Synthesis of aryl thiocyanates via copper-catalyzed aerobic oxidative cross-coupling between arylboronic acids and KSCN

Article abstract of DOI:10.1055/s-0033-1338939

A new protocol for the preparation of aryl thiocyanates by the cross-coupling reaction of arylboronic acids with KSCN salt is described. The coupling reaction was catalyzed by 20 mol% of copper acetate in the presence of 2.0 equivalents 4-methylpyridine serving both as ligand and base under 0.2 MPa of molecular oxygen. A variety of arylboronic acids, including those with substituents at ortho position, were suitable under the reaction conditions. Georg Thieme Verlag Stuttgart · New York.

Full text of DOI:10.1055/s-0033-1338939

LETTER
▌1443
lSetyternthesis of Aryl Thiocyanates via Copper-Catalyzed Aerobic Oxidative
Cross-Coupling between Arylboronic Acids and KSCN
Synthesis of Aryl Thiocyanates
Nan Sun,a Han Zhang,a Weimin Mo,a Baoxiang Hu,a Zhenlu Shen,a Xinquan Hu*a,b
a
College of Chemical Engineering and Materials Science, Zhejiang University of Technology, Hangzhou 310014, P. R. of China
Fax +86(571)88320103; E-mail: xinquan@zjut.edu.cn
b
State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences,
Lanzhou 730000, P. R. of China
Received: 16.03.2013; Accepted after revision: 17.04.2013
Organic thiocyanates are pivotal intermediates for the
Abstract: A new protocol for the preparation of aryl thiocyanates
synthesis of various sulfur-containing compounds, such
by the cross-coupling reaction of arylboronic acids with KSCN salt
as thiocarbamates, thioethers, disulfides, and heterocy-
is described. The coupling reaction was catalyzed by 20 mol% of
copper acetate in the presence of 2.0 equivalents 4-methylpyridine
cles.9 Some organic thiocyanates also exhibited interest-
serving both as ligand and base under 0.2 MPa of molecular oxygen. ing biological activities.10 Comparing with the numerous
reports on the synthesis of alky thiocyanates,11 the effi-
A variety of arylboronic acids, including those with substituents at
ortho position, were suitable under the reaction conditions.
cient methods for the preparation of aryl thiocyanates are
Key words: aryl thiocyanates, arylboronic acids, potassium thio-
cyanate, copper acetate, oxidative cross-coupling reaction
rather limited. For example, the classic Sandmeyer-type
procedure (nucleophilic substitution of aryl diazonium
salts with thiocyanate salt) was less efficient in the synthe-
sis of aryl thiocyanates because of the competitive forma-
Nowadays, the synthesis of heteroatom-substituted aro- tion of thermodynamically more stable aryl
matic compounds by transition-metal-catalyzed carbon– isothiocyantaes.12 Although many efforts have been made
heteroatom (C–X) bond formation was prevailing in mod- to improve the reaction selectivity, only some special cop-
ern organic synthesis. Among the massive works in this per catalysts or complex thiocyanation reagents showed
area, the Cham–Lam reaction, that is, the copper-promot- desired results with aryl diazonium salts.13 On the other
ed oxidative cross-coupling of arylboronic acids with het- hand, the direct electrophilic thiocyanation of arenes was
eroatom nucleophiles, was one of the most attractive another common approach for the synthesis of aryl thio-
interests, based on the readily availability of arylboronic cyanates, while it was found that the electrophilic thiocy-
acids and the less expense of copper salts. Moreover, the anation was strictly suitable for those activated arenes.14
reaction conditions were relatively mild.1 Since the initial Moreover, the reaction activity and selectivity were obvi-
reports in 1998,2 many researches focused on the Chan– ously affected by the substituents on aromatic rings. In ad-
Lam reaction and significant progress has been achieved dition, other traditional methods such as nucleophilic
over the past decade. Recently, it has become a powerful substitution of aryl halides with thiocyanate salts under
tool for introducing heteroatom-containing functionalized the promotion of copper salts,15 coupling of aryllithium
groups into aromatic compounds.3 Among the wide appli- compounds with Zn(SCN)2 under the assistance of NCS16
cations of the Chan–Lam reaction in C–X bond forma- and cyanation of arylsulfenyl chlorides,17 were also inves-
tions, it was vital for the functionalization of aromatic tigated for the preparation of aryl thiocyanates. However,
compounds with sulfur derivatives by directly construct- most of them were confronted with the various drawbacks
ing C–S bonds. With the promotion of some simple cop- including the poor access to starting materials, use of toxic
per salts in either a stoichiometric or a catalytic amount, a reagents, or harsh reaction conditions. Thus developing a
wide range of unsymmetric aryl thioethers were prepared more general and facile method for the synthesis of aryl
by coupling of arylboronic acids and/or derivatives with thiocyanates is highly desirable.
thoils,4 thioamides,5 thioimides,6 and diaryldisulfides.7
Herein, we reported a general and efficient protocol for
More recently, Vicic and Zhang8 utilized the CF3S– anion
the preparation of aryl thiocyanates from arylboronic ac-
as sulfur nucleophile in the coupling with arylboronic ac-
ids and KSCN in the presence of a catalytic amount of
ids to form the corresponding trifluoromethylarylthio-
Cu(OAc)2 with molecular oxygen as a terminal oxidant
ethers. However, the direct aryl C–S bond formation
(Scheme 1).
between arylboronic acids and the thiocyanate nucleo-
phile for the construction of aryl thiocyanates has not been
reported yet.
Cu(OAc)2 (20 mol%), O2 (0.2 MPa)
4-methylpyridine (2.0 equiv)
KSCN
Ar B(OH)2
+
Ar SCN
MeCN, 80 °C, 12 h, 3 Å MS
16 examples
up to 91% yield
SYNLETT 2013, 24, 1443–1447
Advanced online publication: 15.05.2013
Scheme 1
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
DOI: 10.1055/s-0033-1338939; Art ID: ST-2013-W0236-L
© Georg Thieme Verlag Stuttgart · New York

