CsF-Celite, an efficient solid state reagent for the syntheses of thioesters and thioethers

Article abstract of DOI:10.1007/s00706-005-0351-6

Coupling reactions of a number of aliphatic, aromatic, and heterocyclic compounds bearing an acidic hydrogen atom attached to sulfur, with alkyl, acyl, benzyl, or benzoyl halides in acetonitrile with cesium fluoride-Celite are described. This procedure is convenient, efficient, and practical for the preparation of thioethers and thioesters. Springer-Verlag 2005.

Full text of DOI:10.1007/s00706-005-0351-6

Monatshefte f €u r Chemie 136, 1583–1589 (2005)
DOI 10.1007/s00706-005-0351-6
CsF-Celite, an Efficient Solid State
Reagent for the Syntheses
of Thioesters and Thioethers
1
2
2
Syed T. A. Shah , Khalid M. Khan , Hidayat Hussain ,
2
1;ꢀ
Safdar Hayat , and Wolfgang Voelter
1
Abteilung f €u r Physikalische Biochemie des Physiologisch-chemischen
Instituts der Universit €a t T €u bingen, D-72076 T €u bingen, Germany
HEJ Research Institute of Chemistry, International Center for Chemical Sciences,
University of Karachi, Karachi 75270, Pakistan
2
Received December 3, 2004; accepted January 14, 2005
Published online August 10, 2005 # Springer-Verlag 2005
Summary. Coupling reactions of a number of aliphatic, aromatic, and heterocyclic compounds bearing
an acidic hydrogen atom attached to sulfur, with alkyl, acyl, benzyl, or benzoyl halides in acetonitrile
with cesium fluoride-Celite are described. This procedure is convenient, efficient, and practical for the
preparation of thioethers and thioesters.
Keywords. Cesium fluoride-Celite, Thioethers, Thioesters.
Introduction
A variety of organic reactions have recently been reported to be catalysed by
cesium fluoride-Celite. The syntheses of carboxylic esters [1], ꢀ-lactones [2], N-
alkylation of anilines, carboxamides, and nitrogen heterocyclic compounds [3], and
ring opening of epoxides [4] are among the reactions which are facilitated.
The importance of the fluoride ion as a catalyst for the promotion of various
types of base-catalyzed reactions in organic synthesis has been previously recog-
nized [5–8]. In particular, the work of Clark and Miller [9–12] revealed that the
fluoride ion effects the coupling reaction because of its high capability of hydrogen
bond formation. As reagents, generating fluoride ions, potassium [6], cesium [1],
and tetraalkylammonium fluorides were used so far. However, it is not easy to
handle these hygroscopic reagents and the reproducibility of these reactions is in-
variably poor. Recently, poorly hygroscopic reagents generating fluoride ions were
designed allowing cesium fluoride to be absorbed on Celite [1]. The effect of
ꢀ Corresponding author. E-mail: wolfgang.voelter@uni-tuebingen.de

1
584
S. T. A. Shah et al.
cesium fluoride-Celite might be two-fold [13]: (a) activation of the hydroxyl group
by the fluoride ion whose ionic character is large owing to the low charge=surface
area ratio of the cesium cation [14] and (b) activation of the alkyl or acyl halide
groups by the Lewis acid type effect.
Thioethers are, in general, formed by reaction of thiols or thiophenols under
basic conditions with alkyl or benzyl halides. Thioesters are formed and cleaved in
the same way as oxygen esters, however, they are more reactive against nucleo-
philic substitution [15] and used as ‘‘activated esters’’. The conversion of thiols to
thioethers is usually achieved by reaction of thiolates with organic halides [16].
The yields and reaction conditions depend on the solvent, the basic catalyst, and
the acidity of the thiol. However, these reactions require often very long refluxing
times and suffer from low yields [16].
Several other methods have been employed to prepare thioethers and thioesters,
which include palladium(0)-mediated alkylation [17], phase-transfer catalysis [18],
catalysis via platinum(II) complexes of bis(diphenylphosphino)methane [19],
via bis(triphenylstannyl)-tellurides [20], palladium-catalyzed reactions of stannyl
sulfides with aryl bromides [21], ligand-transfer reactions of aryl thiocyanates
[
22, 23], fluorodestannylation of organotin sulfides [24], montmorillonite clay-
catalyzed acylation of thiols, etc. [25], or benzyl-type protection of thiols using
trifluoroacetic acid [26]. Recently, Yin and Pidgeon [27] reported a high yield
method for the preparation of unsymmetrical sulfides by using very strong basic
n-butyllithium.
Results and Discussions
In extension of our work on the reactivity of CsF-Celite [2, 3, 28], we wish to
report the utility of this reagent for the synthesis of thioethers and thioesters in
good yields (Scheme 1). Tables 1 and 2 list a series of the CsF-Celite-assisted
couplings of aliphatic and aromatic thiols into thioethers and thioesters using a
variety of alkyl, acyl, benzyl, and benzoyl halides, catalyzed by this inexpensive,
non-corrosive solid base, allowing simple work up of the reaction mixtures.
In general, to a stirred solution of thiol (1.0 mol) and CsF-Celite (1.5 mol) in
acetonitrile, alkyl, acyl, benzyl, or benzoyl halides (2.0 mol) were added. Then, the
mixture was stirred at room temperature or reflux up to completion of the reaction
(
TLC analysis). The reaction mixture was filtered and the solvent evaporated. The
product was purified, whenever necessary, by column chromatography on silicagel
using appropriate solvent systems like dichloromethane and petroleum ether, etc.
as eluents, to afford pure thioethers or thioesters (Scheme 1).
Scheme 1

