Direct trifluoromethylthiolation of alcohols under mild reaction conditions: Conversion of R-OH into R-SCF3

Article abstract of DOI:10.1002/chem.201402679

A direct process for the trifluoromethylthiolation of allylic and benzylic alcohols under mild conditions has been developed. A wide range of free alcohols underwent nucleophilic substitution in the presence of stable CuSCF₃ and BF₃·Et₂O to give the corresponding products in good to excellent yields.

Full text of DOI:10.1002/chem.201402679

DOI: 10.1002/chem.201402679
Communication
&
Synthetic Methods
Direct Trifluoromethylthiolation of Alcohols under Mild Reaction
Conditions: Conversion of RÀOH into RÀSCF3
Pavlo Nikolaienko, Roman Pluta, and Magnus Rueping*[a]
The direct conversion of Csp3ÀX (X=halogen) by nucleophilic
Abstract: A direct process for the trifluoromethylthiolation
displacement has been reported.[9h,12]
of allylic and benzylic alcohols under mild conditions has
The analogous direct transformation of alcohols into the cor-
been developed. A wide range of free alcohols underwent
responding SCF3 has not yet been accomplished. Two proto-
nucleophilic substitution in the presence of stable CuSCF3
cols have been described in the literature that either require
and BF3·Et2O to give the corresponding products in good
the conversion of the hydroxyl group into an O-(N,N-diethyl-
to excellent yields.
amido)phosphite followed by a subsequent reaction with an
SCF3-based reagent, or conversion into the appropriate thiol
followed by a subsequent trifluoromethylation (Scheme 1).[13]
Functional-group exchange is remarkably relevant in modern
synthetic chemistry. Among the wide array of established
transformations, the introduction of fluorine and fluorine-con-
taining groups is prominent owing to the wide utilization of
the resulting products in the pharmaceutical and agrochemical
industries.[1] Particularly, perfluoroalkylthio groups are of inter-
est because they exhibit improved properties, including en-
hanced lipophilicity, substantial electron-withdrawing effects,
and better metabolic stability.[2] The first member of the
perfluoroalkylthio groups, the trifluoromethylthio group, is
a stable pseudohalogenic group with special biological proper-
ties.[3] Furthermore, use of this group as a key intermediate in
the synthesis of trifluoromethylsulfoxides and triflates has
been demonstrated.[4] Based on these important features, the
incorporation of a SCF3 group into biologically active mole-
Scheme 1. Csp3ÀSCF3 bond formation approaches starting from alcohols.
cules is desirable.
Although the trifluoromethylthio group is known,[5] trifluoro-
methylthiolation has only recently attracted increased interest.
In particular, new methods for a fast, efficient, and straightfor-
ward insertion have attracted attention.
Owing to their wide availability, alcohols are favorable sub-
strates and a direct exchange[14] with a SCF3 group under mild
reaction conditions would be desirable. Therefore, we decided
to examine a direct trifluoromethylthiolation of alcohols, in
which no prefunctionalization would be necessary.
Initial work employed metal–SCF3 compounds or the use of
Me4NSCF3 as stable salts.[5] The direct introduction of the SCF3
moiety by utilizing F3CSCl or F3CSSCF3 has also been described.
However, owing to their high toxicity and gaseous condi-
tions,[6] more-convenient and shelf-stable analogues were de-
veloped.[7] Subsequently, electrophilic,[8] nucleophilic,[9] and rad-
ical[10] approaches for the direct trifluoromethylthiolation with
SCF3-containing reagents were reported.[11] However, most of
these methods deal with CspÀS and Csp2ÀS bond formation,
whereas the creation of Csp3ÀS bonds remains less explored.
Herein, we report a straightforward trifluoromethylthiolation
of hydroxyl groups employing CuSCF3 as readily available SCF3
source. CuSCF3 is a light-stable and a nontoxic nucleophilic tri-
fluoromethylthiolating reagent, which can be easily synthe-
sized in multigram scale.[5b]
We chose diphenyl methanol (1a) as a model substrate to
search for the optimal reaction conditions. When CuSCF3 and
AgSCF3 were used without any additive, no conversion was ob-
served even at elevated temperatures (Table 1, entries 1 and 2).
Acetate- and carbonate-protected alcohols 2a and 3a were
also not active, and only very low conversions were observed
(Table 1, entries 3 and 4). Thus, we concluded that the use of
Brønsted or Lewis acids may be required. However, the main
challenge was to uncover reaction conditions that would allow
activation of the starting material, but would not cause de-
composition of CuSCF3. Brønsted acids, such as methane sul-
[a] P. Nikolaienko, R. Pluta, Prof. Dr. M. Rueping
Institute of Organic Chemistry
Institution 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/chem.201402679.
