Mild and efficient palladium-catalyzed direct trifluoroethylation of aromatic systems by C-H activation

Article abstract of DOI:10.1002/anie.201510555

The introduction of trifluoroalkyl groups into aromatic molecules is an important transformation in the field of organic and medicinal chemistry. However, the direct installation of fluoroalkyl groups onto aromatic molecules still represents a challenging and highly demanding synthetic task. Herein, a simple trifluoroethylation process that relies on the palladium-catalyzed C-H activation of aromatic compounds is described. With the utilization of a highly active trifluoroethyl(mesityl)iodonium salt, the developed catalytic method enables the first highly efficient and selective trifluoroethylation of aromatic compounds. The robust catalytic procedure provides the desired products in up to 95 % yield at 25 °C in 1.5 to 3 hours and tolerates a broad range of functional groups. The utilization of hypervalent reagents opens new synthetic possibilities for direct alkylations and fluoroalkylations in the field of transition-metal-catalyzed C-H activation.

Full text of DOI:10.1002/anie.201510555

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International Edition: DOI: 10.1002/anie.201510555
German Edition: DOI: 10.1002/ange.201510555
Trifluoroethylation
Mild and Efficient Palladium-Catalyzed Direct
À
Trifluoroethylation of Aromatic Systems by C H
Activation
+
+
˝
Balµzs L. Tóth , Szabolcs Kovµcs , Gergo Sµlyi, and Zoltµn Novµk*
Angewandte
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1988
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Abstract: The introduction of trifluoroalkyl groups into
be achieved by various transition-metal-catalyzed coupling
aromatic molecules is an important transformation in the
field of organic and medicinal chemistry. However, the direct
installation of fluoroalkyl groups onto aromatic molecules still
represents a challenging and highly demanding synthetic task.
Herein, a simple trifluoroethylation process that relies on the
reactions.[8] Nickel- and palladium-catalyzed Suzuki-type
reactions have enabled the trifluoroethylation of aromatic
halides,[9] tosylates,[10] and diazo compounds.[11] In contrast,
À
the direct introduction of trifluoroethyl groups by C H bond
À
functionalization or C H activation is very rare. Only two
À
palladium-catalyzed C H activation of aromatic compounds
synthetic methods for the direct installation of trifluoroethyl
À
is described. With the utilization of a highly active trifluoro-
ethyl(mesityl)iodonium salt, the developed catalytic method
enables the first highly efficient and selective trifluoroethylation
of aromatic compounds. The robust catalytic procedure
provides the desired products in up to 95% yield at 258C in
1.5 to 3 hours and tolerates a broad range of functional groups.
The utilization of hypervalent reagents opens new synthetic
possibilities for direct alkylations and fluoroalkylations in the
groups at aromatic cores by transition-metal-catalyzed C H
activation have been reported. Recently, Ackermann and co-
workers described the first trifluoroethylation that proceeds
À
by nickel-catalyzed C H activation, utilizing 8-aminoquino-
line as the directing group and ICH2CF3 as the alkylating
agent (Scheme 1a).[12] Liu and co-workers described a Cat-
ellani-type palladium-catalyzed alkenylation/trifluoroethyla-
tion cascade reaction[13] and the synthesis of numerous
olefinated trifluoroethyl arenes (Scheme 1b). Considering
À
field of transition-metal-catalyzed C H activation.
À
T
he potential of transition-metal-catalyzed C H activation
in organic chemistry has been extensively exploited over the
last decade.[1] Both C H activation of aromatic systems
[1,2]
À
3
[3]
À
and C(sp ) H functionalization with the aid of directing
groups have been evaluated, and several methods offer
solutions for the construction of new carbon–carbon bonds.
In contrast to the vast array of methods available for
À
transition-metal-catalyzed arylation by C H activation, the
À
alkylation of aromatic molecules through C H bond activa-
tion is less explored.[4] Fluorine-substituted alkyl groups are of
particular importance for the pharmaceutical and agrochem-
ical industries as well as for materials science, as the electronic
properties and the lipophilicity, bioavailability, and metabolic
stability of compounds can be fine-tuned by substitution with
fluorine.[5] Therefore, the development of new synthetic
methods for the installation of trifluoromethyl groups is an
emerging area of synthetic organic chemistry. Despite the
great number of trifluoromethylation methods already avail-
able,[6] the introduction of other alkyl groups bearing CF3
groups has hardly been explored. The simplest alkyl homo-
logue that contains a CF3 moiety is the trifluoroethyl group.
