Revealing the Activity of π-Acid Catalysts using a 7-Alkynyl Cycloheptatriene

Article abstract of DOI:10.1002/chem.201703817

A compound that isomerizes into distinct products depending on the particular Lewis acid or Br?nsted acid catalyst used is disclosed. One product can only be obtained with the softest π-acids, such as Au, Pt, Ga, or In complexes. Another is formed only wi

Full text of DOI:10.1002/chem.201703817

A Journal of
Accepted Article
Title: Revealing the Activity of pi-Acid Catalysts Using a 7-Alkynyl
Cycloheptatriene
Authors: Vincent Gandon, Marie Vayer, Regis Guillot, and Christophe
Bour
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Revealing the Activity of -Acid Catalysts Using a 7-Alkynyl
Cycloheptatriene
Marie Vayer,[a] Regis Guillot,[a] Christophe Bour,*[a] and Vincent Gandon*[a],[b]
Abstract:
A
compound that isomerizes into distinct products
Brønsted acids such as HOTf or HNTf2, as well as hard Lewis
acids.[12] Changing the phosphine of (R3P)Au+ for a more -
accepting phosphite or related ligands makes the gold ion more
electrophilic and requalifies the complex (i.e. ((RO)3P)Au+) as an
activator for simple alkenes and allenes.[13,14]
depending on the Lewis or Brønsted acid catalyst used is disclosed.
One product can only be obtained with the softest -acids such as Au,
Pt, Ga or In complexes. Another is formed only with harder -acids
such as Ag or Cu salts. The formation of the third one requires even
harder -acids or protons. This simple benchmark reaction allows to
predict the category of transformations that can be catalyzed by a new
complex. It also informs whether protons have been accidentally
generated in the reaction mixture.
The addition of nucleophiles to alkynes, alkenes and allenes
is a well-established synthetic strategy. It has been made possible
under mild conditions by using carbophilic catalysts able to
deprive C-C multiple bonds from part of their electron density,
turning them electrophilic. These catalysts are referred to as -
acids.[1] Even though C-C  bonds are considered as soft
according to the HSAB principle,[2] the -acid denomination is
actually not reserved to the softest Lewis acids. Depending on the
ability of the substrate to give rise to a stabilized carbocationic
intermediate, even Brønsted acids can sometimes activate C-C
-bonds. In the case of simple substrates, such as unconjugated
alkynes, the softest -acids are usually the most efficient ones.
Among them, complexes of the relativistic elements Pt, Au, and
Hg have shown a great versatility.[1,3,4] These species do not
simply act as “large soft protons”.[4a,5] They are also able to
stabilize incipient positive charges in the nucleophilic addition
transition state (e.g. kinetic alkynophilicity), as well as the
carbocation resulting from it.[1,3,6] In particular, in 1,n-enyne
systems, the nucleophilic addition of the alkene to the alkyne can
lead to vinylcyclopentenes through a metal-stabilized carbene-
like nonclassical carbocation (Scheme 1, eq 1).[7] Although such
a mechanism is typical for soft -acids, there is no well-defined
border between soft and hard ones since the ancillary ligands, the
oxidation state, the charge and the counterions of the metal ions
can tune their activity.[8] For instance, whereas phosphine ligated
(R3P)Au+ ions readily follow eq 1,[7,9] they are sometimes unable
to trigger hydroarylations of alkenes[10] or allenes[11] as in eq 2.
The latter reaction, on the other hand, can be catalyzed by
Scheme 1. Prototypical Examples of Reactions Catalyzed by -Acids.
These guidelines suffer from many exceptions and there is no
simple reaction that would allow to predict the “carbene-like” or
the “large proton-like” behavior of a new -acid. In addition, the
use of some Lewis acids can be the source of protons that can
outcompete the -acid as the true catalyst.[15] We report herein a
substrate which sheds light on these issues. It combines the
chemistry of enynes and that of hydroarylations shown in Scheme
1.
Instead of a simple enyne fragment, we have used a 7-alkynyl
cycloheptatriene framework as in 1 (Scheme 2). Echavarren et al
described the Au-catalyzed cycloisomerization of such
compounds into indenes.[16] Fluxional nonclassical Au-stabilized
9-barbaralyl cations[17] are likely intermediates in this reaction. In
the case of our model substrate 1, they would correspond to 4, 4’,
and other isomers. Depending on the catalyst, 1 can give rise to
either indenes 7 or 9, or to the hydroarylation products 14 and 16.
