Catalytic Use of Low-Valent Cationic Gallium(I) Complexes as π-Acids

Article abstract of DOI:10.1002/adsc.201701081

Transformations of alkene and alkyne substrates relevant to π-Lewis acid catalysis have been performed using low-valent Ga(I) species for the first time. [Ga(I)(PhF)₂]⁺[Al(OR^F)₄]^? and gallium dichloride (i. e. [Ga(I)]⁺[GaCl₄]^?) proved to be efficient catalysts for cycloisomerizations, Friedel-Crafts reactions, transfer hydrogenations, and reductive hydroarylations. Their activity is compared to more common Ga(III) complexes. This study shows that even the readily available and yet overlooked gallium dichloride salt can be a more active π-Lewis acid catalyst than gallium trichloride or other Ga(III) species. (Figure presented.).

Full text of DOI:10.1002/adsc.201701081

Title: Catalytic Use of Low-Valent Cationic Gallium(I) Complexes as π-
Acids
Authors: Zhilong Li, Guillaume Thiery, Martin Lichtenthaler, Regis
Guillot, Ingo Krossing, Vincent Gandon, and Christophe Bour
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10.1002/adsc.201701081
Advanced Synthesis & Catalysis
UPDATE
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Catalytic Use of Low-Valent Cationic Gallium(I) Complexes as
π-Acids
Zhilong Li,a Guillaume Thiery,a Martin R. Lichtenthaler,b,c Régis Guillot,a Ingo
Krossing,c Vincent Gandon,a,* and Christophe Boura,*
a
Institut de Chimie Moléculaire et des Matériaux d’Orsay, CNRS UMR 8182, Université Paris-Sud, Université Paris-
Saclay, Bâtiment 420, 91405 Orsay cedex, France
Phone: (+33)169157651
E-mail: christophe.bour@u-psud.fr, vincent.gandon@u-psud.fr
Department of Chemistry, University of California, Berkeley, USA
Institut für Anorganische und Analytische Chemie and Freiburger Materialforschungszentrum (FMF), Albert-Ludwigs-
b
c
Universität Freiburg, Albertstr. 21, 79104 Freiburg, Germany
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
(Scheme 1).[6,7] Nevertheless, these complexes remain
Abstract. Transformations of alkene and alkyne substrates
relevant to -Lewis acid catalysis have been performed able to cleave some Csp3-O bonds (e.g. deprotection
using low-valent Ga(I) species for the first time.
[Ga(I)(PhF)2]+[Al(ORF)4]− and gallium dichloride (i.e.
[Ga(I)]+[GaCl4]−) proved to be efficient catalysts for
cycloisomerizations, Friedel-Crafts reactions, transfer
hydrogenations, and reductive hydroarylations. Their
activity is compared to more common Ga(III) complexes.
This study shows that even the readily available and yet
overlooked gallium dichloride salt can be a more active -
Lewis acid catalyst than gallium trichloride or other Ga(III)
species.
of Ar-OMe or acetal groups) and are incompatible
with nitro groups.
Keywords: Alkenes; Alkynes; Gallium; Homogeneous
catalysis; Low-valent species; -Lewis acids.
Scheme 1. Ga(III) and In(III) complexes used as -Lewis
acids.
Another way to modulate the activity would be to
play on the oxidation state of the metal. This
approach is well-established in gold catalysis, where
Au(I) and Au(III) species can lead to different
reaction outcomes.[8] In contrast with Ga(III)- and
In(III)-based catalysts, Ga(I) complexes have
remained little exploited in homogeneous molecular
catalysis. Unlike low-valent In(I) salts used in a wide
range of organic transformations as catalysts (e.g.
Salts of Group 13 metals such as GaX3 and InX3 are
increasingly used in homogeneous catalysis to
promote the formation of C-C, C-X, and C-H
bonds.[1] This renewed interest can be notably
explained by the recent discovery of their ability to
catalyze nucleophilic additions across C-C -
bonds.[2,3] This reactivity of -Lewis acids was
traditionally believed to be the prerogative of late
transition metals such as Ru, Rh, Ir, Pt or Au, but it is
now established that alkynes, alkenes and allenes can
be also activated by soft main group metals.[4] In spite
of encouraging results obtained in this area with
gallium and indium complexes, functional group
tolerance remains an issue, so it is still necessary to
develop and screen new catalysts,[5] or to test existing
species that have not been considered as -Lewis
acids. In that respect, we have recently demonstrated
InX-catalyzed
allylations,[9]
allenylation,[9]
propargylations,[9] Reformatsky reactions,[10] 1,2-
additions[11] or radical cyclizations[12]), the use of
Ga(I) has been mainly limited to stoichiometric
reactions,[13] with a notable recent exception for the
formation of C-C bonds from boron-based
pronucleophiles and various electrophiles.[14] The in
situ generated [18]-crown-6–Ga(I)·(dioxane)nOTf
complex was used for its ambiphilic nature, since
Ga(I) is both a Lewis acid[15] (by its three vacant p
orbitals) and a Lewis base[16] (by its lone pair).
