Cobalt-Catalyzed Hydroarylations and Hydroaminations of Alkenes in Tunable Aryl Alkyl Ionic Liquids

Article abstract of DOI:10.1021/acs.orglett.8b02688

Tunable aryl alkyl ionic liquids (TAAILs) are a promising class of imidazolium- or triazolium-based ionic liquids. Contrary to "standard" all-alkyl ionic liquids, these carry an aryl ring together with a linear or branched alkyl chain. Their application i

Full text of DOI:10.1021/acs.orglett.8b02688

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Cobalt-Catalyzed Hydroarylations and Hydroaminations of Alkenes
in Tunable Aryl Alkyl Ionic Liquids
Felix Schroeter, Swantje Lerch, Maria Kaliner, and Thomas Strassner*
Physikalische Organische Chemie, TU Dresden, Bergstrasse 66, 01062 Dresden, Germany
S
* Supporting Information
ABSTRACT: Tunable aryl alkyl ionic liquids (TAAILs) are a
promising class of imidazolium- or triazolium-based ionic liquids.
Contrary to “standard” all-alkyl ionic liquids, these carry an aryl ring
together with a linear or branched alkyl chain. Their application in the
cobalt-catalyzed hydroarylation/hydroamination of alkenes and ani-
lines is presented. The catalytic system is tolerant toward air and is
scalable and reusable. It has been successfully used for the synthesis of
pharmacologically relevant primary to tertiary aryl amines.
n the quest for more sustainability in chemistry, ionic
liquids (ILs) are regarded as one of the most potent
Scheme 1. Representative Catalyst Systems for the
Markovnikov Hydroarylation/Hydroamination of Activated
Olefins and Anilines
O
solvents of the future,1−4 e.g., for application in catalysis.4−6
Their negligible vapor pressure minimizes emission into the
environment, because of which they are often labeled as “green
solvents”.7,8 At the same time, they are highly thermally,
chemically, and electrochemically stable. Most importantly,
due to their modular design, their properties can be fine-tuned
and thus can be tailored to a specific task.9−12 For this
endeavor, we use a new generation of ionic liquids, the tunable
aryl alkyl ionic liquids (TAAILs),13−19 with the additional
opportunity of fine-tuning the electronic structure of the cation
through the aryl ring substitution pattern.20,21 For example, the
increased solubility of metal salts in TAAILs has been applied
to the extraction of noble metals and rare earth elements.22
The ability of ionic liquids to dissolve both metal salts and
organic substrates allows access to intriguing reusable catalytic
systems.4−6,23 The utility of TAAILs has previously been
demonstrated in the industrially relevant platinum-catalyzed
hydrosilylation of olefins.24
At first, the required TAAILs had to be synthesized in
reasonable quantities following the general synthetic procedure
(Scheme 2). An improved protocol for the synthesis of the aryl
Scheme 2. Modular Approach for the Synthesis of TAAILs
Similarly, hydroaminations and hydroarylations are among
the most sought-after reactions because of their high atom
economy.25−30 In particular, we were interested in the
intermolecular addition of anilines to olefins (Scheme 1).
Common approaches for this reaction include the use of
ligand-modulated late transition metals,31−40 strong Brønsted
acids,41−47 and earth-abundant metals.48−51 There are some
reports on the use of ionic liquids in this type of reaction,52−57
especially for the reaction of the more reactive acetylenes58 and
activated olefins59 or for reactions (co)catalyzed by Brønsted
acids.60−62
Our approach combines the use of earth-abundant metal
catalysts, namely, cobalt chloride, with the fine-tuning ability of
the TAAILs. We present an easy-to-use catalytic system that is
tolerant toward air, reusable, and allows product isolation by
distillation. The scope has been explored, showing that the
system enables access to pharmacologically relevant substituted
aromatic amines.
imidazoles allowed facile scale-up and purification by
distillation. All TAAILs were accessible in three steps from
commercial starting materials in quantities of up to 500 g. The
different counterions, aryl substituents, and alkyl chains of the
TAAILs can be widely varied in a modular fashion. An
overview of the synthesized TAAILs is given in Table S1 (see
the Supporting Information (SI)).
