Exploring the Limits of π-Acid Catalysis Using Strongly Electrophilic Main Group Metal Complexes: The Case of Zinc and Aluminium

Article abstract of DOI:10.1002/chem.202001376

The catalytic activity of cationic NHC-Zn^II and NHC-Al^III (NHC=N-heterocyclic carbene) complexes in reactions that require the electrophilic activation of soft C?C π bonds has been studied. The former proved able to act as a soft π-L

Full text of DOI:10.1002/chem.202001376

Accepted Article
Accepted Article
Title: Exploring the Limits of π-Acid Catalysis Using Strongly
Electrophilic Main Group Metal Complexes: the case of Zinc and
Aluminum
Authors: Jiaxin Tian, Yan Chen, Marie Vayer, Alexandre Djurovic,
Régis Guillot, Refka Guermazi, Samuel Dagorne, Chritophe
Bour, and Vincent Gandon
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Chemistry - A European Journal
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Exploring the Limits of -Acid Catalysis Using Strongly
Electrophilic Main Group Metal Complexes: the case of Zinc and
Aluminum
[
a]
[a]
[a]
[a]
[a]
[b]
Jiaxin Tian, Yan Chen, Marie Vayer, Alexandre Djurovic, Régis Guillot, Refka Guermazi,
[
b]
[a]
[a,c]
Samuel Dagorne, Christophe Bour,* and Vincent Gandon*
Abstract: The catalytic activity of cationic NHC-Zn(II) and NHC-Al(III)
complexes in reactions that require the electrophilic activation of soft
C-C  bonds has been studied. The former proved able to act as a
soft -Lewis acid in a variety of transformations. The benefit of the
bulky IPr NHC ligand was demonstrated by comparison with simple
ZnX
2
salts. The tested NHC-Al(III) catalyst is not able to activate C-C
+

bonds but simple AlX
2
ions were found potent in some cases.
Scheme 1. Nonclassical carbocation intermediates in -acid-catalyzed
transformations of enynes
Introduction

-Acid catalysis, i.e. the electrophilic activation of C-C  bonds
[
1]
with a Lewis acid, is a powerful way to transform alkyne, alkene
or allene derivatives into valuable building blocks. Gold and
platinum complexes have been the most versatile -acids used
so far.[ Other elements such as gallium and indium are also well-
known in this field. In fact, not all -activators behave the same
way. Whereas a simple proton can promote the addition of water
to alkynes through a vinyl cation intermediate, what makes gold,
platinum, gallium and indium complexes rather unique is their
capacity to mediate the addition of a C-C  bond to another C-C
Likewise, we reported that the reaction of compound 1 with
catalysts of the gold, platinum, mercury, gallium and indium series
leads to its skeletal reorganization into the regioisomeric indenes
2]
2
and 3 in 1,2-dichloroethane (DCE) or toluene (Scheme 2). With
[3]
harder Lewis acids of the silver, gallium, copper, and calcium
series, or even a Brønsted acid such as HOTf, the bicyclic product
4
is obtained. This occurs through the rearrangement of the 7-
alkynylcycloheptatriene moiety into a phenylallene,[
7 ]
which
undergoes a rapid hydroarylation by the pendant phenyl group. In
DCE, a solvent that can give rise to superacids such as HSbF in
the presence of AgSbF or SbF , the
,[8],[9] or by using HNTf
conversion of 4 into the tricyclic compound 5 is observed with time,
which corresponds to the hydroarylation of the styrene moiety
through a benzylic carbocation.[5]
nonclassical carbocation.[
2],[
4
]
The

bond through
a
6
cycloisomerization of enynes is a prototypical example of such a
reactivity (Scheme 1, eq 1).[2a],[3c] The classification of Lewis acids
between the softest ones, which will be good for mediating enyne
cycloisomerizations, and harder ones, that will be more potent for
C-C  bond functionalization through classical carbocations, can
be easily made by using the cycloheptatriene test that we
6
5
2
developed.[
5
]
As described by Echavarren et al, 7-
alkynylcycloheptatrienes are treated like enynes by gold(I)
complexes (Scheme 1, eq (2)).[6]
[
a]
J. Tian, Y. Chen, Dr M. Vayer, A. Djurovic, Dr C. Bour, Prof. V.
Gandon
Institut de Chimie Moléculaire et des Matériaux d’Orsay (ICMMO),
CNRS UMR 8182, Université Paris-Saclay, Bâtiment 420, 91405
Orsay cedex, France.
E-mail: vincent.gandon@universite-paris-saclay.fr
Dr. S. Dagorne, Dr. R. Guermazi
Institut de Chimie de Strasbourg, CNRS-Université de Strasbourg, 1
rue Blaise Pascal, 67000 Strasbourg, France.
