A mild copper-catalyzed aerobic oxidative thiocyanation of arylboronic acids with TMSNCS

Article abstract of DOI:10.1039/c4ob02208d

A facile and efficient transformation of arylboronic acids to their corresponding aryl thiocyanates has been successfully developed. Based on the CuCl-catalyzed oxidative cross-coupling reaction between arylboronic acids and trimethylsilylisothiocyanate (TMSNCS) under oxygen atmosphere, the transformation can be readily conducted at ambient temperature. The newly-developed protocol provided a competitive synthetic approach to aryl thiocyanates that can tolerate a broad range of reactive functional groups and/or strong electron-withdrawing groups.

Full text of DOI:10.1039/c4ob02208d

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DOI: 10.1039/C4OB02208D
A Mild Copper-catalyzed Aerobic Oxidative Thiocyanation of
Arylboronic Acids with TMSNCS
Nan Sun,a,* Liusheng Che,a Weimin Mo,a Baoxiang Hu,a Zhenlu Shena and Xinquan Hua,b,*
a College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310014, China
b State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical
Physics, Chinese Academy of Sciences, Lanzhou 730000, China
E-mail: sunnan@zjut.edu.cn and xinquan@zjut.edu.cn
Received Date (to be automatically inserted after your manuscript is accepted if required
according to the journal that you are submitting your paper to)
ABSTRACT: A facile and efficient transformation of arylboronic acids to their corresponding
arylthiocyanates has been successfully developed. Based on the CuCl-catalyzed oxidative cross-
coupling reaction between arylboronic acids and trimethylsilylisothiocyanate (TMSNCS) under oxygen
atmosphere, the transformation could be readily conducted at ambient temperature. The newly
developed protocol provided a competitive synthetic approach to aryl thiocyanates, which can tolerate a
broad range of reactive functional groups and/or strong electron-withdrawing groups.
Introduction
As an important family member in the sulfur chemistry, organic thiocyanate compounds can be
commonly found in natural products and many of them exhibit biological activities to some extent.1
More important, organic thiocyanates are usually served as the synthetic precursors, which could be
conveniently converted into various sulfur-containing derivatives, including sulfur heterocycles,2
thioethers,3 disulfides,4 thiols,5 thiocarbamates,6 and so on.7 Because of the importance, a number of
1

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methods have been reported to synthesize organic thiocyanates in the past decades. However, very
DOI: 10.1039/C4OB02208D
limited methods are efficient and practical in the synthesis of aryl thiocyanates. In fact, aryl
thiocyanates could be prepared via two classic routes. One is the direct electrophilic thiocyanation of
arenes by thiocyanogen or its equivalents,9 and the other is the nucleophilic substitution of aryl
diazonium salts by thiocyanate salts (Sandmeyer-type reaction).10 Unfortunately, both approaches could
not avoid some inherent drawbacks, such as the use of high toxic reagents, low regio- and chemo-
selectivity, as well as narrow substrate scope. Although great efforts have been devoted to the
improvement of these classic processes during the past years, they are largely relied on the use of some
special reagents and thus far from the potential application.11 In view of all the above reasons, some
approaches have also been investigated for the synthesis of aryl thiocyanates, which include the copper-
promoted nucleophilic replacement of aryl iodides with thiocyanate salts,12 thiocyanation of aryl
organometallic compounds (aryllithium, arylthallium and arylzinc compounds)13 and cyanation of
organosulfur compounds.14 However, these methods have also been encountered some difficulties, such
as, the limited availability of starting materials, the poor functional group compatibility, harsh reaction
conditions and/or low yields.
Over the past decades, great interests have been attracted on the transition-metal-mediated aryl carbon-
heteroatom bond (C-X) formations, which are becoming powerful tools for the access of heteroatom-
functionalized aromatic compounds.15 In particular, the aryl C-X bond formation by the copper-
mediated oxidative cross-coupling of arylboronic acids or their derivatives with heteroatom
nucleophiles, i.e. Cham-Lam reaction, has received considerable attentions, because of the readily
availablility of arylboronic acids, the lower cost of copper source and the relatively mild reaction
conditions.16 Since its initial report in 1998, significant progress has been achieved on Cham-Lam
reaction and a wide range of heteroatom-functionalized aromatic compounds, such as arylamines,17
arylesters,18 arylthioesters,19 and others,20 have been successful synthesized form arylboronic acids
(and/or their derivatives) and the corresponding heteroatom-containing nucleophiles by this strategy.
