Cationic gallium(III) halide complexes: A new generation of π-Lewis acids

Article abstract of DOI:10.1002/chem.201201202

Ga-neration X: Well-defined neutral and cationic gallium(III) halide complexes have been synthesized and evaluated in catalysis. Both the (NHC)GaX₃/AgSbF₆ catalytic mixture and isolated (NHC)GaX₂⁺ species function as exquisite π-Lewis acids in typical GaX₃-catalyzed reactions (see scheme). The cationic complexes are more active than GaX₃ and yet more resistant to hydrolysis, which allows lower catalytic loadings and faster reactions. Copyright

Full text of DOI:10.1002/chem.201201202

COMMUNICATION
DOI: 10.1002/chem.201201202
Cationic GalliumACHTUNGTRENNUNG
Acids
Shun Tang, Julien Monot, Ahmad El-Hellani, Bastien Michelet, Rꢀgis Guillot,
Christophe Bour, and Vincent Gandon*[a]
Gallium
N
Among these, a few examples of N-heterocyclic carbene–
GaX3 adducts have been reported.[15] However, the stabiliza-
tion of GaX3 by a strongly coordinating ligand will certainly
be detrimental to its reactivity. To generate an active spe-
cies, one could think of opening a vacant site on gallium by
halide abstraction, that is, by using a silver salt [Eq. 2].
used in a variety of catalytic transformations. These salts are
strong oxo-, thio- or azaphilic s-Lewis acids, yet they are
also able to activate simple olefins and alkynes [Eq. 1].[1,2]
For instance, as s-Lewis acids, they catalyze nucleophilic ad-
ditions to epoxides,[3] 1,4-addition to a, b-unsaturated car-
bonyl compounds,[4] Nazarov reactions,[5] skeletal rearrange-
ment of aldehydes,[6] allylation of imines,[7] sulfidation of al-
kynes or alkenes,[8] as well as C C bond formation involving
[9]
À
À
C H, N H,
acids, GaX3 salts catalyze inter- and intramolecular nucleo-
philic additions to alkynes and alkenes.[12]
Yet such additives are also, to some extent, moisture- and
light-sensitive, and can interfere with the catalytic process.
The use of isolated complexes of type L1 L2GaX2+, in which
L1 is a strongly coordinating ligand and L2 is a weakly coor-
dinating ligand (ergo exchangeable by the substrate), could
prevent the systematic use of a silver salt. To the best of our
knowledge, there are no applications of such well-defined
complexes in catalysis. They would be particularly interest-
ing in the field of p-catalysis, in which expensive noble
metal complexes (Au, Pt, Ir, Rh, and so on) are mostly
used.[16,17]
Several (NHC)GaX3 adducts (1a–f, X=Cl, Br, I) were
prepared by the free carbene method[15b,c,18] and proved
indeed much easier to handle than GaX3 salts (Scheme 1).
In the absence of an additional ligand, we could not iso-
late the reaction product of (IPr)GaCl3 with AgSbF6. In-
In spite of their capabilities, galliumACTHNUTRGNEUNG(III) halides are not
widely used and keep giving rise to mistrust from the syn-
thetic chemistsꢀ community. The main reasons are: 1) the
difficulty in handling these species due to their highly hygro-
scopic nature, and 2) the presupposed ill-defined nature of
the intermediates, which makes the anticipation and the ra-
tionalization of the results quite difficult.[13] The issue of hy-
groscopy is indeed a critical one; even after taking precau-
tions, hydration products can be observed, sometimes in
large amounts.[12h] Ga(OH)Cl2, which would arise from ad-
ventitious hydrolysis, has even been proposed as the active
species of a coupling reaction catalyzed by GaCl3.[14] Thus,
high loadings are usually required, typically between 10 and
100 mol%, to ensure a sufficient concentration of intact cat-
alyst. In this context, the use of well-defined air- and mois-
ture-stable L·GaX3 adducts seems highly desirable to ensure
further development of GaIII-catalyzed transformations.
