Selective synthesis of thiodiglycol dicarboxylic acid esters via p-TsOH/C-catalysed direct esterification

Article abstract of DOI:10.1007/s12039-012-0296-3

The esterification of thiodiglycol and long alkyl-chain carboxylic acids is reported. Reaction of thiodiglycol with carboxylic acid via p-TsOH/C-catalysed direct esterification afforded thiodiglycol dicarboxylic acid esters in good yields and chemoselectivity. The use of immobilized p-TsOH on activated carbon as catalyst is crucial for the transformation. Indian Academy of Sciences.

Full text of DOI:10.1007/s12039-012-0296-3

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J. Chem. Sci. Vol. 124, No. 5, September 2012, pp. 1087–1090. ꢀ Indian Academy of Sciences.
Selective synthesis of thiodiglycol dicarboxylic acid esters
via p-TsOH/C-catalysed direct esterification
DAHONG JIANG and MIN HUANG∗
Department of Chemistry and Life Science, Guangdong University of Petrochemical Technology,
Maoming 525000, China
e-mail: min_huang@163.com
MS received 10 November 2011; accepted 10 April 2012
Abstract. The esterification of thiodiglycol and long alkyl-chain carboxylic acids is reported. Reaction of
thiodiglycol with carboxylic acid via p-TsOH/C-catalysed direct esterification afforded thiodiglycol dicar-
boxylic acid esters in good yields and chemoselectivity. The use of immobilized p-TsOH on activated carbon
as catalyst is crucial for the transformation.
Keywords. Thiodiglycol dicarboxylic acid esters; catalytic esterification; p-TsOH on activated carbon;
antioxidant.
1. Introduction
2. Experimental
The long alkyl-chain esters of thiodiglycol are widely 2.1 Preparation of p-TsOH/C
employed in various commercial applications. For
The active carbon (25 g) was immersed in 100 mL of
instance, they have been proposed as lubricating agents,
softening agents, emulsifiers, dispersing agents, and
wetting agents.1 Recently, their analogues are reported
to be used as antioxidant for polymers.2 Thus, their syn-
thesis has attracted interest from synthetic chemists.3
Due to the simple procedure and readily available pre-
cursors, acid-catalysed direct esterification has been
well-established as a means of formation of esters from
alcohols and acids.4 However, the method employing
acid-catalysed direct esterification of carboxylic acids
with thiodiglycol to form the corresponding esters is
quite limited. It is well-known that dithiane, linear
polymeric ethers and oxathiane are formed under the
conditions,5 making the method less adaptable to the
synthesis of thiodiglycol dicarboxylic acid esters.
Solid-supported acids are recognized as environ-
mentally benign catalysts for esterification, and they
also afford great convenience in terms of prepara-
tion and purification of esters.6 Here, we report a
chemo-selective synthesis of thiodiglycol long alkyl-
chain dicarboxylic acid esters via p-TsOH/C-catalysed
esterification of carboxylic acids with thiodiglycol.
p-TsOH in water (25%) for 30 h, and then filtrated. The
solid was dried in oven at 110◦C for 5 h and used with-
out any further manipulation. The loading of p-TsOH
on carbon was determined as 0.203 g per gram of active
carbon by comparing the weights of active carbon and
the catalyst.
2.2 General procedure
Thiodiglycol 1 (1.5 equiv), carboxylic acid 2 (1.0 equiv)
and p-TsOH/C (4 w% of acid) were mixed with 20 mL
of xylene. The mixture was refluxed for 2 h until no
water was carried out by xylene. The reaction mixture
was cooled to room temperature and filtered. Precipi-
tate appeared by the addition of ethanol to the filtrate.
The crude product was filtered and further purified by
recrystallization in ethanol which afforded 3.
2.3 Characterization of products
2.3a 3a: White solid. m.p. 48–50◦C 1H NMR
(300 MHz, CDCl3) δ 4.21 (2H, m), 2.29 (2H, t, J =
5.7 Hz), 1.59 (2H, t, J = 5.4 Hz), 1.23–1.26 (16H, m),
0.85 (3H, r, J = 5.1 Hz); 13C NMR (75 MHz, CDCl3)
δ173.8, 63.4, 32.0, 30.7, 29.7, 29.6, 29.5, 29.4, 29.3,
25.0, 24.9, 22.8, 14.3; MS-EI (m/z) 86, 69, 57, 43.
∗For correspondence
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Dahong Jiang and Min Huang
2.3b 3b: White solid. 58–60◦C 1H NMR (300 MHz,
