Gold-Catalyzed Cyclisation by 1,4-Dioxidation

Article abstract of DOI:10.1002/chem.201900996

Amide-substituted diynes were cyclized in the presence of a cationic gold catalyst and an external nucleophile leading to 1-indenones and 1-iminoindenones. The electron-donating features of the nitrogen atom enable the formation of a reactive ketene iminium ion, which can be trapped by either diphenyl sulfoxide or anthranil as nucleophiles in a subsequent oxidation step, providing substituted inden-1-on-3-carboxamides.

Full text of DOI:10.1002/chem.201900996

A Journal of
Accepted Article
Title: Gold-Catalyzed Cyclisation by 1,4-Dioxidation
Authors: Vanessa Claus, Lise Molinari, Simon Büllmann, Jean Thusek,
Matthias Rudolph, Frank Rominger, and A. Stephen K.
Hashmi
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To be cited as: Chem. Eur. J. 10.1002/chem.201900996
Link to VoR: http://dx.doi.org/10.1002/chem.201900996
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10.1002/chem.201900996
Chemistry - A European Journal
Gold-Catalyzed Cyclisation by 1,4-Dioxidation: Synthesis of
Vanessa Claus,a Lise Molinari,a Simon Büllmann,a Jean Thusek,a Matthias Rudolph,a
Frank Rominger,a[+] A. Stephen K. Hashmia,b,*
[a] M. Sc. V. Claus, Dr. L. Molinari, M. Sc. S. Büllmann, M. Sc. J. Thusek, Dr. M.
Rudolph, Dr. F. Rominger, Prof. Dr. A. S. K. Hashmi
Organisch-Chemisches Institut
Heidelberg University
Im Neuenheimer Feld 270, 69120 Heidelberg (Germany)
Fax: (+49)-6221-54-4205
E-mail: hashmi@hashmi.de
Homepage: http://www.hashmi.de
[b] Prof. Dr. A. S. K. Hashmi
Chemistry Department, Faculty of Science
King Abdulaziz University, Jeddah 21589, Saudi Arabia
[+] Crystallographic investigation
Keywords: gold, ynamides, diphenylsulfoxide, cyclization, diynes
Abstract:
Amide-substituted diynes were cyclized in the presence of a cationic gold catalyst and
an external nucleophile leading to 1-indenones and 1-iminoindenones. The electron-
donating features of the nitrogen atom enable the formation of a reactive ketene
iminium ion, which can be trapped by either diphenyl sulfoxide or anthranil as
nucleophiles in a subsequent oxidation step, providing substituted inden-1-on-3-
carboxamides.
In the highly active field of gold catalysis,[1] reactions applying diyne systems as starting
materials have become increasingly important.[2] The most common reactivity pattern
includes the intermolecular or intramolecular attack of a nucleophile on one of the triple
bonds activated by a π-coordinated gold complex. The resulting intermediates can
undergo valuable cascade reactions to form highly interesting products.[3] If the diyne
contains at least one terminal alkyne, it can additionally be activated by a σ-bonded
gold complex in the so-called dual activation mode.[1c] This leads to the formation of
highly reactive gold vinylidenes which can undergo further transformations (Scheme
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1).[4] The highly electrophilic sp-carbon center of the vinylidene can for example be
trapped by an intramolecular nucleophilic attack of an phenyl ring to form a
dipenzopentalene (III) or by an C(sp3)-H bond insertion leading to benzofulvene
substrates (IV).
We envisioned that if we substitute the gold acetylide by an ynamide, we could form a
ketene iminium ion instead, which can then likewise can be attacked by different
intramoleculary offered nucleophiles (Scheme 2). Ynamides, with their polarized
structure due to the electron-donating features of the nitrogen atom, are highly reactive
and together with the distinction of the two sp-carbon atoms they show interesting
reactivities.[5] To our surprise, when we conducted our initial experiments, we always
obtained a product with two additional oxygen atoms[6] in place of the desired
substituted dibenzopentalene (Scheme 3). It seems that the ketene iminium ion
intermediate is way more stable and the electrophilicity of the sp-carbon is significantly
lowered. Therefore, the intermediate can be attacked by an external O-nucleophile,
which in this case could be the oxygen of a tosyl group of another substrate molecule,
leading to an interesting substituted 1-indenone structure in the further reaction.
Functional 1-indenones are wide spread structural motifs in natural products and highly
bioactive molecules.[7] Furthermore, they are important intermediates in the synthesis
of natural products and pharmaceuticals.[8] Especially 2-aryl-substituted 1-indenones
show outstanding pharmaceutical and biological features.[9] Since these structures are
so highly favorable, there are already many protocols for their synthesis published in
the last decades.[10] Nevertheless, a mild procedure for the synthesis of functionalized
indenones with readily available starting materials is still of high interest. There is only
one example known for a gold-catalyzed protocol starting from 2-alkynylarylketones.[11]
