Copper-catalysed ring-opening trifluoromethylation of cyclopropanols

Article abstract of DOI:10.1039/c5ob00808e

A copper-catalysed ring-opening trifluoromethylation reaction of cyclopropanols has been developed. Various β-trifluoromethyl ketones are obtained in good to excellent yields under mild reaction conditions. The present method also exhibits good functional-group compatibility. The mechanism of this new ring-opening trifluoromethylation reaction was investigated by radical trapping reactions.

Full text of DOI:10.1039/c5ob00808e

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DOI: 10.1039/C5OB00808E
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Copper‐Catalysed Ring‐Opening Trifluoromethylation of
Cyclopropanols†
Received 00th January 20xx,
Accepted 00th January 20xx
Xia‐Ping He, Yong‐Jin Shu, Jian‐Jun Dai,* Wen‐Man Zhang, Yi‐Si Feng and Hua‐Jian Xu*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Copper‐catalysed ring‐opening trifluoromethylation reaction of
cyclopropanols has been developed. Various β‐trifluoromethyl
ketones are obtained in good to excellent yields under mild
reaction conditions. The present method also exhibits good
functional‐group compatibility. The mechanism of this new ring‐
opening trifluoromethylation reaction was investigated by radical
trapping reactions.
The trifluoromethyl (CF3) group is an important structural motif in
many medicinally and biologically important compounds.1 Thus,
considerable attention has been paid to the development of
efficient methods for the introduction of CF3 group into molecules
in recent years.2 In comparison with the significant progress
achieved in C(sp2)‐CF3 and C(sp)‐CF3 bond formation,3,4 the
construction of C(sp3)‐CF3 is relatively less investigated. Recent
advances in this field focus on: (1) the conversion of
prefunctionalized alkyl halides,5 silane6 and boronic acid7 to the
corresponding C(sp3)‐CF3 compounds; (2) trifluoromethylation of
alkenes;8 (3) direct C(sp3)‐H tirfluoromethylation.9 On the other
hand, β‐trifluoromethyl ketones are important yet synthetically
difficult molecules.10 Traditional method such as nucleophilic
Scheme 1 The formation of β‐trifluoromethyl ketones involving C‐C
bond cleavage strategy.
diaryl allylic alcohols (Scheme 1, eq. 2). Despite these important
advances, it remains important to develop new methodology for
the formation of CF3‐containing compounds using C‐C bond
cleavage strategy.14
‐
addition of “CF3 ” reagent to enones suffers from the poor
regioselectivity.11 To solve these problems, recent attention has
+
been directed to the use of “CF3 ” reagent combing with transition
Cyclopropanols are easily prepared with the development of
Kulinkovich reaction.15 Importantly, cyclopropanols can undergo
synthetically useful ring‐opening transformations for the formation
of β‐substituent (e.g., halogen, aryl, alkyl etc.) ketones.16 In this
context, we are interested in the construction of β‐trifluoromethyl
ketones from cyclopropanols. From the hypothesis that the ring‐
opening trifluoromethylation of cyclopropanols would not only
expands the concept and utility of C‐C bond cleavage
trifluoromethylation, but also enable the development of a facile
and new route to the synthesis of β‐trifluoromethyl ketones, we
herein report the copper‐catalysed trifluoromethylation reaction of
cyclopropanols (Scheme 1, eq. 3). Moreover, our study also adds a
new example of cyclopropanols transformation chemistry.
metal via a radical process. For example, Shibata et al. recently
reported copper‐mediated conjugate addition trifluoromethylation
+
of α,β‐unsaturated ketones with the Umemoto type “CF3 ”
reagent.12 Recent breakthroughs for the construction of β‐
trifluoromethyl ketones have been achieved by rearrangement
trifluoromethylation reactions involving C‐C bond cleavage.13 For
instance, Tu et al. described the semipinacol rearrangement
trifluoromethylation of allylic alcohols (Scheme 1, eq. 1). Wu and Li
also reported neophyl rearrangement trifluoromethylation of α,α‐
School of Medical Engineering, School of Chemistry and Chemical Engineering,
Hefei University of Technology, Hefei 230009, P. R. China.
E‐mail: daijj@hfut.edu.cn; hjxu@hfut.edu.cn
† Electronic supplementary information (ESI) available: Experimental procedures,
characterization data, and 1H 13C and 19F NMR charts. See
DOI: 10.1039/x0xx00000x
We started our investigations using 1a as the probe substrate in
the presence of a catalytic amount of CuI (20 mmol %) and Togni
reagent 2a in MeOH at 60 °C (Table 1, entry 1). To our surprise, the
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J. Name., 2013, 00, 1‐4 | 1
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Table 2 Substrate scope of cyclopropanolsa,b
Table 1 Optimization of the reaction conditionsa
View Article Online
DOI: 10.1039/C5OB00808E
O
Cu(CH3CN)4PF6 (20 mol %)
R
OH
R
CF3
MeOH (2 mL)
Ar, 60 oC, 24 h
R = aryl, alkyl, vinyl
3a-p
1a-p
O
O
O
CF3
CF3
Temp
Time
(h)
Yield
(%)b
CF3
Entry Copper catalyst
Solvent
(°C)
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
40
60
F3C
MeO
3a 81%
3b 72%
3c 65%
1
2
CuI
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
DMA
24
24
24
24
24
