Copper-Mediated Aromatic 1,1-Difluoroethylation with (1,1-Difluoroethyl)trimethylsilane (TMSCF2CH3)

Article abstract of DOI:10.1002/asia.201600577

A new method for the formation of 1,1-difluoroethyl copper species (“CuCF₂CH₃”) with 1,1-difluoroethylsilane (TMSCF₂CH₃) has been developed. The “CuCF₂CH₃” species can be applied to the efficient 1,1-difluoroethylation of diaryliodonium salts under mild conditions, affording (1,1-difluoroethyl)arenes in good to excellent yields. This convenient procedure tolerates a wide range of functional groups and thus serves as a practical synthetic tool for the introduction of CF₂CH₃ group(s) into complex molecules.

Full text of DOI:10.1002/asia.201600577

DOI: 10.1002/asia.201600577
Communication
1,1-Difluoroethylation
Copper-Mediated Aromatic 1,1-Difluoroethylation with (1,1-
Difluoroethyl)trimethylsilane (TMSCF2CH3)
Xinjin Li,[a, b] Jingwei Zhao,[b] Yunze Wang,[b] Jian Rong,[b] Mingyou Hu,[b] Dingben Chen,[b]
Pan Xiao,[b] Chuanfa Ni,[b] Limin Wang,*[a] and Jinbo Hu*[b]
pounds with sodium difluoroethylsulfinate (DFES-Na) has been
Abstract: A new method for the formation of 1,1-difluor-
disclosed by the group of Baran[4] (Scheme 1a). Furthermore,
oethyl copper species (“CuCF2CH3”) with 1,1-difluoroethyl-
different methods to activate the benzylic CÀH bond are em-
silane (TMSCF2CH3) has been developed. The “CuCF2CH3”
species can be applied to the efficient 1,1-difluoroethyla-
tion of diaryliodonium salts under mild conditions, afford-
ing (1,1-difluoroethyl)arenes in good to excellent yields.
This convenient procedure tolerates a wide range of func-
tional groups and thus serves as a practical synthetic tool
for the introduction of CF2CH3 group(s) into complex mol-
ecules.
Fluorinated compounds have been widely used in the fields of
pharmaceuticals, agrochemicals, and functional materials, as
the introduction of fluorine atom(s) into organic molecules
often results in dramatic changes in the physical properties,
the chemical reactivity, and especially the biological activity.[1]
Scheme 1. Strategies for the synthesis of (1,1-difluoroethyl)arenes.
Among various fluorinated moieties, the difluoroethyl group
(CF2CH3) is of high importance because it mimics the steric
ployed to afford 1,1-difluoroethylarenes[5] (Scheme 1b). Very re-
and electronic features of a methoxy group and improves the
bioactivity and metabolic stability of important molecules.[2]
cently, Hammond and co-workers reported the gold-catalyzed
For example, the replacement of a methoxy group by a difluor-
oethyl group of a triazolothienopyrimidine molecule as a urea
transporter UT-B inhibitor shows remarkable advantages in
terms of potency and metabolic stability.[2b] However, the in-
vention of reagents and methods for the synthesis of (1,1-di-
fluoroethyl)arenes remains largely unexplored. Traditional
methods for the synthesis of such compounds convert ketones
into the corresponding difluoroalkyl groups using nucleophilic
fluorination reagents such as N,N-diethylaminosulfur trifluoride
(DAST).[3] However, the reaction conditions usually cannot tol-
erate a large number of functional groups. Recently, a radical
method for 1,1-difluoroethylation of heteroaromatic com-
dihydrofluorination of terminal arynes with a DMPU/HF com-
plex for the synthesis of difluoroethylarenes[6] (Scheme 1c).
During the last few years significant progress has been
made in copper-mediated or -catalyzed difluoroalkylation for
the selective introduction of CF2 group(s) onto aromatic
rings.[7,8] Recently, we disclosed the direct preparation of the
“CuCF3” reagent with phenyl trifluoromethyl sulfoxide
(PhSOCF3) via the treatment of CuCl and 2 equivalents of
tBuOK and its application to trifluoromethylation of aryl hal-
ides.[9] We envisioned that a copper-mediated difluoroethyla-
tion via the “CuCF2CH3” species[10] could provide an alternative
method for the synthesis of (1,1-difluoroethyl)arenes
(Scheme 1d), a process that has not been reported to date.
