Copper-catalyzed aromatic trifluoromethylation via group transfer from fluoral derivatives

Article abstract of DOI:10.1002/adsc.201000825

Starting from a readily available fluoral derivative, catalytic aromatic trifluoromethylation has been successfully achieved. A small amount of copper(I) iodide-phenanthroline complex catalyzed the cross-coupling reactions of aryl/heteroaryl iodides with the O-silylated hemiaminal of fluoral (trifluoroacetaldehyde) to provide trifluoromethylated arenes in moderate to high yields.

Full text of DOI:10.1002/adsc.201000825

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DOI: 10.1002/adsc.201000825
Copper-Catalyzed Aromatic Trifluoromethylation via Group
Transfer from Fluoral Derivatives
Hideaki Kondo,a Masahiro Oishi,a Kenichi Fujikawa,a and Hideki Amiia,b,*
a
Department of Chemistry, Graduate School of Science, Kobe University, 1–1 Rokkodai-cho, Nada-ku, Kobe 657-8501,
Japan
b
Department of Chemistry and Chemical Biology, Graduate School of Engineering, Gunma University, 1-5-1 Tenjin-cho,
Kiryu, Gunma 376-8515, Japan
Fax : (+81)-277-30-1280; e-mail: amii@chem-bio.gunma-u.ac.jp
Received: November 3, 2010; Revised: March 13, 2011; Published online: May 17, 2011
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201000825.
Abstract: Starting from a readily available fluoral
derivative, catalytic aromatic trifluoromethylation
has been successfully achieved. A small amount of
copper(I) iodide-phenanthroline complex catalyzed
the cross-coupling reactions of aryl/heteroaryl
iodides with the O-silylated hemiaminal of fluoral
(trifluoroacetaldehyde) to provide trifluoromethy-
lated arenes in moderate to high yields.
CF3ZnX/CuX,[5j–l] CF3SO2Cl/Cu,[5m] FSO2CF2CO2Me/
CuI,[5n] FSO2CF2CF2OCF2CO2Me/CuI,[5o] CF3CO2Na/
CuI,[5p,q] CF3CO2Me/CsF/CuI,[5r] ClCF2CO2Me/KF/
CuI,[5s] CF3SiEt3/KF/Cu,[5t,u] and [CF3SPh2]+OTfÀ/
Cu.[5w] For most of the aromatic trifluoromethylations
using CF3Cu, stoichiometric (sometimes, excess)
amounts of copper reagents are required to complete
the cross-coupling reactions.[5a–m,o–v,6–8] Recently, de-
tailed studies on transition metal-catalyzed aromatic
trifluoromethylation have been described.[6–12] In
order to develop a practical and environmentally
benign process, it is desirable to reduce the quantity
of heavy metal reagents employed in these transfor-
Keywords: aromatic substitution; copper; cross-cou-
pling; retro reactions; trifluoromethylation
Trifluoromethylated aromatic compounds (Ar-CF3)
are substances of considerable interest in various in-
dustrial fields. The high lipophilicity, strong electron-
withdrawing ability, and charasteristic size of the tri-
fluoromethyl group are key influences in biologically
active molecules.[1] Due to these attractive properties,
Ar-CF3 structural motifs have been widely employed
in the design of pharmaceuticals, agrochemicals, dyes,
liquid crystals, and polymers (Scheme 1).[2] Therefore,
the development of highly efficient methodologies for
aromatic trifluoromethylation is of significant impor-
tance for wide fields of science and technology.
With respect to the high regiochemical fidelity in
aromatic substitution, the cross-coupling of aryl hal-
ides with organometallic reagents is one of the most
powerful and versatile tools in organic synthesis.[4] Tri-
fluoromethylcopper (CF3Cu), which is unstable and
needs to be generated in situ, acts as a prominent
cross-coupling participant of aromatic trifluoromethy-
lation.[5–8] In order to generate CF3Cu species, there
have been several protocols using CF3I/Cu,[5a–d]
CF3Br/Cu-anode,[5e] CF3N(NO)Tf/Cu,[5f] CF2Br2/Cu/
Scheme 1. Selected examples of trifluoromethylated phama-
DMAc,[5g,h]
(CF3)2Hg/Cu,[5i]
CF3CdX/CuX,[5j,k] ceuticals.
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Hideaki Kondo et al.
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mations.[5n,12–18] Recently, Watson and Buchwald et al.
have demonstrated the palladium-catalyzed cross-cou-
pling reactions of aryl chlorides with trifluoromethyl-
Table 1. Optimization of the reaction conditions.
ACHTUNGTRENNUNG
(triethyl)silane (CF3SiEt3).[14] In 2009, we reported an
efficient procedure for aromatic trifluoromethylation
using a diamine ligand, which makes possible a reac-
tion using catalytic copper.[15] In the presence of a
small amount of CuI and 1,10-phenanthroline (phen),
the cross-coupling reactions of iodoarenes with
CF3SiEt3 proceeded smoothly to afford trifluorome-
thylated aromatics in good yields. Despite its efficien-
cy for catalytic aromatic trifluoromethylation,[14,15]
CF3SiEt3 is a highly expensive reagent with limited
availability.[7c] Therefore, more convenient CF3 sour-
ces have been required for cross-coupling reactions.