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Communication
Table 1. Optimization of reaction conditions for trifluoromethylthiolation
of benzylic alcohols.[a]
Table 2. Trifluoromethylthiolation of benzylic alcohols under the opti-
mized reaction conditions.[a]
Entry
R
CuSCF3
[equiv]
Additive
[(equiv)]
Reaction time
[h]
Yield[b]
[%]
Entry
4
Ar
R1
R2
Yield[b]
[%]
1
2[d]
3[d]
4[e]
5
6
7
8
9
10
11
12
13
14
15
H
H
Ac
CO2Et
H
H
H
H
H
H
H
H
H
H
H
1
1
1
1
1
1
1
1
1
1
1
1
2
1.5
3
–
–
–
–
12
12
12
12
1
1
1
1
1
1
1
0.5
0.5
0.5
0.5
n.d.
n.d.
n.d.
n.d.
<5[f]
<5[f]
<5[f]
n.d.
n.d.
n.d.
12
1
2
3
4
5
6
7
8
9
10
11
12
13
14
4a
4b
4c
4d
4e
4 f
4g
4h
4i
4j
4k
4l
4m
4n
C6H5
p-biphenyl
o-FC6H4
C6H5
C6H5
C6H5
C6H5
C6H5
p-MeO-C6H4
Me
Me
Me
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
93
90
83
85
50[c]
96
98
99
97
93
90
91
88
51
p-FC6H4
MsOH (1.5)
TsOH (1.5)
TFA (1.5)
Sc(OTf)3 (0.1)
Bi(OTf)3 (0.1)
In(OTf)3 (0.1)
BF3·Et2O (0.25)
BF3·Et2O (1)
BF3·Et2O (1)
BF3·Et2O (2)
BF3·Et2O (2)
p-CF3C6H4
p-MeOC6H4
2,4,6-Me3C6H2
3,4-MDOC6H3
3,4,5-(MeO)3C6H2
3,4-(MeO)2C6H3
[d]
[d]
3,4-MDOC6H3
H
H
H
21
61
96 (93)[c]
95
methyl ferrocene
5-methylthienyl
C6H5
2-furyl
15
16
4o
4p
Ar, R1:
H
91
98
[a] All reactions were carried out under air atmosphere on a 0.1 mmol
scale. Reactions were stopped by addition of three equivalents of Et3N.
OTf=triflate. [b] Yield based on 1H or 19F NMR spectroscopy with internal
standards mesitylene and C6H5CF3, respectively. n.d.=not determined.
[c] Yield after purification. [d] CuSCF3 and AgSCF3 were tested at 808C.
[e] DMF was used as solvent at 1008C. [f] Formation of a substantial
amount of F3CSH was detected.
Ar, R1:
H
17
18
4q
4r
C6H5
C6H5
iPr
Me
90
98
R1, R2: cyclohexyl
[a] Reaction conditions:
1 (0.4 mmol), CuSCF3 (0.6 mmol), BF3·Et2O
(0.8 mmol), CH3CN (2 mL), see the Supporting Information for details.
[b] Yields after purification by column chromatography. [c] 48 h reaction
time; [d] MDO=methylendioxy (-OCH2O-).
fonic acid (MsOH), toluene sulfonic acid (TsOH), and trifluoro-
acetic acid (TFA) gave a substantial amount of highly volatile
F3CSH (detected by NMR spectroscopy) without product for-
mation (Table 1, entries 5–7). Metal triflates also did not act as
efficient catalysts (Table 1, entries 8–10). To our delight, boron
trifluoride etherate (BF3·Et2O) was effective. The first attempt
with a catalytic amount of BF3·Et2O gave only 12% yield. Fur-
ther optimization (Table 1, entries 12–15) revealed that two
equivalents of BF3·Et2O are optimal, and the desired trifluoro-
methylthiodiphenylmethane (4a) was obtained in 93% yield
(Table 1, entry 14).
and 1r furnished the corresponding products 4q and 4r in ex-
cellent yields (Table 2, entries 17 and 18).
Following the successful direct trifluoromethylthiolation of
benzylic alcohols, we turned our attention to allylic alcohols as
more flexible substrates with further functionalization poten-
tial.[15] Initially, the same reaction conditions were applied for
1,3-diphenylallylic alcohol 5a and the corresponding trifluoro-
methylallyl thioether 6a was obtained in 96% yield within
30 min (Table 3, entry 1). Encouraged by this result we investi-
gated the trifluoromethylthiolation of other 1,3-disubstituted
allylic alcohols, 5. It was found that substrates 5b–g proceed
in the transformation to give the desired products in high to
excellent yields and high regioselectivity (Table 3, entries 2–7).