Nevertheless, in this study and others related to
+
+
that cationic species of type LGaX2 or LInX2 ,
where L is a N-heterocyclic carbene (NHC) such as
IPr, have a higher stability and are more active and
selective than the corresponding GaX3 and InX3 salts
1
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10.1002/adsc.201701081
Advanced Synthesis & Catalysis
[Al(ORF)4]− is an even weaker coordinating anion
than [SbF6]−,[19] [IPrGaCl2]+[Al(ORF)4]− actually
seemed less reactive than [IPrGaCl2]+[SbF6]− as it
could not promote the addition of anisole in both
DCE or toluene (entries 5 and 6). This could be due
to a faster hydrolysis of the former by adventitious
water under such reaction conditions, as a small
amount of the cationic gallium dihydroxide
[{IPrGaCl(µ-OH)2}2·H2O]2+{[Al(ORF)4]−}2 could be
isolated and characterized by X-ray diffraction.[20]
The corresponding silver salts Ag+[SbF6]− and
[Ag(DCM)3]+[Al(ORF)4]− (DCM = dichloromethane)
molecular catalysis, it is mostly the -Lewis acidity
that has been exploited, i.e. the catalytic activation of
C-X bonds (X = N, O, Cl, Br). A Clue that Ga(I)
species could be employed as -Lewis acid for
applications in molecular catalysis actually comes
from the activation of allylboron substrates by
vertical neighboring In(I) species.[9b,17] Another clue
comes from polymer chemistry, since low-valent
univalent
cationic
complexes
of
type
[Ga(I)(PhF)2]+[Al(ORF)4]− (RF= C(CF3)3) can activate
isobutylene towards the formation of highly reactive
polyisobutylene (HR-PIB, Scheme 2).[18] Herein, we
report the first use of Ga(I) complexes as -acid
catalysts for molecular applications.
[9a]
proved inactive (entries 7 and 8). With the Ga(I)-
containing salt “GaI”,[21] 2a was obtained selectively
[22]
(entries 9 and 10). In toluene, Ga2Cl4
led to 2a
(entry 11), but 3a was obtained as the major product
in DCE (entry 12). This is in sharp contrast with
GaCl3, with which 3a was a minor product (entry
13).[23] On the other hand, the use of GaCl3 and
[Ag(DCM)3]+[Al(ORF)4]− to form the putative highly
electrophilic [GaCl2]+[Al(ORF)4]− species, provided
3a in both DCE and toluene (entry 14-15).
Table 1. Reaction optimization for Ga-catalyzed
dihydroarylation of arenyne 1a with anisole.
Scheme 2. Activation of C-C  bonds by Ga(I) complexes.
En Cat.
try
Solvent Yield of Yield of
We started our investigation with a benchmark
reaction that we have used previously to evaluate the
ability of Ga(III) Lewis acids to activate alkynes and
alkenes using a single substrate (Table 1).[2a,2h] The
cycloisomerization of arenyne 1a can lead to the
dihydronaphthalene derivative 2a which, in the
presence of a nucleophile such as anisole, can give
2a [%]b
3a [%]b
+
1
2
3
4
5
6
7
8
9
[IPrGaCl2] [SbF6] −
DCE
92
95
5
c
c
-
+
[IPrGaCl2] [SbF6] −
-
toluene
DCE
[Ga(PhF)2]+[Al(ORF)4]−
[Ga(PhF)2]+[Al(ORF)4]−
91
0
toluene
DCE
81
0
+
[IPrGaCl2] [Al(ORF)4]−
90
89
NR
NR
99
98
79
15
77
-
c
c
+
[IPrGaCl2] [Al(ORF)4]−
toluene
DCE
DCE
0
-
-
rise to the tetrahydronaphthalene 3a.[6] In a preceding
Ag+[SbF6]−
Textmarke nicht definiert.a]
study,[Fehler!
