Electrochemical ipso-Thiocyanation of Arylboron Compounds

Article abstract of DOI:10.1002/adsc.201900156

An operationally simple electrochemical method for the transition-metal-free ipso-thiocyanation of arylboronic acids and aryl trifluoroborates has been developed. The SCN electrophile is generated in situ by anodic oxidation of thiocyanate anions, which avoids formation of salt waste and prevents unwanted side reactions arising from chemical oxidants. The reaction proceeds regiospecifically, and the scope extends to non-activated aromatic systems. (Figure presented.).

Full text of DOI:10.1002/adsc.201900156

Title: Electrochemical ipso-Thiocyanation of Arylboron Compounds
Authors: Marco Dyga, Davit Hayrapetyan, Raja Rit, and Lukas J.
Goossen
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10.1002/adsc.201900156
Advanced Synthesis & Catalysis
Very Important Publication
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DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Electrochemical ipso-Thiocyanation of Arylboron Compounds
Marco Dyga, Davit Hayrapetyan, Raja K. Rit, and Lukas J. Gooßen*
*
Evonik Chair of Organic Chemistry, Fakultät für Chemie und Biochemie, Ruhr-Universität Bochum
Universitätsstr. 150, ZEMOS, 44801 Bochum, Germany
E-mail: lukas.goossen@rub.de
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
substitution reactions at the aromatic ring to form the
C–S bond (Scheme 2, top).
Abstract. An operationally simple electrochemical
method for the transition-metal-free ipso-thiocyanation
of arylboronic acids and aryl trifluoroborates has been
developed. The SCN electrophile is generated in situ by
anodic oxidation of thiocyanate anions, which avoids
formation of salt waste and prevents unwanted side
reactions arising from chemical oxidants. The reaction
proceeds regiospecifically, and the scope extends to non-
activated aromatic systems.
Cyanations of thiols,[7] aryldisulfides,[7,8] or
thioethers[9] are limited by starting-material
availability. Thiocyanations of arenediazonium
salts[10] or aryl halides[11] are efficient, but rely on the
heavy metal copper as a mediator.[12] Only some
electron-deficient aromatic systems can also be
thiocyanated by metal-free substitutions of aryl
halides.[13]
Keywords: thiocyanation; thiocyanogen; boronic acids;
trifluoroborates; electrochemistry
Known syntheses of aryl thiocyanates
electrophilic
cyanation
SCN source
Organothiocyanates are versatile building blocks in
the synthesis of biologically active compounds
containing aryl-sulfur bonds,[1] such as thioethers,[2]
thiols,[3] disulfides,[4] and various heterocycles
(Scheme 1).[5] They can also serve as starting
materials in the synthesis of poly- and perfluoroalkyl
thioethers, which have attracted considerable
attention in medicinal chemistry due to their high
lipophilicity.[6]
SCN, [Cu]
SCN, [Ox.]
in situ (SCN)2
[Cu], O2
KSCN, [Cu]
KSCN or
Me3SiNCS
X = I, Br
This work: electrochemical thiocyanation of arylboron compounds
NH4SCN
[B] = B(OH)2, BF3K
RMgX
NaN3
Scheme 2. Overview on syntheses of aryl thiocyanates.
TMS-CF3
H+, H2O
[Red.]
Electrophilic aromatic thiocyanations are non-
regiospecific and are traditionally performed with
complex reagents, such as N-thiocyanatosuccinimide,
or strong oxidants, e.g. Br2 or K2S2O8, to generate the
pseudohalogen (SCN)2 from thiocyanate salts.[14]
Some of the downsides of this approach, e.g. poor
atom economy, or uncontrolled formation of
brominated by-products or SCN polymers, can be
overcome by generating (SCN)2 using anodic
oxidation of SCN–.[15] However, the electrochemical
generation of (SCN)2 has never been utilised in a
regiospecific thiocyanation process (Scheme 2,
bottom). In this context, boronic acids appeared to be
the most advantageous aryl sources.[16] They are
[Pd]
NEt3, 
Scheme 1. Aryl thiocyanates as key building blocks.