CsF-Celite
1585
Table 1. Synthesis of thioethers using CsF-Celite
No.
Substrate
Reagent
Product
Comp.
Yield=%
b
1
2
3
4
5
6
7
8
9
CH CH SH
2
CH O CCH CH Br
2
CH O CCH CH SCH CH
3
1
2
60
3
3
2
2
3
2
2
2
2
b
CH (CH ) SH
3 2 4
C H CH Cl
6
CH (CH ) SCH C H
2 6 5
81
5
2
3
2 4
b
CH (CH ) SH
3 2 3
C H CH Cl
6
CH (CH ) SCH C H
2 6 5
3
88
5
2
3
2 3
b
C H CH SH
2
CH ¼CHCH Br
C H CH SCH CH¼CH
4
66
6
5
2
2
6
5
2
2
2
b
C H SH
6 5
C H CH Br
2
C H SCH C H
6 2 6 5
5
85
6
5
5
b
C H SH
6 5
CH CHICH
3
C H SCH(CH )
3 2
6
78
3
6
5
b
4-CH OC H SH
4
C H CH Br
2
4-CH OC H SCH C H
2 6 5
7
81
3
6
6
5
3
6
4
b
CH C H SH
4
C H CH Br
2
CH C H SCH C H
2 6 5
8
92
3
6
6
5
3
6
4
b
4-NO C H SH
4
CH CHICH
3
4-NO C H SCH(CH )
3 2
9
61
2
6
3
2
6
4
b
1
1
1
1
0
1
2
3
4-NO C H SH
4
CH ¼CHCH Br
4-NO C H SCH CH¼CH
10
11
12
13
75
2
6
2
2
2
6
4
2
2
b
4-NO C H SH
4
CH CH I
3
4-NO C H SCH CH
3
60
2
6
2
2
6
4
2
b
4-NO C H SH
4
C H CH Cl
2
4-NO C H SCH C H
2 6 5
77
2
6
6
5
2
6
4
b
4-CH OC H SH
4
4-CH OC H CH Cl
4
4-CH OC H SCH C H OCH -4
3
86
3
6
3
6
2
3
6
4
2
6
4
b
1
1
4
5
6
14
15
16
90
a
74
a
1
81
a
b
rt for 1–8 h; reflux at 82 C for 2–48 h
ꢁ
Table 2. Syntheses of thioesters using CsF-Celite
No.
Substrate
C H SH
Reagent
Product
Comp.
Yield=%
a
1
2
3
4
5
6
7
8
C H COCl
6 5
C H SOCC H
6 5
17
18
19
20
21
22
23
24
88
6
5
6
5
b
CH C H SH
4
C H COBr
6 5
CH C H SOCC H
6 5
75
3
6
3
6
4
a
C H SH
6
CH COCl
3
C H SOCCH
3
85
5
6
5
b
4-CH OC H SH
4
CH O CCH CH COCl
2
4-CH OC H SOCCH CH CO CH
3
89
3
6
3
2
2
3
6
4
2
2
2
b
4-CH OC H SH
3
CH COCl
3
4-CH OC H SCO CH
3
88
6
4
3
6
4
2
b
4-CH OC H SH
4
4-CH OC H COCl
6
4-CH OC H SOCC H OCH -4
6
81
3
6
3
4
3
6
4
4
3
b
4-CH OC H SH
3
C H COCl
6
4-CH OC H SOCC H
4
71
6
4
5
3
6
6
5
b
4-NO C H SH
2
C H COCl
6
4-NO C H SOCC H
6 5
49
6
4
5
2
6
4
b
9
25
26
27
83
b
1
1
0
1
79
b
75
b
1
2
28
80
a
b
rt for 1–8 h; reflux at 82 C for 2–4 h
ꢁ

