Mendeleev Commun., 2006, 16(5), 250–251
Catalytic thiocyanation of aryldiazonium salts in the presence of copper salts
Irina P. Beletskaya,*a Alexander S. Sigeev,a Alexander S. Peregudovb and Pavel V. Petrovskiib
a Department of Chemistry, M. V. Lomonosov Moscow State University, 119992 Moscow, Russian Federation.
Fax: + 7 495 939 3618; e-mail: beletska@org.chem.msu.ru
b A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, 119991 Moscow, Russian Federation
DOI: 10.1070/MC2006v016n05ABEH002403
Aryldiazonium fluoroborates readily react with potassium thiocyanate in the presence of the CuI/CuII/Phen catalytic system to give
arylthiocyanates in high yields.
Arylthiocyanates are of interest as compounds with high bio-
logical activity1 and convenient sources of ArS–.2 Methods for
synthesising these compounds involve the cyanation of sulfinates
and Bunte salts3 or rhodanation of activated arenes with alkaline-
metal rhodanides in the presence of an oxidant.4 Yet another
approach involves iodine replacement in aryl iodides on treat-
ment with copper(I) rhodanide.5 Furthermore, a synthesis of
arylthiocyanates based on the reaction of arylsulfenyl halides
with potassium cyanide was also reported.6 However, unlike in
nitrile synthesis, Sandmeyer-type reactions are uncommon in
syntheses of these compounds. There have been only a few
examples of studies that deal with reactions of aryldiazonium
salts with alkaline-metal thiocyanates on treatment with stoichio-
metric amounts of FeIII salts,7(a) copper powder7(b) or copper(II)
acetate.7(c)
The nature of the phase-transfer catalyst does not affect the
aryl thiocyanate yield. What is more, the solubility of potassium
thiocyanate is sufficient for performing the reaction without a
phase-transfer catalyst (Table 1, entries 5–7). The replacement
of phenanthroline with α,α'-bipyridyl decreases the yield of 2a
insignificantly (Table 1, entries 7, 8). Analogous results were
previously observed in similar reactions with potassium cyanide8
and bromide.8(b)
As we showed previously in a similar reaction with potas-
sium bromide,8(b) the yield of the product strongly depended on
the nature of the anion in the catalyst. Maximum product yields
were achieved with copper bromides, whereas the use of copper
fluoroborates decreased the yields despite their higher solubility.
Copper bromides can be either used as ready salts or generated
separately from suitable copper salts just before the reaction.
Since the use of the respective copper salts is difficult in the case
of thiocyanates, we prepared them in situ from (MeCN)4CuBF4
and Cu(BF4)2 by reactions with KSCN and then used them as
catalysts in the reaction. However, we observed that in the case
of rhodanation, the nature of the anion in the catalyst did not
affect the yield and ratio of the reaction products (Table 1,
entry 9).
In continuation of our studies on catalytic cyanation8 and
halogenation8(b) of aryldiazonium salts, we developed a method
for synthesising arylthiocyanates based on the reaction of aryl-
diazonium salts with potassium thiocyanate involving coca-
talysis by CuI and CuII salts.
We used the thiocyanation of 4-fluorophenyldiazonium fluoro-
borate 1a with potassium thiocyanate as the model reaction for
catalyst screening. The fluorine label allowed us to monitor the
progress of the reaction and the composition of the products by
means of 19F NMR spectroscopy.
N2BF4
[Cu]/L/PTC
+ KSCN
solvent
Since the oxidation potential of SCN– is close to that of I–,
it could be expected that this reaction would occur without a
catalyst. In fact, the formation of 4-fluorophenyl thiocyanate 2a
is observed under these conditions, but its yield is extremely
low, whereas fluorobenzene 3 is the major reaction product
identified (Table 1, entry 1). Addition of 10 mol% copper(I) salts
and 1,10-phenanthroline increases the yield of 2a significantly
(Table 1, entry 2). On the other hand, the use of CuII tetra-
fluoroborate (20%) proved to be inefficient under these condi-
tions as the yield of aryl rhodanide was as low as 27% (Table 1,
entry 3). As before,8 the two-component CuI/CuII catalyst was
the optimum variant (Table 1, entry 4); the yield of ArSCN
could be increased by performing the reaction at a lower tem-
perature (Table 1, entry 5).
F
1a
SCN
+
F
F
2a
3
Scheme 1
Under the optimum conditions found, we synthesised a series
of aryl thiocyanates containing electron-donating substituents
2a–d and electron-withdrawing substituents 2e–i. In a typical
procedure, a solution of an aryldiazonium salt in acetonitrile is
added at 0–5 °C to KSCN (6 mmol), (MeCN)4CuBF4 (0.3 mmol),
Cu(BF4)2 (0.3 mmol), 1,10-phenanthroline (0.3 mmol) in aceto-
nitrile.† Note that the effect of the substituent is much stronger
Table 1 Conditions and catalyst systems (solvent, MeCN).a
Yieldb (%)
CuI
CuII
(mol%) (mol%)
The H and 13C NMR spectra for the aryl thiocyanates synthesised
†
1
Entry
L
PTC
T/°C
match those reported in the literature. Aryldiazonium fluoroborates were
synthesised by diazotization of appropriate amines with sodium nitrite in
a 15% HBF4 solution at 0–5 °C.
2a
3
1
2
3
4
5
6
7
8
9c
0
10
0
10
10
10
10
10
10
0
0
—
Dibenzo-18-crown-6 20
10
53
27
77
84
82
83
76
76
36
40
45
19
8
phen Dibenzo-18-crown-6 20
phen Dibenzo-18-crown-6 20
phen Dibenzo-18-crown-6 20
phen Dibenzo-18-crown-6
phen TEBACl
phen —
bpy Dibenzo-18-crown-6 20
phen — 20
General procedure. A solution of (MeCN)4CuBF4 (0.3 mmol) and
Cu(BF4)2 (0.3 mmol) in acetonitrile (1 ml) and then 1,10-phenanthroline
(0.3 mmol) were added in a two-necked flask under argon to a suspension
of KSCN (6 mmol) in acetonitrile (3 ml). After stirring for 10 min, the
reaction mixture was cooled with an ice bath, and a solution of a diazo-
nium salt (3 mmol) in acetonitrile (3–10 ml) was added with vigorous
stirring in such a way that the temperature did not exceed 0–5 °C. After
gas evolution had ceased, cooling was removed; the reaction mixture was
allowed to heat to room temperature and poured into diethyl ether
(50 ml). The precipitate was filtered off and washed two times with
diethyl ether. The combined organic fractions were concentrated in
vacuo. Column chromatography (SiO2, hexane–EtOAc, 95:5 or 90:10
for 2h and 2i) gave a pure aryl thiocyanate, either as a yellow oil or as
yellow (2h) or orange (2i) crystals.
20
10
10
10
10
10
10
0
0
0
9
12
14
17
a10% PTC, (MeCN)4CuBF4, Cu(BF4)2 and ligand were added to a suspen-
sion of KSCN (2 mmol) in 1 ml of the solvent. A solution of compound 1a
in 2 ml of the solvent was then added over a period of several minutes to the
resulting mixture. According to 19F NMR data. CuI and CuII rhodanides
were synthesised separately just before the reaction from the appropriate
copper borofluorides and potassium rhodanide.
b
c
250 Mendeleev Commun. 2006