Synthesis of seven-membered ring containing difluoromethylene unit by Sc(OTf)3-catalyzed activation of single C-F bond in CF3 group

Article abstract of DOI:10.1246/cl.190280

A Sc(OTf)₃-catalyzed activation of a single C-F bond in a CF₃ group is described. This reaction has two interesting features: (1) the synthesis of difluoromethylene compounds through Lewis acid-catalyzed activation of a single C-F bond in a CF₃ group, and (2) the selective formation of a seven-membered ring over a five-membered ring.

Full text of DOI:10.1246/cl.190280

Received: April 5, 2019 | Accepted: April 22, 2019 | Web Released: May 30, 2019
CL-190280
Synthesis of Seven-membered Ring Containing Difluoromethylene Unit by
Sc(OTf)3-catalyzed Activation of Single C-F Bond in CF3 Group
Naoya Hisano, Daiki Kimura, and Keiji Mori*
Department of Applied Chemistry, Graduate School of Engineering, Tokyo University of Agriculture and Technology,
2-24-16 Nakacho, Koganei, Tokyo 184-8588, Japan
E-mail: k_mori@cc.tuat.ac.jp
Lalic (2016): Transition metal catalyst
A Sc(OTf)3-catalyzed activation of a single C-F bond in a
Ph3SiH
CF3 group is described. This reaction has two interesting
features: (1) the synthesis of difluoromethylene compounds
cat. SIPrCuCl
cat. Pd(OAc)2
H
F
through Lewis acid-catalyzed activation of a single C-F bond in
a CF3 group, and (2) the selective formation of a seven-
membered ring over a five-membered ring.
F
F
F
F
Gschwind and Konig (2017) and Jui (2018) : Photocatalyst
photocatalyst
F
R
SET reduction
Keywords: C–F bond activation
| Seven-membered ring |
F
F
F
F
F F
Difluoromethylene compound
Yoshida and Hosoya (2016): Lewis acid-mediated reaction
TMS
SiPh2H
SiPh2F
SiPh2F
F
Recently, many organic chemists have shifted their attention
to organic fluorine compounds. The main reason for this trend is
the important effects fluorine has on pharmaceuticals, that is, the
introduction of a fluorine atom to organic molecules signifi-
cantly enhances their biological activities compared to a non-
fluorine molecule.1 Among the known organic fluorine mole-
cules, those with a difluoromethylene unit have attracted much
attention due to their prominent biological activities, such as
Tafluprost, Gemcitabine, and Docetaxel analog.2
Ph3CBF4
F
F
+
F
F
F
F
Scheme 1. Strategies for activation of single C-F bond in an
aromatic CF3 group.
Our previous work (Lewis-acid catalyzed C–H bond functionalization)
EWG
EWG
EWG
EWG
Δ
or
H
EWG
EWG
Acid catalyst
H
H
To access the target structure, the mono functionalization of
a C-F bond3-5 in a CF3 group would be a reliable choice because
of the high accessibility of compounds with the CF3 group. Most
of the successful methods reported so far have relied on the
cleavage of a C-F bond with the aid of an adjacent activating
group, such as a carbonyl group6 or a vinyl group.7 Reports of
the single C-F bond functionalization of a simple CF3 group,
particularly an aromatic CF3 group, are scarce.8 This is difficult
to achieve because the C-F bond dissociation energy gradually
decreases as the number of fluorine atoms decreases, that is, the
activation of a C-F bond in a CF2 group is easier than that in
a CF3 group. Leckta and co-workers, in a pioneering work in
1997,9 accomplished the synthesis of a fluorine containing
difluoromethylene unit through an intramolecular fluorine trans-
fer from a CF3 group to a vinyl cation generated from a
diazonium salt. However, the chemical yield was quite low
(20%). After almost 20 years, four research groups independ-
ently reported effective methods. The key reaction in three of the
four groups was a single-electron transfer by a transition metal
(Pd-Cu catalysis by the Lalic group)10 and a photoredox catalyst
(fac-Ir(ppy)3 by Gschwind and König groups,11a and N-phenyl-
phenothiazine by the Jui group) (Scheme 1).11b A Lewis acid
version was developed by the Yoshida and Hosoya group, in
which installation of an ortho-silyl group relative to a CF3 group
was essential.12 The achievement of a simple Lewis acid-
catalyzed single C-F bond activation is not a trivial issue, and the
development of a novel and simple method is strongly desired.
Recently, our group has been interested in the hydride shift
triggered C(sp3)-H bond functionalization, namely, internal
redox reaction (Scheme 2).13 In the course of development of
new transformations by way of group transfer instead of hydride
[1,5]-H shift
6-endo-
X
R1
X
R1
X
R1
cyclization
X = NR2, O.
EWG = CN, CO2R, etc.
This work
O
N
N
O
O
O
5
N
30 mol%
Sc(OTf)3
Benzene
O
O
+
N
N
7
5
O
N
F
O
O
R
R
R
O
F
F
F
F
1
2
3
major
minor
Scheme 2. Formation of difluoromethylene-containing seven-
membered ring by single C-F bond activation.
shift,13l we examined fluorine group transfer. Instead of
achieving the desired reaction, we found a Lewis acid-catalyzed
activation of a single C-F bond in an aromatic CF3 group
starting from 1,3-dimethyl barbituric acid 1 having an ortho-CF3
benzyl group at 5-position. This reaction has two interesting
features: (1) the synthesis of difluoromethylene compounds
through Lewis acid-catalyzed activation of a single C-F bond
in a CF3 group, and (2) the selective formation of a seven-
membered ring over a five-membered ring. We report herein the
details of this unique reaction.
Table 1 summarizes the results of screening for the reaction
conditions. When 1a was treated with 30 mol % of Yb(OTf)3 in
refluxing ClCH2CH2Cl, the reaction did not proceed, and 1a was
completely recovered (Entry 1). Both Gd(OTf)3 and La(OTf)3
were also ineffective (Entries 2 and 3). Gratifyingly, Hf(OTf)4
exhibited excellent catalytic performance, and C-F bond activa-
tion followed by hydrolysis occurred to give indanone derivative
Chem. Lett. 2019, 48, 771–774 | doi:10.1246/cl.190280
© 2019 The Chemical Society of Japan | 771