Received: August 22, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b02688
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
We initially screened several commercially available metal
salts in the reaction of styrene and aniline in the ionic liquid 1a
(1-butyl-3-(2-methylphenyl)imidazolium bis(trifluorometh-
anesulfonyl)imide) (Table 1). The reaction mainly yielded
leads to an almost instant color change from red to green,
indicating the formation of a cobalt chloride aniline complex.
The product ratio 2a:3a was approximately 3:2, regardless of
the metal salt being used. This selectivity is similar to other
reports.60−62 For a high selectivity toward the monoalkeny-
lated products, a 3-fold excess of aniline was required. The
excess aniline can be reisolated after the catalytic run.
Table 1. Screening of Different Catalysts
Next, we screened several ionic liquids as solvent for the
reaction (Table 2). The choice of the counterion (X) (Scheme
2) is very important for the catalytic performance of the system
(Table 2, entries 1−7). Only the TAAILs containing the
weakly coordinating counterions NTf2 (Table 2, entry 1) and
BArF (Table 2, entry 6) were able to mediate the reaction. This
dependence on the counterion has been reported for related
catalytic systems.43 Upon variation of the chain length (R2)
(Scheme 2), we found a pronounced influence on the catalytic
activity (Table 2, entries 8−12). In the undecyl-substituted IL
1k, the chain length is at an optimum (Table 2, entry 11).
Similarly, the choice of the aryl substitution pattern (R1)
(Scheme 2) is important for an efficient catalytic conversion
(Table 2, entries 13−19). For optimum performance, one
ortho substituent has to be present. Among the tested ionic
liquids, 1q was the most efficient solvent for the reaction
(Table 2, entry 17). Again, the product ratio of 2a:3a was
approximately 3:2 in all TAAILs, which shows that the
selectivity is not governed by the catalytic system but by the
intrinsic properties of the substrates. We also compared the
performance of other “standard” ionic liquids (Table 2, entries
20−22). In general, these performed worse compared to the
TAAILs. The reaction does not proceed in the absence of an
ionic liquid (Table 2, entries 23 and 24), although the catalyst
is completely dissolved in the latter mixtures. This shows that
the highly polar environment, which is formed by the ionic
liquids,63 is necessary for an efficient catalytic reaction.26,64,65
Screening of the catalyst load and reaction temperature
showed that although there are some changes in selectivity the
highest yields are still obtained by employing 2 mol % catalyst
at 140 °C reaction temperature (Table S2, see SI). At higher
2a/3a
2a/3a
no.
cat.
yield/%
no.
cat.
yield/%
a
a
b
1
2
3
4
5
6
7
8
FeCl3·6H2O
CoCl2·6H2O
NiCl2·6H2O
MoO2Cl2
RuCl3·3H2O
AuCl
18:16 (37)
16:14 (39)
13:12
7:6
17:15 (31)
10:8
9
10
11
12
13
14
15
16
LiNTf2
4:4
3:3
2:0
7:6
7:7
13:11
16:13
12:10
b
none
Fe(acac)2
FeSO4·7H2O
Fe(C2O4)
Co(OAc)2·4H2O
Co(NO3)2·6H2O
Ni(OAc)2·4H2O
a
LaCl3·H2O
10:10
8:8
c
HOTf
a
b
c
2a, yield after 24 h. After 24 h. 5 mol %. Legend: 1a = 1-butyl-3-
(2-methylphenyl)imidazolium bis(trifluoromethanesulfonyl)imide.
the hydroarylation product 2a and the hydroamination product
3a. Among several earth-abundant metal salts and noble metal
salts, iron(III) chloride, cobalt(II) chloride hexahydrate, and
ruthenium(III) chloride trihydrate performed best (Table 1,
entries 1, 2, and 5). Other transition metal chlorides, as well as
triflic acid, were less efficient (Table 1, entries 3, 4, and 6−8).
