An efficient synthetic route to aryl thiocyanates from arenesulfinates

Article abstract of DOI:10.1081/SCC-100104044

An efficient procedure for the conversion ot the inexpensive and readily available arenesulfinate (sodium) salts in three steps into the corresponding aryl thiocyanates is described. Overall yields are high, and the presence of both electron-with drawing and electron-donating substituents is well tolerated.

Full text of DOI:10.1081/SCC-100104044

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AN EFFICIENT SYNTHETIC
ROUTE TO ARYL
THIOCYANATES FROM
ARENESULFINATES
Ian W. J. Still a & Iain D. G. Watson a
a Department of Chemistry, University of Toronto
at Mississauga, Mississauga, Ontario, L5L 1C6,
Canada
Version of record first published: 09 Nov 2006.
To cite this article: Ian W. J. Still & Iain D. G. Watson (2001): AN EFFICIENT
SYNTHETIC ROUTE TO ARYL THIOCYANATES FROM ARENESULFINATES, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic
Organic Chemistry, 31:9, 1355-1359
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SYNTHETIC COMMUNICATIONS, 31(9), 1355–1359 (2001)
AN EFFICIENT SYNTHETIC ROUTE TO
ARYL THIOCYANATES FROM
ARENESULFINATES
Ian W. J. Still* and Iain D. G. Watson
Department of Chemistry, University of Toronto at
Mississauga, Mississauga, Ontario, Canada L5L 1C6
ABSTRACT
An efficient procedure for the conversion of the inexpensive
and readily available arenesulfinate (sodium) salts in three
steps into the corresponding aryl thiocyanates is described.
Overall yields are high, and the presence of both electron-
with drawing and electron-donating substituents is well
tolerated.
As part of our ongoing investigation of the reactivity of aryl thiocya-
nates, we sought a general syntheticprocedure that did not suffer from some
of the drawbacks of existing methods. The introduction of the -SCN group
by direct (electrophilic) substitution, for example, while effective, is
restricted to aromatic compounds bearing an electron-donating group.1
The two-step conversion of aromatic thiols to thiocyanates developed by
Harpp et al.2 is also very useful, but is limited by the availability of the
necessary arenethiols. Traditional methods of direct electrophilic thiocyana-
tion frequently involve the use of heavy metal catalysts, or the use of
* Corresponding author.
1355
Copyright & 2001 by Marcel Dekker, Inc.
www.dekker.com

1356
STILL AND WATSON
unstable reagents like thiocyanogen (SCN)2 or thiocyanogen halides XSCN,
and often proceed with poor yields.3,4 While nucleophilic replacement of
halide by thiocyanate ion is an efficient and convenient method, it is
restricted to aryl halides bearing powerful o- or p-electron-withdrawing
substituents.
The starting materials used for our approach were the readily available
and easily handled sodium salts of arenesulfinic acids, which are often much
less expensive than the corresponding thiols. The general approach we deve-
loped and now describe is conveyed by the sequence shown below. No
isolation of the presumed intermediate arenesulfenyl chlorides after the
second step is necessary. The arenethiols produced in step one were gener-
ally found to be sufficiently pure (1H NMR) to proceed directly to the
second stage. After a number of tests involving the use of both tin and
zincin aqueous HCl, we found that the initial step in the sequenec was
best achieved with refluxing tin and 6 M HCl. The results are summarized
in Table 1.
The thiols isolated on reduction of the arenesulfinates were subjected
initially to reaction with SO2Cl2 in dichloromethane at 0ꢀC, in the presence
of a catalytic amount of triethylamine.2 Replacement of the dichloro-
methane by acetonitrile, followed by slow addition of trimethylsilyl cyanide
(TMSCN), led rapidly to the formation of the desired thiocyanate (Table 2).
The thiocyanates were readily isolated by Kugelrohr (bulb-to-bulb)
distillation or, in the case of 3c and 3f, by flash chromatography on silica
gel, followed by recrystallization or sublimation, respectively. The inter-
mediate arenethiols are all known compounds whose identity was confirmed
Table 1. Reduction of Sodium Arenesulfinates to Thiols with Sn/HCl
ArSO2Na
Substituent
ArSH
Isolated Yield (%)
1a
1b
1c
1d
1e
1f
None
4-CH3
4-Cl
4-F
3-CF3
4-OCH3
2a
2b
2c
2d
2e
2f
79
81
83
78
84
96