we found that
the selective
[Ag(DCM)3]+[Al(ORF)4]−
GaId
[IPrGaCl2]+[SbF6]−
catalyzes
DCE
-
-
transformation of 1a into 3a in 1,2-dichloroethane
(DCE) (entry 1) or toluene (entry 2) at 80 °C. In DCE
at 80 °C, we were pleased to see that
[Ga(PhF)2]+[Al(ORF)4]− could also be used as catalyst
to give 2a in 91% yield (entry 3). However, in
contrast with [IPrGaCl2]+[SbF6]−, the formation of 3a
was very slow. Gratifyingly, 3a could be obtained
selectively in toluene in 81% yield (entry 4).
10 GaId
toluene
toluene
DCE
DCE
DCE
11 Ga2Cl4
12 Ga2Cl4
13 GaCl3
14 [GaCl2] [Al(ORF)4]−
15 [GaCl2] [Al(ORF)4]−
11
77
14
56
80
+
e
e
+
toluene
-
a)
Reaction conditions: 1,6-arenyne 1a and anisole
(3 equiv) in the indicated solvent (0.2 M) in the presence
of catalyst at the indicated temperature for 24 h. b) Isolated
Importantly,
NMR
studies
revealed
that
c)
[Ga(PhF)2]+[Al(ORF)4]− was not subject to
disproportionation under the reaction conditions (see
the Supporting Information). Since the Ga(III) and
the Ga(I) complexes have different counterions,
[IPrGaCl2]+[Al(ORF)4]− was also tested. Even though
yields.
Generated in situ by using IPrGaCl3 and
Ag+[SbF6]− or [Ag(DCM)3]+[Al(ORF)4]−.
20 mol%.
d)
e)
Generated
in
situ
by
using
GaCl3
and
[Ag(DCM)3]+[Al(ORF)4]−
2
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10.1002/adsc.201701081
Advanced Synthesis & Catalysis
Given the good activity of [Ga(PhF)2]+[Al(ORF)4]− in
toluene and Ga2Cl4 in DCE for the dihydroarylation
of 1a, various 1,6-arenynes and aromatic
nucleophiles were then tested (Scheme 3). The
corresponding dihydroarylation products were
isolated in good to excellent yields. The results
obtained with Ga2Cl4 are markedly better for 3c-e.
With these optimal reaction conditions, we explored
the scope of the Ga(I)-catalyzed transfer
hydrogenation (Scheme 4). A range of di- or
trisubstituted acyclic or cyclic alkenes were
hydrogenated in good to high yields. With a few
exceptions
(5c,
5g,
5l,
5n),
[Ga(PhF)2]+[Al(ORF)4]−proved more efficient than
Ga2Cl4. Remarkably, methoxy, and nitro groups were
not affected by these conditions (5j-5m). Again, these
Ga(I) species proved superior to GaCl3. For instance,
whereas the nitro derivative 5m was obtained in 94%
and 78% yield with [Ga(PhF)2]+[Al(ORF)4]− and
Ga2Cl4 respectively, GaCl3 furnished it in 45% yield
only.
Scheme 3. Bimolecular dihydroarylation of 1,6-arenynes
with
aromatics.
Conditions
A:
[Ga]
=
[Ga(PhF)2]+[Al(ORF)4]− in toluene; Conditions B: [Ga] =
Ga2Cl4 in DCE.
The use of [Ga(PhF)2]+[Al(ORF)4]− and Ga2Cl4 as
catalysts was then extended to transfer hydrogenation
of alkenes,[24] using 1,4-cyclohexadiene (1,4-CHD) as
dihydrogen source.[25] We started this study with the
reduction of 1,2-diphenylpropene 4a (Table 2). At
80 °C in DCE, the reduction was efficient with
[Ga(PhF)2]+[Al(ORF)4]− (entry 1), but sluggish with
Ga2Cl4 (entry 2). With the latter, the yield increased
significantly in toluene (entry 3). Raising the
temperature to 110 °C improved the yield to 94%
with Ga2Cl4 (entry 4) and to 73% with
[Ga(PhF)2]+[Al(ORF)4]− (entry 5).
Table 2. Reaction optimization for Ga(I)-catalyzed transfer
hydrogenation of (E)-1,2-diphenylpropene 4a.
