Dibromoindium(iii) cations as a π-Lewis acid: Characterization of [IPr·InBr2][SbF6] and its catalytic activity towards alkynes and alkenes

Article abstract of DOI:10.1039/c5cc00740b

[IPr·InBr₂][SbF₆] (2) (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) has been synthesized and characterized in the solid state. This complex proved to be a very active catalyst for hydroarylations, transfer hydrogenations, and cycloisomerizations.

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Dibromoindium(III) Cation as -Lewis Acid:
Characterization of [IPr·InBr2][SbF6] and
Catalytic Activity Towards Alkynes and Alkenes
Cite this: DOI:
Bastien Michelet,a Jean-Rémy Colard-Itté,a Guillaume Thiery,a Régis Guillot,a
Christophe Bour*a and Vincent Gandon*a
10.1039/x0xx00000x
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
[IPr·InBr2][SbF6] (2) (IPr = 1,3-bis(2,6-diisopropylphenyl) Lewis acid properties due to its unsaturated character. We are not
imidazol-2-ylidene) has been synthetized and characterized in aware of any such compound characterized in the solid state.
the solid state. This complex proved to be a very active Secondly, the formation of a donor-acceptor adduct between a NHC
catalyst for hydroarylations, transfer hydrogenations, and and a main group Lewis acid can give rise to easy-to-handle species.
cycloisomerizations.
For instance, whereas GaCl3 is highly hygroscopic, [IPr·GaCl3] (IPr =
1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) is bench-stable.8 In
Indium(III) salts of type InX3 (X = Cl, Br, I, OTf, NTf2) are enjoying
increasing use in homogeneous molecular organic catalysis.1 In
particular, Corey et al. showed that InBr3 and InI3 have a strong affinity
for alkynes and trigger elegant cationic polycyclizations.2 In a recent
paper, this team reported that a catalytic mixture of InI3 and AgY (10
mol% each, Y = SbF6 or B[C6H3-3,5-(CF3)2]4) was an even more active
initiator for such cyclizations than InI3 or InBr3.3,4 These two
that respect, it would be particularly interesting to use [IPr·InBr3] (1)
instead of the hygroscopic InBr3 Lewis acid.9 This air-stable species is
one of the very few indium complex supported by NHC ligands.10 If the
bromide abstraction can be achieved and provide a well-defined
species, a unique opportunity to study the behavior of [L·InBr2]+
cations as
-Lewis acids would be at hand.
+
components are supposed to give rise to InX2 cations, yet no analysis
Addition of one equiv. of AgSbF6 to [IPr·InBr3] (
1
) in CD2Cl2 at room
could be performed on the weakly soluble material, and the putative
[InX2][Y] species could not be separated from AgX. The higher activity
temperature led to the immediate formation of a grey precipitate.
Energy dispersive spectroscopy was used to qualitatively analyze the
chemical composition of this precipitate which reveals mainly the
+
of InX2 has been explained by the participation of the two vacant p-
1
orbital of indium in the coordination of the orthogonal -orbitals of
presence of AgBr. All H signals of the carbene moiety were found
the alkyne cascade initiator in a crisscrossed geometry. On the other
shifted, notably the singlet corresponding to the olefinic backbone of
the imidazolium ring, which was found at 7.60 ppm after addition of
+
hand, we have shown that complexes of type L·GaCl2 , where L is a N-
the silver salt instead of 7.35 ppm in 1
. The 19F NMR spectra showed a
heterocyclic carbene (NHC), are more active than GaCl3 towards
alkynes, even though they display only one vacant orbital.5 We
broad signal at -123.6 ppm, which is typical of a fluorine-bridged
5f,11
reasoned that the synthesis and characterization of a well-defined
… …
[M
F
SbF5] fragment in which each fluorine is in rapid exchange.
+
indium complex of the (NHC)·InX2 series and the evaluation of its
-
Thus, this reaction is likely to provide the desired complex
2
of
acidity would shed light on that matter and advance the field for
formula [IPr·InBr2][SbF6], which could also be described as the
additional reasons: firstly, only a few indium(III) complexes of type (1
-
fluorine-bridged [IPr·InBr2(µ-F)SbF5].