Aryl thiocyanates are typically synthesised by
cyanation of arylsulfur compounds such as thiols or
thioethers, or by nucleophilic or electrophilic
1
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10.1002/adsc.201900156
Advanced Synthesis & Catalysis
commercially available in a structural variety by far
exceeding that of thiols or disulfides, and are
accessible via numerous synthetic strategies ranging
from Friedel-Crafts processes and the metalation of
aryl halides all the way to late-stage catalytic C–H
borylations.[17]
The only established approach to couple boronic
acids with nucleophiles is the Chan-Evans-Lam
coupling. However, for thiocyanations, it requires
large amounts of copper in combination with an
increased partial oxygen pressure.[18] To the best of
our knowledge, there are only three examples of
acetoxylation.[21] Alternatively, the nucleophile can
be oxidised to an electrophilic intermediate which is
capable of regiospecific substitution reactions. We
were optimistic that such an approach could be used
for thiocyanation, since the oxidation potential of
SCN– (+0.77 V vs. standard hydrogen electrode,
SHE)[22] is much lower than that of most arenes, e.g.
anisole (+2.00 V vs. SHE),[23] or aryl boronates, e.g.
phenyl trifluoroborate (+2.08 V vs. SHE).[21c]
Our mechanistic blueprint for the regiospecific
thiocyanation of arylboron compounds is outlined in
Scheme 3. SCN– is oxidised to the pseudohalogen
(SCN)2, which then reacts with the arylboron
compound to give the aryl thiocyanate. This process
metal-free
ipso-sulfurations
of
aryl-boron
compounds: The synthesis of diphenyl disulfide and
diphenyl sulfide from various phenyl borates using S8, may be aided by coordination of SCN– to the boronic
S2Cl2, or (PhSO2)2S as sulfur sources,[16c] the
synthesis of sulfonamides from boronic acids and
diethylaminosulfur trifluoride (DAST),[16d] and the
synthesis of Ph2B–SCN, in which formation of
PhSCN has been proposed as a side reaction.[16b]
However, this side product was not isolated.
acid, which gives an anionic species and thus
increases the local concentration of both reagents at
the anode and, at the same time, activates the boron-
based leaving group.
In order to verify our hypothesis, we chose the
reaction of 2-methoxyphenylboronic acid 1a with
NH4SCN as the model system (Table 1). To our
delight, formation of the desired thiocyanate 3a was
observed in encouraging yield using an undivided cell
and simple Pt sheet electrodes (Entry 1).
In light of the synthetic utility of aryl thiocyanates,
an electrochemical thiocyanation of arylboron
compounds would be a valuable addition to the
toolkit of synthetic chemists, in particular if it could
be conducted within a simple, undivided cell
releasing only H2 as a by-product (Scheme 3). In a
recent review, Zhu and Falck have similarly
identified the area of metal-free deborylative aryl–S
bond formation as an important but underexploited
field.[16a]
Table 1. Optimisation of the conditions for thiocyanation
of arylboronic acids and aryl trifluoroborates.
SCN source (10 eq.)
Pt/Pt, constant current
undivided cell
Solvent, RT
NH4SCN
1a or 2a
3a
Pt-Pt, undivided cell
[B]
Solvent
SCN
source
I
3a
constant current electrolysis
[mA] [%]
1
2
3
4
5
6
B(OH)2 MeCN
NH4SCN
80
"
"
"
"
22
7
12
0
47
0
"
"
"
"
"
HFIP
DMF
MeOH
DMSO
"
"
"
"
"
Pt
Pt
(SCN)2
H2
C
A
T
H
O
D
E
A
N
O
D
E
AcOH/
"
-2e
-[B+]
+2e
H2O (9:1)
7
"
MeCN/
"
"
43
HCO2H
(9:1)
SCN
SCN
2H
8
9
"
AcOH
"
"
"
"
100
60
72(68)
64
59
55
74
56
0
[B] = B(OH)2, BF3
"
"
NaSCN
KSCN
NH4SCN
"
10
11
12
13
14
"
"
"
"
Scheme 3. Proposed mechanism for the electrochemical
thiocyanation.
"
"
"
"
NH4SCNa) 80
H
"
NH4SCN
"
15 B(OH)2
16 BF3K
"
"
"
"
0
80
"
0
79
39
Several strategies for electrochemical arene
substitution have been developed.[19] The aromatic
ring can be oxidised to a radical, which is then
attacked by a nucleophile to give the substituted
arene. This strategy has been applied in a wide range
of C–C and C–heteroatom bond forming reactions.[20]
Regiospecific oxidation of arylboron compounds to
aryl radicals has been utilised in reactions such as
hydroxylations, aminations, fluorinations, and
17
"
MeCN/
HCO2H
(9:1)
"
NH4SCNb)
18
"
NH4SCNc)
"
83(78)
Reaction conditions: Solutions of 0.50 mmol of 1a / 2a and
5.00 mmol of the thiocyanate source in 10 mL of solvent
were electrolysed at constant current (12 F) using 2×1 cm
2
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10.1002/adsc.201900156
Advanced Synthesis & Catalysis
Pt sheet electrodes. GC yields using mesitylene as internal
current density of up to 40 mA/cm² is ideal (Entries
11-12). Under these conditions, 0.5 mmol of 1a were
converted within less than 2 hours at room
temperature, leading to 68% yield of 3a along with
the boroxine as the only by-product detected by GC-
MS. The reaction proceeded regiospecifically; the
electronically and sterically favoured para isomer
was not detected. A control experiment using anisole
as the starting material did not give detectable
amounts of thiocyanated compounds (Entry 14).
