The green method for regiospecific ring opening of epoxide with dimercaptoethane

Article abstract of DOI:10.1080/10426500701313888

A simple, efficient, and environmentally friendly one-step regiospecefic ring opening of strained epoxides with dimercaptoethane as a nucleophile in nearly saturated aqueous potassium carbonate is reported. The synthesized ,'-dihydroxydithioether compound

Full text of DOI:10.1080/10426500701313888

This article was downloaded by: [Pennsylvania State University]
On: 02 June 2014, At: 02:32
Publisher: Taylor & Francis
Informa Ltd Registered in England and Wales Registered Number: 1072954
Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH,
UK
Phosphorus, Sulfur, and Silicon
and the Related Elements
Publication details, including instructions for
authors and subscription information:
http://www.tandfonline.com/loi/gpss20
The Green Method
for Regiospecific Ring
Opening of Epoxide with
Dimercaptoethane
Seyed Mohammad Seyedi a , Hamid Sadeghian a &
Masoomeh Rezai a
a Department of Chemistry, Faculty of Sciences ,
Ferdowsi University of Mashhad , Mashhad, I. R.,
Iran
Published online: 15 Jun 2007.
To cite this article: Seyed Mohammad Seyedi , Hamid Sadeghian & Masoomeh
Rezai (2007) The Green Method for Regiospecific Ring Opening of Epoxide with
Dimercaptoethane, Phosphorus, Sulfur, and Silicon and the Related Elements, 182:8,
1709-1716, DOI: 10.1080/10426500701313888
To link to this article: http://dx.doi.org/10.1080/10426500701313888
PLEASE SCROLL DOWN FOR ARTICLE
Taylor & Francis makes every effort to ensure the accuracy of all the
information (the “Content”) contained in the publications on our platform.
However, Taylor & Francis, our agents, and our licensors make no
representations or warranties whatsoever as to the accuracy, completeness,
or suitability for any purpose of the Content. Any opinions and views
expressed in this publication are the opinions and views of the authors, and
are not the views of or endorsed by Taylor & Francis. The accuracy of the
Content should not be relied upon and should be independently verified with
primary sources of information. Taylor and Francis shall not be liable for any
losses, actions, claims, proceedings, demands, costs, expenses, damages,

and other liabilities whatsoever or howsoever caused arising directly or
indirectly in connection with, in relation to or arising out of the use of the
Content.
This article may be used for research, teaching, and private study purposes.
Any substantial or systematic reproduction, redistribution, reselling, loan,
sub-licensing, systematic supply, or distribution in any form to anyone is
expressly forbidden. Terms & Conditions of access and use can be found at
http://www.tandfonline.com/page/terms-and-conditions

Phosphorus, Sulfur, and Silicon, 182:1709–1716, 2007
Copyright © Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500701313888
The Green Method for Regiospecific Ring Opening of
Epoxide with Dimercaptoethane
Seyed Mohammad Seyedi
Hamid Sadeghian
Masoomeh Rezai
Department of Chemistry, Faculty of Sciences, Ferdowsi University
of Mashhad, Mashhad, I. R. Iran
A simple, efficient, and environmentally friendly one-step regiospecefic ring open-
ing of strained epoxides with dimercaptoethane as a nucleophile in nearly
saturated aqueous potassium carbonate is reported. The synthesized β,βꢀ-
1
dihydroxydithioether compounds were confirmed by elemental analysis, H NMR,
13
C NMR, and MS spectral studies.
INTRODUCTION
The ring opening of epoxides1 with various nucleophils is an impor-
tant synthetic transformation in organic chemistry. The preparation of
β-hydroxy thioethers is generally realized by the action of thiolates2
or thiols in the presence of metal salts such as Lanthanum3, Cobalt,4
Titanium5, complex of Gallium-Lithium bis-naphtoxide6 and Lewise
acids ZnCl2,7 and InCl3.8 Ring opening reaction by thiols was also car-
ried out in the presence of alumina9 and polyethylene glycol.10 This
reaction has also been reported in hexafluoroisopropanol without us-
ing catalyst.11 The ring opening of oxiranes with dimercaptoethane has
been recently performed in the presence of benzyltrimethylammonium
hydroxide (Triton B) as catalyst.12 Highly regioselective ring opening
of epoxides by thiophenols in ionic liquids has also been reported.13
Herein, we report the ring opening reaction of two mole equivalents of
epoxides with dimercaptoethane in saturated aqueous solution of potas-
sium carbonate to prepare corresponding β,βꢀ-dihydroxy dithioethers.
Having two secondary hydroxy groups, make these compounds suscep-
tible reagents to synthesize new thiocrown ethers possessing various
sidearms.14
Received November 18, 2006; accepted February 1, 2007.
Address correspondence to Seyed Seyedi, Department of Chemistry, School of Sciences,
Ferdowsi University of Mashhad, Mashhad, 41775, Iran. E-mail: smseyedi@yahoo.com
1709

