Gold-catalyzed Intermolecular Oxidations of 2-Ketonyl-1-ethynyl Benzenes with N-Hydoxyanilines to Yield 2-Aminoindenones via Gold Carbene Intermediates

Article abstract of DOI:10.1002/anie.201606467

Gold-catalyzed oxidations of 2-ketonyl-1-ethynyl benzenes with N-hydroxyanilines yield 2-aminoindenone derivatives efficiently. Experimental data suggests that this process involves an α-oxo gold carbene intermediate, generated from the attack of N-hydroxyaniline on furylgold carbene intermediate, rather than the typical attack of oxidants on π-alkynes.

Full text of DOI:10.1002/anie.201606467

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Communications
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International Edition: DOI: 10.1002/anie.201606467
German Edition: DOI: 10.1002/ange.201606467
Heterocycle Synthesis
Gold-catalyzed Intermolecular Oxidations of 2-Ketonyl-1-ethynyl
Benzenes with N-Hydoxyanilines to Yield 2-Aminoindenones via Gold
Carbene Intermediates
Bhanudas Dattatray Mokar, Deepak B. Huple, and Rai-Shung Liu*
Abstract: Gold-catalyzed oxidations of 2-ketonyl-1-ethynyl
benzenes with N-hydroxyanilines yield 2-aminoindenone
derivatives efficiently. Experimental data suggests that this
process involves an a-oxo gold carbene intermediate, gener-
ated from the attack of N-hydroxyaniline on furylgold carbene
intermediate, rather than the typical attack of oxidants on p-
alkynes.
Notably, these new alkyne oxidations do not involve
attack of the N-hydroxyanilines on the gold–p-alkynes.[6–8]
Gold-catalyzed intermolecular reactions of terminal alkylal-
kynes with N-hydroxyanilines generally follow proceed by
O attack [Eq. (3)], thus yielding indoles preferably.[7] The
preference for N attack is noted for terminal arylalkynes and
1,6-enynes to afford easily hydrolyzed nitrones [Eq. (3)].[8]
The strategy of this work involves an initial formation of the
oxonium intermediate III which is attacked subsequently by
the N-hydroxyaniline to generate the a-oxo gold carbene IV
T
he gold-catalyzed intermolecular oxidations of alkynes
with either pyridine-based oxides or nitrones, have been
intensively investigated.[1] The resulting a-oxo gold carbenes
[Eq. (2)] by cleavage of the C O and N O bonds. Exper-
imental data to support this atypical mechanism and to
exclude nitrone intermediates (V)[9] are described in detail.
=
ꢀ
ꢀ
are versatile in various reactions, including X H additions
(X = O, NR),[2] C H functionalizations,[3] cyclopropana-
ꢀ
tions,[4] and annulation reactions.[5] These alkyne oxidations
are synthetically appealing as readily available alkynes
replace a-oxo diazo species as the initial reagents. The
reported oxidations proceed exclusively by an attack of highly
ꢀ
basic N O oxides at gold–p-alkynes, as depicted in Equa-
tion (1). Catalytic oxidations from alkynes, through a distinct
and novel mechanism, are highly significant and challenging.
In seeking a breakthrough, we report the gold-catalyzed
oxidations of 2-ketonyl-1-ethynylbenzenes with N-hydroxy-
anilines to yield 2-aminoindenones efficiently [Eq. (2)].
Herein, N-hydroxyaniline serves as one oxygen donor to
oxidize terminal alkynes into ketones before becoming
aniline. This reaction sequence produces water as a byproduct,
thus fulfilling atom economy.