1444
N. Sun et al.
LETTER
We initiated our investigation by exploring the cross-cou- petitive to that of L7. The results of screening other Cu(II)
pling reaction of phenylboronic acid (1a) and KSCN un- salts, such as Cu(TFA)2 and CuCl2, showed that phenyl
der aerobic conditions (Table 1). To our delight, the thiocyanate (2a) was obtained in 73% and 56% yields, re-
coupling reaction could proceed in acetonitrile under the spectively (Table 1, entries 16 and 17). CuSO4 was found
catalysis of Cu(OAc)2 in the presence of O2. After reacted to be inefficient in this transformation (Table 1, entry 18).
at 80 °C for 12 hours, the desired coupling product phenyl Noteworthy, Cu(I) salt (CuCl) also could promote the re-
thiocyanate (2a)18 was obtained in 43% yield by using 20 action, and a 73% yield of phenyl thiocyanate (2a) was re-
mol% of Cu(OAc)2, one equivalent of K2CO3, and 3 Å sulted under the same reaction conditions (Table 1, entry
molecular sieves under 0.2 MPa of O2 (Table 1, entry 1). 19). The reaction efficiency was also greatly affected by
To improve the yield of the coupling product 2a, different other reaction conditions such as reaction solvent and
types of ligands were screened, and the results were listed moisture. For example, when the reaction was carried out
in Table 1 (entries 2–11). The results showed that the re- in THF and DMF, the yields of 2a were decreased to 30%
action was slightly variable with some N,N-bidentate li- and 78%, respectively, and no product was formed in
gands, such as tetramethylethylenediamine (L1), DCE (Table 1, entries 20–22). Moreover, without 3 Å mo-
phenanthroline (L2), and bipyridine(L3), and even lecular sieves, the yield 2a was decreased to 80% (Table
dropped the selectivity with N,O-bidentate ligand 8-hy- 1, entry 23). Unfortunately, the attempt to cut down the
droxyquinoline (L4, Table 1, entries 2–5). However, the loading of Cu(OAc)2 was not very successful (Table 1, en-
reaction efficiency was significantly improved by adding try 24). Finally, the optimal conditions were concluded as
N-monodentate ligands such as piperidine (L5) and pyri- the reaction being carried out with 20 mol% of Cu(OAc)2
dines L6–9 (Table 1, entries 6–10). Among these mono- and two equivalents of 4-methylpyridine in MeCN at 80
dentate ligands, the most efficient 4-methylpyridine (L7) °C for 12 hours in the presence of oxygen (0.2 Mpa), to-
afforded 74% yield of 2a (Table 1, entry 8). No improve- gether with 3 Å molecular sieves.
ment was found with the sterically hindered N-monoden-
tate ligand 2,4,6-collidine (L10), which might be ascribed
to its unsteadily coordination with Cu(II) (Table 1, entry
11). Considering ligand L7 was also a commonly used or-
ganic base, we decided to replace base K2CO3 by increas-
ing the amount of 4-methylpyridine to two equivalents.
With the optimized conditions in hand, we investigated
the scope of this new transformation (Scheme 2). The ar-
ylboronic acids bearing with either electron-rich or mod-
erate electron-deficient substituents worked well under
the previous procedure and afforded their corresponding
aryl thiocyanates 2a–m in moderate to good yields. The
This change remarkably improved the yield of 2a to 91%
ortho-substituted arylboronic acids also could smoothly
(Table 1, entry 12). Similar results were also observed by
react with KSCN to form the corresponding thiocyanates
replacing the K2CO3 with ligands L6, L8, and L9 (Table
2c,f,k,l in moderate yields.
1, entries 13–15), however, these results were less com-
Table 1 Optimization Studies for the Preparation of Phenyl Thiocyanate by Copper-Catalyzed Oxidative Cross-Coupling of Phenyl Boronic
Acid and KSCN under Aerobic Conditionsa
[Cu] (20 mmol%)
L, O2 (0.2 MPa)
KSCN
B(OH)2
SCN
+
base, solvent, 3 Å MS
1a
2a
Me
N
N Me
N
N
N
Me Me
N
N
OH
L1
L2
L3
L4
Me
Me Me
N
O
N
H
N
N
N
N
N
L5
L6
L7
L8
L9
L10
Entry
[Cu]
Ligand (equiv)
Base (equiv)
Solvent
Yield (%)b
1
2
3
4
5
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
–
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
MeCN
MeCN
MeCN
MeCN
MeCN
42
44
43
43
19
L1 (0.2)
L2 (0.2)
L3 (0.2)
L4 (0.2)
Synlett 2013, 24, 1443–1447
© Georg Thieme Verlag Stuttgart · New York