Their transformation could take place through the -complex 5,
followed by 6 after a 1,2-H shift, and then indene 7 after a second
1,2-H shift. Alternatively, the -complex 8 could be formed,
leading directly to indene 9 after a 1,2-alkyl shift. These pathways
are supported by M06//QZVP/6-311+G(2d,p) computations (see
the Supporting Information). Besides, 7-alkynyl cycloheptatrienes
can undergo valence tautomerism to the norcaradiene form,[18]
which would be 2 in our case. Computations support its
transformation into the allyl cation 10, and its isomerization into
indene 9 though intermediates 11 and 12. Since 10 is less
delocalized than the barbaralyl cations, one can anticipate that
this pathway will be less favorable with the softest -acids, which
is indeed corroborated by computations. Finally, under hard acidic
conditions, norcaradienes are known to isomerize into
phenylallenes.[19] In our case, compound 3 would lead to 14 after
[a]
[b]
M. Vayer, Dr. R. Guillot, Dr. C. Bour, Prof. V. Gandon
Institut de Chimie Moléculaire et des Matériaux d’Orsay, CNRS
UMR 8182, Univ. Paris-Sud, Université Paris-Saclay, 91405 Orsay
(France)
E-mail: christophe.bour@u-psud.fr; vincent.gandon@u-psud.fr
Prof. V. Gandon
Institut de Chimie des Substances Naturelles, CNRS UPR 2301,
Univ. Paris-Sud, Université Paris-Saclay, 1, av. de la Terrasse,
91198 Gif-sur-Yvette (France)
Supporting information for this article is given via a link at the end of
the document.
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10.1002/chem.201703817
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allene hydroarylation through the allyl cation 13, and possibly 16
after alkene hydroarylation through 15. These steps should be
favored by -acids even harder than those leading to 14, or even
by protons. Thus, the chemoselectivity of the isomerization of 1
should reveal the category to which a -acid belongs.
1
AuCl
-/-
0
2
AuCl3
-/-
0
3
[(Ph3P)Au][NTf2]
[(Me4-tBu-XPhos)Au][NTf2]
[(JohnPhos)Au(MeCN)][SbF6]
(Ph3P)AuCl/AgOTf[a]
100/0
90/10
80/20
90/10
75/25
85/15
60/40
85/15
100/0
58/42
68
78
76
64
59
59
69
42
48
72
4
5
6
[a]
7
(Ph3P)AuCl/AgNTf2
[a]
8
(Ph3P)AuCl/AgBF4
[a]
9
(Ph3P)AuCl/AgSbF6
[a]
10
11
12
(phosphite)AuCl/AgSbF6
[a]
(JohnPhos)AuCl/AgSbF6
[a]
IPr·AuCl/AgSbF6
[a] Same charge for each component. [b] Isolated.
1
Of note, indenes 7a and 9a are easily identifiable by H NMR of
the crude (Figure 1).
Scheme 2. Isomerization Pathways of Compound
1 (R = CH2XCH2Ph)
Supported by DFT Computations.
Compound 1a (Table 1) was easily synthesized in 2 steps from
dimethyl propargyl malonate. At 80 °C in 1,2-dichloroethane
(DCE) or toluene, no reaction took place with the hard oxophilic
acids HCl, AlCl3, TiCl4, or SnCl4 (5 mol%). The reaction of 1a was
then carried out in the presence of various Au(I) complexes. In
contrast with the alkyl- and aryl-substituted 7-alkynyl
cycloheptatrienes studied by Echavarren,[16] no reaction took
place with AuCl or AuCl3 (entries 1 and 2). With 3 mol% of cationic
phosphine-Au(I) complexes, indenes 7a and 9a were obtained,
the former being the major one (entries 3-5). When gold(I)
chloride complexes activated with silver salts were used (entries
6-12), only indenes were obtained as well. The 7a/9a ratio
depended on the ligand/counterion combination.
Figure 1. Diagnostic 1H NMR Peaks of 1a, 7a and 9a.
Importantly, the selectivity for indenes with gold(I) complexes
generated in situ using silver salts was the same, no matter if the
anion metathesis had been performed prior to adding 1a or
directly in its presence. This suggests that the cyclization is faster
than the anion metathesis since the silver salts alone led to
different products (Table 2). With AgSbF6, no reaction took place
in toluene (B, entries 1-3). In DCE (A), a mixture of 14a and 16a
was obtained at a catalytic charge of 3 or 5 mol% (entries 1 and
2), but only 16a was isolated at 7 mol% (entry 3).[20] This
transformation was monitored by 1H NMR in CD2Cl2,[21] which
clearly revealed the subsequent formation of 3a, 14a (E and Z
isomers), and finally 16a (Figure 2). The selective formation of
Table 1. Au(I)-Catalyzed Skeletal Rearrangement of 1a.