Fused five-membered azacyclic ketones are present in
bioactive molecules, and two selected examples, VI and VII,
are provided in Figure 1. The compound VI (MR16924)
represents one prominent example of the tripentone family
and shows remarkable antitubulin effects over tested lines
Figure 1. Bioactive indene molecules containing nitrogen.
including leukemia, central nervous system, ovarian and
breast cancers,[10] whereas the fluorazone derivatives VII are
nontoxic, anti-proliferative agents in melanoma cells.[11] Our
new syntheses lead to the 2-aminoindanonyl derivatives VIII–
XI. The synthesis of indeno-[2,1-b]-pyrroles VIII has
attracted considerable attention because of their structural
similarity to fluorazones (VII). Notably, VIII can be con-
verted into other bioactive molecules (IX–XI), which have
[*] B. D. Mokar, Dr. D. B. Huple, Prof. Dr. R.-S. Liu
Department of Chemistry, National Tsing-Hua University
Hsinchu, 30013, Taiwan (ROC)
E-mail: rsliu@mx.nthu.edu.tw
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201606467.
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Table 2: Catalytic synthesis of indeno[2,1-b]pyrrol-8(1H)-ones.
been claimed in a US patent to possess analgesic or anti-
inflammatory activities.[12a] Our target VIII serves an inter-
[12b,c]
ꢀ
mediate for useful S N-containing heterocycles.
Table 1 presents optimized reactions conditions for 1-
alkynoyl-2-ethynylbenzene (1a) with N-hydroxyaniline (2a;
1.2 equiv) in the presence of various gold catalysts (5–
8 mol%). We tested the reactions with LAuCl/AgNTf2 [L =
P(tBu)2(o-biphenyl), 5 mol%] in 1,2-dichloroethane (DCE;
Table 1: Catalyst screening over various gold catalysts.
Entry
Catalyst[b] (mol%)
t [h]
Solvent
Yields [%][c]
1a
3a
4a
1
2
3
4
5
6
7
8
9
LAuCl/AgNTf2 (5)
LAuCl/AgNTf2 (8)
LAuCl/AgOTf (8)
LAuCl/AgSbF6 (8)
PPh3AuCl/AgNTf2 (8)
IPrAuCl/AgNTf2 (8)
AgNTf2 (8)
LAuCl/AgNTf2 (8)
LAuCl/AgNTf2 (8)
LAuCl/AgNTf2 (8)
18
8
16
8
16
12
24
10
12
36
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCM
toluene
1,4-dioxane
–
–
–
–
–
–
82
–
–
–
12
–
10
–
–
–
–
–
–
8
82
94
68
85
80
90
–
88
80
44
[a] [substrate]=(0.05m). [b] L=P(tBu)2(o-biphenyl), [c] Yields are those
for product isolated after silica chromatography.
thus yielding the desired 4d and 4e, respectively, in satisfac-
tory yields (entries 3 and 4). For the alkyl-substituted
analogues 1 f–h (R = cyclopropyl, methyl, and n-butyl), the
corresponding products 4 f–h were produced in excellent
yields (entries 5–7). These catalytic reactions worked well
with various 4- or 5-phenyl-substituted substrates 1i–l (X = F,
Cl or Y= Cl, OMe), thus giving the compounds 4i–l with
yields exceeding 84% (entries 8–11). Entries 12–15 show the
compatibility of these reactions with various 4-substituted N-
hydroxyanilines (2b–e; X = Me, Cl, F and CO2Et), thus
delivering the compounds 4m–p in 85–91% yields.
To our pleasure, the scope of these reactions became
considerably expanded by using various aryl- or heteroaryl
ketones (5a–k) to yield the desired 2-aminoindenones 6
efficiently (Table 3). Gold-catalyzed reactions of the phenyl-
ketone-derived substrates 5a–e with 2a afforded 6a–e in 72–
92% yields (entries 1–5). The 3,4-dimethoxyphenylketone
derivative 5e showed the best efficiency at a brief period
(1 h). We tested the reactions of 2- and 3- substituted thienyl
and furylketones (5 f,g and 5h,i) and they delivered the
desired 2-aminoindenones 6 f–i in satisfactory yields
(entries 6–9). For 2-benzofuryl- and 2-pyridinylketone deriv-
atives, 5j and 5k, respectively, led to the corresponding
products 6j (85%) and 6k (45%; entries 10 and 11). We
prepared the alkenylketone substrates 5l–n, which afforded
the desired products 6l–n in 62–90% yields (entries 12–14).
