Activation of Lewis acid catalysts in the presence of an organic salt containing a non-coordinating anion: Its origin and application potential

Article abstract of DOI:10.1039/b712060e

In the presence of a soluble organic salt containing non-coordinating anion (e.g., [bmim][SbF₆] or [NR₄][SbF₆]), the catalytic activity of Lewis acid (MX_n) was dramatically enhanced due to the anion exchange between the Lewis acid and organic salt. The Royal Society of Chemistry.

Full text of DOI:10.1039/b712060e

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COMMUNICATION
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Activation of Lewis acid catalysts in the presence of an organic salt
containing a non-coordinating anion: its origin and application
potential{
a
Jin Hong Kim, Ji Woong Lee, Ueon Sang Shin, Jin Yong Lee, Sang-gi Lee and Choong Eui Song*
a
a
a
b
a
Received (in Cambridge, UK) 6th August 2007, Accepted 17th August 2007
First published as an Advance Article on the web 29th August 2007
DOI: 10.1039/b712060e
In the presence of a soluble organic salt containing non-
coordinating anion (e.g., [bmim][SbF ] or [NR ][SbF ]), the
catalytic activity of Lewis acid (MX ) was dramatically
only rare earth metal triflates but other types of Lewis acid
catalysts can also be activated just by mixing them with an organic
salt containing ‘‘non-coordinating anions’’.
6
4
6
n
The Lewis acidity of a metal ion is expected to increase with
decreasing nucleophilicity of its counter anion, that is, metal salts
with less nucleophilic counter anions are more electrophilic. For
example, the stronger Lewis acidity of those Sc-complexes having
enhanced due to the anion exchange between the Lewis acid
and organic salt.
Lewis acids-catalyzed organic reactions play important roles in
1
modern organic synthesis. However, their relatively low turnover
4
2
6 5 4
very weakly coordinating anions, such as [B(C F ) ] rather than
2
OTf , has already been confirmed by their much higher activity in
5
many Sc-ion promoted electron transfer reactions. In addition, we
numbers often inhibit their application. Recently, several Lewis
acid-catalyzed organic reactions have been conducted in ionic
2
liquids (ILs) with great success. In most of the examples studied,
also observed the dramatic anion effect of ILs on the catalytic
switching from an organic solvent to an IL not only facilitated the
recycling of the catalyst but also led to positive changes in the
course of the reaction (e.g., significant rate acceleration).
3
activity in the Sc(OTf) -catalyzed Friedel–Crafts alkylation
3b
3h
reactions, and alkenylations. The catalytic activity generally
increased with decreasing nucleophilicity of the IL anion.
Considering all of these reported results, we postulated that the
more Lewis-acidic (i.e., more electrophilic) metal species 2 might be
generated in situ by anion exchange between the metal triflate and
2
ILs containing very weakly coordinating anions such as PF
6
or
2
SbF6 (Scheme 1).
To obtain evidence for the anion exchange between metal
triflates and ILs 1, we first carried out the reaction of Sc(OTf)
with 5 equivalents of Merrifield’s resin-bound imidazolium
3
In our earlier studies on catalysis in ILs, we also found that the
3
rates of the metal triflate-catalyzed C–C bond forming reactions
3
b
3h,i
e.g. Friedel–Crafts alkylation and alkenylation reactions of
(
6
3
c
hexafluoroantimonate
Scheme 2). After stirring for 4 h, the resin was filtered, washed
thoroughly with THF, MeOH, H O, and again with MeOH, and
3
2 2
in CH Cl at room temperature
aromatic compounds, and Diels–Alder reactions ) were drama-
tically accelerated up to several hundred-fold in the presence of an
IL containing very weakly coordinating anions such as
(
2
1
9
dried in vacuo. The solid-state F NMR spectra of the filtered
resin 4 clearly indicated the anion exchange between the OTf anion
of Sc(OTf) and SbF anion of resin 3. As shown in Fig. 1, the
[
bmim][SbF
6 6
] (1a) or [bmim][PF ] (1b) (bmim = 1-butyl-3-
3b,h,i
methylimidazolium). In some cases,
even the reactions that
3
6
were not possible to conduct in conventional organic solvents
proceeded smoothly in ILs. However, to date, the exact reasons
behind this observed positive ‘‘ionic liquid effect’’ have not yet been
elucidated. In this research, we performed for the first time detailed
studies to elucidate the origin of the ‘‘ionic liquid effect’’ and found
that an soluble organic salt having a ‘‘non-coordinating anion’’
7
filtered resin 4 contains the OTf anion as well as the SbF
6
anion.