Meanwhile, fluoral (trifluoroacetaldehyde) and its
derivatives are one of the most available and
common organofluorine compounds. Hemiaminals of
fluoral are stable compounds and they are known to
Entry
Base
Solvent
Yield [%][a]
1
2
4
5
6
7
CsF
CsF
CsF
CsF
CsF
KF
DMF
NMP
DMSO
DME
diglyme
diglyme
7
17
20
33
77
3
[a]
NMR yield, which was calculated by 19F NMR integra-
tion of product 3a relative to the trifluoromethoxybenze
internal standard.
be convenient sources of trifluoromethyl anion.[19,20] and DME, which were frequently employed in the
O-Silylated hemiaminal derivative 2 was readily pre- catalyzed cross-coupling reactions, but they were with
pared from commercially available CF3CH(OH)- a limited success (entries 2–5). In particular, when uti-
ACHTUNGTRENNUNG
(OEt) (hemiacetal of fluoral) and morpholine.[20a] Pre- lizing diethylene glycol dimethyl ether (diglyme) as a
viously, Langlois and Billard reported the nucleophilic solvent, the CuI-phen complex showed high catalytic
trifluoromethylation of aldehydes and ketones by the activity and the yield of 3a increased upto 77%
use of compound 2 as a CF3 anion equivalent.[20b] In (entry 6). Diglyme could play an important role to
spite of the great synthetic utility, to the best of our dissolve CsF efficiently by chelation.[22] The combina-
knowledge, there has been no successful report of tion of CsF as a base and diglyme as a solvent was ef-
aromatic trifluoromethylation using O-silyl hemiami- fective for the catalytic aromatic trifluoromethylation;
nal 2.[21] Herein, we disclose a new catalytic procedure the use of KF as a base in place of CsF resulted in the
for aromatic trifluoromethylation by the use of formation of 3a in only 3% yield (entry 7).
trifluoro
G
Representative examples of the formation of tri-
cross-coupling partner (Scheme 2).
fluoromethylarenes 3 are summarized in Table 2.
Our initial studies were focused on a survey of suit- Using 10 mol% of the copper(I) complex prepared in
able reaction conditions such as ligand, base, and sol- situ from CuI and phen in a molar ratio of 1:1 worked
vent for the Cu-catalyzed aromatic trifluoromethyla- well for cross-coupling of aryl iodides 1 with O-silylat-
tion (Table 1).
ed hemiaminal
2 to afford the corresponding
When a mixture of 4-nitroiodobenzene (1a) and O- trifluoro
AHCTUNGTRENNUNG
silylated hemiaminal 2 and KF in DMF was heated at vance, selective aromatic trifluoromethylation was ac-
808C for 24 h in the presence of a copper complex complished; less than 1% yield of pentafluoroethylat-
(0.1 equiv. to 1a) prepared in situ from CuI and 1,10- ed by-product[5k] was observed in each case. Iodoben-
phenanthroline (phen), the cross-coupling product 3a zenes 1a–g endowed with electron-withdrawing
was obtained in 7% yield (entry 1). Several attempts groups on the aryl rings underwent catalytic trifluoro-
to transform 1a to the trifluoromethylarene 3a were methylation smoothly to provide the desired cross-
made by using solvents such as DMF, NMP, DMSO, coupling products in good yields (entries 1–8). In the
Scheme 2.
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Copper-Catalyzed Aromatic Trifluoromethylation via Group Transfer from Fluoral Derivatives
Table 2. Scope of Cu-catalyzed aromatic trifluoromethylation.
[a]
NMR yield, which was calculated by 19F NMR integration of product 3a relative to the trifluoromethoxybenze
internal standard.[a] NMR yields, which were calculated by 19F NMR integration of products 3 relative to the
trifluoromethoxybenzene internal standard.
The reaction was carried out in the presence of CuI (5 mol%) and phen (5 mol%).
A complex mixture was obtained.
The reaction was carried out in the presence of CuI (20 mol%) and phen (20 mol%).