+
L)n·InX2
displaying monocoordinating ligands have been
characterized in the solid state.6 Most compounds actually display
2-L bidentate ligands and are 8- or 12-electron species of type
+
+ 7
(
2-L)·InX2 or (2-L)2·InX2 . To the best of our knowledge, none of
them has found application in molecular catalysis. On the other hand,
+
a 6-electron complex of type (1-L)·InX2 should display interesting
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Table 1. In(III)-catalyzed dihydroarylation of arenyne 3 with anisole.
Entry
[In]
AgY
Solvent
4:5
Yield of
5 (%)b
-
(% conv)a
-
c
1
2
3
4
5
6
7
8
InCl3
InBr3
InBr3
1
1
2
1
1
1
1
-
-
DCE
DCE
DCE
DCE
DCE
DCE
32/13
18/81
-
20/80
29/71
12
65
Scheme 1. Reaction of [IPr·InBr3] with AgSbF6.
AgSbF6
-
AgSbF6
-
AgSbF6
AgSbF6
AgBF4
AgPF6
d
-
After filtration of the precipitate and slow evaporation, colorless
crystals suitable for X-ray diffraction were collected (82% isolated
yield). X-ray structure analysis confirmed the general structural
62
68
15e
Toluene 78/13
d
THF
DCE
DCE
-
-
-
-
-
-
assignment (Figure 1).12 In
significantly shorter than in
2
, the In1–Br bond distances are
d
9
d
1
(2.4379(5) Å (av) vs 2.496(2) Å (av)). The
10
a Determined by GC. b Isolated yields. c Trace. d No reaction. e 4 is isolated in
same is true for the In1–C1 bond (2.178(2) Å vs 2.204(8) Å (av)). The
In1–F1 distance (2.189(2) Å) is quite long for a standard In–F bond
(expected around 2.05 Å).13 As a result of the sharing of F1 between
In1 and Sb1, the Sb1–F1 bond is also longer than the other five Sb–F
75% yield.
The reaction was first carried out in 1,2-dichloroethane (DCE) at 80 °C.
With a catalytic amount of the poorly soluble InCl3, only traces of
products were monitored by gas chromatography (entry 1). With the
bonds (1.954(2) Å vs 1.856(3) Å (av)). Thus, complex
1 represents a
rare example of a perfluoroanion monocoordinated to a p-block
soluble InBr3, product
mixture and isolated in 12% yield only (entry 2). The joint use of InBr3
and AgSbF6 provided as the major product in 65% isolated yield
(entry 3).16 Complex
showed no catalytic activity (entry 4). On the
other hand, a catalytic mixture of and AgSbF6 gave similar results as
InBr3/AgSbF6 (entry 5). It is worthy of note that in the two-component
reactions, the indium complex (InBr3 or ) and AgSbF6 were premixed
5 was obtained as the minor component of the
metal,5f,14 and, as noted above, a rare example of a well-defined (1
L)·InX2+ complex.
-
5
1
1
1
at room temperature for 5 min to ensure the formation of the
corresponding cationic species (a precipitate forms instantly).
Moreover, AgSbF6 shows no activity in this reaction. That the cationic
species
evidenced by using it directly (entry 6). The products distribution and
the isolated yield of were found quite close to those of the two-
component experiment. In toluene, the /AgSbF6 system was much
2 is generated under these catalytic conditions was further
5
1
less effective (entry 7) and in THF, no reaction took place (entry 8).
Replacement of AgSbF6 by AgBF4 or AgPF6 was also detrimental to
the reactivity (entries 9 and 10).
