Transition-metal-free cross-coupling of thioethers with aryl(cyano)iodonium triflates: A facile and efficient method for the one-pot synthesis of thiocyanates

Article abstract of DOI:10.1039/c5cc00875a

A novel transition-metal-free cross-coupling method for the one-step synthesis of thiocyanates via the C-S bond cleavage of readily available thioethers with aryl(cyano)-iodonium triflates as the cyanating agent is developed. This process features relatively broad substrate scopes, less-toxic hypervalent iodine reagents, mild operating conditions, excellent functional group compatibilities, and affords various thiocyanates in moderate to good yields. This journal is

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DOI: 10.1039/C5CC00875A
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Transition-metal-free cross-coupling of thioethers with
aryl(cyano)iodonium triflates: a facile and efficient
method for the one-pot synthesis of thiocyanates
Cite this: DOI: 10.1039/x0xx00000x
Received 00th January 2012,
Accepted 00th January 2012
Dan Zhu, Denghu Chang and Lei Shi*
DOI: 10.1039/x0xx00000x
www.rsc.org/
A novel transition-metal-free cross-coupling method for the
one-step synthesis of thiocyanates via the C S bond
−
cleavage of readily available thioethers with aryl(cyano)-
iodonium triflates as the cyanating agent is developed. This
process features relatively broad substrate scopes, less-toxic
hypervalent iodine reagents, mild operating conditions,
excellent functional group compatibilities, and affords
various thiocyanates in moderate to good yields.
Organic thiocyanates are wellꢀknown in the area of organosulfur
chemistry and have attracted considerable attention, since they are
not only the substructures or building blocks of bioactive natural
products1 and pesticides,2 but also the versatile synthetic precursors
and useful intermediates for the synthesis of sulfurꢀcontaining
compounds such as thiols,3 thioethers,4 disulfides,5 trifluoromethyl
sulfides6 and thiocarbamates.7 On the one hand, thiocyanates can
usually be obtained by the nucleophilic or electrophilic thiocyanation
of the corresponding organic molecules with stoichiometric amounts
of inorganic thiocyanates (Scheme 1, eqn. (1)),8 which commonly
requires more careful control of the reaction conditions to avoid the
undesirable formation of isothiocyanates. On the other hand, the
cyanation of sulfurꢀcontaining compounds, such as thiol derivatives
(Scheme 1, eqn. (2)9 and (3)10) and arylsulfinate salts (Scheme 1,
eqn. (4)),11 provides a beneficial complement and perfection for the
aforementioned transformation. However, most of these cyanation
methods are confronted with various drawbacks and limitations,
including the utilization of highly poisonous and notorious cyanating
agents (e.g. KCN, CuCN, Zn(CN)2 and TMSCN), the preꢀ
functionalization or preꢀactivation of substrates and harsh reaction
conditions.
Scheme 1 Existing methods to prepare aryl thiocyanates.
In the past few years, great achievements have been made in the
efficient introduction of C≡N moieties via alternative cyano
sources12 into molecules. More recently, Wang and coꢀwokers13
reported an efficient method for iron(II)ꢀcatalyzed direct cyanation
of arenes with aryl(cyano)ꢀiodonium triflates as the cyanating
agents, which were developed by Stang and coꢀworkers.14 Herein,
we report a oneꢀstep, metalꢀfree synthetic route to the synthesis of
thiocyanates between readily available thioethers and aryl(cyano)ꢀ
iodonium triflates as the cyano source under mild conditions
(Scheme 1, eqn.
(5)). This protocol offers several advantages of relatively broad
substrate scopes and the utilization of lessꢀtoxic hypervalent iodine
reagents as the cyanide source. To the best of our knowledge, the
present study is the first example of accessing the thiocyanates via
the unusual C−S bond cleavage of thioethers under metalꢀfree
condition, although the cleavage of C−S bonds has been extensively
studied with a vast range of transitionꢀmetal catalysts.15
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DOI: 10.1039/C5CC00875A
Table 1 Optimization of the reaction conditionsa
Table 2 Scope of thioether derivativesa,b
Solvent
(1mL)
Cyanating
agent
Entry
Yieldb (%)
1
2
3
4
5
6
DMF
THF
2a
2a
2a
2a
2a
2b
0
0
S
S
S
S
S
CN
CN
CN
CN
CN
CN
F
Cl
Toluene
DCM
65
69c
77
93
3b 88%
3e 43%
3a 93%
3d 62%
3c 75%
3f 59%
S
S
O2N
Br
NC
CH3CN
CH3CN
S
S
S
CN
CN
CN
CN
CN
OMe
O
O
3h 57%
3g 20%
3j 69%
3i 71%
S
S
F
CN
MeO
a The reactions were conducted with 0.5 mmol of 1a and 0.75
mmol of 2a-b for 12 h. b Isolated yields. c Stirred at 40 oC.
3k 42%
3l 71%
Cl
We initiated our studies by investigating the reaction of
thioanisole (1a) and 1.5 equiv of 4ꢀfluorophenyl(cyano)iodonium
triflate (2a) in various commonly used organic solvents for 12 h
under nitrogen atmosphere. To our delight, it was found that with
CH3CN as the solvent, the reaction provided a 77% yield of the
desired product (Table 1, entry 5). In contrast, none of the expected
product was formed in DMF and THF (Table1, entries 1 and 2), or
relatively low yields of product were obtained in toluene and DCM
(Table 1, entries 3 and 4). Further improvement in the yield was
achieved by replacing 4ꢀfluorophenyl(cyano)iodonium triflate (2a)
with 3,5ꢀdi(trifluoromethyl)phenyl(cyano)iodonium triflate (2b) as
the cyanating agent (Table1, entry 6). Consequently, the reaction
condition described in entry 6 was selected as the standard condition