Therefore, the preparation of “CuCF2CH3” involved in the cross-
coupling reaction was considered first.
[a] X. Li, Prof. Dr. L. Wang
Key Laboratory of Advanced Materials, Institute of Fine Chemicals
East China University of Science and Technology
130 Meilong Road, Shanghai, 200237 (China)
E-mail: wanglimin@ecust.edu.cn
At the onset of our investigation, we employed 1,1-difluor-
oethyl phenyl sulfone (PhSO2CF2CH3, 1a) or sulfoxide
(PhSOCF2CH3, 1b) as the CF2CH3 source. However, the condi-
tions for the generation of “CuCF3”[9] did not give the
“CuCF2CH3” species, and only the unreacted starting material
(1a or 1b) and the byproduct 1,1-difluoroethene (CF2 =CH2)
were detected by 19F NMR spectroscopy. The low reactivity of
sulfone and sulfoxide reagents (1a and 1b) could be attribut-
[b] X. Li, Dr. J. Zhao, Y. Wang, J. Rong, Dr. M. Hu, Dr. D. Chen, P. Xiao, Dr. C. Ni,
Prof. Dr. J. Hu
Key Laboratory of Organofluorine Chemistry
Shanghai Institute of Organic Chemistry
Chinese Academy of Sciences
345 Lingling Road, Shanghai, 200032 (China)
E-mail: jinbohu@sioc.ac.cn
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À
ed to the difficulty in breaking the CÀS bond to give CH3CF2
was completely consumed and the amount of “[Cu(CF2CH3)2]À”
increased. Consequently, when 1.6 equivalents of 1c was used,
the total yield of the “CuCF2CH3” species was increased to 98%
(Table 1, entry 12), with 66% contribution from “L·CuCF2CH3”
(L=DMF, Cl) and 32% from “[Cu(CF2CH3)2]À”.
species. Thereafter, we attempted the reaction using (1,1-di-
fluoroethyl)trimethylsilane (TMSCF2CH3, 1c)[11] as the difluor-
oethyl precursor. To the mixture of CuCl (0.2 mmol) and tBuOK
(2 equiv) in DMF at room temperature, reagent 1c was added
dropwise at the same temperature under an argon atmos-
phere (Table 1, entry 1). Indeed, the formed 1,1-difluoroethyl
With an efficient method for the generation of “CuCF2CH3”
in hand, we next investigated the 1,1-difluoroethylation reac-
tion of aryl iodides with “CuCF2CH3”. However, only a small
amount of the difluoroethylated product was detected, even
though we used both electron-deficient and electron-rich aryl
iodides.[7c,h] The low efficiency of the cross-coupling reaction
between aryl iodides and “CuCF2CH3” is probably due to the
fact that the oxidative addition process usually requires high
temperature and/or long reaction time. Thus, we envisioned
that diaryliodonium salts, a more reactive version of aryl io-
dides in metal-catalyzed or -mediated reactions,[7l,14] could
react with “CuCF2CH3” for the synthesis of (1,1-difluoroethyl)ar-
enes. After optimization of the reaction conditions, the reac-
tion of diaryliodonium salts with “CuCF2CH3” neutralized by
Et3N·3HF[9,10c,15] (0.5 equiv) could provide the desired products
in good yields at room temperature for 1 hour (Scheme 2).
Table 1. Screening of formation of “CuCF2CH3”.[a]
Entry
Initiator [equiv]
Solvent
T
Yield[b] [%]
1
2
3
4
5
6
7
8[c]
9
10
11[d]
12[e]
tBuOK (2.0)
tBuONa (2.0)
CsF (2.0)
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
NMP
DMA
DMF
DMF
RT
RT
RT
RT
38
0
0
tBuOK (2.5)
tBuOK (1.5)
tBuOK (2.0)
tBuOK (2.0)
tBuOK (2.0)
tBuOK (2.0)
tBuOK (2.0)
tBuOK (2.0)
tBuOK (2.0)
11
35
67
65
53
52
60
82
98
RT
08C
À108C
08C
08C
08C
08C
08C
[a] Reaction conditions: 1c (0.2 mmol), CuCl (0.2 mmol), solvent (1 mL).
[b] Yields were determined by 19F NMR spectroscopy using PhCF3 as an
internal standard. [c] CuI was used. [d] 1c (0.26 mmol) was used.