That procedure still lacks selectivity as the corresponding isoquinoline is obtained as
a side product only, isolated in low yields. Protocols for the synthesis of 2-aryl-3-
carboxamide indenones are not known at all. Therefore, we present a direct and mild
gold(I)-catalyzed oxidative cyclization of tosylamide-substituted diynes leading to
substituted 1-indenones.[12]
Scheme 1. Dual activation of diynes and further reactions
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Scheme 2. Formation of a ketene iminium ion
no:
Ts
Ts
N
N
Ts
N
O
10 mol% IPrAuNTf2
toluene, 60 °C, 6 h
O
1a
2a
, 34%
Scheme 3. Initial experiment
Triggered by the results in our initial experiments, we first conducted a screening with
diyne 1a and different external oxygen donors (Table 1). First we used 3,5-
dibromopyridine-N-oxide as an oxidant and could selectively obtain the diketone 3a as
a major product, the desired product 2a was observed only in traces (entry 2). It is
known, that pyridine-N-oxides can generate α-oxo gold carbene intermediates with
alkynes in the presence of a gold catalyst.[13] Oxidation of the carbene with a second
molecule of N-oxide can then lead to the formation of the diketone moiety.[14] Therefore
we tested different catalyst/oxidant/solvent combinations, which are already known to
avoid the formation of the undesired diketone.[15] But neither changing the oxidant to
the sterically more hindered 8-ethylquinoline- and 8-isopropylquinolin-N-oxide, nor
replacing the IPr ligand by the more bulky JohnPhos (entry 5) or XPhos ligand (entry
4), provided a higher yield. Next, we used the diphenyl sulfoxide as an oxidant and
could achieve a significant increase of the yield of 2a and no undesired 1,2-diketone
was observed (entry 7), although this undesired side-reaction is known.[16] Now we
-
changed the counter ion to the SbF6 anion and obtained the product in an excellent
yield (entry 8). When we applied the same catalyst in combination with an N-oxide
instead of the diphenyl sulfoxide, exclusively the diketone was obtained in excellent
yields. When conducting the reaction at room temperature, a small decrease of yield
was observed. However, increasing the amount of the oxidant did not lead to any
improvement.
Table1. Optimization of the Reaction Conditions
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Yield
2a
60 °C 34%
Entry Catalyst
A-O
Solvent
T
3a
0%
[b]
[b]
1
2
3
4
5
6
7
8
9
6
7
IPrAuNTf2
IPrAuNTf2
IPrAuNTf2
----
toluene
toluene
H2O
2 eq. 4a
2 eq. 4b
2 eq. 4b
2 eq. 4b
2 eq. 4c
2 eq. 4d
2 eq. 4d
2 eq. 4a
2 eq. 4d
3 eq. 4d
60 °C trace 79%
80 °C 0%
0%
XPhosAuCl/AgNTf2
JohnPhosAuCl/AgNTf2
JohnPhosAuCl/AgNTf2
IPrAuNTf2
DCE
rt
rt
rt
0%
0%
0%
81%
89%
83%
0%
DCE
DCE
DCM
DCM
DCM
DCM
DCM
40 °C 76%
40 °C 91%[c] 0%
[(IPr)Au(NCMe)]SbF6
[(IPr)Au(NCMe)]SbF6
[(IPr)Au(NCMe)]SbF6
[(IPr)Au(NCMe)]SbF6
rt
rt
0%
95%
0%
86%
40 °C 90%
0%
1
[a] Reactions were run on a 250 mmol scale. Yields were determined by H NMR spectroscopy;
hexamethylbenzene was used as an internal standard, if not stated otherwise. [b] 10 mol% of the catalyst
was used. [c] Isolated yield.
After optimizing the reaction conditions, we focused on the scope of the reaction (Table
2). First, we varied the aryl unit connected to one of the two alkynes. To our delight,
electron-donating groups, such as tert-butyl (2b) and methoxy (2d) groups in para-
position were tolerated well. Changing to a mesityl group (2c) lead to a significant
decrease of the yield, probably caused by a higher steric hindrance. If a fluorine in para
position as an electron-withdrawing group is applied, the product was obtained in an
excellent yield (2e). A 2-thienyl group (2f) as model for an electron-rich heterocycle
and the 1-naphthyl group (2g) were also tolerated in the reaction. When we tested
aliphatic groups, the product could be obtained in moderate yields using an n-hexyl
group (2h), whereas a tert-butyl group (2i) leads to the decomposition of the starting
material. As a next step, we varied the backbone of the diyne. If a [1,3]-dioxole
backbone is applied, decomposition of the starting material was observed (2j). When
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using an additional methoxy (2k) or fluorine group (2l) in the backbone, the reaction
was proceeding well and excellent yields could be obtained.
Table 2. Scope with respect to the N-methyltosylynamides
Furthermore, we extended the scope by varying the amide (Table 3). Since, for
reasons of stability, an electron-withdrawing group is needed, we started by switching
from a methyl group to a benzyl group, which was tolerated extremely well; 2m was
obtained in excellent yield. In order to verify the connectivity of the indenone structure,
we conducted an X-ray crystal structure analysis of a single crystal of 2m (Figure 1).[17]
A thiophen-2-ylmethyl group (2n) only led to a decomposition of the starting material.