LETTER
Synthesis of Aryl Thiocyanates
1445
Table 1 Optimization Studies for the Preparation of Phenyl Thiocyanate by Copper-Catalyzed Oxidative Cross-Coupling of Phenyl Boronic
Acid and KSCN under Aerobic Conditionsa (continued)
[Cu] (20 mmol%)
L, O2 (0.2 MPa)
KSCN
B(OH)2
SCN
+
base, solvent, 3 Å MS
1a
2a
Me
N
N Me
N
N
N
Me Me
N
N
OH
L1
L2
L3
L4
Me
Me Me
N
O
N
H
N
N
N
N
N
L5
L6
L7
L8
L9
L10
Entry
[Cu]
Ligand (equiv)
Base (equiv)
Solvent
Yield (%)b
6
7
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(TFA)2
CuCl2
L5 (0.4)
L6 (0.4)
L7 (0.4)
L8 (0.4)
L9 (0.4)
L10 (0.4)
L7 (2.0)
L6 (2.0)
L8 (2.0)
L9 (2.0)
L7 (2.0)
L7 (2.0)
L7 (2.0)
L7 (2.0)
L7 (2.0)
L7 (2.0)
L7 (2.0)
L7 (2.0)
L7 (2.0)
K2CO3
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
DCE
52
K2CO3
61
8
K2CO3
74
9
K2CO3
64
10
11
12
13
14
15
16
17
18
19
20
21
22
23c
24d
K2CO3
68
K2CO3
42
–
–
–
–
–
–
–
–
–
–
–
–
–
91 (90)
84
77
88
73
56
CuSO4
<10
73
CuCl
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
n.r.
30
THF
DMF
78
MeCN
MeCN
80
76
a Reaction conditions: phenyl boronic acid (10 mmol), KSCN (11 mmol), [Cu] (2 mmol), solvent (25 mL), 80 °C, 12 h, 3 Å MS (2.5 g) and O2
(0.2 MPa) in 250 mL Teflon-lined autoclave.
b GC yields with anisole as the internal standard. The datum in parentheses was the isolated yield.
c Without 3 Å MS.
d Conditions: 1.5 mmol of Cu(OAc)2 were used.
Notably, chloro and bromo groups survived well in the corresponding thiocyanates 2n and 2o, however, the de-
standard procedure, leading to aryl thiocyanates 2j,k,l, sired products were obtained in relative lower yields (59%
which could be used for further potential derivations. The and 25%, respectively). When the currently developed
strong electron-withdrawing groups obviously decreased procedure was applied to the cross-coupling between 4-
the reaction efficiency. The 4-methoxycarbonyl- and 4- nitrophenyl boronic acid and KSCN, only little product 2p
acetyl-substituted phenyl boronic acids could afford the was resulted (less than 10% yield according to GC–MS)
© Georg Thieme Verlag Stuttgart · New York
Synlett 2013, 24, 1443–1447

