Facile and versatile synthesis of alkyl and aryl isothiocyanates by using triphosgene and cosolvent

Article abstract of DOI:10.1080/00397911.2013.783600

A mild and efficient method for the conversion of alkyl and aryl amines to isothiocyanates via dithiocarbamates has been developed using (CH ₃)₂CO-CS₂ as co-solvent and triphosgene as dehydrosulfurization reagent. High yields, mild reaction conditions and excellent functional group compatibility make it become a versatile synthetic method for the preparation of isothiocyanates compared with reported methods. [Supplementary materials are available for this article. Go to the publisher's online edition of Synthetic Communications for the following free supplemental resource(s): Full experimental and spectral details.]

Full text of DOI:10.1080/00397911.2013.783600

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Facile and Versatile Synthesis of Alkyl
and Aryl Isothiocyanates by Using
Triphosgene and CoSolvent
Pengfei Liu a , Chunyan Li a , Jingwei Zhang a & Xiaoyong Xu a
a Shanghai Key Laboratory of Chemical Biology, School of Pharmacy ,
East China University of Science and Technology , Shanghai , China
To cite this article: Pengfei Liu , Chunyan Li , Jingwei Zhang & Xiaoyong Xu (2013) Facile and
Versatile Synthesis of Alkyl and Aryl Isothiocyanates by Using Triphosgene and CoSolvent, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry,
43:24, 3342-3351
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Synthetic Communications1, 43: 3342–3351, 2013
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2013.783600
FACILE AND VERSATILE SYNTHESIS OF ALKYL AND
ARYL ISOTHIOCYANATES BY USING TRIPHOSGENE
AND COSOLVENT
Pengfei Liu, Chunyan Li, Jingwei Zhang, and Xiaoyong Xu
Shanghai Key Laboratory of Chemical Biology, School of Pharmacy, East
China University of Science and Technology, Shanghai, China
GRAPHICAL ABSTRACT
Abstract A mild and efficient method for the conversion of alkyl and aryl amines to isothio-
cyanates via dithiocarbamates has been developed using (CH3)2CO-CS2 as co-solvent and
triphosgene as dehydrosulfurization reagent. High yields, mild reaction conditions and
excellent functional group compatibility make it become a versatile synthetic method for
the preparation of isothiocyanates compared with reported methods.
[Supplementary materials are available for this article. Go to the publisher’s online
edition of Synthetic Communications1 for the following free supplemental resource(s):
Full experimental and spectral details.]
Keywords Amines; co-solvent; dehydrosulfurization; isothiocyanates; triphosgene
INTRODUCTION
Isothiocyanates are often encountered in natural products and have various
biological activities such as antimicrobial,[1] antiproliferatives,[2] antitumor,[3] enzyme
inhibitors for the HIV virus[4] and reagent in Edman peptide sequencing,[5] and other
biological assays of DNA and protein.[6] Isothiocyanates are also versatile synthetic
intermediates whose strong electrophilicity enable them to readily take part in nucleo-
philic addition and cycloaddition reactions. They are starting materials of sulfur- and
nitrogen-containing organic compounds with extensive bioactivities which can be
used as agrochemicals and pharmaceuticals.[7]
For this reason, much effort has been devoted to develop efficient methods for
the synthesis of isothiocyanates and their derivatives. Isothiocyantes can be obtained
from different starting materials such as tert-alcohols,[8] amines,[9] nitrile oxides,[10]
Received October 28, 2012.
Address correspondence to Xiaoyong Xu, Shanghai Key Laboratory of Chemical Biology, School
of Pharmacy, East China University of Science and Technology, Shanghai 200237, China. E-mail: xyxu@
ecust.edu.cn
3342

