Efficient direct 2,2,2-trifluoroethylation of indoles via C-H functionalization

Article abstract of DOI:10.1039/c5cc00519a

A novel highly C3 selective metal free trifluoroethylation of indoles using 2,2,2-trifuoroethyl(mesityl)-iodonium triflate was developed. The methodology enables the introduction of a trifluoroethyl group in a fast and efficient reaction under mild conditions with high functional group tolerance. Beyond the synthetic developments, quantum chemical calculations provide a deeper understanding of the transformation. This journal is

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Efficient Direct 2,2,2-Trifluoroethylation of
Indoles via C-H Functionalization§†
Cite this: DOI: 10.1039/x0xx00000x
Gergely L. Tolnai,aǂ Anna Székely,aǂ Zita Makó,a Tamás Gáti,b János Daru,c
Tamás Bihari,c András Stirling*c and Zoltán Novák*a
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
A novel highly C3 selective metal free trifluoroethylation of trifluoromethyl diazomethane.9 Although, the unique electronic
indoles using 2,2,2-trifuoroethyl(mesityl)-iodonium triflate properties of the CF3CH2X (X= Br, I, OTf) compounds limit their use as
was developed. The methodology enables the introduction of simple electrophiles,10 recently several cross-coupling approaches
the trifluoroethyl group in a fast and efficient reaction under were developed for the introduction of trifluoroethyl group into
mild conditions with high functional group tolerance. Beyond aromatic systems.11 The direct C-H functionalization with
the synthetic developments, quantumchemical calculations trifluoroethyl group is seldom explored. Very recently Ackermann
provide deeper understanding of the transformation
.
described the first nickel catalyzed trifluoroethylation process utilizing
an 8-aminoquinoline directing group and trifluoroethyliodide as alkyl
source.12
A
Catellani type palladium catalyzed cascade
The presence of fluorine in organic molecules often provides
advantageous properties to the molecules, as fluorinated functional
groups can beneficially modify electronic properties of the
compounds, improve their metabolic stability, and enhance their
lypophilicity. Therefore the development of new synthetic methods
for the installation of the fluorine and fluorous functional groups is an
emerging field of synthetic organic chemistry. 1
trifluoroethylation was developed by Liu for the synthesis of
olefinated trifluoroethyl arenes.13 A unique example for transition
metal catalyst free direct trifluoroethylation of heterocycles is Baran’s
radical alkylation strategy using zinc sulfinate salts.14
In our work, we aimed to develop a new alkylation methodology
for the direct trifluoroethylation of indoles with the aid of
a
Modification of lead compounds with fluorous functional groups
to fine tune their biological activity is one of the most frequently used
strategies in medicinal chemistry. Aside from the simple fluorine atom
and CF3 group, the presence of other related small fluorous functional
groups such as trifluoromethoxy,2 trifluoromethylthio3 or structurally
similar trifluoroethyl4 groups on an aromatic core structure has also
gained exceptional attention from the fields of synthetic organic and
medicinal chemistry.
electrophilic trifluoroethyl synthon originated from hypervalent
iodonium salts. Introduction of hypervalent iodonium salts as useful
reagents in organic chemistry15 opens new possibilities for the
formation of carbon-heteroatom and carbon-carbon bonds via C-H
activation.16 Besides numerous synthetic applications,15a,17 the use of

3 iodo compounds provides an emerging transition metal catalyzed
strategy for the direct functionalization of indoles.18 The pioneering
work of Umemoto in the field of fluoroalkyliodonium salts19 inspired
us to use hypervalent iodine reagents for the trifluoroethylation of
indoles. These reagents were used mostly for the fluoroalkylation of
heteroatom20 and soft carbon21 nucleophiles to build C-N, C-O, C-S
and C(sp3)-C(sp3) bonds.
Due to the importance of the indole framework in the structure of
drug candidates much attention has been focused on the synthesis of
fluoroalkylated indole derivatives. In general, trifluoroethylated
indoles are accessible through indole core functionalization via
multistep syntheses (3-6 synthetic steps)5 or ring constructions such
For the realization of the desired transformation we synthesized
as
a novel radical Fisher indole type synthesis developed by
2,2,2-trifuoroethyl(mesityl)-iodonium triflate salt (1a) as
a new
Antonchik.6 Compared to trifluoromethylation, the synthetic
transformations to access trifluoroethylated compounds via C-C bond
formation are underdeveloped. The main, but still uncommon
methods for the construction of trifluoroethyl group involve the
reagent from bis(trifluoroacetatoxy)(2,2,2-trifluoroethyl)iodide and
mesitylene in 94 % isolated yield on multigram scale. We strategically
chose the mesityl group (Mes) as a frequently used aromatic system
for the synthesis of aryliodonium salts with high stability and activity.