1446
N. Sun et al.
LETTER
Clark, C. G.; Saubern, S.; Adams, J.; Winters, M. P.; Chan,
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Cu(OAc)2 (20 mol%), O2 (0.2 MPa)
4-methylpyridine (2.0 equiv)
B(OH)2 KSCN
+
SCN
MeCN, 80 °C, 12 h, 3 Å MS
R
R
1
2
SCN
SCN
Ph
SCN
SCN
R
R
2a (90%)
2b (91%) 2c R = 2-OMe (70%)
2d R = 3-OMe (78%)
2f R = 2-Me (74%)
2g R = 3-Me (80%)
2h R = 4-Me (85%)
2e R = 4-OMe (80%)
SCN
SCN
SCN
SCN
F
Cl
Br
Cl
Cl
2i (78%)
2j (78%)
2k (81%)
2l (73%)
SCN
SCN
SCN
MeO
SCN
F3C
O2N
2p (<10%)
(5) Prokopcova, H.; Kappe, C. O. J. Org. Chem. 2007, 72, 4440.
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72, 1241. (c) Taniguchi, N. Synlett 2006, 1351.
O
O
2m (80%)
SCN
2n (59%)
2o (25%)
SCN
SCN
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452. (d) Iranpoor, N.; Firouzabadi, H.; Nowrouzi, N.
Tetrahedron 2006, 62, 5498; and the references cited in
these papers.
2q (80%)
trace
no product
Scheme 2 The reactions were conducted under the optimized condi-
tions; isolated yields are given
and no pure 2p was finally isolated. Fused arylboronic
acid can also serve as a good reaction substrate, and 2-
naphthyl thiocyanate 2q was obtained in 80% yield when
2-naphthylboronic acid was applied. It should be noted
that this reaction system is not suitable for both vinyl-
boronic acid and alkylboronic acids as substrates.
In conclusion, we present here a new protocol for the
preparation of aryl thiocyanates. The method was based
on the Cu(OAc)2-catalyzed coupling of arylboronic acids
with KSCN by using molecular oxygen as a terminal oxi-
dant. The transformation provides a facile and practice
route for the synthesis of aryl thiocyantes. The mechanism
of this transformation was undergone in our laboratory.
(12) (a) Guy, R. G. In The Chemistry of Functional Groups: The
Chemistry of Cyanates and Their Thio Derivatives; Patai, S.,
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Sigeev, A. S.; Peregudov, A. S.; Petrovskii, P. V. Mendeleev
Commun. 2006, 250. (c) Barbero, M.; Degani, I.;
Acknowledgment
We gratefully acknowledge the financial support of the Major Pro-
ject of Science and Technology Department of Zhejiang Province
(No. 2010C11039) and the Key Innovation Team Project of
Zhejiang Province, China (No. 2009R50002).
Diulgheroff, N.; Dughera, S.; Fochi, R. Synthesis 2001, 585.
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Donyadari, E.; Shiri, M.; Kruger, H. G. Catal. Commun.
2012, 26, 34. (b) Khazaei, A.; Zolfigol, M. A.; Mokhlesi,
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106. (c) Iranpoor, N.; Firouzabadi, H.; Khalili, D.; Shahin,
R. Tetrahedron Lett. 2010, 51, 3508. (d) Bhalerao, D. S.;
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Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.SnoIufproig
m
iotSrat
ungIifoop
r
t
References and Notes
(1) For recent reviews, see: (a) Rao, K. S.; Wu, T. S.
Tetrahedron 2012, 68, 7735. (b) Qiao, J. X.; Lam, P. Y. S.
Synthesis 2011, 829.
(2) (a) Chan, D. M. T.; Monaco, K. L.; Wang, R. P.; Winters, M.
P. Tetrahedron Lett. 1998, 39, 2933. (b) Lam, P. Y. S.;
Synlett 2013, 24, 1443–1447
© Georg Thieme Verlag Stuttgart · New York

