Copper-catalyzed trifluoromethylthiolation of primary and secondary alkylboronic acids

Article abstract of DOI:10.1021/ol502132j

A Cu-catalyzed trifluoromethylthiolation of primary and secondary alkylboronic acids with an electrophilic trifluoromethylthiolating reagent is described. Tolerance for a variety of functional groups was observed.

Full text of DOI:10.1021/ol502132j

Letter
pubs.acs.org/OrgLett
Copper-Catalyzed Trifluoromethylthiolation of Primary and
Secondary Alkylboronic Acids
Xinxin Shao, Tianfei Liu, Long Lu,* and Qilong Shen*
Key Laboratory of Organofluorine Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Ling-Ling
Road, Shanghai 200032, China
S
* Supporting Information
ABSTRACT: A Cu-catalyzed trifluoromethylthiolation of primary and
secondary alkylboronic acids with an electrophilic trifluoromethylthiolating
reagent is described. Tolerance for a variety of functional groups was observed.
Scheme 1. Previous Direct Trifluoromethylthiolation
Strategies for Alkylthioethers
ue to its high lipophilicity (πR = 1.44) and strong
electronegativity, the trifluoromethylthio group (CF3S−)
D
is generally considered a privileged structural motif that is
frequently employed in new drug and agrochemical design and
routinely evaluated for fine-tuning biological properties of
leading compounds.1 Consequently, development of methods
for the introduction of the trifluoromethylthio group into small
molecules is of great interest for both academic and industrial
chemists.2,3 In recent years, several elegant methods involving
transition metal catalysis or metal-free conditions for trifluoro-
methylthiolation of aryl halides, boronic acids, and terminal
alkynes employing a nucleophilic CF3S reagent such as CF3SAg,
CF3SCu, or CF3SNMe4 have been established by the research
groups of Buchwald,4 Vicic,5 Qing,6 and Weng.7 Alternatively,
direct trifluoromethylthiolation of indoles,8 aryl boronic acids,9
or alkynes11 with an electrophilic trifluoromethylthiolating
reagent12 were reported by Billard, Shibata, Rueping, and
us.9a,10 More recently, Gooßen reported a Sandmeyer-type
trifluoromethylthiolation of arenediazonium salts with NaSCN
and TMSCF3 under mild conditions.13 These new methods or
reagents allow the introduction of the trifluoromethylthio group
at a late stage of the synthesis of the drug molecules.
In contrast to these significant achievements of trifluoro-
methylthiolation that typically involved the formation of
C(sp2)−SCF3 or C(sp)−SCF3 bonds, processes that facilitate
the construction of C(sp3)−SCF3 bonds from unactivated alkyl
substrates remain much less developed. Alkyltrifluoromethyl-
thio-ethers can be accessed through classical halogen−fluorine
exchange of polyhalogenomethyl thioethers or the trifluorome-
thylation of sulfur-containing compounds such as disulfides,
thiocyanates, and thiols via radial pathways.14 Alkyl trifluoro-
methylthio ethers could also be generated by the electrophilic
trifluoromethylation of thiolates.15 These indirect strategies are
of interest but generally require preformed sulfur precursors.
Direct nucleophilic trifluoromethylthiolation of alkyl halides are
reported using MSCF3 (M = Me4N+, Ag(I), Cu(I), or Hg(II))
reagents (Scheme 1).16 However, these methods were mainly
applied to activated alkyl halides such as benzyl, allyl, or
propargyl halides or α-haloesters or ketones.17 Few examples for
the nucleophilic trifluoromethylthiolation of simple alkyl halides
were reported previously.16m More recently, Billard et al.
reported the preparation of the air and moisture stable
trifluoromethanesulfanylamides that were capable of trifluoro-
methylthiolation of alkenes, Grignard or lithium reagents.11a,18
Qing reported that the same reagent enabled trifluoromethyl-
thiolation of allylsilane mediated by acid chloride.12b,c Mean-
while, several groups independently reported trifluoromethyl-
thiolation of β-keto esters using different electrophilic trifluoro-
methylthiolating reagents in high yields.8b,9a,12e−g Hu, Wang, and
Rueping independently reported trifluoromethylthiolation of α-
diazoesters from the formation of α-trifluoromethylthiolated
esters.19 Despite these important advances, numerous challenges
such as starting material availability, functional group tolerance,
and operational simplicity remain. Herein, we report the first Cu-
catalyzed trifluoromethylthiolation of primary (1°) and secon-
dary (2°) alkylboronic acids with an electrophilic trifluoro-
methylthiolating reagent developed in our laboratory. The
reaction is compatible with a variety of functional groups.