Scheme 4. Scope of the Ga(I)-catalyzed transfer
hydrogenation of alkenes. Conditions A: [Ga]
=
[Ga(PhF)2]+[Al(ORF)4]−; Conditions B: [Ga] = Ga2Cl4.
PMP = para-methoxyphenyl; PNP = para-nitrophenyl.
Entry [Ga]
Solvent
T
Yield of
We next investigated the potential of the Ga(I)
complexes in hydrogenative cyclizations (Scheme
[°C] 5a [%]b
1
2
3
4
[Ga(PhF)2]+[Al(ORF)4]− DCE
80
80
80
110 94
110 73
61
37
61
Textmarke
nicht
definiert.a]
5).[Fehler!
Again,
Ga2Cl4
Ga2Cl4
Ga2Cl4
DCE
toluene
toluene
[Ga(PhF)2]+[Al(ORF)4]− and Ga2Cl4 proved to be
catalytically active. The cyclization/reduction
products were isolated in good to high yields. Again,
[Ga(PhF)2]+[Al(ORF)4]− led to better yields than
Ga2Cl4.
5
a)
[Ga(PhF)2]+[Al(ORF)4]− toluene
Reactions conditions: 4a (1 equiv) and 1,4-CHD
(1.2 equiv) in the indicated solvent (0.2 M) in the presence
of the catalyst (5 mol%) for 24 h. b) Isolated yields.
3
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10.1002/adsc.201701081
Advanced Synthesis & Catalysis
Experimental Section
General procedure for Ga(I)-catalyzed transfer
hydrogenation (Scheme 4). Under argon, the alkene
(0.25 mmol, 1 equiv) was introduced into a glass tube
containing a magnetic stirrer. The catalyst (5 mol%,
0.0125 mmol) and then dry degassed solvent
(0.25 mol.L-1) were added into the reaction vessel.
The tube was capped with a rubber septum. 1,4-CHD
(0.3 mmol, 30 L, 1.2 equiv) was added through the
septum with a Hamilton® syringe. The mixture was
then heated at 110 °C and the reaction progress was
monitored by GC. After completion, the reaction
mixture was filtered over a pad of celite, which was
rinsed with DCM or Et2O. Volatiles were removed by
rotary evaporation under vacuum. The crude was
purified by flash column chromatography on silica
gel (eluent: cyclohexane/ethyl acetate).
Scheme 5. Ga(I)-catalyzed hydrogenative cyclizations of
arenyne. Conditions A: [Ga] = [Ga(PhF)2]+[Al(ORF)4]−;
Conditions B: [Ga] = Ga2Cl4. a) 6 equiv of 1,4-CHD; dl :
meso = 1/1 .
Acknowledgements
We thank the China Scholarship Council (CSC), the Deutsche
Forschungsgemeinschaft (DFG; GEPRIS-No.: 288099196),
CNRS, ministère de l’Enseignement Supérieur et de la Recherche,
ENS Paris-Saclay, Université Paris-Sud and Institut
Universitaire de France (IUF) for the support of this work
Lastly, we briefly investigated the cycloisomerization
of enyne 7 (Table 3).[Fehler! Textmarke nicht definiert.e,Fehler!
Textmarke nicht definiert.,26]
This reaction is highly
challenging since Ga(III) species tend to encourage
the polymerization of the substrate. With GaCl3 and
[IPrGaCl2]+[Al(ORF)4]−, 8 was obtained in a low 17%
and 32% yield respectively (entries 1 and 2, full
conversion but high level of polymerization).[27] With
[Ga(PhF)2]+[Al(ORF)4]−, the yield increased to 45%
(entry 3), and reached 60% with Ga2Cl4 (entry 4).
References
[1] (a) M. Yamaguchi, S. Matsunaga, M. Shibasaki, B.
Michelet, C. Bour, V. Gandon, e-EROS, 2014, 1. (b) M.
K. Gupta, T. P. O’Sullivan, RSC Adv. 2013, 3, 25498.
(c) M. S. Singh, K. Raghuvanshi, Tetrahedron 2012, 68,
8683. (d) M. Yamaguchi, Y. Nishimura, Chem.
Commun. 2008, 35. (e) R. Amemiya, M. Yamaguchi in:
Acid Catalysis in Modern Organic Synthesis (H.