“[Cu(CF2CH3)2]À” (27%, 19F NMR) was produced. [e] 1c (0.32 mmol) was
used. “[Cu(CF2CH3)2]À” (32%, 19F NMR) was produced.
copper species (“CuCF2CH3”) exhibits three signals in the
19F NMR spectrum: À67.0 ppm (A); À68.6 ppm (B) and
À73.2 ppm (C) (q, 3JF-H =28.2 Hz).[12] Similar to “CuCF3” spe-
cies,[13] the species A and B containing one CF2CH3 group, pre-
sumably complexed by DMF and chloride, denoted as
“L·CuCF2CH3” (L=DMF, ClÀ), and the signal at À73.2 ppm (C) is
tentatively assigned to “[Cu(CF2CH3)2]À”. After screening of vari-
ous initiators, we found that both tBuONa and CsF did not
generate the “CuCF2CH3” species; the former mainly gave the
protonated product (CH3CF2H) and the latter failed to activate
the C-Si bond of TMSCF2CH3 (Table 1, entries 2–3). In addition,
it was found that 2 equivalents of tBuOK were beneficial to
produce the “CuCF2CH3” species (Table 1, entries 4–5). The low
yield of “CuCF2CH3” generated from 1c is probably due to its
low thermal stability at room temperature. When the reaction
was carried out at 08C (Table 1, entry 6), the yield of
“CuCF2CH3” was increased to 67%. However, a lower tempera-
ture did not increase the yield (Table 1, entry 7). Furthermore,
the employment of CuI failed to improve the yield of
“CuCF2CH3” (Table 1, entry 8). After solvent screening, it was
found that NMP or DMA was inferior to DMF (Table 1, en-
tries 9–10).
Scheme 2. Scope of 1,1-difluoroethylation of diaryliodonium salts with
“CuCF2CH3”. Reaction conditions: diaryliodonium salts (0.4 mmol),
“CuCF2CH3” (1.5 equiv) neutralized by Et3N·3HF (0.5 equiv), DMF (4 mL).
Yields were of isolated products. [a] Yields were determined by 19F NMR
spectroscopy using PhCF3 as an internal standard.
In an attempt to improve the yield of “CuCF2CH3”, we in-
creased the loading of 1c under the optimized conditions.
When the amount of 1c was increased to 1.3 equiv, the yield
of the “CuCF2CH3” species was correspondingly enhanced to
82% (Table 1, entry 11). It should be noted that in this case, 1c
Generally, both electron-rich and -deficient aryl groups of di-
aryliodonium salts (2) were successfully transformed to give
the corresponding (1,1-difluoroethyl)arenes (3) in 40% to 91%
yields. This method tolerates various functional groups, such
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as tert-butyl (3b), methoxyl (3d), halides (3e and 3 f), acetyl
(3l), ester (3m), nitro (3n), trifluoromethyl (3o), formyl (3p),
hydroxyl (3q), nitrile (3r), affording the desired products in
moderate to good yields. It is noted that the (difluoroethyl)ar-
enes bearing methyl group(s) (3c, 3h, 3i and 3j) were ob-
tained in good yields, which is impossible to accomplish by
the activation of benzylic CÀH bonds.[5] The employment of
some aryl(mesityl)iodonium salts (2i, 2k–2s, 2u), owing to
their straightforward preparation, provided the selective trans-
fer of the smaller aryl groups,[16,17] even though a little amount
of (mesityl)difluoroethyled compound was detected by GC-MS.
Gratifyingly, the reaction worked well with a sterically hindered
dimesityliodonium salt (2j) under the optimized conditions, af-
fording the difluoroethylated product (3j) in a yield of 67%. In
addition, heteroaromatic iodonium salts (2s and 2t) were also
employed to provide the corresponding difluoroethyled heter-
oarenes (3s and 3t) in moderate to good yields. Furthermore,
a difluoroethylated estrone derivative (3u) was also obtained
by the reaction of an estrone-derived iodonium salt (2u) with
“CuCF2CH3” under the optimized conditions.
In the copper-mediated 1,1-difluoroethylation reaction, aryl
groups instead of the mesityl group of aryl(mesityl)iodonium
salts (Ar¼ mesityl) are transferred to form the difluoroethylated
products due to the steric effect of mesityl group, irrespective
of electronic properties. Subsequently, we examined the elec-
tronic effect using an anisyl(p-nitrophenyl)iodonium salt, the
result of which revealed preferential transfer of the more elec-
tron-deficient aryl group (Scheme 4).
