An Improved Protocol for the Synthesis of α-Trifluoromethylthio-Substituted Ketones by Copper-Mediated Trifluoromethylthiolation of α-Bromo Ketones

Article abstract of DOI:10.1002/ejoc.201403221

An efficient and practical approach to α-trifluoromethylthio-substituted ketones was developed. The trifluoromethylthiolation of (bpy)Cu(SCF₃) (bpy = 2,2′-bipyridyl) with various α-bromo ketones afforded the desired α-trifluoromethylthio-substituted ketones in good yields. The reaction tolerates more functionally than previously reported methods and demonstrates efficient scalability and practicality. An improved protocol for the copper-mediated trifluoromethylthiolation of α-bromo ketones by using (bpy)Cu(SCF₃) (bpy = 2,2′-bipyridyl) to afford the corresponding α-trifluoromethylthio-substituted ketones in good to excellent yields is presented. The procedure tolerates electron-withdrawing and electron-donating groups on the aromatic rings.

Full text of DOI:10.1002/ejoc.201403221

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201403221
An Improved Protocol for the Synthesis of α-Trifluoromethylthio-Substituted
Ketones by Copper-Mediated Trifluoromethylthiolation of α-Bromo Ketones
Yangjie Huang,[a] Xing He,[a] Haohong Li,*[a] and Zhiqiang Weng*[a]
Keywords: Synthetic methods / Trifluoromethylthiolation / Copper / Ketones / Fluorine
An efficient and practical approach to α-trifluoromethylthio-
substituted ketones was developed. The trifluoromethylthiol-
ation of (bpy)Cu(SCF3) (bpy = 2,2Ј-bipyridyl) with various α-
bromo ketones afforded the desired α-trifluoromethylthio-
substituted ketones in good yields. The reaction tolerates
more functionally than previously reported methods and
demonstrates efficient scalability and practicality.
with trifluoromethylsulfenyl chloride yielded the α-SCF3-
substituted ketones. Recently, the groups of Shen[16] and
Rueping[17] reported the electrophilic trifluoromethylthiol-
ation of β-keto esters to give the corresponding α-trifluoro-
methylthiolated carbonyl compounds in good to excellent
yields. Li and Zard showed that α-trifluoromethylthio-sub-
stituted ketones can be synthesized from trifluoromethylthi-
olation of α-bromo ketones with O-octadecyl-S-trifluoro-
thiolcarbonate.[18]
Introduction
The introduction of the trifluoromethyl group (CF3) into
an organic molecule often profoundly modifies the meta-
bolic stability, lipophilicity, bioavailability, biological ac-
tivity, and selectivity of lead compounds.[1] In this context,
the development of efficient and operationally simple routes
with which to access molecules containing the CF3 group
has potential for pharmaceutical and agrochemical applica-
tions.[2,3] Among them, carbonyl compounds bearing a tri-
fluoromethyl group at the α position constitute an interest-
ing class of synthons that are useful for the synthesis of
biologically active compounds.[4–6]
Conventionally, α-trifluoromethyl carbonyl compounds
are synthesized by the reactions of silyl enol ethers[7] and
enolates[8] (premade by using a strong base such as lithium
diisopropylamide)[9] or by electrophilic trifluoromethylation
of β-keto esters[10] with a trifluoromethylating reagent. Al-
ternatively, these compounds can be readily synthesized by
oxidative trifluoromethylation of olefins with S-(trifluoro-
methyl)diphenylsulfonium triflate[11] or with CF3SO2Na.[12]
Additionally, an excellent example of nucleophilic trifluoro-
methylation of α-halogenated ketones by using fluoroform-
derived CuCF3 under mild conditions was recently reported
by Grushin and co-workers.[13]
Looking at the importance of CF3S substituents in phar-
maceutical, agricultural, and advanced material prod-
ucts,[19,20] the preparation of α-trifluoromethylthio-substi-
tuted ketones from readily available starting materials by
a simple and convenient approach is consequently highly
desirable. To this end, we recently reported the copper-cata-
lyzed trifluoromethylthiolation of α-bromo ketones with el-
emental sulfur and CF3SiMe3 to generate α-trifluorome-
thylthio-substituted ketones.[21] The catalyst system was
particularly effective for the trifluoromethylthiolation of α-
bromo ketones having either electron-donating groups or
electron-neutral groups on the aromatic rings. Nevertheless,
trifluoromethylthiolation of α-bromo ketone derivatives
with a copper catalyst remained difficult for α-bromo
ketones possessing electron-withdrawing groups on the
phenyl ring (Scheme 1). For instance, under catalytic condi-
tions, 4-(2-bromoacetyl)benzonitrile afforded the trifluoro-
methylthiolated product in 13% yield, whereas reaction of
4-(2-bromoacetyl)anisole afforded the product in 88%
yield. Consequently, there remains a clear need for further
development of efficient synthetic methods for the tri-
fluoromethylthiolation of α-halo ketones that demonstrate
a broad substrate scope.
However, despite this promising precedence for the syn-
thesis of α-trifluoromethyl carbonyl compounds, methods
to access α-trifluoromethylthio-substituted ketones have
only scarcely been studied. Pioneering studies conducted by
Bayreuther and Haas[14] and by Kolasa[15] have demon-
strated that reactions of ketones or ethyl benzoylacetate
In connection with our recent studies on fluorinated cop-
per reagents, we initiated an investigation on the nucleo-
philic trifluoromethylthiolation of aryl halides,[22] alkyl hal-
ides,[23] and allylic bromides[24] with (bpy)Cu(SCF3) (1, bpy
= 2,2Ј-bipyridine). In this paper, we report an improved
[a] Department of Chemistry, Fuzhou University,
Fuzhou China, 350108
E-mail: zweng@fzu.edu.cn
http://chem.fzu.edu.cn/szdw/teacherinfo.aspx?id=99
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201403221.
7324
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Eur. J. Org. Chem. 2014, 7324–7328