Yamamoto, K. Ishihara, Eds.), Wiley-VCH, Weinheim,
2008, pp. 347-375. (f) Lewis Acids in Organic
Synthesis (H. Yamamoto, Ed.), Wiley-VCH, Weinheim,
2008. (g) J. Augé, N. Lubin-Germain, J. Uziel,
Synthesis 2007, 1739. (h) T.-P. Loh, G.-L. Chua, Chem.
Commun. 2006, 2739. (i) R. Amemiya, M. Yamaguchi,
Eur. J. Org. Chem. 2005, 5145. (j) M. Yamaguchi, in:
Main Group Metals in Organic Synthesis (H.
Yamamoto, K. Oshima, Eds.), Wiley-VCH, Weinheim,
2004, pp. 307-322. (k) C. G. Frost, J. P. Hartley, Mini-
Rev. Org. Chem. 2004, 1, 1. (l) D. C. Barman, Synlett,
2003, 2440.
Table 3. Ga(I)-catalyzed cycloisomerisation of enyne 7.
Entry [Ga]
E/Z Yield(%)a
3/2 17
8/1 32
3/2 45
1
2
3
GaCl3
[IPrGaCl2]+[Al(ORF)4]−
b
[Ga(PhF)2]+[Al(ORF)4]−
Ga2Cl4
4
2/1 60
a)
b)
Isolated yields, full conversion reached in each case.
in situ by using IPrGaCl3 and
[Ag(DCM)3]+[Al(ORF)4]−.
Generated
[2] For selected examples of the use of Ga(III) halides as
π-Lewis acids, see: (a) H. Inoue, N. Chatani, S. Murai,
J. Org. Chem. 2002, 67, 1414. (b) N. Chatani, H. Inoue,
T. Kotsuma, S. Murai, J. Am. Chem. Soc. 2002, 124,
10294. (c) B. P. Peppers, S. T. Diver, J. Am. Chem. Soc.
2004, 126, 9524. (d) V. Mamane, P. Hanne, A. Fürstner,
Chem. Eur. J. 2004, 10, 4556. (e) S. I. Lee, S. H. Sim,
S. M. Kim, K. Kim, Y. K. Chung, J. Org. Chem. 2006,
71, 7120. (f) S. M. Kim, S. I. Lee, Y. K. Chung, Org.
Lett. 2006, 8, 5425. (g) E. M. Simmons, R. Sarpong,
Org. Lett. 2006, 8, 2883. (h) H.-J. Li, R. Guillot, V.
Gandon, J. Org. Chem. 2010, 75, 8435. (i) F. de Jesus
In conclusion, we have developed the first catalytic
use of cationic Ga(I) species in homogeneous
molecular -acid catalysis. Compared to other
cationic Ga(III) complexes in hydroarylation,
hydrogenative cyclization or alkene transfer
hydrogenation, the activity of [Ga(PhF)2]+[Al(ORF)4]−
and Ga2Cl4 is competitive or markedly superior.
Moreover, functional groups that are typically
incompatible with GaCl3 or other Ga(III) species
proved compatible with these soft Ga(I)-based
catalysts.
4
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10.1002/adsc.201701081
Advanced Synthesis & Catalysis
Cortez, R. Sarpong, Org. Lett. 2010, 12, 1428. (j) J. S.
[13] S. P. Green, C. Jones, A. Stasch, R. P. Rose, New J.
Chem. 2007, 31, 127.
Yadav, B. V. S. Reddy, B. Padmavani, M. K. Gupta,
Tetrahedron Lett. 2004, 45, 7577. (k) J. S. Yadav, B. V.
S. Reddy, S. K. Biswas, S. Sengupta, Tetrahedron Lett.
2009, 50, 5798. (l) S. Pascual, C. Bour, P. de Mendoza,
A. M. Echavarren, Beilstein J. Org. Chem. 2011, 7,
1520.
[14] B. Qin, U. Schneider, J. Am. Chem. Soc. 2016, 138,
13119.
[15] (a) J. M. Slattery, A. Higelin, T. Bayer, I. Krossing,
Angew. Chem. Int. Ed. 2010, 49, 3228. (b) A. Higelin,
C. Haber, S. Meier, I. Krossing, Dalton Trans. 2012, 41,
12011. (c) A. Higelin, S. Keller, C. Göhringer, C. Jones,
I. Krossing, Angew. Chem. Int. Ed. 2013, 52, 4941. (d)
A. Higelin, U. Sachs, S. Keller, I. Krossing, Chem. Eur.
J. 2012, 18, 10029.
[3] For selected examples of the use of In(III) halides as π-
Lewis acids, see: (a) A. Fꢀrstner, V. Mamane, J. Org.