1710
S. M. Seyedi et al.
Many efforts are developed to perform chemical reactions in aque-
ous media to safe environmental conditions.15 Water is cheap, readily
available, and nontoxic, resulting in an economic process that has clear
advantages as an environmentally friendly solvent alternative in or-
ganic synthesis.16
RESULTS AND DISCUSSION
The ring opening of the starting epoxides (1a–i) was regionspecific
by nucleophilic attack on the terminal carbon atoms affording a sec-
ondary diols. The water solubility of epoxides are decreased in the
presence of high concentrations of potassium carbonate lead to de-
crease epoxide ring opening percentage by the protic solvent.17 Dimer-
captoethan is deprotonated by carbonate anion (rather than water
deprotonation) lead to formation of water soluble dimercaptid anion,
which can react with epoxides in the boundary surface of aqueous
and organic phase. This is a suitable condition of the SN2 mecha-
nism resulted nucleophilic attack on low substituted site.17 Obviously,
the ring opening of epoxides (1a–i) afforded corresponding dihydroxy
dithioethers as a mixture of isomeric diastreomers.12 It was notable that
in low concentrations of potassium carbonate, the formation of mono-
substituted ethandiols—instead of formation of corresponding β,βꢀ-
dihydroxydithioethers—was increased. This was obvious when the re-
sults were checked by TLC and sodium metaperiodate-benzidine test of
1,2-diols.18
The β,βꢀ-dihydroxydithioethers (3a–i) were prepared by the action
of two mole equivalents of epoxides (1a–i) with deporotonated dimer-
captoehane (2), which was formed by proton abstracting of carbon-
ate anion under reflux condition and vigorous stirring (Scheme 1). All
SCHEME 1

Ring Opening of Epoxide with Dimercaptoethane
1711
TABLE I Thiolysis by Dimercaptoethane of Epoxides (1a–i) in
Aqueous Potassium Carbonate to Give Beta, Betaꢀ-dihydroxy
Dithioether
Beta, Betaꢀ-dihydroxy
Dithioether
Time Yield
Epoxides
(hour)
(%)
1a
1b
1c
5
4
3
94
97
91
1d
1e
3
3
96
96
1f
3
3
96
99
1g
1h
1i
3
3
96
89

1712
S. M. Seyedi et al.
dihydroxy compounds were obtained in excellent (Table I) and indi-
cated high regioselectivity by showing one spot on TLC, which did
not need further purification. The dihydroxy compounds (3a–g) were
isolated by decanting and (3h–i) by filtering follow by washing with
water.
CONCLUSION
The method used here is a simple, efficient, and environmentally
friendly procedure with excellent yields, high regioselectivity, and no
need for any organic solvents either for reaction medium or extract-
ing the products. Therefore, the work follows the basic aims which are
important to green chemistry.19
EXPERIMENTAL
Melting points were recorded on an Electrothermal type 9100 melt-
ing point apparatus. 1H NMR (500 MHz) and 13C NMR (125 MHz)
were obtained by using a Bruker Avance DRX-500 Fourier trans-
former spectrometer. Chemical shifts are reported in ppm (δ) downfield
from tetramethylsilane (TMS). Electron impact (EI) mass spectra were
recorded on a Varian Match 7A spectrometer. Elemental analysis was
obtained on a Thermo Finnigan Flash EA microanalyzer. All chemicals
were purchased from Merck and Fluka Co. and used without further
purification.
General Procedure for Ring Opening of Epoxides 1a-i
with Dimercaptoethane (2)
To a solution of potassium carbonate (50 g, 350 mmol) in water (65
ml) was added dimercaptoethane (2) (13.5 ml, 160 mmol) followed by
epoxide (300 mmol). The mixture was refluxed in oil bath while stirring
vigorously. The completion of reaction was checked by TLC (silica gel
60 F254, benzene-ethylacetate 50:50).
After completion, the mixture was cooled, and the precipitated prod-
ucts (3h–i) were filtered, washed with water (3 × 50 ml), and dried in
oven at 50–55◦ for 4h. The liquid products (3a–g) in aqueous phase were
decantated, washed with water (3 × 50 ml), and then heated in oven
at 80–90◦ for 2h to eliminate water. The remainder of the carbonate
solution was reused in subsequent runs.
1-(2-[(2-Hydroxypropyl)sulfanyl]ethylsulfanyl)-2-propanol 3a
Colorless oil (Found: C, 45.91; H, 8.57; S, 30.27. C8H18O2S2 requires
C, 45.68; H, 8.62; S, 30.49%); δH 1.25 (6H, d, J 6.3, 2 × CH3), 2.51 (2H,

