Oxidative trifluoromethylation of arylboronates with shelf-stable potassium (trifluoromethyl)trimethoxyborate

Article abstract of DOI:10.1002/chem.201102652

Introducing CF₃: Arylboronic acid pinacol esters are converted into the corresponding benzotrifluorides with the easy-to-use one-component trifluoromethylating reagent potassium (trifluoromethyl)trimethoxyborate, mediated by copper acetate under an oxygen atmosphere (see scheme). Copyright

Full text of DOI:10.1002/chem.201102652

COMMUNICATION
DOI: 10.1002/chem.201102652
Oxidative Trifluoromethylation of Arylboronates with Shelf-Stable Potassium
(Trifluoromethyl)trimethoxyborate
Bilal A. Khan, Annette E. Buba, and Lukas J. Gooßen*[a]
Trifluoromethyl-substituted (hetero)arenes are structural
motifs often encountered in pharmaceuticals, agrochemicals,
and organic materials, as the CF3 substituent profoundly af-
fects their chemical and physical properties.[1] In medicinal
chemistry, trifluoromethyl groups are widely employed to
impart higher metabolic stability, increased lipophilicity, and
stronger dipole moments to druglike molecules.[1a–d] Exam-
ples for top-selling products featuring trifluoromethyl
groups are the pharmaceuticals celecoxib, dutasteride, fluox-
etine, and sitagliptin, as well as the agrochemicals beflubuta-
mid, diflufenican, and norfluazon.[1]
Traditional methods to access benzotrifluorides typically
require harsh conditions and display a low substrate scope,
so that the introduction of a trifluoromethyl group is usually
confined to the beginning of a synthetic sequence. Examples
of trifluoromethylation strategies are the Swarts reaction of
benzotrichlorides using HF or SbF5, the fluorination of car-
boxylic acid derivatives with SF4, and the radical trifluoro-
methylation of arenes with bis(trifluoroacetyl)peroxide or
Langloisꢀ reagent (Scheme 1, a and b).[2] In recent years,
substantial research has been devoted to the development
of trifluoromethylation reactions that allow the selective in-
troduction of CF3 groups into functionalized, late-stage syn-
thetic intermediates.[3]
Scheme 1. Strategies for the introduction of trifluoromethyl groups.
pling of aryl iodides with trifluoromethyl silanes or borates.
Grushin[10] and Sanford[11] disclosed stoichiometric transfor-
mations of Pd–CF3 complexes, and Buchwald developed the
first Pd-catalyzed coupling of aryl chlorides with triethyl(tri-
fluoromethyl)silane.[12]
Based on pioneering work on Cu– and Pd–perfluoroalkyl
complexes by McLoughlin, Yagupolskii, Burton, Kondo,
Chambers, Grushin, and others,[4] four types of processes
have been devised (Scheme 1, c–f). Most studied are cou-
pling reactions of aryl halides with nucleophilic CF3 reagents
(Scheme 1, c), most of which involve the use of stoichiomet-
ric amounts of copper–CF3 complexes.[4b,d] These may also
be generated in situ from copper salts and Ruppertꢀs reagent
(CF3SiMe3)[5] or by decarboxylation of copper trifluoroace-
tates.[6] Chen and Wu reported the use of fluorosulfonyldi-
fluoroacetic acid for the transformation using catalytic
amounts of copper.[7] The groups of Amii[8] and Gooßen[9]
reported copper-catalyzed processes, which allowed the cou-
Palladium complexes were also found to catalyze regio-
À
specific C H functionalizations of arenes with electrophilic
trifluoromethylating reagents (Scheme 1, d). Thus, Yu re-
ported the ortho-trifluoromethylation of donor-substituted
arenes with Umemotoꢀs reagent,[13] and Sanford described a
Pd-catalyzed coupling of arenes with perfluoroalkyl io-
dides.[14] The reaction of arylboronic acids with Togniꢀs and
Umemotoꢀs reagent by Shen and Liu, respectively, are ex-
amples of coupling reactions of aryl nucleophiles with elec-
trophilic CF3 sources (Scheme 1, e).[15] Oxidative couplings
of aryl nucleophiles with nucleophilic CF3 reagents
(Scheme 1, f) were first reported by Qing, who developed a
Chan–Lam–type coupling[16] of arylboronic acids with Rup-
[a] B. A. Khan, A. E. Buba, Prof. Dr. L. J. Gooßen
Fachbereich Chemie - Organische Chemie
Technische Universitꢁt Kaiserslautern
Erwin-Schrçdinger-Strasse Geb. 54
67663 Kaiserslautern (Germany)
pertꢀs reagent.[17] The authors proposed
a
mechanism
À
(Scheme 2) involving the formation of reactive Cu CF3 spe-
Fax : (+49)631-205-3921
E-mail: goossen@chemie.uni-kl.de
cies (B) that undergo transmetallation with aryl borates,
À
À
generating aryl Cu CF3 species (C). The benzotrifluoride
products are liberated by reductive elimination.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201102652.
Chem. Eur. J. 2012, 18, 1577 – 1581
ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1577