Prof. V. Gandon
[
[
b]
c]
Laboratoire de Chimie Moléculaire (LCM), CNRS UMR 9168, Ecole
Polytechnique, Institut Polytechnique de Paris, route de Saclay,
91128 Palaiseau cedex, France.
Supporting information for this article is given via a link at the end of
the document.
Scheme 2. Isomerization pathways of the 7-alkynylcycloheptatriene
compound 1 (E = CO Me; LA = Lewis Acid; DCE = 1,2-dichloroethane)
2
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One interest of this test is to identify new soft -acids, as we did
in the gallium(I) and indium(I) series.[5],[10] Another interest is to
show the efficiency of an anion exchange. Indeed, since silver
(entries 9 and 11). It is also worthy of note that the activation of
ZnBr
it led to compound 4 (entry 14), i.e. the same as the one obtained
in our previous study with AgSbF
alone.[6a] No reaction took place
with the aluminum complex D with or without AgSbF
alone (entries 15-20). As an indication of the formation of HSbF
2 6
as one of these zinc salts with AgSbF was not efficient, as
salts such as AgSbF
6
, which is a widely used halide abstractor,
6
gives a different result than the softest Lewis acids (4 vs 2/3), this
6
, or AlBr
3
test allows to validate the formation of the desired cationic species
6
+
(
such as IPr·InBr
2
from IPr·InBr
3
and AgSbF
6
). Lastly, this test
in DCE, compounds 4 and 5 were isolated when using a mixture
of AlBr and AgSbF (entries 21-22).
informs if the catalytic conditions release protons (5).
3
6
In this study, we have used the cycloheptatriene test and
other reactions to analyze the catalytic behavior of other
complexes of the main group metal series. We were especially
interested in broadening the scope of application of -acid
catalysis to inexpensive metals such as zinc and aluminum.
Table 1. Skeletal reorganization of 1.
Regarding zinc, only simple salts of type ZnX
far as -acids.[11] We have studied the case of the NHC-stabilized
zinc(II) species [IPr·Zn(C
)]+[B(C ]- (Table 1, B) and
IPr·ZnBr (THF) (Table 1, C). Synthesized by some of us, the
aryl-zinc complex B has a higher fluoride ion affinity (FIA) than
B(C and efficiently catalyzes alkene, alkyne and CO
hydrosilylation. Complex C has thus far not been reported but the
2
have been used so
6
F
5
6 5 4
F )
[12]
2
6
F
5
)
3
2
Entry
cat.
x
Solvent
Time
[h]
2/3/4/5
Yield
[%][
a]
related IMes·ZnCl
mixture has been used for the cycloaddition of CO
and IPr·ZnCl
amination of CO
aware of their use as catalyst for cycloisomerization reactions. We
2
(THF) is known,[ 13 ] the SIPr/ZnBr
2
catalytic
to epoxides,[
14]
1
2
3
4
5
6
7
8
9
10
A
3
DCE
20
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
58/42/0/0
72
88
89
2
2
has been employed to catalyze the reductive
.[15] Regarding aluminum complexes, we are not
B
3
DCE
100/0/0/0
2
B
3
toluene
DCE
100/0/0/0
16
]
have tested the IMes·AlBr
3
complex D[
to allow direct
B
10
100/0/0/0
92
comparison with C. Both new complexes C and D were
characterized by X-ray diffraction analyses.[17]
-[b]
C
10
DCE
-
C/AgSbF
6
6
10/10
10/10
10
DCE
93/7/0/0
62
Results and Discussion
-[c]
C/AgSbF
toluene
DCE
-
ZnCl
2
-
-[b]
33
-[c]
As mentioned above, 7-alkynylcycloheptatrienes rearrange into
indenes under gold catalysis.[6a] So did compound 1[5] when
treated for instance by the cationic gold complex A (Table 1, 3
mol%), displaying a IPr NHC ligand, in DCE at 80 °C for 20 h
ZnBr
2
2
10
DCE
86/14/0/0
ZnBr
10
toluene
DCE
-
(
7
entry 1). A 58/42 mixture of indenes 2 and 3 has been isolated in
2% overall yield. The cationic IPr-stabilized aryl zinc complex B
11
12
13
14
ZnI
2
2
10
83/17/0/0
34
was then tested (entry 2). To our delight, under similar conditions,
indene 2 was isolated as a sole product in 88% yield. The same
result was obtained in toluene (entry 3). A better yield of 92% was
reached when using 10 mol% of catalyst (entry 4). The IPr
substituted zinc bromide complex C proved unreactive (entry 5).
-[c]
ZnI
10
toluene
DCE
-
Zn(OTf)
2
10
-
-[d]
49
-[b]
ZnBr
2
/AgSbF
6
10/10
20
DCE
0/0/100[e]/0
However, a catalytic mixture of C and AgSbF
provided a 93/7 mixture of 2 and 3, isolated in 62% yield (entry 6).