stead, we obtained a trace amount of the gallium
hydroxide 3, presumably resulting from the reaction of the
expected unsaturated cationic gallium(III) dichloride with
ACHTUNGTRENNUNG(III) di-
AHCTUNGTRENNUNG
[a] S. Tang, Dr. J. Monot, A. El-Hellani, B. Michelet, Dr. R. Guillot,
Dr. C. Bour, Prof. Dr. V. Gandon
ICMMO, UMR CNRS 8182
Universitꢁ Paris-Sud
91405 Orsay (France)
Fax : (+) +331694747
E-mail: vincent.gandon@u-psud.fr
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201201202.
Scheme 1. Neutral and cationic galliumACTHNUTRGNEUNG(III) tri- and dihalides.
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Table 1. Bimolecular reaction catalyzed by cationic gallium
generated in situ.
ACHTUNGTREN(NGNU III) dihalides
adventitious water. The unusual structure of 3 could be es-
tablished by single crystal X-ray structure analysis (see the
Supporting Information).[19]
The desired cationic galliumACTHNUTRGNEUGN(III) dichlorides 2a–d were
obtained by reproducing the above reaction in the presence
of electron-rich and -poor benzonitrile derivatives
(Scheme 1).[20,21,22] They were isolated in nearly quantitative
yields after simple filtration of AgCl as kinetically stable yet
quite moisture sensitive solids. Two of them, 2a and 2c,
were crystallographically characterized (see Figure 1 and
Entry
L
X
a
b
t
Yield
[%][a]
A
A
[h]
1
2
3
4
5
6
7
8
–
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Br
I
10
5
5
5
5
1
1
1
5
5
5
5
5
5
5
0
0
5
10
15
1
2
3
7
7
7
7
7
7
7
10
48
2
2
2
20
20
20
2
2
2
88[b]
IPr (1a)
IPr (1a)
IPr (1a)
IPr (1a)
IPr (1a)
IPr (1a)
IPr (1a)
IPr (1a)
IPr (1b)
IPr (1c)
SIPr (1d)
IMes (1e)
SIMes (1 f)
PPh3 (1g)
0[c]
92 (38)[d]
99 (68)[d]
99 (92)[d]
63
93
96
99
99
95
70
80
67
9
10
11
12
13
14
15
Cl
Cl
Cl
Cl
2
2
2
2
[e]
–
[a] Yield of the isolated product. [b] At 808C. [c] No reaction at 808C.
[d] Conversion after 0.5 h indicated in parentheses. [e] Traces.
Figure 1. ORTEP diagram of 2c with thermal ellipsoid at 50% probabili-
ty level (hydrogens and SbF6À omitted for clarity).
with entry 1 because a lower amount of gallium could be
used, full conversion was reached in only 2 h, and the tem-
perature could be lowered to 408C. Since none of the two
steps of the tandem reaction is promoted by AgSbF6
Supporting Information).[19] The main feature of these tetra-
À
hedral gallium species is the N Ga bond distance of 1.95 ꢃ
with 2,4,6-trimethoxybenzonitrile versus 1.99 ꢃ with 2,4,6-
triflurobenzonitrile, reflecting the weakest strength of inter-
action of the electron-poor derivative.