Herein the byproduct 7l was obtained in a minor proportion
(20%) because of a N-attack of the hydroxyaniline on 5l
(entry 12).[8] An electron-rich heteroaryl group avoids this
byproduct because the oxomium intermediate III forms
rapidly [Eq. (2)].
10
[a] 1a (0.05m). [b] L=(1,1’-biphenyl-2-yl)di-tert-butylphosphine. [c] Yield
is that of the isolated product after silica chromatography. DCE=1,2-
dichloroethane, DCM=dichloromethane, Tf=trifluoromethanesulfonyl.
258C, 18 h), thus yielding the indenone 3a and indeno[2,1-
b]pyrrol-8(1H)-one 4a in 12 and 82% yield, respectively
(entry 1). A large loading (8 mol%) of this catalyst enabled
complete transformation of 1a into 4a in 94% yield (entry 2).
In a separate experiment, LAuCl/AgNTf2 (5 mol%) allowed
complete conversion of 3a into 4a in 98% yield. A change of
silver salts as in LAuCl/AgOTf and LAuCl/AgSbF6, each at
8 mol%, afforded 4a in 68 and 85% yield, respectively
(entries 3 and 4). PPh3AuCl/AgNTf2 and IPrAuCl/AgNTf2
[IPr= 1,3-bis(2,6-diisopropylphenyl)
imidazole-2-ylidene]
were also effective to give 4a in 80 and 90% yield,
respectively (entries 5 and 6). Notably, AgNTf2 alone was
not catalytically active at all (entry 7). Other solvents like
toluene and 1,4-dioxane were less efficient for the LAuCl/
AgNTf2 catalyst (entries 9 and 10). The molecular structures
of 3a and 4a were confirmed by X-ray diffraction studies.[13]
We assessed the scope of reaction of such indeno[2,1-
b]pyrrol-8(1H)-one syntheses with various 2-alkynoyl-1-ethy-
nylbenzenes (1b–l) and the N-hydroxyanilines 2b–e using the
LAuCl/AgNTf2 catalyst [L = P(tBu)2(o-biphenyl), 8 mol%;
Table 2]. The results in entries 1–2 show the compatibility of
this reaction with 1b and 1c, bearing R = 4-XC6H4 (X = OMe
and CF3), thus giving the desired products 4b and 4c,
respectively, in satisfactory yields (78–85%). The reactions
were extended to the substrates 1d and 1e (R = 2-thienyl, H),
Alkyl-substituted phenylketones are not suitable sub-
strates because they form nitrone species,[14] thus giving
distinct isoindole products.[9b] Such 3-alkyl indenones were
2
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Table 3: Various arylketone- and alkenylketone-derived substrates.
8-methylquinoline oxide and aniline in dry DCE, yielding the
desired [18O]6a in 37% yield, with a small loss of the 18O
content [Eq. (6)]. This observation indicates the intermediacy
of the a-oxo gold carbene A, which is subsequently attacked
by aniline to form the C- and O-bound a-amino enolates B
and C, respectively, to enable an aldol reaction. Herein, the
oxo group of A arises partly from the ketone group of [18O]5a,
and is indicative of an indirect attack as the major route.
Although 6a can be produced alternatively from this three-
component coupling, as depicted in [Eq. (6)], the efficiency is
poor because of the hydroamination product 5a–I as well as
an imination of 5a at its phenyl ketone. These imination
products are easily hydrolyzed into the ketone 5a–H during
the workup.
[a] [substrate]=(0.05m). [b] L=P(tBu)2(o-biphenyl), [c] Yields are those
for products isolated after silica chromatography.
