Direct catalytic trifluoromethylthiolation of boronic acids and alkynes employing electrophilic shelf-stable N-(trifluoromethylthio)phthalimide

Article abstract of DOI:10.1002/anie.201307484

A new and safe method for the synthesis of N-(trifluoromethylthio) phthalimide, a convenient and shelf-stable reagent for the direct trifluoromethylthiolation, has been developed. N-(Trifluoromethylthio) phthalimide can be used as an electrophilic source of F₃CS ⁺ and reacts readily with boronic acids and alkynes under copper catalysis. The utility of CF₃S-containing molecules as biologically active agents, the mild reaction conditions employed, and the high tolerance of functional groups demonstrate the potential of this new methodology to be widely applied in organic synthesis as well as industrial pharmaceutical and agrochemical research and development. Shelf life: A new and safe method for the synthesis of N-(trifluoromethylthio)phthalimide has been developed. It serves as a convenient and shelf-stable reagent for the direct copper-catalyzed trifluoromethylthiolation of readily available boronic acids and alkynes. Copyright

Full text of DOI:10.1002/anie.201307484

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Communications
DOI: 10.1002/anie.201307484
Synthetic Methods
Hot Paper
Direct Catalytic Trifluoromethylthiolation of Boronic Acids and
Alkynes Employing Electrophilic Shelf-Stable N-(trifluoro-
methylthio)phthalimide
Roman Pluta, Pavlo Nikolaienko, and Magnus Rueping*
Abstract: A new and safe method for the synthesis of N-
(trifluoromethylthio)phthalimide, a convenient and shelf-
stable reagent for the direct trifluoromethylthiolation, has
been developed. N-(Trifluoromethylthio)phthalimide can be
used as an electrophilic source of F3CS+ and reacts readily with
boronic acids and alkynes under copper catalysis. The utility of
CF3S-containing molecules as biologically active agents, the
mild reaction conditions employed, and the high tolerance of
functional groups demonstrate the potential of this new
methodology to be widely applied in organic synthesis as
well as industrial pharmaceutical and agrochemical research
and development.
of aryl halides and alkyl halides with these reagents has been
known for a long time.[8] Recent studies described the
palladium-catalyzed cross-coupling reaction between aryl
halides and AgSCF3/R4NI.[6] This reaction was also performed
with Me4NSCF3 in the presence of a nickel catalyst.[9] We
recently described a copper-catalyzed trifluormethylthiola-
tion of vinyl halides to yield various vinyl trifluoromethyl
thioethers employing Me4NSCF3.[10]
Daugulis and co-workers[11] recently reported a protocol
À
for the copper-catalyzed direct sulfenylation of aromatic C H
bonds with auxiliary directing groups using disulfides. In
contrast, Zhang and Qing[12] have described the oxidative
trifluoromethylthiolation of terminal alkynes and aryl bor-
onic acids starting from elemental sulfur and the Ruppert–
Prakash reagent. This methodology does not require the
existence of a preformed CF3S moiety in the reagent. The
above-mentioned protocols employ all nucleophilic CF3S
sources, which imply the use of an oxidant or the use of
electrophilic substrates, thus making the range of direct
trifluoromethylthiolation limited.[6–13]
Compounds containing perfluoroalkylated groups have
received increased attention because of their widespread
biological and therapeutic properties.[1] These substituents
have been involved in the manufacturing processes of block-
buster drugs and highly effective agrochemicals. Among the
enormous range of fluorinated compounds known to date,
those containing perfluorinated alkylthiogroups are valuable
for the pharmaceutical and agrochemical industries.[2] Such
compounds possess unique and important physical, chemical,
and biological properties compared to the parent nonfluori-
nated ones. The most prominent among them is the trifluor-
omethylthio group (CF3S). It plays an important role because
of its strong electron-withdrawing effects and high lipophi-
licity, which increases the ability to cross lipid membranes and
create opportunities for the modification of known and new
drugs.[3] Additionally, trifluoromethylthioethers (CF3SR) are
also key intermediates in the preparation of trifluoromethyl-
sulfoxides and sulfones.[4] Methodologies to synthesize com-
plex organic molecules containing these functional groups are
now vital for both academic and industrial research.[5]
Unfortunately, simple approaches for the direct incorporation
of a CF3S group into organic molecules are still limited, and
therefore the development of safe and efficient method-
ologies is highly desirable.