The singlet peak at 282.5 ppm corresponds to the OTf anion
1
9
(
compare with the solid-state F NMR spectrum (Inset (a) of
8
Fig. 1) of resin-bound imidazolium triflate, which was prepared
independently). The formation of the superacidic species 2 was also
2
6
2
6
detected from the analysis of the filtrate. Due to the instability of
such as PF
or SbF
activates the metal triflate catalysts,
the superacidic species 2, the same exchange experiment was
9
-THF (1 : 1, v/v). After stirring for 4 h,
causing them to become more electrophilic by in-situ anion
exchange, which dramatically enhances their catalytic activity.
Moreover, on the basis of this result, we demonstrated that not
carried out in d -DMSO–d
6
8
the resin was filtered off and the filtrate was subjected directly to
1
9
19
F NMR, and FAB-MS analysis. The F NMR spectrum of the
2
filtrate clearly (Fig. 2) showed that it contains the SbF
6
moiety
a
Department of Chemistry, Institute of Basic Science, Sungkyunkwan
together with other unidentifiable species. The two coupling
University, Suwon 440-746, Korea. E-mail: s1673@skku.edu;
Fax: (+82) 31-290-7075; Tel: (+82) 31-290-5964
Division of Nano Sciences (BK21)/Department of Chemistry, Ewha
b
Womans University 11-1 Deahyun-dong, Seodaemun-gu, Seoul, 120-750,
Korea
{
Electronic supplementary information (ESI) available: Experimental
procedure and characterization data for all new compounds. See DOI:
0.1039/b712060e
Scheme 1 In-situ formation of more electrophilic metal species 2 from
metal triflates and 1a by anion exchange.
1
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Chem. Commun., 2007, 4683–4685 | 4683

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such as 2, which were formed from the anion exchange between
the metal triflate and ILs.
On the basis of our proposed anion-exchange mechanism for
the activation of the catalyst, not only rare earth metal triflates but
n
other types of Lewis acids, MX , would be activated merely by
mixing them with any soluble organic salts containing non-
coordinating anions. To demonstrate the validity of this concept,
we next carried out a series of C–C bond forming reactions such as
Diels–Alder reaction, Friedel–Crafts alkylation and Friedel–Crafts
Scheme 2 The anion exchange reaction between Sc(OTf)
3
and the
Merrifield’s resin-bound imidazolium hexafluoroantimonate 3.
alkenylation using various Lewis acid catalysts (Sc(OTf)
TMS-OTf) in the presence of a quaternary ammonium salt having
the SbF anion (e.g., [NBu ][SbF ]) instead of the corresponding
imidazolium salt 1a. As we hoped, in all cases, [NBu ][SbF
produced a similar rate acceleration to [bmim][SbF ] (1a). Thus,
3 3
, InCl or
6
4
6
4
6
]
6
the Diels–Alder reaction of naphthoquinone and cyclohexadiene
using 0.2 mol% of Sc-catalyst in CH Cl was completed within
2
2
3
0 min in the presence of 1 equiv. of [NBu
4 6
][SbF ], whereas the
same reaction without [NBu ][SbF ] or 1a is extremely sluggish
4
6
(
Scheme 3). Sc(III) triflate-catalyzed Friedel–Crafts alkylation of
benzene with alkenes also proceeded readily in the presence of
NBu ][SbF ], whereas the same reactions did not occur in
dichloroethane. Thus, a quantitative yield of cyclohexylbenzene
was obtained from benzene and cyclohexene using Sc(OTf) and
NBu ][SbF ] after 24 h (Scheme 4). In the Friedel–Crafts
alkenylation of arenes with alkynes, the addition of [NBu ][SbF ]
[
4
6
3
[
4
6
1
9
Fig. 1 Solid-state F NMR spectra of polymer 4. (Inset (a) is the solid-
4
6
1
9
state F NMR spectrum of the resin-bound imidazolium triflate, while
also activated the catalysts markedly (Table 1). In the absence of a
19
inset (b) is the solid-state F NMR spectrum of resin 3.)
salt with a non-coordinating anion, Sc(OTf) was totally inactive
3
for the alkenylation of p-xylene with an electron-deficient alkyne
1
0
such as p-chlorophenylacetylene (entry 1). In contrast, in the
presence of 5 equiv. of 1a or [NBu ][SbF ], the reaction was
4
6
completed within 13 h and afforded the desired product in good
yield (entries 2 and 3). More strikingly, metal chlorides such as
InCl
entry 9), were also activated in the presence of the ammonium
salt. For example, InCl (entry 4) and TMS-OTf (entry 7) showed
very low activity for the alkenylation of p-xylene with 1-phenyl-1-
propyne, whereas in the presence of 1a or [NBu ][SbF ], the same
3
(entry 6) or even non-metallic triflates such as TMS-OTf
(
3
4
6
1
1
reactions were completed within 1–3 h (entries 5, 6, 8 and 9). We
also carried out the same reaction using several quaternary
2
2
6 4
ammonium salts bearing different anions such as PF , BF and
2
OTf to clarify ‘‘the anion effect’’ on the activation of Lewis acid
catalyst (in this case, TMS-OTf). As we expected, the Lewis acidity
of the TMS-cation increased with decreasing nucleophilicity of the
1
9
Fig. 2
6
F NMR spectrum of the filtrate in d -DMSO. (Inset (a) is the
19
spectrum of 1a); (,) the coupling between F and the NMR-active Sb-
19
nucleus (I = 5/2); (.) the coupling between F and the NMR-active Sb-
nucleus (I = 7/2).)