Ring Opening of Epoxide with Dimercaptoethane
1713
dd, J 8 and 20, 2 × CH2S), 2.73 (2H, dd, J 4.1 and 12.5, 2 × CH2S),
2.78 (4H, s, SCH2CH2S), 2.92 (2H, br, 2 × OH), 3.87 (2H, m, 2 × CH);
δC 21.96 (2 × q), 32.41 (2 × t), 41.40 (2 × t), 65.92 (2 × d); m/z EI 210
(M+), 165 (12%) C6H13OS2, 151 (58) C5H11OS2, 119 (65) C5H11OS, 75
(100) C3H7S.
1-(2-[(2-Hydroxybutyl)sulfanyl]ethylsulfanyl)-2-butanol 3b
Colorless oil (Found: C, 50.20; H, 9.34; S, 26.82. C10H22O2S2 requires
C, 50.38; H, 9.30; S, 26.90%); δH 0.97 (6H, t, J 7.5, 2 × CH3), 1.54 (4H,
m, 2 × CH3CH2), 2.52 (2H, dd, J8 and 20, 2 × CH2S), 2.75 (2H, dd, J4.1
and 12.5, 2 × CH2S), 2.78 (4H, s, SCH2CH2S), 2.93 (2H, br, 2 × OH),
3.61 (2H, m, 2 × CH); δC 9.82 (2 × q), 28.87 (2 × t), 32.24 (2 × t), 39.46
(2 × t), 71.02 (2 × d); m/z EI 238 (M+), 179 (18%) C7H15OS2, 133 (94)
C6H13OS, 105 (95) C4H9OS, 75 (100) C3H7S.
2-(2-[(2-Hydroxy-2-phenylethyl)sulfanyl]ethylsulfanyl)-1-
phenyl-1-ethanol 3c
Colorless oil (Found: C, 64.68; H, 6.57; S, 19.09. C18H22O2S2 requires
C, 64.63; H, 6.63; S, 19.17%); δH 2.55-2.93 (4H, m, 4 × CH2S), 2.76 (4H,
s, SCH2CH2S), 3.45 (2H, br, 2 × OH), 4.74 (2H, m, 2 × CH), 7.37 (10
H, m, 2 × Ph); δC 32.63 (2 × t), 42.78 (2 × t), 77.65 (2 × d), 128.74
(2 × d), 130.02 (d), 130.88 (2 × d), 139.84 (d); m/z EI 334 (M+), 317
(11%) C18H21OS2, 227 (23) C11H15OS2, 213 (28) C10H13OS2, 196 (72)
C10H12S2, 136 (92) C4H8OS2, 121 (96) C8H9O, 107 (100) C7H7O.
1-(2-[(2-Hydroxy-3-isopropoxypropyl)sulfanyl]ethylsulfanyl)-
3-isopropoxy-2-propanol 3d
Colorless oil (Found: C, 51.43; H, 9.21; S, 19.39. C14H30O4S2 requires
C, 51.50; H, 9.26; S, 19.64%); δH 1.21 (12H, d, J 6.1, 2 × CH3CHCH3)
2.69 (2H, dd, J7 and 19.5, 2 × CH2S), 2.77 (2H, dd, J5.2 and 17.4, 2 ×
CH2S), 2.84 (4H, s, SCH2CH2S), 2.92 (2H, br, 2 × OH), 3.47 (2H, dd,
J 6.3 and 15.7, 2 × CH2O), 3.55 (2H, dd, J 4.1 and 13.5, 2 × CH2O),
3.65 (2H, m, 2 × CH3CHCH3), 3.90 (2H, m, 2 × CH); δC 22.45 (2 × q),
32.99 (2 × t), 36.14 (2 × t), 67.05 (2 × d), 71.02 (2 × t), 72.75 (2 × t);
m/z EI 326 (M+), 297 (11%) C12H25O4S2, 209 (48) C8H17O2S2, 177 (57)
C8H17O2S, 103 (88) C5H11O2, 59 (100) C3H7O.
1-(Tert-butoxy)-3-[(2-[3-(tert-butoxy)-2-hydroxypropyl]
sulfanylethyl)sulfanyl]-2-propanol 3e
Colorless oil (Found: C, 54.43; H, 9.71; S, 17.98. C16H34O4S2 requires
C, 54.20; H, 9.67; S, 18.09%); δH 1.18 (18H, s, 2 × C4H9), 2.68 (2H, dd,
J7 and 19.5, 2 × CH2S), 2.76 (2H, dd, J5.2 and 17.4, 2 × CH2S), 2.83
(4H, s, SCH2CH2S), 2.87 (2H, br, 2 × OH), 3.46 (2H, dd, J 6.3 and 15.7,