However, two additional trifluoroethylaryliodonium salts were
trifluoromethylation of
trifluoromethyl group onto alkenes,8 and the utilization of gaseous
a
benzylic position,7 the addition of
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prepared, but the phenyl (1b) and 4-tolyl (1c) derivatives were
obtained with lower efficiency (68% and 83% respectively).
After having the designed reagents in hand, we turned our
attention to the possible C-H functionalization of unprotected indoles
to develop a powerful synthetic methodology to enable C-C bond
formation without the necessity of protecting groups. First, we
studied the trifluoroethylation reaction of indole (2a) with 1a in
dichloromethane (CH2Cl2) at 25 °C with or without transition metal
catalyst (Cu(II), Pd(II), Ag(I) salts). To our delight the reaction of
unsubstituted indole provided the desired C3 trifluoroethylated
product 3a (Scheme 1.). In contrast with the known iodonium salt
based transition metal catalyzed indole functionalization
methodologies, we found that the presence of transition metal
catalyst had deleterious effect on the trifluoroethylation reaction. 22
a
a
Scheme 2. Substrate scope of the transformation. without base, 2 equiv. of
iodonium salt
Scheme 1. Adjustment of reactivity by different bases. 2 Equivalents of base, 1
equivalent of 1a reagent. Isolated yield of the mixture. Determined by 19F
b
c
NMR measurements. d 1.3 Equivalent of iodonium salt (1a) was used
the scope with electron withdrawing groups on the indole skeleton.
To our delight the presence of fluorine, nitro, cyano and
trifluoromethyl group on the indole core were well tolerated, and the
desired products (3r-v) were isolated in 31-94 % yield after 1.5 - 4
hours reaction time. Demonstrating the power of the methodology
we examined the trifluoroethylation of indoles bearing functional
groups suitable for cross coupling chemistry (Cl, Br, I, boronic ester;
Scheme 3.). Each halogen substituted indoles gave the desired
products (3w-3ee) in good to excellent yields (61-90 %), as well as the
boronic ester derivative (3ff, 83%).
Despite of the high reactivity of the polyfluoroalkyl 3 iodonium
compounds toward heteroatom nucleophiles20a,20b trifluoroethylation
did not occur on the indole nitrogen due to its lower nucleophilicity.
Besides these beneficial features, we observed the formation of
significant amount of N-trifluoroethylated indolino-indole derivative
(4). The presence of trifluoromethanesulfonic acid, generated in the
reaction, accelerates the dimerization of indole, and the formed
indoline moiety undergoes straightforward N-trifluoroethylation.22 It
was anticipated, that the amount of this side product could be
decreased by adding an appropriate base such as 2,6-di-tert-
butylpyridine (DTBPy) and target compound 3a was isolated in 84 %
yield.22 Next, we explored the synthetic utility of this reaction by
examining the substrate scope in CH2Cl2 at 25 °C.22 Indole and alkyl
indoles reacted smoothly with 1a under our conditions to provide
trifluoroethyl indoles 3a-3e in 10 minutes reaction time (Scheme 2.).
In case of 3-methylindole, formation of any trifluoroethylated product
were not observed (not shown). More electron rich alkoxyindoles were
also successfully functionalized in a rapid reaction (10-60 minutes) and
the desired compounds (3f-j) could also be isolated in good to
excellent yields (46-96%). Although, the indole nitrogen does not
require protection, the tested N-methyl and N-TIPS derivatives were
also successfully trifluoroethylated (3k-m). Nucleophilic functional
groups connected to the indole frame such as free hydroxyl, amino
and carboxyl functions could be masked with different protecting
groups for the efficient trifluoroethylation (3n-p) of indole in position
3 to obtain valuable heterocyclic building blocks. Indole containing
TMS protected alkyne was also converted to its corresponding
trifluoroethylated compound (3q) in 84 % yield, opening the way for
further synthetic transformations through the acetylene part. To
further explore the real synthetic power of the method we examined
Scheme 3. Trifluorethylated indoles for cross-coupling chemistry.