CDCl3) δ4.21 (2H, m), 2.29 (2H, t, J = 5.7 Hz), 1.59
(2H, t, J = 5.1 Hz), 1.23–1.26 (20H, m), 0.86 (3H,
r, J = 5.1 Hz); 13C NMR (75 MHz, CDCl3) δ 173.9,
63.5, 30.0, 29.9, 29.8, 29.7, 29.6, 29.5, 29.4, 25.2, 23.0,
14.4; MS-EI (m/z) 86, 71, 57, 43, 41.
Table 1. Catalyst effects on esterification of thiodiglycol
with lauric acid.a
O
CH2CH2OH
S
CH2CH2OH
CH2CH2O CC11H23
S
CH2CH2O CC11H23
O
Catalyst
140oC
C11H23COOH
+
1
2a
3a
Entry
Catalyst
Yieldb (%)
2.3c 3c: White solid. 58–61◦C 1H NMR (300 MHz,
CDCl3) δ4.20 (2H, m), 2.29 (2H, t, J = 5.7 Hz), 1.59
(2H, t, J = 5.1 Hz), 1.23–1.26 (24H, m), 0.86 (3H, r,
J = 5.0 Hz); 13C NMR (75 MHz, CDCl3) δ174.0, 63.6,
30.0, 29.9, 29.9, 29.8, 29.6, 29.5, 29.4, 25.2, 23.0, 14.4;
MS-EI (m/z) 104, 86, 71, 57, 44, 41.
1
2
3
4
5
6
H2SO4
H3PO4
Ti(On-Bu)4
SnCl4
p-TsOH
p-TsOH/C
70
52
0
0
83
85
aReaction conditions: 1 (1.5 equiv), 2a (1.0 equiv), catalyst
(2 w% of carboxylic acid), xylene (20 mL), 140◦C, 2 h.
bIsolated yield
2.3d 3d: White solid. 57–60◦C 1H NMR (300 MHz,
CDCl3) δ4.20 (2H, m), 2.29 (2H, t, J = 5.7 Hz), 1.59
(2H, t, J = 5.4 Hz), 1.23–1.26 (28H, m), 0.86 (3H, r,
J = 5.1 Hz); 13C NMR (75 MHz, CDCl3) δ174.0, 63.5,
32.1, 31.7, 30.9, 29.9, 29.7, 29.5, 29.3, 25.1, 24.9, 22.9,
14.3; MS-EI (m/z) 104, 86, 71, 57, 43.
when active carbon-supported p-TsOH was employed
as catalyst. To optimize the reaction conditions, we
judiciously screened the azeotropical agents, catalyst
loading and with results summarized in table 2. Xylene
was proved to be clearly superior to other azeotropical
agents, as hexane and toluene gave the product in lower
yields. Balancing the yield and the cost, 4 w% of cata-
lyst loading was selected for the sequent studies. An
attempt to decrease the ratio of thiodiglycol to lauric
acid to 1:1 gave much lower yield.
With the optimized reaction conditions in hand, we
applied this methodology for the preparation of some
other long alkyl-chain esters of thiodiglycol (table 3).
The conversion of carboxylic acids containing 12 to 18
carbons to the corresponding thiodiglycol esters is per-
formed smoothly under the reaction conditions. Neither
thiodiglycol monocarboxylic acid esters nor its poly-
meric ethers were detected. Though the formation of
1,4-oxathiane can not be eliminated, it has no effect on
the yields of esters, suggesting that the esterification
might be much faster than the intramolecular condensa-
tion. This method did not necessitate special use of the
technique of separation of catalyst. After completion of
the reactions, the catalyst was separated through a sim-
ple filtration and could be reused several times without
significant decrease in catalytic activity.
3. Results and discussions
Generally, direct esterification needs either removal of
water generated in the reaction or large excess amounts
of one of the reactants to shift the equilibrium between
reactants and products.7 In our preliminary studies on
acid-catalysed direct esterfication of long alkyl-chain
carboxylic acids with thiodiglycol, we have first exa-
mined the reaction of excess amount of thiodiglycol
1 with lauric acid 2a using xylene to remove water
azeotropically. Some results from that study are sum-
marized in table 1. Although thiodiglycol lauric acid
ester 3a was obtained in moderate yield using con-
densed sulphuric acid as catalyst (entry 1, table 1), large
amount of by-product which was determined as 1,4-
oxathiane 4 was observed. The compound 4 was con-
sidered the product of intramolecular condensation of
thiodiglycol (scheme 1). We felt that the acid catalyst
played an important role in terms of chemo-selectivity
in this transformation. An attempted survey of catalysts
was conducted. Phosphoric acid was proved to be less
efficient (entry 2, table 1) and both Ti(On-Bu)4 and
SnCl4 provided no detectable amounts of the desired
product (entry 3 and 4, table 1). Fortunately, p-TsOH
was found to be an efficient catalyst for the esterifi-
cation (entry 5, table 1). We were pleased to find that
this transformation provided 3a in 85% yield (entry 6,
table 1) and the amount of 4 decreased dramatically
S
CH2CH2OH
S
CH2CH2OH
H2SO4
140oC
1
4
O
Scheme 1. Formation of 1,4-oxathiane.