1714
S. M. Seyedi et al.
2 × CH2O), 3.54 (2H, dd, J 4.1 and 13.5, 2 × CH2O), 3.86 (2H, m, 2 ×
CH); δC 27.45 (2 × q), 33.09 (2 × t), 36.24 (2 × t), 67.76 (2 × d), 71.62
(2 × t), 73.15 (2 × t); m/z EI 354 (M+), 223 (53%) C9H19O2S2, 166 (54)
C5H10O2S2, 137 (60) C4H9OS2, 57 (100) C4H9.
1-Butoxy-3-(2-[(3-butoxy-2-hydroxypropyl)sulfanyl]
ethylsulfanyl)-2-propanol 3f
Colorless oil (Found: C, 54.16; H, 9.73; S, 18.22. C16H34O4S2 requires
C, 54.20; H, 9.67; S, 18.09%); δH 0.89 (6H, t, J8, 2 × CH3), 1.18–1.69
(8H, m, 2 × CH2CH2), 2.64 (2H, dd, J1.1, 7 and 19.5, 2 × CH2S), 2.70
(2H, dd, J5.2 and 17.4, 2 × CH2S), 2.76 (4H, s, SCH2CH2S), 2.90 (2H,
br, 2 × OH), 3.42(2H, dd, J 6.3 and 15.7, 2 × CH2O), 3.49 (2H, dd, J 4.1
and 13.5, 2 × CH2O), 3.46 (4H, t, J 8, 2 × CH2CH2O), 3.87 (2H, m, 2 ×
CH); δC 14.44 (2 × q), 19.30 (2 × t), 31.2 (2 × t) 33.15 (2 × t), 36.08 (2 ×
t), 67.70 (2 × d), 71.53 (2 × t), 72.89 (2 × t); m/z EI 354 (M+), 223 (43%)
C9H19O2S2, 191 (63) C9H19O2S, 57 (100) C4H9.
1-(Allyloxy)-3-[(2-[3-(allyloxy)-2-hydroxypropyl]sulfanylethyl)
sulfanyl]-2-propanol 3g
Colorless oil (Found: C, 52.16; H, 8.18; S, 19.78. C14H26O4S2 requires
C, 52.14; H, 8.13; S, 19.89%); δH 2.68 (2H, dd, J7.1 and 13.8, 2 × CH2S),
2.76 (2H, dd, J4.2 and 17.8, 2 × CH2S), 2.81 (4H, s, SCH2CH2S), 3.50
(2H, dd, J 6.2 and 15.9, 2 × CH2O), 3.55 (2H, dd, J 4.1 and 13.7, 2 ×
CH2O), 3.95 (2H, m, 2 × CH), 4.05 (4H, d, J 5.6, 2 × CH2 =CHCH2O),
5.06 (2H, br, 2 × OH), 5.22 (2H, dd, J 10 and 1.2, 2 × CH2 =CH), 5.30
(2H, dd, J 17 and 1.2, 2 × CH2 =CH), 5.86–5.94 (2H, m, 2 × CH2 =CH);
δC 33.06 (2 × t), 36.08 (2 × t), 69.33 (2 × d), 70.11 (2 × t), 72.77 (2 × t),
117.99 (2 × t), 134.68 (2 × d); m/z EI 322 (M+), 247 (13%) C10H15O3S2,
207 (30) C8H15O2S2, 175 (54) C8H15O2S, 147 (57) C6H11O2S, 71 (62)
C4H7O, 41 (100) C3H5.
1-(2-[(2-Hydroxy-3-phenoxypropyl)sulfanyl]ethylsulfanyl)-3-
phenoxy-2-propanol 3h
White solid (Found: C, 61.13; H, 6.72; S, 16.07. C20H26O4S2 requires
C, 60.88; H, 6.64; S, 16.25%); m.p. 79◦C (from carbon tetrachloride); δH
2.39 (2H, br, 2 × OH), 2.71 (2H, dd, J7.2 and 15.5, 2 × CH2S), 2.79 (2H,
dd, J4.2 and 17.8, 2 × CH2S), 2.85 (4H, s, SCH2CH2S), 4.04 (2H, dd, J
6.2 and 15.8, 2 × CH2O), 4.12 (2H, m, 2 × CH), 4.15 (2H, dd, J 4.1 and
13.7, 2 × CH2O), 6.84-7.38 (10 H, m, 2 × Ph); δC 33.10 (2 × t), 35.80
(2 × t), 69.91 (2 × t), 70.92 (2 × d), 114.90 (2 × d), 121.09 (d), 120.74
(2 × d), 159.23 (d); m/z EI 394 (M+), 287 (85%) C13H19O3S2, 243 (86)
C11H15O2S2, 211 (90) C11H15O2S, 75 (100) C6H5.