Entry
cat.
7a/9a
Yield [%][b]
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16a was also observed with SbF5 (entry 4). With AgNTf2 and
AgOTf, only 14a was obtained in both solvents (entries 5 and 6).
Table 2. Other Acid-Catalyzed Rearrangements of 1a.
Entry
cat.
x[a]
Cond[b]
Product[c,d]
Yield
[%][e]
1
2
AgSbF6
AgSbF6
3
5
A/B
A/B
14a[f]:16a
(90:10)/-
36/0
57/0
Figure 2. 1H NMR Monitoring of the AgSbF6-Catalyzed Skeletal Rearrangement
of 1a.
14a:16a
(90:10)/-
3
4
5
6
7
AgSbF6
SbF5
7
5
5
5
5
A/B
A
16a/-
71/0
41
It has been shown that active cationic mono-coordinated gold(I)
complexes can be generated from the corresponding chloride and
Cu(OTf)2 instead of a silver salt.[22] However, the anion metathesis
is slow, which means that the activator can compete with the gold
catalyst. With LAuCl/Cu(OTf)2 catalytic mixtures (entries 7 and 8),
7a was obtained as minor product and 14a as the major one.
Clearly the former results from the activity of the gold ion (see
Table 1) and the latter from the copper salt. Indeed, when used
alone, Cu(OTf)2 provided 14a only (entry 9). A similar result was
obtained with [(MeCN)5Cu][SbF6]2 (entry 10). Compound 14a was
16a
AgNTf2
AgOTf
A/B
A/B
A
14a/14a
14a/14a
81/70
52/77
82
(JohnPhos)AuCl/
Cu(OTf)2
7a:14a
(20:80)
8
(Ph3P)AuCl/
Cu(OTf)2
5
A
7a:14a
(20:80)
87
also
obtained
selectively
with Bi(OTf)3,
In(NTf2)3,[23]
Ca(NTf2)2/nBu4NPF6[24] and HOTf (entries 11-14). With HNTf2 in
DCE, a 37:63 mixture of 14a and 16a was isolated, but only 14a
formed in toluene (entry 15).
9
Cu(OTf)2
5
5
5
5
5
5
5
A/B
A
14a/14a
14a[g]
96/98
74
10
11
12
13
14
15
[(MeCN)5Cu][SbF6]2
Bi(OTf)3
A/B
A/B
A
14a/14a
14a/14a
14a
91/72
73/81
70
This first set of results validated the above hypotheses about the
peculiarities of type 1 compounds and encouraged us to test other
Lewis acids. At this point of the study, the reactivity of 1a had
identified only Au(I) complexes as truly soft -acids. If the test is
relevant, other species known to isomerize enyne systems
through nonclassical carbocations should also lead to 7a/9a. This
was indeed the case with PtCl2,[1] a cationic Pt(II) complex,[25]
GaCl3,[26] Hg(OTf)2[9a] and InCl3 (entries 16-20),[27] which also led
selectively to indenes. The use of gallium complexes of type
IPr·GaX2+ as -acids has been described, and it was shown that
these species are harder than GaCl3.[28] In a logical manner, the
Ga+ complex gave no indene (entry 21). Cationic gallium -acids
are usually less reactive when stabilized by a nitrile.[28a] By using
such a complex, we could selectively obtain allene 3a, which is
the putative intermediate to 14a and 16a (Scheme 3). No reaction
took place when 3a was treated with Ph3PAuCl/AgSbF6 (5 mol%).
Only the most electrophilic gold complex, i.e. the phosphite-
derived one, sluggishly transformed 3a into 14a (33% yield), but
not into indene 7a or 9a. With AgOTf, 14a was isolated in 78%
yield.
In(NTf2)3
Ca(NTf2)2/nBu4NPF6
HOTf
A
14a
69
HNTf2
A/B
14a:16a
(60:40)/14a
54/80
16
17
PtCl2
5
5
A/B
-/7a:9a
(55:45)
0/54
99
[(o-tolyl)2-C,P
Pt(MeCN)2][SbF6]
B
7a:9a
(60:40)
18
19
20
21
GaCl3
5
5
5
5
A/B
A
7a/7a
7a
42/63
21
Hg(OTf)2
InCl3
A/B
A
7a/7a
63/85
96
IPr·GaCl3/AgSbF6
14a[f]:16a
(80:20)
[a] Same charge for each component. [b] A = DCE; B = toluene. [c]
Ratio based on the NMR spectra of the crude. [d] 14a has E
configuration unless otherwise noted. [e] Isolated. [f] E/Z-14a:
55/35. [g] E/Z-14a: 50/50.