2
2
2
counter anion of salts in the order of SbF
6
. PF
6
. BF
4
.
2
OTf (entries 9–12 in Table 1). According to the ab initio
calculations, just as expected, the atomic charge at Si of
1
9
patterns between F and two NMR-active Sb-nuclides (I = 5/2
19
and 7/2) that were observed in the F NMR spectrum of the
filtrate are comparable with those in the spectrum of 1a (inset (a)
of Fig. 2).
In addition, FAB-MS analysis provided more direct evidence
for the existence of superacidic species 2 in the filtrate. In the FAB-
6 3
MS spectrum, the molecular ion peak for Sc(SbF ) (m/z = 751.6,
7
53.6) was clearly detected, along with peaks for Sc(OTf) (SbF )
2
6
2 6
and Sc(OTf) (SbF )?3DMSO (see ESI{). Based on these results, it
can be concluded that the rate accelerations observed in metallic
Lewis acid-catalyzed reactions in ILs containing very weakly
2
2
or SbF
6
coordinating anions such as PF
6
may originate from
Scheme 3 Sc(OTf) -catalyzed Diels–Alder reaction of naphthoquinone
3
the in situ generation of more highly Lewis-acidic metal species
with cyclohexadiene (for experimental details, see ESI{).
4
684 | Chem. Commun., 2007, 4683–4685
This journal is ß The Royal Society of Chemistry 2007

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This work was supported by a Korea Research Foundation
Grant (KRF-2005-005-J11901 for C. E. Song) funded by
MOEHRD, and by grants R01-2006-000-10426-0 (KOSEF for
C. E. Song and S.-g. Lee), R11-2005-008-00000-0 (SRC program
of MOST/KOSEF for C. E. Song).
Notes and references
1
(a) Lewis Acids in Organic Synthesis, ed. H. Yamamoto, Wiley,
New York, 2000, vol. 1 and 2; (b) S. Kobayashi, Chem. Rev., 2002, 102,
3
Scheme 4 Sc(OTf) -catalysed Friedel–Crafts alkylation of benzene and
2227.
cyclohexane (for experimental details, see ESI{).
2
For recent reviews, see: (a) C. E. Song, M. Y. Yoon and D. S. Choi,
Bull. Korean Chem. Soc., 2005, 26, 1321; (b) S. Luo, L. Zhu,
A. Talukdar, G. Zhang, X. Mi, J.-P. Cheng and P. G. Wang, Mini-
Rev. Org. Chem., 2005, 2, 177; (c) K. Binnemans, Chem. Rev., 2007, 107,
Table 1 Lewis acid-catalyzed Friedel–Crafts alkenylation of p-xylene
with alkyne
a
2
592, and references therein.
Our recent examples: (a) C. E. Song and E. J. Roh, Chem. Commun.,
000, 837; (b) C. E. Song, W. H. Shim, E. J. Roh and J. H. Choi, Chem.
3
2
Commun., 2000, 1695; (c) C. E. Song, W. H. Shim, E. J. Roh, S.-g. Lee
and J. H. Choi, Chem. Commun., 2001, 1122; (d) D. W. Kim, C. E. Song
and D. Y. Chi, J. Am. Chem. Soc., 2002, 124, 10278; (e) D. W. Kim,
C. E. Song and D. Y. Chi, J. Org. Chem., 2003, 68, 4281; (f) E. J. Kim,
S. Y. Ko and C. E. Song, Helv. Chim. Acta, 2003, 86, 894; (g) S.-g. Lee,
Y. J. Zhang, P. Z. Yu, H. Yoon, C. E. Song, J. H. Choi and J. Hong,
Chem. Commun., 2003, 2624; (h) C. E. Song, D. Jung, S. Y. Chung,
E. J. Roh and S.-g. Lee, Angew. Chem., Int. Ed., 2004, 43, 6183; (i)
M. Y. Yoon, J. H. Kim, D. S. Choi, U. S. Shin, J. Y. Lee and C. E. Song,
Adv. Synth. Catal., 2007, 349, 1725; (j) D.-S. Choi, D. H. Kim, U. S. Shin,
R. R. Deshmukh, S.-g. Lee and C. E. Song, Chem. Commun., 2007,
3467; (k) D.-S. Choi, J. H. Kim, U. S. Shin, R. R. Deshmukh and
C. E. Song, Chem. Commun., 2007, 3482.
Weakly coordinating anions: (a) I. Krossing and I. Raabe, Angew.