24
24
24
24
24
24
24
24
24
12
24
24
25
29
CuTc
O
O
O
3
CuCl
65
CF3
CF3
CF3
4
CuBr
60
Cl
EtO
Me
Ph
3f 55%
3d 78%
3e75%
5
CuSCN
trace
trace
82
6
Cu(PP3)3Br
Cu(CH3CN)4PF6
Cu(CH3CN)4PF6
Cu(CH3CN)4PF6
Cu(CH3CN)4PF6
Cu(CH3CN)4PF6
Cu(CH3CN)4PF6
Cu(CH3CN)4PF6
Cu(CH3CN)4PF6
Cu(CH3CN)4PF6
Cu(CH3CN)4PF6
none
O
O
OMe O
7
8c
9d
10
11
12
13
14
15
16
17
59
CF3
CF3
CF3
CF3
CF3
tBu
50
3i 73%
3g 75%
3h 62%
39
NMP
34
O
O
O
MeO
DMSO
CH3CN
1,4‐dioxane
MeOH
MeOH
MeOH
trace
29
CF3
CF3
S
59
3l 75%
3j 58%
3k 71%
48
O
O
50
O
CF3
CF3
NR
S
3m 72%
3o 83%
3n 70%
O
a Unless otherwise noted, the reaction was conducted with 1a (0.2 mmol),
Togni reagent 2a (1.5 equiv), copper catalyst (20 mol %), MeOH (2 mL)
under argon atmosphere, 60 °C, 24 h. b Yields were determined by 19F NMR
spectroscopy with trifluoromethoxybenzene as an internal standard. c Togni
reagent 2b was used. Umemoto reagent 2c was used. DMA = N,N‐
dimethylacetamide, NMP = N‐methyl‐2‐pyrrolidone, DMSO = dimethyl
CF3
3p 64%
d
a
Reaction conditions: 1 (0.2 mmol), Cu(CH3CN)4PF6 (20 mol %), Togni
reagent 2a (1.5 equiv), and MeOH (2 mL) under argon atmosphere, 60 °C,
sulfoxide. NR = No reaction.
24 h. b Yields of isolated products are shown.
β‐trifluoromethyl ketone 3a was directly formed in 25% yield.
Testing of different copper catalysts showed that Cu(CH3CN)4PF6
can dramatically improve the yield of 3a to 82% (Table 1, entry 7).
The reaction was also effective with the use of Togni reagent 2b
and Umemoto reagent 2c, providing the desired product 3a in 59%
and 50% yield, respectively (Table 1, entries 8 and 9). However,
when the reaction was conducted in other solvents such as DMA,
NMP, DMSO, CH3CN, or 1,4‐dioxane the yields of 3a were inferior
(Table 1, entries 10‐14). We also noted that the reaction was not
progressing effectively at lower reaction temperature or for shorter
reaction time, presumably due to the incomplete conversion of 1a
(Table 1, entries 15 and 16). Finally, it is important to mention that
the control experiment conducted in the absence of copper catalyst
gave no desired product of 3a (Table 1, entry 17).
can tolerate well electron‐donating groups such as alkyl (3f, 3g),
ether (3a, 3e, 3h) as well as electron‐withdrawing groups such as
trifluoromethyl (3c), phenyl (3i). Notably, this reaction can also
tolerate well aryl halide groups such as Cl (3d), enabling additional
functional handle at these positions via transition‐metal‐catalysed
cross‐coupling techniques.17 Remarkably, pharmaceutically
interesting heterocyclic substrates (3l, 3m) were found to be
compatible in this reaction, and gave the trifluoromethyl‐containing
ketones with synthetically useful yields. Importantly, we found that
the ring‐opening trifluoromethylation of 3p could be conducted
without noticeable ring expansion product.15g Finally, aliphatic
substituted cyclopropanols were also applicable to give
corresponding compounds in good yields (3n, 3o).
To test this newly developed copper‐catalysed ring‐opening
trifluoromethylation methodology in a pharmaceutical context as
well as preparative organic synthesis, a drug molecule ibuprofen 4
that contains C−COOH site was converted to the corresponding β‐
trifluoromethyl ketone. As shown in Fig. 1, ibuprofen 4 can be easily
Having optimized the reaction conditions, we next studied the
scope of the copper catalysed ring‐opening trifluoromethylation of
cyclopropanols, and the results are summarized in Table 2. We
found that a variety of cyclopropanols can be converted to the
desired product in good to excellent yields. The present reaction
2 | J. Name., 2012, 00, 1‐4
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Conclusions
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In summary, we have developeDdOI: t1h0.e1039r/iCn5gO‐oBp0e08n0in8gE
trifluoromethylation of cyclopropanols using Cu(CH3CN)4PF6
and Togni reagent 2a under mild reaction conditions.18 A wide
variety of synthetically useful β‐trifluoromethyl ketones were
obtained. Further investigations of this reaction system are
currently underway in our laboratory.
We gratefully acknowledge financial support from the National
Natural Science Foundation of China (Nos. 21472033, 21402036 and
21272050), the program for New Century Excellent Talents in
University of the Chinese Ministry of Education (NCET‐11‐0627),
and the project funded by China Postdoctoral Science
(2014M551793).
Fig.
1
Application of the copper‐catalysed ring‐opening
trifluoromethylation protocol to pharmaceutically relevant molecule.
Notes and references
converted to the corresponding cyclopropanol 1q in two steps:
acid‐catalysed esterification and Kulinkovich reaction.16 By using the
new reaction described in the present study, 1q can be converted
to 3q in 88% yield.
Note that ketones are known to undergo diverse transformations.
We considered that the present reaction combined with further
functional group conversions could provide new opportunity for the
introduction of CF3 group into the organic molecules to produce
potentially useful building blocks in the field of medicinal chemistry.
For example, the CF3‐containing alcohol 5 formed from 3e in good
yields (Fig. 2).
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