Figure 1. Crystal structure of
2 (thermal ellipsoids at 30% probability level;
hydrogen atoms omitted for clarity). Selected bond lengths (Å) and angles (°):
In1-Br1 2.4333(5), In1-Br2 2.4425(5), In1-F1 2.189(2), In1-C1 2.178(2), Sb1-F1
1.954(2), Sb1-F2 1.851(2), Sb1-F3 1.867(2), Sb1-F4 1.842(4), Sb1-F5 1.855(2),
Sb1-F6 1.865(2); In1-F1-Sb1 137.1(1).
Complex
1 does not only represent a stable alternative to the
hygroscopic InBr3. In some cases, the [IPr·InBr2]+ cation proved more
+
active than InBr2 . When 1,2-dimethoxybenzene or 1-
(phenylsulfonyl)indole were used instead of anisole in the
The catalytic activity of
compared to that of simple indium salts. The reaction of arenyne
with anisole was used as a prototype of a cationic cascade since it
involves the activation of an alkyne to give and then the activation
of the alkene moiety of to give the tandem product
(Table 1).15
2 as a -Lewis acid was evaluated and
transformation of
isolated yields of
3
, full conversion was reached faster and the
3
6
and were better with the former (Table 2).
7
4
4
5
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Table 2. In(III)-catalyzed bimolecular dihydroarylation of arenyne 3 with 1,2-
dimethoxybenzene and 1-(phenylsulfonyl)indole.
Hydrogenative cyclization of
3
in the presence of 1,4-CHD was also
tested (Table 4). Again, the activity of the neutral indium species was
+
marginal (entries 1 and 2). With InBr2 , the transformation required 6 h
to reach completion at 80 °C and the isolated yield of 14 was 58%
(entry 3). On the other hand, the reaction was over in 2 h with
[IPr·InBr2]+ and the isolated yield was improved to 73% (entry 4).
Table 4. In(III)-catalyzed hydrogenative cyclizations of arenyne 3.
Entry [In]
Ar-H
t (h)
Yield
(%)a
1
2
3
4
InBr3
1
1,2-dimethoxybenzene
1,2-dimethoxybenzene
1-(phenylsulfonyl)indole
1-(phenylsulfonyl)indole
24
2
24
16
6 (45)
6 (82)
7 (69)
7 (83)
InBr3
1
a Isolated yields.
Entry
[In]
x
t (h)
Yield of
14 (%)a
We next examined whether catalytic transfer hydrogenation of
alkenes was possible (Table 3). The reduction of cyclohexenylbenzene
1
2
3
4
InBr3
1
InBr3
0
0
7
7
24
24
6
12
0
58
73
8
was attempted using 1,4-cyclohexadiene (1,4-CHD) as hydrogen
transfer agent.5e This reaction does not work with the neutral indium
1
2
a Isolated yields.
species InBr3 and
1 (entries 1 and 2), nor with AgSbF6 alone. Yet it
becomes very efficient with InBr2+ or [IPr·InBr2]+ (entries 3 and 4), both
displaying similar activities.
Lastly, the cycloisomerization of the enyne 15 was attempted
(Scheme 3).4c,17 When carried out with a catalytic mixture of [IPr·GaCl3]
and AgSbF6 or [IPr·InBr3] and AgSbF6, full conversion was reached but
the desired product 16 was obtained in low amount and admixed with
a complex mixture of unidentified products. Another cationizing
agent was then tested: silver polyfluoro-tert-butyloxyaluminate
[Ag][Al(pftb)4] (pftb = OC(CF3)3) which displays one of the most
weakly coordinating anions known.18 While the joint use of [IPr·GaCl3]
and [Ag][Al(pftb)4] still gave rise to decomposition, 16 could be
obtained very cleanly with [IPr·InBr3] and [Ag][Al(pftb)4].19 This result
shows that the NHC indium complex can surpass the gallium one and
reveals an interesting counterion effect in this chemistry.
Table 3. In(III)-catalyzed transfer hydrogenation of cyclohexenylbenzene 8.
Entry [In]
x
Conv of
9 (%)a
1
2
3
4
InBr3
1
InBr3
0
0
7
7
1
0
99
94
1
a Conversion determined by GC.