Received: July 19, 2014
Published: September 8, 2014
© 2014 American Chemical Society
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Letter
Alkyl boronic acids are valuable cross-coupling partners since
they are crystalline compounds that can be easily prepared from
alkenes or alkyl halides and are compatible with a variety of
functional groups.20 Direct trifluoromethylthiolation of alkyl
boronic acids under mild conditions using abundant, cheap metal
catalysts would provide a new strategy for the construction of
Csp3−SCF3 bond. Cu catalysts would be an ideal choice since
they have been extensively employed for a variety of functional
group manipulations. Unlike numerous reported reactions of aryl
and alkenyl boronic acids such as the Suzuki−Miyaura reaction21
and Chan−Lam−Evans reaction,22 few Cu-catalyzed cross-
coupling reactions of alkyl boronic acids have been reported.23
We began the study by examining the reaction between
phenethylboronic acid and the electrophilic reagent 1 as a model
reaction to survey the reaction conditions. When the reaction
was conducted at 35 °C using 20 mol % Cu(MeCN)4PF6 and 40
mol % 2,2′-bipyridine as the catalyst in diglyme, the conditions
that were efficient to promote the Cu-catalyzed trifluoromethyl-
thiolation of arylboronic acids, no desired product was detected
by GC-MS or 19F NMR spectroscopy (Table 1, entry 1).
improved to 60% (Table 1, entry 3). Further screening of Cu
salts showed that when CuCl2·2H2O was used as the catalyst, the
product yield improved to 73% (Table 1, entry 5). The reaction
time could be significantly shortened to 12 h with a slightly
increased yield when the temperature was increased to 120 °C
(Table 1, entries 6−7). The amount of the alkyl boronic acid was
important for the yield of the desired product. When 1.3 or 2.0
equiv of alkyl boronic acid were used, the yield decreased to 71%
and 60%, respectively (Table 1, entries 8−9). The effects of the
ligands and bases were further studied. Reactions were less
effective when 4,4′-dimethoxy-2,2′-bipyridine L2, 4,4′-tert-butyl-
2,2′-bipyridyl L3, or 1,10-phenanthroline L4 was used as the
ligand (Table 1, entries 10−12). We further studied the influence
of bases on the reaction. Reactions in the presence of KOAc,
K3PO4, Na3PO4, or Na2CO3 gave the corresponding trifluoro-
methylthiolated product in lower yields than those in the
presence of K2CO3 (Table 1, entries 13−16). Likewise, using
other Cu salts such as copper(I) thiophene-2-carboxylate
(CuTc), Cu(OAc)2, and CuBr·SMe2 led to slightly lower yields
(Scheme 2, entries 17, 19−20). Interestingly, using CuBr2 as the
To our delight, when the mixture was heated to 80 °C for 36 h,
the trifluoromethylthiolated product was formed in 35% yield,
along with side products such as alkyl iodide and alkane from
proto-deboronation (Table 1, entry 2). When the amount of
alkyl boronic acid was increased to 1.5 equiv, the yield was
Scheme 2. Scope of Cu-Catalyzed Trifluoromethylthiolation
of 1° Alkyl Boronic Acids
a
,b
a
Table 1. Optimization of Reaction Conditions
a
Reaction conditions: alkylboronic acid (0.75 mmol), reagent 1 (0.5
mmol), CuCl2·2H2O (20 mol %), bpy (40 mol %), and K2bCO3 (2.0
equiv) in 1,2-dichloroethane (2.5 mL) at 120 °C for 12 h. Isolated
c
yield. CuTc was used as the catalyst and ((2-phenylpropan-2-
yl)oxy)(trifluoromethyl)sulfane was used as the electrophilic trifluoro-
methylthiolating reagent in toluene.
a
Reaction conditions: alkyl boronic acid (0.15 mmol), reagent 1 (0.1
mmol), CuX (20 mol %), ligand (40 mol %), and base (2.0 equiv) in
solvent (0.5 mL) at temperatures indicated in Table 1 for 12 h; yields
were determined by 19F NMR analysis of the crude reaction mixture
catalyst was equally effective (Table 1, entry 18). The yield
decreased significantly to 30% when the reaction was conducted
in the absence of ligand (Table 1, entry 21). Likewise, no desired
trifluoromethylthiolated product was observed in the absence of
the Cu catalyst (Table 1, entry 22). The effects of the solvents
were also evaluated. Reactions in toluene were slightly less
effective, while reactions in polar solvents such as NMP, acetone,
THF, or acetonitrile led to <35% yields (Table 1, entries 23−27).