[b]
[c]
[d]
[e]
Silyl hemiaminal 2 (3 equiv.) and CsF (3 equiv.) were used.
case of 3-iodonitrobenzene (1b), 5 mol% catalyst of iodobiphenyl 1i with 2 to deliver trifluoromethylat-
loading was sufficient to complete the cross-coupling ed biphenyl 3i in 53% yield (entry 10). The reactions
reaction (entry 3). Even ortho-substituted iodoarene of 1j and 1k possessing electron-donating groups such
1c underwent the cross-coupling reaction to produce as butyl and ethoxy groups with O-silylated hemiami-
2-nitro(trifluoromethyl)benzene (3c) in 47% yield nal 2 provided p-butyl(trifluoromethyl)benzene (3j)
(entry 4).
and p-ethoxy(trifluoromethyl)benzene (3k) in 40%
Not only a nitro group, but cyano and ester groups and 57% yields, respectively (entries 11 and 13). Intri-
in 1d and 1e were tolerable under the nucleophilic tri- guingly, iodoarene 1l bearing a hydroxy group partici-
fluoromethylating conditions (entries 5 and 6). How- pated in the catalytic aromatic trifluoromethylation
ever, the reaction of 4-iodoacetophenone (1f) with 2 upon treatment with excess amounts of silyl hemiami-
gave a complex mixture containing 1,2-adduct of the nal 2 and CsF (3 equiv. to 1l) (entry 14). Both 1- and
À
À
carbonyl moiety of 1f (entry 7). C Cl and C Br func- 2-iodonaphthalenes 1m and 1n underwent the group-
tionalities in 1g and 1h were compatible with the pres- transfer trifluoromethylation to afford the corre-
ent reaction conditions to provide the corresponding sponding trifluoromethylnaphthalenes 3m and 3n in
trifluoromethyl arenes 3g and 3h, respectively (en- high yields (entries 15 and 16). Furthermore, heteroar-
tries 8 and 9). Of significant interest, the present pro- ene 1o partook in the catalytic trifluoromethylation to
tocol worked well for both electron-deficient and give 3o in 75% yield (entry 17).
electron-rich iodoarenes; employing CuI-phenanthro-
line complex engendered the cross-coupling reaction trifluoro
One of the key features is that an aromatic
methylation that is catalytic in copper has
AHCTUNGTRENNUNG
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Hideaki Kondo et al.
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Scheme 3. Proposed mechanism for the Cu-catalyzed cross-coupling between aryl iodides 1 and fluoral derivative 2.
been accomplished; a small amount of copper com- Experimental Section
plex is enough to complete the cross-coupling. The
catalytic formation of trifluoromethylarenes 3 can be General Procedure for Catalytic Aromatic
explained by assuming the pathway pictured in Trifluoromethylation
Scheme 3. First, the fluoride ion-induced reaction of
An oven-dried Schlenk tube was charged with CuI
O-silyl hemiaminal 2 with CuI-diamine complex 4
(0.030 mmol, 5.7 mg), 1,10-phenanthroline (0.030 mmol,
gives copper alkoxide 5. Next, the migration of the
trifluoromethyl group in 5 readily takes place leading
to the elimination of N-formylmorpholine and the
generation of trifluoromethyl-copper(I) complex 6.[23]
Finally, s-bond metathesis[24] between CF3Cu complex
(6) and Ar-I (1) allows the formation of trifluorome-
thylarenes 3 and the regeneration of CuI-diamine
complex 4, which is reusable in the aromatic trifluoro-
methylation again.
In conclusion, we have demonstrated that a small
amount of CuI-phenanthroline complex catalyzes the
CF3-group transfer cross-coupling reactions of aryl/
heteroaryl iodides[25,26] with O-silylated hemiaminal 2
to give trifluoromethylated arenes in moderate to
high yields. The substrate scope of the present meth-
odology is broad and a variety of functional groups
are tolerated. Starting from a readily available CF3
source, catalytic aromatic trifluoromethylation has
been successfully achieved. Thus, the ingenious design
of the catalytic reactions expands remarkably the
scope of organofluorine compounds in practical syn-
thetic applications.
5.4 mg) and CsF (0.60 mmol, 67.3 mg). The tube was evacu-
ated and backfilled with argon, and then diglyme (0.60 mL),
4-nitroiodobenzene (1a, 0.30 mmol, 74.7 mg), and O-silylat-
ed hemiaminal 2 (0.60 mmol, 154 mg) were added. The re-
sulting reaction mixture was stirred at 808C for 24 h. After
the reaction was complete, the mixture was cooled to room
temperature, diluted with water, and extracted with ethyl
acetate. The combined organic layers were washed with
brine, dried over MgSO4, filtered, and the volatiles were re-
moved under vacuum. The residue was purified by chroma-
tography on silica gel to yield trifluoromethyl arene 3a. The
identity and purity of the known products were confirmed
1
by H, 19F NMR and GC-MS analysis. See the Supporting
Information for full details.
Acknowledgements
The financial support of the Ministry of Education, Culture,
Sports, Science and Technology of Japan (Grant-in-Aid for
Scientific Research), ꢀJapan Science and Technology Agency
(Research for Promoting Technological Seeds), Ube Founda-
tion, Asahi Glass Foundation, Naito Foundation, and Astellas
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Copper-Catalyzed Aromatic Trifluoromethylation via Group Transfer from Fluoral Derivatives
Foundation is gratefully acknowledged. We would like to
thank Prof. Masahiko Hayashi (Kobe University) for his
useful suggestions.
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