Two other alkenes were also reduced using the in situ generated
[IPr·InBr2]+ cation as catalyst and 1,4-CHD as hydrogen transfer agent
(Scheme 2). For the transformation of the acyclic substrate 10, a
temperature of 80 °C had to be applied. With the more reactive cyclic
alkene 12, the reaction took place at rt.
a
b
Scheme 3. In(III)-catalyzed cycloisomerization of enyne 15
determined by GLC analysis).
( Isolated yield;
In conclusion, this work demonstrates that the hygroscopic InBr3 salt
can be advantageously replaced by the bench-stable [IPr·InBr3]
complex to generate the catalytically active [IPr·InBr2]+ cation upon
treatment with AgSbF6. This species, which is a rare type of cationic
+
(
1-L)·InX2 complex, has been characterized in the solid state and
proved to be a powerful

-Lewis acid. In various cases, we could show
+
its superior activity compared to InBr2 , even with alkyne substrates,
even though it displays a single vacant site. We also found one type of
reaction for which [IPr·InBr2]+ proves more selective than the
previously described [IPr·GaCl2]+, provided it is generated from
[IPr·InBr3] and [Ag][Al(pftb)4] instead of AgSbF6. This interesting
counterion effect is being explored in our laboratory.
Scheme 2. In(III)-catalyzed transfer hydrogenation of alkenes 10 and 12
b
(a Isolated yield; determined by 1H NMR using p-anisaldehyde as
internal standard)
.
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This work was supported by UPS, CNRS, MESR, and IUF.
Notes and references
a Université Paris-Sud, ICMMO (UMR CNRS 8182), LabEx CHARM3AT,
91405 Orsay cedex, France. Fax: (+)33169154747. Tel:
(+)33169153931 E-mail: vincent.gandon@u-psud.fr. Homepage:
www.polycata.u-psud.fr.
†
Dedicated to Prof. A. M. Echavarren on the occasion of his 60th
birthday.
†† Electronic Supplementary Information (ESI) available: Synthetic
procedures and crystallographic details for
2
.
For ESI and
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,
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M. Webster and W. Zhang, Inorg. Chem., 2008, 47, 9691; (d) C.
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4 For other examples of the joint use of catalytic amounts of InX3 and a
silver salt in carbonyl-ene, Mukayama-aldol, and cycloisomerization
reactions, see respectively: (a) J.-F. Zhao, T.-B. W. Tjan and T.-P.
Loh, Tetrahedron Lett., 2010, 51, 5649; (b) J.-F. Zhao, B.-H. Tan and
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(a) H. Grützmacher and H. Pritzkow, Organometallics, 1991, 10, 938;
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15
Z.-X. Yu, J. Org. Chem., 2012, 79, 8527.
This reaction also works well with GaCl3 (see ref 5a) or
5 (a) H.-J. Li, R. Guillot and V.Gandon, J. Org. Chem., 2010, 75, 8435;
[IPr·GaCl2][SbF6] (see ref 5b). On the other hand, the aluminium
counterparts of these species are not active.
(b) S. Tang, J. Monot, A. El-Hellani, B. Michelet, R. Guillot, C. Bour
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Monot, R. Guillot, C. Bour and V. Gandon, Inorg. Chem., 2013, 52
,
slight excess of AgY compared to the indium complex is to ensure
complete cationization. The yields are usually slightly better with a
5/7 ratio than with a 5/5 one.
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19
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,
Neither [IPr·InBr3] nor [Ag][Al(pftb)4] alone are not active in this
reaction.
43.
6
+
We are aware of only two 12-electron InX2 ions: (a)
[InI2(Me2SO)4][InI4]: F. W. B. Einstein and D. G. Tuck, J. Chem.
Soc. D: Chem. Commun., 1970, 1182; (b) [InCl2(Ph3PO)4][InCl4]: W.
T. Robinson, C. J. Wilkins and Z. Zeying, J. Chem. Soc. Dalton
Trans. 1990, 219.
4 | J. Name., 2012, 00, 1-3
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