This is clearly not AgSbF that is responsible for this reaction
since when used alone, AgSbF leads to a different product type
6
(10 mol% each),
15
D
DCE
-
-
-
-
-
-
1
6
7
D
20
toluene
DCE
-[b]
6
6
1
D/AgSbF
20/20
20/20
20
-[b]
6
6
[
5]
(
Scheme 2, compound 4). Even though it could not be isolated,
18
D/AgSbF
toluene
DCE
-[b]
+
]- as active
this clearly supports the formation of [IPr·ZnBr] [SbF
6
species. The same transformation could not be carried out in
toluene (entry 7). Nevertheless, the involvement of protons
cannot be invoked in DCE since neither 4 nor 5 were detected.
Importantly, these NHC-stabilized species B and C proved much
19
AlBr
AlBr
AlBr
AlBr
3
3
3
3
-[b]
20
20
toluene
DCE
-[b]
2
1
2
/AgSbF
6
20/20
20/20
0/0/68[e]/32
0/0/90[e]/10
66[f]
40
more efficient than the simple zinc salts ZnCl
Zn(OTf) (entries 8-13), with which low yields and
regioselectivities were at best obtained with ZnBr or ZnI in DCE
2 2 2
, ZnBr , ZnI and
2
2
/AgSbF
toluene
6
2
2
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Scheme 3. Calculated intermediates and transition states for the formation of
[
[
a] Isolated. [b] No reaction. [c] Very low conversion, trace of indene product detec it ne dd e. nes from a 7-alkynylcycloheptatriene complex.
d] Phenylallene observed, 20% yield. [e] 1:1 E/Z mixture. [f] Determined in situ by 1
Br as internal standard.
H
NMR using CH
2
2
The computed free energies corresponding to Scheme 3 are
collected in Table 2. While the free energies of activation all seem
reasonable, there is no kinetic preference for one indene or the
other in the gold series since TSbc and TSbe lie virtually at the
same value (14.0 and 14.3 kcal/mol respectively). Thus, a low
selectivity is expected between the regioisomers, which is indeed
the case between 2 and 3 when catalyst A is used (Table 1, entry
1). On the other hand, in the zinc series, the values are markedly
different: 18.1 kcal/mol for TSbc and 14.7 kcal/mol for TSbe. A high
regioselectivity in favor of the indene displaying the alkyl group at
the  position of the phenyl ring is therefore expected, which is
corroborated by the selective formation of 2 (Table 1, entry 2).
In contrast with the other reactions studied herein, the skeletal
reorganization of 7-alkynylcycloheptatrienes did not receive a
detailed computational treatment. Thus, the mechanism of the
indene formation promoted by the model gold and zinc NHC
fragments shown in Scheme 3 was computed at the BP86/def2-
QZVP(Au)-
atoms)//BP86/LANL2DZ(Au,Zn)-6-31G(d,p)(other atoms) (see
the Supporting Information for details). The 7-
alkynylcycloheptatriene complex was used as starting
6-311+G(2d,p)(other
Table 2. Computed free energies (G298, kcal/mol) relatively to a.
Entry
Computed
structure
Au series
Zn series
a
compound. Based on these calculations, the formation of the
indene scaffold of complex g, corresponding to 2 in Table 1, can
be explained as follows: the binding of the metal fragment to the
triple bond triggers the nucleophilic attack of the middle double
bond of the 1,3,5-triene fragment to give the cyclopropylcarbenoid
b. The latter rearranges into the bicyclic allyl cation e. A 1,2-H shift
transforms e into the Wheland-type complex f. A 1,2-proton shift
finally gives rise to the indene complex g. For the formation of the
other indene complex d, corresponding to product 3 in Table 1,
the deciphered mechanism involves the rearrangement of b into
the diallyl cation c, which undergoes a 1,2-methyl shift to give d
1
2
3
4
5
6
7
8
9
TSab
b
11.8
3.1
10.6
7.1
TSbc
c
14.0
-19.6
0.8
18.1
-25.4
-23.6
-60.3
14.7
-4.9
TScd
d
-55.4
14.3
-10.3
1.2
(
pink pathway).
TSbe
e
TSef
f
5.9
10
11
12
-28.8
-23.1
-59.6
-34.8
-29.6
-64.1
TSfg
g
Although the same kind of carbenoid intermediates are expected
in the gold and zinc series, the zinc-derived ones seem less
reactive towards double bonds. Echavarren et al reported that
compound 6 transforms into tautomeric barbaralanes 7 and 8
when treated with the gold catalyst A (Scheme 4). They
interconvert
rapidly
through
a
strain-assisted
Cope
rearrangement.[ They likely arise from the trapping of the
putative barbaralyl gold carbene. Using B, the cyclopropanation
was not observed, although the same barbaralyl carbene explains
6a]
6
the formation of the indene isomers. Again, the D/AgSbF mixture
did not promote any reaction in this case.