Synthesis of α-Trifluoromethylthio-Substituted Ketones
Table 1. Optimization of the Cu-mediated trifluoromethylthiolation
of 4-(2-bromoacetyl)benzonitrile.[a]
Entry
Solvent
Temp. [°C]
Time [h]
Yield [%][b]
1
2
3
4
5
6
7
8
9
CH3CN
CH3CN
THF
DMF
NMP
CH3OH
DMSO
CH2Cl2
CH2Cl2
CH2Cl2
25
80
80
120
120
80
120
25
16
16
16
16
16
16
16
16
16
10
66
49
76
47
56
4
Scheme 1. Methods for the synthesis of α-SCF3-substituted
ketones. EWG = electron-withdrawing group.
synthetic procedure for the preparation of α-trifluoro-
methylthio-substituted ketones by using 1 as the trifluoro-
methylthiolation reagent.
7
81
88
83
50
50
10
[a] Reaction conditions: 1 (0.060 mmol), 2a (0.050 mmol), solvent
(1.0 mL), N2 atmosphere. [b] Yields were determined by 19F NMR
spectroscopy with PhOCF3 as an internal standard.
Results and Discussion
Initially, the trifluoromethylthiolation of (bpy)Cu(SCF3)
(1, 1.2 equiv.) with 4-(2-bromoacetyl)benzonitrile (2a) was
selected as the model reaction to determine the optimum
reaction parameters (Table 1). Upon performing the reac-
tion at 25 °C in CH3CN, an acceptable yield of 4-[2-(tri- (Table 2, Entries 7–11). Even if a free phenolic hydroxy
fluoromethylthio)acetyl]benzonitrile (3a, 66%) was ob- group was present as an electron-donating substituent in
tained in 16 h (Table 1, Entry 1). Further experiments indi- the para position of the aromatic ring, desired ketone 3l was
cated that raising the reaction temperature to 80 °C did not formed in 32% yield (as determined by 19F NMR spec-
increase the yield of 3a (49%; Table 1, Entry 2). The use of troscopy; Table 2, Entry 12). Additionally, halide functional
THF as a solvent at 80 °C for 16 h resulted in an improved groups (F, Cl, and Br) in aromatic α-bromo ketones 2m–p
yield of the product (76%; Table 1, Entry 3). Using DMF were well tolerated, and these substrates reacted smoothly
or N-methylpyrrolidone (NMP) as the solvent at 120 °C for to afford the corresponding products in 60–90% yields
16 h generated a modest yield of 3a (47 and 56%, respec- (Table 2, Entries 13–16). This particular feature shows one
tively; Table 1, Entries 4 and 5). A rather low conversion important advantage of this protocol, because the haloge-
was observed with CH3OH and DMSO as the solvent nated products can be further utilized in well-established
(Table 1, Entries 6 and 7). Surprisingly, if a noncoordina- cross-coupling reactions. 3-Bromoacetylpyridine 2q and 3-
ting solvent such as CH2Cl2 was used at 25 °C for 16 h, the (2-bromoacetyl)-2H-chromen-2-one 2r as heterocyclic α-
yield of 3a increased to 81% (Table 1, Entry 8). Therefore, bromo ketones also reacted to give desired products 3q and
CH2Cl2 was determined to be the superior solvent for the 3r in yields of 71 and 61%, respectively (Table 2, Entries 17
reaction, and the yield of desired trifluoromethylthiolated and 18). Interestingly, this transformation is not limited to
product 3a was further improved to 88% by raising the re- aromatic α-bromo ketones. Aliphatic α-bromo ketone 2s
action temperature to 50 °C (Table 1, Entry 9). These find- also participated in trifluoromethylthiolation to furnish 3s
ings show that the present protocol greatly improved the in 87% yield (Table 2, Entry 19). Additionally, the reaction
efficiency of the trifluoromethylthiolation in comparison to of N-methyl-2-bromoacetanilide (2t) also proceeded
our previous synthesis,[21] with respect to substrates bearing smoothly, and desired product 3t was isolated in 72% yield
electron-withdrawing groups.
(Table 2, Entry 20). Notably, α-chloro ketones 2u and 2v
We further investigated the trifluoromethylthiolation of also underwent trifluoromethylthiolation to afford corre-
(bpy)Cu(SCF3) (1) with several α-bromo ketones to exam- sponding products 3u and 3v in good yields (80 and 76%,
ine the scope and limits of the process. The results are sum- respectively, as determined by 19F NMR spectroscopy) if
marized in Table 2. In general, the reactions of 1 with α- Bu4NI (2.0 equiv.) was used to activate the substrates
bromo ketones 2a–f possessing electron-withdrawing sub- (Table 2, Entries 21 and 22). This method was also ex-
stituents (e.g., CN, NO2, CF3, and CO2Me) in the para and panded to the trifluoromethylthiolation of secondary α-
meta positions proceeded smoothly to afford corresponding bromo ketones 2w and 2x. A chloro group or a bromo
products 3a–f in good yields (77–83%; Table 2, Entries 1– group in the meta or ortho position was tolerated, and cor-
6). Notably, these functional groups, which were problem- responding product 3w or 3x was afforded in 74 or 76%
atic in our previous method, were well tolerated by the pres- yield, respectively (Table 2, Entries 23 and 24).
ent reaction conditions. Electron-donating para-, meta-, and
To demonstrate the practical utility, the reaction of 1 and
ortho-methoxy- or -amino-substituted aromatic α-bromo 2a was performed on a 2.23 mmol scale. As shown in
ketones 2g–k were also found to react successfully, and de- Scheme 2, desired product 3a was obtained in 82% yield
sired ketones 3g–k were produced in 82–93% yields (0.450 g).
Eur. J. Org. Chem. 2014, 7324–7328
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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7325