1
586
S. T. A. Shah et al.
When thiophenol and benzyl bromide were allowed to react in DMF or ace-
tonitrile, S-benzylthiophenol could be collected in excellent yield (92% or 85%)
after reflux for 28 h. Most of the reactions were carried out in acetonitrile, but
sometimes DMF was used as a solvent (Table 1, entries 5 and 9). These experi-
ments indicate that the cesium fluoride-Celite-catalyzed reaction is not very much
dependent on the solvent used. Application of this method on some heterocyclic
compounds with mercapto groups, e.g. 2-mercaptobenzoxazole or 2-mercapto-2-
thiazoline, supplied the corresponding thioethers in very good yields proving that
the methodology is equally applicable on aromatic as well as heterocyclic com-
pounds (Table 1, entries 14–16). We also extended our present discovery to the
formation of thioesters by using CsF-Celite to catalyze the reaction between mer-
captanes, thiophenols, and acyl groups in very good yield (Table 2).
In conclusion, the CsF-Celite-assisted reactions provide an easy access to
thioethers as well as thioesters in good yields. In most cases the products were
obtained just by filtration and evaporation of the filtrates, while in some cases (7,
24–26) they were isolated by column chromatography. In short, this approach is an
efficient, convenient, inexpensive, non-corrosive, and practical method for prepar-
ing thioethers and thioesters. As this methodology has several advantages, it is a
valuable addition to existing methods for preparing thioethers and thioesters. The
physical properties and NMR spectra of the prepared compounds agreed with those
reported in literature [29–44], and were furthermore confirmed by comparing the
data with those of authentic samples. The hitherto unknown compounds were
characterized through their sharp melting points, spectroscopic techniques, and
their elemental analyses.
Experimental
Melting points were determined with a B €u chi SMP-20 apparatus. IR spectra (KBr discs) were recorded
on a Bruker FT-IR IFS 48 spectrometer and EI mass spectral data on a Varian MAT 711 (70 eV)
1
13
spectrometer (data are tabulated as m=z). H and C NMR spectra were performed in CDCl contain-
3
ing ca. 1% TMS as internal standard on a Bruker AC 250 (250 MHz and 62.9MHz) spectrometer.
Chemical shifts are reported in ꢁ (ppm) and coupling constants are given in Hz. The progress of all
reactions was monitored by TLC on 2.0ꢂ5.0 cm Al sheets, precoated with silicagel 60F
to a
54
2
thickness of 0.25mm (Merck, Darmstadt, Germany). The chromatograms were visualized under
ultraviolet light (254–366 nm). Elemental analyses were found to agree favourably with the calculated
values (C, H, S).
Ethanethiol, thiophenol, 1-pentanethiol, 4-methoxythiophenol, 4-nitrothiophenol, 2-mercaptoben-
zoxazole, 2-mercaptobenzothiazole, 2-mercapto-2-thiazoline, benzyl bromide, 4-nitrobenzyl bromide,
allyl bromide, acetyl chloride, benzoyl chloride, 4-methoxybenzoyl chloride, CsF, Celite 521, and other
chemicals are commercially available (Fluka, Aldrich, Munich, Germany). Anhydrous acetonitrile was
purchased from Merck, Darmstadt, Germany, and used without purification. The CsF-Celite was
prepared by stirring an aqueous solution of CsF with Celite 521 at room temperature for 20min [1].
Typical Procedure for Syntheses of Thioethers and Thioesters
3
To a stirred solution of 1.0 mol thiol and 1.5 mol CsF-Celite in 20cm of acetonitrile, 2.0 mol alkyl,
acyl, benzyl, benzoyl halides, etc. were added. Then the mixtures were stirred at room temperature or
reflux up to completion of the reactions, indicated by monitoring. The reaction mixtures were filtered,
the solvents evaporated, and the residues dissolved in ethyl acetate. Precipitates were filtered off,
3
washed with 20 cm of ethyl acetate and the filtrates evaporated under reduced pressure. The products