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We attribute this difference to the higher steric crowd around the
zinc carbene center due to the trigonal geometry around the metal,
vs a linear geometry with gold, which will place the bulky NHC
further away from the incoming double bond.
Although the hydroarylation pathway to 4 (Scheme 2) was not
observed with compound 1 under zinc catalysis (except Table 1,
entry 14), such a reaction was independently tested using
arenyne 11 (Table 3). This compounds was previously shown to
undergo
RuCl (CO)
metal-catalyzed
, [RuCl (CO)
hydroarylation
with
2
PtCl ,
.[ 18 ] The
[
2
n
L
m
]
2
2
n
L
m
]
2
/AgOTf and GaCl
3
reaction was very efficient with zinc, especially with the NHC-
stabilized species (entries 1 and 2). Of note, the hydroarylation
catalyzed by ZnBr
-4 vs 5), even though these simple salts could not compete with
the NHC-complexes. Adding AgSbF (1/1 mol% ratio) could
enhance the activity of ZnBr (entries 3 vs 6), but not that of ZnI
and AlBr (entries 7 and 9 vs 4 and 5). The NHC-Al complex D left
the starting material intact (entry 9). Since the reaction can be
2 2 3
or ZnI is more efficient than with AlBr (entries
Scheme 4. Cycloisomerization of 6.
3
6
2
2
This lack of reactivity could be due to the trigonal geometry
around zinc (vs linear for gold), which brings the bulky NHC and
the C F ligands closer to the carbene center. To validate this
6 5
3
conducted in the presence of AgSbF
efficiency (entry 10), we can postulate the formation of IMesAlBr
ions which are yet inactive for this transformation.
6
alone with moderate
hypothesis, DFT computations were performed at the
aforementioned level of theory (Scheme 5). The formation of the
polycyclic scaffold of compounds 7 and 8 shown in Scheme 4
involves two main steps: formation of the cyclopropylcarbenoid i
and the electrophilic cyclopropanation at the carbene center to
give j. While the first step is exergonic with gold, it remains, as
found before, endergonic but attainable with zinc. There is yet a
major difference between gold and zinc in the cyclopropanation
step, the corresponding transition states lying at 13.4 kcal/mol and
+
2
Table 3. Hydroarylation of 11.
20.6 kcal/mol respectively.
Entry
cat.
Yield [%][a]
1
2
3
4
5
6
7
8
9
B
95
96
61
50
27
79
50
-[c]
22
55
C/AgSbF
6
ZnBr
ZnI
AlBr [
ZnBr
ZnI
D/AgSbF
AlBr /AgSbF
AgSbF
2
2
b]
3
2
/AgSbF
6
2
/AgSbF
6
6
3
6
10
6
[
a] Isolated. [b] 20 mol%. [c] no reaction.
The transfer hydrogenation of alkenes using 1,4-cyclohexadiene
derivatives as H
2
surrogate is a very convenient process that can
be catalyzed by gallium[ or boron complexes.[20] The use of zinc
in this field seems to be undocumented.[ 21 ] The reduction of
compound 13 (Table 4) was previously studied with the NHC-
19]
Scheme 5. Calculated key intermediates and transition states for the
cycloisomerization of compound 6 (G298, kcal/mol).
+
-
gallium complex [IPr·GaCl
product 14 in 67% yield.[19] Here, also we observed an increased
2 6
] [SbF ] , leading to the reduced
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efficiency with the zinc catalyst B, which provided 14 in 80% yield
hence the low yield in entry 1. In that respect, the zinc species
used in this study do not tolerate the presence of water and
therefore do not efficiently catalyze the hydration of p-methyl
(
entry 1). Catalyst C, ZnBr
yields did not exceed 68% (entries 2, 5, and 6). It is striking to
note that no reaction took place with ZnBr or ZnI alone (entries
and 4) or with the aluminum species used with AgSbF (entries
and 8). The NHC-zinc complexes also proved much more potent
2 2 6
and ZnI with AgSbF worked, but the
2
2
phenylacetylene, which is
catalysts.[23]
a
major difference with gold
3
7
6
Table 5. Hydroalkoxylation of 15.
than the corresponding gold complex (entry 9).
Table 4. Transfer hydrogenation of 13.
Entry
cat.
Yield [%][a],[c]
1
2
3
4
5
6
7
8
B
31
84
71
65
17
17
21
10
C/AgSbF
6
Yield [%][a]
ZnBr
ZnI
ZnBr
ZnI
D/AgSbF [
AlBr
/AgSbF [
2
Entry
cat.