Chem., Int. Ed., 2004, 43, 2066; (b) C. A. Reed, Acc. Chem. Res., 1998,
31, 133; (c) S. H. Strauss, Chem. Rev., 1993, 93, 927; (d) W. Beck and
K. Suenkel, Chem. Rev., 1988, 88, 1405.
(a) S. Fukuzumi, J. Yuasa and T. Suenobu, J. Am. Chem. Soc., 2002,
124, 12566; (b) I. Kazuaki, K. Manabu and Y. Hisashi, Synlett, 1996, 3,
265; (c) J. Yuasa, T. Suenobu, K. Ohkubo and S. Fukuzumi, Chem.
Commun., 2003, 1070.
Entry
R
1
R
2
Catalyst
Additive (5 equiv.) t/h Yield (%)
c
1
2
3
p-ClPh
p-ClPh
p-ClPh
Ph
Ph
Ph
Ph
Ph
Ph
Ph
H
H
H
Sc(OTf)
Sc(OTf)
Sc(OTf)
3
None
1a
NBu
48
0
c
3
3
12 63
13 61
4
SbF
SbF
SbF
6
6
6
b
4
Me InCl
Me InCl
Me InCl
Me TMS-OTf None
Me TMS-OTf 1a
Me TMS-OTf NBu
Me TMS-OTf NBu
Me TMS-OTf NBu
Me TMS-OTf NBu
b
3
3
3
None
1a
NBu
72
3
3
2
1
7
94
88
9
93
95
89
46
5
b
5
b
6
4
7
8
9
4
5
6
4
4
4
4
2
1
1
1
a
0
1
2
PF
BF
OTf
6
2
2
2
Ph
Ph
4
3
For experimental details, see ESI. 20 mol% of InCl was used.
See ref. 3h.
c
For the preparation of Merrifield’s resin-bound imidazolium hexafluor-
oantimonate 3, see ESI{.
7 The solid F NMR of 3 (Inset (b) of Fig. 1) and 4 did not show
[
TMS][SbF ] was found to be much more positive than that of
6
19
19
TMS-OTf and eventually close to the charge of naked TMS-
a fine coupling pattern between F and two NMR-active Sb-nuclides
I = 5/2 and 7/2) which can be attributed to the inhomogeneous
chemical environment of the SbF moiety in the polymer matrix).
12
(
cation. These experimetal and theoretical results strongly indicate
that the anionic nature of organic salts plays a crucial role for the
activation of Lewis acids. Thus, the acidity of an metallic or non-
metallic Lewis acid could be very simply tuned by proper selection
of counter anions of salts.
6
8
For the preparation of Merrifield’s resin-bound imidazolium triflate, see
ESI{.
9 Since species 2 was shown to be unstable in the absence of electron
donor ligands, we carried out the exchange experiment in the presence of
DMSO.
In summary, the origin of the acceleration effect of ILs in Lewis
acid-catalyzed reactions was clearly elucidated for the first time.
An imidazolium IL having very weakly coordinating anions such
10 T. Tsuchimoto, T. Maeda, E. Shirakawa and Y. Kawakami, Chem.
Commun., 2000, 1573.
11 The alkenylation of p-xylene with 1-phenyl-1-propyne using TMS-OTf
as a catalyst was also carried out with a reduced amount (1 equiv.) of
2
2
as PF6 or SbF6 activates metal triflate catalysts so that they
become more electrophilic (i.e., more Lewis acidic) by anion
exchange, which dramatically enhances the catalytic activity. As
[
NBu
4 6
][SbF ]. However, the reaction proceeded more slowly than when
5
obtained after the same reaction time (2 h).
equiv. of the same additive was used and thus only 45% yield was
19
F
+
12 The calculated atomic charges at Si of TMS from TMS-OTf and TMS-
evidence for the anion exchange, solid- and solution-state
a
NMR and FAB-MS analyses were conducted. Moreover, it was
also demonstrated that several types of Lewis acid can be
activated, according to the catalyst activation mechanism, merely
by mixing them with a soluble organic salt containing non-
coordinating anions. Furthermore, the acidity of Lewis acids can
be very simply tuned by proper selection of counter anions of
soluble organic salts. We believe that the present study will open
up new perspectives for Lewis acid chemistry.
SbF
6
in gas/dichloromethane/DMSO are as following (see ESI{ for full
data):
This journal is ß The Royal Society of Chemistry 2007
Chem. Commun., 2007, 4683–4685 | 4685