The simplest electrophilic CF3S reagent, gaseous
CF3SCl,[14] is toxic and corrosive and thus not safe to use.
Shelf-stable, easy-to-handle reagents have been recently
developed. One was described by Billard and Langlois[15] (1;
Scheme 1), the second by Lu and Shen[16] (2) and third by
Shibata[17] (3). These reagents have been employed for the
functionalization of alkynes, boronic acids, Grignard reagents,
or b-ketoesters and enamines. N-(trifluoromethylthio)phthal-
imide (4) is a known compound which acts as an electrophilic
source for the trifluoromethylthio group. The synthesis of 4
was initially developed by Munavalli et al.[18] but to date only
a very limited use of this reagent has been shown. Moreover,
its previously described synthesis requires the use of the
unfriendly and difficult-to-work with trifluoromethylsulfenyl
chloride.
Since the field of direct electrophilic trifluoromethyl-
thiolation reactions and the synthetic utility of such trans-
formations is still limited, we decided to disclose a more
efficient pathway toward 4, examine its properties, and
Several methodologies for the direct trifluoromethythio-
lation have been described. Nucleophilic trifluoromethylth-
iolation reagents including AgSCF3 and CuSCF3 are known
and well studied. They can easily be prepared from silver
fluoride[6] and after metal exchange with CuBr.[7] The reaction
[*] M. Sc. R. Pluta, M. Sc. P. Nikolaienko, Prof. Dr. M. Rueping
Institute of Organic Chemistry, RWTH Aachen University
Landoltweg 1, 52074 Aachen (Germany)
E-mail: Magnus.Rueping@rwth-aachen.de
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201307484.
Scheme 1. Electrophilic trifluoromethylthiolating agents.
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Table 1: Stability of 3[a] in different solvents and in the presence of
additives.
Entry Solvent
d(19F)
3d
7d
[ppm]
(RT)
(RT)
(12 h, 508C)
1
[D6]DMSO
À48.6
À48.6
À48.6
À49.7
À49.0
n.c.
n.c.
n.c.
n.c.
n.c.
n.c.
n.c.
n.c.
n.c.
n.c.
n.c.
–
–
n.c.
n.c.
2[b]
3[c]
4[d]
5[d]
6
[D6]DMSO
[D6]DMSO
[D7]DMF
CDCl3
CDCl3/DBU
À49.0 dec.: 40%[e] dec.[f]
–
n.c.
Scheme 2. Electrophilic trifluoromethylthiolating agents.
7
8
9
CDCl3/lutidine À49.0
n.c.
n.c.
n.c.
n.c.
n.c.
n.c.
n.c.
n.c.
–
–
–
n.c.
n.c.
n.c.
n.c.
–
CDCl3/Et3N
CDCl3/D2O
[D8]toluene
[D8]THF
À49.0
À49.0
À44.5
À50.6
À50.4
À50.1
À51.3
dec.: <5%
n.c.
10[d]
11[d]
12[d]
13[d]
14
n.c.
n.c.
n.c.
n.c.
explore its synthetic utility in direct trifluoromethylthiola-
tions of alkynes and boronic acids.
[D6]acetone
CD3CN
CD3OD
To develop an efficient synthesis of 4 we decided to
perform a series of studies. The cyclic voltammogram of N-
chlorophthalimide showed a reduction peak at À0.45 V
versus SCE in acetonitrile whereas the nucleophilic trifluoro-
methylthio reagents AgSCF3 and CuSCF3 were oxidized at
potentials of + 0.37 V and + 0.81 V, respectively. This data
showed the possibility of a redox transformation of formal
F3CSÀ to F3CS+ and reduction of formal Cl+ to ClÀ (see the
Supporting Information for spectra). Therefore, we decided
to firstly explore the reaction of N-chlorophthalimide with
AgSCF3 in acetonitrile (Scheme 2).