2
1
2
3
4
5
6
7
8
9
B
80
2
/AgSbF
6
C/AgSbF
6
63
-[b,c]
2
/AgSbF
6
ZnBr
ZnI
ZnBr
ZnI
D/AgSbF
AlBr
/AgSbF [
IPr·AuCl/AgSbF
2
b]
-[b,c]
6
2
b]
6
3
2
/AgSbF
6
64
2
/AgSbF
6
68
[a] Isolated. [b] 20 mol% each. [c] All these reactions work equally
well in the presence of 2.2 equiv of 2,6-di-tert-butylpyridine.
-[b,c]
-[b,c]
37
6
The preferred behavior of the zinc species as soft -Lewis acids
rather than hard -Lewis acids was further confirmed using the
-ketoester 17a (Table 6). It is known to undergo carbonyl-olefin
metathesis by -Lewis acid activation of the carbonyl group.
c]
3
6
[1a]

6
[
24]
[
25 ]
[ 26 ]
Schindler et al have shown that FeCl
3
and GaCl
3
are
[
a] Isolated. [b] No reaction. [c] 20 mol% each.
particularly efficient mediators of such transformations, including
We next turned our attention to the formation of C-O bonds.
Compound 15 is an interesting substrate for the evaluation of -
acids (Table 5). As shown by Michelet, Genêt et al, with AuCl or
that of 17a. However, catalysts B, C/AgSbF , ZnBr2 and ZnI2
6
proved unable to trigger the metathesis (entries 1-4). Only harder
+
ZnX ions formed by halide abstraction with AgSbF provided the
6
AuCl
3
as catalysts, the two alcohol functionalities react with the
cyclic isomers 18 and 19 in good yields (entries 5 and 6). On the
other hand, the reaction was over in 4 h at 50 °C with the putative
AlBr2+ ion generated in situ, and the isolated yield was in the same
range (entry 8). Schindler and coworkers recently reported a
preprint describing an aluminum-based heterobimetallic ion pair
as catalyst to promote carbonyl-olefin ring-closing metathesis.[27]
alkyne moiety to furnish the bicyclic ketal 16.[22] A distinctbehavior
3
was reported by us when GaCl was used as catalyst, as only one
hydroalkoxylation took place, giving rise to a cyclic enol.[3d] With
the zinc species, similarly to gold, only product 16 was observed.
It is complex C and AgSbF
entry 2). ZnBr and ZnI were rather deactivated this time by the
presence of the silver additive (entries 3/4 vs 5/6). The latter had
little influence on the activity of D or AlBr , which remained not
6
that gave the best result (84% yield,
2
2
Table 6. Carbonyl-Olefin Metathesis of 17a.
3
sufficiently carbophilic to efficiently activate the alkyne
functionality (entries 7 and 8). These hydroalkoxylations could
well be catalyzed by an unsuspected Brønsted acid, but as
mentioned above, the cycloheptatriene test has not detected the
release of protons. Accordingly, it is important to note that the use
of 2.2 equiv of 2,6-di-tert-butylpyridine had no effect on the
Entry
cat.
18/19
Yield [%][a]
reaction outcome. It is again, for entry 2, likely that
-[b]
1
2
3
B
-
-
-
+
]- is the active species. We can also infer that the
[
IPr·ZnBr] [SbF
6
C/AgSbF
6
-[b]
-[b]
OH groups in 15 can be acidic enough to promote the cleavage
of the Zn-C bond in B, yielding a less reactive zinc alkoxyde,
ZnBr
2
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-[b]
Conclusions
4
5
6
7
8
ZnI
ZnBr
ZnI /AgSbF
D/AgSbF
AlBr
/AgSbF [
2
-
2
/AgSbF
6
58/42
82/18
-
90
84
-[d]
79
Starting from the cycloheptatrienyl test, we have revealed that
cationic NHC-Zn(II) complexes can be exquisite -acids in various
reactions such as skeletal rearrangements, hydroarylations,
hydroalkoxylations and transfer hydrogenations. They can even
compete with NHC-Au(I) or NHC-Ga(III) catalysts sometimes,
although this study has delineated some of their limitations
compared to gold and gallium species. This work also shows that
NHC-stabilized zinc ions are more active than simple zinc halides.
2
6
[
c]
6
c]
3
6
67/33
[
a] Isolated. [b] No reaction. [c] reaction performed with 20 mol% of
AlBr /AgSbF at 50 °C in 4 h. [d] no reaction.
3
6
Due to its proximity with zinc and gallium in the periodic table,
aluminum was also tested. This time, simple AlX
+
2
ions proved
While the carbonyl-olefin metathesis discussed above is not
related to -acid catalysis, it leads to alkenes that can be activated
by -acids. The tandem carbonyl-olefin/transfer hydrogenation of
more active than a NHC-Al(III)+ species. They are much less
versatile than the zinc complexes but have shown a promising
activity in a tandem process at the border between - and -Lewis
acid catalysis. Study of the use of Zn and Al complexes in
polycyclizations reactions is underway in our laboratory.