FACILE SYNTHESIS OF ISOTHIOCYANATES
3343
Scheme 1. Synthesis of isothiocyanate.
isocyanides,[11] and isocyanates.[12] Among these starting materials, amines are
generally selected to obtain isothiocyantes because of broad resource and versatility.
Thiophosgene is the oldest dehydrosulfurization reagent but it is a highly toxic
irritant. In an alternative strategy, different thiocarbonyl-transfer reagents and
dithiocarbamic acid salt desulfurization reagents were used instead of toxic thiophos-
gene. For example, iodine can be used to convert amino coumarin to corresponding
coumarin isothiocyanate in one pot.[13] A series of isothiocyanates were prepared in
good yields by the decomposition of dithiocarbamates using chlorosilanes such as
Me3SiCl, Me2SiCl2, MeSiCl3, and SiCl4.[14] Peptide coupling reagents can be used
as versatile reagents for the formation of aliphatic isothiocyanates in the solid phase
from the corresponding solid-phase anchored aliphatic primary amines.[15] However,
some of methods suffered from poor yields, inconvenient workup procedures, and
the use of expensive catalysts or high toxic reagents.
Bis(trichloromethyl)carbonate (BTC), usually called triphosgene or solid phos-
gene, being a substitute for phosgene and diphosgene in synthesis,[16] is safe and con-
venient to be used with good selectivity and yields under mild conditions.[17] The
chemical properties of the solid phosgene allow it to have very widespread use. The
use of triphosgene as a dehydrosulfurization reagent in the preparation of isothiocya-
nate has been reported by Chaskar and coworkers in 2008.[18] Most isothiocyanates
bearing electron-donating or electron-withdrawing goups were obtained in good
yields by their method. However, it is not suitable for anilines bearing strong electron-
withdrawing groups such as cyano and nitro groups. Besides, the authors did not
mention if alkyl isothiocyanates could be synthesized by their method. Munch and
coworkers reported a method using di-tert-butyldicarbonate by which both alkyl
and aryl isothioncanates can be prepared in good yields except for triphenylmethyl
isothioncanate.[19] It is well known that some strong electron-withdrawing substitu-
ents are important pharmacophores in agrochemicals and pharmaceuticals; thus it
is necessary to find good methods to synthesize them.
Herein we report a facile method for the synthesis of isothiocyanates bearing
strong electron-withdrawing groups that uses triphosgene efficiently and is suitable
for the conversion of other electron-donating heterocyclic and alkyl amines
(Scheme 1). The notable features of this reaction are its high efficiency, excellent
functional group compatibility, operational simplicity, inexpensive catalyst, easily
available starting materials, and mild reaction conditions.
RESULTS AND DISCUSSION
The synthesis of dithiocarbamates from amines was more challenging and often
required longer reaction times and excess CS2 and base to effect complete conversion.
In the exploratory studies, a strong electric-withdrawing amine, p-nitro aniline, was

3344
P. LIU ET AL.
Scheme 2. Suggested mechanism for the synthesis of isothiocyanates from the corresponding amines using
triphosgene.
selected to investigate the effect of solvents. p-Nitro aniline, and carbon disulfide in
the presence of 1,4-diazabicyclo[2.2.2]-octan (DABCO) reacted in different solvents
at room temperature (Table 1). There was even no precipitation of product (b) in
ethanol after 24 h. In other solvents, the reaction did not finish after 24 h and only
a little precipitation was afforded. The yield using toluene as solvent was good, but
it was not optimal because of its toxicity and pungent smell. Acetone was less efficient
Table 1. Effect of solvent on the synthesis of p-nitrophenyl isothiocyanate
Entry
Solvent
Yield of b (%)
1
2
EtOH
THF
CH3CN
No reaction
13
30
25
28
23
70
42
50
69
75
81
85
90
86
80
47
3
4
CH2Cl2
CHCl3
5
6
EtOAc
Toluene
(CH3)2CO
7
8
9
(CH3)2CO–CS2 ¼ 5:1
(CH3)2CO–CS2 ¼ 1:1
(CH3)2CO–CS2 ¼ 1:2
(CH3)2CO–CS2 ¼ 1:3
(CH3)2CO–CS2 ¼ 1:4
(CH3)2CO–CS2 ¼ 1:5
(CH3)2CO–CS2 ¼ 1:6
(CH3)2CO–CS2 ¼ 1:7
(CH3)2CO–CS2 ¼ 1:10
10
11
12
13
14
15
16
17