Without electric current, no reaction took place
(Entry 15).
a)
b)
standard, isolated yields in parentheses. 4.00 mmol.
1.50 mmol. c) 2.00 mmol.
Solvent optimisation proved to be the key to high
conversion and selectivity, with best yields achieved
in glacial acetic acid (Entries 2-8). This Brønsted-
acidic solvent is likely to facilitate H2 generation at
the cathode. The presence of water shut down the
reaction (Entry 6).[24]
Table 2. Scope of the electrochemical thiocyanation.
NH4SCN
The reaction was conducted with several arylboron
compounds. The yields obtained followed Mayr’s
nucleophilicity scale vs. benzhydrylium cations (see
the SI for details).[25] The highest reactivity was
observed for the bench-stable and easy-to-prepare
potassium aryltrifluoroborate 2a.[26] With an adapted
reaction protocol, using 4 equiv. NH4SCN in a 9:1
mixture of acetonitrile and formic acid, 3a was
obtained in 78% yield (Entry 18).
Pt-Pt, undivided cell, 80 mA
constant current electrolysis
CH3CN/HCO2H (9:1)
1 or 2
3
25 °C, 2 h
A: 68%a)
B: 78%b)
A: 93%b)
B: 67%b)
A: 0%b)
3c,
A: 95%b)
B: 70%b)
Having thus established effective and robust
protocols, we went on to evaluate the scope of our
electrochemical thiocyanation (Table 2). The reaction
variant starting directly from boronic acids (Method
A) is applicable to various substituted electron-rich
substrates. Starting from trifluoroborates (Method B),
an even broader range of aryl thiocyanates can be
synthesised, including sterically hindered o,o-
disubstituted compounds. Its performance with regard
to electron density extends all the way to potassium
phenyltrifluoroborate (2j). Whereas the p-
fluorophenyl derivative 2c still gives reasonable
yields, aryltrifluoroborate salts with electron-
withdrawing substituents (2w-y) do not give any
conversion. The chlorinated thiocyanate 3h was
obtained in good yield starting from both 1h and 2h.
Interestingly, while the synthesis of n-butyl
thiocyanate (3z) was not successful under our
conditions, the thiocyanation of potassium
benzyltrifluoroborate (2s) afforded thiocyanate 3s in
62% yield, proving that this reaction is not strictly
limited to aromatic substrates. With the exception of
3a,
3e,
3b,
3d, A: 100%b)
B: 54%b)
A: 86%a)
B: 70%b)
A: 89%a)
3h,
A: 0%b)
3f,
3g, A: 100%b)
3i, A: 66%a)
3j,
B: 72%b)
B: 34%b)
A: 8%a)
B: 83%b)
A: 0%b)
B: 60%b)
A: 38%a)
B: 64%b)
3k,
3l,
3m,
3n, B: 81%b)
3o, B: 67%b)
A: 91%b)
B: 47%b)
A: 0%b)
B: 72%b)
3p,
3q,
3r, B: 28%b)
3s, B: 62%b)
a
few extremely electron-rich arenes, the
trifluoroborates gave higher yields than boronic acids,
presumably due to their higher nucleophilicity, as
outlined above.
A: 0%b)
B: 47%b)
A: 0%b)
B: 74%b)
3t, A: 91%b)
3u,
3v,
A gram-scale reaction starting from 20 mmol of p-
methoxyphenylboronic acid 1b afforded thiocyanate
3b in 96% yield, demonstrating that the reaction can
also be performed at higher concentrations for
preparative applications.
Several control experiments were performed to
elucidate the reaction mechanism (see SI). A solution
of pre-formed (SCN)2 was reacted with 2c, affording
the corresponding thiocyanate 3c in only 32% yield
along with polymeric parathiocyanogen. While
formation of 3c is in agreement with the proposed
reaction mechanism via (SCN)2, the low yields
demonstrate the advantages of a controlled formation
of this reactive intermediate via anodic oxidation.
A: 0%a)
A: 0%a)
3z,
3w,
3x, B: 0%b)
3y, B: 0%b)
B: 0%b)
B: 0%b)
Reaction conditions: Electrolysis for 2 h at 80 mA, isolated
yields. (A) 0.50 mmol of 1, 5.00 mmol of NH4SCN; (B)
0.50 mmol of 2, 2.00 mmol of NH4SCN. a) 10 mL of acetic
acid. b) 9 mL of CH3CN and 1 mL of formic acid.
Further experiments revealed that NH4SCN is the
optimal SCN source (Entries 9-10), that Pt is the most
effective electrode material (see SI), and that a
3
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10.1002/adsc.201900156
Advanced Synthesis & Catalysis
Quaternisation of the boronic acid with SCN– was
confirmed by in situ NMR spectroscopy.