1
7 was thus attempted with 1,4-cyclohexadiene (1,4-CHD) as
hydrogen donor (Table 7). This tandem process was recently
reported by us using IPr·GaCl + ions as catalyst.[28] In this reaction,
7 is first transformed into cyclopentenes (such as 18/19), which
then undergo transfer hydrogenation to cyclopentanes 20. Thus,
-activation of the carbonyl is required in the first step, and then
2
1
Experimental Section

a -activation of the C=C bond is required in the second one.
While zinc remained inefficient (entry 1), aluminum salts proved
active. After screening the time and the equivalents of 1,4-CHD,
we found that the reductive cyclization of 17a, could be carried
General Considerations. Unless otherwise stated, commercially available
reagents were used as received. [(IPr)Au(NCMe)]SbF
from Aldrich. Solvents were purified by distillation under argon from the
following: tetrahydrofuran (THF) and diethyl ether (Et O):
6
(A) was obtained
2
out in the presence of in situ generated AlBr +
sodium/benzophenone; toluene (tol): sodium; 1,2-dichloroethane (DCE):
calcium hydride; and degassed by freeze-pump-thaw technique. Complex
B was synthesized following a reported procedure.[12] The following
compounds were previously described: 1-5,[5] 6-8,[6a], 11-12,[18a],13-14,[19]
2
ions (entries 2 and
3, 82% yield for the optimized condition). With Ar = p-fluoro- and
p-chloro-benzene, the aluminum-catalyzed reaction in DCE
proved to be more efficient when compared to the gallium-
catalyzed one; the ethyl 2-arylcyclopentane-carboxylates 20b-c
were isolated in 44% and 60% yield respectively (entries 4-5), vs
5-16,[22] 17a-c, 18-19 and 20a-c.[28] NMR spectra of the new precatalysts
C and D are shown in the Supporting Information.
1
1
Hand 13C NMR spectra
were recorded on Bruker 250, 300 and 360 MHz spectrometers at 20 °C.
1H NMR chemical shifts are reported in ppm using residual solvent peak
: δ = 5.32 ppm). Data for 1
as reference (CHCl : δ = 7.27 ppm, CH Cl H
3 2 2
NMR are presented as follows: chemical shift δ (ppm), multiplicity (s =
singlet, d = doublet, t = triplet, m = multiplet, br = broad), coupling constant
J (Hz) and integration; 13C NMR spectra were recorded at 63, 75, 91 or
100 MHz using broadband proton decoupling and chemical shifts are
2
3% and 50% respectively in our previous study.[28] These good
results contrast with those shown in Table 4, but cyclic alkenes
are usually more reactive.
Table 7. Hydrogenative carbonyl-ene metathesis of 17.
reported in ppm using residual solvent peaks as reference (CHCl
7.16 ppm, CH Cl : δ = 53.84 ppm). Infrared spectra were recorded on a
FTIR spectrometer (Perkin-Elmer spectrum one, NaCl pellets or Bruker
3
: δ =
7
2
2
-
1
Vertex 70 ATR Pike Germanium) and are reported in cm . Flash
chromatography (FC) was performed with 40-63 µm silica gel with
mixtures of ethyl acetate (EtOAc) and cyclohexane (Cy). Thin layer
chromatography was carried out using Merck TLC Silica gel 60 F254 and
visualized by short-wavelength ultraviolet light as well as by p-
anisaldehyde stain.
Entry
Ar
Cat.
Time
h)
Yield [%][a],[b]
Procedure
for
the
Skeletal
Rearrangement
of
(
Alkenylcycloheptatrienes 1 and 6 (Table 1 and Scheme 4): under argon
atmosphere, 1 or 6 (0.15 mmol, 1.0 equiv), the catalyst (3-20 mol%) and
AgSbF (0-20 mol%) were charged in a 10 mL tube equipped with a Teflon-
6
coated magnetic stir bar. Distilled DCE or toluene (1.0 mL, 0.15 M) was
added and the tube was sealed. The reaction tube was immersed and
stirred in a preheated oil bath at 80 °C until completion. The reaction
mixture was filtered through a pad of celite (thoroughly rinsed with Et O)
2
and concentrated under reduced pressure. Purification by FC (Cy/EtOAc)
afforded the corresponding product.
1
2
3
4
5
Ph
ZnBr
2
24
4
-[c]
Ph, 17a
Ph, 17a
4-ClPh, 17b
4-FPh, 17c
AlBr
AlBr
AlBr
AlBr
3
68 (20a)
82 (20a)
44 (20b)
60 (20c)
3
24
24
24
3
3
Procedure for the Hydroarylation of Arenyne 11 (Table 3): under argon
atmosphere, arenyne 11 (0.20 mmol, 1.0 equiv), the catalyst (10 mol%)
[
a] Isolated. [b] cis/trans dr > 4/1. [c] complex mixture.