Chem. 2002, 67, 6264. (b) A. Fꢀrstner, V. Mamane,
Chem. Commun. 2003, 2112. (c) J. Augé, N. Lubin-
Germain, J. Uziel, Synthesis, 2007, 1739. (d) N. Sakai,
K. Annaka, A. Fujita, A. Sato, T. Konakahara, J. Org.
Chem. 2008, 73, 4160. (e) K. Surendra, W. Qiu, E. J.
Corey, J. Am. Chem. Soc. 2011, 133, 9724. (f) W-W.
Qiu, K. Surendra, E. J. Corey, Org. Lett. 2011, 13,
5893. (g) Y. Kwon, H. Cho, S. Kim, Org. Lett. 2013,
15, 920. (h) Z.-L. Shen, S.-Y. Wang, Y.-K. Chok, Y.-H.
Xu, T.-P. Loh, Chem. Rev. 2013, 113, 27. (i) L.
Alonso-Marañón, M. Monserrat Martinez, L. A.
Saraneses, J. Pérez Sestelo, Org. Biomol. Chem. 2015,
13, 379.
[16] (a) M. C. Kuchta, J. B. Bonanno, G. Parkin, J. Am.
Chem. Soc. 1996, 118, 10914. (b) E. S. Schmidt, A.
Jockisch, H. Schmidbaur, J. Am. Chem. Soc. 1999, 121,
9758. (c) N. J. Hardman, P. P. Power, J. D. Gorden, C.
L. B. Macdonald, A. H. Cowley, Chem. Commun. 2001,
1866. (d) M. Asay, C. Jones, M. Driess, Chem. Rev.
2011, 111, 354 and references therein. (e) D. Dange, S.
L. Choong, C. Schenk, A. Stasch, C. Jones, Dalton
Trans. 2012, 41, 93.
[17] For examples see: (a) S. Kobayashi, H. Konishi, U.
Schneider, Chem. Commun. 2008, 2313. (b) U.
Schneider, H. T. Dao, S. Kobayashi, Org. Lett. 2010,
12, 2488. (c) U. Schneider, Y.-Y. Huang, A.
Chakrabarti, H. T. Dao, N. Morita, S. Kobayashi, Pure.
Appl. Chem. 2012, 84, 2417.
[4] V. Michelet, P. Y. Toullec, J.-P. Genꢁt, Angew. Chem.
Int. Ed. 2008, 47, 4268.
[5] C. Bour, V. Gandon, Coord. Chem. Rev. 2014, 279, 43.
[6] (a) S. Tang, J. Monot, A. El-Hellani, B. Michelet, R.
Guillot, C. Bour, V. Gandon, Chem. Eur. J. 2012, 18,
10239. (b) A. El-Hellani, J. Monot, S. Tang, R. Guillot,
C. Bour, V. Gandon, Inorg. Chem. 2013, 52, 11493. (c)
A. El-Hellani, J. Monot, R. Guillot, C. Bour, V.
Gandon, Inorg. Chem. 2013, 52, 506. (d) C. Bour, J.
Monot, S. Tang, R. Guillot, J. Farjon, V. Gandon,
Organometallics, 2014, 33, 594. (e) B. Michelet, G.
Thiery, C. Bour, V. Gandon, J. Org. Chem. 2015, 80,
10985. (f) C. Bour, V. Gandon, Synlett Account 2015,
26, 1427. (g) B. Michelet, S. Tang, G. Thiery, J. Monot,
H. Li, R. Guillot, C. Bour, V. Gandon, Org. Chem.
Front. 2016, 3, 1603.
[18] (a) M. R. Lichtenthaler, A. Higelin, A. Kraft, S.
Hughes, A. Steffani, D. A. Plattner, J. M. Slattery, I.
Krossing, Organometallics 2013, 32, 6725. (b) M. R.
Lichtenthaler, S. Maurer, R. J. Mangan, F. Stahl, F.
Mönkemeyer, J. Hamann, I. Krossing, Chem. Eur. J.
2015, 21, 157.
[19] (a) I. Krossing, I. Raabe, Chem. Eur. J. 2004, 10,
5017. (b) I. Krossing, A. Reisinger, Coord. Chem. Rev.
2006, 250, 2721.
[20] CCDC-1567727.
[21] In the solid state, the composition of “GaI” can be
[7] B. Michelet, J.-R. Colard-Itté, G. Thiery, R. Guillot, C.