With the optimized conditions (Table 1, entry 7) in hand, we
then studied the substrate scope of the Cu-catalyzed trifluoro-
b
with an internal standard. 1.0 equiv of alkylboronic acid was used.
c
Reaction was conducted at corresponding temperature for 36 h. DCE
d
e
= 1,2-dichloroethane. Alkyl boronic acid (1.3 equiv) was used. Alkyl
boronic acid (2.0 equiv) was used.
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Organic Letters
Letter
methylthiolation of 1° alkylboronic acids, as summarized in
Scheme 2. A variety of alkyl boronic acids reacted with
electrophilic trifluoromethylthiolating reagent 1 to give the
corresponding trifluoromethylthiolated alkanes in good to
excellent yields. Challenging functionalized alkyl boronic acids
were compatible with the reaction conditions. Reactions of alkyl
boronic acids with functional groups such as enolizable ketone,
ester, alkene, nitro, amide, cyano, fluoride, chloride, and bromide
occurred in good yields (Scheme 2, 3b−q). Moreover, a
structurally complicated biologically active estrone derivative
also reacted with trifluoromethylthiolated reagent 1 to give the
corresponding trifluoromethylthiolated product in good isolated
yield (Scheme 2, 3q). Notably, the yield for the reaction of 6-
bromohexylboronic acid and undec-10-enylboronic acid with
reagent 1 on a 5 or 10 mmol scale dropped significantly to give
the corresponding product in 41% and 44% yield, respectively.
The main side products observed for the reaction with undec-10-
enylboronic acid were alkyl chloride (22%) and iodide (34%) as
determined by 1H NMR spectroscopy and GC/MS of the crude
mixture. In contrast, the same reaction in a 1.0 mmol scale gave
the desired product and two side products in 71%, 15%, and 28%
yield, respectively (Scheme 2, 3d).
Scheme 3. Scope of Cu-catalyzed Trifluoromethyl-thiolation
of 2° Alkyl Boronic Acids
a
,b
a
Reaction conditions: alkylboronic acid (0.75 mmol), reagent 1 (0.5
mmol), CuTc (20 mol %), bpy (40 mol %), and K2CO3 (2.0 equiv) in
b
1,2-dichloroethane (2.5 mL) at 120 °C for 12 h. Isolated yield.
Encouraged by these excellent results, we next turned our
attention to 2° alkylboronic acids that are more challenging
substrates in organic synthesis. The reaction conditions were
slightly modified, and we found the highest yields were obtained
when CuTc was used as the catalyst. Other Cu salts such as
CuBr2, CuSCN, CuI, CuBr, Cu(OAc)2, CuCN, CuBr·SMe2, or
CuCl2·2H2O were slightly less effective. However, much less side
products were observed when CuTc was used as the catalyst
(Table 2). Under these conditions, several cyclic and acyclic 2°
the current method compared to the previous method using a
Grignard or lithium reagent is its tolerance for a variety of
functional groups. Thus, potentially, it will provide a general
method for the construction of any desired trifluoromethylthio-
substituted alkyl building blocks. Further investigation of the
more challenging stereospecific trifluoromethylthiolation of 2°
alkyl boronic acids and mechanistic studies of the reaction are
underway in our laboratory.
ASSOCIATED CONTENT
* Supporting Information
Experimental procedures, characterization data. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
Table 2. Optimization of Reaction Conditions for
Trifluoromethylthiolation of 2° Alkyl Boronic Acids
S
ab
,
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: shenql@sioc.ac.cn.
*E-mail: lulong@sioc.ac.cn.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors gratefully acknowledge the financial support from
the National Basic Research Program of China
(2012CB821600), National Natural Science Foundation of
China (21172244/21172245/21372247), Agro-scientific Re-
search in the Public Interest (201103007), the National Key
Technologies R&D Program (2011BAE06B05), Shanghai
Scientific Research Program (10XD1405200), and SIOC for
financial support.
a
Reaction conditions: alkylboronic acid (0.15 mmol), reagent 1 (0.1
mmol), Cu catalyst (20 mol %), bpy (40 mol %), and K2CO3 (2.0
b
equiv) in 1,2-dichloroethane (2.5 mL) at 120 °C for 12 h. Yields were
determined by 19F NMR with an internal standard.
alkyl boronic acids could be successfully trifluoromethylthiolated
in good yields, as shown in Scheme 3. More importantly, ketones,
esters, and N-protected piperidines were well tolerated with the
reaction conditions (Scheme 3, 4f−h). In addition, the reaction
was not significantly affected by the ring size of the cyclic 2° alkyl
boronic acids. Five-, six-, seven-, and twelve-membered cyclic
alkyl boronic acids all reacted with reagent 1 to give the
corresponding trifluoromethylthiolated products in good iso-
lated yields (Scheme 3, 4a−b, 4d−e).
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