Sandmeyer trifluoromethylthiolation of arenediazonium salts with sodium thiocyanate and Ruppert-Prakash reagent

Article abstract of DOI:10.1039/c3sc53076k

In the presence of copper thiocyanate, sodium thiocyanate and the inexpensive, easy-to-use trifluoromethylating reagent Me₃Si-CF ₃, diazonium salts are smoothly converted into the corresponding aryl trifluoromethyl thioethers. Combin

Full text of DOI:10.1039/c3sc53076k

Showcasing research from Prof. Lukas J. Gooßen’s laboratory,
Technische Universität Kaiserslautern, Kaiserslautern, Germany.
As featured in:
Sandmeyer trifluoromethylthiolation of arenediazonium salts
with sodium thiocyanate and Ruppert-Prakash reagent
Trifluoromethyl thioethers are obtained directly from aryl
and heteroaryl diazonium salts, sodium thiocyanate and the
inexpensive, easy-to-use trifluoromethylating reagent Me3Si–CF3
in the presence of a copper thiocyanate catalyst. The preparative
utility of this Sandmeyer-type trifluoromethylthiolation is
demonstrated by the synthesis of 22 aryl and heteroaryl
trifluoromethyl thioethers bearing various functionalities from
the corresponding anilines.
See Lukas J. Gooßen, et al.,
Chem. Sci., 2014, 5, 1312.
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Sandmeyer trifluoromethylthiolation of
arenediazonium salts with sodium thiocyanate and
Cite this: Chem. Sci., 2014, 5, 1312
Ruppert–Prakash reagent†
*
Gregory Danoun, Bilguun Bayarmagnai, Matthias F. Gruenberg and Lukas J. Goossen
´
In the presence of copper thiocyanate, sodium thiocyanate and the inexpensive, easy-to-use
trifluoromethylating reagent Me3Si–CF3, diazonium salts are smoothly converted into the corresponding
aryl trifluoromethyl thioethers. Combined with diazotisation, this convenient and inexpensive method
allows the straightforward synthesis of aryl or heteroaryl trifluoromethyl thioethers from the
corresponding anilines.
Received 6th November 2013
Accepted 2nd January 2014
DOI: 10.1039/c3sc53076k
www.rsc.org/chemicalscience
electrophilic
processes
are
the
reactions
of
tri-
Introduction
uoromethanesulfonamides with aryl-magnesium or lithium
reagents reported by Billard et al. (Scheme 1, C)10 and the
copper-mediated reaction of arylboronic acids with hypervalent
iodine–SCF3 reagents by Lu and Shen (D).11 Nucleophilic tri-
uoromethylthiolations include, for example, the palladium-
catalysed triuoromethylthiolation of aryl halides with sensitive
AgSCF3 by Buchwald et al.,12 and the nickel-catalysed coupling
of aryl halides either with the similarly unstable Me4NSCF3 by
Zhang and Vicic,13 or with stable, but laborious-to-prepare
copper–triuoromethylthiolate complexes by Huang (E).14 C–H
functionalizations are exemplied by the copper-mediated
ortho-triuoromethylthiolation of benzamides with CF3S–SCF3
by Daugulis et al. (F).15 Oxidative triuoromethylthiolations
have been reported by Qing et al.,16,17 who treated boronic acids
with the Ruppert–Prakash reagent (TMSCF3) and sulfur in the
In recent years, methods for the introduction of uorine-con-
taining groups into organic molecules have attracted great
attention within organic synthesis, as they can impart desirable
properties to bioactive compounds.1 Substantial progress
has recently been achieved in the eld of late-stage tri-
uoromethylations,2
whereas
the
corresponding
tri-
uoromethylthiolations are less developed.3 In general,
triuoromethylthio groups induce even higher lipophilicity
than triuoromethyl substituents (Hansch constant p ¼ 1.44
versus 0.88),4 and are more bulky. This allows a more effective
transport of drug molecules through lipid membranes, thereby
increasing their bioavailability. Thus, SCF3 groups are oen
seen as key functionalities of many pharmaceutical and agro-
chemical products, such as tiorex, toltrazuril (Baycox®) or
vaniliprole.5
Traditional strategies
As shown in Scheme 1, several access routes exist for the
formation of triuoromethyl thioethers. Traditional strategies
for the introduction of SCF3 groups include halogen–uorine
exchange reactions of trihalogenomethyl thioethers (A),6 as well
as triuoromethylations of sulfur-containing compounds such
as thiols,7 thiocyanates8 and disuldes,9 all of which have to be
synthesised in additional steps (B). More modern, one-step
triuoromethylthiolation methods can be divided into four
main categories: electrophilic (C and D), nucleophilic (E) and
radical (F), as well as oxidative cross-couplings (G). Examples of
¨
FB Chemie-Organische Chemie, TU Kaiserslautern, Erwin-Schrodinger-Str. Geb. 54,
D-67663 Kaiserslautern, Germany. E-mail: goossen@chemie.uni-kl.de; Fax: +49 631
205 3921
† Electronic supplementary information (ESI) available: procedural and spectral
data. See DOI: 10.1039/c3sc53076k
Scheme
groups.
