Highly selective trifluoromethylation of 1,3-disubstituted arenes through iridium-catalyzed arene borylation

Article abstract of DOI:10.1002/anie.201106673

The old one two: A sequential iridium-catalyzed borylation and copper-catalyzed trifluoromethylation of arenes is described (see scheme; Pin=pinacol). The reaction is conducted under mild reaction conditions and tolerates a variety of functional groups. T

Full text of DOI:10.1002/anie.201106673

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Angewandte
Communications
DOI: 10.1002/anie.201106673
À
C H Activation
Highly Selective Trifluoromethylation of 1,3-Disubstituted Arenes
through Iridium-Catalyzed Arene Borylation**
Tianfei Liu, Xinxin Shao, Yaming Wu, and Qilong Shen*
Dedicated to Professor Weiyuan Huang on the occasion of his 90th birthday
The trifluoromethyl group has evolved into an indispensable
structural motif[1] for the pharmaceutical industry because of
its unique size, electronic properties, and excellent metabolic
stability, as evidenced by a large number of trifluoromethyl-
ated pharmaceuticals and drug candidates, such as the
antidepressant Prozac,[2] the fungicide Trifloxystrobin,[3] and
the herbicide Fusilade.[4] Consequently, development of new
and efficient methods to introduce the trifluoromethyl group
to small molecules are of high interest.[5]
Toward this end, several copper- and palladium-catalyzed
trifluoromethylations of aryl halides or aryl boronic acids
have been reported recently.[6–10] For example, Amii and co-
workers reported the first copper-catalyzed trifluoromethyl-
ation of electron-poor aryl iodides with the Ruppert–Prakash
reagent, CF3SiEt3, in the presence of CuI/1,10-phenanthroline
(Scheme 1).[7] Buchwald and co-workers reported a palla-
dium-catalyzed trifluoromethylation of aryl chlorides by use
of sterically hindered electron-rich ligands.[8] Qing and Chu,
and Buchwald and co-workers independently reported the
copper-catalyzed trifluoromethylation of aryl boronic acids
under oxidation conditions.[9,10] While these methods over-
come the shortcomings of the classic Swarts reaction[11] or the
[CuCF3] strategy,[7,12] which requires harsh reaction condi-
tions, stoichiometric amounts of organometallic reagents, and
occurs with limited substrate scope, the trifluoromethyl group
introduced by these catalytic methods was typically placed at
À
the position of the C X (X = halides or boron) bond of the
prefunctionalized arenes. Such a prefunctionalization of
substrates limits the application of these methodologies for
the late-stage modification of drug candidates for structure–
activity relationship (SAR) studies. A strategy that could
À
À
convert the C H bond of arenes into C CF3 bonds is thus
highly desirable (Scheme 1). Yu and co-workers recently
reported
a palladium-catalyzed directed ortho trifluoro-
methylation of arenes,[13] but a method for the regioselective
À
trifluoromethylation of arenes at a C H bond without a
directing group is unknown.
Recent work from our laboratory has demonstrated that
the copper-catalyzed trifluoromethylation of aryl and vinyl
boronic acids with Togniꢀs reagent proceeded under very mild
reaction conditions;[14] Hartwig, Marder, Smith, Maleczka,
and their co-workers have independently reported a series of
reactions involving the iridium-catalyzed borylation of arenes
and subsequent functionalization of the resulting boronic
ester.[15] The regioselectivity of the products from the iridium-
catalyzed borylation of arenes results from steric, rather than
electronic control. For example, reactions of 1,3-disubstituted
arenes under these conditions generate 3,5-disubstituted
arene boronates. We envisioned that if these two catalytic
À
reactions could be combined, a tandem C H activation/
trifluoromethylation strategy could be realized for a highly
meta-selective preparation of trifluoromethyl arenes from
readily available 1,3-disubstituted arenes. Herein, we report
the development of such a one-pot method for the trifluoro-
methylation of arenes using the combination of the iridium-
catalyzed borylation of arenes and copper-catalyzed trifluoro-
methylation of the resulting aryl boronates. The utility of this
method is demonstrated by the efficient trifluoromethylation
of some drug candidates through this one-pot meta trifluoro-
methylation. The introduction of a trifluoromethyl group into
a molecule of interest at a late stage of a synthesis using mild
reaction conditions could accelerate the discovery of a lead
compound.