Without added catalyst, the yield after 24 h is as low as 3%
(Table 1, entry 10). Metal salts other than chlorides were also
less active (Table 1, entries 11−16). We chose to further
optimize the reaction using CoCl2·6H2O since it is an
inexpensive and abundant metal salt, which also showed a
slightly better performance after 24 h reaction time, where
complete conversion of styrene was observed, compared to
FeCl3·6H2O. Dissolving CoCl2·6H2O in the catalytic mixture
a
Table 2. Screening of the Ionic Liquid
no.
2a/3a yield/%
no.
2a/3a yield/%
IL, R1 = 2-Me, R2 = C4H9, (X)
1a (X = NTf2)
1b (X = OMs)
1c (X = OTs)
1d (X = TFA)
1e (X = Br)
IL, R2 = C11H23, X = NTf2, (R1)
1m (R1 = 2-MeO)
1n (R1 = 2-EtO)
1o (R1 = 4-Br)
1p (R1 = 4-OMe)
1q (R1 = 2,4-Me2)
1r (R1 = H)
1
2
3
4
5
6
7
39:22
0:0
0:0
0:0
0:0
13
14
15
16
17
18
19
47:16
54:27
34:23
37:23
59:29
4:3
1f (X = BArF
)
26:26
0:0
1g (X = DCA)
1s (R1 = 2,4,6-Me3)
other solvents
30:18
IL, R1 = 2-Me, R2, X = NTf2
1h (R2 = C3H7)
8
9
10
11
12
15:11
13:9
47:24
55:25
14:10
20
21
22
23
24
[UndecMim][NTf2]
[EMim][NTf2]
[P66614][NTf2]
PhNO2
none
19:15
4:2
13:6
0:0
1i (R2 = C6H13)
1j (R2 = C9H19)
1k (R2 = C11H23)
1l (R2 = C16H33)
0:0
a
OMs − methane sulfonate, OTs − p-toluene sulfonate, TFA − trifluoroacetate, BArF − tetrakis(3,5-bis(trifluoromethylphenyl))borate, DCA −
dicyanamide, UndecMim − 1-methyl-3-undecylimidazolium, EMim − 1-ethyl-3-methylimidazolium, P66614 − trihexyltetradecylphosphonium.
B
DOI: 10.1021/acs.orglett.8b02688
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
temperatures, 2a is obtained as the sole product, which is
caused by decomposition of 3a, as indicated by prolonging the
reaction time at 140 °C (Table S3, see SI). Adding excess
amounts of water or performing the reaction in air only leads
to a slight decrease in yield (Table S4, see SI). In the latter
case, a color change from green to pink indicates oxidation to
Co(III). Water-free cobalt(II) chloride is an equally efficient
catalyst (Table S4, entry 4). To our delight, the reaction is
scalable. A 5-fold increase of catalyst and reactants only led to a
slight decrease in yield (Table S4, entry 5). It has to be
emphasized that the amount of the TAAIL 1q was not
increased, rendering the reaction even more efficient.
Table 4. Substrate Scope with Styrene Derivatives and
Anilines
a
c
no.
R1
R2
R3
2 yield/%
3 yield/%
The fact that all reactants and products are volatile, while the
ionic liquid and the metal salt are not, enables the catalytic
system to be reused (Table 3). Distillation of the reaction
b
1
H
H
H
H
H
H
H
H
H
H
tBu
H
H
H
H
H
H
H
H
H
H
Me
H
2a, 47
2b, 67
2c, 76
2d, 41
2e, 58
2f, 46
2g, 47
2h, 54
2i, 60
2j, 60
2k, 52
3a, 22
3b, (8)
3c, (1)
3d, 12
3e, (6)
3f, (15)
3g, 47
3h, (7)
3i, (5)
3j, (10)
3k, 9
2
3
4
5
6
7
8
9
4-OMe
4-NO2
4-Me
2,4-Me2
4-Cl
a
Table 3. Reusability
2-F
2-Me
4-iPr
H
10
11
a
b
b
run
conversion /%
run
conversion /%
H
1
2
3
96
97
95
4
5
6
95
84
85
b
c
Isolated yields. On a 5 mmol scale, In parentheses: not isolated.