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and AgSbF
6
(0 to 10 mol%) were charged in a 10 mL tube equipped with
cryoprotectant and then flashfrozen in a nitrogen-gas stream at 100 K. For
compounds, the temperature of the crystal was maintained at the selected
value by means of a 700 series Cryostream cooling device to within an
accuracy of ±1K. The data were corrected for Lorentz polarization, and
absorption effects. The structures were solved by direct methods using
a Teflon-coated magnetic stir bar. Dry degassed 1,2-DCE (1.33 mL, 0.15
M) was added and the tube was sealed. The reaction tube was immersed
and stirred in a preheated oil bath at 80 °C for 24 h. The reaction mixture
was filtered through a pad of celite (thoroughly rinsed with Et O) and
2
concentrated under reduced pressure. The crude was purified by flash
column chromatography on silica gel (Cy/EtOAc).
[
29 ]
2
SHELXS-97
and refined against F by full-matrix least-squares
[
30 ]
techniques using SHELXL-2018
with anisotropic displacement
parameters for all non-hydrogen atoms. Hydrogen atoms were located on
a difference Fourier map and introduced into the calculations as a riding
model with isotropic thermal parameters. All calculations were performed
by using the Crystal Structure crystallographic software package WINGX.
Procedure for the Hydrogenation of Alkene 13 (Table 4): under argon
atmosphere, alkene 13 (0.20 mmol, 1.0 equiv) was introduced into a 10
mL tube equipped with a Teflon-coated magnetic stir bar. The catalyst (10
[31].
The crystal data collection and refinement parameters are given in
6
mol%), AgSbF (0 to 10 mol%) and then dry degassed 1,2-DCE (1.33 mL,
Table S1 (Supporting Information). CCDC 1977549 (D) and 1977550 (C)
contains the supplementary crystallographic data for this paper. These
data can be obtained free of charge from the Cambridge Crystallographic
0
.15 M) were added into the reaction vessel. The tube was capped with a
rubber septum. 1,4-CHD (0.24 mmol, 23 μL, 1.2 equiv) was added through
the septum with a Hamilton® syringe. The reaction tube was stirred for 24
h a preheated oil bath at 80 °C. The reaction mixture was filtered over a
Data
Centre
via
http://www.ccdc.cam.ac.uk/Community/Requestastructure.
2
pad of celite, which was rinsed with Et O. Volatiles were removed by rotary
evaporation under vacuum. The crude was purified by flash column
chromatography on silica gel (Cy/EtOAc).
Preparation of IPr·ZnBr
glovebox, anhydrous ZnBr
of dry degassed THF. A solution of 1,3-bis(2,4,6-trimethylphenyl)-imidazol-
-ylidene (303.4 mg, 1.0 mmol) in 3 mL of THF was then slowly added.
The mixture was stirred at rt for 12 h. The resulting solution was then
concentrated to 5 mL. After the addition of 20 mL of pentane, a white solid
precipitated, which was filtered off and washed three times with pentane
and dried in vacuo for 2 h. Yield: 514 mg, 0.85 mmol (85%). Crystals
2
(THF) (C): in a Schlenk flask placed in the
2
(225.8 mg, 1.0 mmol) was dissolved in 15 mL
Procedure for the Hydroalkoxylation of Arenyne 15 (Table 5): under
argon atmosphere, arenyne 15 (0.20 mmol, 1.0 equiv) was introduced into
a 10 mL tube equipped with a Teflon-coated magnetic stir bar. The catalyst
2
6
(10 mol%), AgSbF (0 to 10 mol%) then dry degassed 1,2-DCE (1.33 mL,
0.15 M) were added into the reaction vessel and the tube was sealed. The
reaction tube was immersed and stirred in a preheated oil bath at 50 °C
for 24 h. The reaction mixture was filtered through a pad of celite
suitable for X-ray diffraction analysis were obtained by recrystallization in
1
2
CH Cl
2
/hexane at -20 °C. H NMR (300 MHz, CD
2
Cl
2
): δ 7.13 (s, 2H), 6.99
(
thoroughly rinsed with Et
The crude was purified by flash column chromatography on silica gel
Cy/EtOAc).
2
O) and concentrated under reduced pressure.
(
s, 4H), 3.72-3.62 (m, 4H), 2.35 (s, 6H), 2.10 (s, 12H), 1.84-1.76 (m, 4H).