However, in this case, the expected redox reaction turned
out to be quite challenging because of the formation of
a substantial amount of F3CSSCF3 (5) as the main product,
thus suggesting a radical mechanism. Evaluation of various
reaction conditions such as solvent and temperature did not
show any improvement. Interestingly, when CuSCF3 in
acetonitrile was used, the desired product was formed in
high yield with almost no formation of 5; approximately 3%,
as determined by 19F NMR analysis of the crude reaction
mixture. This result might be explained by a nonradical
n.c.
[a] NMR experiment conditions: 4 (9.25 mg, 0.037 mmol) and ethyl
trifluoroacetate (internal standard, 5.3 mg, 0.037 mmol) in 0.75 mL of
appropriate solvent. Final concentration of reagent was 50 mm.
Decomposition as detected by signals with respect to the internal
standard. [b] 21.09 mg (5 equiv) of TFA was added. [c] 11.1 mg
(15 equiv) of D2O was added. [d] (Trifluoromethyl)benzene (5.5 mg,
0.037 mmol) was used as an internal standard. [e] 40% decomposition
after 15 min. [f] Complete decomposition after 24 h. DBU=1,8-
diazabicyclo[5.4.0] undec-7-ene, DMF=N,N-dimethylformamide,
DMSO=dimethylsulfoxide, n.c.=no change, dec.=decomposition,
THF=tetrahydrofuran.
3 h) showed no signs of decomposition. The presence of
transition-metal ions (e.g. Cu+, Cu2+, Fe2+, Fe3+, Pd2+) also
does not decompose 4.
To our delight, 4 was totally inactive toward acidic
conditions (e.g. AcOH, MsOH, TsOH, TfOH) and the
presence of water. In contrast, 4 can be sensitive to strong
bases. In the presence of weak bases such as lutidine there was
no decomposition, and Et3N as a moderate base gave less than
5% decomposition after 12 hours at 508C. In the presence of
a strong base such as DBU, 4 decomposed in short time.
Noteworthy, by 19F NMR spectroscopy we realized that
during this process new species were formed: F3CSSCF3
(d = À45.6 ppm), F3CSOH (d = À5.2 ppm), F3CSO2H (d =
À
pathway. Insertion of CuSCF3 into the N Cl bond could form
a copper(III) species, which undergoes reductive elimination
to provide the desired product (Scheme 2). There is no
evidence for the formation of the amino/CuIII/SCF3 species,
but Dobbie and Emeldus reported the preparation of
trifluoromethylthio(bistrifluoromethylamino)mercury,[19]
thus showing the possibility to form aminometallic SCF3
species and indicating that such a copper intermediate could
be feasible.
À
À81.8 ppm), and the corresponding anion F3CSO2 (d =
À86.1 ppm).[20]
Subsequently, the reaction was scaled up to over 5.0 grams
and the desired product 4 was isolated as colorless crystals in
90% yield after flash chromatography on silica gel and fully
characterized by spectroscopic techniques. The compound 4
melts without decomposition at 115–1168C (lit. 115–1178C)
and could be easily sublimed at 808C/1 mbar. To gain
evidence for the potential utility and possible limitations of
this reagent, we investigated its stability. Accordingly, 4 is not
light sensitive and it is stable in both the solid state and in
solution. NMR studies showed no decomposition of the
reagent under aerobic conditions in solvents such as
[D6]DMSO, [D7]DMF, CDCl3, [D8]toluene, [D8]THF,
[D6]acetone, CD3CN, and CD3OD (Table 1). Standing in
solution at room temperature for seven days, or heating of the
solutions at 508C for 12 hours (DMF and toluene at 1008C/
With this shelf-stable electrophilic trifluoromethylthiolat-
ing reagent 4 in hand, we started to test its usefulness in
reaction with representative C nucleophiles such as boronic
acids and terminal acetylenes. Initial attempts showed that
there was no product formation when heating in diglyme or
DCE (see the Supporting Information, and Tables 2 and 3)
Hence, we turned our attention to copper catalysis which is
widely applied in transformations involving boronic acids and
alkynes. We observed that the reactions proceeded only in the
presence of copper(I) salts whereas copper(II) salts showed
no activity. Further examination of various commercially
available copper sources revealed that CuCl and CuI were the
most appropriate for the reaction with boronic acids and only
CuI showed good catalytic activity for the functionalization of
alkynes. Interestingly, the ligand had a strong influence on the
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reactivity of the copper catalyst. The unsubstituted bipyridine
was effective, and the trifluoromethylthiolated products were
obtained in high yields. However, when different substituted
bipyridines and phenantrolines were applied, yields dramat-
ically dropped. Other factors such as solvent, base and
temperature also had an influence on the reaction outcome
and were successfully optimized (see the Supporting Infor-
mation). With the optimized reaction conditions in hand, 4
was used with a broad range of boronic acids as well as
alkynes (Tables 2 and 3).