1
Strategies for the introduction of trifluoromethylthio
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Table 1 Optimisation of the reaction conditionsa
presence of CuSCN and silver carbonate (G). Very recently,
Zhang and Vicic developed
a
similar oxidative tri-
uoromethylthiolation using Me4NSCF3 as the source of SCF3.18
Although the above approaches provide viable routes for the
formation of triuoromethyl thioethers, they all entail short-
comings, such as the laborious multi-step preparation of
starting materials or the use of expensive, air-sensitive or poorly
available reagents. As an alternative, we present a cheap and
straightforward synthesis of triuoromethyl thioethers via a
Sandmeyer-type reaction.
Entry
Sulfur source
Additive
Solvent
Yield of 2b [%]
1c
2c
3c
4
5
6
7
8
9
10
11
12d
13e
14f
15g
S8
CsF
CsF
CsF
CsF
CsF
CsF
Cs2CO3
Cs2CO3
Cs2CO3
—
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
DMF
Acetone
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
5
Lawesson's reagent
Na2S
NaSCN
KSCN
Traces
0
30
Traces
Traces
98
81
18
0
0
0
34
98
67
Sandmeyer trifluoromethylthiolation
NH4SCN
NaSCN
NaSCN
NaSCN
NaSCN
—
NaSCN
NaSCN
NaSCN
NaSCN
In the context of our work on new triuoromethylation reac-
tions,19,20 we have developed an effective synthesis of benzotri-
uorides via a Sandmeyer reaction.21 The key advantage of this
reaction over related processes22 is that the Cu–CF3 reagents are
generated in situ from simple triuoromethyl silanes or borates.
An analogous reaction concept, in which easily accessible dia-
zonium salts are converted into the corresponding tri-
uoromethyl thioethers via a redox-neutral reaction involving a
nucleophilic CF3 reagent in combination with a sulfur source,
appeared to be a plausible and attractive way of introducing
triuoromethylthio groups (Scheme 2, bottom). Clark et al. have
shown the principal feasibility of Sandmeyer tri-
uoromethylthiolations starting from preformed CuSCF3.
However, they found that their laboriously prepared CuSCF3
complex transfers its SCF3 group only reluctantly to diazonium
salts, so that only a few electron-poor aryl triuoromethyl thi-
oethers could be accessed in reasonable yields.23
a
Reaction conditions: 0.5 mmol CuSCN, 2 equiv. additive, 1.5 equiv.
sulfur source, 2 mL solvent, RT, dropwise addition of 0.5 mmol 1 in
b
2
mL of solvent, then
2
equiv. TMSCF3, 12 h.
Yields were
determined by 19F NMR using 1,3-diuorobenzene as an internal
c
d
e
standard. TMSCF3 added before 1. Without CuSCN. 1 equiv.
f
g
Cs2CO3. 0.5 equiv. CuSCN. 0.1 equiv. CuSCN. Lawesson's reagent ¼
2,4-bis(4-methoxyphenyl)-1,3,2,4-dithiadiphosphetane-2,4-dithione.
directly be converted into the corresponding aryl tri-
uoromethyl thioethers (Scheme 2, top). However, this
appeared to be merely a theoretical possibility, since nucleo-
philic CF3 sources are known to react smoothly with copper
salts.2e,19–21 Thus, one would expect that any triuoromethylat-
ing reagent capable of substituting a cyano group in an aryl
thiocyanate would also react with CuSCN intermediates to give
unwanted Cu–CF3 or Cu–SCF3 species. Nevertheless, we were
Simply combining Clark's process with the in situ formation of
the Cu–SCF3 reagents from a triuoromethylating and a sulfu-
rising agent thus did not appear to be a promising strategy
towards
a one-step triuoromethylthiolation process. This
assumption was supported by a series of test experiments (Table 1,
entries 1–3).