Yield determined by GC-MS.
a
b
a
Purification by vacuum distillation. Conversion of styrene,
Scheme 3. Reaction of Indene and Aniline
determined by GC.
mixture under high vacuum at 120 °C is possible due to the
high thermal stability of the TAAILs, as indicated by TG
measurements on closely related TAAILs.66 The system can be
reused up to 4 times without loss of activity. Then, the
achieved conversion drops by 10%.
a
Under the optimized reaction conditions, several aniline
derivatives were reacted with styrene derivatives (Table 4).
The highest yield of 76% was achieved for 4-nitroaniline
(Table 4, entry 3). In this case, nearly no hydroamination
product was observed. In general, the selectivity and reactivity
are determined by the electronic and steric properties of the
aniline. This reaction protocol also allows for the efficient and
scalable synthesis of new and promising agents for the
treatment of cancer. For example, 2i was obtained in 60%
yield in a single step. It has been shown to inhibit the
proliferation of several related cell lines.67 The reaction of N-
methyl aniline with styrene also preferably yields the
hydroarylation product 2j (Table 4, entry 10). Other styrene
derivatives, such as 4-tert-butylstyrene (Table 4, entry 11) and
indene (Scheme 3), were also tolerated.
The reaction was also performed using norbornene as a
substrate (Table 5). In general, the selectivity toward the
hydroamination product was higher. For example, the
selectivity with 4-nitroaniline as substrate in this reaction is
completely inverted, yielding nearly quantitative amounts of
the hydroamination product 3o (Table 5, entry 3). Other
terminal or unstrained cyclic aliphatic alkenes were unreactive
under these reaction conditions.
Isolated yields.
a
Table 5. Substrate Scope with Norbornene and Anilines
b
no.
R1
R2
2 yield/%
3 yield/%
b
1
H
H
H
H
H
2m, 34
2n, 33
2o, (0)
2p, 25
2q, (8)
3m, 54
3n, 18
3o, 97
3p, 74
3q, 60
2
3
4
5
4-OMe
4-NO2
2-Me
H
Me
a
b
Isolated yields. In parentheses: not isolated. Yield determined by
GC-MS.
have a strong impact on the catalyst efficiency. The aryl ortho-
substituted TAAILs outperformed the “standard” aryl alkyl
ionic liquids by far. The selectivity toward the hydroamination
and hydroarylation product is determined by the electronic
and steric properties of the different substrates, providing
access to pharmacologically relevant structures.
In conclusion, we have developed a versatile catalytic system
for the hydroarylation and hydroamination of aryl amines with
alkenes in TAAILs. An efficient scale-up procedure allows
access to sufficient quantities of the solvent. Among several
transition metal salts, the inexpensive cobalt(II) chloride
performed best. The choice of the ionic liquid was shown to
C
DOI: 10.1021/acs.orglett.8b02688
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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(22) Strassner, T.; Schulz, T.; Bernhard, G.; Raff, J.; Lehmann, F.
(Technische Universitaet Dresden, Germany; Helmholtz-Zentrum
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b02688.
■
S
Supplemental figures and tables, experimental details,
characterization data, and NMR spectra for all
synthesized compounds (PDF)
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: thomas.strassner@chemie.tu-dresden.de.
ORCID
Thomas Strassner: 0000-0002-7648-457X
Notes
The authors declare no competing financial interest.
(23) Zazybin, A.; Rafikova, K.; Yu, V.; Zolotareva, D.; Dembitsky, V.
M.; Sasaki, T. Metal-containing ionic liquids: current paradigm and
applications. Russ. Chem. Rev. 2017, 86, 1254−1270.
ACKNOWLEDGMENTS
This work was funded by the Deutsche Forschungsgemein-
schaft (priority program SPP 1708).
■
(24) Schulz, T.; Strassner, T. Biphasic platinum catalyzed hydro-
silylation of terminal alkenes in TAAILs. J. Organomet. Chem. 2013,
744, 113−118.
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DOI: 10.1021/acs.orglett.8b02688
Org. Lett. XXXX, XXX, XXX−XXX