+
alone,[25] we suppose that (IPr)GaCl2 is the active species
of the catalytic cycle. An excess of silver with respect to gal-
lium increases the rate of the tandem process, probably by
increasing the concentration of the monocationic gallium
species rather than forming an elusive di- or tricationic galli-
um-centered complex (Table 1, entries 4 and 5).[26] This
effect is even more spectacular when using only 1 mol% of
(IPr)GaCl3 (Table 1, entries 6–8). A two- or three-fold
excess of AgSbF6 markedly increased the yields of the
tandem process (up to 96%), although the reaction time
was longer. We stress that it is not possible to carry out this
(or probably any other) reaction with GaCl3 in such a low
catalytic amount. In the rest of the screening, a 1.4-fold
excess of silver was used. For instance, with 5 mol% of
(IPr)GaCl3 and 7 mol% of AgSbF6, the product was isolated
nearly quantitatively after 2 h (Table 1, entry 9). Similar re-
sults were obtained with the bromide and iodide derivatives
(Table 1, entries 10 and 11). Other (NHC)GaCl3 adducts
could be used, however the yields were markedly lower
(Table 1, entries 12–14). As a matter of comparison, the tri-
phenylphosphine adduct was tested but this proved ineffi-
cient (Table 1, entry 15).[27] Thus, while the peculiar selectivi-
ty of GaCl3 is maintained, the (IPr)GaCl3/AgSbF6 catalytic
To test these neutral (1a–f) and cationic gallium com-
plexes (2a–d), we carried out a one-pot cycloisomerization/
Friedel–Crafts tandem that we recently reported.[12h] This re-
À
action involves consecutive triple C C bond activation (cy-
À
cloisomerization step) and a double C C bond activation
(Friedel–Crafts-type step). Although it stops at the cycliza-
tion step of the starting arenyne with neutral and cationic
complexes of AuI, PtIV, RuII, and InIII, it is catalyzed all the
way by GaCl3 or GaBr3, provided at least 10 mol% of these
salts and a thoroughly dried solvent are used to avoid exces-
sive catalyst decomposition and/or triple bond hydration of
the starting arenyne.[23] Typically, this transformation re-
quires 10 h in refluxing DCE to reach completion.
For instance, the reaction of arenyne 4 with anisole gives
rise to product 5 in 88% yield with GaCl3 as catalyst
(Table 1, entry 1).[12h] As expected, (IPr)GaCl3 was found to-
tally unreactive (Table 1, entry 2). On the other hand, the
adjunction of 5 mol% AgSbF6 to 5 mol% of (IPr)GaCl3 al-
lowed the formation of 5 in 92% yield (Table 1, entry 3).[24]
This result represents a significant improvement compared
+
mixture is more active and the putative (IPr)GaCl2 active
species is less fragile.
To avoid the systematic handling of the silver additive, we
next evaluated the isolated cationic species 2a–d (Table 2).
Complex 2a, which exhibits an electron-rich nitrile, did not
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Cationic GalliumACHTUNGTRENNUNG(III) Halide Complexes
COMMUNICATION
Table 2. Bimolecular reaction catalyzed by isolated gallium
lides.
(III) diha-
Table 3. Intramolecular reaction catalyzed by isolated or in situ-generat-
ed gallium
ACHTUNGTRENNUNG
Entry
Cat.
T
a
b
Yield
[%][a]
[8C]
[mol%]
ACHTUNGTRNE[NUNG mol%]
Entry
L or Cat.
T
a
b
Yield
[%][a]
1
2
3
4
5
6
7
8
2a
2a
2b
2b
2c
2d
2a
2b
2c
2d
40
40
40
40
40
40
80
80
80
80
5
5
5
5
5
5
5
5
5
5
0
5
0
5
0
0
0
0
0
0
0[b]
30
36
81
81
55
A
A
ACHTUNGTRNE[NUNG mol%]
1
2
3
4
5
6
7
8
IPr (1a)
2b
2c
2d
IPr (1a)
2b
40
40
40
40
80
80
80
80
5
5
5
5
5
5
5
5
7
0
0
0
7
0
0
0
91
57
81
55
92
54
93
52
[c]
–
79
82
72
2c
2d
9
10
[a] Yield of the isolated product.