Previously, we have described a novel method for the synthesis of aryl thiocyanates from arylboronic
2

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acids and KSCN by Cu(OAc)2-catalyzed aerobic oxidative aryl C-S bond formation (Scheme 1, A). It
DOI: 10.1039/C4OB02208D
is a straightforward approach that can access a number of aryl thiocyanates in good to excellent yields.
However, the reaction was not well favorable for the electrophilic substituents (such as ester, ketone and
aldehyde groups) in arylboronic acids, because of strong nucleophilic character of KSCN. Moreover the
reaction was inert in the transformation of those arylboronic acids bearing with strong electron-
withdrawing groups (such as, CN and NO2), together with the autoclave operation (0.2 Mpa of O2 as
o
terminal oxidant) and the elevated reaction temperature (80 C). And thus, we are greatly interesting in
developing a more general and facile procedure to overcome these problems. In this work, we proposed
a new method for the preparation of aryl thiocyanates from arylboronic acids by using
trimethylsilylisothiocyanate (TMSNCS) as thiocyantion reagent (Scheme 1, B). In the presence of NaF,
thiocyanation reaction of arylboronic acids with TMSNCS could proceed at room temperature with a
catalytic amount of CuCl and under balloon oxygen. Under the newly developed procedure, a broad
range of functionalized aryl thiocyanates could be obtained in moderate to good yields. Noteworthy,
TMSNCS was commercially available, and has already been used as reliable NCS- nucleophile source
in the ring-opening of oxiranes22 and nucleophilic substitution of alkyl halides23 under the promotion of
n-Bu4NF (TBAF), while its application in the metal-catalyzed aromatic thiocyanation reaction has never
been reported.
Scheme 1 Copper-catalyzed aerobic oxidative thiocyanation of arylboronic acids
Results and discussion
We initiated our investigation by choosing the cross-coupling of 4-methylphenylboronic acid (1a) and
TMSNCS to form 4-methylphenyl thiocyanate (2a) as model reaction and the results were summarized
3

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in Table 1. At first, the reaction was carried out in acetonitrile by employing 20 mol% Cu(OAc)2 as
DOI: 10.1039/C4OB02208D
catalyst, O2 as terminal oxidant, 20 mol% tetramethylethylenediamine (TMEDA, L1) as ligand, 4 equiv.
of K2CO3 as base and 3Å molecular sieves as desiccant. After stirred at room temperature for 12 h
under a balloon of O2 atmosphere, only a trace of thiocyanation product 2a was observed according to
GC-MS detection, and the residual was mainly the starting material, 4-methylphenylboronic acid (Entry
1, Table 1). Pleasantly, when 1 equiv. of TBAF was added into the reaction system, the thiocyanation
reaction occurred quite smoothly, and the desired product 2a was obtained in 58% yield (Entry 2, Table
1). It is obvious that the additive TBAF promoted the cleavage of N-Si bond in TMSNCS to free the
active NCS- nucleophile. With this preliminary result in hand, we then further investigated the other
effective factors on this thiocyanation reaction, including catalysts, ligands, additives and solvents.
Screening experiments of catalysts revealed that quite a lot of copper compounds, either Cu(II) salts
(CuSO4, CuBr2 and CuCl2) or Cu(I) salts (CuCl and CuI), were able to catalyze this oxidative cross-
coupling reaction, and the thiocyanation product 2a could be obtained ranging from 50 to 61% yields
(Entries 3-7, Table 1). Considering that CuCl provided a relative better result on this transformation, it
was selected as catalyst for our further research. More experiments showed that the reaction efficiency
was significantly depended on the ligand property. For example, replacing ligand L1 with
dimethylethylenediamine (L2), the yield of 2a was dramatically decreased to 43% (Entry 8, Table 1).
