Cooperative effect of silver in copper-catalyzed trifluoromethylation of aryl iodides using Me3SiCF3

Article abstract of DOI:10.1021/om200204y

An effective model of cooperative effect of silver for the coppercatalyzed trifluoromethylation of activated and unactivated aryl iodides to trifluoromethylated arenes using Me₃SiCF₃ was achieved with a broad substrate scope.

Full text of DOI:10.1021/om200204y

NOTE
pubs.acs.org/Organometallics
Cooperative Effect of Silver in Copper-Catalyzed Trifluoromethylation
of Aryl Iodides Using Me3SiCF3
Zhiqiang Weng,*,†,‡ Richmond Lee,§ Weiguo Jia,§ Yaofeng Yuan,† Wenfeng Wang,† Xue Feng,*,‡ and
Kuo-Wei Huang*,§
†Department of Chemistry, Fuzhou University, Fujian, People's Republic of China
‡Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore
§KAUST Catalysis Centre and Division of Chemical and Life Sciences and Engineering, King Abdullah University of Science and
Technology, Thuwal 23955-6900, Kingdom of Saudi Arabia
S Supporting Information
b
ABSTRACT: An effective model of cooperative effect of silver for the copper-
catalyzed trifluoromethylation of activated and unactivated aryl iodides to trifluor-
omethylated arenes using Me3SiCF3 was achieved with a broad substrate scope.
well-defined Cu(I)ꢀCF3 complexes has also been demon-
1. INTRODUCTION
strated.34,42 However, these methods are not favorable for
Trifluoromethylated aromatics are one of most important
large-scale practical applications because of the use of harsh
reaction conditions, toxic or expensive sources of the CF3 group,
and low yields of the trifluoromethylated products. Metal-
catalyzed trifluoromethylation of aryl halides is not unprece-
classes of fluorinated organic compounds, which play a key role
in pharmaceutical and fine chemical industries.1,2 For instance,
the highly commercially successful antidepressant fluoxetine
(under the names of Prozac and Sarafem) and a number of
herbicides, e.g., fluazifop-butyl (Fusilade), fluometuron (Cotoran),
and acifluorfen (Blazer), all contain the CF3 group on the
dented, but reports are scarce. In 1989, Chen and Wu reported
the use of fluorosulfonyldifluoroacetic acid for the transforma-
tion using catalytic amounts of copper.26 More recently, Amii
aromatic ring. Only a few methods have been reported for the
and co-workers demonstrated a catalytic system by employing
catalytic trifluoromethylation of haloarenes (eq 1), even though
CuI, 1,10-phenanthroline, and CF3SiEt3 as the trifluoromethyla-
there is a great commercial driving force for developing such
tion source.6 Contravening the conventional wisdom of a highly
transformations.3ꢀ8
difficult reductive elimination of aryl and ꢀCF3 groups from Pd,
Buchwald and co-workers successfully demonstrated a Pd-cata-
lyzed model for chloroarenes using BrettPhos as the ligand and
CF3SiEt3 as the trifluoromethylating agent.7 While the work by
Conceivably, nucleophilic trifluoromethylation using metal-
the Amii and Buchwald groups using Cu and Pd catalysts and
CF3 reagents, e.g., Grignard reagents (CF3MgX) and organo-
Et3SiCF3 are remarkable breakthroughs, low yields were ob-
lithium (CF3Li),9ꢀ11 is one of the more successful strategies for
served when the more economical Me3SiCF3 (Ruppert’s
trifluoromethylation reactions. Unfortunately, inherent in these
reagent)43 was employed, presumably due to its self-decomposi-
approaches are a number of limitations, especially in the forma-
tion in the presence of fluoride under those reaction
tion of polymeric materials, difluorocarbene, and fluoride species,
conditions.44 Herein, we report a cooperative catalytic model
due to the strong repulsive interactions of the filled orbital on the
carbanion with the lone pairs of the fluorine substituents.
of silver-assisted copper-catalyzed trifluoromethylation of aryl
iodides using Me3SiCF3.