While searching for another straightforward strategy to
introduce triuoromethylthio groups, we reasoned that it
should be advantageous to rst connect the aryl-C–S and then
the S–CF3 bonds. In search for a viable reaction pathway, we
struck upon a report by Langlois et al. in which they demon-
strated that aryl thiocyanates can be triuoromethylated with
Ruppert–Prakash's reagent.8 We reasoned that if we performed
a Sandmeyer thiocyanation24 in the presence of a nucleophilic
triuoromethylation reagent, the arenediazonium salts might
intrigued by the prospects offered by
uoromethylthiolation process and decided to evaluate its
feasibility.
a one-pot tri-
Results and discussion
Development of a Sandmeyer triuoromethylthiolation
We systematically investigated the reaction of 4-methoxy-
benzenediazonium tetrauoroborate with sodium thiocyanate
and TMS–CF3 as a model system in the presence of various
copper catalysts (see ESI†). As expected, anisole and 4-methoxy-
benzotriuoride were formed in most cases, while the desired
triuoromethyl thioether 2 was only a minor product. However,
when slowly adding the diazonium salt 1 and TMSCF3 to a
mixture of sodium thiocyanate, CuSCN and CsF in acetonitrile,
the desired triuoromethyl thioether 2 was obtained in an
encouraging 30% yield, along with a residual aryl thiocyanate
intermediate (Table 1, entry 4). Under these conditions, the
formation of 4-(triuoromethyl)anisole was no longer observed.
Scheme 2 Approaches to Sandmeyer trifluoromethylthiolations.
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Table 2 Scope of the trifluoromethylthiolation of arenediazonium
Further experiments revealed that sodium thiocyanate is the
most effective reagent, whereas many other thiocyanate salts
suppressed the subsequent triuoromethylation step (entries 5
and 6). A decisive step-up in the yields was achieved when
replacing CsF, which is commonly used to activate TMSCF3,
with Cs2CO3 (entry 7). Acetonitrile was conrmed to be the most
effective solvent, which corresponds well with the ndings for
other Sandmeyer reactions (entries 8 and 9).25
saltsab
Control experiments revealed that the reaction does not
proceed if the copper mediator, the basic additive or sodium
thiocyanate are omitted (entries 10–12). Reducing the amount
of base to one equivalent led to decreased yields (entry 13). Even
when the amount of copper was reduced to 50 mol%, the aryl
triuoromethyl thioether 2 was formed in a near-quantitative
yield, with traces of the aryl thiocyanate as the only detectable
by-product (entry 14). Reasonable yields were obtained even
with only 10 mol% of copper (entry 15), which suggests that
future catalyst generations will allow the metal loading to be
lowered to truly catalytic amounts.
Scope of the new transformation
Having thus found an effective protocol for the tri-
uoromethylthiolation of arenediazonium salts, we next inves-
tigated its scope. As can be seen from the examples in Table 2,
various arenediazonium tetrauoroborates were smoothly
converted into the corresponding aryl triuoromethyl thio-
ethers in moderate to excellent yields. In contrast to the reaction
of diazonium salts with preformed CuSCF3, the new process is
in no way limited to strongly electron-decient derivatives.
Common functionalities including ester, ether, amino, keto,
carboxylate and cyano groups were tolerated. Substrates con-
taining chloro, bromo or even iodo substituents were tri-
uoromethylthiolated selectively at the position of the
diazonium group. Various heterocycles such as quinoline,
thiophene, benzothiazole and carbazole were also smoothly
converted. Most products were directly obtained in a sufficiently
pure form to allow their straightforward isolation. Only in a few
cases did traces of the protodediazotisation products compli-
cate the purication of the crude products. These results
demonstrate the utility of the new reaction for the late-stage
a
Reaction conditions: 1 mmol of arenediazonium tetrauoroborate in
4 mL MeCN and 2 equiv. of TMSCF3 were slowly added to 0.5 equiv.