[a] Yield of the isolated product. [b] No reaction. [c] Traces.
provide the tandem product, nor its carbocyclization precur-
sor (Table 2, entry 1). Product 5 could be obtained nonethe-
less in 30% yield when using AgSbF6 as additive (Table 2,
entry 2). Here, the main role of the silver salt is probably
not to generate a gallium-centered dication but to abstract
the nitrile itself. Pertaining to this hypothesis, we could ser-
endipitously collect single crystals of [(2,4,6-trimethoxyben-
zonitrile)2Ag]SbF6 and characterize them by using X-ray dif-
fraction.[28] With the benzonitrile derivative 2b, without
silver salt, we were pleased to find product 5 in the reaction
mixture and isolate it in an encouraging 36% yield (Table 2,
entry 3). Again, the yield could be greatly improved to 81%
with AgSbF6 (Table 2, entry 4). Of particular interest, the
same yield could be reached without the silver additive
when using complex 2c, which exhibits the more labile tri-
fluorobenzonitrile ligand (Table 2, entry 5). On the other
hand, the pentafluoronitrile derivative 2d gave rise to
a lower yield of 55% (Table 2, entry 6). Increasing the tem-
perature to 808C was beneficial with 2b and 2d (Table 2,
entries 8 and 10), but not with 2c, which remained the more
active catalyst (Table 2, entry 9). Apart from anisole, other
nucleophiles were tested and gave very good results (see the
Supporting Information).
An intramolecular reaction was next investigated
(Table 3). At 408C, the (IPr)GaCl3/AgSbF6 mixture gave the
best yield of the series (91%, Table 3, entry 1) and outper-
formed the cationic complexes 2b–d (Table 3, entries 2–4).
Again, among the latter, 2c proved to be the most effi-
cient (81%, Table 3, entry 3). Interestingly, at 808C, no
amelioration was observed (Table 3, entries 5, 6, and 8),
except for 2c, which performed similarly to the Ga/Ag mix-
ture (93% yield, Table 3, entry 7).
Lastly, we carried out the cyclization of enyne 8 in the
presence of anisole (Scheme 2).[29] By using either the Ga/
Ag mixture or the cationic catalyst 2c in DCE at 808C, we
could isolate the regioisomers 9a and 9b from many other
Scheme 2. GalliumACTHNUTRGNEUNG(III)-catalyzed 1,6-enyne cyclization.
unidentified by-products in less than 30% yield. Gratifying-
ly, the selectivity could be greatly improved in toluene.[30]
In conclusion, the combination of (NHC)GaX3 complexes
and AgSbF6 advantageously replaces hygroscopic GaX3
salts. While the peculiar selectivity of the salts is maintained,
the air-stable NHC adducts allow better yields at lower tem-
peratures and faster reaction times. Catalytic amounts as
low as 1 mol% can be used, which is not possible with
simple galliumACTHUNTGRNEUNG(III) halides. Besides, a “silver-free” proto-
col[31] based on the use of well-defined cationic gallium hal-
ides has been developed. The advantages of having one
metal instead of two in the reaction mixture have been
clearly recognized in the field of gold catalysis.[32] This paves
the way for further developments in GaIII catalysis in gener-
al, and in (asymmetric) p-acid catalysis in particular. The
synthesis of air- and moisture-stable, and yet active, cationic
gallium species is also a goal actively pursued in our labora-
tory.
Experimental Section
General procedure (Table 2): The arenyne 4 (0.25 mmol, 1 equiv) and
anisole (3 equiv) were mixed in a screw-cap vial under argon. 1,2-Di-
chloroethane (0.25m) was added and the mixture was stirred for 5 min.
The galliumACTHNUTRGNEUG(N III) catalyst (5 mol%) was added and the mixture was
stirred at 40 or 808C for 4 h. The mixture was quenched with Et3N (1
drop). The reaction mixture was diluted with diethyl ether (10 mL). The
solution was filtered on a short pad of silica, which was rinsed with dieth-
Chem. Eur. J. 2012, 00, 0 – 0
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V. Gandon et al.
yl ether (5 mL). After evaporation, the crude was purified by flash chro-
matography (92:8 cyclohexane/EtOAc).