However, a cyclopropyl group (2o) was tolerated well, delivering a good yield of the
corresponding indenone. Changing the electron-withdrawing group to a nosyl group
(2p), provided a good yield, too. An oxazolidinone group was also tolerated, 2q was
obtained in moderate yields.
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Table 3. Scope with respect to the amide
Scheme 4. Reaction with anthranil as a weak nucleophile
To extend this new reactivity, we also varied the external nucleophile. Since anthranil
is known to react as a nitrene donor,[18] comparable to the oxygen atom transfer from
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diphenyl sulfoxide, we decided to apply this nucleophile together with our standard
diyne 1a. To our delight, the reaction proceeded well and the corresponding diimine
5a could be obtained in moderate yields. We also proved the structure of the product
by single crystal X-ray structure analysis (Figure 1).[17]
As mentioned, the reaction most likely[19] is initiated by a nucleophilic attack of the β-
carbon of the amide-substituted alkyne at the π-activated alkyne in a 5-endo-dig
fashion, promoted by the electron donating features of the nitrogen atom (Scheme 5).
This leads to the formation of the ketene iminium ion IIb. This intermediate can be
attacked at the electrophilic sp-carbon by the oxygen atom of diphenyl sulfoxide,
giving rise to intermediate IIIb. In the next step, the gold carbene intermediate IVb is
formed and diphenyl sulfide is released. The gold carbene IVb can be attacked by a
second diphenyl sulfoxide molecule, leading to the desired indenone product 2 by
releasing another diphenyl sulfide molecule. An alternative pathway via a gold α-oxo
carbene is also possible. To proof our mechanistic proposal, we intended to trap the
different carbene species with an alkene function in a cyclopropanation reaction.
Trapping the gold carbene IVb with an alkene function in appropriate distance
proceeded well and the corresponding cyclopropanation product 6q was obtained in
moderate yields (Scheme 4).
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Scheme 5. Proposed reaction mechanism
However, proving that no α-oxo carbene (of type VI) is involved, is significantly more
challenging. Nevertheless, diphenyl sulfoxide is not known for forming gold α-oxo
carbenes in intermolecular alkyne oxidation reaction pathways.[13] We synthesized an
diyne substrate 1r, which offers an alkene function in the right distance for trapping a
possible α-oxo-carbene. To our delight it was possible to isolate the corresponding
indenone 2r in a good yield with respect to the oxidant, the alkene function remained
untouched, no 7r was detected (Scheme 7). In our screening we always obtained the
diketone as a main product when using an N-oxide, thus we assumed that with these
oxidants the reactions proceeded via the α-oxo carbene. To show that the carbene can
be trapped by the alkene, we tried the same reaction with the N-oxides 4a-4c. The
gold-catalyzed cyclopropanation reaction of a similar system is already known from
simple en-ynamide-type substrates.[14e] Unfortunately, we only obtained complex
product mixtures and the desired cyclopropanation product could not be isolated. As
there are many different functional groups in our system, the unselective reaction can
easily be explained.
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Figure 1. Solid state molecular structures of 2m (left), 3a (right) and 6q (bottom)
Scheme 6. Trapping the gold carbene by a cyclopropanation reaction
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Scheme 7. Mechanistic studies
In conclusion, we developed a mild and simple gold-catalyzed protocol for the
synthesis of functionalized 1-indenones. The gold-catalyzed cyclization of the amide-
substituted diyne to the indenone products most likely proceeds as predicted, i.e. via
the ketene iminium ion intermediate. However, the sp-carbon of this intermediate is by
far not electrophilic enough to be trapped by an aryl or tert-butyl group like it is known
for gold vinylidenes.[20] But we were able to trap the ketene iminium ion with external
nucleophiles giving rise to highly desirable 1-indenone and 1-iminoindenone
structures. To the best of our knowledge, there is no synthesis established for
achieving this specific substitution pattern. The reaction is tolerant to most of the tested
functional groups and leads to a broad range of products. For further applications the
biological activity of those products should be distinguished.
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10.1002/chem.201900996
Chemistry - A European Journal
This article is protected by copyright. All rights reserved.

10.1002/chem.201900996
Chemistry - A European Journal
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