Considering its structural features, this group resembles OCF3
and SCF3 moieties, which are frequently used in medicinal
chemistry.[7] Furthermore, trifluoroethyl groups connected to
heteroatoms are present in several drugs, such as Flecainide
(OCH2CF3), Polythiazide (SCH2CF3), or Quazepam
(NCH2CF3). However, the synthesis and medical applications
of trifluoroethyl-substituted aromatic compounds have not
been studied in much detail.
À
Scheme 1. Existing methods for trifluoroethylation by C H functionali-
zation. BDMAE=bis(2-dimethylaminoethyl)ether.
the reaction conditions of these transformations, a simpler
functionalization method was developed by Baran and co-
workers for the fluoroalkylation of selected nitrogen hetero-
cycles, which was based on a zinc sulfinate as the active
reagent (Scheme 1c).[14] Although these state-of-the-art tri-
fluoroethylation methods have demonstrated the possibility
of directly introducing trifluoroethyl groups into aromatic
systems, the narrow substrate scope, the forcing reaction
conditions, the formation of complex reaction mixtures, and
occasional selectivity problems still suggest the need for
further method development to efficiently introduce fluo-
À
The installation of trifluoroethyl groups at aromatic cores
with concomitant formation of new carbon–carbon bonds can
roalkyl groups into aromatic cores by C H activation.
We anticipated that the application of electrophilic
fluoroalkylation reagents[15,16] in combination with transi-
À
tion-metal-catalyzed C H activation would be useful owing
[*] B. L. Tóth,[+] Dr. Sz. Kovµcs,[+] G. Sµlyi, Dr. Z. Novµk
MTA-ELTE “Lendület” Catalysis and Organic Synthesis Research
Group, Institute of Chemistry
to their high reactivity. The utilization of a highly active
trifluoroethylation agent could solve this challenging syn-
thetic problem and open new synthetic possibilities for the
direct trifluoroethylation and other alkylation reactions of
aromatic systems.
Eçtvçs University, Faculty of Science
Pµzmµny PØter stny. 1/A, H-1117 Budapest (Hungary)
E-mail: novakz@elte.hu
Anilides are perfect substrates for this study owing to the
Homepage: http://zng.elte.hu/
[+] These authors contributed equally to this work.
À
efficient directing ability of the amide moiety in C H
activation, and their straightforward and versatile transform-
ability. We envisioned that dimeric organopalladium com-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201510555.
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plexes of anilides could react with a hypervalent iodonium
salt, benefitting from synergistic effects between the neigh-
boring palladium atoms.
The feasibility of this concept was confirmed by the
stoichiometric reaction of mesityl(trifluoroethyl)iodonium
triflate and the trifluoroacetate-bridged dimeric complex of
2-methylacetanilide and palladium (Scheme 2). To our
delight, the desired product was formed rapidly in CH2Cl2
at 258C and isolated in 76% yield.
(TFA) led to full conversion (100%, entry 3). Amongst
various palladium catalysts, PdCl2 was completely inactive,
whereas the use of either Pd(TFA)2 or Pd2(dba)3 led to high
conversion within 24 hours (entries 4–6). We also tested
several solvents (entries 7–11), but only 1,2-dichloroethane
proved to be a suitable medium for the coupling (100%
conversion, entry 11) aside from dichloromethane.
The progress of the trifluoroethylation of 2-methylaceta-
nilide under the optimized reaction conditions was monitored
by in situ IR analysis, in particular by monitoring the
characteristic peak of the product at 1638 cmÀ1. We observed
the straightforward formation of the desired trifluoroethy-
lated coupling product after the addition of TFA, and the
reaction was complete in three hours. (Figure 1).[17]
Scheme 2. Proof-of-concept studies: Trifluoroethylation in the presence
of the shown palladacycle dimer with mesityl(trifluoroethyl)iodonium
triflate in CH2Cl2 at 258C.