Ligands phenanthroline (L3), 8-hydroxyquinoline (L4) and 2,4,6-collidine (L5) were even found to
retrain the reaction (Entries 9-11, Table 1). Ligand 4-(N,N-dimethylamino)pyridine (DMAP, L6) could
be almost comparable to L1, and the reaction with L6 afforded 2a in 58% yield (Entry 12, Table 1). To
our delight, when we switched additive TBAF to inorganic F- source, NaF, the yield of 2a was greatly
improved to 93% (Entry 13, Table 1). Other inorganic F--containing salts, KF and NH4F, also can
achieve higher yields of 2a with 83% and 81%, respectively (Entries 14 and 15, Table 1). We reckoned
that these inorganic F--containing salts have lower solubility than TBAF in reaction solvent (CH3CN),
and thus the concentrate of active NCS- nucleophile was kept in a low level by in situ generating in a
slower rate, which favored the reaction selectivity. In addition, it was found that reaction solvent and/or
4

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moisture could be also crucial to the reaction efficiency. When the reaction was carried out in DMF, the
DOI: 10.1039/C4OB02208D
yield of 2a was dropped to 24%, whilst trace product was resulted in DCE, THF and MeOH (Entries 16-
19, Table 1). On other hand, 3Å MS played a marginal role in the reaction (87% yield without 3 Å MS,
Entry 20, Table 1). Further optimized experiments indicated that 2 equiv. of phenylboronic acid
(comparison to TMSNCS) was essential to achieve high yield (Entry 21, Table 1). Cutting down the
loading of catalyst (CuCl) led to extremely longer reaction time while maintaining the reaction
selectivity (Entry 22, Table 1). It is noteworthy to point that O2 was necessary for this transformation.
When the reaction was set up under N2 atmosphere, only a little amount of 2a was observed (Entry 23,
Table 1). With the above experimental results, the optimal condition was concluded that the reaction
was carried out with 20 mol% of CuCl, 20 mol% of L1, 1 equiv. of NaF and 4 equiv. of K2CO3 in
CH3CN at room temperature for 12 h and under oxygen balloon, together with 3Å molecular sieves.
Table 1 Optimization studies for the copper-catalyzed cross-coupling of 4-methoxylphenylboronic acid
and TMSNCS a
Ligand:
Entry
[Cu]
Ligand (equiv.)
L1(0.2)
Additive (equiv.)b
-
Solvent
CH3CN
CH3CN
CH3CN
Yield (%)c
Trace
58
1
2
3
Cu(OAc)2
Cu(OAc)2
CuSO4
L1(0.2)
TBAF(1.0)
TBAF(1.0)
L1(0.2)
50
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DOI: 10.1039/C4OB02208D
4
5
CuCl2
CuBr2
CuCl
CuI
L1(0.2)
L1(0.2)
L1(0.2)
L1(0.2)
L2(0.2)
L3(0.2)
L4(0.2)
L5(0.4)
L6(0.4)
L1(0.2)
L1(0.2)
L1(0.2)
L1(0.2)
L1(0.2)
L1 (0.2)
L1(0.2)
L1(0.2)
L1(0.2)
TBAF(1.0)
TBAF(1.0)
TBAF(1.0)
TBAF(1.0)
TBAF(1.0)
TBAF(1.0)
TBAF(1.0)
TBAF(1.0)
TBAF(1.0)
NaF(1.0)
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
DMF
55
60
61
6
7
51
8
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
43
9
8
10
11
12
13
14
15
16
17
18
19
20e
21f
5
Trace
56
93(89)d
83
KF(1.0)
NH4F(1.0)
NaF(1.0)
81
24
NaF(1.0)
DCM
Trace
Trace
Trace
87
NaF(1.0)
THF
NaF(1.0)
MeOH
CH3CN
CH3CN
NaF(1.0)
NaF(1.0)
66
69(12 h)
89(30 h)
13
22g
23h
CuCl(0.1)
CuCl
L1(0.1)
L1(0.2)
NaF(1.0)
NaF(1.0)
CH3CN
CH3CN
a
Reaction condition: 4-methoxyl phenylboronic acid 1a (2.0 mmol), TMSNCS (1.0 mmol), [Cu] (20 mol%),
b
K2CO3 (4.0 mmol), O2 balloon, 2.5 g 3Å MS, solvent (5 ml), room temperature, 12h; GC yield by internal
c
standard method with biphenyl as internal standard, and the yield in parentheses was isolated yield. TBAF was
used as a 1 mol/L solution in THF, and NaF, KF and NH4F were AR grade. d 4-Methylphenyl thiocyanate 2a was
somewhat volatile. e Without 3Å MS. f 1.5 mmol of 4-methoxylphenylboronic acid were used. g 10 mol% of CuCl
were used. h Under a N2 balloon.
