Polymer-supported thiocyanate as new, versatile and efficient polymeric reagent for conversion of alkyl halides to corresponding alkyl thiocyanates under mild conditions

Article abstract of DOI:10.1002/jccs.200700183

Crosslinked poly(N-methyl-4-vinylpyridinium)thiocyanate [P ₄-Me]SCN (II) is prepared easily and is used as a new polymeric reagent for synthesis of alkyl thiocyanates from alkyl halides under mild and nonaqueous conditions. The used polymeric reagent usually can be removing quantitatively and be regenerated.

Full text of DOI:10.1002/jccs.200700183

Journal of the Chinese Chemical Society, 2007, 54, 1299-1302
1299
Polymer-supported Thiocyanate as New, Versatile and Efficient Polymeric
Reagent for Conversion of Alkyl Halides to Corresponding Alkyl
Thiocyanates under Mild Conditions
Mohammad Ali Karimi Zarchi*
Department of Chemistry, College of Science, Yazd University, P. O. Box 89195741, Yazd, Iran
4
Crosslinked poly(N-methyl-4-vinylpyridinium)thiocyanate [P -Me]SCN (II) is prepared easily and is
used as a new polymeric reagent for synthesis of alkyl thiocyanates from alkyl halides under mild and
nonaqueous conditions. The used polymeric reagent usually can be removing quantitatively and be regen-
erated.
Keywords: Polymer-supported thiocyanate; Alkyl halides; Alkyl thiocyanates; Polymeric re-
agent.
INTRODUCTION
alkyl thiocyanates with metal thiocyanates such as sodium
13
1
or potassium thiocyanate. Very recently Caineli et al.
4
1
In 1963, Merrifield introduced a modified technique
that overcame the problems associated with classical multi-
step synthesis in solution. This technique has been used in
the production of large numbers of products. However, re-
cently the chemistry of functional polymers has received
great attention and became a practical method for the effi-
reported a polymeric reagent for synthesis of alkyl thiocya-
nates. This polymeric reagent was prepared by treating the
chloride form of amberlyst A-26 (a macroporous resin con-
taining quaternary ammonium groups) with aqueous potas-
sium thiocyanate. This polymeric reagent converts a few
alkyl halides to the corresponding alkyl thiocyanates in
2
-7
12
cient preparation of novel chemical libraries. These poly-
meric reagents are generally used in single-step reactions;
their main advantage over non-polymeric reagents is their
insolubility in the reaction medium and consequently their
easier work-up by a simple filtration. The reactions often
can be driven to completion using excess amounts of re-
agents without the fear of separation of the excess reagents
from the products. The used polymeric reagents usually can
be removed quantitatively and be regenerated. In addition,
anions bound to the macroporous resin have the advantage
that they often react successfully in non-polar solvents.
These reagents usually can be prepared from commercial
anion exchange resins or polymers such as crosslinked
poly(4-vinylpyridine) as supports. Recently we have re-
ported several different types of polymer-supported re-
agents from crosslinked poly(4-vinylpyridine) and used in
benzene, under reflux conditions. Also, Hodge et al. re-
ported that amberlyst A-26 converted a few alkyl halides to
the corresponding alkyl thiocyanates (seven examples) un-
der the same conditions of the last report. They observed
that in some reactions the isothiocyanates are the only
products and in a few reactions isothiocyanates are also
formed as by-products.
1
5
Recently Kamal et al. reported preparation of a new
“task-specific” ionic liquid (TSIL), 1-n-butyl-3-methyl-
imidazolium thiocyanate ([bmim] SCN) for the synthesis
of alkyl thiocyanates from alkyl halides (nine examples). In
this method, the workup of the reaction mixtures is diffi-
cult.
In general these reagents suffer at least from one of
the following drawbacks: 1) aqueous medium has to be
used; 2) yield of the reactions are low; 3) the reactions must
be carried out in the presence of phase transfer catalysts; 4)
the workup of the reaction mixtures is difficult; and 5) or
the by-product of alkyl isothiocyanate is formed.
In this study we report an efficient and simple method
8
-10
organic synthesis.
A literature search shows that there are a few reports
11-15
in the preparation of alkyl or aryl thiocyanates,
and the
most general one involves conversion of alkyl halides to
*
Corresponding author. Tel: (+98)3518211670-9; Fax: (+98)3518210644; E-mail: makarimi@yazduni.ac.ir

