Carbene-Induced Intra- vs Intermolecular Transfer-Fluoromethylation of Aryl Fluoromethylthio Compounds under Rhodium Catalysis

Article abstract of DOI:10.1021/acscatal.5b00853

The intra- vs intermolecular transfer-fluoromethylation of aryl fluoromethylthio compounds is proposed. Finely designed ArSCF₃ (1a) nicely releases its trifluoromethyl (CF₃) group intermolecularly under rhodium catalysis, whereas a difluoromethylated analogue, ArSCF₂H compound 1b shows intramolecular reaction. (Chemical Presented).

Full text of DOI:10.1021/acscatal.5b00853

Letter
pubs.acs.org/acscatalysis
Carbene-Induced Intra- vs Intermolecular Transfer-
Fluoromethylation of Aryl Fluoromethylthio Compounds under
Rhodium Catalysis
Ibrayim Saidalimu, Etsuko Tokunaga, and Norio Shibata*
Department of Nanopharmaceutical Sciences, Nagoya Institute of Technology, Gokiso, Showa-ku, Nagoya 466-8555, Japan
S
* Supporting Information
ABSTRACT: The intra- vs intermolecular transfer-fluorome-
thylation of aryl fluoromethylthio compounds is proposed.
Finely designed ArSCF3 (1a) nicely releases its trifluoromethyl
(CF3) group intermolecularly under rhodium catalysis, whereas
a difluoromethylated analogue, ArSCF2H compound 1b shows
intramolecular reaction.
KEYWORDS: trifluoromethylation reagent, difluoromethylation reagent, rhodium catalyst, intramolecular, intermolecular
otable success witnessed in recent synthetic fluorine
omethylation product 3a, Stevens rearrangement12 product,
Nchemistry is obviously related to the development of new was observed, even in the absence of nucleophiles. On the
other hand, a difluoromethylated analogue, ArSCF2H com-
pound 1b behaves rather differently to 1a. Intramolecular
transfer-difluoromethylation on the oxygen atom proceeded
providing O−CF2H 3b, even in the presence of nucleophiles
(Scheme 1). Cationic, radical, and carbene mechanisms are
fluoro-functionalization reagents, such as fluorination and
trifluoromethylation reagents, and their usage under new
catalytic systems supported by the meticulous work of organic
chemists involved in fluorine chemistry and organometallics.1,2
Electrophilic trifluoromethylation reagents have been one of
the most awaited reagents for years.3,4 They have been
developing relatively slowly, probably due to the difficulty in
generating a trifluoromethyl cation (+CF3), which is affected by
its high group electronegativity (3.45).5 Several shelf-stable
reagents have been reported for this purpose: diaryl-
(trifluoromethyl)sulfonium salts (1984, Yagupolskii),6 chalco-
genium salts (1990, Umemoto),7 hypervalent iodine com-
pounds (2006, Togni),8 (trifluoromethyl)sulfoximinium and 5-
thiophenium salts (2008, 2010, Shibata).9 They are effective for
the electrophilic-type trifluoromethylation of a wide range of
nucleophiles, and some of them are now commercially
available. It is not surprising that researchers are continuously
eager for new fluoro-functionalization reagents, because new
reagents often encourage an encounter with efficient synthetic
methodology useful for the synthesis of sought-after organo-
fluorine compounds on the drug market.10 In this context, we
disclose herein a different strategy based on the in situ
generation of “unstable/reactive +CF3 equivalents” from a shelf-
stable aryl-trifluoromethylthio compound, ArSCF3, instead of
Scheme 1. Intra- vs Intermolecular Transfer-
Fluoromethylation of ArSCF3 and ArSCF2H Compounds
under Rhodium Catalysis
proposed to understand the difference of the reaction pathways
depend of the CF2X based on the discussions of Stevens
rearrangement.