Y. Huang, X. He, H. Li, Z. Weng
SHORT COMMUNICATION
Table 2. Trifluoromethylthiolation of α-bromo ketones 2 by (bpy)
Cu(SCF3) (1).[a]
Table 2. (Continued)
[a] Reaction conditions: 1 (0.30 mmol), 2 (0.25 mmol), CH2Cl2
(5.0 mL), 50 °C, 16 h, N2. [b] Yield of isolated product. [c] The
yield was determined by 19F NMR spectroscopy with PhOCF3 as
an internal standard. [d] Addition of KF (0.50 mmol, 2 equiv.).
[e] Addition of Bu4NI (0.50 mmol, 2.0 equiv.).
Scheme 2. Scalability of the trifluoromethylthiolation of 2a.
Conclusions
We disclosed herein the copper-mediated trifluoro-
methylthiolation of readily available α-bromo ketones as an
improved protocol for the synthesis of various α-trifluoro-
methylthio-substituted ketones, useful and versatile syn-
thons for further synthetic transformations, in good-to-high
yields. The reaction conditions appeared to be highly com-
patible with a wide range of functional groups, and the ease
of the protocol is highly convenient for synthetic chemistry.
Experimental Section
1
General Information: H NMR, 19F NMR, and 13C NMR spectra
were recorded by using a Bruker AVIII 400 spectrometer. 1H NMR
and 13C NMR chemical shifts are reported in parts per million
(ppm) downfield from tetramethylsilane, and 19F NMR chemical
shifts were determined relative to CFCl3 as the external standard.
The residual solvent peak was used as an internal reference
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Synthesis of α-Trifluoromethylthio-Substituted Ketones
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(CHCl3: δH = 7.26 ppm, δC = 77.0 ppm). HRMS were obtained
with a Waters GCT-TOF at the Shanghai Institute of Organic
Chemistry. (bpy)Cu(SCF3) (1)[22] and α-bromo ketones 2f[25] and
2t[26] were prepared according to published procedures. Other rea-
gents were received from commercial sources. Solvents were freshly
dried and degassed according to published procedures[27] prior to
use. Column chromatography purifications were performed by
flash chromatography by using Merck silica gel 60.
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General Procedure for the Trifluoromethylthiolation of α-Bromo
Ketones with (bpy)Cu(SCF3): α-Bromo ketone 2 (0.50 mmol), [(bpy)
Cu(SCF3)] (1; 192 mg, 0.60 mmol, 1.2 equiv.), and CH2Cl2
(5.0 mL) were added to a reaction tube with Teflon screw cap
equipped with a stir bar. The mixture was stirred at 50 °C for 16 h.
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Acknowledgments
Financial support from the National Natural Science Foundation
of China (NSFC) (grant number 21372044), the Research Fund for
the Doctoral Program of Higher Education of China (grant
number 20123514110003), the Scientific Research Foundation for
the Returned Overseas Chinese Scholars, State Education Ministry,
P. R. China (grant number 2012-1707), the Science Foundation of
the Fujian Province, China (grant number 2013J01040), and
Fuzhou University (grant numbers 022318, 022494) is gratefully
acknowledged.
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