FACILE SYNTHESIS OF ISOTHIOCYANATES
3345
Table 2. Synthesis of isothiocyanates using triphosgenea
Entry
1
Substrate (a)
Product (c)
Yield (%)b
93[24a]
2
3
96[24a]
80[24a]
4
5
6
95[24a]
85[24a]
92[24a]
7
8
90[24a]
95[18]
94[24a]
95[18]
9
10
11
76[24b]
(Continued )

3346
P. LIU ET AL.
Table 2. Continued
Entry
12
Substrate (a)
Product (c)
Yield (%)b
86[24c]
13
14
92[18]
75[24d]
15
16
76[24e]
98[24f]
17
18
19
88[24g]
90[9g]
94[24g]
20
21
93[24h]
91[24i]
22
23
90[18]
95[18]
(Continued )

FACILE SYNTHESIS OF ISOTHIOCYANATES
3347
Table 2. Continued
Entry
24
Substrate (a)
Product (c)
Yield (%)b
95[11j]
25
26
86[24k]
75[24l]
27
28
29
88[24c]
87[24m]
91[24m]
30
90[24n]
31
32
85[24n]
82
(Continued )

3348
P. LIU ET AL.
Table 2. Continued
Entry
Substrate (a)
Product (c)
Yield (%)b
33
34
89[24i]
82[24j]
aReagents and conditions: amine (3 mmol), DABCO (9 mmol), CS2 (15 mL), (CH3)2CO (3 mL), BTC
(1 mmol).
bYield of isolated product.
than toluene, but its dissolving capacity for p-nitro aniline was much better. After the
optimization of the reaction conditions, it was found that the yield dramatically
increased up to 90% when (CH3)2CO-CS2 was used as the cosolvent in the formation
of b. Acetone dissolved aniline well, and carbon disulfide made more dithiocarbamate
salt precipitate. The most appropriate volume ratio of (CH3)2CO–CS2 is 1:5.
Aromatic amine and triethylenediamine were stirred at room temperature in a
mixed solvent composed of acetone and carbon bisulfide to form dithiocarbamate
salt, which was subsequently treated with BTC to obtain isothiocyanates. In general,
it was proposed that dithiocarbamate salt reacted with BTC to form dithiocarba-
mate chloroformic anhydride, which decomposed to isothiocyanate and released
COS and HCl. In this proposed reaction, only 1 mol of BTC was needed to afford
3 mol of isothiocyanate.
With the optimum reaction conditions (Table 1, entry 12) determined, the sub-
strate scope of the reaction was then investigated (Table 2). Amines with different
electronic and steric demands were tested. Electron-rich, deficient, and neutral
amines were all suitable substrates, and the corresponding isothiocyanates were
afforded (1c–34c) in good to excellent yields. Anilines substituted in the ortho, meta,
or para positions all provided the corresponding isothiocyanates. The mild reaction
conditions allowed for the transformation of aryl amines containing a range of func-
tional groups, including ester, amide, and nitrile groups (15c–17c). Notably, even for
the substrates bearing bromide, iodine, hydroxy, or carboxyl (10c–14c), isothio-
cyantes with good yields were still obtained, thus providing opportunities for further
transformation. Interestingly, p-nitro and p-cyano isothiocyanate (17c–18c) were
acquired in 90% and 88% yields respectively, but it was reported that they could
not be obtained using Chaskar’s method. Triphenylmethyl isothiocyanate (27c)
was obtained in 75% yield, which also could not be afforded by the method in
Munch’s article. Naphthalene is an important substrate for the synthesis of dye
and pharmaceuticals.[20] Using our method, 1-naphthylamine was used as a starting
material to give corresponding isothiocyanate (24c) in 95% yield. In addition, pyridyl
isothiocyanates (27c–31c) were also obtained from corresponding amines in good
yields (85%–91%). Various aryl amine and aliphatic amines also reacted at the same
condition to give the corresponding isothiocyanates (33c–34c) in 82–89% yields,
which cannot be prepared using only BTC by Chaskar’s method. It suggested that
this method is a general method for the preparation of isothiocyanates that avoids