We employed DFT calculations to understand the mechanism and
the selectivity of the present reaction. First we considered the
reaction between indole (2a) and 2,2,2-trifluoroethyl(mesityl)-
iodonium triflate (1) (Scheme 4.). The dissociation of the triflate anion
proved to be slightly exergonic (-0.9 kcal·mol-1) suggesting
a
dissociated resting state. The rate-determining step is the
trifluoroethylation of the indole-ring. The next step is the
deprotonation of the σ-complex by the basis with a 18.3 kcal·mol-1
barrier. Both steps are highly exergonic indicating irreversible
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transformations. Formation of 1- and 2-substituted indoles is much
less favourable which explains that they could not be observed in
experiment.22 We have also calculated the free energy barrier heights
for the all the substrates indicated in Schemes 2 and 3.22 Within the
expected accuracy the calculations nicely correlate with experiment
providing confidence in the conclusion. The formation of 3-indolyl-
trifluorethyl-mesityl-iodonium salt from 1a and 2a as another possible
intermediate of the reaction and subsequent product formation
through reductive elimination have been also taken into
consideration. This reaction pathway has been safely excluded on the
basis of the prohibitively high barrier (52 kcal·mol-1) for the formation
of the key intermediate from 1a and 2.
Scheme 5. GC Conversion of trifluoroethyl indole derivatives obtained from
reactions carried out in the presences of different amines. Activation energies of
trifluoroethylation of indole derivatives and bases are indicated.
The readily accessible new 2,2,2-trifluoroethyl(mesityl)-iodonium
triflate reagent ensures the straightforward trifluoroethylation of
indoles in position 3 under very mild conditions in a rapid reaction (10-
240 minutes reaction time). The excellent functional group tolerance
of the developed transformation enables the access of chemically
diverse compound classes with potential medicinal interest.
Additionally, with the aid of DFT studies we revealed the mechanistic
steps of the reaction, and explained the important role of basic
additives in the transformation. Beyond these mechanistic studies
and the synthesis of 3-trifluoroethyl indoles with high versatility the
developed methodology opens new doors to the synthesis of other
trifluoroethylated aromatic and heterocyclic compounds via C-H
functionalization or transition metal catalyzed C-H activation. The
study of these synthetic possibilities currently undergoes in our
laboratory.
Scheme 4. Free energy profile in kcal·mol-1 of the transformation based on DFT
calculation
To explain the special efficiency of DTBPy as base in the reaction
we have also calculated the activation barriers of the alkylations of
different amine bases such as TEA, DIPEA, 2,6-lutidine and DTBPy. It
was found that the activation energies of the N-trifluoroethylation of Notes and references
these bases with 1a were 18.4 kcal·mol-1, 22.4 kcal·mol-1 and 23.6
a
MTA-ELTE “Lendület” Catalysis and Organic Synthesis Research
kcal·mol-1 respectively which are in the same range as the barriers of
the C alkylations of the studied indoles. However, computation results
showed that the N-trifluoroethylation of DTBPy required extremely
high activation energy (36.2 kcal·mol-1) due to the significant steric
repulsion.22 These results indicate that variations in the barrier heights
of trifluoroethylations can strongly influence the outcome of the
reaction, which is important from a synthetic standpoint. Additional
experiments have convincingly confirmed this prediction as shown in
Scheme 5. In this scheme we collected the conversions of four indole
Group, Institute of Chemistry, Eötvös University, Faculty of Science,
Pázmány Péter stny. 1/A, 1117, Budapest, Hungary. E-mail:
novakz@elte.hu; web: http://zng.elte.hu.
b
Servier Research Institute of Medicinal Chemistry, Záhony utca 7. H-
1031 Budapest, Hungary.
c
Research Centre for Natural Sciences of the Hungarian Academy of
Sciences, Department of Theoretical Chemistry, Magyar Tudósok
körútja 2. 1117 Budapest Hungary E-mail: stirling.andras@ttk.mta.hu.
§
This paper is dedicated to Prof. József Rábai on the occasion of his
derivatives in the presence of different bases. From left to right the 65th birthday
reactivity (TS barriers of trifluoroethylation) of the indole derivatives
†
Electronic Supplementary Information (ESI) available: [detailed
are tuned by varying their substituents. Going down in the columns, experimental procedures, characterization of products and the details of
the barriers of the N-alkylation increase. The trends obtained for the DFT calculations]. See DOI: 10.1039/b000000x/
all possible combinations of indole-base pairs demonstrate that the
ǂ
Both authors contributed equally to this work
efficiency of a substrate-base combination is determined by the This project was supported by the “Lendület” Research Scholarship of
activation barrier differences of the competing N- and C-alkylations. the Hungarian Academy of Sciences (LP2012-48/2012), TÉT-10-1-2011-
Clearly, alkylations of the bases and substrates are competitive 0245 and by OTKA K-101115.The Authors thank to Brian M. Stoltz at
reactions, and the yields support the postulated kinetic control.
Caltech for the proofreading of this manuscript. Generous donation of
In conclusion we have developed a transition metal free direct C-H indole derivatives is also acknowledged to Servier Research Institute of
trifluoroethylation reaction, which enables the selective Medicinal Chemistry.
trifluoroethylation of a heterocyclic system for the first time resulting
in C(sp2)-C(sp3) bond formation.
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