LETTER
Synth. Commun. 1996, 26, 3413. (c) Clark, J. H.; Jones, C.
Synthesis of Aryl Thiocyanates
1447
CDCl3): δ = 110.5, 124.4, 129.5, 130.06, 130.23. HRMS
(EI): m/z calcd for C7H5NS [M]+: 135.0143; found:
135.0140. CAS Reg. No. 5285-87-0.
W.; Duke, C. V. A.; Miller, J. M. J. Chem. Soc., Chem.
Commun. 1989, 81.
(16) Takagi, K.; Takachi, H.; Sasaki, K. J. Org. Chem. 1995, 60,
6552.
2-Methoxyphenyl Thiocyanate (2c)
Yellow oil. IR (neat): ν = 2153 cm–1. 1H NMR (500 MHz,
CDCl3): δ = 3.94 (s, 3 H), 6.94–7.59 (m, 4 H). 13C NMR (125
MHz, CDCl3): δ = 56.2, 110.5, 111.4, 113.1, 122.1, 129.9,
130.5, 156.5. HRMS (EI): m/z calcd for C8H7NOS [M]+:
165.0248; found: 165.0252. CAS Reg. No. 14372-66-8.
4-Fluorophenyl Thiocyanate (2i)
(17) (a) Still, I. W. J.; Watson, I. D. G. Synth. Commun. 2001, 31,
1355. (b) Kagabu, S.; Sawahara, K.; Maehara, M.; Ichihashi,
S.; Saito, K. Chem. Pharm. Bull. 1991, 39, 784. (c) Harpp,
D. H.; Friedlander, B. T.; Smith, R. A. Synthesis 1979, 181.
(18) General Procedure for the Preparation of Aryl
Thiocyanates
Colorless oil. IR (neat): ν = 2153 cm–1. 1H NMR (500 MHz,
CDCl3): δ = 7.15–7.59 (m, 4 H). 13C NMR (125 MHz,
CDCl3): δ = 110.6, 117.6 (d, J = 22.7 Hz), 119.2 (d, J = 3.3
Hz), 133.2 (d, J = 8.9 Hz), 163.6 (d, J = 250.1 Hz). HRMS
(EI): m/z calcd for C7H4NSF [M]+: 153.0048; found:
153.0048. CAS Reg. No. 2924-02-9.
To a Teflon-lined stainless steel autoclave (250 mL) was
added aryl boronic acid (10 mmol), KSCN (1.07 g, 11
mmol), Cu(OAc)2 (0.36 g, 2 mmol), 3 Å MS (2.5 g), and
MeCN (25 mL). The autoclave was closed and charged with
oxygen to 0.2 MPa. The autoclave was then set into the
preheated (80 °C) oil bath. After the reaction proceeded
within 12 h, the autoclave was cooled to r.t. and carefully
depressurized. The mixture in the autoclave was filtered to
remove the catalyst and 3 Å MS and washed with MeCN.
The combined filtrate was concentrated under reduced
pressure. The residual was purified by flash chromatography
through a silica column using PE–EtOAc as the eluent under
the protection of N2 to afford the desired products. The
products were characterized by IR, 1H NMR, 13C NMR, and
HRMS.
2-Bromophenyl Thiocyanate (2k)
Pink oil. IR (neat): ν = 2161 cm–1. 1H NMR (500 MHz,
CDCl3): δ = 7.25–7.74 (m, 4 H). 13C NMR (125 MHz,
CDCl3): δ = 109.6, 121.8, 127.1, 129.07, 129.44, 130.1,
133.6. HRMS (EI): m/z calcd for C7H4NSBr [M]+: 212.9248;
found: 212.9238. CAS Reg. No. 55757-32-9.
4-(Methoxycarbonyl)phenyl Thiocyanate (2n)
White soild; mp 62.7 °C. IR (KBr): ν = 2153, 1708 cm–1. 1H
NMR (500 MHz, CDCl3): δ = 3.96 (s, 3 H), 7.57–8.12 (m, 4
H). 13C NMR (125 MHz, CDCl3): δ = 52.6, 109.1, 128.5,
130.47, 130.93, 131.21, 165.8. HRMS (EI): m/z calcd for
C9H7NO2S [M]+: 193.0198; found: 193.0203. CAS Reg. No.
1879-22-7.
Phenyl Thiocyanate (2a)
Yellowish oil. IR (neat): ν = 2173 cm–1. 1H NMR (500 MHz,
CDCl3): δ = 7.43–7.56 (m, 5 H). 13C NMR (125 MHz,
© Georg Thieme Verlag Stuttgart · New York
Synlett 2013, 24, 1443–1447

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