Scheme 1. Previous metal-catalyzed approaches to trifluoromethyl
arenes.
[*] T. Liu, X. Shao, Dr. Y. Wu, Prof. Dr. Q. Shen
Key Laboratory of Organofluorine Chemistry
Shanghai Institute of Organic Chemistry
Chinese Academy of Sciences
345 Lingling Road, Shanghai 200032 (China)
E-mail: shenql@sioc.ac.cn
[**] The authors thank Ms. Xiangqing Jia of the China Pharmaceutical
University for conducting some experiments. We also thank the Key
Program of the National Natural Science Foundation of China
(21032006, 20632070), the National Natural Science Foundation of
China (21172245), and the National Basic Research Program of
China (2010CB126103, 2012CB821602) for financial support.
The trifluoromethylation of 4-biphenyl pinacolboronate
with Togniꢀs reagent[16] was chosen at the start of our
investigation as a model reaction to identify conditions for
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201106673.
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Angew. Chem. Int. Ed. 2012, 51, 540 –543

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the conversion of pinacolboronate esters into trifluoro-
methylarenes. We first examined if our previously developed
reaction conditions, wherein 5 mol% of CuI and 10 mol% of
1,10-phenanthroline (L1) were used as the catalyst, were
suitable for the reaction of 4-biphenyl pinacolboronate with
Togniꢀs reagent (Table 1). Not surprisingly, the reaction
occurred much more slowly than those with aryl boronic
acids. Only 80% conversion with a 58% yield (19F NMR
spectroscopy) was observed after 24 hours at 458C (Table 1,
entry 1). Switching the copper salt from CuI to copper(I)
presence of 1.0 equivalent of water when LiOH was used as
the base, whereas only 28% yield was observed in the absence
of water (Table 1, entries 6 and 10). Reactions in the presence
of other bases such as Li2CO3 or NaOH, however, afforded
the desired product in less than 45% yield (Table 1, entries 8–
9). Finally, it was found that reactions in CH2Cl2 were much
faster than those in other solvents and proceeded to full
conversion after 4 hours at 458C to give the desired product in
97% yield, as determined by 19F NMR spectroscopy (Table 1,
entries 10–16). Other dinitrogen ligands such as 2,9-dimethyl-
1,10-phenanthroline (L2), 2,2’-bipyridine (L3), or tetra-
methylethylenediamine (TMEDA) were tested, but reac-
tions under these conditions formed less than 81% of the
trifluoromethyl biphenyl (Table 1, entries 17–19). In the
absence of the copper catalyst, less than 5% of the
trifluoromethylated product was observed.
Table 1: Optimization of the CuI/L1-catalyzed trifluoromethylation of
4-biphenyl pinacolboronate with Togni’s reagent.[a,b]
With the optimized reaction conditions for the trifluoro-
methylation of aryl pinacolboronate in hand, we studied a
tandem sequence to convert 1,3-disubstituted arenes or
heteroarenes into the corresponding trifluoromethylated
arenes or heteroarenes. The borylation of 1,3-disubstituted
arenes was conducted with 0.7 equivalents of bis(pinacola-
to)diboron (B2pin2) in the presence of 0.25 mol% of [{Ir-
(cod)OMe}2] (cod = 1,5-cyclooctadiene) and 0.5 mol% of
di-tert-butylbipyridine (dtbpy) in THF at 808C for 24 hours.