“CuꢀCF3” species generated stoichiometrically in situ from
early examples of (1) di- or trifluorinated methanes,12ꢀ17 (2)
2. RESULTS AND DISCUSSION
group 12 trifluoromethyl derivatives, M(CF3)2 (where M = Zn,
Cd, or Hg),18ꢀ20 (3) trifluoroacetic and fluorosulfonyldifluor-
oacetic acid derivatives,21ꢀ33 and (4) Ruppert’s or Ruppertꢀ
Prakash reagent34ꢀ36 and its ethyl derivative, CF3SiEt3,37 have
shown very promising reactivities toward trifluoromethylation of
aryl halides. Prolific reports on the utility and expansion of these
methods are well comprehensively compiled in numerous ex-
cellent reviews.38ꢀ41 Important progress in the utilization of
As part of our interest in the development of metal-catalyzed
cross-coupling reactions and heterobimetallic catalysis,45,46 we
continue to pursue new methodologies by combining the
different reactivities of two metals for trifluoromethylation of
Received: March 6, 2011
Published: May 02, 2011
r
2011 American Chemical Society
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dx.doi.org/10.1021/om200204y Organometallics 2011, 30, 3229–3232
|

Organometallics
NOTE
Table 1. Impact of Reaction Parameters on the Trifluoro-
Table 2. Catalytic Trifluoromethylation of Aryl Halides
methylation of Aryl Iodides
entry
variation from the “standard conditions”
standard
yielda (%)
1
2
71
<2
<5
18
0
no CuI
3
10% NiCl2(DME) [instead of CuI]
1.33 equiv of KF [instead of AgF]
4
5
1.33 equiv of t-Bu4NF 3H2O [instead of AgF]
3
6
AgOTf [instead of AgF]
<2
41
58
54
<5
7
no NaOt-Bu [instead of 20% NaOt-Bu]
DMSO [instead of NMP]
8
9
DMF [instead of NMP]
10
rt, 20 h [instead of 90 °C, 6 h]
a Determined by 19F NMR relative to 4-fluorobenzonitrile as an internal
standard. Yields are based on aryl iodides as the limiting reagent.
aryl iodides. We began our studies by examining the reactions of
aryl iodide and Me3SiCF3 in the presence of CuI, AgF, NaOt-Bu,
and N,N0-dimethylethylenediamine (dmeda) (Table 1). It was
intuitive to predict that AgF may promote the reaction due to the
high affinity between the silver cation and the iodide as well as
that between the fluoride ion and silicon. Indeed, it was found
that 1-iodonaphthalene coupled with Me3SiCF3 at 90 °C in
N-methylpyrrolidone (NMP) to produce 1-(trifluoromethyl)-
naphthalene in 71% yield in the presences of AgF (entry 1). In
the absence of CuI, essentially none of the desired ArꢀCF3 bond
formation was observed (entry 2). NiCl2(DME) was proved to
be inefficient for the trifluoromethylation of aryl halides (entry 3).
It was noted that the use of KF furnished the target compound
only in a significantly lower yield (18%, entry 4), while Amii et al.
have shown that it performs well in similar reactions with
a L1: N,N0-dimethylethylenediamine; L2: 1,10-phenanthroline. b Deter-
mined by 19F NMR. c Isolated yield.
Et3SiCF3.6 Other fluoride sources, such as t-Bu4NF 3H2O, are
3
totally inactive (entry 5). Using AgOTf instead of AgF does not
promote the reaction with copper (entry 6). These results
suggest that both CuI and AgF are essential for efficient catalysis.
NaOt-Bu was identified to be necessary for higher yields (entry 7).
The coupling reactions proceed with less efficiency in DMF and
DMSO (entries 8 and 9).
This new methodology was further found to be successfully
applied to catalyze the trifluoromethylation of a range of
iodoarenes with Me3SiCF3, affording the desired trifluoromethy-
lated aromatics in moderate to excellent yields (Table 2). It was
found noteworthy that activated electron-poor aryl iodides as
well as deactivated electron-rich ones (e.g., entries 9, 10) are both
suitable reaction substrates. Functional groups, such as cyano,
nitro, acetals, and the trifluoromethyl group, are tolerated.
In an effort to elucidate the role that AgF plays to significantly
enhance the yields for the Cu-catalyzed trifluoromethylation
using Me3SiCF3, we designed and prepared dinitrogen-ligated
trifluoromethyl silver(I) and copper(I) complexes and tested
their reactivities toward aryl iodides. We focused our attention on
bathophenanthroline ligands. Adopting the literature method for
the synthesis of perfluoroorganosilver(I) compounds,47 the
colorless sliver(I) complex (bathophenanthroline)AgCF3 (1)
Figure 1. ORTEP diagram of (bathophenanthroline)AgCF3 (1) with
thermal ellipsoids at the 30% probability level. Selected bond lengths
(Å) and angles (deg): Ag(1)ꢀC(25) 2.092(6), Ag(1)ꢀN(1) 2.389(5),
Ag(1)ꢀN(2) 2.263(5), N(1)ꢀAg(1)ꢀN(2) 70.57(17).
was successfully synthesized in 74% yield from a reaction mixture
of AgF, Me3SiCF3, and bathophenanthroline in THF. The
analogous copper complex (bathophenanthroline)CuCF3 (2)
was prepared in 64% yield from a reaction of Cu(Ot-Bu),
Me3SiCF3, and bathophenanthroline as orange solids.