[1] For reviews on the chemistry of organothiocyanates,
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The versatility of the products as synthons was
demonstrated by several follow-up reactions of p-
methoxyphenyl thiocyanate 3b (Scheme 4). The
trifluoromethyl thioether 4 was obtained via reaction
with TMSCF3,[6b] reactions with aryl and alkyl
Grignard reagents afforded thioethers 5 and 6, and
cycloaddition with sodium azide led to tetrazole 7,[5c]
which is a substructure widely employed as a
carboxylic acid bioisostere.[27]
a
c
b
d
4, 63%
6, 96%
5, 93%
7, 90%
[4] E. Hoggarth, W. A. Sexton, J. Chem. Soc. 1947, 0,
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3b
Scheme 4. Transformations of thiocyanate 3b. a) TMSCF3,
Cs2CO3. b) PhMgBr. c) MeMgCl. d) NaN3, ZnCl2.
In
conclusion,
the
transition-metal-free,
electrochemical thiocyanation of arylboronic acids
and derivatives provides a straightforward synthetic
entry to aryl thiocyanates. The underlying strategy
consists
of
electrochemically
generating
pseudohalogens that undergo facile nucleophilic
substitution reactions with organometallic reagents.
This may open up many synthetic opportunities for
other transformations currently achieved via Chan-
Evans-Lam-type mechanisms using stoichiometric
amounts of copper.
[7] a) K. Yamaguchi, K. Sakagami, Y. Miyamoto, X.
Jin, N. Mizuno, Org. Biomol. Chem. 2014, 12,
9200–9206; b) T. Castanheiro, M. Gulea, M.
Donnard, J. Suffert, Eur. J. Org. Chem. 2014, 7814–
7817.
[8] H. F. Gilbert, J. Am. Chem. Soc. 1980, 102, 7059–
7065.
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Chang, L. Shi, Chem. Commun. 2015, 51, 7180–
7183.
Experimental Section
Standard procedure for the ipso-thiocyanation of
arylboronic acids or aryltrifluoroborate salts
[10] a) J. W. Dienske, Recl. Trav. Chim. Pays-Bas 1931,
50, 407–414; b) I. P. Beletskaya, A. S. Sigeev, A. S.
Peregudov, P. V. Petrovskii, Mendeleev Commun.
2006, 16, 250–251.
[11] H. Suzuki, H. Abe, Synth. Commun. 1996, 26, 3413–
3419.
[12] H. Miyamoto, C. Sakumoto, E. Takekoshi, Y.
Maeda, N. Hiramoto, T. Itoh, Y. Kato, Org. Process
Res. Dev. 2015, 19, 1054–1061.
[13] a) J. Miller, A. J. Parker, J. Am. Chem. Soc. 1961,
83, 117–123; b) D. Landini, A. Maia, F. Montanari,
J. Chem. Soc., Perkin Trans. 2 1983, 0, 461–466.
[14] Methods using pre-formed electrophilic SCN
reagents: a) F. D. Toste, V. De Stefano, I. W. J. Still,
Synth. Commun. 1995, 25, 1277–1286; b) D. Wu, J.
Qiu, P. G. Karmaker, H. Yin, F.-X. Chen, J. Org.
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Liebigs Ann. Chem. 1919, 419, 217–322; d) E.
An oven-dried 20 mL electrolysis cell was charged with
0.5 mmol of 1, 5 mmol (if starting from boronic acids) or 2
mmol (if starting from trifluoroborates) of NH4SCN and 10
mL of solvent (see Table 2). The mixture was stirred for 5-
10 minutes to obtain a homogeneous solution. The
platinum sheet electrodes (2×1 cm) were inserted at a
distance of 0.5 cm, and the reaction mixture was stirred at
a constant current of 80 mA for 2 hours. Silica gel was
added to the resulting reaction mixture and the volatiles
were removed under vacuum. The resulting solid was
directly subjected to flash chromatography (SiO2,
cyclohexane/ethyl acetate gradient), yielding the
corresponding thiocyanate 3.
Acknowledgements
Funded by the Deutsche Forschungsgemeinschaft (DFG, German
Research Foundation) under Germany’s Excellence Strategy –
EXC-2033 – Projektnummer 390677874 and GO 853/11-1.
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10.1002/adsc.201900156
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10.1002/adsc.201900156
Advanced Synthesis & Catalysis
COMMUNICATION
Electrochemical ipso-Thiocyanation of Aryl
Boronates
Adv. Synth. Catal. Year, Volume, Page – Page
NH4 SCN
constant current
undivided cell
20 examples
Marco Dyga, Davit Hayrapetyan, Raja K. Rit,
Lukas J. Gooßen*
[B] = B(OH)2, BF3K
6
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