All tested arylboronic acids (6a–h and 6l,m) reacted with
4 and the corresponding trifluoromethylthiolated arenes 7a–h
and 7l,m were obtained in good to very high yields (54–95%,
Table 2). Of note, the reaction conditions were tolerant to
different substituents. When o-substituted boronic acids were
used, no significant steric hindrance effect was found and the
products were isolated in good yield. A slight decrease in the
yield was observed when o-iodoboronic acid was used as
compared to the o-bromo derivative. Also, the vinyl boronic
acids 6i–k could be used and the corresponding trifluoro-
methlythioethers 7i–k were obtained in good yield.[10]
The terminal alkynes 8 also reacted with 4, but to achieve
good conversion, the reaction mixture needs to be heated up
to 808C for 15 hours (see Table 3 and the Supporting
Information). Under the optimized reaction conditions we
examined the substrate scope of this transformation (Table 3).
Both electron-rich and electron-deficient terminal alkynes
could be converted into their corresponding alkynyl trifluor-
omethyl sulfides 9 in good yields. The optimized reaction
conditions allowed the transformation of various terminal
alkynes containing a wide range of functional groups, includ-
ing amino, ester, keto, nitro, ethers, heterocyclic, and aliphatic
compounds. Additionally, a bromide substituent was compat-
ible with the reaction conditions, which offers the opportunity
for further modifications.
Finally, to demonstrate the synthetic utility of our
methodology we prepared the protected bis(trifluoromethyl-
thiolate)d binol 7l, which can be oxidized to the correspond-
ing disulfone 10 in high yield (Scheme 3). The latter product
could be used as an intermediate for catalyst preparation.[21]
Additionally, oxidation of the alkyne derivative 9b gave the
trifluoromethylsulfoxide 11, a useful building block for
organic synthesis.
In summary, we have developed for the first time a new
and safe method for the synthesis of N-(trifluoromethylthio)-
phthalimide (4), a convenient and shelf-stable reagent for the
direct trifluoromethylthiolation of readily available boronic
acids and alkynes. Because of the utility of the CF3S group in
biologically active agents, the mild reaction conditions
Table 2: Reaction scope of the copper-catalyzed trifluoromethylthiola-
tion of boronic acids.[a]
Table 3: Reaction scope of the copper-catalyzed trifluoromethylthiola-
tion of alkynes.[a]
[a] Reaction conditions: 6 (1.0 equiv), 4 (1.15 equiv), CuCl (10 mol%),
bpy (20 mol%), and K2CO3 (2.0 equiv), in DME at 458C for 18 h. Yield of
the isolated product after column chromatography. [b] Used 0.5 equiv of
6l. bpy=4,4’-bipyridine, DME=dimethoxyethane.
[a] Reaction conditions: 8 (1.0 equiv), 4 (1.2 equiv), CuI (10 mol%), bpy
(20 mol%), and Cs2CO3 (2.0 equiv), in DCE at 808C for 15 h. Yield of the
isolated product after column chromatography. [b] 8m (0.5 equiv) was
used. DCE=1,2-dichloroethane, m-CPBA=meta-chloroperbenzoic acid.
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Scheme 3. Oxidation of the trifluoromethylthioether products.
employed, and high tolerance of functional groups, the
method developed has the potential of being widely applied
in organic synthesis as well as industrial pharmaceutical and
agrochemical research and development. Further research
toward trifluoromethylthiolations under mild reaction con-
ditions as well as asymmetric reactions is ongoing in our
laboratories. Results will be reported in due course.
Received: August 26, 2013
Published online: January 21, 2014
Keywords: alkynes · boron · copper · homogeneous catalysis ·
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synthetic methods
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