ARENESULFINATES
Table 2. Conversion of ArSH to ArSCN
1357
B.p.b/torr or
B.p./torr or m.p.
(solvent) (ref.)
ArSH Substituent ArSCN Yielda (%) m.p. (solvent)
2a
2b
2c
2d
2e
2f
None
4-CH3
4-Cl
4-F
3-CF3
4-OCH3
3a
3b
3c
3d
3e
3f
74
86
87
80
85
83
110ꢀC/0.5 torr
100ꢀC/0.41 torr
110ꢀC/0.7 torr7
130ꢀ–132ꢀC/19 torr7
30ꢀ–31ꢀC (EtOH) 34ꢀ–35ꢀC(hexanes)2
75ꢀC/0.0034 torr 101ꢀ–103ꢀC/14 torr7
90ꢀC/0.05 torr
31ꢀ–32ꢀC (subl.) 33ꢀ–34ꢀC (EtOH)4
58ꢀC/0.01 torr8
aAll yields were of isolated, pure product;
bKugelrohr oven temperature.
1
by comparison of their H NMR spectra and physical constants with the
literature data (see Experimental). All six aryl thiocyanates reported are
known compounds whose b.p./m.p. compare closely with literature values.
Spectral data (1H NMR and IR) are in full agreement with the expected
structures (see Experimental). All six thiocyanates displayed the very char-
acteristic IR band associated with the –SC ꢁ N group, in the range ꢀ2154–
2162 cmꢂ1
.
The method described here provides a facile and versatile route to aryl
thiocyanates that complements existing procedures. The overall isolated
yields for the three-step process are in the range of 60–80%.
EXPERIMENTAL
General
The starting arenesulfinate (sodium) salts were supplied by Parish
Chemical Co., Provo, UT, except for 1b (Aldrich). Trimethylsilyl cyanide
was supplied by Aldrich and used without further purification.
Dichloromethane was dried by distillation from calcium hydride. The acet-
onitrile used was HPLC grade Baker Analyzed Reagent. Flash chromato-
graphy was conducted on silica gel (35–70 m particle size) from Toronto
Research Chemicals.
1H NMR spectra were recorded in CDCl3 at 200 MHz on a Varian
Gemini-200 instrument and IR spectra were recorded on a Nicolet Avatar
360 FT-IR spectrometer. Mass measurements were obtained using a Micro-
mass 70-250S (double focusing) instrument, operating at 70 eV. Melting
points are uncorrected.

1358
STILL AND WATSON
Representative Procedures
Reduction of Arenesulfinates
Granular tin (15.0 g, 0.12 mol), pretreated by exposure to concentrated
HCl for 30 min, was placed in a mixture of conc. HCl (25 mL) and water
(25 mL) at 0ꢀC and sodium benzenesulfinate (1.00 g, 4.82 mmol) was added.
After refluxing for 24 h, almost all the tin had disappeared and the cooled
solution was extracted (3ꢃ) with dichloromethane, washed with sat’d NaCl,
and dried (Na2SO4). Concentration of the solution afforded the almost pure
1
benzenethiol (2a) (421 mg, 79%) as a pale yellow liquid; H NMR: ꢁ 7.31–
7.14(m, 5H), 3.44(s, 1H), lit.5 ꢁ 7.28–6.94(m, 5H, 3.39(s, 1H); IR (film):
ꢀ 2567 cmꢂ1
.
Spectral data for 2b, yellow solid, m.p. 38ꢀ–40ꢀC, lit.5 m.p. 42ꢀ–44ꢀC;
1H NMR: ꢁ 7.23–7.01(dd, 4H, J ¼ 8.0 Hz), 3.38(s, 1H), 1it. (5) ꢁ 7.24–6.88
(dd, 4H, J ¼ 8.4 Hz), 3.35(s, 1H); IR (KBr): ꢀ 2562 cmꢂ1
.
Spectral data for 2c, off-white solid, m.p. 50ꢀ–51ꢀC, lit.5 m.p. 49ꢀ–
51ꢀC; H NMR: ꢁ 7.21(s, 4H), 3.45(s, 1H), lit5 ꢁ 7.20(s, 4H), 3.43(s, 1H);
1
IR (KBr): ꢀ 2576 cmꢂ1
.
1
Spectral data for 2d, colorless liquid; H NMR: ꢁ 7.32–6.83(m, 4H),
3.42(s, 1H), lit.5 ꢁ 7.28–6.73(m, 4H), 3.42 (s, 1H); IR (film): ꢀ 2558 cmꢂ1
.
Spectral data for 2e, pale yellow liquid; 1H NMR: ꢁ 7.52(s, 1H), 7.47–
7.34(m, 3H), 3.58(s, 1H), lit.6 ꢁ 7.4(m, 4H), SH not reported; IR (film):
ꢀ 2575 cmꢂ1
.
Spectral data for 2f, yellow liquid; 1H NMR: ꢁ 7.26(d, 2H, J ¼ 8.8 Hz),
6.80(d, 2H, J ¼ 8.8 Hz), 3.78(s, 3H), 3.36(s, 1H), lit.5 ꢁ 7.18(d, 2H, J ¼ 8.8
Hz), 6.72(d, 2H, J ¼ 8.8 Hz), 3.75(s, 3H), 3.32(s, 1H); IR (film): ꢀ 2560 cmꢂ1
.
Conversion of Arenethiols to Aryl Thiocyanates
Based on the procedure of Harpp et al.,2 4-methylbenzenethiol
(p-thiocresol) (4.00 g, 32.0 mmol) was first treated with SO2Cl2 (2.9 mL,
35.0 mmol) at 0ꢀC in dry dichloromethane (50 mL) under N2. Triethylamine
(4 drops) was also present. Stirring of the solution at 0ꢀC for 30 m was
accompanied by the evolution of HCl gas. The CH2Cl2 was removed by
evaporation and replaced with acetonitrile (50 mL), followed by the slow
addition of TMSCN (4.7 mL, 35.0 mmol). The resulting brown solution
was stirred at 25ꢀC for 1 h and concentrated. Distillation of the crude resi-
due on the Kugelrohr apparatus at 100ꢀC/0.41 torr gave 4-methylphenyl
thiocyanate (3b) as a clear, colorless liquid (4.13 g, 86%); 1H NMR:
ꢁ 7.43(d, 2H, J ¼ 8.0 Hz), 7.24(d, 2H, J ¼ 8.1 Hz), 2.38(s, 3H); IR (film):
ꢀ 2156 cmꢂ1
.