The resulting arylboronate esters were converted into the
trifluoromethylated arenes by evaporation of the volatile
materials, dissolution of the residue in CH2Cl2, addition of
10 mol% of CuTc, 20 mol% of 1,10-phenanthroline,
1.1 equivalents of Togniꢀs reagent, and 2.0 equivalents of
LiOH·H2O, and then heating at 458C for 4–8 hours.
Entry Copper salt
Ligand Base
Additive Solvent
t
Yield
[h] [%][c]
1
CuI
L1
K2CO3
K2CO3
K2CO3
K2CO3
LiOtBu
LiOH
LiOtBu H2O
NaOH 2H2O
Li2CO3 2H2O
LiOH H2O
LiOH H2O
–
–
–
–
–
–
diglyme 24 58
diglyme 24
diglyme 24
diglyme 24
diglyme 48 <5
diglyme 48
diglyme 48
diglyme 48
diglyme 48
diglyme 48
2
3
[Cu(OTf)]2·C6H6 L1
(MeCN)4CuPF6 L1
62
13
78
4
5
6
7
8
9
10
11
CuTC
CuTC
CuTC
CuTC
CuTC
CuTC
CuTC
CuTC
L1
L1
L1
L1
L1
L1
L1
L1
28
33
13
42
52
54
A variety of 1,3-disubstituted arenes were subjected to
À
the C H activation/trifluoromethylation conditions to give
1,4-
48
dioxane
DME
DMF
CH2Cl2 24 >97
CH2Cl2
CH2Cl2
CH2Cl2 48
CH2Cl2 24
CH2Cl2
the
corresponding
5-trifluoromethyl-1,3-disubstituted
12
13
14
15
16
17
18
19
CuTC
CuTC
CuTC
CuTC
CuTC
CuTC
CuTC
CuTC
L1
L1
L1
L1
L1
L2
LiOH H2O
LiOH H2O
LiOH H2O
LiOH H2O
LiOH H2O
LiOH H2O
48
48
37
14
arenes in good to excellent yields, as summarized in
Table 2. Arenes containing ester, protected phenoxy, chlo-
ride, and cyano groups were compatible with the reaction
conditions and afforded the products in yields of 50–90%
(Table 2, entries 1–8). In addition, the reactions of a number
of heteroarenes gave the trifluoromethylated product with
excellent selectivity and yields. For example, the reaction of
a 2,6-disubstituted pyridine produced the corresponding
4-trifluoromethyl pyridine in 90% yield (Table 2, entry 9).
Reactions of benzofuran and benzothiophene generated the
2-trifluoromethyl products in 72% and 75% yields, respec-
tively (Table 2, entries 10 and 11). The reaction of Boc-
protected indole gave the corresponding 3-trifluoromethyl-
substituted product in good yield (Table 2, entry 12).
4
4
97
86[d]
47
TMEDA LiOH H2O
L3 LiOH H2O
10
81
4
[a] Reaction conditions: 4-biphenyl pinacolboronate (0.2 mmol), Togni’s
reagent (0.2 mmol), CuX (10 mol%), ligand (20 mol%) and base
(0.4 mmol) in specified solvent (1.0 mL) at 458C. [b] Reactions listed in
entries 1–13 were conducted in Schlenk tubes with 5 mol% of CuX and
10 mol% of ligand, whereas reactions listed in entries 14–19 were
conducted in sealed bombs. [c] Yields were determined by 19F NMR
analysis of the crude reaction mixture with 1-fluoronaphthalene as an
internal standard. [d] 10 mol% of the ligand was used.