for investigation of the reaction scope.
Cl
S
S
Cl
Cl
S
CN
CN
CN
Cl
Cl
F
Cl
3n 59%
3m 63%
3o 6%
S
S
CN
CN
CN
CN
S
( )9
CN
S
S
S
S
N
3q 68%
3p 36%
3r 22%
CN
H2N
OHC
HO
3u 0%
3t 0%
3s 0%
a
The reactions were conducted with 0.5 mmol of 1a-u and 0.75
mmol of 2b for 12 h. b Isolated yields.
To assess the ability and efficiency of Csp3−S bond cleavage in
this protocol, various alkyl phenyl sulfides, such as ethyl, isopropyl,
benzyl, cyclopropane and phenyl sulfides were subsequently reacted
with 3,5ꢀdi(trifluoromethyl)phenyl(cyano)iodonium triflate (2b)
under the aboveꢀoptimized reaction condition, and the results are
summarized in Table 2. In all cases studied, the reaction proceeded
smoothly to afford the corresponding thiocyanates 3a and 3i in
moderate to excellent yields under metalꢀfree condition.
With the highly efficient condition in regard to the cleavage
of S−CH3 bond in hand, we next commit to explore the scope
of this reaction with respect to a variety of thioether derivatives.
As shown in Table 3, the thioanisole derivatives bearing
electronꢀdonating substituents, gave slightly better yields than
a The reactions were proceeded in the standard condition.
b Isolated yields. c The yield was calculated based on one side.
Scheme 2 Diversity in C−S bond cleavagea,b
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those bearing electronꢀwithdrawing substituents, such as nitryl preparation of other useful sulfurꢀcontaining compounds are still
3e), cyano (3f). Gratifyingly, diverse functional groups, underway in our laboratory.
including halides (3b 3c and 3d), cyano (3f), acetyl (3g), ester The work was supported by the “Hundred Talents Program” of
3h), nitro (3e), alkyl (3i) and alkoxy (3j), were very well Harbin Institute of Technology (HIT), the NSFC (21202027), the
(
,
(
tolerated under the optimal reaction condition, furnishing the NCET (NCETꢀ12ꢀ0145), and the “Technology Foundation for
desired products in generally moderate to excellent yields. Selected Overseas Chinese Scholar” of Ministry of Human
However, thioanisoles containing an active hydrogen atom such Resources and Social Security of China (MOHRSS).
as OH, NH2, or aldehyde did not undergo the same reactions
( , 3t and 3u). Furthermore, the position of attached
3s
substituent groups on the phenyl ring did not affect the
efficiency of this transformation (3b 3l and 3m). In addition to Interdisciplinary Science, Harbin Institute of Technology, Harbin 150080,
Notes and references
Institute of Organic Chemistry, The Academy of Fundamental and
,
P. R. China. Tel: +86ꢀ451ꢀ86403860; Eꢀmail: lshi@hit.edu.cn
Electronic Supplementary Information (ESI) available: Detailed
synthetic procedures and characterization of new compounds. See
DOI: 10.1039/c000000x/
monoꢀsubstituted thioanisoles, bisꢀsubstituted thioanisole was
also compatible with this novel reaction, generating the
corresponding thiocyanate (3n) in 59% yield. It was noteworthy
†
that
a
miserable yield of 6% was observed when
1o was employed as substrate,
pentachlorothioanisole
(
)
1
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with 3,5ꢀdi(trifluoromethyl)phenyl(cyano)iodonium triflate (2b
)
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yields, respectively. Finally, we also detected this reaction with
alkyl thioethers under the standard condition, and the expected
thiocyanate (3p) could be isolated in a relatively low yield of
36%.
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Scheme 3 Proposed mechanism for the reaction.
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On the basis of a previous report by Sanford and coꢀworkers,16
a
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possible mechanism is depicted in Scheme 2. Initially, the oxidation
of thioethers with aryl(cyano)ꢀiodonium triflates gives rise to the
corresponding sulfonium salts A and aryl iodides. After subsequent
nucleophilic substitution of A with triflate (OTfꢀ), the expected
thiocyanate products could be released with alkyl triflate as
byproduct to finish the reaction. To prove our mechanistic
hypothesis, we repeated the model reaction (Table 1, entry 6), and to
our delight, the existence of MeOTf (R2 = Me) and 1ꢀiodoꢀ3,5ꢀ
bis(trifluoromethyl)benzene could been observed in the crude
mixture by GCꢀMS analysis. Furthermore, when the reaction of
thioanisole (1a) and 3,5ꢀdi(trifluoromethyl)phenyl(cyano)iodonium
triflate (2b) was carried out in CDCl3, we could also observe the
generation of MeOTf and 1ꢀiodoꢀ3,5ꢀbis(trifluoromethyl)benzene by
13C NMR analysis. These observations lend very strong support to
our mechanistic proposal.
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In summary, we have developed
a
transitionꢀmetalꢀfree
transformation of readily available thioethers into the corresponding
thiocyanates through the Csp3−S bond cleavage. This process
features relatively broad substrate scopes, lessꢀtoxic hypervalent
iodine reagents, mild operating conditions, excellent functional
group compatibilities, and affords various thiocyanates in moderate
to good yields. Notably, this protocol is not only complementary to
existing methods of thiocyanate formation, but also offers an
opportunity to extend the applications of lessꢀtoxic hypervalent
iodine reagents in organic synthesis. Further investigations on
related reactions involving hypervalent iodine reagents for the
9
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