onic acid substrate, making the addition of an excess of base
or molecular sieves unavoidable. We envisioned that replac-
ing the boronic acids with the corresponding pinacol esters
should effectively suppress the protodeborylation, and that
the use of K CTHNUGTRENUN[GN CF3BACHTUNGTRENNUGN
+A (OMe)3]À as a one-component trifluoro-
Scheme 2. Tentative mechanism of the oxidative trifluoromethylation.
methylating reagent might allow the development of a sim-
plified and more effective trifluoromethylation method. Pi-
nacol borates are commonly used synthons for the late-stage
functionalization of complex molecules, because they can be
generated under mild conditions by Pd-catalyzed cross-cou-
pling of the corresponding aryl halides with bis(pinacolato)-
This last strategy appears to possess a high potential for
late-stage functionalizations of complex molecules, as the
coupling proceeds close to room temperature and works
well for a broad range of substrates. However, the prototype
protocol makes use of a [Cu
N
diboron,[21] by Ir-catalyzed C H functionalization of
À
be handled in a glove-box, requires Ag2CO3 as the oxidant,
and involves the slow addition of the boronic acid to a five-
fold excess of Ruppertꢀs reagent. Buchwald disclosed an im-
proved protocol, in which many of these initial shortcomings
were overcome.[18] Nevertheless, they also used Ruppertꢀs
arenes,[22] or by briefly heating the corresponding boronic
acids with pinacol.[23] Besides, many of them are commer-
cially available.
We selected 1-naphthylboronic acid pinacol ester (1a) as
a model substrate for catalyst development, knowing from
our investigations on other couplings of boronic acid deriva-
tives that 1-naphthyl boronates are particularly susceptible
to protodeborylation.[24] With this model substrate, all prod-
ucts and potential side products, including protodeborylation
product 3b, homocoupling product 5b, methoxylation prod-
uct 4b, and halogenation products, would be detectable by
gas chromatography. In analogy to the Buchwald system, we
initially used a copper(II)/1,10-phenanthroline catalyst, di-
chloroethane as the solvent and molecular oxygen as the ox-
À
reagent, which acts as a source of CF3 in combination with
a hygroscopic fluoride salt. It is less expensive than most
electrophilic CF3 reagents, but volatile (b.p.=458C), sensi-
tive to moisture, and prone to decomposition with formation
of difluorocarbene and fluorosilicon compounds.
As an alternative one-component trifluoromethylation re-
agent, Rçschenthaler and our group have recently intro-
duced the bench-stable, crystalline potassium (trifluorome-
thyl)trimethoxyborate.[9,19] It smoothly transfers trifluoro-
methyl anions to copper catalysts and thus allows the syn-
thesis of benzotrifluorides from aryl iodides. Recently, this
meanwhile commercially available reagent has also been
used to transfer CF3 groups to carbonyl compounds.[20]
Herein, we report a straightforward, user-friendly proto-
col for the conversion of arylboronic acid derivatives into
the corresponding benzotrifluorides, based on potassium
(trifluoromethyl)trimethoxyborate as the CF3 source
(Scheme 3).
idant, but employed K
(Table 1).
+A (OMe)3]À as the CF3 source
CHUTGTNRENNUG[CF3BACHTUNGTRENNUGN
Table 1. Development of the catalyst system.[a]
Cu source Oxidant Add. Solvent
Yield [%]
2a
3a 4a 5a
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
CuOAc
CuF2
Cu
Cu
Cu
Cu
N
phen
C2H4Cl2 17
C2H4Cl2 12
C2H4Cl2 13
C2H4Cl2 20
52
23
–
–
–
–
–
–
–
–
–
–
–
–
–
–
22
39
60
45
29
26
36
21
45
70
37
–
8
16
24
33
–
23
–
–
32
–
42
–
–
18
–
CsF
MS
–
–
–
–
–
–
–
–
–
–
–
–
–
THF
CH3CN
NMP
25[b]
50[b]
58
79
21
25
15
0
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
O2
O2
O2
0
50
0
–
30
–
Scheme 3. Oxidative trifluoromethylations of arylboronic acid deriva-
tives.
16[b] Cu
80
20
–
Both Qing and Buchwald found protodeborylation to be
the main side reaction in the Cu-mediated oxidative trifluor-
omethylation of arylboronic acids with a combination of
Ruppertꢀs reagent and CsF as a CF3 source. Thus, all re-
agents and additives have to be dried carefully. However,
moisture and protons are inevitably introduced with the bor-
[a] Reaction conditions: 1a (0.50 mmol), Cu source (1 equiv), additive
(1 equiv), solvent (2.5 mL), 808C, 1.5 h. Yields were determined by GC
analysis using n-tetradecane as the internal standard. Add.=Additive,
BQ=p-benzoquinone, phen=1,10-phenanthroline, DDQ=2,3-dichloro-
5,6-dicyano-p-benzoquinone, MS=4 ꢃ molecular sieves. NMP=N-meth-
ylpyrrolidone. [b] Reaction at 608C for 3 h.
1578
www.chemeurj.org
ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 1577 – 1581