Table 1. Examination of reaction conditions.a
O
Unsuitable substrates
O
O
O
F
O
F
O
O
N
N
MeO
OMe
O
O
N
OH
N
N
O
O
O
N
F
F
F
30 mol%
catalysts
O
O
O
N
N
F
F
F
F
F F
+
F
F
F F
O
N
solvent
O
CF3
1a
O
F
F
2a
Figure 1. Importance of the 1,3-dimethyl barbituric acid
moiety.
3a
yield (%)b
Run
Catalyst
Solvent
O
2a
3a
1a
N
N
O
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
Yb(OTf)3
Gd(OTf)3
La(OTf)3
Hf(OTf)4
Sc(OTf)3
TiCl4
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
Benzene
0
0
0
0
42
7
0
0
0
0
45
0
0
0
0
0
0
0
66
20
7
0
0
0
0
8
35
24
0
0
0
98
98
94
0
O
O
N
30 mol%
Sc(OTf)3
O
O
O
N
N
+
O
N
Benzene
O
CF3
R
R
R
O
F
F
1
2
3
0
O
O
O
O
O
O
O
N
N
N
N
52
98
71
94
94
Me
F
O
O
O
O
N
N
N
N
SnCl4
O
Me
F
F
F
F
F
F
F
F
BF3¢OEt2
B(C6F5)3
TfOH
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
2b
3b: 13%)
2c
3c: trace)
2d
3d
2e: 36% (3e: trace)
Figure 2. Substrate scope.
Trace
0
Toluene
C6H5CF3
CH3CN
THF
O
O
O
0
N
N
30 mol%
Sc(OTf)3
O
O
O
98
98
93
N
O
N
+
N
N
Benzene
O
O
O
F
F
F
2a
F
Pyridine
0
2a: 18%
3a: 60%
aUnless otherwise noted, all reactions were conducted with
0.10 mmol of 1a in the presence of 30 mol % of catalyst in
solvent (1.0 mL) at refluxing temperature. Isolated yield.
19
O
b
N
N
F
O
N
O
O
O
F
N
Sc(OTf)3
CDCl3
O
F
F
F
[Sc]
F
F
F
3a in good chemical yield (66%, Entry 4). Sc(OTf)3 also
showed high catalytic ability, but a slightly different result was
observed. Not only indanone 3a (five-membered ring), but also
seven-membered ring adduct 2a with a difluoromethylene unit
was obtained, and the latter one was the major product (3a: 20%,
2a: 42%, respectively, Entry 5). The structures of 2a and 3a
were determined by X-ray analysis.14 As described above,
careful substrate design and special setting of the reaction
conditions were required to activate a single C-F bond in an
aromatic CF3 group.10-12 The present reaction realized this
difficult transformation without special precautions. Another
interesting feature of this reaction was the selective formation of
a seven-membered (middle-sized) ring over a five-membered
ring. To improve this abnormal product selectivity (2a/3a),
further screening for the reaction conditions was conducted.
Examination of the acid catalysts suggested that Sc(OTf)3 was
the catalyst of choice. Some strong Lewis acids, such as TiCl4,
BF3¢OEt2, B(C6F5)3, and Brønsted acid (TfOH), did not promote
the reaction at all (Entries 6-10). Then, solvent screening was
conducted with Sc(OTf)3 as the optimal catalyst, and it was
revealed that the choice of solvent had a significant effect on
the selectivity of the two products (2a/3a). Selectivity was
improved in benzene, and seven-membered ring 2a was obtained
in 45% yield (Entry 11). In sharp contrast, indanone 3a was
furnished exclusively in the case of toluene and C6H5CF3 (35%
and 24%, respectively, Entries 12 and 13). Polar solvents, such
as CH3CN, THF, and pyridine, completely inhibited the reaction,
and 1a was recovered (Entries 14-16).
F
1a
A
Figure 3. Confirmation of kinetic control and 19F NMR
experiment.
The employment of 1,3-dimethyl barbituric acid was
indispensable to achieve the reaction (C-F bond activation)
(Figure 1). Various 1,3-dicarbonyl moieties, such as dimethyl
malonate, piperidine-2-6-dione, and pyrolidine-2-5-dione, and
carboxylic acid and amide moiety resulted in the recovery of the
starting materials.
The substrate scope of this seven-membered ring formation
is illustrated in Figure 2. The selective formation of seven-
membered rings was observed when substrates 1b-d with
methyl and fluorine groups on the aromatic ring were used, and
corresponding difluoromethylene compounds 2b-d were ob-
tained in moderate chemical yields (34-47%). The construction
of a tetracyclic adduct was also attainable from a naphthyl-type
substrate (2e: 36%).
To obtain some insights into the reaction mechanism,
additional experiments were conducted (Figure 3). Exposure of
seven-membered ring adduct 2a to the optimized reaction
conditions resulted in the formation of indanone 3a (60%). This
suggests that seven-membered ring adduct 2a was the kinetic
product. Although the strengths of each peak were quite weak
due to the low solubility of Sc(OTf)3, two new peaks with 1:2
ratio were observed in the 19F NMR spectrum by mixing 1a and
772 | Chem. Lett. 2019, 48, 771–774 | doi:10.1246/cl.190280
© 2019 The Chemical Society of Japan