FACILE SYNTHESIS OF ISOTHIOCYANATES
3349
using special reagent 1,10-(ethane-1,2-diyl)dipyridinium bistribromide (EDPBT),[21]
highly toxic thiophosgene,[22] and expensive agent 1,1-thiocarbonyldiimidazole.[23]
CONCLUSION
In summary, a mild method for a rapid and convenient preparation of isothio-
cyanates in good yields and purity without laborious workup has been developed.
This method efficiently afforded alkyl and aryl isothiocyanates in good to excellent
yields. Because of the avoidance of highly toxic thiophosgene and chloroformate, the
operation process is simple and environmentally friendly.
EXPERIMENTAL
All melting points (mp) were obtained with a Buchi melting-point B540
¨
instrument and are uncorrected. NMR spectra were recorded in dimethylsulfoxide
(DMSO-d6) (1Hat 400 MHz and13Cat 100 MHz) using tetramethylsilane (TMS) as
the internal standard on a Bruker WP-400SY (400-MHz) spectrometer. Chemical
shifts are reported in d (parts per million) values. Analytical thin-layer chromato-
graphy (TLC) was carried out on precoated plates (silicagel 60 F254), and spots were
visualized with ultraviolet (UV) light. All chemicals were reagent grade and used as
supplied unless otherwise specified.
CS2 (50 mL) was added to a stirring mixture of DABCO (30 mmol) and amine
(10 mmol) in acetone (10 mL). The resulting mixture was further stirred at room tem-
perature to achieve dithiocarbamate, and the process of the reaction was monitored
by TLC. After completion of the reaction, the solution was filtered and the solid was
washed with acetone. After it was dried in an infrared-ray oven, the salt was dissolved
in dried CHCl3 (10 mL) and the mixture was chilled to 0–5 ꢀC in an ice bath. BTC
(1010 mg, 3.4 mmol), dissolved in CHCl3 (5 mL), was added dropwise for 30 min with
constant stirring. The reaction mixture was then allowed to reach room temperature
and stirred for a further 4 h. Triethylenediammmonium hydrochloride was filtered
off, and the filtrate was evaporated in vacuo to afford the desired isothiocyanate in
good purity. All products were identified by comparison with authentic samples. Ana-
lytical data of isothiocyanate (32c): Light yellow solid; mp 45–46 ꢀC. IR (KBr): 2944,
2133, 1709, 1540, 1470, 1314, 1275, 1190, 1034, 917. 1H NMR (400 MHz, DMSO-d6):
d 4.27 (q, J ¼ 7.2 Hz, 1H), 2.67 (m, 1H), 1.73 (m, 1H), 1.31 (t, J ¼ 7.2 Hz, 1H). 13C
NMR (100 MHz, DMSO-d6): d 161.5, 136.7, 134.5, 133.2, 131.6, 126.9, 61.0, 26.1,
24.8, 22.1, 14.6. HRMS (ESI): m=z [M þ H]þ calcd. for C12H14NO2S2: 268.0466;
found: 268.0465.
ACKNOWLEDGMENTS
This work was financially supported by the National Natural Science
Foundation of China (21272071), the National Basic Research Program of China
(973 Program, 2010CB126100), and the National High Technology Research and
Development Program of China (863 Program, 2011AA10A207). This work was
also partly supported by the National Key Technology R&D Program of China
(2011BAE06B01), Shanghai Leading Academic Discipline Project Project No. B507,

3350
P. LIU ET AL.
and the Fundamental Research Funds for the Central Universities and Special Fund
for Agro-scientific Research in the Public Interest (201103007).
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