Oxidative Trifluoromethylation of Arylboronates
COMMUNICATION
Table 2. Scope of the Cu-mediated trifluoromethylation.[a]
The desired product was formed in detectable quantities,
along with large amounts of by-products resulting from pro-
todeborylation, methoxylation, and homocoupling (Table 1,
entry 1). We did not observe any halogenation by-products.
Control experiments soon revealed that neither the phenan-
throline ligand nor the presence of molecular sieves or fluo-
ride salts had a beneficial influence on the reaction outcome
(entries 2–4). In contrast, the choice of solvent was crucial
to minimize protodeborylation, as well as homocoupling
(entries 4–8). In low polarity solvents, such as dichloro-
ethane and THF, the yields of trifluoromethylated product
remained low and the rates of side reactions remained
almost unaffected (Table 1, entries 4 and 5). Better results
were obtained in aprotic polar solvents, such as acetonitrile
and NMP (Table 1, entries 6 and 7). The best results were
achieved when using DMSO, in which 1-trifluoromethyl
naphthalene was formed in 79% yield (Table 1, entry 8).
Under these conditions, neither homocoupling nor protode-
borylation was observed. The only by-product was 1-me-
thoxynaphthalene (4a, 21%), which results from the trans-
fer of a methoxy group rather than a trifluoromethyl group
Product
Yield
[%][b]
Product
Yield
[%][b]
2a
2c
73
63
2b 49
2d 99
2e
2g
70
63
2 f
68
2h 84
2i
71
59
2j
2l
71
63
2k
from K
further reduced by employing other CF3 salts, such as K+
[CF3B
(OEt)3]À. With this reagent, only 37% of the benzotri-
+A (OMe)3]À. This side reaction could not be
CHTUNGTRENNUNG[CF3BACHTUNGTRENNUNG
2m 44
2n 49
2p 71
A
ACHTUNGTRENNUNG
fluoride 2a was obtained, along with 58% 1-ethoxynaphtha-
lene. However, we hope that in the future this side reaction
can be suppressed by using chelating substituents at the
boron.
2o
67
Among the copper salts tested, copper acetate gave the
best results (entries 8–11). Beside molecular oxygen, silver
carbonate could also be used as the oxidant, whereas other
oxidants were ineffective (entries 12–15). A reaction temper-
ature of 608C proved optimal (entry 16). At lower tempera-
tures, incomplete conversion was observed, while at temper-
atures higher than 808C the trifluoromethylating reagent
started to decompose.
2q
2s
46
40
2r
2t
63
36
[a] Reaction conditions: Potassium (trifluoromethyl)trimethoxyborate
(216 mg, 1.00 mmol), Cu(OAc)2 (90.9 mg, 0.50 mmol), arylboronic acid
pinacol ester 1 (0.50 mmol), DMSO (2.5 mL), 608C, 16 h. Naph=1-naph-
AHCTUNGTRENNUNG
thyl. [b] Isolated yields.
The protocol is very easy to perform. The stable solids
aryl pinacol borate, copper(II) acetate, and potassium (tri-
fluoromethyl)trimethoxyborate were dissolved in DMSO
and the reaction mixture was briefly purged with oxygen
and stirred at 608C at ambient oxygen pressure for three
hours. Then, it was diluted with diethyl ether, washed with
aqueous sodium bicarbonate, and the crude product was
separated from the by-products by column chromatography.
The optimized protocol allowed the selective synthesis of
a wide range of products in moderate to good yields, and
many functional groups, such as ether, amide, fluoride,
ketone, ester, cyano, and alkynyl groups were tolerated
(Table 2). Throughout, the scope of this protocol is compa-
rable with those reported for the related oxidative trifluoro-
methylation procedures of boronic acids with Ruppertꢀs re-
agent.[17,18] Particularly electron-rich substrates, such as com-
pounds 2c–2h, were converted in good yields. These com-
pounds are only accessible by treating the boronic acid with
expensive electrophilic trifluoromethylation reagents[15] or
from the corresponding aryl iodides under harsh reaction
conditions.[25] The indole derivative 2n, an electron-rich het-
erocycle, was converted in moderate yield, which is in the
same order as with the previously reported procedures.
Moderately electron-deficient substrates, such as compounds
2l, 2o, 2p, and 2s, were converted smoothly. In contrast to
the other oxidative trifluoromethylation methods, sterically
hindered ortho,ortho-disubstituted boronates, such as 2b
and 2q, were successfully trifluoromethylated. The opti-
mized reaction conditions were mild enough to be tolerated
by electrophilic carbonyl groups. Compound 2r was ob-
tained in good yield, along with only 10% of the corre-
sponding trifluoromethylated alcohol, which originates from
nucleophilic addition of the CF3 group to the carbonyl
group. This represents a frequently observed side reaction
common to most nucleophilic trifluoromethylations of car-
bonyl-group-containing substrates.
In conclusion, the use of the crystalline, shelf-stable, and
easy-to-handle potassium (trifluoromethyl)trimethoxyborate
as a source of CF3 nucleophiles in combination with copper
acetate as the mediator, molecular oxygen as the oxidant,
Chem. Eur. J. 2012, 18, 1577 – 1581
ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
1579