Ring Opening of Epoxide with Dimercaptoethane
1715
2-(2-[(2-Hydroxycyclohexyl)sulfanyl]ethylsulfanyl)-1-
cyclohexanol 3i
White solid (Found: C, 58.13; H, 9.06; S, 22.07. C14H26O2S2 requires
C, 57.89; H, 9.02; S, 22.08%); m.p. 72◦C (from carbon tetrachloride); δH
1.25–1.54 (8H, m, 2 × CH2CH2), 1.73 (4H, m, 2 × CH2CHS), 2.08 (4H,
m, 2 × CH2CHOH), 2.33 (2H, br, 2 × OH), 2.27–2.44 (2H, m, 2 × CHS),
2.80 and 2.83 (4H, two singlets, 2 × SCH2CH2S), 317-3.41 (2H, m, 2 ×
CH); δC24.22 (2 × t), 26.03 (2 × t), 30.72 and 31.11 (two peaks, 2 × t),
32.87 and 32.97 (two peaks, 2 × t), 33.83 (2 × t), 53.33 and 53.42 (two
peaks, 2 × d), 72.41 and 72.79 (two peaks, 2 × d); m/z EI 290 (M+), 273
(10%) C14H25OS2, 159 (94) C8H15OS, 99 (95) C6H11O, 83 (100) C6H11.
REFERENCES
[1] (a) S. Garzynski, Synthesis, 1984, 629; (b) C. Bonini and G. Righi, Synthesis, 1994,
225; (c) W. M. Dai, J. Wu, and A. Wu, Tetrahedron Lett., 1998, 39, 4091; (d) I. C.
Gonza´lez and C. J. forsyth, Tetrahedron Lett., 2000, 41, 3805; (e) O. A. Ozhogina,
Tetrahedron Lett., 2002, 43, 553; (f) K. Surendra, N. S. Krishnaveni, Y. V. D.
Nageswar, and K. R. Rao, J. Org. Chem., 2003, 68, 4994; (g) A. Solladie´-Cavallo,
P. Lupattelli, and C. Bonini, J. Org. Chem., 2005, 70, 1605; (h) R. M. Jong, J. J. W.
Tiesinga, A. Villa, L. Tang, D. B. Janssen, and B. W. Dijkstra, J. Am. Chem. Soc.,
2005, 127, 13338.
[2] (a) E. J. Corey, D. A. Clark, G. Goto, A. Marfat, C. Mioskowski, B. Samuelsson, and
S. Hammarstorm, J. Am. Chem. Soc., 1980, 102, 1436; (b) M. F. Sebban, P. Vottero,
A. Alagui, and G. Dupuy, Tetrahedron Lett., 2000, 41, 1019; (c) S. Patai, Ed., The
Chemistry of The Thiol Group, (John Wiley, New York, 1974).
[3] A. E. Vougiokas and H. B. Kagan, Tetrahedron Lett., 1987, 28, 6065.
[4] J. Iqbal, A. Pandey, A. Shukla, R. R. Srivastava, and S. Tripathi, Tetrahedron, 1990,
46, 6423.
[5] (a) S. Y. Ko and K. B. Sharpless, J. Org. Chem., 1986, 51, 5413; (b) Y. Belokon, M.
Flego, N. Ikonnikov, M. Moscalenko, and M. North, Tetrahedron Asymm., 1996, 7,
851; (c) Y. Jiang, L. Gong, X. Feng, and W. Hu, Tetrahedron, 1997, 53, 14327; (d) J.
Wu, X. L. Hou, L. J. Xia and M. H. Tang, Tetrahedron Asymm., 1998, 9, 3431.
[6] (a) R. M. Hanson and K. B. Sharpless, J. Org. Chem., 1986, 51, 1922; (b) K. Narasaka,
N. Iwasawa, M. Inoue, T. Yamada, and M. Nakashima. J. Am. Chem. Soc., 1989, 111,
5340. (c) K. Mikami, M. Terada, and T. Nakai, J. Am. Chem. Soc., 1990, 112, 3949; (d)
H. Sasai, T. Arai, Y. Satow, K. N. Houk, and M. Shibasaki, J. Am. Chem. Soc., 1995,
117, 6194; (e) T. Arai, Y. M. A. Yamada, N. Yamamoto, H. Sasai, and M. Shibasaki,
Chem. Eur. J., 1996, 2, 1368; (f) T. Aria, H. Sasai, K. Aoe, K. Okamura, and M.
Shibasaki, Angew. Chem. Int. Ed. Engl., 1996, 35, 104.
[7] (a) D. Amantini, F. Fringuelli, F. Pizzo, S. Tortoioli, and L. Vaccaro, Synlett, 2003,
15, 2292; (b) F. Fringuelli, F. Pizzo, S. Tortoioli, and L. Vaccaro, J. Org. Chem., 2003,
68, 8248.
[8] F. Fringuelli, F. Pizzo, S. Tortoioli, and L. Vaccaro, Adv. Synth. Catal., 2002, 344,
379.
[9] (a) G. H. Posmer and O. Z. Rogers, J. Am. Chem. Soc., 1977, 99, 8208; (b) A. S. Rao,
S. K. Paknihar and J. G. Kirtane, Tetrahedron, 1983, 39, 2323.
[10] A. K. Maiti and P. Behattacharyya, Tetrahedron, 1994, 50, 10483.