As expected, the same isomeric product was obtained in the
case of allylic alcohols 5b and 5c (Table 3, entries 2 and 3). Ter-
tiary allylic alcohol 5h and primary alcohol 5i were also tested
and gave the corresponding trifluoromethylthioethers 6h and
6i in high yields (Table 3, entries 8 and 9). In most cases a 4:1
volume ratio of dichloromethane and acetonitrile was chosen
owing to the solubility of CuSCF3 and the starting allylic alco-
hols.
With the optimized conditions in hand, we explored the
substrate scope of this reaction. Benzylic trifluoromethylthiola-
tion of a variety of alcohols, 1a–r, proceeded smoothly to give
the desired products 4a–r with moderate to very high yields.
Primary, secondary, and tertiary alcohols were tested and the
corresponding trifluoromethylthioethers 4a–r were obtained in
all cases. Substrates bearing electron-donating groups, 1a, 1b,
and 1 f–k, which stabilize the carbocation intermediate, gave
higher conversion within short reaction times (Table 2, en-
tries 1, 2 and 6–11). Pleasingly, substrates 1c–e bearing elec-
tron-withdrawing substituents, were also tolerated, although
with lower yields and longer reaction times (Table 2, entries 3–
5). Ferrocene derivative 1l (Table 2, entry 12), as well as differ-
ent heterocyclic derivatives, such as thiophene and furan deriv-
atives 1m and 1n (Table 2, entries 13 and 14), were compatible
with the new reaction conditions and gave the trifluorometh-
ylthiolated products 4l,n in good yields. Tertiary alcohols 1q
To confirm the mechanism of the reaction, optically pure al-
lylic alcohol 5b was submitted to the typical reaction condi-
tions (Scheme 2, [Eq. (1)]). Only the racemic product, 6b, was
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no prefunctionalization of the alcohols is necessary. A wide va-
riety of differently substituted substrates were applied in the
reaction with CuSCF3 to give the corresponding products in
good to high yields, within short reaction times. The devel-
oped reaction procedure shows broad scope, uses safe and
nontoxic metals and reagents, and can be carried out under
mild reaction conditions, making it a useful reaction for prepa-
rative organic synthesis.
Table 3. Trifluoromethylthiolation of allylic alcohols.[a]
Entry
5
6/6’
Ratio
Yield[b]
[%]
[6/6’]
1
2
3
4
5
6a
6b
6’c
6d
6e
–
96[c]
90
Experimental Section
100:0
11:89
88:12
96:4
General procedure for the direct trifluoromethylthiolation of
benzylic and allylic alcohols
For benzylic alcohols: A vial equipped with a septum and a stirring
bar was charged with CuSCF3 (98.7 mg, 0.6 mmol), the relevant
benzylic alcohol (0.4 mmol), and CH3CN (4 mL), and the vial was
closed. Borontrifluoride etherate (0.1 mL, 0.8 mmol) was added
dropwise with vigorous stirring.
87
95
For allylic alcohols: A vial equipped with a septum and a stirring
bar was charged with CuSCF3 (131.4 mg, 0.8 mmol) and the rele-
vant allylic alcohol (0.4 mmol). The reaction vessel was closed and
CH3CN/CH2Cl2 (1:4, 4 mL) was added. Then, Borontrifluoride ether-
ate (0.1 mL, 0.8 mmol) was added dropwise with vigorous stirring.
73
6
7
6’f
3:97
3:97
95
91
The solution was stirred at room temperature for 0.5 h until a pre-
cipitate was formed and the starting material was consumed (in
the case of incomplete transformation the reaction time was pro-
longed). The suspension was filtered through a short pad of silica,
the filter cake was rinsed with additional Et2O (2ꢁ5 mL), and the
solvent was concentrated in vacuum. The crude product was puri-
fied by column chromatography on silica gel with pentane/diethyl
ether mixtures as eluent.
6’g
8
9
6h
6i
97:3
97:3
85
81
[a] Reaction conditions: Alcohol 5 (0.4 mmol), CuSCF3 (0.8 mmol), BF3·Et2O
(0.8 mmol), CH3CN/CH2Cl2 (1:4, 4 mL), 0.5–2 h, see the Supporting Infor-
mation for details. [b] Yields after column chromatography. [c] 1 mmol of
CuSCF3 and 2 mL CH3CN as solvent were used.
Keywords: alcohols · nucleophilic substitution · thioethers ·
trifluoromethyl sulfenylation
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Received: March 30, 2014
Published online on July 14, 2014
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