Late-stage modification of drug candidates is valuable
for structure–activity relationship (SAR) studies since the
complex target molecules are otherwise more challenging to
obtain. We selected several pharmaceutical compounds to
illustrate the advantage of late-stage trifluoromethylation
compared to conventional synthesis to access complex
trifluoromethylated molecules (Scheme 2). Biologically
active molecules such as Vitamins B3 and E, Vitamin E
nicotinate, carbohydrates, and steroids are all compatible
thiophene-2-carboxylate (CuTc) resulted in a much faster
reaction with full conversion after 24 hours (Table 1, entry 4);
other copper salts such as [Cu(OTf)]2·benzene and [Cu-
(MeCN)4]PF6 did not improve the reaction yield (Table 1,
entries 2 and 3). Interestingly, the reaction in the presence of
water was much faster, probably because of the faster
transmetalation of the aryl pinacolboronate to copper
(Table 1, entry 5 versus 7 and entry 6 versus 10). For example,
the reaction proceeded to full conversion after 48 hours in the
À
with the tandem C H activation/trifluoromethylation proce-
dure. It is worth mentioning that for all trifluoromethylated
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Table 2: Trifluoromethylation of arenes by iridium-catalyzed borylation.
compounds shown in Scheme 2, the trifluoromethylation was
regioselective, and no constitutional isomers were observed in
most cases. Only in the case of estrone were two isomers
isolated in 1:1 ratio of inseparable isomers. The generally high
À
regioselectivity of the sequential iridium-catalyzed C H
borylation and copper-catalyzed trifluoromethylation is an
advantage over the other known trifluoromethylation reac-
tions.
In summary, we have demonstrated the first copper-
catalyzed trifluoromethylation of arylboronate esters and a
Entry Product
Yield
[%][a]
Entry Product
Yield
[%][a]
À
sequential iridium-catalyzed C H activation borylation and
1
2
90
75
8
9
70
90
copper-catalyzed trifluoromethylation of arenes with a vari-
ety of functional groups. The advantage of this tandem
procedure was demonstrated by application to a number of
biologically active molecules. Mechanistic studies and syn-
thetic applications of these transformations are ongoing in
our laboratory.
3
4
5
6
7
75
80
87
70
50
10
11
12
13
14
72
Experimental Section
[{Ir(cod)(OMe)}2]
(1.7 mg,
0.25 mol%),
dtbpy
(1.45 mg,
75
0.540 mol%), and B2pin2 (186 mg, 0.730 mmol) were placed into an
oven-dried sealed bomb. The bomb was evacuated and refilled with
Ar three times. Under a positive flow of argon, dry THF (2.0 mL) and
arene (1.0 mmol) were added. The reaction was stirred at 808C and
monitored by GC/MS until the disappearance of the arene (24 h).
After filtration through a short plug of celite, the volatiles were
removed under vacuum and 4.0 mL of CH2Cl2 was added. The
solution was transferred by a syringe into an oven-dried sealed bomb
that contained CuTc (19 mg, 0.10 mmol), 1,10-phenanthroline
(36 mg, 0.20 mmol), LiOH·H2O (83 mg, 2.0 mmol), and Togniꢀs
reagent (363 mg, 1.10 mmol) under Ar. The reaction system was
quickly degassed through three freeze/pump/thaw cycles and refilled
with Ar. The reaction was stirred at 458C and monitored by 19F NMR
spectroscopy until the disappearance of Togniꢀs reagent (typically
8 h). Brine (25 mL) and CH2Cl2 (10 mL) were added and the organic
phase was separated. The aqueous phase was extracted with CH2Cl2
(5 ꢁ 10 mL) and the combined organic phases were dried over
anhydrous Na2SO4, and concentrated in vacuo. The product was
purified by flash chromatography on silica gel with n-pentane, and
further purified by Kugelrohr distillation.
67[b]
65[b]
50
[a] Yields of isolated product from the one-pot procedure (1.0 mmol
scale). [b] Borylation with 1.0 mol% [{Ir(cod)OMe}2] and 2.0 mol%
dtbpy.
Received: September 20, 2011
Published online: November 30, 2011
À
Keywords: C H activation · copper · fluorine ·
homogeneous catalysis · iridium
.
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