Benzylic C-H trifluoromethylation of phenol derivatives

Article abstract of DOI:10.1039/c5cc07011b

Phenol derivatives were trifluoromethylated using copper/Togni reagent. In dimethylformamide, the benzylic C-H bond at the para position of the hydroxyl group was selectively substituted with a CF₃ group. In contrast, aromatic C-H trifluorometh

Full text of DOI:10.1039/c5cc07011b

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Benzylic C–H trifluoromethylation of phenol derivatives
Hiromichi Egami,a Takafumi Ide,a Yuji Kawatoa and Yoshitaka Hamashima*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Phenol derivatives were trifluoromethylated using copper/Togni 1). Herein, we describe Cu-catalyzed benzylic Csp3–H and
reagent. In dimethylformamide, the benzylic C–H bond at the para aromatic Csp2–H trifluoromethylation of substituted phenol
position of the hydroxyl group was selectively substituted with a derivatives, using Togni reagent II.3j,10
CF3 group. In contrast, aromatic C–H trifluoromethylation occurred
OH
OH
in alcoholic solvents. Practical utility of the reactions was
demonstrated by application to the synthesis of a potent enoyl-
acyl carrier protein reductase (FabI) inhibitor.
R1
R2
R1
R2
R3
F3C
I
O
CuI
+
O
R3
DMF, 40 °C, 1 h
R4 CF3
R4
Introduction of trifluoromethyl group(s) into organic molecules
often results in significant improvements in metabolic stability,
hydrophobicity, and lipophilicity.1,2 Thus, trifluoromethyl
substitution has attracted much attention in the
pharmaceutical and agrochemical fields, and the development
of efficient trifluoromethylation methodology is of continuing
interest.3
1
Togni reagent II
2
OH
R1
F3C
R2
R3
F3C
I
O
[(MeCN)4Cu]PF6
t-BuOH, 40 °C, 1 h
R4
Togni reagent I
3
Scheme 1 This work: C–H trifluoromethylations.
Although phenols are widely found as structural
components of bioactive compounds, only a few attempts to
introduce a CF3 group into phenols have been reported. In
For screening of the reaction conditions, we selected phenol
2008, Togni and co-workers disclosed a trifluoromethylation of derivative 1a bearing two methyl groups at the ortho- and
sodium phenolate derivatives.4 However, the reaction was para-positions and a tert-butyl group at the other ortho-
hard to control and various types of by-products were formed. position as a test substrate (Table 1). Preliminary experiments
They also fortuitously found that reaction of 2,4,6- revealed that reactions of less substituted phenols tended to
trimethylphenol with Togni reagent I3j,5 gave a Csp3–H involve undesired side reactions, including oxidative
trifluoromethylated product.4 However, only one substrate dimerization and oligomerization, due to higher reactivity of
was examined, and neither the reaction time (4 days) nor the ortho and para positions of phenols. The bulky tert-butyl group
chemical yield was practically useful.
was introduced to suppress such undesired reactions, since
To date, various types of trifluoromethylation have been the tert-butyl group can be removed under acidic conditions
reported in the literature.3 Among them, direct C–H (vide infra). First, various metal salts were tested in CH2Cl2 at
trifluoromethylation is the most atom-economical strategy. 40 °C (entries 1-7). The reaction proceeded cleanly in the
Compared to aromatic C–H substitution reactions via either presence of 20 mol % of CuI or CuOAc. It should be noted that
metal-catalyzed C–H activation6 or addition of
a
a CF3 group was selectively incorporated at the para-methyl
trifluoromethyl radical7, direct Csp3–H trifluoromethylation has group to give 2a in 65 and 63% yield, respectively (entries 1
been less well studied.8 During the course of our investigations and 4); substitution at the ortho-methyl group and at the
on trifluoromethylation chemistry,9 we found two types of aromatic C–H group was negligible. Interestingly, a divalent
solvent-dependent trifluoromethylation of phenols (Scheme copper species, Cu(OAc)2, afforded 2a in 78% yield (entry 5).
On the other hand, the reaction did not proceed with iron salts
(entry 7). MeCN, DMSO, and NMP were found to be good
a.School of Pharmaceutical Sciences, University of Shizuoka, 52-1 Yada, Suruga-ku,
Shizuoka, Japan. E-mail: hamashima@u-shizuoka-ken.ac.jp
†Electronic Supplementary Informaꢀon (ESI) available: Experimental details and
spectroscopic data. See DOI: 10.1039/x0xx00000x
solvents (entries 8-10). Among the solvents screened, DMF
was the solvent of choice, and 2a was isolated in excellent
yield (93%) under the described conditions (entry 11). In
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contrast, the use of MeOH resulted in formation of the trifluoromethylation reactions, was also a good substrate of
aromatic trifluoromethylation product 3a as the major product our reaction (2n).