CsF-Celite
1587
were purified whenever necessary by column chromatography on silicagel using various solvent sys-
tems like CH Cl and petroleum ether, etc. as eluents, to afford pure thioethers or thioesters.
2
2
Benzyl phenyl sulfide (5)
ꢁ
ꢁ
1
Solid; mp 42–43 C (Ref. [31] 43–44 C); H NMR (250MHz, CDCl ): ꢁ ¼ 4.3 (s, 2H), 7.45–7.25 (m,
3
1
3
1
1
0H) ppm; C NMR (63 MHz, CDCl ): ꢁ ¼ 39.25, 126.45, 127.12, 128.44, 128.89, 129.52, 136.40,
3
þ
37.40 ppm; EIMS: m=z ¼ 200 (M ).
2
-Propyl 4-nitrophenyl sulfide (9)
ꢁ
ꢁ
1
Solid; mp 46–47 C (Ref. [34] 46–47 C); H NMR (250 MHz, CDCl ): ꢁ ¼ 1.40 (d, J ¼ 6.65 Hz, 6H),
3
1
3
3
.60 (m, 1H), 7.45 (d, J ¼ 8.92 Hz, 2H), 8.23 (d, J ¼ 8.92 Hz, 2H) ppm; C NMR (63 MHz, CDCl ):
3
þ
ꢁ
¼ 22.15, 36.64, 124.38, 127.62, 145.13, 147.55ppm; EIMS: m=z ¼ 197 (M ).
Allyl 4-nitrophenyl sulfide (10)
ꢁ
ꢁ
1
Solid; mp 38–39 C (Ref. [35a] 38–39 C); H NMR (250MHz, CDCl ): ꢁ ¼ 3.57 (m, 2H), 5.14–5.55
3
1
3
(m, 2H), 5.94 (m, 1H), 7.54 (d, J ¼ 9.10Hz, 2H), 8.08 (d, J ¼ 9.12Hz, 2H) ppm; C NMR (63 MHz,
þ
CDCl ): ꢁ ¼ 53.12, 119.01, 123.54, 125.92, 131.71, 145.00, 146.58 ppm; EIMS: m=z ¼ 195 (M ).
3
Ethyl 4-nitrophenyl sulfide (11)
ꢁ
ꢁ
1
Solid; mp 40–42 C (Ref. [35b] 42–43 C); H NMR (CDCl ): ꢁ ¼ 1.40 (t, J ¼ 7.53Hz, 3H), 3.00 (q,
3
1
3
J ¼ 7.36Hz, 2H), 7.31 (d, J ¼ 8.93Hz, 2H), 8.21 (d, J ¼ 8.80Hz, 2H) ppm; C NMR (63 MHz,
þ
CDCl ): ꢁ ¼ 13.73, 25.15, 123.77, 126.29, 144.63, 147.81 ppm; EIMS: m=z ¼ 183 (M ).
3
Benzyl 4-nitrophenyl sulfide (12)
ꢁ
ꢁ
1
Solid; mp 128–129 C (Ref. [33] 128–129 C); H NMR (250 MHz, CDCl ): ꢁ ¼ 4.22 (s, 2H), 7.36 (d,
3
3
1
J ¼ 9.10Hz, 2H), 7.00–7.51 (m, 5H), 8.43 (d, J ¼ 8.67 Hz, 2H) ppm; C NMR (63 MHz, CDCl ):
3
þ
ꢁ
¼ 37.41, 122.80, 126.35, 127.62, 128.88, 129.27, 135.42, 145.22, 147.35ppm; EIMS: m=z ¼ 245 (M ).
Phenylthio acetate (19, C H OS)
8
8
1
13
Liquid [42a, 42b]; H NMR (250 MHz, CDCl ): ꢁ ¼ 2.41 (s, 3H), 7.41–7.43 (m, 5H) ppm; C NMR
3
þ
(
63 MHz, CDCl ): ꢁ ¼ 30.11, 127.87, 129.12, 129.35, 134.37, 193.90ppm; EIMS: m=z ¼ 152 (M ).
3
Methyl 4-[(4-methoxyphenyl)thio]-4-oxobutanoate (20, C H O S)
1
2 14 4
1
Liquid; H NMR (250MHz, CDCl ): ꢁ ¼ 2.51 (t, J ¼ 6.00Hz, 2H), 3.21 (t, J ¼ 6.10Hz, 2H), 3.56 (s,
3
1
3
3
H), 3.62 (s, 3H), 6.98–7.15 (m, 4H) ppm; C NMR (63 MHz, CDCl ): ꢁ ¼ 31.21, 40.51, 52.22,
3
þ
5
5.54, 111.75, 117.69, 136.87, 160.22, 172.50, 196.23 ppm; EIMS: m=z ¼ 254 (M ).
4
-Methoxyphenylthio benzoate (23)
ꢁ
ꢁ
1
Solid; mp 96–98 C (Ref. [40] 97.5–99.5 C); H NMR (250 MHz, CDCl ): ꢁ ¼ 3.80 (s, 3H), 6.96–7.41
3
0
0
0
13
(
m, 4H, H-2,3,5,6), 7.45–7.60 (m, 3H, H-3 ,4 ,5 ) ppm; C NMR (63 MHz, CDCl ): ꢁ ¼ 55.45,
3
þ
1
15.15, 117.53, 127.42, 128.34, 133.35, 135.59, 136.59, 160.84, 190.95 ppm; EIMS: m=z ¼ 231 (M ).
4
-Nitrophenylthio benzoate (24)
ꢁ
0 0 0 0
13
,4 ,5 ), 7.72 (m, 2H, H-2,6), 8.03 (m, 2H, H-2 ,6 ), 8.30 (m, 2H, H-3,5) ppm; C NMR (63 MHz,
ꢁ
1
Solid; mp 125–127 C (Ref. [40] 126–127 C); H NMR (250MHz, CDCl ): ꢁ ¼ 7.58–7.66 (m, 3H, H-
3
0
3
CDCl ): ꢁ ¼ 123.88, 127.78, 128.69, 135.10, 135.80, 135.85, 136.25, 148.30, 188.01 ppm; EIMS:
3
þ
m=z ¼ 259 (M ).
1
,3-Benzoxazol-2-yl thiophene-2-carbothioate (26, C H NO S )
12 7 2 2
ꢁ
1
Solid; mp 110–112 C; H NMR (250MHz, CDCl ): ꢁ ¼ 7.01 (dd, J ¼ 3.74, 4.8 Hz, 1H), 7.44 (dd,
3
1
3
J ¼ 1.17, 4.75Hz, 1H), 8.22 (dd, J ¼ 1.29, 3.59Hz, 1H), 7.35 (m, 2H), 8.21 (m, 2H) ppm; C NMR