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With the optimal conditions in hand, we then investigated the substrate scope of this transformation. As
DOI: 10.1039/C4OB02208D
listed in Table 2, not only arylboronic acids bearing with electron-donating groups (1a-1f) could afford
the desired products in good yields (Entries 1-6, Table 2), but those arylboronic acids with electron-
withdrawing groups (1g-1m) also worked well under the standard procedure and their corresponding
aryl thiocyanates were obtained in moderate to good yields (Entries 7-13, Table 2). It is specially noted
that the substrates with strong electron-withdrawing groups (CN and NO2) could smoothly convert to
the desired 4-cyanophenyl thiocyanate 2l (62%) and 4-nitrophenyl thiocyanate 2m (45%) (Entries 12
and 13, Table 2), while these two substrates were almost inert in our previously report.21 Generally, the
yield of ortho-substituted thiocyanates was a slightly lower than its para- and meta-substituted analogs,
which was ascribed to its steric hindrance (Entries 1-3 and 4-6, Table 2). Halogen-substituted
arylboronic acids were good reaction substrates, and their corresponding thiocyanates 2g-2j were
obtained in good yields (74-84%), which could be potentially used for further transformations. To our
delight, the novel reaction system was also favorable for the synthesis of aryl thiocyanates containing
electrophilic ester, ketone and aldehyde groups and these thiocyanates (2n-2p) could be obtained in
moderate to good yields (57-73%), which was attributed to non-nucleophilic property of TMSNCS
(Entries 14-16, Table 2). Moreover, reactive vinyl group could be survived well under the present
procedure, leading to 2q in 72% yields. Unfortunately, as the same results in our previous report, we
found that this reaction system is still not suitable in the synthesis of both vinyl thiocyanates and
aliphatic thiocyanates. In addition, the protocol was also inefficient in the transformation of heteroaryl
boronic acids to their thiocyanates. When the current developed procedure was applied to the cross-
coupling between pyridin-4-ylboronic acid and TMSNCS, only a little amount of 4-thiocyanatopyridine
was resulted (less than 10% yield according to GC-MS) and no pure product was finally isolated.
Table 2 CuCl-catalyzed aerobic oxidative thiocyanation of arylboronic acids at room temperature a
CuCl (20 mol%), O2 (balloon)
TMEDA (20 mol%), NaF, K2CO3
TMSNCS
+
SCN
B(OH)2
R
R
CH3CN, RT, 12 h, 3Å MS
1
2
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b
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Entry
1
1(R)
Product
Yield (%)
2
DOI: 10.1039/C4OB02208D
1a
(4-CH3)
2a
89
1b
(3-CH3)
2
2b
88
1c
(2-CH3)
3
4
82
76
2c
1d
(4-OMe)
2d
1e
(3-OMe)
5
6
74
72
2e
2f
1f
(2-OMe)
1g
(4-F)
7
8
2g
2h
84
81
1h
(4-Cl)
SCN
Br
1i
(2-Br)
9
78
74
2i
2j
1j
10
(2,4-diCl)
1k
(4-CF3)
11
12
13
2k
2l
79
62
45
1l
(4-CN)
1m
(4-NO2)
2m
1n
14
15
73
68
2n
2o
(4-COOMe)
1o
(4-COMe)
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DOI: 10.1039/C4OB02208D
1p
(4-CHO)
16
17
57
2p
2q
1q
72
SCN
(4-CH=CH2)
a Reaction conditions: arylboronic acid (2.0 mmol), TMSNCS (1.0 mmol), CuCl (20 mol%), TMEDA (20 mol%),
NaF(1.0 mmol), K2CO3 (4.0 mmol), O2 balloon, 2.5 g 3Å MS, CH3CN (5 ml), room temperature, 12 h. b Isolated
yields.
In order to further investigate the scope of this reaction, we also tested the reaction of various
phenylboronic acid derivatives, pinacol 4-methylphenylboronate
3
and potassium 4-
methylphenyltrifluoroborate 4, with TMSNCS under our proposed reaction system (Scheme 2).