(entry 14). While both monovalent and divalent copper species
Table 2 Substrate scopea
were equally effective catalysts in CH2Cl2, CuI gave a better
yield than Cu(OAc)2 in DMF (entries 11 and 12). It is
noteworthy that 10 mol % of CuI was enough for this
transformation; no reaction occurred in the absence of copper
catalyst (entries 13,15).
OH
OH
R1
R2
R3
R1
R2
R3
F3C
I
O
CuI (10 mol%)
+
O
DMF, 40 °C, 1 h
R4 CF3
R4
1
(1.5 equiv.)
OH
2
Table 1 Screening of the reaction conditionsa
OH
OH
OH
cat. (20 mol %)
Togni reagent II
(1.5 equiv.)
OH
tBu
OMe
tBu
NHCbz tBu
CO2Et
OH
tBu
tBu
tBu
40 °C, 1 h
CF3
CF3
CF3
CF3
CF3
1a
2a
3a
2b: 81%
2c: 86%
2d: 86%
OH
OH
OH
Entry
1
Catalyst
CuI
Solvent
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
MeCN
DMSO
NMP
Yield of 2a (%)b
tBu
tBu
TES tBu
CO2Et
OMe
65
38
2
2
[(MeCN)4Cu]PF6
CuTc
3
51
CF3
CF3
CF3
4
CuOAc
Cu(OAc)2
Cu(OTf)2
Fe(OAc)2
CuI
63
2e: 79%
2f: 59%
2g: 62%
5
78
R
OH
OH
OH
6
28
tBu
S
tBu
tBu
7
trace
72
8
9
CuI
50
CF3
CF3
CF3
10
11
12
13d
14
15
CuI
61
93c
2k: 53%b
2h, R = OMe : 84%
2i, R = H: 51%
2j: 78%
CuI
DMF
Cu(OAc)2
CuI
DMF
83
OH
OH
OH
DMF
93c
5e
tBu
tBu
CuI
MeOH
DMF
-
N.R.f
a The reactions were carried out with catalyst (20 mol %) and Togni reagent II (1.5
CF3
CF3
2m: 72%
CF3
2n: 75%
b
2l: 81%
equiv.) at 40 °C for 1 h on a 0.2 mmol scale, unless otherwise mentioned.
Determined by 1H NMR analysis with 1,1,2,2-tetrachloroethane as an internal
standard. c Isolated yield. d Run with 10 mol % CuI. e Compound 3a was obtained
in 25% yield. f No reaction.
a
The reactions were carried out with CuI (10 mol %) and Togni reagent II (1.5
equiv.) in DMF at 40 °C for 1 h on a 0.2 mmol scale, unless otherwise mentioned.
b Run at 90 °C.
Having established the optimal reaction conditions, the
scope of the reaction was investigated by using various phenol
derivatives (Table 2). Phenols having an electron-donating
heteroatom group were good substrates, and 2b and 2c were
obtained in high yields. Various functional groups, such as
To confirm the usefulness of the present reaction, we
examined transformation of the product 2a (Scheme 2). The
tert-butyl group was readily removed under acidic conditions
to give
4 in quantitative yield. After triflation, Suzuki-Miyaura
cross-coupling reaction was conducted under the described
ester,
α,β-unsaturated ester and silyl, were tolerated in the
conditions, giving
6
in 89% yield.
reaction (2d-f). C–H trifluoromethylation at the para-benzylic
position of the phenols occurred in preference to aromatic
trifluoromethylation under our reaction conditions, even for
substrates with another aromatic ring (2h-j). Encouraged by
these results, we investigated the reaction of 1k having a
secondary alkyl C–H bond. Although the reaction became
slower, probably due to steric repulsion, an acceptable yield
was obtained when the reaction was carried out at 90 °C. The
presence of a tert-butyl group is not essential, and reactions of
Pd(PPh3)4
PhB(OH)2
Na2CO3
OH
OTf
Ph
Tf2O
pyridine
R
toluene
100 °C, 10 h
CF3
CF3
CF3
6: 89%
2a: R = tBu,
5: 96%
H2SO4
toluene
120 °C
4: R = H
quant.
naphthyl substrate 1l and 2,4,6-trimethylphenol 1m proceeded Scheme 2 Further transformations of the product.
smoothly to afford the corresponding products 2l and 2m in 81
We also applied this reaction to achieve an efficient
synthesis of a potent enoyl-acyl carrier protein reductase
(FabI) inhibitor 9
, which is effective against resistant bacteria11
and 72% yield, respectively. In addition, 2,6-di-tert-butyl-4-
methylphenol (BHT), which is a well-known radical scavenger
that has been used for mechanistic studies of several
(Scheme 3). Thus, starting from 2b, removal of the tert-butyl
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tBu
OH
group gave compound
7
in 88% yield. Simple addition reaction
to 2,6-difluoropyridine provided
in good yield under basic conditions. Finally, the
methyl group was removed by treatment with BBr3, affording
of the obtained phenol
diaryl ether
7
tBu
HO
8
CuI (10 mol %)
Togni reagent II
(1 equiv.)