Synthesis of thiodiglycol dicarboxylic acid esters
Table 2. Reaction conditions optimization.
1089
Azeotropical
agent
Catalyst loading
(w% of acid)
Ratio
(thiodiglycol to acid)
Entry
Yielda (%)
1
2
3
4
5
6
7
8
xylene
hexane
toluene
xylene
xylene
xylene
xylene
xylene
2
2
2
1
4
6
8
4
1.5:1
1.5:1
1.5:1
1.5:1
1.5:1
1.5:1
1.5:1
1:1
85
55
68
78
92
94
93
71
aIsolated yield
Table 3. p-TsOH/C-catalysed esterification of thioglycol with long chain carboxylic
acid.a
Entry
Carboxylic acid
Ester of thioglycol
Yieldb (%)
O
CH2CH2O CC11H23
S
1
C11H23COOH 2a
92
CH2CH2O CC11H23
O
3a
O
CH2CH2O CC13H27
2
3
4
C13H27COOH 2b
C15H31COOH 2c
C17H35COOH 2d
95
91
96
S
CH2CH2O CC13H27
O
3b
O
CH2CH2O CC15H31
S
CH2CH2O CC15H31
O
3c
O
CH2CH2O CC17H35
S
CH2CH2O CC17H35
O
3d
aReaction conditions: 1 (1.5 equiv), 2 (1.0 equiv), catalyst (4 w% of acid), xylene
(20 mL), 140◦C, 2 h. bIsolated yield
4. Conclusions
of the methodology for the construction of various long
alkyl-chain esters of thiodiglycol.
In conclusion, we have developed a chemoselective
heterogeneous p-TsOH/C-catalysed route to long alkyl-
chain esters of thiodiglycol via direct esterification
of thiodiglycol with carboxylic acid. The high effi-
cient and reusable catalyst renders the transformation
highly valuable from both environmental and economi-
cal points of view. This illustrates the potential utility
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