1
300 J. Chin. Chem. Soc., Vol. 54, No. 5, 2007
Karimi Zarchi
for synthesis of alkyl thiocyanates from alkyl halides using
a polymer-supported thiocyanate under mild and nonaque-
ous conditions with high yields (Scheme I).
few reactions isothiocyanates were formed. However in
our study we observed that thiocyanates are the only prod-
ucts, and isothiocyanates as by-products were not formed.
The advantages of our method are: 1) the reaction can be
performed in non-aqueous medium and at room tempera-
ture; 2) an excess of the reagent can be used; 3) the product
can be obtained by simple filtration and evaporation of the
solvent; 4) the polymeric reagent can be regenerated; and
5) waste and byproducts are minimized. The last advantage
takes place because the thiocyanate ion is a bidentate ion
and tightly bonds to the pyridinium pendant group of the
polymer at the nitrogen site, which results is the formation
of the thiocyanates and therefore alkyl isothiocyanates are
not formed.
RESULTS AND DISCUSSION
4
Crosslinked [P -Me]SCN (II) can be prepared easily
and used as a mild and efficient polymeric reagent for con-
version of alkyl halides to the corresponding alkyl thiocya-
nates in n-hexane at room temperature. It was observed that
n-hexane is the best solvent among other non-polar organic
solvents. This polymeric reagent is stable and can be stored
for months at room temperature without losing its activity.
In these conversions the best molar ratio for reagent to
substrate was found to be 2:1. The order of reactivity with
closely related compounds was iodide > bromide > chlo-
ride, and the reactivity of primary alkyl halides was higher
than secondary and tertiary alkyl halides. The representa-
tive results are summarized in Table 1.
In conclusion, we demonstrated the synthesis of a
4
new polymer-supported thiocyanate, [P -Me]SCN (II) and
its application as a reaction medium reactant for the con-
version of alkyl halides to their alkyl thiocyanates at room
temperature in moderate to high yields. More importantly
there is no formation of any side-product in this process,
and the product can be obtained by simple filtration and
evaporation of the solvent.
Recently Cainelli and Manescalchi have described
the reactions of polymer-supported thiocyanate with few
1
3
alkyl halides. They reported that larger substrates gener-
ally required more vigorous reaction conditions that the
smaller substrates in order to obtain good yields of product.
Our results also support this observation. This may be be-
cause the larger molecules have more difficulty gaining ac-
cess to the active sites in the polymer. Another report in this
EXPERIMENTAL
General
Crosslinked poly(4-vinylpyridine) (2% divinylben-
zene) and all of the alkyl halides were commercial products
(Fluka). All alkyl thiocyanate products are known com-
pounds and were identified by comparison of their spectra
1
2
field is described by Harrison and Hodge. Their results
also support this view, but they observed that in some reac-
tions the isothiocyanates are the only products, while in a
Scheme I a) Preparation of polymer-supported reagent, b) Preparation of alkyl thiocyanates, c) Regeneration of
polymeric reagent

Polymer-supported Thiocyanate
J. Chin. Chem. Soc., Vol. 54, No. 5, 2007 1301
Table 1. Conversion of alkyl halides to corresponding alkyl
a
thiocyanates by using [P -Me]SCN (II) in n-hexane at
were recorded by a Perkin-Elmer IR-157-G and a Hitachi
R-2413, 60 MHz spectrophotometer, respectively. The ca-
pacities of the reagents were determined by potentiometric
titration and gravimetric methods.
4
room temperature
b
c
Yield (%)
Entry
1
Substrate
Product
Br
SCN
95
75
99
93
91
86
Preparation of crosslinked poly(N-methyl-4-vinyl-
4
pyridinium)thiocyanate, [P -Me]SCN (1)
Br
SCN
2
3
4
5
6
The crosslinked poly(4-vinylpyridine) (4.24 g) was
treated with methyl iodide (11.36 g), in n-hexane (10 mL).
The mixture was allowed to stir at room temperature for 24
hr. The resulting suspension was filtered and washed suc-
cessively with distilled water and n-hexane. Then it was
dried at 40 °C under vacuum to obtain crosslinked poly(N-
methyl-4-vinylpyridinium)iodide (I) (5.70 g).
Br
Br
Br
Cl
SCN
Br
Cl
Br
Cl
SCN
Cl
SCN
The crosslinked poly(N-methyl-4-vinylpyridinium)
iodide (4.50 g) was added to 40 mL of 3 M solution of po-
tassium thiocyanate and slowly stirred with a magnetic stir-
rer for 24 hr. The mixture was filtered off and washed suc-
cessively with distilled water until the filtrate gave a nega-
Cl
Cl
SCN
SCN
Cl
Cl
-
7
54
tive test for SCN . It was then washed with ether and dried
overnight at 40 °C under vacuum to give poly(N-methyl-
Br
SCN
4
4-vinylpyridinium)thiocyanate, [P -Me]SCN (II) (3.15 g).
OH
OH
The capacity of the polymer was determined by gravimet-
ric and potentiometric titration with 0.1 N solution of silver
nitrate and obtained 3.1 mmol per g of the polymer.
8
9
50
0
O
O
(CH
)
Cl
CCl
(CH ) CSCN
3 3
3
3
SCN
1
0
Trace
General procedure for conversion of alkyl halides to
corresponding alkyl thiocyanates with [P
To a solution of alkyl halide (1 mmol) in n-hexane (10
mL) in a round-bottomed flask (50 mL), [P -Me]SCN (645
4
-Me]SCN (I)
Br
SCN
4
1
1
1
2
10
mg, 2 mmol) was added and the mixture was allowed to stir
at room temperature for 72 hr. The progress of the reaction
was followed by TLC (eluent, n-hexane/ether: 9.5/0.5) or
GLC. When the reaction was completed the mixture was
filtered and washed successively with n-hexane (5 ´ 2 mL).
The solvent was evaporated and the pure product was ob-
tained in moderate to high yields (10-99% yields) (Table
1).
Br
SCN
Ph
Ph
Trace
O
O
1
1
1
a
3
4
5
CH
3
I
CH
3
SCN
50
62
70
C
C
H
H
Br
Br
C
C
H
SCN
SCN
2
5
2
5
H
4
9
4
9
The molar ratio of (II): alkyl halide is 2:1, and the reactions
b
were stirred at room temperature in n-hexane for 72 hr. The
structures confirmed by comparison of the IR and NMR spectra
c
with those of an authentic specimen. Isolated yield.
Preparation of benzyl thiocyanate from benzyl bro-
4
mide by using [P -Me]SCN (II): a typical procedure
To a solution of benzyl bromide (785 mg, 5 mmol)
and physical data with those authentic samples. Reaction
monitoring and the purity of the products were accom-
plished by GLC and TLC. Separation of the products was
and n-hexane (10 mL) in a round-bottomed flask (50 mL)
4
[P -Me]SCN (I) (3.23 g, 10 mmol) was added and the mix-
ture was allowed to stir at room temperature for 72 hr. The
progress of the reaction was followed by TLC (eluent,
1
conducted on a silica gel column. IR and H NMR spectra