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Scheme 5. In(I)- and Ga(I)-Catalyzed Cyclization of 1a into Indene 7a.
Scheme 3. Synthesis and Reactivity of Allene 3a (TFBN
= 2,4,6-
trifluorobenzonitrile).
In conclusion, compound 1a allows the selective formation of
distinct products arising from the activation of an alkyne (1 or 2),
an allene (3), or an alkene (14), each of them deriving from the
preceding one. The softest -acids of the Au, Pt, Hg, Ga, and In
series treat 1a as an enyne and transform it into indenes through
nonclassical “soft carbocations”. The preference for this pathway
is supported by M06 computations with (Me3P)Au+, GaCl3 and
InCl3. Harder -acids transform it into 14a through “harder” allyl
cations derived from an allene (13). Computations confirmed that
the indene pathway is disfavored with AgOTf. With even harder
-acids, 1a is transformed into 16a. Our results in different
solvents and with a broad range of catalysts displaying various
weakly coordinating anions strongly suggest that 16a is obtained
via the participation of protons.[15] Thus, if a new complex converts
1a into 7a (or 9a), it will be probably a good catalyst for skeletal
rearrangement of 1,n-enyne systems as a general rule. If it gives
14a, it will probably be a good hydroarylation catalyst. If it leads
to 16a, it will also be a good hydroarylation catalyst but it is likely
that the activity is explained by hydrolysis of the metal salt.[31] In
particular, this simple reaction could allow to identify surrogates
of noble metal complexes within the main group metal series. Two
of them have been identified thanks to this simple test.
-
Compound 16a was mostly observed with acids of the SbF6
series in DCE. This result supports the hypothesis that 16a is
encouraged by the presence of highly dissociated protons.
Pertaining to this idea, no reaction took place when 1a was
treated with AgSbF6 (7 mol%) and the proton scavenger 2,6-di-
tBu-pyridine (7 mol%). On the other hand, the Au-catalyzed
isomerization of 1a into 7a was not affected by the presence of
2,6-di-tBu-pyridine (see the Supporting Information). Compound
16a was also observed with HNTf2 in DCE, but not with metal
triflimides, which seems to rule out an exclusive proton-catalyzed
sequence between 1a and 16a.
We have applied the benchmark reaction to a new catalytic
system composed of IPr·InBr3 and AgSbF6 (Scheme 4).[29] It gave
rise to indene 7a and the hydroarylation products 14a and 16a.
The presence of both 7a and 14a tell-tales the continuum between
soft and hard acids. The test also reveals that protons are likely
to be formed, which is fully consistent with our finding that HSbF6
is released under such conditions.[29b]
Acknowledgements
This work was supported by UPSud, MENESR, IUF, and CNRS.
We thank David Lebœuf for useful suggestions and Guillaume
Thiery for the synthesis of [In(PhF)2][Al(pftb)4].
Scheme 4. Reaction of 1a in the Presence of a New In(III)/Ag(I) Catalytic
System.
Keywords: alkynes • allenes • carbenes • noble metals •
Encouraged by the above results in the cationic In(III) series, we
speculated that a cationic indium complex with a lower oxidation
state would be softer and less prone to generate protons. We
tested the In(I) complex [In(PhF)2][Al(pftb)4],[30] which, just like
some of the cationic gold(I) complexes shown in Table 1,
converted 1a into 7a selectively in DCE or toluene (Scheme 5).
Other In(I) species such as InCl or InOTf left the substrate
unchanged. We also found that Ga2Cl4, which is actually a
cationic Ga(I) species of formula [Ga][GaCl4],[31] is able to act as
a soft -acid. Thus, the reaction of 1a has allowed to reveal the -
acidic property of Lewis acids that had never been exploited as
such.
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10.1002/chem.201703817
Chemistry - A European Journal
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Marie Vayer, Régis Guillot, Christophe
Bour*, Vincent Gandon*
Page No. – Page No.
Revealing the Activity of -Acid
Catalysts Using a 7-Alkynyl
Cycloheptatriene
A simple benchmark reaction allows to predict the category of transformations that
can be catalyzed by a new complex. Soft -acids follow a carbenoid pathway
leading to indenes from a 7-alkynyl cycloheptatriene. Harder -acids follow a “large
proton” pathway, yielding a hydroarylation product. If these hard species liberate
protons in the reaction mixture, the latter further cyclizes into a tricyclic product.
This article is protected by copyright. All rights reserved.