ARENESULFINATES
1359
1
Phenyl thiocyanate (3a) Colorless liquid; HNMR: ꢁ 7.71–7.43(m);
IR (film): ꢀ 2158 cmꢂ1
4-Chlorophenyl Thiocyanate (3c) White needles, m.p. 30ꢀ–31ꢀC; H
.
1
NMR: ꢁ 7.54–7.35(m); IR (KBr): ꢀ 2157 cmꢂ1
.
4-Fluorophenyl Thiocyanate (3d) Colorless liquid; 1H NMR: ꢁ
7.56(complex d, 2H, J ¼ 8.8, 2.2 Hz), 7.17(complex d, 2H, J ¼ 8.8, 3.6 Hz);
IR (film); ꢀ 2159 cmꢂ1
.
3-Trifluoromethylphenyl Thiocyanate (3e) Pale yellow liquid; 1H
NMR: ꢁ 7.80–7.55 (m); IR (film): ꢀ 2162 cmꢂ1; MS: m/z(%), 203(100),
183(38), 145(43); HRMS calcd. for C8H4F3NS 203.001656, found
203.001759.
4-Methoxyphenyl Thiocyanate (3f) White needles, m.p. 31ꢀ–32ꢀC;
1H NMR: ꢁ 7.51(d, 2H, J ¼ 8.8 Hz), 6.95(d, 2H, J ¼ 8.8 Hz), 3.84(s, 3H);
IR (KBr); ꢀ 2154 cmꢂ1
.
ACKNOWLEDGMENTS
We are grateful for the award of a Natural Sciences and Engineering
Research Council of Canada Undergraduate Summer Research Assistant
award (to IDGW).
REFERENCES
1. Toste, F.D.; De Stefano, V.; Still, I.W.J. Synth. Commun. 1995, 25,
1277.
2. Harpp, D.N.; Friedlander, B.T.; Smith, R.A. Synthesis 1979, 181.
3. Wood, J.L. Org. React. 1946, 3, 240.
4. Bacon, R.G.R.;Guy, R.G. J. Chem. Soc. 1960, 318.
5. Pouchert, C.J.; Campbell, J.R., Eds. The Aldrich Library of NMR
Spectra, Aldrich Chemical Co., Inc.: Milwaukee, WI, 1974: 5, 27–29.
6. Wong, G.B.; Kurtz, D.M.; Holm, R.H.; Mortenson, L.E.; Upchurch,
R.G. J. Am. Chem. Soc. 1979, 101, 3078.
7. Lieber, E.; Enkoji, T. J. Org. Chem. 1961, 26, 4472.
8. Pilgram, K.; Phillips, D.D. J. Org. Chem. 1965, 30, 2388.
Received in the USA July 24, 2000