OH
10: 38%
tBu
TEMPO (1 equiv.)
the target compound
9
in 72% yield.
(1)
+
DMF, 40 °C, 1 h
MeO
OH
tBu
MeO
OH
KOH
H2SO4
toluene
120 °C, 12 h
88%
N
F
N
F
1a
OCF3
DMF, 110 °C
90%
11: 58%
Determined by 19F NMR
F3C
F3C
2b
7
Based on the above results, we consider that the mechanism
of the benzylic Csp3–H trifluoromethylation is as illustrated in
Scheme 5. The active copper ion seems to be the divalent
species,13 since Cu(OAc)2 was also a good catalyst (Table 1,
F
F
N
N
MeO
O
HO
O
BBr3
CH2Cl2
-78 °C–rt, 12 h
72%
entry 12). Treatment of CuI with phenol
Togni reagent may provide copper(II)-phenoxide complex
This complex would be oxidized by Togni reagent to afford a
phenoxy radical species with simultaneous formation of a
1 in the presence of
8
9
F3C
F3C
A
.
Scheme 3 Synthesis of a potent FabI inhibitor 9.
A
B
As described above (Table 1, entry 14), the reaction pathway
changed when MeOH was used as the solvent.[12] This solvent-
dependent product switching seems general, and may be due
to competitive decomplexation of the putative Cu phenoxide
with a large excess of the alcoholic solvent (vide infra).
Although the chemical yield should be improved, meta-
trifluoromethyl radical equivalent. Then, this phenoxy radical
abstracts a hydrogen atom at the para-benzylic position of
another phenol in an intermolecular fashion, affording
intermediate
C
.
An alternative possibility would be
intramolecular hydrogen atom transfer from the para-benzylic
position to the oxygen radical.14 Since the O-
trifluoromethylated product was not detected in the present
reaction, we speculate that the hydrogen atom transfer step is
much faster than O-trifluoromethylation between the
phenoxyl radical and the trifluoromethyl radical, probably due
to steric hindrance of ortho-substituents.15 The rate of
hydrogen atom abstraction is presumed to be sensitive to
steric hindrance, which might be one of the reasons for the
site-selectivity of the reaction. The resulting benzylic radical
would react with a trifluoromethyl radical equivalent, affording
the corresponding product.
substituted compounds 3a, 3b, and 3m were obtained as
major products in moderate yield (Scheme 4).
OH
OH
[(MeCN)4Cu]PF6
(10 mol %)
R1
R2
R1
R2
F3C
I
O
+
O
CF3
t-BuOH, 40 °C, 1 h
1
(1.5 equiv.)
3
OH
OH
tBu
OH
tBu
MeO
CF3
CF3
CF3
3m: 50%
3a: 54%
3b: 48%
Scheme 4 Aromatic C–H trifluoromethylation.
To obtain information regarding the reaction mechanism,
we examined the reaction in the presence of 2,2,6,6-
tetramethylpiperidine 1-oxyl (TEMPO), a well-known radical-
trapping reagent (eq 1). In this case, 38% of dimer product 10
was isolated together with 58% of TEMPO-CF3 adduct 11, while
trifluoromethylation product 2a was obtained in only 8% yield.
Although the 1a-TEMPO adduct was not detected, formation
of dimer product 10 suggests that the benzyl radical
intermediate might be involved in the reaction cycle. In
accordance with this idea, a small amount of 10 was detected
in the crude mixture when solvents other than DMF were used
(Table 1). On the other hand, the reaction of the
corresponding methyl ether derivative having no phenolic
hydroxyl group did not proceed, indicating that the phenolic
hydroxyl group plays a critical role in this trifluoromethylation.
Scheme 5 Proposed catalytic cycle for benzylic C-H trifluoromethylation of
phenols.
In summary, we have developed
a catalytic C–H
trifluoromethylation at the para-benzylic position of phenol
derivatives using a Cu catalyst and Togni reagent II. The solvent
is critical in terms of product switching; the reaction in t-BuOH
provides aromatic C-H trifluoromethylation products, while
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the scope of this reaction, will be reported in due course.
This work was supported by a grant from the NOVARTIS
Foundation (Japan) for Promotion of Science and a grant for
Platform for Drug Discovery, Informatics and Structural Life
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12 Screening of the reaction conditions is described in the
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13 It cannot be ruled out that a Cu (I) species generated through
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4 | J. Name., 2012, 00, 1-3
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