1
588
S. T. A. Shah et al.
(
63 MHz, CDCl ): ꢁ ¼ 111.51, 120.51, 124.12, 125.87, 131.25, 132.24, 142.55, 146.90, 152.84, 157.26,
3
þ
1
63.50, 176.41 ppm; EIMS: m=z ¼ 261 (M ).
4
-Nitrophenyl thiophene-2-carbothioate (27, C H NO S )
1
1
7
3 2
1
Liquid; H NMR (250MHz, CDCl ): ꢁ ¼ 6.71 (dd, J ¼ 3.67, 4.8 Hz, 1H), 7.42 (dd, J ¼ 1.27, 4.8Hz,
3
1
3
1
H), 8.02 (dd, J ¼ 1.27, 3.69 Hz, 1H), 7.55 (m, 2H), 8.15 (m, 2H) ppm; C NMR (63 MHz, CDCl ):
3
þ
ꢁ
¼ 126.87, 129.12, 131.25, 135.24, 138.90, 146.45, 155.25, 177.50 ppm; EIMS: m=z ¼ 265 (M ).
1
,3-Benzoxazol-2-yl 4-nitrobenzenecarbothioate (28, C H N O S)
1
4 8 2 4
1
Liquid; H NMR (250 MHz, CDCl ): ꢁ ¼ 7.25 (m, 2H), 7.55 (m, 1H), 7.95–8.12 (m, 3H), 8.32 (m, 2H)
3
1
3
ppm; C NMR (63 MHz, CDCl ): ꢁ ¼ 110.12, 120.11, 125.02, 125.22, 125.57, 130.25, 143.54,
3
þ
1
44.60, 148.76, 153.30, 163.44, 188.22 ppm; EIMS: m=z ¼ 300 (M ).
Acknowledgements
We are thankful to Deutscher Akademischer Austauschdienst (DAAD, Bonn, Germany) for a scholar-
ship granted to S. T. A. Shah.
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