The finely designed ArSCF3 compound 1a was easily
prepared from readily available ortho-ArSCF3 ethanone 413 by
the procedure shown in Scheme 2. First, 4 was treated with
dimethyl carbonate under basic and reflux conditions to
provide ArSCF3 methyl propanoate 5 with 91% yield. The
target reagent 1a was prepared quantitatively by diazotization of
5 with 4-methylbenzenesulfonyl azide in 10 min. The reagent
1a is stable enough at room temperature and even in MeCN
under reflux for 24 h (see run 23, in Table 1).
+
“shelf-stable CF3 equivalents”. The finely designed ArSCF3
compound, methyl 2-diazo-3-oxo-3-(2-((trifluoromethyl)thio)-
phenyl)propanoate (1a), has a carbenoid generation pendant
on its ortho position. An intermolecular transfer-trifluorome-
thylation from the SCF3 moiety on 1a to carbon nucleophiles
(Nu-H, Nu-SiMe3) proceeds smoothly through a tandem
process consisting of a rhodium carbene intermediate11 and a
cyclized inner salt to furnish CF3-products with the exit of
methyl 3-oxo-2,3-dihydrobenzo[b]thiophene-2-carboxylate (2).
Only a trace amount of an intramolecular transfer-trifluor-
Received: April 25, 2015
Revised: July 4, 2015
© XXXX American Chemical Society
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Letter
basis of these results, a set of optimal reaction conditions was
screened out: 1.5 equiv of 1a, 3 mol % of Rh2(OAc)4, 1.5 equiv
of DBU, and reflux temperature in MeCN (84%, run 18). In all
cases, nontrifluoromethylated cyclized 2 was detected in a large
quantity, and intramolecular transfer-trifluoromethylation
product 3a, i.e., Stevens rearrangement product, was observed
only in a trace amount up to 10% yield (runs 20−22), even in
the absence of 6a (run 22). No reaction was observed in the
absence of Rh2(OAc)4/DBU, and ArSCF3 1a was left intact
(run 23).
Scheme 2. Preparation of Designed Ar-SCF3 Compound
a
Table 1. Optimizations of Reaction Conditions
We proceeded to evaluate the scope of trifluoromethylation
by 1a with a wide variety of substrates 6b−m, 8a−8c (Table 2).
a
Table 2. Scope of Trifluoromethylation of 6 and 8
b
run
cat.
base
DBU
solvent
time (h) yield (%)
1
Rh2(OAc)4
Rh2(OAc)4
Rh2(OAc)4
Rh2(OAc)4
Rh2(OAc)4
Rh2(OAc)4
CuI
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
MeCN
MeOH
toluene
DMF
5
52
2
TEA
24
24
3
trace
trace
trace
18
3
DABCO
LDA
K2CO3
BuOK
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
-----
4
5
24
24
24
24
24
4
6
9
7
31
8
Pd2(dba)3
------
trace
15
9
10
11
12
13
14
15
16
Rh2(OAc)4
Rh2(OAc)4
Rh2(OAc)4
Rh2(OAc)4
Rh2(OAc)4
Rh2(OAc)4
Rh2(OAc)4
Rh2(OAc)4
Rh2(OAc)4
Rh2(OAc)4
Rh2(OAc)4
Rh2(OAc)4
Rh2(OAc)4
-------
81
12
12
12
12
12
12
3
trace
28
12
THF
trace
35
Et2O
DCE
25
c
17
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
76
d
18
2
84
e
19
2
83
f
g
20
24
24
24
24
12
g
21
7
h
g
22
DBU
-----
0
a
The reaction of 6 or 8 with reagent 1a (1.5 equiv) was carried out in
23
0
the presence of Rh2(OAc)4 (3 mol %) and DBU (1.5 equiv) in
CH3CN under reflux (substrate scale is 0.1 mmol). Isolated yields are
indicated. For detailed reaction conditions, see the Supporting
Information. 0.2 mmol scale of substrate was examined. 2.5 equiv
of reagent 1a was used.