CuSCN and 2 equiv. Cs2CO3 bin 4 mL MeCN and stirred for 12 h at RT.
Isolated yields are noted. Yields determined by 19F NMR using
1,3-diuorobenzene as an internal standard.
triuoromethylthiolation
intermediates.
of
highly
functionalised
Cs2CO3 in MeCN in the absence of a nucleophilic CF3 source.
Moreover, preformed aryl thiocyanate quickly reacted to the
corresponding aryl triuoromethyl thioether 2 in the presence
of a mixture of TMSCF3 and Cs2CO3, a process that does not
Mechanistic investigations
In order to gain a deeper mechanistic understanding, the require copper. Thus, a Sandmeyer triuoromethylthiolation
reaction was investigated by 19F NMR. Mixtures of CuSCN, pathway involving CuSCF3 species was ruled out. Based on
TMS–CF3 and Cs2CO3 in MeCN were found to contain CuCF3 these results, we propose a mechanistic cycle, as depicted in
(ꢀ28.1 ppm) and [Cu(CF3)2]ꢀ (ꢀ31.2 ppm), but no CuSCF3 Scheme 3.
species.26 This indicates that the SCNꢀ anion does not react
The diazonium salt is initially converted into the thiocyanate
with TMS–CF3 under the reaction conditions. When NaSCN was via a Sandmeyer process, in which the CuISCN species rst
added to the above mixture, the formation of CF3–copper transfers a single electron to the diazonium salt (I). The
species was no longer observed. This explains why no tri- resulting diazo radical II releases nitrogen gas with the forma-
uoromethylation products are obtained under the optimised tion of an aryl radical III, which takes up the thiocyano group
conditions. Further experiments conrmed that 4-methoxy- from the copper(II) intermediate to form the aryl thiocyanate IV.
benzenediazonium tetrauoroborate (1) is smoothly converted The presence of intermediate radicals was conrmed by the
into the aryl thiocyanate by treatment with CuSCN, NaSCN and nding that the addition of TEMPO resulted in strongly
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washed with water (10 mL) and brine (10 mL). The organic layer
was dried over MgSO4, ltered and concentrated (700 mbar,
40 ꢁC). The residue was further puried by ash chromatog-
raphy (SiO2, diethyl ether–hexane gradient), yielding the corre-
sponding triuoromethyl thioethers.
Acknowledgements
¨
We thank the Landesgraduiertenforderung Rheinland Pfalz and
Nanokat for nancial support, and Umicore for donating metal
catalysts.
Notes and references
Scheme 3 Proposed mechanism.
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In conclusion, we have developed a straightforward, inexpen-
sive and expedient method for the regiospecic conversion of
arenediazonium salts into the corresponding aryl tri-
uoromethyl thioethers. The reaction is broadly applicable to
electron-rich and electron-poor arene- and hetero-
arenediazonium salts and tolerates various functional groups.
The availability of the substrates from the large pool of aromatic
amines, the use of inexpensive reagents and the mild reaction
conditions make this reaction particularly attractive for various
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Experimental section
The standard procedure for the synthesis of triuoromethyl
thioethers from the corresponding arenediazonium salts is as
follows. Under a nitrogen atmosphere, an oven-dried 20 mL
crimp cap vessel with a Teon-coated stirrer bar was charged
with copper thiocyanate (61.4 mg, 0.50 mmol), caesium
carbonate (652 mg, 2.00 mmol) and sodium thiocyanate
(122 mg, 1.50 mmol). Acetonitrile (4 mL) was added via syringe
and the resulting suspension was stirred at room temperature
for 10 minutes. A solution of the arenediazonium tetra-
uoroborate (1.00 mmol) in acetonitrile (4 mL) was added
dropwise via syringe and the reaction mixture was stirred for
another 10 minutes. Triuoromethyl-trimethylsilane (321 mL,
2.00 mmol) was then added via syringe and the mixture was
stirred at ambient temperature for 16 h. The resulting mixture
was ltered through a short pad of Celite (5 g) and rinsed with
diethyl ether (20 mL). The resulting organic solution was
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