Next, we studied the feasibility of the trifluoroethylation
of acetanilide 2b with mesityl(trifluoroethyl)iodonium tri-
flate under catalytic conditions. Thorough optimization
studies in terms of catalyst and catalyst loading, temperature,
solvent, and acidic additives revealed suitable conditions for
this coupling (Table 1). In particular, Pd(OAc)2 (7.5 mol%)
was used as the catalyst in dichloromethane, which is
À
a frequently used reaction medium for C H activation with
Figure 1. Monitoring of the catalytic trifluoroethylation by in situ IR
analysis. The trifluoroethylation of 2-methylacetanilide was performed
with 1 and Pd(OAc)2 (7.5 mol%) in CH2Cl2 at 258C.
hypervalent iodonium salts. To our delight, the reaction
provided the desired product (3b) with 40% conversion. To
improve the efficiency of the coupling, we added Brønsted
acids to the reaction mixture. Whereas the reaction pro-
ceeded with only 20% conversion in the presence of acetic
acid (entry 2), the addition of 1 equiv of trifluoroacetic acid
With the optimized conditions in hand, the substrate
scope was investigated with various acetanilides (Scheme 3).
When simple acetanilide was reacted with the mesityl(tri-
fluoroethyl)iodonium salt, the transformation afforded the
desired ortho-trifluoroethylated product (3a) in 83% yield in
1.5 hours. Substituents in ortho position to the directing group
Table 1: Optimization studies.[a]
À
generally have deleterious effects on C H activation. How-
ever, we found that this transformation tolerates the presence
of ortho substituents very well. Anilides bearing methyl,
methoxy, isopropyl, trifluoroethyl, phenyl, and benzyl groups
in the ortho position were trifluoroethylated with high
efficiency (3b–3g, 58–91% yield). The presence of halogens
(Br, Cl) and protected alcohols was also well tolerated, as
exemplified by the formation of 3h–3j in 82–86% yield.
Furthermore, anilides with a fluoro, chloro, bromo, or
even an iodo substituent in the meta position were trifluoro-
ethylated in excellent yields (3k–3n, 78–95%). Electron-
donating groups, such as methyl, methoxy, and silyl-protected
hydroxy moieties, in meta position were also tolerated, and
the desired products were afforded in excellent yields (3o–
3q). Finally, para-substituted anilides were examined.
Methyl-, isopropyl-, methoxy-, and halogen-substituted ani-
lides provided the expected products 3r–3v in 79–87% yield
under the optimized reaction conditions. Furthermore, the
coupling of disubstituted acetanilides afforded the trifluoro-
ethylated compounds (3w–3y) in similarly good yields (82–
91%).
Entry
Catalyst
Solvent
Acid
Conv. [%][b]
1
2
3
4
5
6
7
8
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
PdCl2
Pd(TFA)2
Pd2(dba)3
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
DMSO
EtOAc
Et2O
–
40
20
100[c]
0
91
97
0
23
50
96
100
AcOH
TFA
TFA
TFA
TFA
TFA
TFA
TFA
TFA
TFA
9
10
11
toluene
1,2-DCE
[a] Reaction conditions: 2b (0.05 mmol, 1 equiv), 1 (1.2 equiv), Pd-
(OAc)2 (7.5 mol%), acid (1 equiv), solvent (0.5 mL), 258C, 24 hours.
[b] Conversions determined by GC analysis. [c] For conversions achieved
with lower Pd catalyst loadings, see the Supporting Information.
dba=dibenzylideneacetone, 1,2-DCE=1,2-dichloroethane, TFA=tri-
fluoroacetic acid.
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À
2ai also proved to be an excellent example for the C H
activation of heterocyclic systems, and trifluoroethylated 3ai
was formed in 49% yield. Interestingly, this substrate under-
went double-bond isomerization, leading to the isoxazole
structure with an exo double bond.
À
We next focused on the ortho C H trifluoroethylation of
anilides with directing groups other than the simple acetyl
moiety; pivalamide and cyclohexylamide groups were also
found to be feasible. The corresponding products 5a and 5b
were thus obtained in excellent yields (Scheme 4).
Scheme 4. Palladium-catalyzed trifluoroethylation of aromatic amides.