a
The reaction of 6a with reagent 1a (1.5 equiv) was carried out in the
presence of base (1.2 equiv) in solvent at reflux temperature. For
b
detailed reaction conditions, see the Supporting Information. 19F
b
c
c
d
e
NMR yield. Used 1.0 equiv of DBU. Used 1.5 equiv of DBU. Used
2.0 equiv of DBU. Used 10 mol % of DBU. Intramolecular product
f
g
3a was also obtained in 6−10% yield, based on the use of 1a.
h
The reaction was performed under 0.1 mmol and/or 0.2 mmol
scales. Indanone carboxylates 6b−i reacted with 1a smoothly
under the optimized conditions, independent of the electronic
nature of the substitution on the benzene ring (MeO, Me, Cl)
or the size of the ester moiety (Me, Et, iPr, Ad, Bn, cHex) to
provide corresponding products 7b−i in good to excellent
yields. Notably, electron-rich dimethoxy-indanone carboxylate
6j also underwent the trifluoromethylation reaction to give the
corresponding product in moderate yield (7j, 69−70%).
Tetralone carboxylates 6k−l were also good substrates for the
trifluoromethylation reaction to furnish desired products 7k−l
in 46−62% yields. The yield of 7m was 23−28% when methyl
2-cyclopentanonecarboxylate 6m was used as substrate. It is
noteworthy that dicyanoalkylidenes 8a−c reacted nicely with
1a to provide the desired products 9a−c in moderate to good
yields independent of the cyclic and acyclic structures, whereas
the bis-trifluoromethylated compound 9aa was predominantly
obtained (51%) instead of 9a (13%) with the 2.5 equiv of 1a.
The ArSCF3 reagent 1a was found to be effective for
Reaction was examined in the absence of 6a.
With the reagent 1a in hand, we attempted the
trifluoromethylation of methyl 1-oxo-2,3-dihydro-1H-indene-
2-carboxylate (6a) by 1a via an intermolecular pathway. The
base, metal, and solvent were varied, and the results are
summarized in Table 1. Treatment of 6a with 1a in the
presence of Rh2(OAc)4 (3 mol %) and 1.2 equiv of DBU in
CH2Cl2 gave the trifluoromethylated product 7a in 52% yield
(Table 1, run 1). Using other organic and inorganic bases such
as triethylamine (TEA), DABCO, LDA, and K2CO3 under the
same reaction conditions led to no reaction or a low yield of
desired 7a (runs 2−6). Copper, palladium, or no-metal catalysis
in the presence of DBU was also examined, but no
improvement was observed (runs 7−9). Solvents were next
screened under the conditions of Rh2(OAc)4 and DBU, and
MeCN gave the best result of 81% (runs 10−16). Further
studies focused on the amount of DBU (runs 17−21). On the
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Letter
intermolecular transfer trifluoromethylation independent of the
substrate family 6 and 8 under the same reaction conditions to
provide the desired CF3-products 7 and 9 within several hours
in good to excellent yields.
The scope of rhodium-catalyzed transfer-trifluoromethylation
from 1a to substrates was next extended to silyl enol ethers 10.
The results are summarized in Table 3. The silyl enol ethers
a
Table 3. Substrate Scope of Trifluoromethylation of 10
Figure 1. Proposed reaction mechanism I: Intra- vs intermolecular
transfer-trifluoromethylation from ArSCF3 1a via a tandem process
consisting of a rhodium carbene intermediate A and a cyclized inner
salt B.
intramolecular Stevens rearrangement to form 3a should
support a concerted or stepwise cationic reaction pathway for
electrophilic trifluoromethylation via +CF3 (Figure 2a), which is
a
The reaction of 10a−i with reagent 1a (2.0 equiv) was carried out in
the presence of Rh2(OAc)4 (5 mol %) in MeCN at reflux temperature.
b
For detailed reaction conditions, see SI. 19F NMR yield. Isolated
yield.