A plausible mechanism is depicted in Scheme 2. Our 18O-
labeling experiment in Equation (5) indicates that the car-
bonyl oxygen atom of the resulting indenone 6a arises
completely from the ketones of 5a, thus excluding the
intermediacy of the nitrone V’. The nitrone V’ is known to
be too unstable to exist for aryl-, alkenyl-, and alkynyl-
substituted arylketones.[15] Herein, we propose prior attack of
the tethered ketones on the gold–p-alkynes D to form the
oxonium species E.[16] This intramolecular 5-exo-dig cycliza-
tion is much more rapid than that for an intermolecular attack
of N-hydroxyaniline at gold p-alkynes, thus avoiding either
indole or nitrone byproducts [Eq. (3)]. An attack of the N-
hydroxyaniline on E yields the O-bound product F, which
might undergo a 1,2-proton transfer to form the ammonium
species G because the nitrogen center is more basic than the
Scheme 1. Chemical transformations. DMS=dimethylsulfoxide,
NBS=N-bromosuccinimide, NIS=N-iodosuccinimide, PPA=poly-
phosphoric acid.
alternatively prepared from 6l through selective hydrogena-
tion of its N-methyl derivative (Scheme 1). The compound 6l
was converted into the indeno[2,1-b]pyrrol-8(1H)-one 4a, by
using a Brønsted acid, efficiently and then into the bromo and
iodo derivatives 9a and 9b, respectively, in good yields.
We performed isotope-labeling experiments to elucidate
the mechanism. As depicted in Equation (4), the treatment of
2-phenylketonyl-1-ethynylbenzene (5a) with N-hydroxyani-
line and C6D5NH2, in a molar ratio 1:1, yielded 6a bearing no
deuterium. This information indicates that N-hydroxyaniline
is the only source of the amino moiety of 6a. Our 18O-labeling
experiment [Eq. (5)] was informative because [18O]5a was
transformed into the product [18O]6a without loss of 18O
content upon comparison of their mass spectra (see Figure S1
in the Supporting Information). Additional mechanistic
insight is inferred from the oxidation of species [18O]5a with
ꢀ
oxygen center. The ammonium centers in G induce an N O
bond cleavage, thus generating A, which is a viable inter-
mediate for the final products 3 or 6 [Eq. (6)]. We envisage
that the released aniline captures A rapidly as this acid–base
=
pair is in close proximity through a NH···O C hydrogen
bonding. This hypothesis rationalizes the observation in
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Scheme 2. Proposed reaction mechanism.
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Equation (4). The feasibility of the E!F!G path is sup-
ported by the oxoamination of [18O]5a with 8-methylquino-
line oxide and aniline [Eq. (6)]. This mechanism is depicted in
the D!E!H transformation.
ꢀ
Before this work, a gold-catalyzed attack of N O-
containing oxides on alkynes was the only path to initiate
alkyne oxidations.[17] We report an atypical route in the gold-
catalyzed intermolecular oxidations of ketonylalkynes with
N-hydroxyanilines. These oxidations involve initial formation
of oxonium species which are subsequently attacked by N-
hydroxyaniline, and lead to oxoamination products. This path
is strongly supported by 18O-labeling experiments. In one
control experiment, our resulting 2-aminoindenones were
alternatively produced from the same ketonylalkynes, 8-
methylquiniline oxide, and aniline, but the efficiency was low.
Gold carbenes generated from oxonium, and possibly other
stable carbocations, may inspire future work on this area.
Acknowledgments
We thank National Science Council, Taiwan, for financial
support of this work.
Keywords: alkynes · carbenes · gold · heterocycles · oxidations
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4
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[14] We prepared a cyclohexyl phenylketone 10, which can form
nitrones species.[15] In the presence of gold catalyst, this alkyl
derivative afforded the isoindole product 11a in 72% yield. Its
molecular structure was confirmed by X-ray diffraction.
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Received: July 4, 2016
8383.
Published online: && &&, &&&&
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Communications
Heterocycle Synthesis
B. D. Mokar, D. B. Huple,
R.-S. Liu*
&&&& — &&&&
Gold-catalyzed Intermolecular
Oxidations of 2-Ketonyl-1-ethynyl
Benzenes with N-Hydoxyanilines to Yield
2-Aminoindenones via Gold Carbene
Intermediates
Under attack: The title reaction efficiently
yields 2-aminoindenone derivatives.
Experimental data suggests that this
process involves an a-oxo gold carbene
intermediate, generated from the attack
of N-hydroxyaniline on a furylgold car-
bene intermediate, rather than the typical
attack of oxidants on p-alkynes.
6
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