1716
S. M. Seyedi et al.
[11] V. Kesavan, D. Bennet-Delpar, and P. J. Be´gue´, Tetrahedron Lett., 2000, 41, 2895.
[12] M. R. Younes, M. M. Chaabouni, and A. Baklouti, Tetrahedron Lett., 2001, 42, 3167.
[13] J. Chen, H. Wu, C. Jin, X. Zhang, Y. Xie, and W. Su, Green Chem., 2006, 8, 330.
[14] (a) V. Guyon, A. Guy, J. Foos, M. Lemaire, and M. Draye, Tetrahedron, 51, 4065
(1995); (b) G. W. Gokel, W. M. Leevy, and E. M. Weber, Chem. Rev., 104, 2723 (2004).
[15] P. T. Anastas, Ed., Green Chemistry: Challenging Perspective, Oxford Sc. Pub.,
Oxford, 1999.
[16] V. Pironti and S. Colonna, Green. Chem., 2005, 7, 43.
[17] F. A. Carey and R. J. Sundberg, Advanced Organic Chemistry, (Kluwer Aca-
demic/Plenum Pub., New York, 2000), 4th. ed., Vol. 1, Chap. 5, pp. 263–295.
[18] E. Stahl, Ed., Thin-Layer Chromatography (Springer-Verlag Berlin, Heidelberg,
New York, 1969), 2nd ed., Chap. Z, p. 885.
[19] J. H. Clark, Green Chem., 2006, 8, 17.