Reaction conditions: 4a–4h (1 mmol), 1 (1.05–1.2 equiv), Pd(OAc)2
(7.5 mol%), TFA (1–5 equiv), CH2Cl2 (1 mL), 258C.
Scheme 3. Palladium-catalyzed trifluoroethylation of anilides. Reaction
conditions: 2a–2y (1 mmol), 1 (1.05–1.2 equiv), Pd(OAc)2
(7.5 mol%), TFA (1–3 equiv), CH2Cl2 (1 mL), 258C.
Utilization of an anilide substrate with a cyclic pyrrolidi-
none scaffold afforded the desired monosubstituted product
in 90% yield (5c). The amides that had been prepared from
ortho-toluidine and 2-chloroacetyl and 2-phenylacetyl chlo-
ride were trifluoroethylated in 70% and 47% yield, respec-
tively (5d, 5e). We also examined the applicability of various
benzamide directing groups with electron-donating or -with-
drawing groups. Only the aniline core was trifluoroethylated
in ortho position under the optimized reaction conditions, and
the corresponding products (5 f–5h) were isolated in good
yields.
Importantly, this novel method is not limited to substrates
bearing electron-donating groups or halogens. Whereas the
presence of electron-withdrawing groups generally has dele-
À
terious effects on C H activation and electrophilic substitu-
tion, this mild trifluoroethylation procedure enables the
direct transformation of such challenging substrates. The
efficiency and broad scope of the reaction was well illustrated
by acetanilides with electron-withdrawing ester, acetyl, and
nitro groups in meta or para position. In all cases, the desired
trifluoroethylated products (3z–3ad) were formed with high
levels of efficiency (77–91%). As a unique substrate for
further cross-couplings, 3-pinacolboratoacetanilide (2ae) was
also successfully trifluoroethylated in 50% yield, demonstrat-
ing the robustness of the method and its compatibility with
functionality associated with cross-couplings and other metal-
catalyzed processes. To further expand the substrate scope, we
tested the suitability of bicyclic aromatic systems and hetero-
cyclic rings. With 1-acetamidonaphthalene, the trifluoroethyl
group was introduced in the 2-position, whereas 2-acetami-
donaphthalene was selectively trifluoroethylated in the
3-position; the desired products 3af and 3ag were obtained
in 79% and 81% yield, respectively.
In conclusion, we have developed a novel method for the
direct palladium-catalyzed trifluoroethylation of aromatic
À
compounds by C H activation. The utilization of a mesityl-
(trifluoroethyl)iodonium salt in stoichiometric amount ena-
bles the highly efficient and selective trifluoroethylation of
anilides at room temperature within a short period of time.
The application of hypervalent reagents opens new possibil-
ities in the field of organic synthesis, in particular when
considering the difficulties commonly associated with the
direct fluoroalkylation of aromatic systems.
The trifluoroethylation was also effective in the case of
N-acetylindoline, which was regioselectively trifluoroethy-
lated in 93% yield (3ah). Furthermore, acetamidoisoxazole
Experimental Section
A 4 mL screw-cap vial was charged with Pd(OAc)2 (0.075 mmol,
7.5 mol%, 16.8 mg), the amide derivative (1 mmol), and mesi-
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tyl(2,2,2-trifluoroethyl)iodonium trifluoromethanesulfonate (1.05–
1.2 equiv), and then equipped with a stir bar. CH2Cl2 (1 mL) was
added as the solvent. TFA (1–5 equiv) was instantly added dropwise,
and the mixture was stirred at room temperature for 1.5–3 h. After
the appropriate time, the mixture was diluted with EtOAc (25 mL)
and extracted with concentrated NaHCO3 solution (2 ” 10 mL) and
brine (10 mL). The organic phase was dried over MgSO4 and
evaporated onto Celite with a rotary evaporator. The crude product
was purified by column chromatography (hexane/EtOAc).
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Acknowledgements
This work was supported by a “Lendület” Research Scholar-
ship of the Hungarian Academy of Sciences (LP2012-48/
2012). We thank Professor Brian M. Stoltz (Caltech) for
proofreading this manuscript.
À
Keywords: anilides · C H activation · iodonium salts ·
palladium · trifluoroethylation
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