1
302 J. Chin. Chem. Soc., Vol. 54, No. 5, 2007
Karimi Zarchi
n-hexane/ether: 9.5/0.5). When the reaction was com-
pleted, the mixture was filtered off and was washed succes-
sively with n-hexane (5 ´ 2 mL). The solvent was then
evaporated to give crystalline benzyl thiocyanate. Yield:
REFERENCES
1
2
. Merrifield, R.-B. J. Am. Chem. Soc. 1963, 85, 2149.
. Sherrington, D. C.; Hodge, P. In Synthesis and Separations
Using Functional Polymers; John Wiley & Sons: Chichester,
1
5
5
17 mg, (95%); m.p: 40-41 °C (lit 39-41 °C); IR (KBr): u
1
988.
-
1
1
cm ) = 2150 (-SCN); and H NMR (CDCl
(
(
3
): d (ppm) = 4.2
3
. Sherrington, D. C.; Hodge, P. In Polymer Supported Reac-
tions in Organic Synthesis; John Wiley & Sons: Chichester,
S, 2H), 7.45 (S, 5H).
1
980.
Regeneration of the polymer-supported thiocyanate
To a solution of potassium thiocyanate (0.50 g) in wa-
ter (20 mL) the cream-coloured spent polymeric reagent
4. Takemoto, K.; Inaki, Y.; Ottenbrite, R. M. In Functional
Monomers and Polymers; Marcel Dekker, Inc.: New York,
1
987.
5. Akelah, A.; Sherrington, D. C. Chem. Rev. 1981, 81, 577.
6. Akelah, A.; Sherrington. D. C. Polym. 1984, 24, 1369.
7. Steven, V. L.; Ion, R. B.; Robert, N. B.; Philip, S. J.; Anderw,
G. L.; Deborah, A. L.; Marcella, N.; James, S. S.; Storer, R.
I.; Stephen, J. T. J. Chem. Soc. Perkin Trans 1 2000, 3815.
(
1.00 g) was added and the mixture was allowed to stir at
room temperature for 24 hr. The mixture was filtered and
was washed with distilled water several times until the fil-
-
trate gave a negative test for SCN . It was then washed with
ether and dried overnight at 40 °C under vacuum to give
8. Tamami, B.; Iranpoor, N.; Karimi Zarchi, M. A. Polym.
993, 34, 2011.
. Tamami, B.; Karimi Zarchi, M. A. Eur. Polym. J. 1995, 31,
15.
0. Karimi Zarchi, M. A.; Zarei, A. J. Chin Chem. Soc. 2005, 52,
09.
1. The Chemistry of Cyanates and Their Thio Derivatives;
1
4
[P -Me]SCN (II) (0.70 g). Determination of its capacity
9
shows the same capacity of the original polymeric reagent.
7
1
3
ACKNOWLEDGEMENT
1
Patai, S., Ed.; John Wiley & Sons: New York, 1977; p 819.
The author is thankful to Yazd University research
council for financial support.
12. Harrison, C. R.; Hodge, P. Synthesis 1980, 299.
13. Cainelli, G.; Manescalchi, F. Synthesis 1979, 141.
14. March, J. Reaction, Mechanisms and Structure; John Wiley
&
Sons: New York, 1985; pp 590-601.
1
5. Kamal, A.; Chouhan, G. Tetrahedron Lett. 2005, 46, 1391.
Received May 5, 2006.
1
6. Furniss, B. S.; Hamnaford, A. J.; Smith, P. W. G.; Tatchell, A.
th
R. Vogels Text Book of Practical Organic Chemistry; 5 ed.,
John Wiley & Sons: New York, 1989.