L. J. Gooßen et al.
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Experimental Section
Representative synthetic procedure—preparation of 1-[{4-(trifluorometh-
yl)-naphthoxy}methyl]benzene (2d):
A 20 mL reaction vessel was
charged with 4-[(1-naphthyloxy)methyl]phenylboronic acid pinacol ester
(1d, 360 mg, 1.00 mmol), copper(II) acetate (90.9 mg, 0.50 mmol), and
potassium (trifluoromethyl) trimethoxylborate (216 mg, 1.00 mmol). An-
hydrous DMSO (2.5 mL) was added, the reaction vessel was briefly
purged with oxygen, and the resulting mixture was stirred at 608C for
16 h at ambient oxygen pressure. After cooling to room temperature, the
reaction mixture was diluted with diethyl ether (20 mL) and washed with
saturated aqueous sodium bicarbonate (20 mL). The aqueous layer was
extracted with diethyl ether (3ꢄ20 mL), and the organic layers were
washed with water and brine, dried over MgSO4, filtered, and concentrat-
ed in vacuum. The residue was purified by column chromatography
(silica gel, n-hexane) to yield 2d (298 mg, 99%) as a colorless solid. M.p.
808C; 1H NMR (400 MHz, CDCl3,): d=8.33–8.39 (m, 1H; Ar-H), 7.82–
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7.86 (m, 1H; Ar-H), 7.70 (d, 3J
(H,H)=8.2 Hz, 2H; Ar-H), 7.53 (m, 2H; Ar-H), 7.49 (d, 3J
8.2 Hz, 1H; Ar-H), 7.39 (t, 3J(H,H)=7.9 Hz, 1H; Ar-H), 6.87 (d, 3J-
ACHTUNGTRENNUNG
G
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
ACHTUNGTRENNUNG
(H,H)=7.3 Hz, 1H; Ar-H), 5.33 ppm (s, 2H; ArCH2O); 13C NMR
(101 MHz, CDCl3,): d=154.1, 141.2, 134.6, 130.1 (q, 2J
(C,F)=31.9 Hz)
3
[10] V. V. Grushin, W. J. Marshall, J. Am. Chem. Soc. 2006, 128, 12644–
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127.5, 127.3, 126.6, 125.7 125.6 (q, J
G
ACHTUNGTRENNUNG
1J(C,F)=271.9 Hz) 120.9, 105.2, 69.2 ppm; 19F NMR (376 MHz, CDCl3):
[11] a) N. D. Ball; J. W. Kampf, M. S. Sanford, J. Am. Chem. Soc. 2010,
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d=À62.49 ppm (s); IR (KBr): n˜ =3073 (Ar-H), 3052 (Ar-H), 1737, 1598,
1462, 1323, 1264, 1238, 1167, 1112, 1095, 1065, 1017, 825, 769 cmÀ1 (C F)
À
; MS (70 eV): m/z (%): 303 (16), 302 (77) [M]+, 159 (100), 143 (39), 115
(54), 109 (21), 89 (11); GC/HRMS (EI): m/z calcd for C18H13F3O:
302.0918 [M]+; found: 302.0908; elemental analysis calcd (%) for
C18H13F3O: C 71.52, H 4.33; found: C 71.39, H 4.45.
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Acknowledgements
We thank T. Knauber and G.-V. Rçschenthaler for helpful discussions,
NanoKat and the DFG (SFB-TRR 88 “3MET”) for financial support,
and the HEC Pakistan (B.A.K.) and Landesgraduiertenstiftung (A.B.)
for fellowships.
À
C C
Keywords: benzotrifluorides
·
boronic acids
·
coupling · copper · trifluoromethylation
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Published online: January 11, 2012
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