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F
O
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N
N
H
O
O
N
N
[Sc]
1
2
O
O
C–F bond
cleavage
+
F
R
R
[Sc]
[Sc]
F
F
F
F
A
B
O
3
N
N
O
3
O
+
F
R
[Sc]
F
C
Figure 4. Proposed reaction mechanism.
Sc(OTf)3 in CDCl3 (1a:Sc(OTf)3 = 1:1 molar ratio), indicating
some interaction taking place between one of the fluorine atoms
and Sc(OTf)3.15 The absence of any changes of the fluorine
peaks upon mixing C6H5CF3 and Sc(OTf)3 (1:1 molar ratio)
suggests that one of the carbonyl oxygens of barbituric acid
supported the formation of a complex of 1a and Sc(OTf)3, like
A in Figure 3.
Based on the above experiments, the proposed reaction
mechanism is illustrated in Figure 4. At first, one of the fluorine
atoms in the CF3 group and a carbonyl oxygen in barbituric acid
coordinate to Sc(OTf)3, resulting in the activation of a C-F
bond. The preferential formation of a seven-membered ring over
a five-membered ring could be explained on the basis of the
principle of least motion.16 From cationic intermediate B,17 the
seven-membered ring formation (red arrow) would occur in one
step while maintaining the structure of B. On the other hand, a
two-step sequence (enolization followed by intramolecular
nucleophilic addition, blue arrow) would be required for the
formation of 3. As a result, seven-membered ring adducts 2 were
obtained in a highly selective manner.
4
5
In summary, we have developed a direct route to hetero-
cycles having a difluoromethylene unit via the activation of a
single C-F bond in the CF3 group. Although it is not clear why
the single C-F bond activation occurred even with strong Lewis
acid catalysis, the reaction described herein offers a new entry in
the C-F bond activation field. Mechanistic studies based on
theoretical calculations are under way in our laboratory, and will
be reported in due course.
Supporting Information is available on https://doi.org/
10.1246/cl.190280.
6
7
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774 | Chem. Lett. 2019, 48, 771–774 | doi:10.1246/cl.190280
© 2019 The Chemical Society of Japan