Figure 2. Proposed reaction mechanism II: (a) Cationic process
providing intermolecular products. (b) Stevens 1,2-migration process
involving radical pair in a solvent-cage providing intramolecular
product 3a.
10a−f with various substituents on the aromatic ring, including
electron-donating (OMe) and electron-withdrawing groups
(Cl, CF3, F) reacted smoothly and led to the corresponding
trifluoromethylation product 11a−f in moderate to good yields.
The sterically demanding naphtyl substrate 10g and indanone
trimethylsilyl ether 10h were also compatible with this
transformation, providing desired α-CF3-ketones 11g and 11h
in 65% and 59% yield, respectively. Pyridine derivative 10i was
tolerated under the same reaction conditions to provide 10i in
15% yield. All the reactions of silyl enol ethers 10 proceeded
nicely, and the yields of products 11 were moderate to good.
Competitive intramolecular Stevens rearrangement providing
3a was observed only in 5−10% yield (Table 3).
The proposed reaction mechanism is shown in Figure 1. Ar-
SCF3 1a initially reacts with Rh2(OAc)4 providing a rhodium
carbene intermediate A, which cyclizes into a reactive inner salt
B.11 An intermolecular transfer-trifluoromethylation predom-
inantly proceeds from the salt B to NuH or Nu-SiMe3 as
outlined in the figure to provide CF3-products, Nu-CF3 with 2
after a workup process. Intramolecular Stevens rearrangement
of the CF3 group in B furnishing 3a is considerably inferior
relative to the desired intermolecular transfer-trifluoromethyla-
tion, even in the absence of nucleophiles (up to 10%).
still one of the matter of debates in fluorine chemistry over
decades.15 The mechanism of Stevens 1,2-rearrangement of
stabilized ammonium ylides and sulfonium ylides is proposed
to occur via an intramolecular homolytic dissociation-
recombination processes involving radical pair in a solvent-
cage, by crossover experiments and stereochemical inves-
tigations.14 If the transfer trifluoromethylation in this system
involves a radical related process, the intramolecular Stevens
1,2-migration should predominantly be observed to form 3a
over intermolecular reaction (Figure 2b).
We were next interested in a difluoromethylated analogue,
ArSCF2H 1b, because a difluoromethyl group is also important
in medicinal chemistry, due to the isosteric relation between
CF2H group and OH and NH groups through hydrogen
bonding.16,17 The ArSCF2H 1b was prepared according to the
modified procedure based on Scheme 2 (see, Supporting
Information for details), and the reaction was examined.
Interestingly, the reactivity of 1b was rather different from 1a:
the intramolecular reaction of 1b predominantly proceeded to
complete the O−CF2H product 3b in 48−56% yield, even in
the presence of nucleophile 6a. Neither intramolecular C−
CF2H products like 3a nor intermolecular products 12a, b were
observed (Scheme 3a). To see the generality of this reactivity
pattern, the reaction of 1b with other nucleophiles (6b, 4-
hydroxybenzenesulfonic acid) was also attempted, and similar
It should be mentioned that the present system displays very
different reactivity from the established Stevens rearrangement
using thioethers with rhodium carbenoids furnishing intra-
molecular 1,2-migration products.12c,d,14 The fact of the
preference of unprecedented intermolecular trifluoromethyl
transfer to carbon nucleophiles 6, 8, and 10 over an
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Letter
Scheme 3. Reaction of ArSCF2H 1b under Rhodium
Catalysis (a, b) and Its Proposed Mechanism (c)
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acscatal.5b00853.
Experimental procedures and NMR spectra (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: nozshiba@nitech.ac.jp.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was financially supported in part by the Platform
for Drug Discovery, Informatics, and Structural Life Science
from MEXT Japan, the Advanced Catalytic Transformation
(ACT-C) from the Japan Science and Technology (JST)
Agency, and Hoansha Foundation.
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