Suitable crystals of 1 and 2 for X-ray crystallography were
obtained by recrystallization in THF/n-hexane at ꢀ25 °C. Their
molecular structures are shown in Figures 1 and 2. Both
structures show a three-coordinated trigonal-planar geometry
in which the Ag(I) or Cu(I) center is bound by one neutral
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Organometallics
NOTE
Scheme 1. Proposed Reaction Pathway for the
Cu/Ag-Catalyzed Trifluoromethylation of Aryl Iodides
3. CONCLUSION
In summary, we have demonstrated a cooperative effect of
silver for the copper-catalyzed trifluoromethylation of activated
and unactivated aryl iodides to trifluoromethylated arenes using
the more desirable Me3SiCF3 reagent. The present methodology
can tolerate a variety of functional groups. We have also prepared
the well-defined dinitrogen-ligated trifluoromethyl Ag(I) and
Cu(I) complexes 1 and 2 to elucidate their cooperative interac-
tions. Work on kinetic analysis and the extension to more
practical applications is ongoing in our laboratories and will be
reported in due course.
Figure 2. ORTEP diagram of (bathophenanthroline)CuCF3 (2) with
thermal ellipsoids at the 30% probability level. Selected bond lengths
(Å) and angles (deg): Cu(1)ꢀC(25) 1.907(9), Cu(1)ꢀN(1) 2.024(8),
Cu(1)ꢀN(2) 2.096(8), N(1)ꢀCu(1)ꢀN(2) 79.3(3).
bidentate bathophenanthroline ligand and one anionic CF3
group. To the best of our knowledge, complex 1 is the first
example of an isolable neutral Ag(I)ꢀCF3 complex, although
certain ionic forms have been reported in the literature.47 The
metalꢀCF3 distance in 1 (2.092(6) Å) is found to be significantly
longer than that of 2 (1.907(9) Å) and that of a NHCꢀCu
complex (IPr)CuCF3 (2.022(4) Å).34 The NꢀMꢀN angle in 1
(70.57°) is significantly smaller than that in 2 (79.3°).
Since 1 was readily formed by mixing AgF, Me3SiCF3, and
bathophenanthroline, we propose that the AgꢀCF3 bond was
first generated under our Cu-catalyzed trifluoromethylation
conditions. It is noted that in contrast to copper and palladium
the investigations on the utilization of silver in trifluoromethyla-
tion reactions are limited, although “Ag(I)CF3” species are
known for their use in nucleophilic substitutions.47,48 We sought
to evaluate the reactivity of silver complex 1 for the trifluor-
omethylation of aryl halides. The reaction of 1 with 5 equiv of
4-iodobenzonitrile, however, formed no coupling products in
NMP after 3 h at 90 °C (eq 2). In sharp contrast, the reactions of
copper complex 2 with 5 equiv of 4-iodobenzonitrile in NMP
proceeded smoothly and formed the coupling product in 81%
yield after 30 min (eq 3). The results suggest the possibility of
transferring the CF3 group from 1 to a Cu center to allow the
coupling reactions to proceed. Indeed, we confirmed that the
reaction of the well-documented copper iodide complex 349 with
1 yielded complex 2 in 80% yield at room temperature (eq 4).
4. EXPERIMENTAL SECTION
General Procedures. All reactions were carried out using conven-
tional Schlenk techniques under an inert atmosphere of nitrogen or
argon with an MBraun Labmaster 130 inert gas system. NMR spectra
were measured on Bruker ACF300 300 MHz FT NMR spectrometers
(1H at 300.14 MHz, 13C at 75.43 MHz). 19F NMR spectra (282.38
MHz) were recorded using hexafluorobenzene (C6F6) as an internal
standard (δ 0 ppm). Elemental analyses were performed by the
microanalytical laboratory in-house. All chemicals were obtained
from Sigma-Aldrich or Strem Chemicals unless stated otherwise.
