (2H, CH2, s), Yellow oil
7.171 (2H, d, J = 8), 7.421 (2H, d,
J = 8)
O
S
CH3
Br
5
3,050 (C–H, ArH), 2,922 (C–H, CH2 and CH3),
1,689 (C=O), 1,515, 1,415, (C=C), 1,353, 1,132,
957, 815, 726, 632
2.332 (3H, CH3, s), 2.353 (3H, CH3, s), Yellow oil
4.102(2H, CH2, s), 7.115(2H, d,
J = 7.6), 7.187 (2H, d, J = 7.6)
O
S
CH3
H3C
6
7
[P4-VP] % DVB
[P4-VP]SCOCH3
3,100 (C–H, ArH), 2,922 (C–H, aliphatic), 1,596,
1,556, 1,490 (C=C), 1,414 (C=N), 1,220, 1,089,
994, 817, 611
–
–
White
powder
(not
meltable)
3,030(C–H, ArH), 2,952 (C–H, aliphatic), 1,640
(C=O, thioacetate ion), 1,517 (C=C), 1,472
(C=N), 1,301, 1,188, 1,123 (C–S), 959, 842, 678,
623
Green
powder
(not
meltable)
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J IRAN CHEM SOC (2012) 9:767–774
771
Fig. 1 FT-IR spectrum of
S-(2-phenylethyl) thiocyanate
Fig. 2 FT-IR spectrum of S-(4-
nitrophenylmethyl) thioacetate
spectra of [P4-VP] 2 % DVB and [P4-VP]SCOCH3 are
1
listed in Table 2 and the FT-IR and H-NMR spectra of
phase synthesis of S-alkyl thioacetates from alkyl halides.
S-alkyl thioacetates were obtained in good to excellent
yields (71–95 %) and high purity by simple filtration and
evaporation of the solvent. Exploitation of this reagent to
improve the performance of other nucleophilic displace-
ment is under investigation.
S-(2-phenylethyl) thioacetate and S-(4-nitrophenylmethyl)
thioacetate are shown in Figs.1 –6.
In Table 3, other reported methods for the preparation of
S-alkyl thioacetates are compared with this method. As
demonstrated, the reaction time developed will be shorter
than previously reported methods. This can probably be
attributed to the local concentration of thioacetate ion
species inside the pores.
Conclusion
In summary, it was demonstrated that cross-linked poly
(4-vinylpyridine)-supported sodium thioacetate can act as
an efficient polymeric reagent for the suspended solution
Cross-linked poly (4-vinylpyridine)-supported sodium
thioacetate has been introduced as an efficient S-acylating
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J IRAN CHEM SOC (2012) 9:767–774
Fig. 3 1H-NMR spectrum of S-(2-phenylethyl) thiocyanate
Fig. 5 1H-NMR spectrum of S-(2-phenylethyl) thiocyanate
Fig. 6 Expanded 1H-NMR spectrum of S-(4-nitrophenylmethyl)
thioacetate
Fig. 4 1H-NMR spectrum of S-(4-nitrophenylmethyl) thioacetate
conditions, ready availability, fast reaction rates and simple
reaction workup.
reagent of alkyl halides to give S-alkyl thioacetate esters in
high to excellent yields. The present method has the
advantages of operational simplicity, mild reaction
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J IRAN CHEM SOC (2012) 9:767–774
773
Table 3 Comparison of different reported methods for synthesis of S-alkyl thioesters
Entry
1
Reaction conditions
Product
Yield (%)
73
References
[52]
O
O
O
O
O
O
O
O
O
N
DMSO, H2O
MW, 900 W,
S
S
S
S
S
S
S
S
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
S
40 s
2
3
4
5
6
7
8
79
93
92
85
87
94
87
[52]
[53]
[53]
O
N
MeI, H2O
5 h, US
S
PEG400, Na2CO3
CH2Cl2, rt, 1.5 h
O
C
PhCH2Cl
+
SH
CH3
O
PEG400, Na2CO3
PhCH2Br
+
SH
H3C
CH Cl , rt
, 1.5 h
2
2
Entry 1 in Table 1
Entry 2 in Table 1
[54]
[P4-VP]SCOCH3
PhCH2Cl
CH3CN, rt, 55 min
[P4-VP]SCOCH3
PhCH2Br
CH3CN, rt, 40 min
KSCOCH3, rt
PhCH2Cl
Acetone , 17 h
[50]
-
+
P
SCOCH3
PhCH2Br
P
n-hexane, rt, 2 h
= Amberlyst A-26
9
n-C8H17SCOCH3
n-C8H17SCOCH3
88
90
[53]
[50]
O
PEG400, Na2CO3
n-C8H17Br+
SH
H3C
CH Cl , rt
, 3 h
2
2
10
-
+
P
SCOCH3
n-C8H17Br
P
n-hexane, rt, 2 h
= Amberlyst A-26
11
n-C8H17SCOCH3
87
Entry 15 in Table 1
[P4-VP]SCOCH3
n-C8H17Br
CH3CN, rt, 200 min
7. A. Kameyama, Y. Kimura, T. Nishikubo, Macromolecules 30,
6494 (1997)
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