1
3C NMR (91 MHz, CD
1.3, 18.1. FT-IR (neat, cm ): 3119, 2920, 1609, 1544, 1488. This
complex appears in NMR as a 95:5 mixture of IPr·ZnBr (THF) and the
corresponding dimer [IPr·ZnBr
.[13b]
2
2
Cl ): δ 140.2, 135.9, 134.4, 129.5, 123.9, 69.0, 25.6,
(
-
1
2
2
General Procedure for the Carbonyl-Olefin Metathesis of 17a (Table
): under argon atmosphere, 17a (0.20 mmol, 1 equiv) was introduced into
a 10 mL tube equipped with a Teflon-coated magnetic stir bar. The catalyst
10 mol%), AgSbF (0 to 10 mol%) then dry degassed 1,2-DCE (1.33 mL,
.15 M) were added into the reaction vessel and the tube was sealed. The
2 2
]
6
Preparation of IPr·AlBr
solution of 1,3-bis(2,4,6-trimethylphenyl)-2,3-dihydro-1H-imidazole (100
mg, 0.326 mmol) in 5 mL of dry degassed Et O was slowly added to a
solution of anhydrous AlBr (85.9 mg, 0.322 mmol) in 3 mL of Et O at 25 °C.
3
(D): in a Schlenk flask placed in the glovebox, a
(
6
0
2
reaction tube was immersed and stirred in a preheated oil bath at 80 °C
for 24 h. The reaction mixture was filtered through a pad of celite
3
2
The formed brown suspension was allowed to stirred at room temperature
for 24 h. The off-white product was collected by filtration, washed with
hexane (4 mL) twice, and dried in vacuo for 3 h. Yield: 167 mg, 0.292 mmol
(
thoroughly rinsed with Et
The crude was purified by flash column chromatography on silica gel
Cy/EtOAc).
2
O) and concentrated under reduced pressure.
(
(
90 %). Crystals suitable for X-ray diffraction analysis were obtained by
1
recrystallization in CH
7
CDCl
1)
2
Cl
2
/hexane at -20 °C. H NMR (360 MHz, CDCl
3
): δ
General Procedure for the Hydrogenative Carbonyl-Olefin Metathesis
of 17: under argon atmosphere, 17 (0.2 mmol, 1 equiv) was introduced
into a 10 mL tube equipped with a Teflon-coated magnetic stir bar. The
.20 (s, 2H), 7.03 (s, 4H), 2.36 (s, 6H), 2.15 (s, 8H). 13C NMR (91 MHz,
): δ 140.9, 135.5, 133.4, 129.7, 124.9, 21.4, 18.2. FT-IR (neat, cm-
3
: 2911, 1610, 1538, 1481.
6
catalyst (20 mol%), AgSbF (0 to 20 mol%) and then dry degassed 1,2-
DCE (1 mL, 0.2 M) were added into the reaction vessel. The tube was
capped with a rubber septum was added through the septum with a
Hamilton® syringe. The resulting mixture was stirred for 24 h in a
preheated oil bath at 50 °C, unless otherwise specified. The reaction
Acknowledgements
mixture was diluted with Et
2
O, filtered through a pad of celite (thoroughly
We are grateful to the China Scholarship Council (CSC, PhD
grants for JT and YC), CNRS, MESRI (PhD grant for MV),
Université Paris-Sud, ANR ANR-18-CE07-0033-01 (HICAT) and
Ecole Polytechnique for their support of this work. We are thankful
to the Umicore Company for a generous gift of metal complexes.
rinsed with Et O) and concentrated under reduced pressure by rotary
2
evaporation. Purification by FC over silica gel (Cy/EtOAc) afforded desired
products 20a-c. For 20a-c, the cis/trans diastereomeric ratio was
1
determined by H NMR analysis of the crude reaction mixture.
X-Ray Analysis: X-ray diffraction data for compounds C and D were
collected by using a VENTURE PHOTON100 CMOS Bruker diffractometer
with Micro-focus IuS source Cu Kα radiation. Crystals were mounted on a
CryoLoop (Hampton Research) with Paratone-N (Hampton Research) as
Keywords: Zinc • Aluminum • NHC ligand • Alkynes •
Cycloisomerization • Carbonyl-ene metathesis
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Chemistry - A European Journal
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Entry for the Table of Contents
FULL PAPER
Jiaxin Tian, Yan Chen, Marie Vayer,
Alexandre Djurovic, Régis Guillot, Refka
Guermazi, Samuel Dagorne, Christophe
Bour,* Vincent Gandon*
Page No. – Page No.
Exploring the Limits of -Acid
Catalysis Using Strongly Electrophilic
Main Group Metal Complexes: the
case of Zinc and Aluminum
+
+
Some NHC-Zn , NHC-Al , as well as other zinc and aluminum cations derived from
simple halide salts have been tested as -acids in various reactions. Some of these
species proved to be excellent catalysts, competing with noble metal complexes.
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