[(Bathophenanthroline)CuI]2 was synthesized according to the
literature.49
(Bathophenanthroline)AgCF3 (1). A mixture of AgF (13.0 mg, 0.102
mmol) and bathophenanthroline (33.2 mg, 0.100 mmol) in THF
(6 mL) was stirred at rt for 10 min. To the resulting suspension was
added a THF (1 mL) solution of Me3SiCF3 (28.0 mg, 0.197 mmol). The
mixture was further stirred at rt for 6 h and then filtered through a layer of
Celite. The filtrate was diluted with 4 mL of n-hexanes. The resulting
solution was then cooled at ꢀ25 °C for 36 h. Colorless needles were
formed, separated, washed with 2 ꢁ 2 mL of n-hexanes, and dried to give
38 mg of 1 (73%): 1H NMR (300 MHz, THF-d8) δ 9.13 (br, 2H), 7.86
(br, 2H), 7.65ꢀ7.54 (m, 12H); 13C{1H} NMR (75.5 MHz, THF-d8) δ
147.9, 146.7, 144.1, 135.9, 127.6, 126.5, 126.4, 124.3, 121.9, 121.7; 19F
NMR (282 MHz, THF-d8) δ 144.3 (d, J = 29.2 Hz). Anal. Calcd for
C25H16AgF3N2: C, 58.96; H, 3.17; N, 5.50. Found: C, 58.54; H, 3.48;
N, 5.04.
(Bathophenanthroline)CuCF3 (2). A solution of NaOt-Bu (18.0 mg,
0.188 mmol) in 2 mL of THF was added to a suspension of CuCl (15.0
mg, 0.152 mmol) in 3 mL of THF, and the resulting light yellow mixture
was stirred at rt for 1 h. The solution was filtered through a layer of
Celite. To this filtrate was added a suspension of bathophenanthroline
(50.0 mg, 0.150 mmol) in 3 mL of THF, followed by addition of a THF
(0.5 mL) solution of Me3SiCF3 (28.0 mg, 0.197 mmol). The mixture
turned red immediately and was further stirred at rt for an additional
5 min. The solution was filtered, and to the filtrate was added 8 mL of
n-hexanes. The resulting solution was then cooled at ꢀ25 °C for 36 h.
The resulting red crystals were washed with 2 ꢁ 2 mL of n-hexanes and
dried to give 45 mg of 2 (64%): 1H NMR (300 MHz, THF-d8) δ 9.18 (d,
J = 3.8 Hz, 2H), 7.91 (s, 2H), 7.72 (d, J = 4.1 Hz, 2H), 7.57ꢀ7.55 (m,
10H); 13C{1H} NMR (75.5 MHz, THF-d8) δ 147.3, 146.6, 143.9, 135.7,
127.6, 126.6, 124.4, 122.2, 122.0; 19F NMR (282 MHz, THF-d8)
These observations strongly support the collaborative effect
by the combination of Ag and Cu for the trifluoromethylation of
iodoarenes: the transmetalation of the in situ generated trifluoro-
methylsilver species with copper iodide forms the trifluoro-
methylcopper species, which is followed by the reaction with
iodoarenes to release the ArꢀCF3 coupling products and to
regenerate copper iodide (Scheme 1).
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Organometallics
NOTE
δ 132.2. Anal. Calcd for C25H16CuF3N2: C, 64.58; H, 3.47; N, 6.03.
Found: C, 64.12; H, 3.58; N, 5.59.
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General Procedures for Trifluoromethylation Reactions. A
light yellow mixture of CuI (2.9 mg, 0.015 mmol, 10 mol %) and NaO-
t-Bu (3.0 mg, 0.030 mmol) in 0.5 mL of NMP is a reaction vial was stirred
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Crystal Structure Analyses. Suitable crystal of 1 and 2 were
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average 2.5 THF; in 2, the asymmetric unit contains two molecules of
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Supporting Information. Product analysis and crystal
b
data for 1 and 2. This material is available free of charge via the
Internet at http://pubs.acs.org.
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Corresponding Author
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’ ACKNOWLEDGMENT
We acknowledge the financial support from National Natural
Science Foundation of China (21072030) and Fuzhou Univer-
sity (022318) to Z.W. and from KAUST to K.-W.H. Assistance
with the crystallographic analysis was provided by Dr. L. L. Koh
and Ms. G. K. Tan. R.L. received a KAUST Provost Fellowship,
for which he is grateful.
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