120, 4338; Angew. Chem. Int. Ed. 2008, 47, 4268; e) A. Fꢄrstner,
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[17] GaCl3 is about 20 times cheaper than AuCl3.
[18] We attempted to use (IPr)AgCl as NHC transfer agent (H. M. J.
Wang, I. J. B. Lin, Organometallics 1998, 17, 972). However, its reac-
tion with GaCl3 led to (IPr)2AgGaCl4. The structure of this product
was ascertained by X-ray crystallography (see the Supporting Infor-
mation).
[19] CCDC-865775 ([(IPr)2Ag]GaCl4), CCDC-865776 (1d), CCDC-
865777 (1c), CCDC-865778 (1e), CCDC-865779 (2a), CCDC-865780
(2c), CCDC-865781 (3) contain the supplementary crystallographic
data for this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.cam.a-
c.uk/data_request/cif..
Acknowledgements
This work was supported by CNRS, MESR, and UPS (Chaire d’Excel-
lence). We thank S. E. Denmark and P. de Frꢁmont for useful discussions.
Keywords: cycloisomerization
·
gallium
·
heterocyclic
carbenes · homogeneous catalysis · Lewis acids
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Cole, A. I. McKay, Dalton Trans. 2012, 41, 946; d) M. L. Cole, S. K.
Furfari, M. Kloth, J. Organomet. Chem. 2009, 694, 2934.
[30] Under the same conditions, product 10 was isolated when using tert-
butyl-JohnphosAuCl (C. Nieto-Oberhuber, S. López, A. M. Echavar-
ren, J. Am. Chem. Soc. 2005, 127, 6178) with no incorporation of
anisole. For a similar result, see ref. [29b] and S. Porcel, A. M. Echa-
[16] For reviews, see: a) A. S. K. Hashmi, Chem. Rev. 2007, 107, 3180;
b) E. Jimꢁnez-NfflÇez, A. M. Echavarren, Chem. Rev. 2008, 108,
3326; c) D. J. Gorin, B. D. Sherry, F. D. Toste, Chem. Rev. 2008, 108,
3351; d) V. Michelet, P. Y. Toullec, J.-P. GenÞt, Angew. Chem. 2008,
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Cationic GalliumACHTUNGTRENNUNG(III) Halide Complexes
COMMUNICATION
varren, Angew. Chem. 2007, 119,
2726; Angew. Chem. Int. Ed.
2007, 46, 2672.
We cannot exclude contamination by silver, however, silver is not an
active catalyst here.
[32] See inter alia: S. Gaillard, J. Bosson, R. S. Ramón, P. Nun, A. M. Z.
Slawin, S. P. Nolan, Chem. Eur. J. 2010, 16, 13729, and the references
therein.
[31] Silver is needed once for the gen-
eration of the cationic species but
no longer in the catalytic tests.
Received: April 9, 2012
Published online: && &&, 2012
Chem. Eur. J. 2012, 00, 0 – 0
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V. Gandon et al.
Gallium Catalysis
S. Tang, J. Monot, A. El-Hellani,
B. Michelet, R. Guillot, C. Bour,
V. Gandon* ................... &&&& — &&&&
Cationic GalliumACHTUNGTRENNUNG(III) Halide
Complexes: a New Generation of p-
Lewis Acids
Ga-neration X: Well-defined neutral
and cationic gallium(III) halide com-
plexes have been synthesized and eval-
uated in catalysis. Both the
typical GaX3-catalyzed reactions (see
scheme). The cationic complexes are
more active than GaX3 and yet more
resistant to hydrolysis, which allows
lower catalytic loadings and faster
reactions.
AHCTUNGTRENNUNG
(NHC)GaX3/AgSbF6 catalytic mixture
+
and isolated (NHC)GaX2 species
function as exquisite p-Lewis acids in
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ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
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These are not the final page numbers!