Unfortunately, both of them afforded the desired product 2a in less than 10% yield with recovering a
large amount of starting materials. Prolonging the reaction time or elevating the reaction temperature to
60 oC was also not facilitated the transformation.
O
B
O
+
+
TMSNCS
TMSNCS
SCN
3
4
2a
(5% yield)
BF3K
SCN
2a
(7% yield)
Scheme 2 CuCl-catalyzed thiocyanation of 4-methylphenylboronic acid derivatives with TMSNCS
under standard reaction conditions
For the purpose of investigation of the reaction mechanism, CuSCN was considered as an intermediate
and its reactivity on this transformation was examined (Scheme 3). When 0.5 equiv. of CuSCN was
reacted with 4-methylphenylboronic acid 1a under the standard procedure, the corresponding
thiocyanate 2a was obtained in 46% yield. Particularly, both the ligand TMEDA and O2 were essential
for this transformation. Without TMEDA or O2, only trace 2a was observed.
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DOI: S10C.1N039/C4OB02208D
K2CO3 (4 mmol)
+
CuSCN
1 mmol
B(OH)2
CH3CN, RT, 12 h, 3Å MS
2a
2 mmol
O2
TMEDA
1 mmol
2a (%)
balloon
46%
balloon
N2
-
trace
trace
1mmol
Scheme 3 Reaction of 4-methylphenylboronic acid with CuSCN
From these results, a plausible reaction mechanism is proposed in Figure 1.24 It was assumed that
LCu(I)Cl A was first exchanged with the in situ generated NCS- anion to form reactive LCu(I)SCN B,
which could be oxidized by O2 to yield bis(µ-oxo)-dicopper (III) complex C. In this step, the existence
of the diamine ligand TMEDA (L) was essential. It would increase electron density on the copper center
by coordination. Once C was resulted, it combined with two molecules of arylboronic acid by O-B
coordinate bond interaction, followed by transferring aryl group to copper atom to afford intermediate
D. The D was unstable and decomposed to two monomeric copper (III) complex E. Finally, the resulted
E underwent facile reductive elimination to the desired product aryl thiocyanates and the LCu(I)-
OB(OH)2 F. The formed F further exchanged with the NCS- anion to form LCu(I)SCN B to complete
the catalytic cycle.
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DOI: 10.1039/C4OB02208D
N
Cu Cl
N
A
TMSNCS + NaF
NCS-
N
N
O2
Cu SCN
NCS-
B
SCN
Cu
N
N
N
O
Cu
OB(OH)2
N
Cu
N
O
F
N
SCN
C
Ar SCN
Ar
B(OH)2
N
OB(OH)2
Ar
B(OH)2
SCN
Cu
N
Cu
N
O
NCS
Ar
N
Cu
N
E
O
N
NCS
Ar
(HO)2B
B(OH)2
SCN
N
Ar
O
Cu
Ar
N
Cu
N
O
N
NCS
(OH)2B
D
Figure 1 Plausible reaction pathway for thiocyanation of arylboronic acids with TMSNCS
Conclusion
In conclusion, we have developed a novel protocol for the copper-catalyzed aerobic oxidative
thiocyanation of arylboronic acids. By using TMSNCS as NCS- nucleophile precursor and in presence
of NaF, this cross coupling reaction could proceed smoothly at room temperature with the promotion of
catalytic amount of CuCl and under an O2 atmosphere. The proposed reaction system was suitable in the
synthesis of a broad range of aryl thiocyanates, especially which contained reactive functional groups,
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such as halo, ester, ketone, aldehyde and vinyl groups, as well as strong electron-withdrawing groups,
DOI: 10.1039/C4OB02208D
such as cyano and nitro groups. We believe that the milder and more efficient reaction conditions
described herein will be widely used for aromatic oxidative thiocyanation.
Acknowledgements
This work was supported by the National Natural Science Foundations of China (21376224). We also
thank the support from “Zhejiang Key Course of Chemical Engineering and Technology” and “Zhejiang
Key Laboratory of Green Pesticides and Cleaner Production Technology of Zhejiang Province” of
Zhejiang University of Technology.
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