[Ga(0)]2[Ga(I)]+[Ga(III)I4]-
or
Bour, V. Gandon, Chem. Commun. 2015, 51, 7401.
+
[Ga(0)]2[Ga(I)]2 [Ga(III)2I6]2- see: B. J. Malbrecht, J.
W. Dube, M. J. Willians, P. J. Ragogna, Inorg. Chem.
2014, 53, 9644.
[8] C. Ferrer, A. M. Echavarren, Angew. Chem. Int. Ed.
2006, 45, 1105.
[9] For selected examples of the use of In(I) halides for
[22] In the solid state, the composition of Ga2Cl4 is
[Ga(I)]+[Ga(III)Cl4]-, see: R. A. Novikov, K. V.
Potapov, D. N. Chistikov, A. V. Tarasova, M. S.
Grigoriev, V. P. Timofeev, Y. V. Tomilov,
Organometallics 2015, 34, 4238.
(asymmetric)
allylation,
allenylation
and
propargylation, see: (a) U. Schneider, S. Kobayashi,
Angew. Chem. Int. Ed. 2007, 46, 5909. (b) A.
Chakrabarti, H. Konishi, M. Yamaguchi, U. Schneider,
S. Kobayashi, Angew. Chem. Int. Ed. 2010, 49, 1838.
(c) Y.-Y. Huang, A. Chakrabarti, N. Morita, U.
Schneider, S. Kobayashi, Angew. Chem. Int. Ed. 2011,
50, 11121. (d) U. Schneider, S. Kobayashi, Acc. Chem.
Res. 2012, 45, 1331 and the references therein.
[23] With 10 mol% of GaCl3, 3a becomes the major
product (88%).
[24] Transfer hydrogenation represents a practical strategy
for performing challenging bond formations by
avoiding the handling of hazardous reagents; for a
review see: D. Wang, D. Astruc, Chem. Rev. 2015, 115,
6621 and references therein.
[10] S. Araki, H. Ito, N. Katsumara, Y. Butsugan, J. Org.
Chem. 1989, 369, 291.
[11] B. C. Ranu, A. Das, Tetrahedron Lett. 2004, 45, 6875.
[25] (a) B. Michelet, C. Bour, V. Gandon, Chem. Eur. J.
2014, 20, 14488. (b) I. Chatterjee, M. Oestreich, Angew.
Chem. Int. Ed. 2015, 54, 1965. (c) I. Chatterjee, Z.-W.
[12] B. C. Ranu, T. Mandal, Tetrahedron Lett. 2006, 47,
2859.
5
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10.1002/adsc.201701081
Advanced Synthesis & Catalysis
Qu, S. Grimme, M. Oestreich, Angew. Chem. Int. Ed.
124, 10294. (c) V. Mamane, P. Hannen, A. Fꢀrstner,
Chem. Eur. J. 2004, 10, 4556. (d) E. M. Simmons, R.
Sarpong, Org. Lett. 2006, 8, 2883. (e) Y. Miyanahana,
N. Chatani, Org. Lett. 2006, 8, 2155. (f) L.-G.; Zhuo,
Y.-C. Shi, Z.-X. Yu, Asian J. Org. Chem. 2014, 3, 842.
2015, 54, 12158. (d) I. Chatterjee, M. Oestreich, Org.
Lett. 2016, 18, 2463. (e) S. Kess, M. Oestreich, Chem.
Sci. 2017, 8, 4688.
[26] For selected examples of the use of Ga(III) and In(III)
halides
as
π-Lewis
acids
for
1,6-enyne
[27] With 10 mol% of GaCl3 instead of 5 mol%, a 55%
yield was reported: see ref 27b.
cycloisomerisation see: (a) H. Inoue, N. Chatani, S. J.
Murai, J. Org. Chem. 2002, 67, 1414. (b) N. Chatani, H.
Inoue, T. Kotsuma, S. Murai, J. Am. Chem. Soc. 2002,
6
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10.1002/adsc.201701081
Advanced Synthesis & Catalysis
UPDATE
Catalytic Use of Low-Valent Cationic Gallium(I)
Complexes as π-Acids
Adv. Synth. Catal. Year, Volume, Page – Page
Zhilong Li, Guillaume Thiery, Martin R.
Lichtenthaler, Régis Guillot, Ingo Krossing,
Vincent Gandon,* Christophe Bour*
7
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