We performed the reaction of diphenyliodonium salt (2a)
with “CuCF2CH3” generated from TMSCF2CH3 in the presence
of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) as a radical in-
hibitor, and found no significant decrease of the reaction effi-
ciency (Scheme 5). The results indicate that the involvement of
To further demonstrate the synthetic application of our 1,1-
difluoroethylation protocol, we sought to apply the method to
the difluoroethylation of medicinally important molecules.
Naproxen, an anti-inflammatory drug, is usually modified to
improve its potency by esterification of the carboxylic acid.[18]
Herein, we synthesized a difluoroethyled naproxen derivative,
substituting a difluoroethyl group for a methoxy group
(Scheme 3). After the hydroxyl derivate of naproxen protected
Scheme 5. Radical trapping experiment and plausible reaction pathways for
1,1-difluoroethylation of diaryliodonium salts. [a] Yields were determined by
19F NMR spectroscopy using PhCF3 as an internal standard.
a radical pathway is unlikely. According to previous report-
s[7l,13a] and our experimental results, we envisioned that the
present copper-mediated 1,1-difluoroethylation reaction of dia-
ryliodonium salts involves a CuI/III process (Scheme 5). The pre-
generated “CuICF2CH3” species performs oxidative addition re-
action with diaryliodonium salts to form the CuIII species. The
formed “ArCuIIICF2CH3” species undergoes reductive elimination
to give the coupling products 3.
In summary, the first copper-mediated 1,1-difluoroethylation
reaction has been developed by using TMSCF2CH3. The pre-
generated “CuCF2CH3” species from TMSCF2CH3 is found to be
remarkably efficient for 1,1-difluoroethylation of diaryliodoni-
um salts under mild conditions, affording electron-rich, elec-
tron-poor, and sterically hindered difluoroethylated arenes in
good to excellent yields. The developed methodology proves
to be highly functional group tolerant and is amenable to the
1,1-difluoroethylation of biologically active molecules: difluor-
oethyled estrone and naproxen derivatives were successfully
synthesized under the optimized conditions. Furthermore, the
Scheme 3. Synthesis of a difluoroethyled naproxen derivative 3v. a) 40% (w/
w) HBr, reflux, 2 h, 86%; b) i) (CF3CO)2O, THF, 08C, 4 h, ii) tBuOH, 08C to RT,
14 h, iii) 28% (w/w) NH3·H2O, 08C to RT, 1 h, 72%; c) Tf2O (1.1 equiv), Et3N,
CH2Cl2, 08C to RT, 1.5 h, 92%; d) [Pd(dppf)Cl2] (4 mol%), (BPin)2 (1.2 equiv),
KOAc (3 equiv), DMSO, 808C, 58%; e) NaIO4 (3 equiv), 1N HCl, THF/H2O (4:1,
v/v), RT, 8 h, 85%; f) i) MesI(OAc)2 (1.1 equiv), BF3·Et2O (2 equiv), CH2Cl2, 08C
to RT, 3 h, then NaBF4 (aq), ii) NaOTf (aq), 52%; g) i) TMSCF2CH3 (2.4 equiv),
CuCl (1.5 equiv), tBuOK (3 equiv), DMF, ii) Et3N·3HF (0.5 equiv), 82%.
as a tert-butyl ester, the pinacol boronic ester group was ac-
complished by a palladium-catalyzed borylation of the corre-
sponding triflate, and then hydrolyzed to the boronic acid. Fi-
nally, the naproxen-derived iodonium salt was obtained and
transformed to the difluoroethyled naproxen derivative (3v) in
82% yield.
Scheme 4. Chemoselectivity of an anisyl(p-nitrophenyl)iodonium salt. [a] The yield was determined by 19F NMR spectroscopy using PhCF3 as an internal stan-
dard.
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Acknowledgements
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This work was supported by the National Basic Research Pro-
gram of China (grant No. 2015CB931900, 2012CB215500), Na-
tional Natural Science Foundation of China (No. 21421002,
21372246, 21272069, 21102163), and Shanghai Science and
Technology Program (15XD1504400). We thank Professor Shiz-
heng Zhu (SIOC) for helpful discussions.
Keywords: fluorinated compounds
·
1,1-difluoroethyl
·
arenes · copper · diaryliodonium salts · silanes
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Manuscript received: April 26, 2016
Accepted Article published: May 4, 2016
Final Article published: June 1, 2016
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